New Standard for Assessment of Structural Integrity ...

Proceedings of the ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering OMAE2009 May 31 - June 5, 2009, Honolulu, Hawaii, USA

Proceedings of the ASME 28th International Conference on Ocean, Offshore and Arctic Engineering OMAE2009 May 31 - June 5, 2009, Honolulu, Hawaii

OMAE2009-79379

OMAE2009-79379 NEW STANDARD FOR ASSESSMENT OF STRUCTURAL INTEGRITY FOR EXISTING LOAD-BEARING STRUCTURES-NORSOK N-006 Gunnar Solland, Det Norske Veritas Norway

Inge Lotsberg Det Norske Veritas Norway

Lars G. Bjørheim StatoilHydro Norway

Gerhard Ersdal Petroleum Safety Authority Norway Norway

Vidar-André Gjerstad ConocoPhillips Norway

Philip Smedley, BP Exploration Operating Company Ltd United Kingdom

ABSTRACT An increasing number of platforms in the Norwegian continental shelf are reaching their design life. For various reasons these platforms will require an assessment of their structural integrity. When performing these assessments the engineer is faced with tasks where little guidance is found in design standards, for several reasons. The two most important being: 1) The analyses that is performed in a typical assessment of existing structure is often applying very advanced techniques and methodology that seldom is used in design of new structures, as the cost of doing advanced analysis is relatively low compared to replacement of an existing structure, but relatively high compared to moderate additions of e.g. steel in the design of a new structure. 2) Design standards are based on theories, methods and experience for structures in a given design life (e.g. fatigue design and corrosion protection design). When this design life is extended, sound methods for ensuring that the structures are still sufficient safe is needed. Such methods will normally be “condition based design”, where inspection, maintenance and repairs are included in the assessment in integrated way. Such methods are not given in normal design standards. For these reasons a new NORSOK standard is developed that gives recommendation on how to deal with the specific aspects that engineers meet when performing assessments of

structures in general, but also specifically for assessment for life extension . The standard is named “Standard for Assessment of Structural Integrity for Existing Load-bearing Structures” and is issued as a NORSOK standard and given the number N-006 [1]. The topics that are covered in the standard include: Shut down and unmanning criteria for platforms not meeting ordinary requirements, specific issues for determination of ultimate capacities by use of non-linear methods, cyclic capacity checks, fatigue life extension, requirements to inservice inspection etc. The paper describes the background and the content of the new standard and it presents examples of recommendations given. The role of the new standard in the Norwegian regulatory system is shown.

INTRODUCTION This paper presents the background and the content of a new NORSOK standard for assessment of existing structures to be issued 2009. BACKGROUND The platforms on the Norwegian continental shelf are aging and structural assessment is needed for one or more of the following reasons: • the documented service life is expired,

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Copyright © 2009 by ASME

• • • • •

the original design requirements are no longer fulfilled e.g. the air gap is reduced to less than what was required due to reservoir subsidence, major modifications or operational changes, increased knowledge e.g. revised metocean data, damages inflicted to the platform structure, reduced structural capacity due to degradation mechanisms like fatigue or corrosion.

Assessment of the structural integrity of existing platforms will require the engineer to deal with certain topics that are not covered by today’s design standards. There are also cases where the platform will neither fulfill the original design requirements nor the current regulations. The need for recommendations and guidance to cover these issued was identified by the Petroleum Safety Authority Norway (PSA) and they encouraged the offshore industry to develop documents that will serve the industry to deal with questions related to life extension. The Norwegian organization for offshore operators, The Norwegian Oil Industry Association (OLF) responded to this initiative and to work with development of procedures and recommendations to deal with the issues related to life extension in general. Within the field of offshore structures they invited companies to bid for development of a national standard for life extensions in the beginning of 2007. Det Norske Veritas (DNV) was contracted to carry out the standard development project in March 2007. This work was supervised by an OLF working group headed by Lars Bjørheim StatoilHydro. It was decided to issue the standard as a separate NORSOK standard with the name “Assessment of Structural Integrity for Existing Load-bearing Structures”. It was given the number N006. The standard was issued for public hearing in September 2008 and is expected to be issued as a NORSOK standard in the spring of 2009. SCOPE FOR THE NEW STANDARD The standard is developed to cover issues relevant for structures in service within the NORSOK N-series. The fundamentals for assessment of structural integrity are given in the NORSOK standards N-001 “Structural Design” [2], N-003 “Action and Action Effects” and N-004 “Design of Steel Structures”. The new standard is intended to cover those aspects that are particularly relevant to reassessment of structures and issues of life extension. The standard is together with the other NORSOK standards aimed to be a self contained document, but it is developed, as much as possible, to be in agreement with relevant ISO standards. The standard covers all structural types of platforms and different types of structural materials, but the emphasis is given to steel jacket structures as the majority of structures that are subjected to assessments in the near future, is of this type.

Contradictory to the principles for life extension of existing structures given by API [5] and by ISO [6] it was agreed that the recommendations in the new NORSOK standard should be aiming to represent the same safety level for personnel as what is the minimum requirements to new platforms according to as it is implicit given in the codes and standards refered to in Norwegian regulations. CONTENT OF THE NEW STANDARD The standard gives supplementary requirements to existing structures. The standard consists of the following chapters: Foreword Introduction 1 Scope 2 Normative and informative references 3 Terms, definitions, abbreviations and symbols 4 Assessment process 5 Data collection 6 Assessment principles for existing structures 7 Check of fatigue limit states (FLS) 8 Check of ultimate limit states (ULS) and accidental limit states (ALS) 9 Requirements to in-service inspection after assessment 10 Documentation of structural reassessment Annex A Commentary (informative) Recommendations for when assessments should be initiated are given in Chapter 4, “Assessment Process”. The assessment process is described by text and illustrated by a flowchart that is also shown in Figure 1 included in Annex A to this paper. Chapter 5 gives requirements to the collection of data and to in-service inspection to assess the as-is condition of the considered structure. The principles for how to assess existing structures are given in Chapter 6. The general principle for fulfillment of ordinary requirements as given in NORSOK N-001 [2] is stated together with requirements for how structures not fulfilling these requirements can continue to operate. Platforms that do not meet todays ULS and ALS requirements for environmental loads as given in NORSOK N-001 [2] are in principle not acceptable for further use. However, a possible mitigating measure is to unman the platform during storms. Recommendations for how to determine the forecasted sea state when unmanning of the platform needs to take place are given in the standard, Basis for the unmanning criteria is that the safety for personnel on a platform that needs to be unmanned during storms should be as good as the safety to personnel on platforms that satisfy ULS and ALS requirements for manned platforms. Examples on how unmanning criteria can be developed are given in the Commentary to the standard. Further, recommendations for how to determine directional wave criteria are also given as this may be of larger importance

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for existing structures than for new ones and this topic is judged not to be satisfactory covered in ordinary design standards. Chapter 7 and 8 gives details for how the fatigue and ultimate limits states should be checked. Examples from these two Chapters are given in the following sections of this paper. Additional requirements to in-service inspection after assessment and to the documentation of the assessment are given in Chapter 9 and 10 respectively. The standard includes a comprehensive informative Commentary that describes the background for some of the requirements and gives examples and other forms of guidance for the use of the standard. An example of this is detailed requirements for determination of the sea state for North Sea locations that will require a platform that do not meet ordinary NORSOK requirements to environmental conditions (10 000 year conditions) to be unmanned. RECOMMENDATIONS RELATED TO FATIGUE For cases where the experienced service life for a structure is longer than the calculated fatigue life it is possible to safely operate the platform by using information about the performance and the inspection results that may be available. This is schematically shown for details that can be inspected in Figure 2 that is enclosed in Annex A. Recommendations for details that can not be inspected are given in a similar way. Piles are an example of such structural elements that is impossible to inspect. In order to improve the fatigue assessment for piles specific requirements to Design fatigue factors (DFF) for calculation of fatigue damage from driving based on driving records are given. The background for this is given in [7]. Chapter 7 “Check of fatigue limit states (FLS)” also includes supplementary recommendations for fatigue analysis, acceptance criteria and improvement methods that are not given in other standards. A number of issues relevant for the fatigue limit states are given in the Commentary to the standard. This also includes considerations on low cycle fatigue during a storm loading that has to be assessed for ULS. Engineers faced with the question that a structure susceptible to fatigue still can be operated safely need to perform several difficult considerations. The guidance offered is believed to help the engineers with these considerations and make it easier for them to make the right decisions. RECOMMENDATIONS RELATED TO ULTIMATE AND ACCIDENTAL LIMIT STATES The basic principles adopted for ULS and ALS checks for existing structures is the same as for new structures. Load and material factors are therefore the same as given in the ordinary NORSOK standards [2] to [4]. However, as the cost of implementing operational limits or structural reinforcements are large for an existing structure compared with performing more accurate analyses, additional requirements for how to perform non-linear analyses for determination of structural capacity are

given. The reason is that these types of analyses are seen to be more frequently used for assessments of existing structures compared to structures at the design stage and that requirements to such analyses are not satisfactory given in the ordinary codes used for design. For the same reason a section on how to show cyclic capacity for structures that is assumed to be used outside linear behavior, is included. Structures that are subjected to repeated or cyclic loading need to be checked for the effects of low cycle fatigue and accumulated plastic strains. For marine structures this will limit the ultimate capacity that can be exploited compared with the static capacity. When ordinary linear analyses are carried out to check ULS or ALS capacity, checks for cyclic capacity can be omitted, but in case of elasto-plastic analyses it is necessary to prove that repeated loading will not lead to failure. Unfortunately, there is no established practice for how this shall be done in ordinary design standards. Even not explicitly given in the general design standards, it is reasonable to assume that a structure will be satisfactory if it is capable to resist all the cycles of the dimensioning storm. The dimensioning storm is the largest sea-state the structure can resist when both the largest loads as well as the cyclic loads in the storm are considered. In the new NORSOK standard the load history to be used for cyclic checks are defined. It is assumed that the remaining cycles are in the same directions as the dimensioning wave. The load-history for the remaining waves in the storm may be assumed to have a maximum value equal to 0.93 of the dimensioning wave, duration of 6 hours and a Weibull shape parameter of 2.0. An example with application of this requirement to a structure that experience wave in deck due to insufficient air gap is shown in Figure 3 in Annex A. One method to perform a cyclic check is to show that the structure will shake down meaning that the structure will carry the loads from the remaining cycles while it is staying within the elastic range. This may in many cases be unduly conservative. The capacity against repeated yielding is in the standard recommended to be checked in a similar way as for high cycle fatigue. One may assume pseudo-elastic stress conditions and use a specific low cycle fatigue curve for the check. Such curve is included in the standard for tubular joints in seawater with cathodic protection. This curve is shown in Figure 4. A further guidance on how to perform cyclic checks by use of general FE-methods and use of Neuber’s rule is given in the Commentary. It is based on methods presented in reference [8]. ROLE OF THE STANDARD IN THE NORWEGIAN REGULATORY SYSTEM Requirements to offshore load-bearing structures on the Norwegian continental shelf are given in the PSA regulations. Functional requirements to structures are given in the “Facility regulation” [9]. The Facility regulation further gives reference to recommended standards for detailed requirements that meet the functional requirements. Among other standards, selected

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NORSOK standards are referred to in the PSA regulations. If an operator for any reason wants to deviate from these standards, the operator is to prove that his alternative methods are as safe as required in the functional requirements. It is expected that PSA also will give reference to NORSOK N-006 in their future editions of the regulations. All NORSOK standards are free to be downloaded from the home page to Standard Norge. The web address is www.standard.no.

[9]

Guidance for Low Cycle Fatigue in Ship Structures,” Published in PRADS2004, Hamburg, Germany. PSA, SFT and NSHD Regulations relating to design and outfitting of facilities etc. in the petroleum activities. “The facility regulations”

CONCLUSIONS A standard in the NORSOK N series are developed to deal with issues specifically relevant for existing structures intended to serve the needs for management of aging structure on the Norwegian continental shelf. The standard gives supplementary requirements to existing structures for issues relevant for life extension. The standard aims to ascertain the same safety level for personnel onboard an existing platform as compared to the minimum requirements for new platforms. ACKNOWLEDGMENTS The development of the NORSOK N-006 standard was funded by Norwegian Oil Industry Association (OLF). The authors would like to thank their respective organizations for supporting the development of NORSOK N-006. It should be noted that particular statements in N-006 and in this paper may not necessarily reflect the individual opinions of the authors or their respective organizations. REFERENCES [1] NORSOK N-006. Assessment of Structural Integrity for Existing Offshore Load-bearing Structures. [2] NORSOK N-001 Structural Design. Rev. 4. February 2004. [3] NORSOK N-003 Actions and action effects. Edition 2, September 2007. [4] N-004 Design of Steel Structures. NORSOK Standard, rev. 02, 2004. [5] American Petroleum Institute, “API RP2A-WSD: Recommended practice for planning, designing and constructing fixed offshore platforms. Working stress design”, 21st Edition (2000) together with Supplement 2, Oct. 2005. [6] International Standards Organisation, “Petroleum and Natural Gas Industries. Fixed Steel Platforms”, 19902:2007. [7] Lotsberg, I., Sigurdsson, G. Arnesen, K. and Hall. M.: Recommended Design Fatigue Factors for Piles subjected to Dynamic Actions from Pile Driving. OMAE 2008-57251. Presented at OMAE 2008 Estoril, Portugal. [8] Heo, H. J., Kang, J. K., Kim, Y. I., Yoo, I. S., Kim, K. S., and Urm, H. S. (2004), “A Study on the Design

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ANNEX A FIGURES

Initiator triggered

Collection of data

Assessment of data

Yes

Perform analysis of structure and compare with requirements

Data sufficient and good enough for execution of analysis

Collect additional data as required. Inspection etc.

No

Assess what additional data are required

Yes

Is it feasible to gather necessary data?

No Perform analysis of structure including mitigation proposal

Compliance with requirements of this standard

No

Assess mitigation alternatives

Plan mitigation

Yes Is mitigation feasible?

Yes

No Structure satisfactory

Structure not satisfactory Figure 1 Flow sheet of the assessment process

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Fatigue analysis triggered

Assess platform data and required analysis model(s)

Perform fatigue analysis

No

Compare calculated action effects with measured (if available)

Are calculated fatigue lives shorter than total design life including DFFs?

Can structural integrity be documented based on in-service inspection?

Yes

Yes

No Perform measurements of action effects Calculated and measured action effects in agreement?

No Perform calibration of action effects

Perform revised fatigue analysis Yes Compare calculated lives with inspection history

Yes

Assess calculated hot spot stress (FE -analysis) and S-N data

Cracks detected in primary joints?

No

Perform calibration of action effects, hot spot stress and S-N data

Perform revised fatigue analysis

Compare calculated lives with inspection history

Plan inspection for the life extension period that fulfills the required safety level.

Figure 2 Assessment with respect to fatigue. Requirements in boxes with dotted frames are optional.

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4.0 Impact height= 0.5

3.5

Impact height= 1.0

Impact height (m)

Impact height= 1.5

3.0

Impact height= 2.0 Impact height= 2.5

2.5

Impact height= 3.0 Impact height= 3.5

2.0 1.5 1.0 0.5 0.0 1

2

3

4

5

6

7

8

9

10

Wave number Figure 3 Impact heights for subsequent waves calculated according to the recommendations in NORSOK N-006 (Calculated for a freeboard of 14 m.)

Hot spot stress range (MPa)

10000

1000

100

10

1 1

10

100

1000

10000

100000

1000000

10000000

Number of cycles

Figure 4 S-N curve for low cycle fatigue for tubular joints in seawater with cathodic protection

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