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Copyright 2006, Offshore Technology Conference This paper was prepared for presentation at the 2006 Offshore Technology Conference held in Houston, TX, U.S.A., 14 May 2006. This paper was selected for presentation by an OTC Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract The offshore industry is presently developing a new recommended practice (RP) that will focus on the Structural Integrity Management (SIM) of existing offshore structures. The proposed API RP 2SIM will be a significant change to existing practice and provide considerably more in-depth guidance for maintaining existing platforms than is available in the present API RP 2A. The key concept of proposed RP will be the use of Risk-Based inspection strategies, which will require the engineer to understand the platforms likelihood of failure and consequence of such a failure. Additionally RP 2SIM will, for the first time, provide the engineer with fitness-for-purpose acceptance criteria against the platforms ultimate load capacity, measured as the Reserve Strength Ratio (RSR). To take full advantage of RP 2SIM provisions, the engineer will require knowledge of the likelihood of platform failure, which is best determined through an understanding of the platforms ultimate strength.

This paper provides an overview of ultimate strength assessments and their role in understanding the structural system response to extreme loads for defining appropriate risk-based inspection strategies and for demonstrating fitness-for-purpose. The paper also reviews future recommended practices (RPs) and regulations, and provides several informative studies to further demonstrate the role of ultimate strength assessments in the SIM of offshore structures.

Introduction SIM is an ongoing life-cycle process for ensuring the continued fitness-for-purpose of offshore structures. The SIM process has evolved over the last 25 years to provide industry and regulatory authorities a means to ensure the continued safe and reliable operation of the aging fleet of offshore platforms around the world. RP developments, in the form of a proposed new API RP for the SIM of offshore structures, will allow the

engineer to use ultimate strength assessments to gain an understanding of the behavior of the structural system. This valuable information can provide a role for the development of risk-based strategies, including setting appropriate intervals between inspections and selecting areas for inspection. The information can also be used to demonstrate fitness-for-purpose and assess the need for risk reduction and/or mitigation.

Offshore structures are traditionally designed on a component-by-component basis, such that under all combinations of design loading every component in the structure has a utilization ratio, derived using the strength formulations from the API RP 2A, of unity or less. However, it is recognized that fixed offshore structures are usually redundant and have a number of different load paths such that failure of one member is unlikely to lead to catastrophic structural collapse, provided that adequate redundancy is available. By utilizing this inherent redundancy found in most offshore structures the likelihood of failure of a platform in an extreme event can be determined.

During the life-cycle of an offshore structure the ultimate capacity is an important attribute that affects the SIM strategy, and can significantly influence the risk levels and operational costs. For example, a minimally braced structure may not have alternative load paths to redistribute forces if a component is damaged or if applied loads are higher than initially anticipated. As a consequence, failure of a single component may be critical to overall integrity relatively intense inspection activity may be required to monitor the structural condition of key load paths. Conversely, a robust structure with alternative load paths through the jacket may be more tolerant of damage or increased loads, offering greater operational flexibility and a much-reduced need for inspection activity to provide the same assurance of fitness-for-purpose.

The key elements of ultimate strength assessments are the application of first principles, technology awareness and an understanding of industry experience and lessons from in-service performance data [Bucknell, et al., 2000], including platform failures in extreme events such as hurricanes Andrew [Puskar, et al., 1994], Lili [Puskar, et al., 2004], Ivan [Puskar, et al., 2005], Katrina and Rita. This awareness is not typical of traditional design contractors and more specialist input may be required. Ultimate strength results are sensitive to the assumptions, and careful evaluation of the results is recommended.

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The Role of Ultimate Strength Assessments in the Structural Integrity Management (SIM) of Offshore StructuresH.S. Westlake, MSL; F.J. Puskar, Energo Engineering Inc.; P.E. O'Connor, BP; and J.R. Bucknell, MSL

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Platform Ultimate Strength Overview The ultimate strength of an offshore structure is usually evaluated using non-linear finite element analysis of a structural model, often termed pushover or collapse analysis. Typically the analysis is undertaken by applying the gravity loading as an initial load step. The concurrent metocean design load for the chosen direction is then applied to the model, and the lateral loading is factored incrementally until the ultimate strength of the structure is reached, typically characterized by a plateau in the global load-deflection behavior of the structural model. Alternatively, the wave height or storm severity is increased rather than factoring the design load. The latter method is often applied if the air gap of the structure is small, such that wave-in-deck loading may be accounted for in the ultimate response of the structure.

The ultimate strength assessment considers load redistribution and allows members and joints, including piles, to undergo plastic deformation, carrying loads past yield or buckling; also loads are redistributed within the system until the structure collapses. Members and joints may exhibit a reduced strength in the form of damage caused by overload, having crossed over buckling or inelastic yielding. In this context, damage is acceptable to individual or groups of members as long as the integrity of the structural system against collapse is not compromised.

An ultimate strength assessment of a platform determines the actual system capacity of the analyzed structure. A structure will have a different ultimate strength for each predominant wave direction; the most important ultimate strength for a structure is the lowest, which is likely to be associated with the weakest direction or the most severe metocean loading.

Reserve Strength Ratio The ultimate strength of an offshore structure is expressed in terms of the Reserve Strength Ratio (RSR), which is a measure of the structures ability to withstand loads in excess of those determined from the platforms design. The RSR is quantified as the ratio of the structures ultimate strength to a reference level load. For structures operating in the Gulf of Mexico the reference level load is determined by the 100-year metocean conditions used for the design of new L-1 high consequence platforms, as defined in API RP 2A.

For each structure there is a separate RSR for each metocean direction, although it is typical for most structures to determine the RSR for three principal directions only, the end-on, broadside and diagonal. It should also be noted that the metocean condition/direction that results in the highest component utilizations or highest base shears may not always produce the lowest platform RSR.

Residual Strength The ultimate strength of an offshore structure in a damaged condition is expressed as the structures residual strength and is highly dependent on the inherent robustness of the structure. The ISO code of practice defines robustness as the ability of a structure to find alternative load paths following failure of one or more key components.

Sources of Reserve Strength and Residual Strength Several sources contribute to the reserve and residual strength, which are a result of explicit and implicit conservatisms made during the design of an offshore structure. These aspects of a structures design have been published on several occasions [UK HSE, Research Report 087], [UK HSE, OTO 97 046], [Lalani, et al., 1993], and are provided here in summary form as reference.

Explicit Design Safety Factors The design of offshore structures is based on traditional

engineering practice, which applies a combination of loads to the structure to determine the internal forces in each brace member. For each member and joint in the structure an allowable strength is provided in the design, and the structure is considered to meet the selected standard if all the individual components satisfy the requirements. All structural recommended practice, whether they are based on permissible stress design (Working Stress Design, WSD) or limit state design (Load and Resistance Factor Design, LRFD), address the design of individual members and joints. Within this design procedure is the premise that failure of one member or joint to satisfy the requirements, constitutes non-compliance with the relevant RP. Explicit safety factors are applied to the strength formulae, which are straightforward to calculate, for example a compression member designed to the API RP 2A WSD, has a safety factor of 1.4 (KL/r=80).

Implicit Design Safety Factors Implicit sources of reserve strength are a result of strength

conservatisms that are outside the control of the designer. Members have reserve strength beyond first yield, which contributes to the global reserve. Assuming that most modern jacket structures have strong joints and that the system failure is dominated by member failure, the implicit safety factor will be dominated by the differences between the effective length factor (K-factor) used in design and the actual K-factor for compression members.

Other sources of implicit safety include the differences between the actual strength of the component and the RP based analytical strength predictions, i.e. the model bias and uncertainty. For members under combined compression and bending the expected value of model uncertainty is close to one.

Expected Material Strength The actual material yield strength is typically higher than

the minimum allowed for in the design of the structure. Actual yield strengths values can be between 5-25% above the specified minimum [Baker, 1973]. This additional yield strength provides an increase in structural capacity not accounted for in the design.

System Redundancy Each structure has an inherent reserve and/or residual

strength, which is directly related to the ability of the structure to provide alternate load paths after failure of a member. This redundancy in the structural system (or robustness) is primarily associated with the arrangement of the braces within the system. A reduction of component capacity does not necessarily imply that the system strength is compromised. This will depend on whether or not the component is participating in the failure sequence that produces the system

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collapse mechanism, or whether the members integrity is required to realize that particular mechanism.

Corrosion Allowance Additional thickness is usually allowed for members in the

splash zone to account for operational corrosion protection requirements. During the life of the structure these design allowances may not be consumed uniformly or completely, leading to additional member strength.

Over-design During the design, members and joints may be sized by the

requirements for construction, load-out, transportation or launch loads. These components are often not optimized and will contribute to the platform reserve strength since they were not included to provide operational strength. Other steel introduced at the design stage for boat impact, fatigue and seismic loads also contributes to the reserve and residual strength.

Legs are often sized to accommodate piles, which pass through the inside. Leg member and associated joint strengths are often under-utilized, especially if minimum wall thickness, impact resistance and leg grouting requirements are specified.

Modeling Techniques During the design, analytical techniques may not be

rigorously employed to model the primary bracing and joints. For example it is not common to take advantage of joint flexibility or use gap elements; also point-to-point lengths may have been used, instead of face-to-face lengths when modeling braces between legs. These design simplifications will provide additional strength to the structural framing.

Secondary Framing Platform design usually focuses on the primary structural

framing of the legs, piles, vertical braces and horizontal braces to resist the design loads. Secondary bracing is used to provide support for items such as launch framing, launch runners, conductor guide framing, etc. In reality, these members provide additional strength to the primary structural framing.

Ultimate Strength Assessments Ultimate strength assessments within a formal SIM process can be used to:

1. Optimize the SIM strategy and update future inspection planning or condition monitoring as appropriate.

2. Determine the continued fitness-for-purpose of the structure in its present condition.

3. Identify and optimize the extent of any required strengthening, repair or other mitigation.

Risk-Based Inspection (RBI) Within the overall SIM process the inspection strategy will determine the frequency for routine periodic inspections of the platform, including underwater and above water inspections. An appropriate interval may be selected using a risk-based approach, which categorizes each platform within a fleet of platforms and considers the likelihood of a platforms failure and the consequences of such a failure. The frequency as well as the scope of inspection is increased for the high-risk platforms.

Inspection Interval The time interval between platform inspections should be

determined in accordance with the overall SIM philosophy. A

risk-based strategy involves the understanding of the platforms likelihood of failure and warrants the application of quantitative methods, such as ultimate strength assessments.

Inspection Area Selection Of critical importance to the effectiveness of each

inspection is the proper and adequate selection of the areas to be examined. It is important to select a sufficient number of inspection areas to provide representative information on the overall structure. Making this selection requires an understanding of the platforms structural behavior and requires an understanding of the platforms susceptibility to damage and the tolerance of the structure to that damage. This understanding is gained through the application of ultimate strength assessments, which will determine which components are highly loaded and would contribute to the collapse of the structure in the event of an overload condition. These members or joints would be prioritized for inspection.

Platform Fitness-for-Purpose Fitness-for-purpose assessment is a key element of the overall SIM process. The purpose of assessment is to establish whether an existing structure remains fit-for-purpose or whether strengthening and/or repair or other mitigation is required. A platform may be considered fit-for-purpose when the risk of failure, considering both likelihood and consequence, is within acceptable levels. For existing structures, it is possible that isolated component failure(s), i.e. loads exceeding the component capacity, will be acceptable, provided that sufficient reserve against overall system failure exists.

Assessment analysis provides a best estimate of the strength of the structure. It seeks to utilize the available reserve strength and redundancy not accounted for in design. In particular, initial yield of individual members or joints may be acceptable provided that sustainable alternative load paths can be demonstrated. The assessment of an existing platform is solely intended to demonstrate fitness-for-purpose; metocean and structural criteria for an existing platform may be significantly different from a new design.

To demonstrate structural fitness-for-purpose using ultimate strength methods there are two types of acceptance criteria available.

Acceptance Metocean Criteria The first is specific metocean loading criteria such as wave

height, current, etc., that the platform should be shown to withstand without collapse. Typically the metocean criteria are referenced to the platforms consequence of failure. If used, the platform is deemed to be fit-for-purpose if it is able to sustain metocean loads equal to or greater than the loads represented by the selected conditions.

Acceptance RSR Alternatively, a minimum acceptable RSR is specified,

which as previously discussed, is a measure of the platform loading relative to loads caused by the 100-year metocean conditions used for new platform design. Similar to acceptance using specific metocean criteria, a range of acceptable RSRs based on the platforms consequence of failure is specified.

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Mitigation and/or Risk Reduction It is important to recognize that not all damage is structurally significant such as light corrosion or slight bow of a member. Equally important, a well-designed platform with load redistribution may be able to function adequately throughout its remaining life, even if one or more of its members or joints have significant structural damage. In this context, evaluation of the ultimate strength of a damaged structure is one step for optimizing the requirement for mitigation and/or risk reduction.

The residual strength (or robustness) is a useful measure for determining damage tolerance for a platform; for a robust structure, damage may result in little immediate risk to the platform. For other less robust structures, even a small damage event may significantly degrade the platforms global capacity, resulting in a high-risk situation, justifying immediate response such as platform de-manning, platform shutdown, or emergency repair. Recommended Practices and Regulations Presently there is no explicit RP requirement to encourage the regular use of ultimate strength methods in the design of fixed offshore platforms for metocean conditions. RPs and regulations for the design of offshore platforms are based on the design of individual members and components and generally have no formal requirement to structural system strength beyond the component requirement. API RP 2A is a component-based RP; therefore the strength of the structure is defined by the strength of the weakest component. System strength is not addressed and benefit can not be taken in design from load redistribution.

Furthermore, traditional RPs allow the setting of inspection intervals of the underwater components, based solely on the consequence of platform failure. This approach ignores the different characteristics of each platform and their tolerance to damage as determined from their inherent reserve and residual strength. API RP 2SIM (Under Development) The original Section 17 of API RP 2A [Wisch, et al., 2004], upon which the proposed API RP 2SIM is based, provided specific metocean criteria for the assessment for Gulf of Mexico platforms. The proposed new RP [OConnor, et al., 2005, Puskar, et al., 2006] will provide alternative acceptance criteria for platform fitness-for-purpose assessments. The criteria will be in the form of acceptable RSRs and will be applicable for the assessment of all platforms. To maintain consistency with the present RP, the acceptance criteria will be consequence-based and differentiate between older and newer platforms, such that platforms designed to API RP 2A 20th edition or later will have more stringent RSR criteria.

The present API RP 2A provides a prescriptive approach for platform inspections. The proposed API RP 2SIM will provide an alternative for a Risk-Based Inspection, where inspection intervals and inspection work scope can be based on the combination of the platforms RSR (likelihood of failure) and platforms the consequence of failure.

ISO The ISO Standard (ISO/DIS 19902, Clause 24) for the design and operation of fixed steel structures has expanded the basis of API RP 2A Section 14 to allow prudent Owners to set inspection intervals through the development of an inspection strategy. ISO stipulates that the development of an inspection strategy must consider factors such as age, existing condition, function, consequence, etc., as well as be technically defensible. Default intervals are provided for Owners that do not wish to, or do not have the requisite data and experience to set a defensible strategy.

Adoption of ISO will make possible intervals longer than API RP 2A for certain platforms, and hence allow rationalization of inspection resources. Consistent with the ISO provisions, RBI allows a first level screening of a fleet of platforms for risk. ISO states that intervals for underwater inspections may be extended beyond the default requirements, provided the Owner can show through SIM that a platform or group of similar platforms are fit-for-purpose during the interval to the next inspection. ISO suggests that in the evaluation stage of the SIM process, consideration should be given to consequence of platform component failure and perceived likelihood of such failure; however, ISO provides specific caution against the use of probability-based methods in the evaluation.

It is presently not clear when the ISO standard will be adopted in the US. However, at the time of this writing, API is supporting studies to assess the use of ISO for US fixed offshore platforms. Code of Federal Regulations Recent amendments to 30 CFR Part 250 requires an annual inspection plan, as specified in paragraph 250.919:

You must develop a comprehensive annual in-service inspection plan covering all of your platforms. As a minimum, your plan must address the recommendations of the appropriate documents listed in 250.901(a). Your plan must specify the type, extent, and frequency of in-place inspections which you will conduct for both the above water and the below water structure of all platforms, and pertinent components of the mooring systems for floating platforms. The plan must also address how you are monitoring the corrosion protection for both the above and below water structure.

These regulatory changes allow platform Owners to set inspection intervals based on risk considerations and to focus their inspection resources on platforms that can benefit the most from frequent inspections. Case Studies To illustrate the role of ultimate strength assessments in the SIM of offshore structures several informative studies are included. Pompano As part of a series of proposed modifications to the Pompano platform, a fitness-for-purpose assessment was required to ensure feasibility. The study included the development of center of gravity (CoG) contour plots to define the technical limits for future possible deck additions. An ultimate strength

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assessment was also required to evaluate the platforms robustness as the structures cellar deck was inundated during the passage of hurricane Ivan.

The Pompano platform is a manned 4-leg drilling fixed steel jacket platform, operating in 1290 ft of water in the Viosca Knoll Block 989, Gulf of Mexico. It was installed in 1994 and currently supports 40 conductors and 11 J-tubes. The platform is anchored to the foundation through 12 skirt piles (3 skirt piles at each corner leg). The structural assessments included SACS strength analysis and USFOS ultimate strength analysis.

SACS Strength Assessment The initial assessment of the structure was performed

using linear-elastic methods against API RP 2A component acceptance criteria. The analyses were performed using SACS software and three different metocean assessment criteria were employed. An appropriate dynamic amplification factor (DAF) was used to account for dynamic effects due to the deep water location. The three sets of criteria allowed comparison of results of the site-specific, API 19th Edition design and the API 21st Section 17 L-1 design level criteria. The results of the assessment are presented in Table 1, and it was observed that some jacket members and primary joints had utilization ratios greater than 1.0.

USFOS Ultimate Strength Assessment To demonstrate the robustness of the Pompano platform

and illustrate the tolerance of the platform to the metocean loads imposed by the assessment criteria, a series of engineering ultimate strength assessments using USFOS software were conducted. For comparative purposes four different assessment criteria were used, which included the API 19th design criteria, API 21st Section 2 L-1 design criteria, API 21st Section 17 L-1 design criteria and API 21st Section 17 L-1 ultimate strength analysis criteria. The ultimate strength assessments were conducted for the three principal wave approach directions and two deck loading cases were considered with a movable rig on Well 1 or Well 13. A DAF was also used for the ultimate strength assessment

The results of the ultimate strength assessments of the Pompano platform are summarized in the Table 2 for the three principal wave approach directions and the two deck-loading scenarios. The RSR or Load Factor shown is the ratio of the base shear at platform collapse to that derived from the assessment criteria. It is observed that the diagonal wave direction has the lowest RSR or load factor in all cases, and the ultimate strength assessment using the API 21st Section 17 L-1 ultimate strength criteria gave the lowest load factor of 1.70 in the diagonal wave direction.

Topsides CoG Contour Development To develop an allowable topsides CoG contour, a series of

additional ultimate strength assessments were performed. Shifting the topsides CoG away from its original position developed the topsides CoG contour. The deck loads due to the topsides CoG shift were distributed to the four corner legs. The allowable distance of CoG shift was then determined by running USFOS pushover analyses until the Load Factor reached an allowable minimum value of 1.60. The topsides

CoG contour development was conducted using the L-1 full population hurricane ultimate strength criteria. The topsides CoG contour was developed for deck capacities of 28,000 Kips, 30,000 Kips and 32,000 Kips.

Hurricane Ivan Assessment The predicted hurricane Ivan event criteria were used for

an ultimate strength assessment of the Pompano Platform. The results from the ultimate strength assessment, shown in Figure 1, indicated that the Pompano substructure had reserve strength above the hurricane Ivan event criteria, which is consistent with the platform surviving the hurricane event.

Conclusions and Recommendations It was possible on the basis of the ultimate strength

assessment engineering carried out for the Pompano substructure and foundations, to conclude the following: The Pompano platform has sufficient capacity to resist the

loads imposed by the API RP 2A L-1 full population hurricane ultimate strength criteria in the present as-is condition.

The Pompano platform has sufficient robustness to sustain the load imposed by an event similar to hurricane Ivan in the present as-is condition.

Depending on the nature and extent of future damage, the damage, as determined from inspection of the jacket, may not require repair. However, monitoring of damage must be considered and included as part of the inspection planning process.

The developed allowable topsides CoG contour provides a technical limit envelope for possible future deck additions.

The use of linear-elastic assessment techniques against component acceptance criteria, i.e., checking on a member-by-member basis to demonstrate fitness-for-purpose would have resulted in expensive strengthening of the topsides to accommodate the additional topside weights proposed. The ultimate strength assessment confirmed that this was not necessary.

Cassia A As part of relatively minor topsides modification project for the Cassia A platform, an estimate of the remaining fatigue life of the structure was performed. The analysis indicated numerous fatigue lives below the 40-year design requirement with several below 5-years. The project was in the process of initiating a multi-million dollar underwater inspection program with a possible outcome of costly strengthening. Using a combination of technology (non-codified fatigue curves and joint flexibility) and worldwide and regional performance data, the project was able to demonstrate that no fatigue issue existed.

To provide further evidence that the structure had residual strength (robustness) and is tolerant to damage, a series of ultimate strength assessments were performed, which progressively removed critical members until the platform collapsed. The guiding principle was to select those members that participate in the system collapse mechanism for the intact structure, as determined from the results of the intact structure

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ultimate strength assessment. Engineering judgment was also called upon to select members for downgrading, particularly when alternative collapse mechanisms could be produced that might lead to lower system strength. As a result the need for a special inspection was removed. The assessment results, shown in Figure 2, were also used to optimize the long-term integrity plan for the facility and to establish a risk-based periodic inspection interval for the platform.

Virgo On September 16th, 2004 the eye of hurricane Ivan passed directly over Virgo. The platform, installed in 1999, is a 14-slot drilling and production jacket and is located in 1139ft water depth at Viosca Knoll Block 823, Gulf of Mexico. It was noticeable from the damage that the waves generated by hurricane Ivan had impacted the cellar deck beams.

Total E&P USA decided to identify areas that might have been highly stressed to allow a more focused inspection. An USFOS ultimate strength assessment of Virgo using the predicted hurricane Ivan metocean criteria was performed. The results from the ultimate strength assessment indicated that the Virgo substructure had reserve strength above the hurricane Ivan event criteria, which is consistent with the platform surviving the hurricane event.

From the load-displacement plots, shown in Figure 3, it was possible to determine the members/joints that were highly loaded during the passage of hurricane Ivan, as shown in Figure 4. From the results the planned follow-up underwater inspection could be tailored to include close visual inspection of the heavily loaded members/joints.

The underwater inspection was performed using a working class remotely operated vehicle (ROV) operating from DSV Ocean Intervention II from October 13 to 16, 2004 in compliance with MMS NTL-2004-G18, API RP 2A-Section 14. The general visual survey did not discover any structural anomalies. A more extensive and focused close visual inspection was carried out on 26 structurally significant welds determined from the USFOS ultimate strength assessment. The close visual survey did not reveal any structural anomalies. Subsiding Platform During the Level I inspection of a platform located in the Gulf of Mexico, it was reported that the structure had a measured subsidence of 12ft. To understand the likelihood of failure of the platform in an extreme storm it was necessary to perform an ultimate strength assessment of the platform. The fixed steel jacket platform is located in greater than 200 ft of water, installed in 1971 it is presently operating with eighteen conductors, two risers and three J-tubes.

Although the structure is categorized as an API L-2 consequence of failure, the assessment considered the API L-1 ultimate strength metocean criteria as being more representative of the extreme storm that the structure might be subjected to and would provide the Owner with a better understanding of the platforms likelihood of failure. The assessment was performed for three principal wave directions and the results are presented in the Table below. It was apparent from the results that the structure would not survive the loads imposed from the API RP 2A Section 17 L-1 full

population hurricane. It was also apparent from the assessment that the deck legs are the weakest part of the structure due to the wave-in-deck force associated with the diagonal wave approach direction.

Direction Deck Inundation Base Shear Load Factor

End On 3.75 ft 4660 kips 1.35

Diagonal 6.0 ft 6480 kips 0.90

Broadside 1.0 ft 4340 kips 1.60

To explore appropriate mitigation and/or risk reduction options, a number of additional ultimate strength assessments were performed that considered deck leg strengthening and or conductor removal.

Since the deck leg failures, shown in Figure 5, are the main failure mechanism, a total of 8 knee braces were proposed to reinforce the deck legs. The introduction of the 8 knee brace members, shown in Figure 6, resulted in a slight (

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Special Inspection Initiators The load at which a structure ultimately collapses may be considerably higher than the load at which first yield or member buckling occurs. This information is valuable to the engineer such that triggers for special inspections can be set. If possible the load at first component failure should be back calibrated to an associated metocean condition (wave height). If the metocean condition is exceeded during the operational life of the structure then a special inspection should be instigated, which would primarily focus on the component known from the ultimate strength assessment to be susceptible to failure. Load Incrementing Representation of the wave forces by a single static design wave force pattern is an engineering model. Better knowledge of a structures sensitivity to the actual load pattern at collapse may be gained by scaling the wave height (and thus load patterns) to better represent the collapse load. Increasing the wave height can lead to other failure modes than those arising from incrementing the load with a constant load pattern, particularly in the case where the wave reaches the cellar deck or main decks.

The actual metocean conditions which would cause failure of the structure will usually be higher than those used in the ultimate strength assessment. The actual conditions may cause inundation of the lowest major deck and result in an increased loading not represented in the assessment, which may alter the failure mechanism, such as causing the deck columns to fail. It is therefore apparent that a structure with a lower RSR with a higher deck elevation has a greater survival chance during an extreme storm than a comparable structure with a higher RSR and lower deck elevation. Platform Reliability The determination of the RSR by factoring the metocean loads is not a measure of the reliability of the structure; it is simply the assessment of the reserve strength of a structure subjected to a specific set of loads. The assessment is crude, as by factoring the metocean loads the collapse of the structure is caused by a set of loads, which in reality will never exist. An RSR in excess of one implies that the structure can withstand the metocean loads considered without collapse and is not readily interpretable in terms of the size of a storm that can be withstood. Design RPs deal with this by stipulating minimum deck elevations or that an air gap should exist between the crest of the design wave and the underside of the deck. A more rational approach is to assess the platform to survive a definite, but longer return period storm, for example 500 or 1,000 years.

Consequently the comparison of RSR values may be misleading; one structure may have a lower RSR that another, however it may have a larger air gap and consequently have an increased survival probability during a severe storm. The true reliability of a structure is related the maximum storm wave that the structure can withstand and this in turn is dictated by the size of the air gap and the way the increasing wave height converts to structural load.

Performance-Based Design Performance-based design, as the title implies, is a performance driven approach to offshore engineering design. It is distinct from risk-based or consequence-based design, introduced in the 21st Edition of API RP2A, in that it does not try to optimize the likelihood of failure on the basis of an understanding the consequence of failure. Instead, the objective of performance-based design is to facilitate the design of structures that have predictable performance in compliance with performance goals selected for the intended life of the platform. Emphasis is provided on the consideration of entire platform life-cycle; from concept, design, operation, through to disposal; potentially after one or more change-of-use or reuse. Performance-based design is commonly used in the seismic design of onshore buildings [FEMA 356].

Effective performance-based design requires the engineer to understand the implications of the performance goals on the design and to allow the risks to be weighed against the rewards. The implementation of performance-based design during a project requires the engineer to use enabling technologies that exist and be flexible enough to think beyond the confines of conventional design practice. To deliver and document a performance-based design the engineer will be required to perform ultimate strength assessments. The outcome may be improvements in the framing configuration of the structure, which improves the robustness (tolerance to damage and to human error). This improvement may be sufficient to allow a RBI strategy, with possible diverless underwater inspections on a ten-year interval, supplemented as required by event driven inspections e.g. following vessel impact or occurrence of a hurricane. All of this could be implemented for a minimum increase in the design costs and a significant reduction in the operational costs. Qualifications The key elements of assessment engineering are the application of first principles, technology awareness and application and an understanding of industry experience and lessons from in-service performance data [Bucknell, et al., 2000], including platform failures in extreme events such as hurricanes Andrew [Puskar, et al., 1994], Lili [Puskar, et al., 2004], Ivan [Puskar, et al., 2005], Katrina and Rita. Although these technologies are relatively mature with proven track records, they are outside of existing guidance and unfamiliar to many design engineers. The technologies, therefore, bring with them a level of additional competency over and above the competency associated with a conventional design approach.

These competencies are not typical of design engineers and more specialist input is usually required. Ultimate strength assessment results are sensitive to the assumptions and careful evaluation of the results is required. It is important that these risks are understood to allow Owners to make informed decisions in the consideration of mitigation and/or risk reduction alternatives. Experience indicates that an increased level of competency is usually necessary to manage a risk-based strategy and assessment engineering technologies and data.

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Conclusions Offshore steel structures are designed on a component rather than system basis. Accordingly, offshore installations have varying ultimate strengths, even if they have been designed to the same RP. Experience from in-service performance [Bucknell, et al., 2000] suggests that well-maintained platforms are more robust and damage tolerant than a component-based design approach would indicate. As a result of this inherent design reserve-strength, large numbers of fixed platforms are seeing safe service well beyond their intended design lives. It is apparent that engineers should use ultimate strength assessments as an important decision-making tool in the design of new structures and, more importantly, during the life-cycle SIM for existing offshore structures.

Through more realistic simulation and visualization of a platforms structural behavior, the engineer gets a better understanding of the structures integrity and susceptibility to damage. This increased knowledge can be used to determine the criticality of components within the structural system and to assess inspection and repair schemes.

The design approach, where the structure is considered as numerous components and checked for compliance with RP prescribed allowable stresses, is not always a rational or cost efficient means of demonstrating the fitness-for-purpose of a structure. In some cases it may stop a project or suggest very costly and often hazardous underwater strengthening, modification or repair that typically has no impact on the reliability of the structure. An assessment approach, in contrast, estimates the capacity of the whole structure as a system. Failure mechanisms and associated safety factors are determined allowing informed risk adjusted decision-making. If strengthening is required it can be targeted to maximize improvement in reliability. Acknowledgements The authors wish to gratefully thank bp and Total E&P USA for giving permission to publish the results of ultimate strength assessments performed on their platforms and used as examples in the paper. The authors also thank Tracy King (MSL) for her critical review and comments.

References API RP 2A-WSD, Recommended Practice for Planning, Design and

Constructing Fixed Offshore Platforms, 19th Edition, August 1991.

API RP 2A-WSD, Recommended Practice for Planning, Design and Constructing Fixed Offshore Platforms, 20th Edition, August 1993.

API RP 2A-WSD, Recommended Practice for Planning, Design and Constructing Fixed Offshore Platforms, 21st Edition, Errata and Supplement 2, October 2005.

Baker, M. J., Variability in the strength of structural steel a study in structural safety. Part 1 material variability, CIRIA Technical Note 44, September 1973.

Bucknell, J., Lalani M., Gebara J., and Puskar F. J., Rationalization and Optimization of Underwater Inspection Planning Consistent with API RP2A Section 14, OMAE00-2073, February 2000.

Code of Federal Regulations., Department of Interior Oil and Gas and Sulphur Operations in the Outer Continental Shelf (OCS) Fixed and Floating Platforms and Structures and Documents Incorporated by Reference, 30 CFR Part 250, Part IV, July 19, 2005.

FEMA 356, Prestandard and Commentary for the Seismic Rehabilitation of Buildings, November 2000. Federal Emergency Management Agency.

ISO/DIS 19902, Draft International Standard, Petroleum and Natural Gas Industries - Fixed Steel Offshore Structures, 2004.

Joint Industry Project, Recommended Practice for Structural Integrity Management (SIM) of Fixed Offshore Platforms, MSL Services Corporation, Final Report planned summer 2006.

Lalani M. New Large-scale Frame Data on the Reserve and Residual Strength of Offshore Structures, ERA Conference, London, 1993.

Notice To Leases (NTL) No. 2004-G18, Damage Caused by Hurricane Ivan, Effective Date October 4, 2004, United States Department of the Interior, Minerals Management Service, Gulf of Mexico OCS Region.

OConnor, P. E., Versowsky, P., Day, M., Westlake, H. S., and Bucknell, J., Platform Assessment: Recent Section 17 Updates and Future API/Industry Developments, Proceedings, Offshore Technology Conference, Paper No. 17699, May 2005.

Puskar, F. J., Aggarwal, R. K., Cornell, C. A., Moses, F., and Petrauskas, C., A Comparison of Analytically Predicted Platform Damage to Actual Platform Damage During Hurricane Andrew, Proceedings 26th Offshore Technology Conference, OTC No. 7473, May 1994.

Puskar, F. J., Ku, A., and Sheppard, R. E., Hurricane Lilis Impact on Fixed Platforms and Calibration of Platform Performance to API RP 2A, Proceedings, Offshore Technology Conference, Paper No. 16802, May 2004.

Puskar, F. J., and Spong, R. E., MMS Study for Hurricane Ivan Fixed Platform Performance, MMS Roject #549, Final Report, January 2006.

Puskar, F. J., Westlake, H. S., OConnor, P. E., and Bucknell, J. R., The Development of a Recommended Practice for Structural Integrity Management (SIM) of Fixed Offshore Platforms, Proceedings, Offshore Technology Conference, Paper No. 18332, May 2006.

UK HSE, Comparison of Reserve Strength Ratios of old and New Platforms Offshore Technology Report OTO 97 046, June 1997.

UK HSE, System-based calibration of North West European annex environmental load factors for the ISO fixed steel offshore structures code 19902, Research Report 087.

Wisch, D. J., Puskar, F. J., Laurendine, T. E., OConnor, P. E., Versowsky, P. E., and Bucknell, J., An Update on API RP 2A Section 17 for the Assessment of Existing Platforms, Proceedings, Offshore Technology Conference, Paper No. 16820, May 2004.

OTC 18331 9

Number of Utilization Ratios > 1.0 Jacket Assessment Criteria

Members Joints Piles

Site-Specific Criteria 20 0 0

API 19th Design 16 2 0

API 21st Sect 17 L-1 Design 1 0 0

Table 1: Pompano SACS Linear-Elastic Assessment Results

Reserve Strength Ratio/Load Factor Assessment Criteria Rig End-on Broadside Diagonal

Well 1 2.40 2.25 1.80 API 21st Sect 17 L-1 Ultimate Well 13 2.00 2.15 1.70

Well 1 2.36 2.08 2.00 API 19th Design

Well 13 2.20 2.00 2.00

Well 1 3.50 3.07 2.65 API 21st Sect 17 L-1 Design Well 13 3.40 2.90 2.70

Well 1 2.45 2.40 1.90 API 21st Sect 2 L-1 Design Well 13 2.40 2.37 1.76

Table 2: Pompano USFO Non-Linear Assessment Results Figure 1: Pompano Maximum Plastic Utilization (Hurricane Ivan Criteria)

Cassia A End-on Pushover

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 1.5 3.0 4.5 6.0 7.5 9.0 10.5 12.0

Global Displacement (ft)

Glo

bal L

oad

Fact

or As-is structureOne above tension member removedOne below tension member removedMultiple members removedLow f atigue lif e members removed

Virgo Pushover

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 3.0 6.0 9.0 12.0 15.0

Global Displacement (ft)

Glo

bal L

oad

Fact

or 1

2

34

Figure 2: Cassia A - Load-Displacement Curve Figure 3: Virgo Load-Displacement Curve (Hurricane Ivan Criteria)

10 OTC 18331

Figure 4: Virgo - Joint Failure at Load Increment No. 2 Figure 5: Subsiding Platform - Maximum Plastic Utilization, Diagonal Direction, 18 wells, L-1 Metocean Criteria

Figure 6: Subsiding Platform - Deck Legs Strengthened by 8 Knee Brace Members

Figure 7: Subsiding Platform - Knee Brace Installation

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