collision and grounding

16th INTERNATIONAL SHIP AND OFFSHORE STRUCTURES CONGRESS 20-25 AUGUST 2006 SOUTHAMPTON, UK VOLUME 2

COMMITTEE V.1

COLLISION AND GROUNDING COMMITTEE MANDATE Concern for structural arrangements on ships and floating structures with regard to their integrity and adequacy in the events of collision and grounding, with the view towards risk assessment and management. Consideration shall be given to the frequency of occurrence, the probabilistic and physical nature of such accidents, and consequences on watertight integrity, structural integrity and environment.

COMMITTEE MEMBERS Chairman:

G. Wang C. Ji P. Kujala S.-Gab Lee A. Marino J. Sirkar K. Suzuki P. Terndrup Pedersen A. W. Vredeveldt V. Yuriy

KEYWORDS Collision, grounding, risk assessment, limit state design, accident scenarios, incident probability, acceptance criteria, internal mechanics, external mechanics.

1

ISSC Committee V.1: Collision and Grounding

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CONTENTS 1.

INTRODUCTION ........................................................................................................ 5

2.

PRINCIPLES AND METHODOLOGY OF COLLISION AND GROUNDING DESIGN STANDARDS ..................................................................... 5 2.1 Accidental limit state design ............................................................................ 6 2.2 Existing design standards................................................................................. 7 2.3 Current trends in design standard development related to accident limits ...... 8 2.4 Recent national and international projects....................................................... 8 2.5 Recommendations .......................................................................................... 10

3.

APPLICATION OF RISK ASSESSMENT METHODOLOGY .............................. 11 3.1 Risk assessment methodology........................................................................ 11 3.2 Application to collision and grounding problems.......................................... 12 3.3 Application to waterway designs ................................................................... 13 3.4 Safety measures and risk control options....................................................... 15 3.5 Recommendations .......................................................................................... 15

4.

LIKELIHOOD OF INCIDENTS, PROBABILISTIC ENERGY DISTRIBUTION 16 4.1 Available approaches ..................................................................................... 16 4.2 Statistics of incidents...................................................................................... 16 4.3 Predictive calculations and energy reference values ..................................... 18 4.4 Recommendations .......................................................................................... 20

5.

MECHANICS OF COLLISION AND GROUNDING ............................................. 20 5.1 General ........................................................................................................... 20 5.2 Internal mechanisms....................................................................................... 20 5.3 Rupture criteria............................................................................................... 22 5.4 External mechanics ........................................................................................ 24 5.5 Influences of fluid in tanks............................................................................. 26 5.6 Coupled internal and external mechanics ...................................................... 26 5.7 Recommendations .......................................................................................... 30

6.

CONSEQUENCES OF COLLISION AND GROUNDING..................................... 30 6.1 Oil outflow ..................................................................................................... 31 6.2 Damage stability............................................................................................. 31 6.3 Ship evacuation .............................................................................................. 32 6.4 Residual strength ............................................................................................ 33 6.5 Post-accidental loads...................................................................................... 34 6.6 Other consequences........................................................................................ 34 6.7 Recommendations .......................................................................................... 35

7.

ESTABLISHMENT OF ACCEPTANCE CRITERIA.............................................. 36

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ISSC Committee V.1: Collision and Grounding 7.1 7.2 7.3 7.4

Principles........................................................................................................ 36 Comparative risk assessment ......................................................................... 36 Absolute risk assessment................................................................................ 37 Recommendations .......................................................................................... 38

8.

DESIGNS AGAINST COLLISION AND GROUNDING ....................................... 39 8.1 Buffer bow...................................................................................................... 40 8.2 Innovative double hull designs and steel sandwich panels............................ 40 8.3 Double hulls ................................................................................................... 42 8.4 Composite and sandwich panels .................................................................... 43 8.5 Aluminum panels ........................................................................................... 43 8.6 Economic considerations ............................................................................... 43 8.7 Recommendations .......................................................................................... 43

9.

OFFSHORE STRUCTURE COLLISION ................................................................. 44 9.1 Existing criteria in offshore design codes ...................................................... 44 9.2 FPSO collision ............................................................................................... 45 9.3 FPSO collision scenarios ............................................................................... 46 9.4 Design events for FPSO collision .................................................................. 47 9.5 Ship - Bridge collision ................................................................................... 47 9.6 Recommendations .......................................................................................... 48

10.

CONCLUSIONS AND RECOMMENDATIONS .................................................... 49

ACKNOWLEDGEMENT ................................................................................................... 50 REFERENCES..................................................................................................................... 50


1.

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INTRODUCTION

This is the fourth time since 1990 that the ISSC has established a special committee to address the issue of ships’ collisions and groundings. The current 2006 Committee V.1 on Collision and Grounding is sequential to the 2003 Committee V.3 on Collision and Grounding, 1997 Committee V.4 on Structural Design against Collision and Grounding, and 1994 Committee V.6 on Structural Design for Pollution Control. This report intends to be both a handbook, which covers past and current research achievements, and a compass, directing us towards further research and development. Research and development in 1990s were characterized by: • Several national and international large model testing projects and pilot simulation studies using nonlinear analysis tools, • Theoretical development of the structural crashworthiness concept and methodology, and • Development of environmentally friendly tank arrangements and structural designs. As of the late 1990s, the foci have been: • The integration of key research achievements into risk-based methodology, • Improved application of advanced simulation tools (FEM), • Concepts to develop relevant rules and regulations, and • Continued development of innovative crashworthy structures. These latest developments are the main topic of the current committee report. This report also provides an overview of the latest research, aiming at key risk assessment components for collisions and groundings. In addition to the traditional focus on oil tankers, this report also addresses collisions that occur with offshore structures, high speed crafts, and innovative double hull designs. Finally, the report looks at the crushing behavior of composite and aluminum panels.

2.

PRINCIPLES AND METHODOLOGY GROUNDING DESIGN STANDARDS

OF

COLLISION

AND

There are no generally accepted collision and grounding design standards. While principles may be based on design objectives (i.e.: oil outflow standards or survivability standards), none are universally accepted.

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Essentially, the principles of collision and grounding design standards would be composed of the following elements: • How and why accidents occur: navigation, accident scenarios, probability of occurrence of certain types of accidents. • What happens (structurally) when a collision, grounding, stranding, or allision occurs: structural mechanics in collisions and groundings. • What are the consequences of structural damage: property damages, environmental damages, and loss of life. • How can each of the above be addressed: accident prevention, minimization of structural damage, mitigation of damage consequence, response to damage and loss of life. The risks of collisions and groundings accompany the shipping industry. Traditionally, these risks are addressed in damage stability and compartment requirements. These rules and regulations are mostly prescriptive in nature, and often address individual events separately. Over the past decades, the structural engineering design community has increasingly applied limit state and risk assessment methodology. 2.1

Accidental limit state design

In ship designs, four accidental limit states are often considered: serviceability, ultimate, fatigue, and accidental limit states. The limit states are conditions that will cause a particular structural member or a system to experience performance failure (Paik and Thayamballi 2003, ISO 2005b). The limit state designs are considered improvements over the traditional allowable stress designs. This is because the limit state designs explicitly consider various conditions under which a structure may fail to function, and account for the uncertainties associated with determining the safety margins. One of the most lucid explanations of the advantages of limited state methods over allowable stress methods was published by the Institute for Research in Construction, National Research Council of Canada (NRC 1982). The accidental limit state represents excessive structural damage due to accidents that affect the safety of a human being, the integrity of structures and the environment. The accident limit state designs may be based on safety (including security for some situations) and environmental objectives. There could be many combinations of these objectives, such as loss of life prevention, injury or loss prevention, property damage prevention and/or mitigation, environmental pollution prevention and/or mitigation. Structural design criteria have been based on meeting these defined objectives. There may be many different methodologies for defining accident limit states, depending on the nature of the range of accident types, which create different loading scenarios. Accident types can range from explosive scenarios like fires, explosions, or blasts, to relatively lower loading rates such as low-speed groundings and collisions.

ISSC Committee V.1: Collision and Grounding 2.2

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Existing design standards

Traditionally, ship collisions and groundings have been regarded as most relevant to damage stability or cargo spill from damaged hulls. Recently, more attention has been given to a vessel’s structural resistance to an accident. Similarly, there is more focus on the impact that structural designs have on the extent of resulting damage and the consequential loss of stability, oil outflow, and residual strength. As early as the 1960s, Japan established a regulation for safely transporting nuclear waste. This regulation clearly specified that the cargo area of carried nuclear waste could not to be breached in a collision with a T-2 tanker; and energy absorption in crashed hull structures could be calculated using Minorsky’s formula. The Germanischer Lloyd (GL) has a class notation COLL that ranks the collision resistance of ships (GL 2004). To date, GL has assigned the COLL notation to about 60 ships. The collision resistance is measured by comparing a vessel’s strengthened side to another vessel’s non-strengthened single hull. Analyses of a struck ship’s energy absorption are based on two different striking bows (with and without a bulb), four draught differences of both striking and struck vessels, and assumed probability of these draft differences. The American Bureau of Shipping (ABS) has a class notation RES for SafeHull vessels that demonstrate adequate residual hull girder strength after a collision or grounding accident. Dozens of tankers have been built with this RES notation. The ABS “Guide for assessing hull-girder residual strength” (ABS 1995) provides guidelines and assumptions for facilitating an assessment of structural redundancy and hull-girder residual strength. This notation requires a ship to maintain a minimum hull girder residual strength after sustaining structural damages in the prescribed most un-favorable condition. This minimum strength will help to prevent or substantially reduce the risk of a major oil spill, ship loss due to a post-accident collapse, or disintegration of the hull during a tow or rescue operation. The International Association of Classification Societies (IACS) has developed a series of Unified Requirements for bulk carriers that directly require adequate structural strength in flooded conditions. Structures of various levels (hull girders, double bottoms, and corrugated bulkheads) are required to prove their capability in flooded conditions. Though events that lead to flooding of holds are not defined, some of these IACS Unified Requirements are intended to design against accidents, including collision or grounding prevention as possible accident scenarios. See also Committee IV.1. There is one set of regulations, ADNR (2003), that are required for anyone navigating on the Rhine River. Side structures must absorb minimum collision energy of 22 MJ in gas tankers’ side structures, when the scantlings are deviated from those prescribed in the rules.

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ISSC Committee V.1: Collision and Grounding Current trends in design standard development related to accident limits

With limited exceptions (GL and ABS), structural designs do not consider collisions and groundings. The International Maritime Organization (IMO) is developing “Goal Based Standards” (GBS) for new ship construction. Traditionally, IMO and various maritime administrations have not developed structural standards. Instead, they have relied on classification societies to develop such standards. However, through GBS, IMO is attempting to define certain “high level” goals that must be met. While this effort is in its early stages, the current discussions at IMO do not include structural performance of ship structures in collisions and groundings. See also Committees IV.1, IV.2. The IMO is developing a procedural concept for approving alternative tank arrangement (IMO 2003b). Submitted by Germany, this IMO document has a basic philosophy of comparing the critical deformation energy from the case of a side collision with a strengthened design to that of a double hull design, complying with the damage stability calculations. IMO (2002) explicitly requires minimum structural crashworthiness for transporting nuclear fuel and nuclear waste on international waters. The International Organization for Standardization (ISO) is developing standards for ships and marine technology, and has drafted general requirements for a limit state assessment (ISO 2005b). Currently, there are two standards under development – one for general requirements, and one for ultimate strength. While the ISO is not currently developing an accidental limit state, it is reasonable to expect that the ISO will develop one at a later time. The recent IACS Common Scantling Rules projects involve re-vamping structural design codes for tankers and bulk carriers. The development clearly shows the tendency of moving towards limit state design, even though collisions and groundings are not yet considered in structural designs. Certain IACS requirements indirectly take into account strength of bulk carriers with flooded compartment. 2.4

Recent national and international projects

Tankers must have double hulls. However, even a struck double hull tanker can sometimes spill oil in some situations. The Japanese national project in the 1990s (see previous ISSC reports) suggested that striking buffer bows would be advantageous in reducing damages to struck vessels. The effectiveness of buffer bow design was investigated in the Shipbuilding Research Association of Japan’s RR76 panel, and Kitamura (2000) showed the effectiveness quantitatively using FEM analysis. In 2001, the Japanese Ministry of Land Infrastructure and Transport launched a national project, the Buffer Bow Project. This multi-year national project includes large-scale model tests (Endo 2004), numerical simulations (Yamada et al, 2005), and trial structural designs.


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Discussions held at IMO’s Maritime Safety Committee, within various technical and research panels of SNAME, and at the Ship Structure Committee, have drawn attention to the need to explore survivability of modern cruise liners in flooded conditions. Historical casualties have shown that a passenger ship may survive a flooding from a stability perspective, but fail structurally. Tagg and Akbar (2004) and Iversen (2005) reported studies on the ultimate sagging capacity of a flooded passenger ship with various assumed grounding and collision damages. Increased still-water loads and wave loads of up to seastate 4 were calculated. The Smith method was used in calculating the residual ultimate sagging capacity, which was then compared with the total hull girder bending moment. These studies indicate that most modern passenger ships should be stable, and structurally capable of surviving the damage that is assumed by the typical range of scenarios. The EU-funded research project HARDER was launched in March 2000 and concluded in May 2003. The HARDER project developed a concept of probabilistic damage stability for all types of ships covered by SOLAS. This project (Rusås and Skjong 2004) has updated damage statistics used in probabilistic damage stability assessments. It has addressed the total risk associated with collision and grounding of passenger ships. Some valuable data became available on collision energy distribution functions for various areas of navigation. The concept has been extensively validated, and different proposals for the Required Subdivision Index R have been worked out to ensure equivalency with the current regulations. These proposals were submitted to IMO-SLF for consideration. The draft revision of SOLAS chapter II-1, parts A, B and B1, is, to a large extent, based on the extensive work carried out by the HARDER project (HARDER 2003). The EU-funded CRASHCOASTER project (Vredeveldt 2001) has established a relationship between the crashworthiness of a ship’s side structure and survivability, with respect to damage stability. The developed method can be used within the current SOLAS regulations. There is a new outlined approach on how to incorporate crashworthiness into damage stability assessment methods in a more rigorous fashion. The EU-funded project “Advanced Composite Steel Sandwich Structures” was initiated in 2000 and continued until the summer of 2003. The project aimed to further improve the sandwich panel properties by implementing local filling, developing and testing reliable design formulations, and designing tools as well as promoting applications of metallic sandwich panels into new areas of the transportation industry (Kujala and Roland 2002). The project included laboratory experiments and numerical analyses. A joint industry project called “Decision Support of System for Ships in Degraded Condition” was launched and is being funded by the EU under Framework Programme 6 (Alsos and Amdahl 2005). One of its objectives is to establish tools for consequence assessment of intentional grounding. The final product of this project will be prototype installations of a Decision Support System and a Man Machine Interface onboard one passenger vessel and one cargo vessel. The purpose of the installations will be to provide guidance to the ship’s master and other decision makers on how to operate the ship once critical damage (such as collision or grounding) has occurred.

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SAFEDOR is a EU-funded project responding to the need for more innovative solutions for better, cleaner, and safer transportation (www.safedor.org, IMC 2005). This entails development of a holistic approach that links risk prevention / reduction to ship performance and cost, treating safety as a lifecycle issue and design objective (Bainbridge et al 2004). The project also implies a focus on risk-based operations and the need for risk based regulations within an integrated risk-based design framework, routinely utilizing first principles tools. SAFEDOR includes several R&D tasks to develop methods and tools to assess accidental and catastrophic scenarios, and to integrate them into the design environment. In China, a research project supported by the National High Technology Research and Development Program was initiated in 2002, and finished in 2005. It focused on researching a series of Floating Production, Storage and Offloading Units (FPSO). One of the project’s sub-projects was researching the crashworthiness of a FPSO side structure. In addition, the Y-shaped side structure was introduced, and its crashworthiness analyzed (Hu and Gu 2005). In the U.S., the Ship Structure Committee (www.shipstructure.org) recently sponsored a series of studies on ship collisions and groundings, including Brown (2002), Brown et al (2004), Sajdak and Brown (2005), and Tikka (2001). These studies developed analytical models for predicting structural damage in collisions and groundings. The U.S. Marine Board of the National Academy of Sciences convened a Committee for Evaluating Double Hull Tanker Design Alternatives, and published a report (Marine Board 2001) that documents the methodology to analyze structural damage in collisions and groundings, and the consequences of these incidents. In Europe, a new Network of Excellence on Marine Structures (MARSTRUCT) was launched in 2004. The network, including collaborators from 17 countries, will work for five years to improve comfort, effectiveness, safety, reliability and environmental performance of ship structures (www.mar.ist.utl.pt/marstruct). The MARSTRUCT work packages include accidental loads, crashworthiness and impact strength, and collision tests that spread in different technical areas. 2.5

Recommendations

The committee recommends that risk assessment methodology be more widely and frequently applied in analyses, and that structural crashworthiness be explicitly taken into account. The committee recommends that performance-based design standards be developed by the community. The “community” should consist of classification societies, international organizations (such as the ISO and IMO), leading researchers in the field, representatives from the ship design and shipbuilding communities, ship owners and operators, professional societies (such as SNAME), governments, and regulators.


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The Committee recognizes that this recommendation cannot be achieved in the near future – at least not in a manner that can be universally embraced as a design standard. However, the Committee believes that this is a technologically feasible goal that should be embraced by the “community” in the long term.

3.

APPLICATION OF RISK ASSESSMENT METHODOLOGY

Risk assessment methodology has been widely recognized and more applied in dealing with collision and grounding issues. While the general concept and methodology for risk assessment was covered in ISSC 2003 V.1, this committee is focusing on the recent applications to various aspects of collision and grounding studies and projects. 3.1

Risk assessment methodology

Risk is often defined as the product of the probability/frequency of unwanted events with the associated consequences. Risk can be measured as loss of life, loss of property (ships and cargoes), environmental pollution, costs of retrieving spilled cargoes, etc. Mitigating risks can be achieved through reducing the probability of accident occurrence and/or minimizing the consequences of such accidents. The Formal Safety Assessment (FSA) methodology helps to identify and evaluate risks, and provides the basis for appropriate rules, regulations, designs, or decisions. However, there is a lack of uniformity in FSA applications. A common methodology application should be defined and formalized (Payer 2004). Risk analysis is a tool that is increasingly applied in the marine and offshore industries to manage safety, health and environmental protection. Collisions and groundings are low probability, high consequence events, especially when tankers are involved. The collision and grounding risk assessment includes the knowledge of accident occurring frequency. This may be estimated in a navigational area, comparing experience or extrapolation from historical data to an evaluation of consequences. The consequences would be measured in terms of structural damage, the number of fatalities and injuries, the amount of material released to sea, the immediate impact on environmental resources, and the subsequent costs of restoration. Risk minimizing measures include a combination of actions that reduce the frequency and consequences of accidents. Those assessing the risk normally prioritize measures that are adopted to reduce the number of hazardous situations that may cause an accident. On the other hand, because the consequences of accidents are so serious, we must develop crashworthy structure design and on-board space arrangement regulations and requirements. Most of the current risk assessments were devoted to some specific aspects of a collision or grounding; and comprehensive accident models that combine the likelihood of being in an incident and the undesirable consequences to life, property and environment are scarce.

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Figure 1 shows the steps in a risk assessment methodology for collision risk analysis. 3.2

Application to collision and grounding problems

Pedersen (2002) presented a risk analysis on a large suspension bridge that can also be applied to fixed offshore structures close to high-density shipping lanes. The model is based on dividing collisions into a number of different phenomena, and the subsequent application of mathematical models to quantify the risk from each category. As a way to reduce the consequences of accidents, Amdahl and Hellan (2004) presented an on-board and shore-based risk support system for disabled ships that are intentionally grounded. Related to this, Ueno et al (2004) have analyzed the possibility of predicting the steady drifting motion of disabled ships in wind, waves and currents. Their analyses and experiments show that there is no general unique solution for the mode of steady drifting ships.

Figure 1: Overview of steps in a comprehensive collision risk analysis.


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Friis-Hansen et al (2004) reported a pilot study that was to formulate a general framework for evaluating the navigational risk in a specific geographic area. The framework is divided into two phases. The first phase is a screening procedure that provides fast identification of critical navigation areas, and defines the number of candidate ships that are potentially involved in collisions or groundings. The second phase is a risk evaluation that takes into account the effects of Risk Control Options adopted to mitigate risks. The screening procedure combines statistics of past accidents with modeled traffic distribution. A comprehensive software risk analysis package (GRACAT) was previously developed at the Technical University of Denmark for calculating the probability of collisions, groundings, and subsequent consequences (Friis-Hansen and Simonsen 2002). This program was further enhanced using Artificial Neutral Network (ANN) that is trained to predict the structural damage in a ship’s side as a consequence of ship to ship collisions (Ravn and Friis-Hansen 2004). The input to the ANN is the absorbed energy, the length of the involved ships, the draught of the struck ship, and the angle of collision. The predicted output is the size of the hole (or holes), which is given as the dimensions of a box. The ANN for damage prediction is used in connection with the risk evaluation of a selected navigational area, where the cost related to oil spilled from a tanker is estimated. The paper compares the long-term accumulated loss caused by oil spills in a given navigational area. In addition, Lützen and Simonsen (2003), Son (2004), and Lehmann and Biehl (2004) have created and applied various risk assessment methods and techniques to collisions and groundings. 3.3

Application to waterway designs

One organization that is using collision and grounding risk analysis tools for the design of waterways is the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA). The IALA has developed the “Risk Management Tool for Aids to Navigation and VTS Authorities”. It is used for the cost effective design of navigational aids in ports and waterways. IALA has also submitted a description of the procedure to the 50th IMO session 2004. The IALA tool was developed to: • Assess the risk in ports and waterways, compared to the risk level that authorities and stakeholders deem acceptable. Some elements that may be considered include those relating to vessel conditions, traffic conditions, navigational conditions, waterway conditions, immediate consequences, and subsequent consequences; • Identify appropriate risk control options to decrease risk to the acceptable level. The available IALA risk control options include: improved co-ordination and planning, training, rules, and procedures. These options include enforcement, radio communications, active traffic management, waterway changes, and information on: navigation, meteorology, and hydrology.

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The complete tool is based on the four approaches described in Table 2 and consists of two levels: • Level One: the preliminary Qualitative Risk Assessment model developed by the U.S. Coast Guard as the “Port and Waterway Safety Assessment model” (PAWSA). It is undertaken by making a subjective assessment of a waterway’s risk level, based on the experience and expert opinion of stakeholders. • Level Two: the more detailed Quantitative Risk Assessment model developed by the Canadian Coast Guard as the “IALA Waterway Risk Assessment Programme” (IWRAP). IWRAP is capable of completing an in-depth study on waterways’ navigation requirements that enable meeting the required risk level. It can provide information on the appropriate risk control options. The key features of the IWRAP program are: • Vessel positional accuracy - determined from a set of rules developed in a marine aids study by the Canadian Coast Guard, • Safety margin, drift angles and bank affect - calculated using formulae developed mainly by PIANC, and • Probabilities of grounding and collision - derived from the formulae presented in Pedersen (1995). IWRAP is a risk analysis program similar to GRACAT (Friis-Hansen and Simonsen 2002), where the waterway is broken into discrete reaches and bends, and available traffic data are used. A number of scenarios are then created using a combination of vessel types and differing requirements for the waterway. The results from these scenarios provide quantitative data for risk assessment within the waterway. The geographical arrangements of any waterway that are set out on a vector chart can be applied directly to the model. The values of traffic management tools in the waterway, such as radio navigation services, aids to navigation, VTS, pilotage, and AIS can be inserted in this chart. The operational traffic pattern in the model includes the number and types of ships using the waterway, ships’ speeds, ships’ critical domains, traffic routes, and the spatial distribution of traffic related to time. Various meteorological and hydrological conditions may also be included. As part of the validation process, IWRAP has been applied to the Straits of Bosporus, Tampa Bay, and parts of the St. Lawrence River, with results indicating a strong correlation between theoretical and actual incident data. Other studies related to the design of waterways using simulations are presented by Gray et al (2003) and Hutchison et al (2003).


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Safety measures and risk control options

The cost of safety measures become important when risk control options are exercised to reduce risks associated with collisions and groundings. The most cost effective risk control options are often associated with the probability of occurrence. Examples of preventive risk control options influenced by ship designers are: • Bridge layout, • Navigational equipment, • Engine and steering control, • Maneuverability, and • Redundancy. Similarly, the ship operator controls risk options such as: • Ship speed, • Manning levels, • Crew attitude and training, and • Maintenance. Finally, the society is responsible for: • Vessel traffic systems, • Pilots, • Traffic lanes, • Aids to navigation, • Introduction of AIS, • Inspection procedures etc. Once a collision or grounding has taken place, the hull material and structural arrangements play important roles for limiting the consequences. 3.5

Recommendations

Future research on procedures to reduce the probability of collisions and groundings should focus on developing risk-based software that is capable of rationally modeling the associated cost, and reducing the risk of each risk control option. The calculated cost of these risk reducing measures must then be compared to the calculated savings made from reducing expenses associated with consequences such as: total number of lost ships, repair to structural damages, environmental pollution, loss of life, loss of reputation, loss of cargo, the loss of revenue, and other losses. Only when such tools have been developed can investments in risk control options and related safety improvements be balanced against the benefits.

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High Speed Crafts (HSC) is another subject that needs further research. Here, the statistical data are scarce, and the risks for these vessels are different from the risks for traditional shipping vessels. HSC vessels have different risks because they have higher speeds, more power, new hull forms, "new" materials, and new passive safety measures. Collision risk is an excellent example of the difference risk assessments. In encounters between two fast ferries or between a fast ferry and a slower vessel, the time it takes to make the necessary assessments and maneuvers reduces drastically. A fast ferry traveling at roughly 50 knots may be seen visually or on radar when it is about eight miles away. This means that there can be less than five minutes before a collision occurs, if the other vessel also is a fast vessel. During the five minutes, the other vessel is first detected, then the course and speed are estimated, and the situation is assessed. This will take some time. Thereafter, there must also be time for evasive maneuvers. The amount of time this takes will depend upon the maneuverability of the vessels. In other words, collision avoidance for HSC is completely different from that of traditional vessels; and existing risk assessment models cannot be applied to HSC with confidence. If wing in ground (WIG) vessels are introduced, then the problems associated with collision prediction will be even greater.

4.

LIKELIHOOD OF DISTRIBUTION

INCIDENTS,

PROBABILISTIC

ENERGY

Here, incidents are defined as events that could or would result in an unintended collision or grounding. In order to control losses related to collisions and groundings, it is important to understand the causative factors of incidents. 4.1

Available approaches

Incident occurrence frequencies may be determined through: 1) statistics from historical data, 2) expert opinions, 3) predictive calculation, or 4) risk analysis, as indicated in Table 1. 4.2

Statistics of incidents

As expected, unbiased statistics are the most reliable data for identifying typical and critical incident cases. Nevertheless, statistics of historical data are not error free. Error sources may be accidental underreporting, information misinterpretation, and incorrect incident categorization regarding ship type, accident type, and severity. Most databases are usually sparsely populated at the tail of the distribution. At the higher end, there are by nature very few casualties, and at the lower end, casualties are generally under reported. Furthermore, conditions surrounding an incident, such as vessel speed, loading condition, environmental condition and so on, are not always recorded, and are sometimes poorly recorded.


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TABLE 1 APPROACHES FOR DETERMINING INCIDENT OCCURRENCE FREQUENCIES AND ENERGIES Approach

Main Advantages

Main Disadvantages

Statistics of incidents

Long been regarded as the only reliable sources

Limitation with incident reports, difficulty in application to the future

Expert opinions

Long been used when limited by data

Subjective

Predictive calculations

Predict unfavorable conditions, inexpensive

Targets known scenarios, limits choice of software/programs, restricted to occurrence probability

Comprehensive risk analysis

Rational, includes consequences

Relies on accident data for benchmarking

We should also be careful when applying statistics from historical data to the future. A lot of events will change over time, and these changes will lead to change in the frequency of unwanted events. Some changes that will occur and affect the frequency of wanted events are (Friis-Hansen et al 2004): • Traffic composition and a greater number of vessels; • Improved navigational equipment; • Larger and faster vessels; • The phase-out of single hull tankers and the increase of double hull tankers. Thus, the following aspects should be born in mind so that the danger of misinterpretation of historical data is minimized (Wang et al 2003): • Statistics are based on past experience, and may not reflect present situations; • Statistics of one geographical location can not be used for another location; • Statistics from cases of damage may negatively impact the use of designs that have not experienced damage. Besides the well-known IMO database, some maritime organizations and agencies develop and maintain their own marine accident databases. Examples of such organizations are: • The United Kingdom Ministry of Transport, Marine Accident Investigation Bureau (MAIB) has made significant progress regarding design and implementation of a marine accident information management system. • The Australian Transportation Safety Board (Australian TSB) has defined system information requirements, and software engineers may begin to build an information management system for their marine division. • The Transportation Safety Board of Canada uses an extensive taxonomy to document data from accident/incident investigations.

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ISSC Committee V.1: Collision and Grounding • •

The United States Coast Guard (USCG) has documented accident/incident data that date back to the 1960’s. Japan’s Maritime Accident Inquiry Agency (MAIA) collects collision and grounding accidents occurring in Japanese waters.

These databases are in various stages of development across governments and organizations. The database taxonomies and data elements are similar. However, various formats and organizations are used. Three types of software have been applied: customized software (MAIB), commercial off-the-shelf (Australian TSB), and software shells. Organizations seem to be moving away from commercial off-the-shelf packages like ACCESS and EXCEL to more customized software. New and updated damage statistics from various databases with 930 grounding accident records were used by Lützen and Simonsen (2003) in an attempt to determine the probability of exceeding the current IMO design requirements. Distributions for the extent of damage, such as damage length, damage height and damage width, were determined. Furthermore, attempts were made at identifying the governing grounding scenarios, and deriving a formula for the relationship between the amount of deformed structure and the energy absorption. The HARDER model (Lützen 2001) is based on a comprehensive review of casualties from different scenarios. New and updated distributions for location, length, penetration, and vertical extent of damage have been drawn from a large database with records of 2,946 casualties, 1,851 collisions, 930 groundings, and 165 other accidents. Other recent comprehensive studies include statistical collision and grounding accident data and analyze the data. These studies are: Zhu et al (2002), Skjong and Vanem (2004), FriisHansen et al (2004), Wu and Liu (2004), and Liu and Wu (2004). The maritime community needs a web-based and global unified incident reporting scheme. 4.3

Predictive calculations and energy reference values

The most commonly used calculation to determine the probability of ship to ship collisions and groundings is based on the work by Fujii et al (1974). This two-step procedure first requires determining the potential number of collision candidates or groundings as if no aversive maneuvers are made. A distribution of ship traffic must be known for this part of the analysis. In the second step, a so-called causation factor is determined that models the effect of crew and equipment related actions to avoid the collision or grounding. This causation factor depends strongly on the available navigational aids in the shipping area, the weather conditions, the visibility, and the equipment on the bridge.


19

Predictive calculations are suitable for providing a history of incident probabilities and typical energy reference values associated with an incident. Data for these types of calculations are the probability density functions for the collision scenario, such as the striking location, the collision angle, and the velocities of the vessels, the loading condition of the striking vessel, and the sizes and types of vessels that strike other vessels. Lützen (2001) gave example data for vessels in world-wide trade and also for vessels in specific European routes. Based on the mathematical model for external collision analysis (see Chapter 5) presented by Pedersen and Zhang (1998), and the data briefly discussed above, the absorbed energy distribution of a ships’ structural crushing has been calculated using a Monte Carlo procedure. Some results are presented in Figure 2, which shows the 50- and 90- percentile value of the energy that is absorbed when a collision takes place. This depicts the striking location along the length of the struck vessel. For a smaller percentile, the difference in absorbed energy relative to the striking location is small. The results presented in Figure 2 are based on predictive calculations using the world fleet as a basis for the composition of the striking vessels. It should be noted that the energy released from crushing during a collision depends strongly on the distribution of ship types and sizes, and therefore will be specific for specific geographic areas. 250

4 .5 4

20 0

3 .5

E [MJ]

E [MJ]

3 2 .5 2 1.5

x/ L = 0 .1 and 0 .9

1

x/ L = 0 .3 and 0 .9

0 .5

150 x=0 .1 x=0 .3

10 0

x=0 .5 x=0 .7

50

x=0 .9

x/ L = 0 .5

0

0 0

20 00 0

4 0 00 0

6 0 00 0

Dis place m e nt [t]

8 00 00

100 00 0

0

10 00 00

2 00 00 0

30 0 00 0

40 00 0 0

Dis place m e nt [t]

Figure 2: The 50-percentile value (left) and 90-percentile value (right) for energy to be absorbed as a function of struck vessel displacement and the striking location.

Several systems are currently being developed to help the Officers On the Watch (OOW). Son et al (2004) have described a system to monitor the collision and grounding risk of ships in real time in order to give guidance to the OOW. Pedersen and Liu (2004) have presented a visualization-based information display system for collision risk assessment in congested waterways. Zhou and Hearn (2004) have developed a Genetic Algorithm to identify the optimal approach sequence of actions necessary to avoid identified potential collisions. Lützen and Friis-Hansen (2003) analyzed the risk-reducing effect of implementing Automatic Identification Systems (AIS) on ships. They used a Bayesian network procedure to estimate the causation factor. Also, based on the assumption that AIS is installed on all ships, Kayano et al (2004) have developed a collision avoidance algorithm for collision avoidance support.

20

4.4


Recommendations

The committee recommends that the maritime community conduct further research in order to clearly define or identify collision and grounding scenarios. The committee also recommends developing, formalizing and unifying a procedure for recording collision and grounding accidents.

5.

MECHANICS OF COLLISION AND GROUNDING

Since the early 1990s, many predictive calculation procedures have been developed for predicting a ship’s response in a collision or grounding. These methods have matured to such a level that they are now being integrated into systems for evaluation and designs. Nevertheless, the question of how to calculate a ship’s response in an accident continues to be central to today’s research and development. 5.1

General

Accident analysis mechanics can be classified into two parts - external mechanics and internal mechanics (Pedersen 1995). The external accident mechanics deal with the ships’ rigid body global motion under the force of the collision or grounding and the hydrodynamic pressures acting on the wet surface. The internal accident mechanics evaluates the ships’ structural failure response during the collision or grounding accident. Those two parts are often treated separately, but in some cases, they are solved together. 5.2

Internal mechanisms

The analysis methods of internal mechanisms can be categorized into four groups: simple formulae, simplified analytical approach, simplified FEM, and nonlinear FEM simulation. Their advantages and disadvantages are summarized in the Table 2 (modified from Wang et al 2003). Simple formulae are best suited to estimate initial energy absorption. The recent extensive studies on structural crashworthiness have produced many new simplified formulae that are more rationally based and are applicable to a wider range of problems, including head-on collision (Zhang, et al 2004), grounding (Zhang 1999, Pedersen and Zhang 2000b), collision and bottom raking of high speed crafts (Simonsen and Tornqvist 2004a, Simonsen et al 2004), and ship to bridge collision (Pedersen et al. 1998, Wang and Yi 1997, Li 1997).


21

TABLE 2 AVAILABLE METHODS FOR INTERNAL MECHANICS (STRUCTURAL RESPONSES)

Analysis efforts

Methods

Results

Modeling

Computation

Energy

Loads

Simple formulae

Fewest

Fewest, hand calculations

X

Simplified Analytical approaches

Few

Few, hand calculations

X

X

Simplified FEM approaches

Some

Some, special programs

X

X

Non-linear FEM simulation

Extensive

Extensive, expensive software

X

X

Stress

X

Simplified analytical methods are best at balancing modeling difficulty with prediction accuracy. The technological advances in the last decade are represented by the establishment of the structural crashworthiness concept and methodology Wierzbicki 19911999, Wang (2002). Applications of this advanced methodology to various collision and grounding situations were summarized extensively by the ISSC 2003 Committee V.3 and Wang et al (2002b). This group of approaches has the advantage of capturing the basic characteristics of structural crashworthiness with minimized structural modeling efforts. A series of methods has been developed using this advanced technology. Some have been yielding results of practical importance (e.g., Zhang 1999, Wang and Ohtsubo 1999, Pedersen and Zhang 2000b, Suzuki et al 2000, 2001, Tikka 2001, Brown 2002, Urban 2003, Simonsen et al 2004, Han et al 2005, Zhang and Wu 1990, Zhu et al 1996, Liang et al 2000, Xiao et al 2001). Zhang et al (2004) reported analyses of plate crushing and ship bow damage in head-on collisions, and reviewed and compared existing experimental and theoretical studies on crushing analyses of plated structures. Simple formulae for determining the crushing force, force-deformation curve and the extent of damage to a ship bow, expressed in terms of ship principal particulars, are derived for longitudinally stiffened oil tankers and bulk carriers. These formulae can be used in a probabilistic analysis of how much damage occurs from ship collisions when a large number of calculations are generally required. Simonsen et al (2004) conducted similar analyses to study the raking damage of bottom plating for high speed crafts. Simplified FEM (e.g., Paik at al 1999) has not been used much. Application of nonlinear FEM simulation has been the main theme of recent studies (Wu et al 2004, Zhang L. et al 2004, Endo et al 2004, Yamado and Endo 2004, Endo 2004, Jiang

22


and Gu 2004, Takaoka et al 2004, Tornqvist and Simonsen 2004, Wang et al 2003, 2002b, Le Sourne et al 2003, Kajaste-Rudnitski et al 2004a, 2004b and 2005, Nolau Neto et al 2004, Jastrzebski et al 2004, Lehmann and Biehl 2004, Konter et al 2004, Oh et al 2005, Liu and Gu 2003, Lee et al 2001, Klanac et al 2005, Ozguc et al 2005, 2006, Hu et al 2005, Alsos and Amdahl 2005, Yamada et al 2005). This trend was clearly demonstrated in the 2nd and 3rd International Conference on Collision and Grounding of Ships. As expected, we will see more FEM simulation applications in the coming years. Rapid advances in computer technology and software capacity have made FEM simulation a preferred choice. Many powerful special-purpose FEM packages, such as DYNA3D, DYTRAN and PAMCRASH, are now available and can account for large deformation, contact between structures, non-linearity in material properties, and rupture. For analyzing a collision or grounding accident involving high non-linearity, contact, friction and rupture, the explicit methodology is suitable. The required calculation efforts are fewer than the commonly used implicit methods. Convergence of calculations is much easier to realize. 5.3

Rupture criteria

It is probably most challenging to model rupture and tearing when applying the structural crashworthiness concept. The structural crashworthiness concept also forms crucial background for the important criteria of crashworthy ships. Advanced FEM packages enable reliable automated simulation of the structural failure process up to when fracture occurs, beyond which software aids, such as a user-defined subroutine, are needed for tracing the initiation and propagation of cracks. Traditionally, we assume that rupture occurs when the equivalent plastic strain in an analyzed structure reaches a critical value. This critical value, sometimes referred to as rupture strain, is related to the strain-stress curves obtained from mechanical tests of uniaxially stretched metal coupons. In the simplified analytical approaches (see Table 2), the rupture strain varies from 1% to 20%, and the determination is normally based on calibration or judgment. Some people have been interested in defining rupture strain for FEM analyses. This critical value is found to be dependent on mesh size. Simonsen and Törnqvist (2004), Okazawa et al (2004), Yamada et al (2005), and Alsos and Amdahl (2005) studied ranges of rupture strain. Refined simulation of fracture initiation and propagation requires that mesh size be small enough. This, in turn, makes the analysis of large ship structures very time consuming and computationally demanding. It is commonly known that a failure criterion based on the equivalent strain is generally not valid in bi-axially loaded plates. Urban (2002), Hiramatsu et al (2002), and Törnquist (2003) reported estimations of critical equivalent plastic strain as a function of the stress triaxiality using model tests and FEM analyses. Several simple failure criteria and damage models were implemented in the explicit finite element code LSDYNA (Törnqvist 2003). Törnqvist and Simonsen (2004) have shown that the so-called combined Rice-Tracey and Cockroft-Latham (RTCL) criteria that account for the tri-axial


23

nature of the fracture provide a good comparison to test results for different materials and various stress/strain states. They tested varying stress and strain states for validating these fracture criteria and damage models. The rupture failure may be explained using the metal forming theory. The maximum strain that the material can sustain is limited by the local plastic instability. At failure, plastic deformations concentrate on local areas. These areas have typical dimensions of plate thickness. The failure process can be divided into diffuse necking and local necking phases. Diffuse necking develops slowly as a result of strain rate hardening, and occurs when the load reaches the maximum value. The final failure occurs by local necking during which the deformation is concentrated on a small area away from where the structure remains almost un-deformed. This theory of localized necking has been developed for thin metal sheets, in which the assumption of plane stress is valid and the failure criteria can be based on the bi-axial principal strain state formulations. For typical ship structures the stress state is tri-axial, which complicates the development of simple rupture criteria. There are various approaches presented to handle the tri-axial rupture criteria. Broekhuijsen (2003) improved the rupture index approach developed by Lehmann and Yu (1998), and presented an equation that describes the effective rupture stain, ενr, for multiaxial stress and strain state as a function of the one-dimensional rupture strain, εR, and the corresponding stress tri-axiality function f(β):

ενr = εR f(β)-1/m Where, f(β) = 2(1+υ)/3 + (1-2υ)β2/3, β = 3σH/σeq, and σH and σeq are the hydrostatic stress and the equivalent von Mises stress, respectively. In this equation, a value of m = 1.4 can be used for steel structures, which corresponds to the typical power law strain hardening exponent n = 0.2. However, identifying rupture location requires information of the actual effective strain,εν, over the entire structure. Lehman and Yu (1998) introduced a convenient Rupture Index, IR, a parameter for micro plastic damage: IR = εν f(β)1/m Rupture is predicted to occur as soon as the following equation is satisfied: IR ≥ εR The proper value for one-dimensional rupture strain, εR, can be obtained by simulating the tensile test coupons with nonlinear FEM calculations. Broekhuijsen (2003) has applied this approach to compare the calculated rupture force with experimental value obtained for 12 mm mild steel specimens quasi-statically punched by a sphere 60 mm in diameter. The

24


experiments show that the Rupture Index approach can be applied to various stress states, ranging from uni-axial to bi-axial states. The welding of steel or aluminum structures will have an effect on the material properties of the welded area. Welds will change the stress concentration factors due to changes in geometry and will also affect the metallurgical properties (e.g., by decreasing or increasing the hardness of the material). Simonsen and Abramowitz (2003) investigated the effect of fractured welds or fractured parent material on the energy absorption of ships’ typical structural subassemblies during deep collapse. They presented experiments and theories on the crushing response of typical strength elements. The theories were created because of the infinitely ductile material response and the consistently modified effect of fracture. Jiang and Roehr (2004) studied the failure criteria for welding lines by both experiments and numerical simulations. 5.4

External mechanics

For the external mechanics, simplified methodology based on rigid-body motion theory (Petersen 1982, Pedersen 1995, Pedersen and Zhang 2000, Brown 2002, Paik et al 1999, Wang and Ohtsubo 1999, Nolau Neto et al 2004, Suzuki et al 2000, 2001, Reich and Roher 2004) was often used. Recent simulation methods enable a full six degrees of freedom simulation for ship motions during groundings or collisions (Matusiak 2002). The most demanding task is to model the contact force between the striking and struck ships. An approximate approach is to determine the contact loads independently from running a nonlinear FEM analysis in advance, and applying these loads to the ship motion analyses (Kajaste-Rudnitski et al 2004a). Simulation tools are also available to enable visualization of a grounding event (Mäesalu and Matusiak 2004). Määttänen (2005) reported a series of tests whose goals were to investigate the motions of colliding ships and their interaction with the surrounding water, see Figure 3. These model tests were designed to resemble full-scale experiments carried out in the Netherlands (Wevers and Vredeveldt 1999). A total of 37 laboratory tests were performed to investigate the phenomena in different collision scenarios. The effects of five parameters were studied. They were collision velocity, collision angle, a bulbous bow, the mass ratio between the colliding ships, and the location of the contact point on the struck ship. All six motion components of both models and collision forces were measured. The penetration depth was calculated based on the relative motions of the models assuming small rotational motions. The test results of these scaled models were found to correlate well with the two full-scale collision tests carried out in the Netherlands. The validation was made for 37 symmetrical model scale tests. The parameters from the model scale tests were at full scale for validation. Because the model scale tests had different mass ratios and initial kinetic energies compared to the full scale tests, regression lines were plotted for the model scale tests.


25

(a) Test setup

(b) Front view of test channel Figure 3: Model test of ship collisions (Määttänen 2005).

The maritime community needs to validate the tools for predicting ship motions in an accident. Data of full-scale collision and grounding experiments are now available; see further information in the committee reports of ISSC 2003 V.3, ISSC 1997 V.4, and ISSC 1994 V.6.

26 5.5

ISSC Committee V.1: Collision and Grounding Influences of fluid in tanks

These real-scale experiments reveal that ship motions are affected by the sloshing forces of partially filled ballast tanks. Tabri et al (2004) demonstrated that better correlations could be achieved when sloshing is also included in the simulation of ship movements. See Figure 4. It is especially important that the second peak in the simulated time history could not be captured if sloshing was excluded from the analysis. The variation of the energy components also revealed the importance of sloshing. See Figure 5. Sloshing "stored" the kinetic energy and thus, less energy became available for deforming structures. It was estimated that in their experiment, 43% of the initial kinetic energy was dissipated by structural deformation, reduced from around 65% without the sloshing water. Arai et al (2002) proposed a new numerical treatment of the boundary condition for accurate and stable assessment of the sloshing impact pressure based on a rectangular gird system. The comparisons of the numerical results with experimental ones confirmed the accuracy of the proposed technique. Water in ballast tanks may also cause different structural behavior of ship structures. This effect is yet to be explored. 5.6

Coupled internal and external mechanics

Studies on coupled internal and external mechanics continue the search for the best balance between computational efforts and complexity in methodology of various mechanisms. Some programs that calculate internal and external mechanics together have already been developed. These include programs developed in the Massachusetts Institute of Technology (Wierzbicki 1991-1999), Virginia Polytechnic Institute and State University (Brown 2002), the Technical University of Denmark (Lützen and Simonsen 2002), and the University of Tokyo (Suzuki et al 2000). Määttänen (2005) compared the plastic deformation energy evaluated by the method based on momentum conservation (Zhang 1999) to the experimentally measured energy. He also reported on experiments that were setup for evaluating ship motions and structural resistance in a collision. The deformation energy was evaluated in two different ways, one based on the integration of a force-penetration curve, and the other was evaluated by calculating the change in kinetic energy during contact. This computational model corresponded well with the method based on the loss of kinetic energy, but gave slightly different results when compared with the model tests. This was probably due to the inaccuracy of the calculated penetration depth.


27

Figure 4: Numerically simulated and experimentally measured time history for collision force (Tabri et al 2004). Due to water sloshing, the first peak is lower and then the second peak occurs.

The elastic vibratory energy in the hull girder during a collision was investigated by Pedersen and Li (2004). They concluded that the elastic hull girder energy usually occupies a small amount of total energy for large commercial ships. This study validates a commonly used assumption for considering the contribution of plastic deformation and neglecting elastic energy. Viscous effects are of minor importance. In many cases, notably collisions, the motion of the struck ship during the contact phase is small, and the inertia forces are the most important contribution. In simplified analysis of collisions, this is usually represented by a constant added mass term (Pedersen & Zhang 2000a, Wang and Ohtsubo 1999, Suzuki et al 2000, 2001). For a reliable risk assessment of collision and grounding events, it is necessary to analyze many scenarios, taking into consideration the real structural configuration of ships and using proper oil outflow simulations to evaluate the environmental impact. Brown (2002) presents a Simplified Collision Model (SIMCOL) to calculate damage extent and oil outflow in ship collisions. The proposed process, which uses physic-based models to predict probabilistic damage in collision, provides a practical means of considering structural design in a regulatory framework, in order to improve safety and environmental

28


performance of ships. SIMCOL is sufficiently fast to be applied to thousands cases of collision as is required for probabilistic analysis. Commercial simulation codes use the Lagrangian (finite element) and Eulerian (finite volume, finite difference) solvers for modeling structures and fluids, respectively. Meshes within each solver can be coupled together for the analysis of fluid-structure interactions. In the Lagrangian solver, the grid points are fixed to the body under analysis and move in space when the body (solid) deforms, resulting in distortion of structural elements. In the Eulerian solver, the grid points are fixed in space, and the material of a body (fluid) under analysis moves through the Eulerian mesh. The mass, momentum, and energy of the material is transported from element to element. There are two types of algorithms for the fluid-structure interaction: general coupling and Arbitrary Lagrangian-Euler (ALE) coupling. The coupling algorithm computes the interaction between the two sets of elements. In general coupling, the coupling surface on the Lagrangian structure is used to transfer the forces between the two solver domains. In ALE, the Eulerian grid points may move in space, whereby the material flows through a moving and deforming Eulerian mesh. The ALE coupling is potentially faster than the general coupling. Numerical collision simulations were performed using ALE coupling of MSC/DYTRAN or LS-DYNA in the 1990s (see 1997 ISSC V.4, 1994 ISSC V.6). Gu & Wang (2001) recently adopted this approach and introduced an inertia equivalent model (constant added mass) that considerably reduced computational time. Other studies are Le Sourne et al (2001), (2003). A development of collision forces and consequential damages of collided ship hulls have been considered by Kajaste-Rudnitski et al (2005). In this scenario, a moving Ro-Ro ship’s bow and underwater bulb hits a side surface of another double bottom Ro-Ro ship, head-on. Both ships suffer considerable damages; and the shear failure of shell elements is taken into account. In this case, when equivalent plastic strain reaches a certain level, the element fails to bear any more loads, and is automatically removed from the mesh, thus leaving a hole. Bow structures have a range of stiffness, from completely rigid to quite flexible. The effect of the surrounding water’s added mass is also studied. Dynamic explicit contact formulation is also used for this analysis. Elastic-plastic steel material with kinematic hardening is used for the thin-walled plating shells. In the hybrid computational method developed by Reich and Rohr (2005), a ship’s grounding process was simulated by combining a quasi hydro-elastic nonlinear Timoshenko beam model for the hull girder structural responses, and a three-dimensional contact problem model for grounding loads. The complex interaction between the ship bottom and the soft seabed was treated as a contact problem and analyzed using a boundary integral formulation. The interaction analysis also integrated the elasto-plastic behavior of structural materials permitting isotropic hardening.


29

6

energy [J]

5

x 10

4

3 A

E

2

A

EKIN

1

ESL

0 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5 time[s]

(a) Striking ship energy [J]

5

20

x 10

18

EB

16

EB KIN

14 12

ESL

10 8

WK

6

EB

4 2

EF

0 -2 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5 time[s]

(b) Struck ship Figure 5: Variation of energy components throughout a collision (Tabri et al 2004): EA, EB - total energy; EAKIN, EBKIN - kinetic energy involved in rigid body motions; ESL energy involved in sloshing; WK - work against damping; EB - bending energy; EF - work against friction

30 5.7

ISSC Committee V.1: Collision and Grounding Recommendations

For typical ship structures, the stress state is tri-axial, complicating the development of proper failure criteria for rupture. There are various approaches presented to handle triaxial rupture failure. Still, validation of the available approaches is fairly limited, especially with real structural configurations. For the proper modeling principles for rupture in nonlinear FE-analysis, we especially need further studies. Ship movements during collision are affected by sloshing in partially filled tanks. Systematic analysis is needed to study the effects of partially filled tanks as well as the effects of filled tanks in the double bottom or filled fore peak tanks in a striking ship, considering the orientation of a collision.

6.

CONSEQUENCES OF COLLISION AND GROUNDING

Disasters at sea caused by collisions and groundings may cause serious problems for the environment, human lives, and property. Collision and grounding accidents can very often lead to shell ruptures. Various aspects should be examined such as fatalities, cargo spills, damage stability, residual strength capability, increased load demands on the hull girder, and economical and social impacts. Indirect costs can be much higher than those strictly connected with the accident. The Earth’s ecological equilibrium is becoming more fragile. It is everyone’s responsibility to contribute to a better world. The shipping industry is continuously improving onboard safety. Statistics show that fewer and fewer ship accidents took place in recent years. Nevertheless, we must not take this as a call for complacency, as accidents of tankers Erika (off the coast of France) and Prestige (off the coast of Spain) remind us. Each accident, every life lost at sea, every case of environmental pollution, is one too many, and no longer tolerated by the public. Therefore, everyone involved with designers, builders, operators, class societies, flag states, and so on, must continue to find ways to further reduce risks and improve quality and safety in shipping (Payer 2004). To minimize the risks associated with collision and grounding accidents, we must improve ship operations to reduce the likelihood of accidents and design stronger ships to minimize losses should an accident occur. More specifically, enhanced navigation systems and bridge procedures reduce the possibility of accidents; whereas improved arrangements of cargo tanks, structural crashworthiness, and stabilized damage minimize the consequences. In addition, emergency response and life saving systems help to reduce the number of fatalities.


31

Oil outflow

The IMO has established a probabilistic methodology to assess protection against oil pollution from damaged tankers (IMO 2003a). The required width of wing ballast tanks remains the same along the ship’s length, because the transverse damage extent does not depend on the longitudinal location. Based on the same methodology, McAllister et al (2003) estimated accidental oil spills from bunker tanks. This study was intended to shed light on the potential oil spill risks of bunker oil tanks that were not regulated by MARPOL. Endo (2004) points out that this IMO approach is not effective in estimating side damage penetration. It is recognized, from collision accidents and FEM simulations, that the transverse damage extent is usually much greater amidship than at fore and aft. Endo proposes to adopt a weighting function that more accurately estimates the distribution and extent of transverse damage along the ship’s length, taking into account the longitudinal location of collisions. This would facilitate an improved tanker arrangement and consequently cause more effective oil outflow prevention. Friis-Hansen and Ditlevsen (2003) noted the effect of applying the future losses to the present. They assumed that the risk profile asymptotically approaches a limit risk profile as the operation time increases. This asymptotic profile is approximated by a lognormal distribution. The risk profile modeling was applied to a study of oil spills from tanker collisions in the Danish straits. They found that the distribution of the oil spill volume per spill could be represented by an exponential distribution both in Oeresund and in the Great Belt. When applied in the Poisson model, a risk profile reasonably close to the standard lognormal profile is obtained. Moreover, based on data pairs (volume, cost) for world-wide oil spills, one infers that the conditional distribution of the costs given the spill volume can be modeled by a lognormal distribution. By un-conditioning the exponential distribution of the single oil spill, a risk profile for the costs is obtained that is indistinguishable from the standard lognormal risk profile. Finally the so-called Life Quality Index is used to quantify the risk acceptance criterion for the pollution of the environment. This NPWI acceptance criterion is applied to the oil spill example. 6.2

Damage stability

Damage stability evaluation in collision or grounding events can be performed within a probabilistic approach. This is done while considering the probability of damage of certain dimensions to a particular compartment or group of compartments, and the probability of surviving the damage scenarios. The HARDER project developed new probability distributions of damage p- and v-factors, studied the survivability of a ship after damage, and established a generalized formula for the survival s-factor based on the highest significant wave a damaged ship can survive, and the probability of that survivable seastate occurring (Tagg and Tuzcu 2002). The total survivability of the ship is calculated as the sum of survivability given all possible damages, and this corresponds to the Attained Subdivision Index A = Σi pi vi si. The Required

32


Subdivision Index R is then formulated to ensure a safety level equivalent to the SOLAS regulations. The subdivision of a ship is considered sufficient if A > R.

Figure 6: Event tree for collision of bulk carriers according to survey of recent accidents.

Skjong and Vanem (2004) used the data from the HARDER project in analyzing the damage stability of generic bulk carriers following a collision. See also Figure 6. The frequency of serious collisions and the frequency of flooding after damage were obtained from accident databases containing records of historical accidents and world fleet statistics. 6.3

Ship evacuation

Collisions and groundings may cause flooding of passenger ship compartments, and expose the vessels to the risk of losing stability and sinking. This could cause disastrous consequences such as the loss of human lives. If preventative measures fail, passengers and crews have to be evacuated, and an orderly and timely evacuation can save the life of many people. Emergency evacuations are considered extremely crucial to passenger ships and Ro-Ro ships. Evacuation analyses shall be performed early in the design process for such ships, with the objective to assess, for all possible damage scenarios, the time necessary for evacuation. Total evacuation time shall be analyzed in all its components (i.e., awareness time, travel time, embarkation time, and launching time) in order to implement a number of risk control options capable of preventing or mitigating life loss. Ship designers are encouraged to achieve enhanced evacuation performances by innovative design on new passenger ships (Vanem and Skjong 2004).


33

Numerical tools (software) are now available for analyzing complex ship and human behavior in an accident. The maritime EXODUS simulates mustering and evacuating passengers and crews. Program FREDYN calculates progressive flooding, and has been utilized by van’t Veer et al (2004) to study the behavior of a damaged large passenger ship in waves. 6.4

Residual strength

A ship may collapse after a collision or grounding because of inadequate longitudinal strength. It is important to keep the residual strength of damaged structures at a certain level in order to avoid additional catastrophic consequences. Prestige’s recent accident shows the importance of a reliable assessment of the damaged vessels’ longitudinal strength in real emergency situations and time pressure. Hull girder collapse can be assessed by comparing the applied extreme bending moment and the residual bending capacity of the damaged hull girder. A measure of the residual bending capacity can be based on either the maximum elastic bending moment corresponding to occurrence of initial yielding (section modulus based residual strength), or the maximum bending moment beyond which the ship will break its back due to extensive yielding and buckling (ultimate bending moment based residual strength). The hull girder section modulus is a well-accepted parameter to measure the longitudinal bending strength, especially where brittle failure modes associated with fracture or unstable buckling is concerned. The ultimate hull girder strength is a better indication of the bending capacity when ductile failure modes are predominant. The ultimate strength can be estimated using the incremental strain approach by calculating the moment-curvature relationship of hull girder, and thence the maximum resisting moment offered by all structural members contributing to the longitudinal strength. Through such an approach, it is possible to trace out the complete sequence of the damaged hull girder’s progressive collapse and then a more realistic safety margin can be realized. For a proper evaluation of the hull girder’s ultimate strength, special-purpose programs, simple formulae, or nonlinear FEM packages can be used. See also Committees II.1 and III.1. To appraise the influence of different damage scenarios, the residual strength index can be calculated as the ratio of the damaged hull’s strength to that of the intact hull. Wang et al (2002a) present some simple equations for a quick evaluation of the residual section modulus of typical commercial ships. Different degrees of damage caused by either a collision or grounding is assumed, and the formulae were derived from an extensive study of 67 ships (double-hull tankers, single-hull tankers, bulk carriers, container carriers). These formulae provide very handy tools for predicting the residual strength of a ship’s hull in an accident, without performing step-by-step detailed calculations. They are also useful as elements in a decision making process related to salvage and rescue, and can be easily

34


integrated into a risk assessment scheme. Ozguc et al (2006) compared residual hull girder ultimate strength of a single-hull and a double-hull bulk carrier with collision damages. Damage to side structures was derived from FEM analyses of various collision scenarios. They also studied effects of corrosion wastage. Kozlyakov and Egorov (1991) reported that for vessels losing the side structures on either port or starboard side, the hull girder section modulus was reduced by about 25%, and could be as high as 47% in container carriers. In addition, the damaged hulls were exposed to additional stresses for losing symmetry in its cross section. The combined action of vertical bending and torsion could cause up to 50 – 80% reduction of the ship’s longitudinal strength. 6.5

Post-accidental loads

In the case of serious accidents such as collisions or groundings with adverse consequences to the structures, there can be weighty repercussions on the residual strength and loads that act on the hull girder after damage. There were limited studies on increased static loads of grounded and collided vessels. Pedersen (1994) created a mathematical model for ships grounding on a sloping rigid shore. He calculated the grounding reaction forces; and found that the longitudinal strength of a grounding ship may not be sufficient. In addition, the strength margin depends on ship size, loading state, the shape of the ground, and the coefficient of friction. Reich and Rohr (2005) demonstrated again that static grounding loads (bending moments and shear forces on hull girders) can be accurately calculated, and properly developed coupled models can capture the global behavior of hull girders in grounding. Tagg and Akar (2004) and Iversen (2005) reported studies on the residual strength of passenger ships in flooded conditions. A typical passenger ferry heeled following some collision scenarios. The longitudinal bending moments on the damaged vessel were calculated using the loading computer, and added to the wave loads under the assumed sea-state to give the total hull girder loads. Wave loads of a flooded vessel are generally regarded less severe than the designed wave loads that are normally similar to those on average North Atlantic routes. The IACS Unified Requirement S17 specifies an 80% design wave-induced bending moment in evaluating the adequacy of a hull girder’s strength when flooded. Chan et al (2001) studied the dynamic motions of wave-induced loads on a damaged Ro-Ro vessel, using both experimental and numerical approaches. The total loads were determined by combining static loads and the most probable extreme wave loads obtained from a short-term statistical method. Flooding of engine room was found to give the worst hull girder loads. Vorobyov and Nilva (1997) and Nilva (1998, 1999 and 2000) investigated the wave loads on a grounded ship using both experimental and numerical approaches. 6.6

Other consequences

Other consequences that need to be addressed include, but are not limited to, fire following collision or grounding, blocked traffic, second collision, and leakage of liquefied natural gas (LNG) and consequential explosion (in case of LNG carriers).


6.7

35

Recommendations

Clear identification of accident scenarios is needed for a more uniform risk assessment, considering the different subjects involved (structural behavior of ship, emergency response, evacuation, rescue, simulation of oil spill, fire protection, effects on environmental resources, restoration time, etc.). Crashworthiness is an aspect of the ships’ structural design that should be given greater consideration. It is an aspect that is not generally considered in current regulations except for GL (see section 2.2) Further studies must be directed towards improving the prediction of a damaged ship’s survivability with reference to both the new loading demand (due to the flooding water and wave loads) and the residual capability of the hull girder (considering the effects of the unsymmetrical bending and of the shear force in the damaged sections). We would prefer to see additional development of simplified analytical approaches for rapid and reliable strength assessments during emergency. Additional developments that we need are joint probability of accident occurrence and probable sea states in an accident. We need to consider areas where collisions and groundings are likely to occur and areas that are normally congested such as harbor approaches and channels. As a result, the significant wave height of the sea state at the time of collision can be estimated in accordance with the findings of, for example, the HARDER project. An example of cumulative probability of the significant wave height is illustrated in Figure 7 (HARDER 2001):

Figure 7: Cumulative probability of significant wave height at time of collision.

36 7.

ISSC Committee V.1: Collision and Grounding ESTABLISHMENT OF ACCEPTANCE CRITERIA

Currently, there is no clear vision on the principles of accident design when considering structural crashworthiness. Obviously, the ultimate goal of applying the structural crashworthiness concept is to reduce risks associated with collisions and groundings. However, a direct unambiguous relationship between crashworthiness and risk reduction is difficult to establish. At the moment, damage claims tend to be disproportionate. For example, masters of some troubled ships were and are being held prisoner by some countries on doubtful legal grounds, because they caused environmental pollution. The international maritime community needs a common understanding of risks and acceptance criteria. 7.1

Principles

There are two classes of hazards to be addressed: loss of life at sea or on shore, and damage to the marine environment (Skjong and Vanem 2004). Both hazards lead to financial costs, especially in the latter case. There is no general consensus among sea-going nations regarding acceptable risks. Currently, two approaches seem to be adopted in the maritime industry: comparative and absolute risk assessments. Both hinge on the definition of risk R, being a multiplication of probability p with consequences C.

R= pC 7.2

Comparative risk assessment

In a comparative risk assessment, a risk comparison is made between an existing system and a new system (Vredeveldt et al 2004, Zhang L et al 2004). The assumption is that the existing system complies with the prevailing law and is, therefore, acceptable from a societal point of view. The new system can therefore be proven to be equivalent, if its inherited risks are calculated in the same fashion, and are comparable with the existing or conventional design. This comparative risk assessment approach avoids the difficulty in defining acceptable failure probabilities in combination with consequences. The recent IMO regulations on damage stability use the attained subdivision index A as a measure of survival probability. This index is defined by the following equation:

A = ∑ pi ⋅ si The probabilities pi of conceivable damages i are prescribed by the regulations. They are based on collision damage statistics, which do not consider crashworthiness explicitly.


37

When the ship survives the given damage i, then the survival parameter si is set to 1. Because the definition is similar to the typical risk format and the collision statistics are used, these recent IMO damage stability regulations are viewed by some as implication that designs complying with these regulations have a risk level acceptable to society. When sufficient survival cases can be found for which the aggregated value of pi si yields a value larger than the required threshold R (the required subdivision index), the ship is judged to comply with the implicitly required safety. The choice of the actual value for R is completed by a comparison with existing ships. Thus, safety equivalence is pursued. The designer can influence the survival si of damages by providing ample compartmentation of the hull. Under SOLAS, the designer can not influence the damage or its associated probability pi. Therefore, from a regulatory viewpoint, providing crashworthiness does not pay off. However, Zhang L et al (2004) report an equivalent safety approach for damage stability of ships, including crashworthiness. Vredeveldt et al (2004) present a similar approach applied on inland waterway chemical tankers, aiming to prevent cargo release after a collision. 7.3

Absolute risk assessment

In various studies on the probabilities and consequences of collisions and groundings, the societal assessment was performed in a more or less absolute sense (Lehmann and Biehl 2004, Friis-Hansen et al 2004, Delautre et al 2005, Vanem and Skjong 2004, Trbojevic 2005a, 2005b). Table 3 is an example risk matrix, which is modified from Lehmann and Biehl (2004). The numbers in the matrix cells indicate a ranking. Values of up to 3 are acceptable. Values of 4 and 5 indicate that efforts must be made to reduce probability and/or consequences, with reasonable costs. Values above 5 are not acceptable, and action must be taken to mitigate risk. The probability of occurrence (or likelihood) can be quantified in different ways. For example, consequences were expressed in terms of equivalent fatalities (3rd column) by Skjong (2001). Risk assessments are not unique for the maritime environment. Other industries, such as chemical industries, face similar challenges. Adequate methodologies are described in various publications, such as (CPR 14E 1997) and (CPR 18E 1999). Recently, a guideline was developed on the application of crashworthy side structures in inland waterway ships (TNO 2005) carrying hazardous cargo. The risk concept is used to balance enlarging cargo tanks against reducing the likelihood of tank penetration, by providing crashworthy tank structures.

38

ISSC Committee V.1: Collision and Grounding TABLE 3 PERCEPTION OF PROBABILITIES AND CONSEQUENCES Equivalent fatalities

Catastrophic

Ship breaks apart and/or sinks

10.00

4

5

6

7

Severe

One or more tanks are penetrated; cargo flows into the sea.

1.00

3

4

5

6

Significant

Cargo tanks are not penetrated, but side or bottom shell plating is penetrated. Fuel oils etc. that are stored in tanks of double side or double bottom spill into sea.

0.10

2

3

4

5

Minor

No damage to marine environment

0.01

1

2

3

4

Extremely remote

Remote

Reasonably probable

Frequent

7.4

Recommendations

The potential of providing a ship structure’s crashworthiness seems underrated at this moment. Figure 8 shows an FN diagram depicting the yearly probability of fatalities on passenger ships due to various causes. It is remarkable that the collision scenario shows a relatively high probability of fatalities. It illustrates the advantage of reducing the vulnerability to collision through improving crashworthiness. Nevertheless, no clear relationship has been found between the ship structure’s crashworthiness and the probability of passenger survival, nor is there a connection to hazardous cargo outflow. The ship’s design can be assessed when the probability distributions of kinetic energy sailing with a known displacement and speed are calculated, and the hull’s crashworthiness is estimated for a given area of navigation. Along these lines, we can develop a method to quantify the effect of crashworthiness on ships’ survivability with respect to damage stability. In the case of passenger ships, the effect that crashworthiness has on the time it takes to sink or capsize should be considered, especially in view of passenger evacuation. Similarly, a method must be developed to quantify the effect crashworthiness has on reducing the probability of cargo outflow.


39

It also seems opportune to work toward a common risk denominator, so that shipping safety performance can be compared to the safety performance of other modes of transportation. In this respect, the civil engineering industry provides very valuable ideas and concepts, such as The International Council for Research and Innovation in Building and Construction (CIB 2001.)

Figure 8: FN diagram of passenger ships carrying more than 3,000 persons.

8.

DESIGNS AGAINST COLLISION AND GROUNDING

Increased safety concerns require improved structural crashworthiness, in particular with respect to grounding and collision accidents. Structural crashworthiness of double hulled tankers can be improved, especially to prevent high-energy collision accidents. Recent investigations on innovative designs demonstrate the tendency to minimize the extent of damage. On the other hand, reducing the striking ship’s stiffness or changing the collision’s form also emerge as potential options for reducing accidental damages. As usual, due consideration was paid towards balancing the need to increase a hull structure’s energy absorption against the need to commercially produce and inspect the hull in the future.

40 8.1

ISSC Committee V.1: Collision and Grounding Buffer bow

Collision damages vary. The degree of damage sometimes depends on vessel positions during the collision, and the relative stiffness between the striking and struck vessels. A right-angle collision can cause the struck vessel’s side shell penetrated and the striking bow crushed. An oblique collision may cause the striking bulbous bow to bend a little while being crushed. Rigid bulbous bows create concentrated damage (penetration), while soft (more flexible) striking bows cause wider and shallower damage to struck side structures. It was concluded that reinforcing side structures more is not cost effective (ASIS 1997). A more feasible option to reduce side structure damage is to have weaker striking bows. This leads to the concept of buffer bows, a very cost effective way of reducing pollution. Buffer bows can be built without increasing cost. For example, transversely framed bows are much softer or more flexible than longitudinally framed bows, and are easier to crush in a collision. Transversely framed bows are also not inferior in taking on wave slamming loads. Bows can also be built with blunter forms so that when they penetrate a struck vessel, the penetration is wider and shallower (Kitamura 2000, Endo 2004, Endo et al 2004, Yamada and Endo 2004, Yamada et al 2005). However, if the ship owner uses the buffer bow, either the buffer bow must be mandatory, or there should be a financial advantage, such as an insurance discount. In making buffer bow regulations, the bow’s form should not be specified. Instead, the maximum load or absorbing energy should be specified. In addition, the buffer bow concept can be achieved, and should be left to the designer. 8.2

Innovative double hull designs and steel sandwich panels

Recently, some new steel double hull structures were invented to achieve better energy absorption capacity. Figure 9 shows some innovative hull designs developed by Schelde Naval Shipbuilding and Ship Laboratory of the Helsinki University of Technology. The intention of these new designs was to prevent early crack occurrence during a collision or grounding. Reported studies are about the Y-type shell structures (Konter et al 2004, Hu et al 2005), and sandwich side shell panels (Naar et al 2002, Klanac et al 2005), see Figure 10. The X-type sandwich panels have been shown to absorb more energy without the occurrence of cracks (Törnqvist and Simonsen 2004).


41

Figure 9: Innovative and traditional hull designs. Klanac et al (2005) have studied several different conceptual crashworthy steel sandwich designs and compared them with traditional existing structures for ship side shells. The problem is approached as a multi-criteria design where the side structure is designed to maximize energy absorption and minimize penetration depth, breadth, cost of production and weight, while satisfying needs of standard service.


60 °

6

42

50 5

180

360

5

100

675

energy [MJ]

(a) Dimensions 10 Outer plating tear Inner hull tear

8

sandwich, 500 mm

6 4

sandwich, 360 mm

conventional

2 0 0,0

0,3

0,5

0,8

1,0

1,3 1,5 penetration [m]

(b) Energy absorption Figure 10: Novel design of a steel sandwich panel (Klanac et al 2005).

8.3

Double hulls

There were also parametric studies whose researchers sought to improve structural designs of double hull tankers. Design parameters such as plate thickness and width of wing ballast tanks were mixed, and the range of the tanker structures’ during collisions or groundings were calculated (Paik et al. 1999, Kitamura 2000, Tikka 2001, Lützen and Simonsen 2002, Sajdak and Brown 2005). When streamlined programs which incorporated simplified analytical methods became available, a large body of accident events and design variations could be analyzed, and the effectiveness of potential improved double hull structures were estimated in probabilistic format.


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Composite and sandwich panels

Urban (Urban 2003) reported laboratory test results on the crushing of six sandwich intersections, which were assembled of an E-glass/polyester laminate and a polyurethane core. The T- and X- intersections represented the connection between the upper deck and side shell and the connection of deck and longitudinal bulkhead in an HSC bow, respectively. Generally, no failure was observed before the ultimate strength was reached. After ultimate strength was reached, white bands of damaged laminate formed across the flanges varying from 1 cm to 5 cm in thickness. These specimens showed good global stability during the crushing tests, and none failed in a global buckling mode. Numerical simulations of these tests strongly correlated with the initial crushing responses, but the predicted ultimate strength and average crushing forces were not satisfactory. As part of the EU SANDWICH project, the behavior of steel sandwich panels under local impact loading was investigated experimentally, numerically, and analytically (Kujala et al, 2004 and Tabri, 2003). Verification data was obtained by a series of laboratory experiments. Different configurations included several coating and filling materials, multiple core designs, and different material properties. This project also included fullscale testing optimization of X-type steel sandwich panels as part of the CRASHCOASTER project. The results revealed good energy absorption capability, without loss of integrity of the side structures (Törngvist and Simonsen, 2004). 8.5

Aluminum panels

Urban also conducted tests on aluminum intersection crushing (Urban 2003). Cracks were observed in the welded intersections of some specimens. The folding mechanisms were similar to steel intersections until the onset of cracks. Once there are cracks, the membrane stretching is relieved by the separation of component flanges, and the plastic hinges’ bending dominated. 8.6

Economic considerations

Vessel owners normally invest more in materials to improve crashworthiness. Boitsov (2002) formulated a simplistic quantitative procedure for evaluating the positive effects of reducing oil spills and the negative effects of reducing payloads. He estimated the consequences of strengthening side structures, hoping that these probabilistic economic impact studies would provide a better basis for deciding to invest more in materials. Similar studies were conducted by Appolonov et al (2002). 8.7

Recommendations

The Committee recommends that the community continue to develop designs that reduce pollution caused by collisions and groundings. Some innovations that may work would include changes to the structural arrangement and using steel sandwich panels.

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In the process of searching for more environmentally friendly vessels, one cannot ignore other considerations, such as economical limits, international regulations, and classification societies’ design rules. Decisions about innovative designs should be based on systematic approaches to balance various demands and requirements. In view of the demand for heightened high speed craft safety, and the emerging commercialization of navy vessel classifications, further development on composite and aluminum panels is needed.

9.

OFFSHORE STRUCTURE COLLISION

The offshore industry has a risk management concept that is totally different from that of the marine industry. (Refer also to Committee V.2 for broader coverage on general offshore related development). As represented by the Recommended Practice of the American Petroleum Institute (API), the offshore industry has established systemic assessment procedures for fixed platforms that address the probability of occurrence, risk ranking, structural analyses, and acceptance criteria. While people become more interested in developing oil and gas in deeper and more remote offshore locations, we see some research and development that focus on collision risk assessment of floating structures, especially ship-shaped FPSOs. This chapter is focused on general methodology and design accident scenarios for FPSOs. This chapter also surveys research and projects on ship-to-bridge collisions. Bridges and offshore installations remain in specific locations, and therefore their risks are managed in a somewhat similar manner. 9.1

Existing criteria in offshore design codes

The API recommends evaluating the structural performance of (fixed) platforms that suggest a high risk to life safety and/or the possibility of failure when there is a fire, blast or accidental loading, see API Recommended Practice 2A-WSD (API 2000). This API RP specifies the following assessment tasks for evaluating the events (fire, blast, and accidental loading) that could occur to the platform over its intended service life and service function(s): • Task 1, assign a platform exposure category for the platform • Task 2, assign risk levels to the probability of the event • Task 3, determine the appropriate level of risk for the selected platform and event • Task 4, conduct further study or analyses to better define the risk, consequence and cost of mitigation • Task 5, reassign a platform exposure category and/or mitigate the risk or the consequence of the event • Task 6, assess structural integrity if the platform is considered high-risk


45

Vessel collision during normal operations is one of the accidental loading possibilities. Such a scenario for platforms in the Gulf of Mexico is one when 1,000-ton supply vessel collides, either head-on or broadside, with the platform at a speed of 0.5 m/s. The vessel is chosen to represent typical OSVs in the U.S. Gulf of Mexico. This API RP requires that the platform survive the initial collision, and meet the post-impact criteria. During the described collision, the offshore structure absorbs energy primarily from localized plastic deformation of the tubular wall, elastic/plastic bending, and elongation of the member. In addition, if the fendering device is fitted, then there is global platform deformation, in addition to ship deformation and/or rotation. After collision, the damaged platform should retain sufficient residual strength to withstand one-year environmental storm loads in addition to normal operation loads. In the North Sea (see, e.g., NTS 1999, ISO 2005a), the collision is described as one from a vessel of 5,000 tons with a drifting speed of 2.0 m/s (DNV 2001). The collision energy is 14 MJ for a broad-side collision and 11 MJ for a head-on collision. There are some limited reports on ship-to-platform collision and the consequential risks to the damaged structures. Experts recognize that ship collisions are not likely to cause the push-over failure of collided platforms that lose some individual structural members, especially in a benign environment. However, minimum structures may see rapid deterioration of the overall structural integrity if impact damages are left un-repaired (Grewal and Lee 2004). 9.2

FPSO collision

An FPSO has a typical tanker shape and therefore uses the structural designs of tankers. There have been several ship impacts with FPSOs. In five incidents, shuttle tankers caused the impact; and one near contact of a shuttle tanker was reported. None of these impacts were critical, and in fact, the consequences were marginal. There are also reports of offloading shuttle tankers colliding with FPSOs in the North Sea. The most severe shuttle tanker impact so far involved energy of 37 MJ (BOMEL 1999). Compared to many fixed installations that are supported by truss constructions, an FPSO has a higher level of structural redundancy, and therefore can survive a high level of collision impact energy (Wang et al 2003). Analyses of collision resistance can be based on fundamental methods and approaches covered in sections 5.2 to 5.4, and Skallerund and Amdahl (2002). Moan et al (2002) reported that the critical energy for penetrating the wing tanker of an FPSO that is 40 m wide and 21 meters deep is about 8 to 18 MJ when the FPSO is struck by a 42,000 DWT tanker; 40 to 55 MJ when the FPSO is struck by a 18,000 DWT tanker; and 57 MJ when its engine room is penetrated by a tandemly moored shuttle tanker. These figures are indicative of the energy’s magnitude. Many other influential factors, such as striking bow designs, also play a role, and therefore collision situations should be treated on a case by case basis.

46


There is no formalized acceptance criterion for an FPSO collision, especially not for structural designs. Relevant studies are limited. Some things that may be considered during structural design are oil spills, residual strength, and the cost of impact, among others. 9.3

FPSO collision scenarios

FPSOs have a risk profile different from fixed platforms and commercial trading tankers; because they are stationed in one location, and are routinely visited by supply boats and shuttle tankers. FPSOs can be struck by these vessels. In addition, passing ships also pose a collision risk if an FPSO is close to a sailing route. Three main scenarios require detailed assessment (Moan et al 2002, Wang et al 2003): • Visiting supply vessels (high frequency and low consequences), • Passing vessel collision (low frequency and high consequences), and • Offloading shuttle tankers (medium frequency with high potential consequences). Accident scenarios for supply vessel collisions may be established based on historical incident data. Because of the frequent supply vessel visits and the potential for accompanying accidents, there are many incident data that are adequate for statistically presenting incident trends. Nevertheless, we should note the accident records interpretation. See also sections 4.1, 4.2. Many incident data are available from the North Sea. For many years, data have been collected from the North Sea, and analyzed. The Health and Safety Executive, United Kingdom (HSE 2004) has published many results of such analyses. For the Gulf of Mexico, collision incident records can be found from the U.S. Minerals Management Services (MMS 2000) and the United States Coast Guard (USCG 1999). MacDonald et al (1999) presented an analysis on collision risks in the Gulf of Mexico. Classification societies such as ABS, DnV, LR and NK also maintain vessel incident databases. Generally, these in-house databases cover other geographical areas as well, but are not confined to offshore installations. In an analysis on a collision between a supply vessel and an FPSO in West Africa, Oh et al (2005) assumed that a collision could occur in three places: on the riser, the protector, and the deflector. They selected to look into collisions that induce large deformations on the framed structure at a colliding speed of 1 m/s. They estimated that the speed was a result of marine equipment failure, or human error. Occurrence probability of passing vessel collisions can not be fully captured using only historical incident data. Very limited data are available. Even though there are some, the presence of an offshore installation will absolutely affect the situation when it is absence. There is increasing interest in developing techniques combining historical data with an analytical model, because historical data can not always be used for predicting the future. This is especially true for passing vessel collisions, because the traffic pattern varies from


47

one location to another. Haugen (1998) described a conceptual route-based model for an annual collision frequency rate per year for a given installation. A few incidents of shuttle tanker collisions occurred, but the likelihood of this type of accident is very difficult to specify from sparse historical data. Often, expert opinions are the only choice that decision makers have when there is very little data of previous incidents. Recently, Chen (2003) and Chen et al (2002, 2003) reported a simulation-based approach that showed promise as a potential alternative of simply collecting expert opinions. This new approach adopts operation simulators widely used in training crews for predicting how close a shuttle tanker may come to an FPSO during an offloading operation. With a large number of simulations of offloading operations, the relative motion between an FPSO and an offloading tanker can be fit into some statistical models. Then the probability of collision can be estimated based on those outcomes. 9.4

Design events for FPSO collision

Moan et al (2002) generated an energy spectrum that showed the cumulative collision frequencies versus the energy generated by a collision, with the event corresponding to a specified annual exceedance probability of e.g. 10-4. The most probable impact locations (bow, stern, side) and impact geometry should be established based on the dimensions and geometry of the structure (FPSO) and of the impacting vessel. It should also account for draught variations, the operational sea-state, and vessel motions. Collision scenarios involve supply ships impacts, relatively less frequent shuttle tanker impacts when in tandem to offloading operations, and passing vessel collisions. Colliding supply ships may cause penetration of the side shell, but it is unlikely to penetrate the inner skin causing oil outflow or overall hull girder failure. Stern impact by a tandem shuttle tanker is a possible event and may cause flooding of the aft machinery room, and damage the aft flare tower. Collisions caused by passing vessels are dependent on the location of the FPSO relative to ship lanes. Studies on sites in the North Sea and the Gulf of Mexico show that the annual impact probability may vary from about 10-3 down to 10-6 or less. Such impacts may be caused by vessels traveling at high speeds that have large mass, and may result in significant impact energy and damage potential. Therefore, we can predict that it will cause flooding and outflow from one or two wing tanks, and a center tank. 9.5

Ship - Bridge collision

A survey of international bridge projects (Pedersen et al 1998) reveals that efforts have been made to minimize the risk of collisions to bridges, bridge users, and the environment. To reduce risk, both bridge safety and navigational safety need to be addressed. A risk analysis requires: • Establishment of a realistic representation of the navigational activity, • Conditions and practice that can be expected once the bridge has been built,

48

ISSC Committee V.1: Collision and Grounding • • • • •

Selection of a design collision scenario (e.g., ship size, speed) to measure collision severity, Assessment of consequences such as structural failure, User fatalities and environmental pollution, Acceptance criteria establishment (preferably prior to commencement of risk analyses), and Evaluation of preventative and protective measures.

Building on the experiences of ship collision analyses for the Great Belt Bridge, Pedersen (2002) presented a symmetrical methodology for addressing collision risk for fixed installations close to high-density shipping lanes. Causation factors and the number of collision candidate ships are taken into account. Traffic of different ship types, length, and loading conditions are based on statistical data. China has been building many new bridges over the Yangzi River and many bays. To provide the design load for bridge piers, analyses of ship to bridge collisions have been completed for most of the bridge construction projects. Also, special collision-protecting structures have been designed and constructed for some of bridges. China has two rules that specify collision force: 1) “Design Rules for Highway and Bridges” (China Railway Publishing House 1985), and 2) “Technical Rules for Engineering of Railway” (China Communication Press 1989). Ship collision forces in the former rules are tabulated and specified according to the river way grade, and are defined in the later rules by the following function: F = γ V sin(W/(C1+C2))0.5, where, γ is the kinetic energy reducing ratio (which is 0.3 during a normal ship to bridge collision, and 0.2 otherwise), V is the velocity of the striking ship, W is the mass of the ship; C1 is the elastic deformation ratio of ship, C2 is the elastic deformation ratio of bridge. In addition, design formulas in AASHTO-1994 and in IABSE were often referred in estimating collision force. The Chinese projects mainly used simplified non-linear collision analyses or full FEM dynamic simulations using DYTRAN or DYNA software. The size and speed of striking vessels were estimated from the voyage statistics and 30~50 years of water-route development layout at the bridge’s location. The speed of the current, wind, and other natural conditions were considered in some cases. Bridges with multi-spans had different collision scenarios specified for piers of main and auxiliary spans. Various vessels, speeds, draft, tides, etc. were assumed. 9.6

Recommendations

The marine community needs to formalize the risk assessment approach for addressing ship-to-floating installation collisions. Further investigations may define design accident scenarios, the probability of shuttle tanker impact, and accidental loads with specified exceedance probabilities.


49

The offshore industry has been using simplified structural models in calculating energy absorption. More sophisticated tools such as the nonlinear FEM are recommended so that structural responses during and after a collision can be more precisely predicted.

10.

CONCLUSIONS AND RECOMMENDATIONS

Crashworthiness: The concept and methodology of structural crashworthiness are maturing and have reached a level where they can be applied in analyses and evaluations of a wide range of collision and groundings. The committee advocates more studies and application, and recommends continued tools refinement to apply to structural crashworthiness. Probability of collision and grounding events: Future research on procedures that reduce collision and grounding probabilities should focus on developing risk-based software. The software should be capable of rationally modeling the cost and risk reduction features of each risk control option. The calculated cost of these risk reducing measures must then be compared with savings from calculated reductions in expenses. When such tools have been developed, we can balance risk control option investments and related safety improvements against the benefits. Risk assessment: Risk assessment approaches are well suited to collision and grounding issues, and are expected to continuously stand in the center of future development. The committee recommends focusing on integrating predictive calculation tools, including the development of streamlined software/programs. The committee knows that collision speeds in damage reports are not certain, and are of questionable credibility. We therefore recommend that the global community move towards unifying procedures to more precisely record incidents and damages. Rule and regulation development: Future rules and regulations on collisions and groundings need to address: • Design incident/accident scenarios, • Responses (of ships, offshore installations, bridges, etc) to an incident/accident, and • Consequences and acceptance criteria. Risk assessment approaches are now viable tools to use for developing rules and regulations. Experiences of offshore industry and other industries provide a wealth of useful information, and may be good references for the shipping industry. The committee identifies some key development directions, which include framework for rule and regulation development, formalized accident scenarios, and definition of damage extents, among others.

50


Predictive calculation approaches: The latest research and development since the early 1990s has produced many useful tools that can predict various aspects of a collision or grounding. Topics that will further refine these methods include: • Rupture strain, • Post-accident loads (both still-water and dynamic loads), • Grounding, and • Influences of ballast water or cargo oil in tanks. The number of these advanced predictive calculation approaches will reply on the development of useful software. Applications: The committee highly recommends more active application of the latest research and development achievements to offshore industry and high-speed crafts. We need further in-depth studies to investigate the behavior of aluminum and sandwich panels. We also need further studies to look into innovative designs that maximize the crashworthiness in an accidental impact.

ACKNOWLEDGEMENT

The committee has received support and valuable comments from many individuals: Jorgen Amdahl, Bart Boon (Standing Committee Liasion), Haibo Chen, Weichen Cui (China Standing Committee member), Igor Davydov (committee member until 2005), G.V. Ergorov (Ukraine Correspondent), Zhiqian Hu, Lyuben Ivanov, Ozgur Ozguc, Bo Simonsen, Jack Spencer, Yoichi Sumi (Japan Standing Committee member), and Leshan Zhang. Diana Combs provided extensive editorial assistance. Leanne Ebow and Jim Speed helped with proof-reading. This committee and committee chairman would like to express our sincere appreciation to all these individuals for their continuous supports and encouragement during the last 3 years.

REFERENCES

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