Manoeuvrability in Adverse Conditions

Proceedings of the ASME 2015 34th International Conference on Ocean, Offshore and Arctic Engineering OMAE2015 May 31-June 5, 2015, St. John's, Newfoundland, Canada

OMAE2015-41628

MANOEUVRABILITY IN ADVERSE CONDITIONS Vladimir Shigunov DNV GL SE Hamburg, Germany

ABSTRACT Slow steaming and regulatory drive towards more energy efficient ships have raised a problem of ensuring sufficient manoeuvrability of ships under adverse weather conditions when installed power is reduced. This paper discusses possible criteria for sufficient manoeuvrability in adverse conditions and proposes practical assessment procedure and examples of its application. Further, the paper outlines necessary developments.

INTRODUCTION Introduction of the Energy Efficiency Design Index (EEDI) to improve energy efficiency of shipping and reduce GHG emissions has raised concerns that some ship designers might choose to simply lower the installed power to achieve EEDI requirements, which can lead to insufficient propulsion and steering abilities to maintain manoeuvrability of ships under adverse weather conditions. Following a proposal from the International Association of Classification Societies (IACS), the following requirement was added to the Reg. 21, Ch. 4 of MARPOL Annex VI: “For each ship to which this regulation applies, the installed propulsion power shall not be less than the propulsion power needed to maintain the manoeuvrability of the ship under adverse conditions as defined in the guidelines to be developed by the Organization.” Work carried out by IACS led to development of first draft guidelines for consideration by IMO in 2011 [1], [2] and updated proposal in 2012 [3], [4]. The guidelines are based on three-level assessment (Level 3, comprehensive assessment, Level 2, simplified assessment and level 1, minimum power lines). The 2012 Interim Guidelines were issued in 2012 [5]. They were revised in 2013 [6] (Level 3 was removed; in Level 2, numerical methods were replaced with model tests; accepted formulation of Level 1 does not relate to propulsion or steering characteristics of ships; besides, environmental conditions were adjusted for small vessels) and approved for Phase 0 of EEDI implementation (until December 31, 2014). In 2014, these 2013 Interim Guidelines were extended into Phase 1 of EEDI implementation (until December 31, 2019). Although 2013 Interim Guidelines is a useful provision to prevent from underpowered ships, some elements can be improved: • Potential conflict should be addressed between strengthening EEDI requirements and 2013 Interim Guidelines, e.g. for bulk carriers with deadweight below 25000 t in Phase 3 of EEDI implementation (after January 1, 2025) [7],

NOMENCLATURE significant wave height hs J propeller advance ratio length between perpendiculars Lpp

n PD

propeller rotation speed required delivered power

PDav T t us vd

available delivered power

vs vw w

β βe βw γ ηH δ

propeller thrust thrust deduction longitudinal ship speed ship design speed ship speed wind speed wake fraction drift angle main wave direction

main wind direction seaway peak parameter hull efficiency rudder angle

Indices: time-average wave-induced forces and moments d rudder forces R s calm-water forces and moments w wind forces and moments

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Copyright © 2015 by ASME

•

Table 1. Criteria and weather conditions for redundancy and duplication of propulsion system according to requirements of classification societies from [11] Class Criteria vw hs

Requirement of model tests in Level 2 assessment is too complex for this level, • Level 1 assessment does not refer to ship-specific propulsion or steering characteristics or environmental forces, • Absence of Level 3 assessment stifles development of innovative propulsion and steering concepts, i.e. contradicts the idea of EEDI introduction. This led to several research initiatives in Germany (project PerSee – Performance of Ships in Seaway), The Netherlands (project Short-sea Shipping Requirements: EEDI & Minimum Power Requirements), Japan (Strategy Research Committee on Minimum Propulsion Power, organised by Japan Society of Naval Architects and Ocean Engineers together with ClassNK) and EU (project SHOPERA – Energy Efficient Safe Ship Operation), aiming at updating the guidelines.

GL GL LR BV ABS DNV

Change and keep heading (weather-vaning) Advance speed ≥ min(7 kn, vd /2)

21 m/s

5.4 m

11 m/s

2.8 m

Steering ability, advance speed ≥ 7 knots Advance speed ≥ 7.0 knots Weather-vaning without drifting Weather-vaning at advance speed ≥ 6 knots

-

-

Bft 5 33 kn Bft 8

corresp. vw 4.5 m corresp. vw

Work by IACS Project Teams on minimum power requirements for manoeuvrability in adverse weather conditions within EEDI regulations started with analysis of functional requirements to manoeuvrability in the open sea and coastal areas [1], [2] and concluded that manoeuvring in coastal waters is more challenging than in the open sea; the resulting criteria for ship propulsion and steering abilities were formulated in [3], [4]: the ship should be able to (1) keep course in waves and wind from any direction and (2) keep advance speed of at least 4.0 knots in waves and wind from any direction. The corresponding weather conditions are not severe, because ship masters do not stay near the coast in an increasing storm and either search for a shelter or leave to the open sea and take a position with enough room for drifting, if escape is impossible. The recommended environmental conditions (wind speed 15.7 m/s at significant wave height 4.0 m for ships with Lpp=200 m, to 19.0 m/s and 5.5 m, respectively, for Lpp=250 m and greater) were derived by benchmarking of tankers, bulk carriers and container ships in the EEDI database against these two criteria. The required minimum advance speed of 4.0 knots was assumed to provide some minimum speed over ground for timely escape of the coastal area, and include some margin to take into account current. Thus, criteria proposed so far for manoeuvrability in seaway can be classified as (1) course-changing and coursekeeping, (2) minimum advance speed and (3) weather-vaning; in addition, (4) low-speed manoeuvrability in strong wind was considered. Few available detailed accident investigations (see also [12]) indicate as the most frequent cause of grounding accidents in an increasing storm waiting at anchor until it starts dragging, and starting the engine too late or at a too low rate to avoid grounding. In several accidents, however, e.g. [13], [14], [15], [16], vessels were not able to move away from the coast or turn into waves and wind despite full engine power applied. In accident [13], full engine power was not available due to failure of one of the engines, in accident [14], forward speed had to be reduced to wait for entrance clearance in a port approach channel, and in accidents [15] and [15][16], full engine power was available and applied; such accidents suggest that there is a

MANOEUVRABILITY CRITERIA Manoeuvrability of ships is presently addressed by IMO Standards for Ship Manoeuvrability, adopted in 2002 [8], which assess turning ability (the ability of ship to turn using hard-over rudder), initial turning ability (i.e. the course-changing ability), yaw-checking ability, course-keeping ability and emergence stopping ability, evaluated in simple manoeuvres in calm water. These Standards have been often criticized for not addressing ship manoeuvring characteristics at low speed, in restricted areas and in adverse weather conditions. Two questions arise: first, whether the acceptance limits of the existing criteria are strict enough to ensure sufficient manoeuvrability also at low speed and in adverse conditions, and second, whether all relevant ship characteristics are covered by the existing criteria or additional criteria are required. Whereas existing experience and knowledge do not provide clear answer to the first question, the answer to the second question is obvious when we note that one of tasks of steering is withstanding environmental forces: Because different ships experience different environmental forces, the ship-specific assessment of ship’s steering and propulsion abilities to withstand these forces appears a necessary part of minimum manoeuvrability requirements. In [9], course-keeping ability in beam wind without waves was studied; the results show that the minimum required rudder area is defined by course-keeping in beam wind for slender ships with large windage area, turning in calm water for bulk carriers and tankers, and both requirements (depending on the loading condition) for general cargo ships. Work [10] proposes two additional criteria: (1) leaving quay at low speed in wind 20 to 30 knots to address low-speed manoeuvrability and (2) 180° course change at 40% of maximum speed and rudder angle 2/3 of maximum in a seaway with significant wave height 6.0 m. Paper [11] puts together requirements of classification societies to redundancy and duplication of propulsion system to address criteria and environment relevant for steering and propulsion in adverse weather conditions, Table 1 ( vd means design speed, vw wind speed, hs significant wave height).

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minimum limit for the propulsion and steering abilities for a ship to be able to leave the coastal area in an increasing storm. Table 2 shows a summary of relevant criteria and weather conditions from several accident reports. Figure 1 shows the dependency of the number of ships at anchor (as percentage of the initial number of vessels at anchor) on the significant wave height during an increasing storm, based on data from [16]. At the significant wave height of 4.5 m, about 80% of the initial number of vessels were still at anchor, whereas at the significant wave height of 6.0 m, the majority of the vessels have already left to the open sea, and only 20% remained at the anchor.

how close he can come to storm, depending on the freeboard, cargo, stability and propulsion and steering characteristics of the vessel. When caught in most violent storms, steering against seaway may be impossible for any vessel, but drifting with seaway can be an option for a limited time. However, power matters to escape the storm and bring the ship into safe weather conditions. According to these interviews, weather-vaning practice in heavy seaway strongly depends on metacentric height: for container ships with large GM and all bulk carriers and tankers, the solution is to avoid beam seaway (thus avoiding violent synchronous rolling) and steer the ship bow into seaway to 30° (sometimes more) off seaway. On the other hand, for container ships with low GM, drifting in beam waves and wind is considered as one of the best weather-vaning methods, if there is enough room available for drifting. Whereas low following and quartering seas are acceptable, severe following and quartering waves are always avoided because of lower freeboard at the stern, possible damage to the rudder, stern slamming and parametric roll. Weather-vaning in heavy weather is related to individual seakeeping characteristics and strength of ships, and depends on windage area, stern geometry, free board at the stern and bow, and bow flare. Manoeuvring in coastal areas was reported in the interviews as more challenging than in the open sea: in principle, any manoeuvre may be necessary in unfavourable seaway direction with respect to the ship. Environmental conditions are however less severe, because ship masters do not remain near the coast in a growing storm, but either search for shelter or leave to open sea. Figure 2 shows wind force (top) and significant wave height (bottom) during leaving coastal areas ( ) according to interviews. The same plots show environmental conditions mentioned as relevant for encountered steering and propulsion problems both in coastal areas and in the open sea ( ); note correlation between these conditions, as well as correlation of the bottom plot in Fig. 2 with Fig. 1. As relevant manoeuvring problems, steering problems were mentioned insignificantly more often than propulsion problems (83% vs. 60% of cases, respectively); insufficient engine power was mentioned more frequently for bulk carriers and tankers, whereas insufficient rudder efficiency more frequently for container vessels. As accompanying problem, propeller racing was mentioned in ballast and low load conditions for all vessels, and even for larger load at high forward speed or wave height. As a very specific manoeuvring problem in restricted waters, manoeuvrability at reduced speed (due to navigational restrictions, e.g. during approaching ports) was mentioned, especially for ships with large windage area (container ships, RoRo vessels, car carriers), but also for bulk carriers. As relevant environmental conditions, strong wind and, sometimes, strong current were mentioned, but usually no large waves because of protected areas. Work [12] suggested that steering and propulsion abilities of ships are challenged by the environment in a very different way in three situations, each of which requires specific criteria:

Table 2. Summary of accident reports Ref. Relevant criteria Environmental Conditions [17] [18] [19]

Propulsion Propulsion Course-changing

Bft 5-7, low waves Bft 10 vw 38-45 knots

[13] [14] [15]

Course-changing Course-changing Propulsion

Bft 9-10, hs > 10 m gale force wind, low waves vw 40 knots, hmax = 4 m

[16]

Propulsion

vw 38-46 knots, hs = 6.0-6.6 m

Figure 1. Number of vessels at anchor (as percentage of the initial number of vessels) vs. significant wave height during an increasing storm according to data in [16]

Well-known statistics of the weather conditions during collisions from the HARDER project indicate that most collisions happen at significant wave heights below 2 m, and that significant wave heights exceed 4 m in very few accidents. Results of the EU-funded project SHOPERA concerning statistics of grounding, collision and stranding accidents in adverse weather conditions show similar conclusions [20]; besides, they show that grounding and contact accidents are dominating accident categories in heavy weather conditions. Accident rates concerning grounding, collision and contact in heavy weather are in the order range of 10-5 to 10-3 per ship per year. Ship types with highest accident frequency are those with large windage area (cruise and RoRo vessels and car carriers) and frequent port calls (general cargo coasters). In the projects PerSee and SHOPERA, interviews of ship masters are conducted regarding ship characteristics and corresponding environmental conditions relevant for manoeuvrability in heavy weather; available interviews for about 50 container ships, bulk carriers and tankers indicate that in the open sea, the captain has more freedom and can decide

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can be neglected. There are also statistical data to define relation between the wave height and average wind speed for the open sea; e.g. following ITTC recommendations, work [21] proposed the following relation: hs = 0.115v1.41 w , where hs is in m and vw in m/s. A possibly important factor is gust wind speed, which can exceed mean wind speed by about two times; thus instantaneous wind forces and moments can exceed mean forces and moments about four times. However, space and time scales of wind gusts are limited and will not influence manoeuvring motions that much. A simple practical solution is to introduce an empirical “gust factor” [24] onto the mean wind speed to account for the influence of wind gusts on steering and propulsion. The mean direction of waves and wind may be considered to deviate, e.g. by 30°, which is frequently observed in practice and is known to be more critical for steering and propulsion than aligned wind and wave directions. Manoeuvring in Coastal Areas Operation in coastal areas places greater requirements on manoeuvrability than in the open sea: the ship may require to perform, in principle, any manoeuvre (as well as maintain the required course), and also maintain some speed over ground to enable leaving the coastal area before the storm escalates. Due to navigational restrictions, this must be possible in waves and wind from any direction, including directions most unfavourable for course changing or course keeping. On the other hand, the corresponding environmental conditions are less severe than in the open sea, because in an increasing storm ship masters look for shelter or leave to the open sea. The above requirements to be able to perform any manoeuvre, to maintain the required course and to maintain some minimum advance speed, can be reduced to the following criteria: the ship must be able to change the course to the required one and keep it, as well as to maintain some minimum advance speed; these three requirements should be fulfilled in waves and wind from any direction. Noting further that if a ship can keep any course with respect to seaway, including courses most unfavourable for course-keeping, than the ship will also be able to perform any course change, the following two practical criteria can be proposed [12]: the ship should be able, in waves and wind from any direction, to keep (C2) a prescribed course and (C3) a prescribed advance speed. These criteria were the background of the Level 3 (comprehensive assessment) of 2012 Interim Guidelines [3], [4], [5], with the required minimum advance speed set to 4 knots to provide sufficient time for leaving the coastal area and some margin for a current. Level 3 was removed from 2013 Interim Guidelines [6], which, however, still contain Level 2 (simplified assessment), empirically derived from the application of Level 3 to container ships, bulk carriers and tankers [4]. Regarding weather conditions to be used with criteria C2 and C3, note that wave heights can also be defined by benchmarking of existing ships against these criteria; wave heights defined using a similar method in [5] and [6] are

Figure 2. Environmental conditions from interviews. Top: wind force during leaving coastal areas ( ) and during encountering steering and propulsion problems in coastal areas and in the open sea ( ); bottom: corresponding significant wave height ( and ), respectively

• • •

manoeuvring in the open sea, manoeuvring in coastal areas, low-speed manoeuvring in restricted areas

Manoeuvring in Open Sea In the open sea, the functional requirement following from the above is the ability of ship to change the heading into a favourable one with respect to the environment and keep this heading. Noting that the ability to maintain heading also ensures the ability to change heading, as a practical, easier to evaluate criterion, the following was proposed in [12]: (C1) the ship should be able to keep heading in head to bow-quartering seaway up to 60° off-bow. The corresponding weather conditions should be severe to extreme. When selecting environmental conditions for criterion C1, it should be noted, that none of ships has sufficient steering and propulsion ability to steer against waves and wind in most severe possible storms. Therefore, to define wave height to be used in the assessment, benchmarking of the existing world fleet against the weathervaning criterion C1 seems a promising way ahead: ships with proven safety record should be able to fulfil the requirement. Sufficient data are available for open sea regarding suitable characteristic wave periods and form of seaway spectrum; current is not relevant for manoeuvring in the open sea and thus

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fidelity assessment (model tests or accurate numerical methods), when it is necessary or advantageous for designer or operator. This is especially relevant for innovative design solutions: in practice, designers often improve only particular parts of the design (e.g. steering or propulsion), and in such cases, hydrodynamic performance of the improved part is investigated in detail in model tests or numerical computations, but other components (e.g. wave or wind forces) are not necessarily studied in detail. Therefore, the procedure should be able to take accurately into account all benefits provided by innovative steering and propulsion solutions, whereas it should allow combining such results with simple assessment methods, e.g. empirical formulae, for those contributions where complex methods are not necessary, not required or not practicable for a particular design. Second, the procedure should be verifiable and should combine results from model tests, numerical computations and empirical formulae in such a way that any numerical or experimental contribution can be verified or replaced, if necessary, during design or approval. This is possible if the procedure is modular (i.e. consists of separate blocks), and each block requires simple computations or simple experiments in well-controlled conditions (for example, steady-state). Note that a methodologically similar approach is used for the different problem of ship capsize in dead ship condition according to [22], [23]: even though seakeeping tests in beam seaway at zero forward speed are much easier to evaluate than transient manoeuvres in seaway, still a simplified, more accurate and more efficient procedure is used, based on series of separate simpler tests in well-controlled conditions (drift in beam wind, roll decay in calm water and roll in regular beam waves) to define separately different elements (heel angle, roll damping and effective wave slope) and then put them together in a simple analytical model. Finally, the procedure should reflect the state-of-the art of the available technology in the industry. It should be as accurate as practicable with the available technology, inexpensive and available for any shipyard and administration. Despite inevitable simplifications, the procedure should facilitate taking all relevant physics into account. Moreover, the procedure should be able to accommodate any new knowledge, which will be produced due to innovation efforts, without the need to revise the procedure. In principle, criteria C1-C6 proposed in the previous section (weather-vaning in open sea, course-keeping and advance speed in coastal areas and course-keeping in wind at low speed) can be directly evaluated using transient model experiments with self-propelled ship models in simulated irregular waves and wind, for all required combinations of wave direction and wave period. For practical design purposes, such an approach is however presently unfeasible, for several reasons: First, available experimental methods are not mature enough yet for such tests. For example, one of problems is scale effects for wind forces: Froude-scaling of the model speed leads to large scale effects in wind forces and moments. Another scale

consistent with the described above weather conditions in other regulations, accident statistics and interviews of ship masters. Still, some further refinement might be necessary, e.g. concerning not considered yet ship types, the dependence of environmental conditions on ship size etc. The relation between wind speed and significant wave height in coastal areas, suggested in [5] (taking into consideration limited fetch in coastal areas) and used also in [6], is confirmed by the available statistics in the project SHOPERA and can be approximated as hs = 0.316vw , where

hs is in m and vw in m/s. More difficult than for the open sea is defining wave periods and spectrum shape to be used with criteria C2 and C3. In [5] and [6], maximum seaway steepness was used to define the wave period, and JONSWAP spectrum with γ = 3.3 (average peak parameter for limited fetch areas) was selected; further research might be necessary to refine these parameters. Current speed may also require further studies.

Low-Speed Manoeuvrability in Restricted Areas Manoeuvrability at low forward speed in strong wind may be critical for ships with large windage area during approaching to and entering ports. Because the speed is limited (due to navigational restrictions), rudder efficiency is reduced; an additional important factor is current, which changes speed through the water and thus, rudder efficiency. Low-speed manoeuvrability criteria may lead to enhanced requirements to steering devices, but they will not impose any restrictions on minimum propulsion power, i.e. there is no potential conflict with EEDI requirements. Note that assessment following such criteria will provide useful guidance to operators, e.g. up to what low speed ships can manoeuvre in a given wind, at what wind force port tug assistance is required etc. As practical criteria, work [12] proposed course-keeping at a specified low speed in strong wind in (C4) shallow water, (C5) shallow water near a bank and (C6) shallow water during overtaking by a quicker ship. According to experience, waves in such situations are usually not large and can be neglected in a practical assessment; thus only wind force and current are to be defined. Work [10] recommends wind speed of 20 knots for general use and 30 knots for ferries and cruise vessels; this agrees with the results of interviews, Fig. 2 (top). PRACTICAL ASSESSMENT PROCEDURE Compliance with the IMO Manoeuvrability Standards [8] is demonstrated in full-scale trials; full-scale trials are however impracticable in adverse weather conditions. Alternatives are model tests and numerical computations. In any case, for a practical assessment procedure, the following is required: First, the procedure should be flexible, i.e. it should allow for alternatives (model tests, numerical methods, empirical formulae or combination of these methods) depending on designer needs. For example, it should allow using of high-

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effect concerns propulsion and steering: propeller loading, and thus rudder forces, are different in model scale; see however the rudder efficiency correction (REC) method [25], which may allow proper scaling of propeller loading and rudder force. Second, repeating tests in hundreds of long realisations of each seaway is necessary for reliable statistical predictions in irregular seaways, which is too expensive. Third, only few facilities exist world-wide, able to carry out such tests in principle, which makes such tests impractical for routine design and approval. Finally, results of such tests depend very much on time history of steering, which leads to too large variability of the results. Impact of this variability is difficult to quantify for regulatory purposes, thus it will be not possible to verify such results, especially in marginal cases (which are the actual cases of interest in approval). Alternatives to such model tests – direct numerical simulations of transient manoeuvres in irregular waves – are not mature enough yet for routine design and approval [26]. Thinking about possible simplifications, note, first, that any practical procedure inevitably involves many simplifications, each of which leads to conservative or non-conservative bias or random error. These are accounted for by empirical corrections and safety margins. When deciding about what effects can be simplified and what not, note that the procedure should take explicitly into account those factors, which are, first, shipspecific and second, relevant for a designer trying to improve steering or propulsion characteristics to fulfil regulations (e.g. rudder, propeller and engine characteristics), whereas empirical corrections and safety margins can be used for factors which designers do not control anyway (e.g. change of thrust deduction and wake fraction due to motions in waves), until more knowledge is available. The practical assessment procedure proposed here is based on neglecting oscillatory forces and moments and thus considering only average in time forces, moments and other variables (propeller thrust, torque and rotation rate, required and available power, drift angle and rudder angle), because time scale of these oscillations is shorter than time scale of manoeuvring motions. Note that oscillatory forces and moments due to waves produce several effects, relevant for manoeuvrability: • At high speeds in stern waves, encounter-frequency motions can induce broaching-to; note however that broaching-to cannot be solved by design measures, but is efficiently handled in operation by speed reduction. • Oscillations of propeller thrust, torque and required power above their time-average values may lead to engine overload. The non-conservativeness with regard to this effect can be taken into account using a safety margin on the available engine power (note that some overload margin is usually taken into account in engine selection). • Propeller pitching due to motions of ship in waves reduces available time-average thrust; besides, it can

lead to a drop of the mean available power due to dynamic response of the engine after a ventilation event. Such effects should be taken into account, if assessment is done for ballast loading conditions in severe seaways or at high forward speed. Neglecting oscillatory forces and moments effectively reduces the evaluation of criteria C1-C6 (see the previous section) to a solution of steady coupled equilibrium equations in the horizontal plane under the action of time-average waveinduced forces and moments (index d ), wind forces and moments ( w ), calm-water forces and moments, including interaction effects ( s ), rudder forces ( R ) and propeller thrust ( T ); forces are projected on the x - and y -axes and moments on the z -axis of the ship-fixed coordinate system: Xs + Xw + Xd + XR + T (1 − t ) = 0 (1)

Ys

+

Yw

+

Yd

+

Ns

+

Nw

+

Nd

− YR Lpp / 2

YR

= 0

(2)

= 0

(3)

Figure 3 shows the coordinate system: origin O in the main section at the water plane; x -, y - and z -axes point towards bow, starboard and downward, respectively (positive rotations and moments with respect to z -axis are clockwise when seen from above). The ship is assumed to sail in the north direction with the speed vs ; its heading deviates from the course by the drift angle β (positive clockwise when seen from above). The main wave and wind directions are described by angles β e and β w , respectively (0, 90 and 180° for waves and wind from the north, east and south, respectively); rudder angle δ is positive to port.

Figure 3. Coordinate system and definitions

Results of the solution of equation system (1)-(3) are the required propeller thrust (from which, advance ratio J , rotation speed n of the propeller, and required PD and available PDav delivered power are found), drift angle β and rudder angle δ . Figure 4 shows examples of application of procedure based on equations (1)-(3) to manoeuvring criteria in coastal areas C2

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and C3 (course-keeping and minimum advance speed, respectively, in waves and wind from any direction) in polar coordinates ship speed (radial coordinate) – seaway direction (circumferential coordinate, head waves and wind come from the top). Line A corresponds to situations when the required delivered power PD is equal to the available delivered power

Moreover, even if simplified methods (e.g. empirical formulae) are used for some or all of the contributions, the entire formulation still remains physically meaningful, and any improvement in numerical or experimental methods for selected components can be taken into account when required. Usually, high-level methods are not required for all components simultaneously: when designers try to improve particular ship characteristics (steering or propulsion), they use high-level assessment methods for certain contributions, and thus can gain advantage from such improvements by using directly the obtained high-level results for relevant components, whereas applying simple methods, e.g. empirical formulae, for the components not concerned in the improvement. When new knowledge (new numerical methods, model test results or new empirical formula) becomes available for certain components, it can be directly used in the assessment. Presently available methods for different components are briefly outlined in the following. Still-water hydrodynamic reactions Xs, Ys and Ns can be defined as functions of drift angle β and ship speed vs in steady-drift model tests or RANSE-CFD simulations (at low speeds, double-body solution is sufficient). Effects of shallow-water, bank or hydrodynamic interaction with another ship during overtaking can also be defined in steady model tests or RANSE-CFD computations. As simplified methods, empirical formulae, see e.g. [27], can be used once validated. Empirical formulae to take into account shallow water, bank effects and hydrodynamic interaction have to be developed yet.

PDav , line B shows the line of required minimum advance speed (here 4.0 knots), and line C limits the dark area, in which the required rudder angle for course-keeping exceeds maximum rudder angle (here 25°). The left plot corresponds to seaway in which the installed power is sufficient to fulfil both criteria C2 and C3 (line A does not cross lines B and C); further to the right, the following combinations of ship and seaway are used for illustration: minimum required installed power is defined by required minimum advance speed in head seaway (line A crosses line B in head seaway) and by required minimum advance speed in bow-quartering seaway (line A crosses line B in bow-quartering seaway); and by course-keeping in beam seaway (line A crosses line C). The advantage of the proposed approach is that the timeaverage effects of different factors (wind, waves, calm water, rudder, propeller and engine) can be computed or measured separately, if necessary with different methods (model tests, computations or empirical formulae). Even if model tests or complex numerical computations are used for some of the contributions, they are defined in stationary setups under wellcontrolled conditions, and then combined in the simple steady model (1)-(3).

Figure 4. Examples of assessment results in polar coordinates ship speed (radial coordinate) – seaway direction (circumferential coordinate, head waves and wind come from the top): line “required power equal to available power” (line A), line “advance speed 4.0 knots” (line B) and line “rudder angle 25°°” (line C) for the following situations (from left to right): sufficient installed power to fulfil both criteria C2 and C3 (line A does not cross lines B and C); power defined by advance speed (line A crosses line B) in head seaway and in bow-quartering seaway; power is defined by steering ability in beam seaway (line A crosses line C)

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Wind forces Xw, Yw and moment Nw can be defined as functions of the apparent wind angle of attack and apparent wind speed in steady wind tunnel tests or RANSE-CFD simulations. As simplified methods, available empirical data, e.g. [28], [29] or data of similar vessels can be used. Time-average (drift) forces Xd, Yd and moment Nd due to waves can be calculated with spectral method as functions of ship speed, mean wave heading with respect to ship centre plane and spectral seaway parameters (significant wave height, mean zero-upcrossing wave period and wave energy spreading relative to the mean wave direction). The required input for such (simple) calculations are the transfer functions of average wave forces Xd, Yd and moment Nd in regular waves. It is interesting to compare the influence of average waveand wind-induced forces and moments on manoeuvrability. Figure 5 shows required installed power, estimated from the required delivered power PD by rewriting eq. (4) below as

MCR = PD ⋅ ( nMCR n ) ; the required installed power is found as 2

maximum along the lines of minimum required advance speed (line A in Fig. 4), marked with “advance speed” in Fig. 5, and along the line of maximum rudder angle (line C in Fig. 4), marked as “course-keeping” in Fig. 5, for a container ship (top) and a VLCC (bottom) with Lpp of 355 and 320 m, respectively, both in full-load condition. For the container ship, results in wind only and in waves only are very close, whereas the required MCR in combined waves and wind is about 40% greater than in only waves or only wind; for the VLCC, assessment results in combined waves and wind are close to the results in waves only, whereas the required power in only wind is significantly lower. The transfer functions of time-average wave-induced forces X d , Yd and moment N d can be defined from model tests in regular waves in a seakeeping basin. Although tests in regular waves are much less expensive than tests in irregular waves, still, the limited availability of suitable basins and rather expensive tests require possible alternatives. In long term, potential and RANSE-CFD flow solvers can provide a highlevel alternative; see Fig. 6 comparing longitudinal and lateral drift forces and yaw moment on a single-point moored tanker computed with software GL Rankine [30] with experiments [31] in regular waves of different frequencies from the direction 45° off-bow. In short term, use of simplified methods based on empirical formulae seems inevitable for practical use. Empirical formulae have already been developed for X d component; their validation, as well as development of empirical formulae for Yd and N d are urgently needed.

Figure 5. Required MCR (as ratio to the actual installed MCR) according to course-keeping (C1) and advance speed (C2) requirements for a container ship (top) and VLCC (bottom) in waves, in wind and in combined waves and wind

and RANSE-CFD simulations for a container ship in Figures 7 and 8, respectively. The importance of course-keeping criterion C2 and thus, of accurate rudder model, for the definition of the required installed power, is illustrated in Fig. 9, showing required installed power for course-keeping (C2) and advance speed (C3) criteria for the already mentioned container vessel and VLCC with the original rudder and with 30% increased and reduced rudder efficiency. Interesting that for the original rudder, advance speed and course-keeping criteria lead to close requirements in this case, whereas for the 30%-reduced rudder efficiency, course-keeping criterion becomes dominating for engine dimensioning at significant wave heights above 3 m, and for the 30%-increased rudder efficiency, advance speed criterion is determining for the installed power at all wave heights. The results indicate that, when course-keeping (C2) and advance speed (C3) criteria are used, required installed power can be reduced if rudder efficiency is increased, but only to a certain limit, at which the minimum required advance speed criterion (C3) becomes dominating. Propeller thrust T is found from eq. (1) when thrust deduction t is known. From the thrust, advance ratio J and rotation speed n of the propeller are found from the open-water propeller curves and specified wake fraction w .

Rudder forces X R , YR in the propeller race depend on ship speed and drift angle, rudder angle and propeller loading. They can be defined in steady towing model tests and RANSE-CFD simulations. As simplified methods, semi-empirical methods are available, e.g. semi-empirical methods by H. Söding in [32], [33], which are compared with experiments for a tanker model

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Nd [N/m²]

Yd [N/m²]

Xd [N/m²]

Figure 6. Quadratic transfer functions of X d (left), Yd (middle) and N d (right) vs. wave frequency ω [rad/s] for a single-point moored tanker in 45° off-bow waves from GL Rankine computations (lines) and experiments [34] (symbols)

Figure 7. YR δ [N/rad] vs. propeller thrust [N] for tanker at forward speed 5.0 (left), 7.5 (middle) and 10.0 (right) knots according to models [32] (- - -) and [33] () compared to measurements [34] at rudder angle 10 ( ), 20 ( ) and 30 ( ) degree

Figure 8. YR [kN] vs. propeller thrust [kN] and rudder angle on a twisted rudder of a 14000 TEU container ship DTC [35] from RANSE-CFD simulations (symbols) and model [33] (lines) at rudder angles 0, 10 and 20°° and drift angles of the ship of 0 (left), 7.5 (middle) and 15 (right) degree

Figure 9. Required MCR as ratio to the actual installed MCR from course-keeping and advance speed criteria for container ship (left) and VLCC (right) with original and 30%-increased and 30%-reduced rudder efficiency

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From the same open-water propeller curves, the required delivered power PD is defined. The open-water propeller curves can be defined in well-established steady model tests or potential or RANSE-CFD simulations. As simplified methods, existing propeller series can be used. Thrust deduction t and wake fraction w influence the resulting required delivered power PD through hull efficiency η H = (1 − t ) (1 − w) . Its calm-water value can be defined from steady model tests and scaled with available empirical methods to full scale; RANSE-CFD simulations in full scale are also possible. As simplified methods, empirical formulae for calmwater hull efficiency are available. However, very little is known about change of t , w and η H in waves. Until more knowledge is available, change of η H in waves can be taken into account through a correction factor, conservatively defined from few available results in model scale. Within the steady approach described by equations (1)-(3), engine model can be taken into account as engine diagram; Figure 10 shows a model for diesel engines used in the computations presented in this paper (note logarithmic scale used for both axes). The horizontal axis corresponds to rotation speed as percentage of rotation speed at MCR and the vertical axis shows power as percentage of MCR. Line 1 is the light propeller curve, corresponding to clean hull in calm water; along this line, shaft power is proportional to n 3 . Line 2 is the heavy propeller curve, assumed for fouled hull in seaway, obtained by increasing the light propeller curve power by a “sea margin” at point M; point M corresponds to MCR and is the layout point for the engine. Line 3 is the torque-speed limit of a diesel engine; along this line, shaft power is approximately proportional to n 2 . Line 4 is the engine overload limit (engine margin), about 10% higher than MCR at point M. Diesel engine is controlled by changing pressure in cylinders; constant effective pressure lines are shown as lines parallel to line 7 (mean effective pressure mep limit of 100%); along these lines, shaft power is proportional to n1 .

If, because of increased added resistance in adverse weather conditions, line 2 is shifted further upwards (e.g. to line 8), maximum engine output will be defined by the intersection point A of line 8 with the torque-speed limit curve 3. At this point, the available delivered power PDav is reduced compared to MCR; for illustration, it was approximated in this paper as 2 (4) P av ( n ) = MCR ⋅ ( n n ) , D

MCR

where nMCR is the rotation rate at MCR. This available delivered power PDav is compared with the required delivered power PD.

OUTLOOK Proposed criteria and practical assessment procedure correspond in general to the present knowledge and technology available in the industry; however, their implementation in the practical design and approval require following research efforts: Manoeuvrability criteria should be benchmarked against the existing fleet and their outcomes should be compared with the results of application of IMO Manoeuvrability Standards [1] to add or remove criteria as necessary; the aim is to keep the number of new criteria to minimum. Environmental conditions to be applied with the criteria: wave height for the criteria concerning open sea and coastal areas, and wind speed for low-speed manoeuvrability criteria can be defined from benchmarking of existing ships against the new criteria at varied wave height and wind speed, statistics of environmental conditions during grounding and stranding [20], and further interviews of ship masters, particularly for other ship types than container ships, bulk carriers and tankers. Besides, wave periods and spectrum shape for coastal areas are required, which need analysis of wave statistics. Practical assessment procedure: fine-tuning of safety margins and empirical factors (maximum rudder angle, available delivered power, propulsion in waves) using direct numerical and model-test simulations of transient manoeuvres in waves. Development and validation of numerical methods: highlevel methods (time-average forces Xd, Yd and moment Nd due to waves at forward speed) and empirical formulae for simplified methods (calm-water reactions Xs, Ys and Ns, including shallow-water and interaction effects, rudder forces XR, YR in propeller race, and time-average forces Xd, Yd and moment Nd due to waves) need to be developed. ACKNOWLEDGMENTS This paper summarises experience gathered by the author during his work in Project Teams PT4-PT7 lead by the International Association of Classification Societies (IACS), Collaborative Projects ULYSSES (Ultra Slow Ships) and SHOPERA (Energy Efficient Safe Ship OPERAtion) and research project PerSee (Performance von Schiffen im Seegang) funded by the Federal Ministry for Economic Affairs and Energy of Germany; the views expressed in this paper are those

Figure 10. Engine model

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of the author and do not necessarily reflect the views of the mentioned organisations.

[14] N. N. (1996) Report on the investigation into the grounding of the passenger roro ferry Stena Challenger on 19 September 1995, Blériot-Plage, Calais, Marine Accidents Investigation Branch [15] N. N. (2012) Report on the investigation of the grounding of the cargo ship Carrier at Raynes Jetty in Llanddulas, North Wales on 3 April 2012, Marine Accidents Investigation Branch [16] Australian Transport Safety Bureau (2008) Independent investigation into the grounding of the Panamian registered bulk carrier Pasha Bulker on Nobbys Beach, Newcastle, New South Wales, 8 June 2007, ATSB Rep. Marine Occurrence Investigation No. 243 [17] N. N. (2002) Report on the investigation of the grounding of mv Willy, Marine Accidents Investigation Branch [18] N. N. (2009) Report on the investigation of the grounding of Astral on Princessa Shoal, East Isle of Wight, 10 March 2008, Marine Accidents Investigation Branch [19] N. N. (2012) Report on the investigation of windlass damage, grounding and accident to person on the ro-ro ferry Norcape, Firth of Clyde and Troon, Scotland, on 2627 November 2011, Marine Accidents Investigation Branch [20] A. Papanikolaou, G. Zaraphonitis, E. Bitner-Gregersen, V. Shigunov, O. El Moctar, C. Guedes Soares, D.N. Reddy and F. Sprenger (2014) Energy efficient safe ship operation (SHOPERA), RINA Conf. Influence of EEDI on Ship Design, London, UK [21] V. Shigunov, V. Bertram (2014) Prediction of added power in seaway by numerical simulation, Proc. 9-th Int. Conf. on High-Performance Marine Vehicles HIPER2014, Athens, Greece [22] IMO (2006) Interim Guidelines for Alternative Assessment of the Weather Criterion, MSC.1/Circ.1200 [23] IMO (2007) Explanatory Notes to the Interim Guidelines for Alternative Assessment of the Weather Criterion, MSC.1/Circ.1227 [24] NATO (2007) Controllability and safety in a seaway, Allied Naval Engineering Publication ANEP-79 [25] M. Ueno, Y. Tsukada (2014) Similarity of rudder effectiveness and speed response of a free-running model ship, Proc. 33-rd Int. Conf. on Ocean, Offshore & Arctic Eng. OMAE 2014 [26] Manoeuvring Committee (2008) Final report and recommendations to 25th ITTC, Proc. 25th ITTC, Vol. I [27] D. Kang, K. Hasegawa (2007) Prediction method of hydrodynamic forces acting on the hull of a blunt-body ship in the even keel condition, J. Mar. Sci. Technol. 12, 1-14 [28] W. Blendermann (1993) Schiffsform und WindlastKorrelations- und Regressionsanalyse von Windkanalmessungen am Modell, Rep. 533, Institut für Schiffbau, Harburg

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[29] T. Fujiwara, M. Ueno, Y. Ikeda (2006) A new estimation method of wind forces and moments acting on ships on the basis of physical component models (in Japanese), J. of the Japan Society of Naval Architects and Ocean Engineers, Vol. 2, pp. 243-255 [30] H. Söding, V. Shigunov, T. E. Schellin, O. el Moctar (2014) A Rankine panel method for added resistance of ships in waves, J. Offshore Mech. Arct. Eng. 136(3) 031601·1-031601·7 [31] Pinkster J. A. (1980) Low Frequency Second Order Wave Exciting Forces on Floating Structures. Publication No. 650, Netherlands Ship Model Basin, Wageningen, The Netherlands [32] H. Söding (1998) Limits of potential theory in rudder flow predictions, Proc. Symp. Naval Hydrodynamics, Washington [33] Brix, J. E. (1993) Manoeuvring Technical Manual. – Seehafen Verlag, Hamburg [34] P. Ottosson and M. Brown (2012) Captive Tests for Investigating Manoeuvrability at Low Speeds, SSPA Rep. No. RE40105446-02-00-A [35] O. el Moctar, V. Shigunov and T. Zorn (2012) Duisburg Test Case: Post-panamax container ship for benchmarking, Ship Technology Research 59(3) 50-64

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