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Copyright 2006, Offshore Technology Conference

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AbstractThe 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 offailure 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 recommendedpractices (RPs) and regulations, and provides several

informative studies to further demonstrate the role of ultimate

strength assessments in the SIM of offshore structures.

IntroductionSIM 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

understanding of the behavior of the structural system. T

valuable information can provide a role for the developmen

risk-based strategies, including setting appropriate inter

between inspections and selecting areas for inspection.

information can also be used to demonstrate fitness-

purpose and assess the need for risk reduction an

mitigation.Offshore structures are traditionally designed on

component-by-component basis, such that under

combinations of design loading every component in

structure has a utilization ratio, derived using the stren

formulations from the API RP 2A, of unity or less. Howe

it is recognized that fixed offshore structures are usu

redundant and have a number of different load paths such

failure of one member is unlikely to lead to catastrop

structural collapse, provided that adequate redundancy

available. By utilizing this inherent redundancy found in m

offshore structures the likelihood of failure of a platform in

extreme event can be determined.

During the life-cycle of an offshore structure the ultimcapacity is an important attribute that affects the SIM strate

and can significantly influence the risk levels and operatio

costs. For example, a minimally braced structure may not h

alternative load paths to redistribute forces if a componen

damaged or if applied loads are higher than initi

anticipated. As a consequence, failure of a single compon

may be critical to overall integrity relatively inte

inspection activity may be required to monitor the struct

condition of key load paths. Conversely, a robust struc

with alternative load paths through the jacket may be m

tolerant of damage or increased loads, offering gre

operational flexibility and a much-reduced need for inspec

activity to provide the same assurance of fitness-for-purpos

The key elements of ultimate strength assessments are

application of first principles, technology awareness and

understanding of industry experience and lessons from

service performance data [Bucknell, et al., 2000], includ

platform failures in extreme events such as hurricanes And

[Puskar, et al., 1994], Lili [Puskar, et al., 2004], Ivan [Pus

et al., 2005], Katrina and Rita. This awareness is not typica

traditional design contractors and more specialist input may

required. Ultimate strength results are sensitive to

assumptions, and careful evaluation of the results

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 OverviewThe 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 untilthe 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 highestcomponent 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 stren

which are a result of explicit and implicit conservatisms m

during the design of an offshore structure. These aspects

structures design have been published on several occas

[UK HSE, Research Report 087], [UK HSE, OTO 97 0

[Lalani, et al., 1993], and are provided here in summary f

as reference.Expl icit Design Safety Factors

The design of offshore structures is based on traditi

engineering practice, which applies a combination of load

the structure to determine the internal forces in each br

member. For each member and joint in the structure

allowable strength is provided in the design, and the struc

is considered to meet the selected standard if all the individ

components satisfy the requirements. All struct

recommended practice, whether they are based on permiss

stress design (Working Stress Design, WSD) or limit s

design (Load and Resistance Factor Design, LRFD), add

the design of individual members and joints. Within

design procedure is the premise that failure of one membejoint to satisfy the requirements, constitutes non-complia

with the relevant RP. Explicit safety factors are applied to

strength formulae, which are straightforward to calculate,

example a compression member designed to the API RP

WSD, has a safety factor of 1.4 (KL/r=80).Impli cit Design Safety FactorsImplicit sources of reserve strength are a result of stren

conservatisms that are outside the control of the desig

Members have reserve strength beyond first yield, wh

contributes to the global reserve. Assuming that most mod

jacket structures have strong joints and that the system fai

is dominated by member failure, the implicit safety factor

be dominated by the differences between the effective len

factor (K-factor) used in design and the actual K-factor

compression members.

Other sources of implicit safety include the differen

between the actual strength of the component and the

based analytical strength predictions, i.e. the model bias

uncertainty. For members under combined compression

bending the expected value of model uncertainty is clos

one.

Expected M ateri al StrengthThe actual material yield strength is typically higher

the minimum allowed for in the design of the structure. Ac

yield strengths values can be between 5-25% above

specified minimum [Baker, 1973]. This additional y

strength provides an increase in structural capacity accounted for in the design.

System RedundancyEach structure has an inherent reserve and/or resi

strength, which is directly related to the ability of the struc

to provide alternate load paths after failure of a member. T

redundancy in the structural system (or robustness)

primarily associated with the arrangement of the braces wi

the system. A reduction of component capacity does

necessarily imply that the system strength is compromi

This will depend on whether or not the componen

participating in the failure sequence that produces the sys

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collapse mechanism, or whether the members integrity is

required to realize that particular mechanism.

Corrosion AllowanceAdditional 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-designDuring 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.Modeli ng TechniquesDuring 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 F raming

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 AssessmentsUltimate strength assessments within a formal SIM process

can be used to:

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

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

3. Identify and optimize the extent of any requiredstrengthening, 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.

I nspection 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

platforms likelihood of failure and warrants the applicatio

quantitative methods, such as ultimate strength assessment

I nspection Area Selection

Of critical importance to the effectiveness of e

inspection is the proper and adequate selection of the area

be examined. It is important to select a sufficient numbe

inspection areas to provide representative information onoverall structure. Making this selection requires

understanding of the platforms structural behavior

requires an understanding of the platforms susceptibility

damage and the tolerance of the structure to that damage. T

understanding is gained through the application of ultim

strength assessments, which will determine which compon

are highly loaded and would contribute to the collapse of

structure in the event of an overload condition. Th

members or joints would be prioritized for inspection.

Platform Fitness-for-PurposeFitness-for-purpose assessment is a key element of the ov

SIM process. The purpose of assessment is to estabwhether an existing structure remains fit-for-purpose

whether strengthening and/or repair or other mitigation

required. A platform may be considered fit-for-purpose w

the risk of failure, considering both likelihood

consequence, is within acceptable levels. For exis

structures, it is possible that isolated component failure(s),

loads exceeding the component capacity, will be accepta

provided that sufficient reserve against overall system fai

exists.

Assessment analysis provides a best estimate of

strength of the structure. It seeks to utilize the avail

reserve strength and redundancy not accounted for in des

In particular, initial yield of individual members or joints m

be acceptable provided that sustainable alternative load p

can be demonstrated. The assessment of an existing platf

is solely intended to demonstrate fitness-for-purp

metocean and structural criteria for an existing platform

be significantly different from a new design.

To demonstrate structural fitness-for-purpose u

ultimate strength methods there are two types of accepta

criteria available.

Acceptance Metocean Cri teria

The first is specific metocean loading criteria such as w

height, current, etc., that the platform should be shown

withstand without collapse. Typically the metocean criteria

referenced to the platforms consequence of failure. If u

the platform is deemed to be fit-for-purpose if it is ablsustain metocean loads equal to or greater than the lo

represented by the selected conditions.

Acceptance RSRAlternatively, a minimum acceptable RSR is specif

which as previously discussed, is a measure of the platf

loading relative to loads caused by the 100-year metoc

conditions used for new platform design. Similar

acceptance using specific metocean criteria, a range

acceptable RSRs based on the platforms consequence

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 foroptimizing 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 theregular 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 willbe 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 de

and operation of fixed steel structures has expanded the b

of API RP 2A Section 14 to allow prudent Owners to

inspection intervals through the development of an inspec

strategy. ISO stipulates that the development of an inspec

strategy must consider factors such as age, existing condit

function, consequence, etc., as well as be technicdefensible. Default intervals are provided for Owners tha

not wish to, or do not have the requisite data and experienc

set a defensible strategy.

Adoption of ISO will make possible intervals longer t

API RP 2A for certain platforms, and hence al

rationalization of inspection resources. Consistent with

ISO provisions, RBI allows a first level screening of a flee

platforms for risk. ISO states that intervals for underw

inspections may be extended beyond the default requireme

provided the Owner can show through SIM that a platform

group of similar platforms are fit-for-purpose during

interval to the next inspection. ISO suggests that in

evaluation stage of the SIM process, consideration shouldgiven to consequence of platform component failure

perceived likelihood of such failure; however, ISO prov

specific caution against the use of probability-based meth

in the evaluation.

It is presently not clear when the ISO standard wil

adopted in the US. However, at the time of this writing, AP

supporting studies to assess the use of ISO for US f

offshore platforms.

Code of Federal RegulationsRecent amendments to 30 CFR Part 250 requires an ann

inspection plan, as specified in paragraph 250.919:

You must develop a comprehensive annual in-ser

inspection plan covering all of your platforms. As a minim

your plan must address the recommendations of

appropriate documents listed in 250.901(a). Your plan m

specify the type, extent, and frequency of in-place inspect

which you will conduct for both the above water and

below water structure of all platforms, and pertin

components of the mooring systems for floating platfor

The plan must also address how you are monitoring

corrosion protection for both the above and below w

structure.

These regulatory changes allow platform Owners to

inspection intervals based on risk considerations and to fo

their inspection resources on platforms that can benefit

most from frequent inspections.

Case StudiesTo illustrate the role of ultimate strength assessments in

SIM of offshore structures several informative studies

included.

Pompano

As part of a series of proposed modifications to the Pomp

platform, a fitness-for-purpose assessment was required

ensure feasibility. The study included the developmen

center of gravity (CoG) contour plots to define the techn

limits for future possible deck additions. An ultimate stren

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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 19

th

Editiondesign 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 Ul timate 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 19thdesign criteria, API 21stSection 2 L-1 design criteria,

API 21stSection 17 L-1 design criteria and API 21 stSection 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 17L-1 ultimate strength criteria gave the lowest load factor of

1.70 in the diagonal wave direction.

Topsides CoG Contour DevelopmentTo 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

population hurricane ultimate strength criteria. The tops

CoG contour was developed for deck capacities of 28,

Kips, 30,000 Kips and 32,000 Kips.

Hur ri cane I van Assessment

The predicted hurricane Ivan event criteria were used

an ultimate strength assessment of the Pompano Platform. results from the ultimate strength assessment, shown in Fig

1, indicated that the Pompano substructure had res

strength above the hurricane Ivan event criteria, which

consistent with the platform surviving the hurricane event.

Conclusions and RecommendationsIt was possible on the basis of the ultimate stren

assessment engineering carried out for the Pomp

substructure and foundations, to conclude the following:

The Pompano platform has sufficient capacity to resistloads imposed by the API RP 2A L-1 full popula

hurricane ultimate strength criteria in the present a

condition.

The Pompano platform has sufficient robustnesssustain the load imposed by an event similar to hurric

Ivan in the present as-is condition.

Depending on the nature and extent of future damage,damage, as determined from inspection of the jacket, m

not require repair. However, monitoring of damage m

be considered and included as part of the inspec

planning process.

The developed allowable topsides CoG contour prova technical limit envelope for possible future d

additions.

The use of linear-elastic assessment techniques aga

component acceptance criteria, i.e., checking onmember-by-member basis to demonstrate fitness-

purpose would have resulted in expensive strengthen

of the topsides to accommodate the additional top

weights proposed. The ultimate strength assessm

confirmed that this was not necessary.

Cassia A

As part of relatively minor topsides modification project

the Cassia A platform, an estimate of the remaining fati

life of the structure was performed. The analysis indic

numerous fatigue lives below the 40-year design requirem

with several below 5-years. The project was in the proces

initiating a multi-million dollar underwater inspecprogram with a possible outcome of costly strengthen

Using a combination of technology (non-codified fat

curves and joint flexibility) and worldwide and regio

performance data, the project was able to demonstrate tha

fatigue issue existed.

To provide further evidence that the structure had resi

strength (robustness) and is tolerant to damage, a series

ultimate strength assessments were performed, w

progressively removed critical members until the platf

collapsed. The guiding principle was to select those mem

that participate in the system collapse mechanism for the in

structure, as determined from the results of the intact struc

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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 indicatedthat 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 PlatformDuring 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 assessm

that the deck legs are the weakest part of the structure du

the wave-in-deck force associated with the diagonal w

approach direction.

Direction Deck Inundation Base Shear Load Facto

End On 3.75 ft 4660 kips 1.35

Diagonal 6.0 ft 6480 kips 0.90Broadside 1.0 ft 4340 kips 1.60

To explore appropriate mitigation and/or risk reduc

options, a number of additional ultimate strength assessm

were performed that considered deck leg strengthening an

conductor removal.

Since the deck leg failures, shown in Figure 5, are the m

failure mechanism, a total of 8 knee braces were propose

reinforce the deck legs. The introduction of the 8 knee b

members, shown in Figure 6, resulted in a slight (