Otc19607 About API Rp 2sk

5/24/2018 Otc19607 About API Rp 2sk

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API RP 2SK: Stationkeeping An Emerging PracticeD.J. Petruska, BP America Production Company, C. Castille, ENI Petroleum, C.A. Colby, Diamond Offshore, J.Stiff, ABS Consulting, B.A. Stone, WorleyParsons Sea, and D.J. Wisch, Chevron ETC

Copyright 2008, Offshore Technology Conference

This paper was prepared for presentation at the 2008 Offshore Technology Conference held in Houston, Texas, U.S.A., 58May2008.This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

AbstractHurricanes in the past five years have provided significant information regarding performance of moored vessels in the Gulfof Mexico in particular for the Mobile Offshore Drilling Units (MODU). Starting with Hurricane Lili in 2002 and endingwith Hurricanes Ivan, Katrina and Rita in 2004 and 2005, the result was twenty-three MODUs having suffered eithercomplete mooring failures or partial mooring failure with the vast majority of these occurring in the 2004 and 2005 hurricaneseason. After the 2005 hurricane season, the API RP 2SK, Design and Analysis of Station Keeping Systems for FloatingStructures [1], Task Group was reactivated to explicitly address the moored MODU issue with the group made up of rigowners, operators, contractors and regulators. A summary of the API RP 2SK Commentary, Gulf of Mexico MODUMooring Practice for Hurricane Season [2], and evolution from API RP 2SK, 3rdEdition, to API RP 95F, Interim Guidancefor Gulf of Mexico (GoM) MODU Mooring Practice 2006 and 2007 Hurricane Season [3, 4] to the Commentary will bepresented. New requirements, adjusted return periods, introduction of formal risk assessment processes and direct linkage toinspection of mooring components are now incorporated. A large Joint Industry Project was also formed that focused ongathering data, assessing MODU performance, calibrating observed performance to code expectations and addressing someanalysis questions. This work provided valuable input to all the above mentioned API documents and others. This paper will

focus on the key information and linkages to the changes and additional expectations for design and operation of mooredMODUs for the GoM during hurricane season. Improvements the MODU owners have made to the fleet will also bediscussed along with options that can help mitigate the risk of a MODU failing and doing damage to infrastructure during ahurricane.

IntroductionMODUs in the Gulf of Mexico are a critical part of the infrastructure required for exploration and development of oil and gasin preparation for the market. Industry standards that allow safe and economic operations are important to the industry,community and regulatory authorities. Much work has been done since the events of the 2004 and 2005 hurricane seasons tobetter understand the causes of moored MODU failures and learn from them. This knowledge has resulted in changes to theindustry standards.

API RP 2SK 3rdEdition establishes a minimum acceptable design code for mooring systems. RP 2SK requires a mobilemooring, like that on a MODU, be designed to a 5-year return period event when operating away from other structures.

When operating in the vicinity of other structures, a 10-year return period event is required. No specific definitions areprovided, however, on what is considered away from and in the vicinity of. Some examples are given for operations inthe vicinity of other structures. RP 2SK also permits using a risk analysis to determine the design return period but in no caseshall it be less than a one year.

Three strong hurricanes (Ivan, Katrina and Rita) entering the Gulf of Mexico within roughly a 12 month period andcarving paths through areas of the Gulf of Mexico with extensive oilfield infrastructure was unprecedented. In the past,hurricanes have caused MODU mooring failures; for example in Andrew and Lili. However, over time, the number ofMODUs working in the deep water has increased along with more infrastructure thus increasing the probability of failure andalso possible consequences of failure (damage).

New criteria, which are risk based, are needed in order to better protect critical infrastructure during GoM hurricanesunless a more prescriptive type of code can be developed. The industrys initial response to the problem was addressed inAPI RP 95F, 1stEdition that was published in May 2006. The minimum design return period was increased to a 10-yearreturn period event, whether far from or in the vicinity of infrastructure. It also required the use of the Central Gulf metocean


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criteria developed by the ABSC MODU Mooring JIP or site specific criteria which includes the 2004 and 2005 hurricaneseason hindcast. Also, in no case is the one-minute wind speed to be less than 64 knots (a threshold category 1 hurricane).95F also recommends a site specific assessment be conducted to evaluate the risk of such operations during hurricane season.

For the 2007 hurricane season, a 2ndEdition of 95F was released that was an improvement of the 1stEdition but still didnot provide all the guidance needed by the industry. The 2ndEdition of 95F also included guidance on post-storm inspectionand recertification of mooring components to ensure the integrity of components from a damaged mooring system. Thisguidance was subsequently incorporated in API RP 2I, 2ndEdition [6], mooring inspection, which was successfully balloted

by API Subcommittee 2 in the spring of 2007. RP2I also dictates that mooring components should go through a morethorough and fully documented quality and inspection process.

A risk-based approach has been developed and incorporated into the Commentary of RP 2SK to replace 95F for the 2008hurricane season and beyond. Proper implementation of the Commentary will aid in the planning of drilling operations andthe designs of MODU mooring systems, which should reduce the consequences to the MODU fleet and offshoreinfrastructure should a repeat of the 2004-2005 hurricane seasons occur.

In parallel with the efforts of the 2SK Task Group, a Joint Industry Project was launched to explicitly study and learn fromthe successes and failures of moored MODUs during the 2004-2005 hurricane seasons. The JIP and the 2SK Task Groupworked closely together in order to both mutually benefit from each other efforts. One of the key objectives of the JIP was topass on results from studies that could benefit the 2SK Task Group in developing the Commentary. Explicit studies that wereconducted and how they benefited the 2SK Task Group will be discussed and how they impacted the 2SK Commentary. Thisincludes development of a consequence based assessment spreadsheet tool, a mooring reliability study, fleet mooringcapability for the original and upgraded mooring systems and comparative risk assessment.

Finally, the response by the industry to the impact of these hurricanes and the evolving practices will be discussed. Evenbefore 95F 1st Edition was published, the MODU owners began making significant upgrades to the mooring systems toaddress the problem. Use of additional mooring lines, Rig Upgrades, and use of alternative mooring systems andcomponents, Component Upgrades have become more common in an effort to reduce the probability of a MODU failure orreduce the likelihood of damage to surrounding infrastructure should a failure occur.

Overview of Moored MODU JIP and Impact on 2SK Task GroupThe MODU Mooring Strength and Reliability Joint Industry Project (JIP) was first formally discussed in January 2005

following the failures that occurred in Hurricane Ivan. The offshore community believed that Hurricane Ivan was an unusualevent, but it was still necessary to take action. In June 2005 formal requests for proposals were sent to a number of potentialcontractors, but assessment of the responses was delayed because of the unexpected arrival of Hurricanes Katrina and Rita.In September ABS Consulting was chosen to lead the JIP with a request to have a fully funded project by November 17 th2005 when a meeting was to be held between industry and the Department of the Interior (see Figure 1).

Unusual Event Shock/Reaction Continuous Improvement

Industry Reaction

Mooring System Upgrades In-Process

2004 2005 2006 2007

Hurricane Ivan

Hurricane Rita

Hurricane Katrina

January, 2005

Joint Industry Project

Formulated to assess

Mooring Reliability

June, 2005

Request for Proposals

issues to Qualified

Contractors

September, 2005

JIP Contract Awarded

November, 2005

Industry Mtg. with Department

of the Interior

JIP Fully Funded

April, 2006

Consequence Checklist

(2006) Developed

March, 2006

New Metocean Data

Developed for GOMEX

"Zones"

June, 2006

API RP95F First Edition

May, 2007

API RP95F

Enhanced Consequence

Checklist (2007)

Unusual EventUnusual Event Shock/ReactionShock/Reaction Continuous ImprovementContinuous Improvement

Industry Reaction

Mooring System Upgrades In-Process

2004 2005 2006 2007

Hurricane Ivan

Hurricane Rita

Hurricane Katrina

January, 2005

Joint Industry Project

Formulated to assess

Mooring Reliability

June, 2005

Request for Proposals

issues to Qualified

Contractors

September, 2005

JIP Contract Awarded

November, 2005

Industry Mtg. with Department

of the Interior

JIP Fully Funded

April, 2006

Consequence Checklist

(2006) Developed

March, 2006

New Metocean Data

Developed for GOMEX

"Zones"

June, 2006

API RP95F First Edition

May, 2007

API RP95F

Enhanced Consequence

Checklist (2007)

Figure 1. Timeline for JIP and Industry Reaction

One of the main objectives of the JIP was to support the 2SK Task Group development of criteria that were reasonable,but with a complete understanding of the impact on fleet operability. One of the first things to become apparent was that the


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metocean design criteria for the GoM needed to be addressed. By March 2006 there were four complete sets of newmetocean criteria based on a new foundation of splitting the Gulf into four zones. These new data for the most severelocation, the Central Zone, were later published in API RP 95F. The JIP work served as the basis for the development of APIRP 2INT-MET [5].

Another task that needed prompt attention was the development of a method of assessing the risk associated with a drillinglocation. It was clear that risk had to become part of the new approval process for operations in the GoM, but there was nosimple way of quickly and uniformly assessing it for any given location. To overcome this challenge, the JIP developed a

Checklist of 25 questions that could be simply answered to help assess the consequences of operating at a specific location.This Checklist, while it had its shortcomings, was developed extremely quickly and was effectively used throughout the 2006hurricane season by both industry and the MMS. A non-numeric version was included as an appendix in API RP 95F 1stEdition. More importantly, the Checklist was completely reworked, putting the calculations onto a more logical foundation,for release in early 2007. The outline of this 2007 version was included in 2ndEdition of API RP 95F, and a further modifiedversion is included in the RP 2SK Commentary. The method described in the RP 2SK Commentary has its limitations, but ithelps the user understand some of the drivers in a risk assessment and gives a consistent method for assessing theconsequences of a mooring failure. It also helps assess various mitigation measures.

Data was gathered on the MODUs that had mooring failures in Hurricanes Katrina and Rita and were summarized in theform of tables giving information on which components failed and where they failed. These data can be used to determinethe probability that different mooring components will be dragged over the seabed after a failure and so can help ascertain thelikelihood of causing subsea equipment damage. This information can be directly used in location risk assessments.

The data also allowed the assessment of some of the MODU failures to determine at what load level the mooring system

failed. This was not a precise exercise since not all of the information was well documented (e.g. presence of loop currents,evacuation tensions, etc.), but by assessing the failures in light of the differing uncertainties in the variables it was possible todetermine the bias in the calculation methods set out in API RP 2SK (The bias is a measure of how conservative orunconservative the calculations are by comparison to actual experience). The study indicated that when reductions inmooring line strength were included to account for allowed degradation and bending over the fairlead, the bias was close tounity: i.e. the calculations were reasonably predicting failures and survivals. They did suggest, however, that full cataloguelisted mooring line strength should not be used in mooring failure risk assessments since this was a likely overestimation ofthe in service performance (due to bending over the fairlead, etc.).

To ascertain the effects of changing requirements on MODU operability, eight MODUs were assessed for four differentmooring arrangements in multiple water depths. In addition, if the MODU was to be upgraded, the effects of the upgradeswere assessed. The intent was never to set a criteria based on fleet capability, but it is important to have a clearunderstanding of the impact of 2SK changes. The results of the study have shown that upgrading the mooring systems andusing improved components (e.g. polyester inserts) can generate improvements over the base system, but these improvementsare not generally going to support the operation of MODUs to operate at the height of the hurricane season, with high returnperiods in the Central Zone. The improvements are important, but they do not give carte blanchefor all operations.

When the reliability implicit in API RP 2SK was last addressed in the mid-1990s, a MODU using a conventionalchain/wire system in 1,000 feet water depth was considered the norm. There was no work assessing semi-taut mooringsystems or polyester inserts in 5,000 feet water depth as these were almost unheard of. It was therefore important to makesure that the safety factors applicable to conventional mooring systems could also be reasonably applied to the newerupgraded systems in much deeper water. To assess this factor, the reliability of 20 combinations (water depth, mooringsystem types) were analyzed. The outcome indicated that the reliability was not greatly affected by the type of mooringsystem, or the water depth, so no immediate changes to the basic standard are needed on that account. However, it was notedthat the metocean zone in which the MODU was operating did have an impact on the reliability. This difference may need tobe addressed at a later date, but it is beyond the sensitivity of the current assessments.

To cap the entire study, the risks associated with mooring failure were compared to those in other sectors of the oilindustry and in other industries altogether. The results show that while MODU mooring failures in hurricanes were morecommon than is reasonably acceptable, the actual risks have not been that high since the consequences have generally been

low. Indeed, the past risk profile is in line with other industries, and improvements in MODU mooring reliability areexpected.

By combining the reliability and bias work with the comparative risk assessment, it is possible to partially assess theimpacts of changing the design return period for higher consequence locations. As stated, there have not been any collisionsbetween a drifting MODU and a major deepwater facility, but it is possible to calculate such a collision probability in theevent of a total mooring failure. The conditional probability of a collision (i.e. conditional on a mooring system failurehaving already occurred) increases inversely with the distance between the MODU and the facility. At 10 miles theconditional probability of a collision between a drifting MODU and a major TLP is approximately 2x10 -3. (Note that whenthe probability of mooring system failure based on a 10 year design is taken into account, this collision probability is asimilar magnitude to the probability of a major ship collision with a manned deepwater facility a far more catastrophicevent.) At one mile the conditional probability is approximately one order of magnitude higher than at 10 miles. Based onthe reliability calculations, and an assumed mooring line break strength of 80% of catalogue value (CBS), the probability offailure of a mooring system reduces by approximately an order of magnitude as the design return period is increased from 10


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years to 50 years. It is, therefore, reasonable to say that if the risk of operating 10 miles from a permanent facility isacceptable if the moorings are designed to 10 year return period, then the risks should be acceptable operating at one milefrom a similar facility with a 50 year designed MODU mooring system. (Note that the use of 80% CBS has been partiallysubstantiated through the bias analysis of Hurricane Katrina and Rita failures. If 100% CBS is used, the difference inprobability of failure between the 10 year and 50 year designs is far greater, thereby making the reliability look much better.)This simple calculation is only valid if the permanent facility is not within a mooring diameter of the MODU, and takes noaccount of the potential to damage pipelines, but the simple example goes some way to justifying the use of 50 years. This

and similar results have been used to help justify some of the methods and values used in the API RP 2SK Commentary.The JIP has been a massive industry effort involving over 3,500 work-hours of participants time in addition to the over

$2.3 million of subscriptions. Some of the results have been directly incorporated into API documents while others will moremethodically work their way in. One of the greatest changes that has occurred with the help of the JIP is the move to a morerisk based location assessment approach for MODU mooring operations. Whereas in the past risk was not considered amajor driver when making decisions about the acceptability of a specific operation at a location, this is no longer true.Careful consideration of the local infrastructure and possible mitigations of the consequences of mooring failure is nowrequired.

Overview of API RP 2SK CommentaryThe 2SK work group was reactivated shortly after completing the 3 rdedition which was released by API in September 2005.The aftermath of Ivan, Katrina and Rita showed that the existing mobile mooring design criteria used for MODUs should bechanged.

For the design of permanent moored systems, API RP 2SK, 3

rd

Edition, recommends the use of a 100-year return periodenvironment. However, if the design life is substantially less than 20 years, a shorter recurrence interval may be justified bya risk analysis which takes into consideration the consequence of a mooring failure. Use of the 100-year design return periodfor permanent facilities has shown to work well with no failures of any sort occurring on a floating production facilitymooring system in the GoM during these hurricane events.

For mooring a mobile floating unit operating away from other structures, the minimum required design condition is the 5-year return period storm per API RP 2SK, 3rdEdition. RP 2SK also allows for the return period to be reduced if justified by arisk analysis but in no case shall be less than one year. For the case when operating in the vicinity of other structures, thedesign environment should have a return period of at least10 years.

RP 2SK design criteria have been risk-based with prescribed design return periods and risk analysis allowed to supportusing a lower value. Thus, for the RP 2SK Commentary to address MODU mooring design for hurricane seasons, a riskbased approach will continue to be used but as the primary means to assess adequacy of a design return period for a particularoperation at a given location. Along with the risk based approach, guidance is provided through consequence categories,within which notional return periods should be based on the likely unmitigated consequence that could result should aMODU mooring system fail.

While it is primarily focused on MODU mooring design requirements for drilling operations, the RP 2SK Commentaryalso applies to stacked rigs.Site Assessment for MODU Mooring

The probability and consequences of a MODU losing station when operating at any location within the US Gulf ofMexico should be assessed. The intent of the assessment process is to determine the characteristics of the area near thedrilling operation and identify options related to mooring component selection, mooring system design, and mitigationopportunities prior to finalizing the mooring design and installing the mooring system. For the planned MODU operation,the mooring system should be associated with acceptable risk parameters, either by minimizing potential consequences ofmooring component or system failure or by reducing the probability of mooring component or system failure.

Information to be obtained by the operator for such assessments will include:1. Local Description (block number, well coordinates, water depth, bathymetry, seabed conditions, hazards, etc.)2. Details of Planned Well Operation (type of well, time of year, duration and confidence in, possible delays, etc.)

3. Site Specific Metocean Data and Source4. Mooring Installation Hazards (restrictions to anchor placement and dragging as appropriate)5. Surface and Subsea Infrastructure (distance and direction, other mooring lines or tendons in the MODU

mooring pattern, mooring lines crossing subsea infrastructure)When collecting data for the site assessment, consideration needs to be given to existing infrastructure and infrastructurethat may be present during the actual MODU operation.

Information related to the mooring system that affects the consequences or mitigates the consequences of the mooringsystem should be included in the assessment. This may include: type of anchor, type of mooring component that ifdragged could damage subsea infrastructure, components used to mitigate the consequence of a mooring failure (buoysor polyester).Risk Based Site Assessment

Risk is defined as:


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Risk = [Probability of an adverse event occurring] x [The consequences associated with that event]

The risk can be reduced either by reducing the probability of experiencing an incident, prevention, or by reducing theconsequences of that incident should it occur, mitigation. For example, the design return period can be increased to reducethe probability of failure. Likewise, the use of suction piles or higher safety factors for drag anchors can help reduce the riskof pipeline damage should the mooring system fail.

The different types of consequences that are associated with MODU mooring failures include:1. Health and Safety2. Environmental3. Financial4. Corporate reputation and image5. Industry reputation and image6. National interest.

Existing procedures for MODU evacuation operations during hurricane season will normally result in the well and pipelinesshut-in, the drilling riser recovered, and the MODU crew evacuated. Likewise, production facilities and pipelines in theprojected cone of the hurricane path are shut-in and evacuated. Consequently, health, safety and environmental consequencesassociated with a MODU mooring failure are relatively low. Corporate and industry reputation and national interest aresubject to individual corporate and regulatory interpretation, and thus an RP cannot provide much guidance in these areas.So while a risk assessment may be performed for all six types of consequences, the one primarily addressed on the RP 2SKCommentary is financial: the direct impact from damage to infrastructure and the delayed or lost production.

The consequences of a MODU mooring failure may be divided into two categories: (1) the consequences of damage to the

surrounding subsea and surface infrastructure and (2) the consequences of damage to the MODU and its mooring system.The purpose of the risk assessment required by the RP 2SK Commentary is to address the consequences of damage tosurrounding infrastructure. For MODU operations in the hurricane season, risk associated with damage to the MODU and itsmooring system must be managed by the operator and the drilling contractor. The MODU operator or owner may conduct arisk assessment to determine the potential for damage to the MODU and its mooring system as well as the impact on theoperators drilling program. However, this assessment process is not addressed explicitly within the RP 2SK Commentary.

The potential consequences to infrastructure of a MODU mooring failure depends on:1. Financial consequence values (cost of repair, replacement and lost production)2. Distances and direction to individual components of infrastructure and the MODU location3. Mitigation strategies4. Different likelihoods of adverse consequences given a mooring failure

The probability of MODU mooring system failure decreases as the design return period increases, i.e. the return period forwhich the mooring system satisfies all of the requirements of API RP 2SK (intact and damaged line tension, anchor load, and

clearance requirements). Consequently, the management of risk to surrounding infrastructure should in general require thatthe design return period increases as the consequence of failure increases; alternately, additional mitigation measures can beput into place. It should be noted that the required return period for a given well location or operation is independent of theduration of the operation and the season of operation. However for a given return period, the intensity of the environmentalconditions (wind, wave, and current) is dependent on the particular site and the season(s) of operations.

Figure 2 shows the general methodology for conducting a risk assessment. Tolerable risk levels should provide a balancebetween absolute safety requirements and cost and benefits of proposed risk reduction measures. Of particular significance,the possibility of changing the season of operation should be considered as a prevention method that can reduce risk toacceptable levels.Assessment Procedures and Criteria

The RP 2SK Commentary provides five consequence categories. The first one covers exceptional operations. Suchoperations may entail using less then the minimum 10-year return period but requires a risk assessment to justify this. As anexample, if a MODU has a damaged mooring system, it may make more sense to repair at the present site than to relocate.

The next three categories cover typical MODU operations and are: C-1: Lower consequences in event of stationkeeping failure

C-2: Medium consequences in event of stationkeeping failure

C-3: Higher consequences in event of stationkeeping failureThe final consequence category covers atypical MODU operations and is discussed further below.


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Start

List Local Infrastructure

Develop Mooring

System Design forRelevant Season

Ascertain consequences

of mooring failure

Calculate probability

of mooring failure

Evaluate the Risk

Is Risk Tolerable?

Stop

Yes

Yes

Can risk be

reduced?

No

No

Operation

Unacceptable

Operation

Acceptable

Documentation

Use PreventionMethods

Use MitigationMethods

Figure 2. Overall Financial Risk Assessment Process.

For MODU operations in the Gulf of Mexico, the RP 2SK Commentary recognizes three methods for assessing theconsequences of mooring system failure and determining the environmental return period to be used in performing the siteassessment of the mooring system. The three methods are:

1. Initial Screening Assessment - Determines the unmitigated consequence category for the location to be evaluatedbased on distance and class of nearby infrastructure. The consequence categories are intended as a starting point forthe assessment of the mooring system only.

2. Basic Risk Assessment- The basic risk assessment is intended to establish the acceptability of the risk and designreturn period.

3. Supplemental Risk Assessment -For higher risk operations or operations for which a more detailed assessment iswarranted, a supplemental risk assessment may be used to determine suitability to drill with a given mooring systemat a specific location. When used, it should include elements from the basic risk assessment. A supplemental riskassessment is always required for exceptional (atypical) MODU operations.

MODU operations always require an initial screening assessment and basic risk assessment to be performed to determinea suitable design return period to be used for a particular site and operation. The results of the basic or supplemental riskassessment always override the results of the initial screening assessment.


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Initial Screening Assessment

A screening assessment assigns the MODU drilling location a consequence category based on production rates of anddistances to nearby production facilities and pipelines. Within the consequence category, a target design return period isprovided for the three typical consequence classes (C-1, C-2, and C-3).

The greater initial investment and future tie-in potential for surface facilities dictates that the consequence assessment forsurface facilities be based on rated production. Pipelines can be repaired in a shorter time and production potentially

rerouted; therefore, the consequence assessment for pipelines may be based on actual throughput rather than rated capacity.If actual pipeline throughput is unavailable, a conservative estimate of pipeline throughput shall be used.

Figure 3 presents recommended unmitigated consequence categories based on distance to surface and subseainfrastructure. The consequence category for a given location may change for other assessment methods, e.g., basic andsupplemental.

R

Well

D 5 NM 10 NM 15 NM

Risk Assessment C-3 C-2 C-2 C-1

Note: d = distance to well center. R = mooring radius = distance between well and most distant anchor; D = mooringdiameter = 2R.

Surface Facilities

RatedCapacity

d R R< d DD < d 5

NM5 < d 10NM

10< d 15NM

15NM < d

>75K BOE C-3 C-2 C-2 C-125-75K C-2 C-2 C-1 C-1

75K BOERisk

AssessmentC-3 C-2 C-2 C-2 C-1

25-75K C-2 C-2 C-2 C-2 C-1 C-1


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mooring system is probably suitable for a consequence class of C-3 or lower. However, at this time, it is still prudent toperform the basic risk assessment to fully and properly assess the location and operation to ensure nothing is overlooked.Until more experience is available, the RP 2SK Commentary will always require the screening and basic risk assessments areperformed for every location to be drilled in the GoM.

Basic Risk Assessment

The purpose of a basic risk assessment is to facilitate planning and follow-up operations that reduce as much as practicallypossible the risk exposure from MODU mooring systems.

A basic risk assessment should, as a minimum, qualitatively assess and fully document the following:1. Potential mooring failure modes2. Probability of mooring failure3. Nearby infrastructure and the potential for damage with various types of failure and consequence of damage4. Operations plans and impact on analysis assumptions5. Mechanical integrity of systems6. Possible mitigation actions to improve reliability and reduce potential consequence of failure

The first three items above are required for infrastructure and design return period evaluation. In determining the designreturn period, the actual infrastructure proximity shall be considered for a given moored MODU operation.

A simple way to estimate the probability of MODU mooring system failure can be taken as the inverse of the return periodcorresponding to the load where failure is expected. In doing this, consideration should be given to actual anchor capacityand mooring line strength. Mooring line strength may be reduced due to component degradation, bending over the fairlead,etc., in comparison to catalog values.

The fourth and fifth items above cover operational planning and evaluation. Additional issues that affect mooring systemreliability and risk exposure should be evaluated. During peak hurricane season, lower risk locations should be given priorityin operational planning. Adequate contingency plans shall be in place with operations near peak hurricane season if the risklevels during peak season are not tolerable.

The last item covers mitigation evaluation. As part of the risk evaluation, possible actions should be evaluated that canreduce the potential for mooring failure and consequence of failure. The design criteria referenced in this document are notintended to preclude reasonable and practical actions that can result in improved mooring systems.

A basic risk assessment may be based on a methodology that determines a mitigated consequence score, which allows thestakeholders to assess, on a relative basis, the consequences of MODU mooring failure associated with the proposedoperations. The intent of this approach is to be more conservative by comparison to a more detailed risk assessment.However, the basic consequence assessment can be completed with the routinely available information and data that shouldbe available to the operator and drilling contractor.

The basic consequence assessment should be based on:1. Consequence factors based on the specific locations (infrastructure that could be damaged in the event of a mooring

failure)2. Consequence factors based on mooring components and system details.

An example of a consequence assessment methodology is the consequence checklist developed by the MODU mooring JIPas discussed above.

In many cases, particularly when considering mooring failures, the detail of the quantification, the magnitude of thenumbers, and level of accuracy is such that the best way to interpret and present the results is through the use of a risk matrix.The selection of levels of probability and consequence depends on the operator, and these must be defined at the start of therisk analysis.

Supplemental Risk Assessment

A supplemental risk assessment may always be used to perform a more detailed assessment of the specific risk elementsassociated with the proposed operations. The risk assessment process contains a series of steps to formally assess the risk atany given location. The steps can be summarized as:

Definition of location and well parameters

Identification of local and distant infrastructure Undertaking a hazard identification (HAZID) study

Probability of mooring failure (reliability analysis)

Quantification of the consequences of failure (e.g., through event tree analysis)

Risk mitigation

DocumentationIn general, a supplemental risk analysis should be quantitative versus qualitative where possible.Atypical Operations

There may be exceptional MODU operations associated with high risks that may require very high environmental returnperiods. MODU operations that may be associated with high risks include but are not limited to:

MODU offset drilling

Tender assisted drilling adjacent to a permanent facility


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MODU operations within a mooring diameter of a permanent facilityBecause of the inherently higher risks generally associated with atypical MODU operations, the risk assessment required

for atypical MODU operations is more rigorous than that for typical MODU operations. A simple checklist assessment is notadequate to properly assess such risks.Other Guidance Provided in the 2SK Commentary

The RP 2SK Commentary provides guidance on other items, including mooring analysis, mooring system improvements,considerations for anchors and anchor selection, hurricane preparedness, mooring system installation and financial risk

assessment.Mooring Analysis - The mooring analysis methodology for hurricane conditions is essentially the same as that specified

in the current RP 2SK and uses information on return period and default metocean as presented above. The Commentarydoes, however, recommend investigation of mooring survival limits and sensitivity checks. This practice is fairly commonfor permanent mooring systems but previously was not common for a mobile mooring. These checks should be done forboth intact and damaged conditions. The plot in Figure 4 shows an example of the utilization factor (ratio of maximum linetension to breaking strength or maximum anchor load to capacity) versus return period. This check allows comparison ofdifferent mooring configurations and also provides useful information for the risk assessment and prevention and mitigationstrategies. Note, however, that the results from such an analysis may not accurately predict mooring system survival becauseof other potential failure modes such as fatigue, wear or corrosion.

Line Tension & Anchor Load - Full Year Hurricane

12100.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20 25 30 35 40 45 50

Return Period (years)

Utilization

Intact -Max Line Tension Intact - Max Anchor LoadDamaged - Max Line Tens ion Dam aged - Max Anchor Load

Figure 4. Mooring Line and Anchor Utilization versus Return Period for Intact and Damaged Conditions.

Mooring Systems Improvements There are several options to improve a mooring systems performance, includingadding more lines, increasing the strength of components or using fiber rope inserts.

Anchor Considerations In the 2004 and 2005 hurricane seasons, anchor drag distances for the windward lines weretypically less than one mile. However, on some occasions, windward anchors were dragged over 20 miles off location.Anchor dragging has resulted in some subsea infrastructure damage and is often one of the relatively bigger risks, thushighlighting the importance of anchor selection and understanding anchor performance and behavior. Guidance is providedfor drag anchors, plate anchors and suction piles to help make a selection for the particular application. Safety factors foranchors remain the same as in RP 2SK but consideration should be given to using higher safety factors and in anchorselection for certain higher risk applications such as the presence of a pipeline in the possible dragging path of an anchor.

Hurricane Preparedness A hurricane preparedness plan shall be developed such that the MODU and its mooring are leftin a state such that survival is optimized and such conditions are documented for future reference. The plan should alsoaddress tracking and recovery of the vessel in case of mooring failure. Loop eddy currents must also be taken intoconsideration. These currents occur in many areas were deepwater floating drilling is being conducted in the GoM and canimpact the ability to adjust mooring line tensions in preparation for a hurricane evacuation. The mooring engineer and


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MODU supervisor must be aware of the Loop / Eddy current situation during hurricane season and make advancepreparations as needed to optimize the mooring systems performance and survivability in the event of a hurricane.

Mooring System Installation It is important that the mooring system is installed per the installation plans. Pertinent as-installed mooring information must be recorded and provided to the operator to determine that the mooring is installed withindesign tolerances. If out of tolerance, verification should be made that the as-installed mooring meets the original safetyfactor requirements. If the as-installed mooring system does not meet the design safety factor requirements, then appropriateaction should be developed and implemented that provide acceptable mooring safety factors. In some cases, changing the

survival payouts is sufficient to meet this requirement.Financial Risk Assessment - The Commentary contains an appendix with guidance on performing a financial risk

assessment. A major advantage of undertaking a risk assessment is that it requires the stakeholders to be involved in theprocess and give consideration to the issues that comprise the potential risk. This effort does not require massive attendanceat, for example, the HAZID workshops, but there is a concern in making any risk assessment process too automated. Riskanalysis is an efficient way of helping to comprehend and evaluate issues that cannot be quantified by conventional means orincorporated into design codes. It is through the process that the participants understand the risks, which in turn, help them todecide on the appropriate acceptance criteria and correct prevention and mitigation measures.

Direct consequence mooring risk assessment is the main issue. This type of assessment may be used to assess the specificprobability and consequences of suffering a mooring failure at a particular location (e.g. probability of mooring failure, andthe consequences and likelihood of the MODU or its mooring system interacting with both near and far infrastructure). Inthis assessment, frequency of occurrence of hazardous events, the likelihood of escalation into further accidents, and themagnitude of potential consequences are evaluated. The methods used could be qualitative or quantitative depending on the

system definition and the objectives of the risk assessment. The initiating event frequencies and the likelihood (conditionalprobabilities) of the chain of events leading to accident scenarios are established using a combination of historical databases,fault tree, event tree, and reliability analysis techniques.

Risk acceptance will be a part of any assessment process and involves deciding whether a risk is tolerable and whetherrisk reduction measures are needed. A balance is required between absolute safety requirements and the effectiveness ofproposed risk reduction measures. Acceptability is generally determined by comparing mooring failure risk against theacceptable risks established for similar or other offshore systems with acceptable operating experience or with thoseestablished by other industries.

Evolution to Commentary and Changes from 95FThe 2SK Commentary is basically a two year evolution from 2SK 3 rdEdition, to 95F 1stEdition and then 2ndEdition and

finally to the Commentary. When 2SK 4thEdition is released, the Commentary will be incorporated into that document alongwith any lessons learned over those years of use and operation between now and when the 4thEdition is released.

Differences between 95F, 1stEdition and 2SK, 3rdEdition are primarily:1. Set the minimum return period at 10 years versus 5 years2. Provide Central GoM metocean hurricane conditions to be used for MODU mooring operations unless current site

specific criteria are available3. Set the minimum one-minute wind speed of 64 knots4. Require, at a minimum, a simple consequence assessment be performedDifferences between 95F, 2nd Edition, and 95F, 1st Edition, are largely:1. Define atypical operations which always require a risk assessment (this included pipelines over a certain size and

within one mooring diameter)2. Define exceptional operations3. Allowed the use of seasonal metocean criteria4. Augment the section on anchor considerations5. Expand on mooring system behavior and weak point analysisDifferences between RP 2SK Commentary and 95F, 2ndEdition, includes:

1. Add consequence categories which provide high level guidance on approximately what the return period should be asa function of infrastructure throughput and distance

2. Set a minimum return period of 50 years for the higher consequence locations3. Provides a default 64kt case to include associated waves and current and provides instructions on how to construct

this case from site specific metocean data4. Provides extensive guidance on how to conduct a financial risk assessment for a typical moored MODU operation.5. Discusses mitigation and prevention strategies for reducing the consequences and likelihood of mooring failure

and how they should always be considered when designing the mooring system and planning and scheduling theoperation.

The benefits of the consequence classes in the RP 2SK Commentary are not of major significance when a basic riskassessment is always required. Nevertheless, they do offer some value:

Sets goal post for the design return period for three different levels of risk. Provides answers to the questions What is important infrastructure? and What is close proximity?


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Sets a the recommended design return period of 50 years for high consequence locations Sets the framework for where RP 2SK, 4 thEdition, may go: a prescriptive design code no longer requiring a risk

assessment for each operationWhen extraordinary events such as the multiple MODU mooring failures that occurred in 2004 and 2005 hurricane

seasons, an evaluation of existing design practice is necessary. Under such circumstances, it is common to apply riskanalysis, best practices and engineering judgment in development of revised standards. This process had led to the evolutionof the RP 2SK Commentary. As experience is gained using this Commentary, it may be possible to develop a prescriptive

standard. This is desirable for the industry but such a standard must adequately serve to reduce the problem (of the numberof MODU stationkeeping failures and thus potential consequence to infrastructure damage) while maintaining a balance withreasonably acceptable risk (drilling cannot be shut down entirely for hurricane season). Thus even if prescriptive designreturn periods are developed for various well defined consequence classes, a risk assessment may always be conducted whichmight verify or justify using a different value.

Seasonal ExtremesThe hurricane season officially spans from June 1stto November 31st. While the exact track, intensity and date of hurricanesto come are not known, the past fifty plus years of Gulf of Mexico hurricane data provides clear trends in the frequency andintensity on a general seasonal basis. Based on the 1950 to 2006 hurricanes in NOAAs database, there is a substantialincrease in hurricane activity from mid-August through mid-October.

For operations during hurricane season that do not run into this approximately two-month peak of the season, thehurricane activity and intensity is far less. This fact allows metocean experts to develop seasonal hurricane criteria that show

substantial reduction in associated wind, wave and currents that can be utilized for MODU mooring design.However, since MODU operations are not associated with well-defined schedules, taking advantage of seasonal hurricanecriteria may not always be easy. Operations that run into June and July show relatively low risk exposure versus the entiretyof the hurricane season. Then, as operations run into August, the exposure risk increases substantially in a short amount oftime. The second major impact is that operations that extend past mid-October show nearly no additional exposure risk.Thus operations that end in the first two months of hurricane season can withstand significantly higher seasonal return periodhurricanes than ones that extend into August, September and October.

For drilling schedule planning, these observations may influence operators at higher risk locations to make use of drillingprograms that avoid the peak of the hurricane season to better manage their exposure risk. This improved planning wouldallow for better utilization of resources to increase robustness for a few key locations with significantly more hurricaneexposure risk.

Response by MODU CommunitySoon after the hurricanes Ivan, Katrina and Rita, there was an immediate drive by both industry and regulators (MMS andUSCG) to improvement the MODU fleets mooring systems. While activities within the JIP and API were in early stages,industry efforts began towards accessing possibilities and means of upgrading stationkeeping systems for MODUs operatingin Gulf of Mexico. This effort occurred while repairs and recovery efforts from hurricanes Katrina and Rita were ongoingand yard and equipment manufacturing backlogs were high with booming global oil projects. With the ultimate API andMMS requirements unknown at the time, many plans were finalized and work started. New metocean criteria accounting forthe intensities of hurricanes Ivan, Katrina and Rita would not be available until after much of the upgrade work had beencompleted. Despite these challenges, there was a determination and commitment to have as much upgrade work done aspossible prior to the peak of next hurricane season in 2006.

Nearly all MODU mooring upgrades that have been completed have focused on adding mooring lines to existing 8 or 9leg mooring systems. Typically, one additional mooring line is added to each column bringing the total number of mooringlines the MODU is capable of deploying to 12. The MODU internally still carries 8 or 9 self-contained lines during locationmoves, and the extra lines are carried on installation vessels. These extra lines are installed similar to pre-installed mooringlines and connected to the rig after the self-contained lines are installed. The extra lines essentially increase the storm

resistance capacity of MODU by approximately 30% to 50%. It should be noted that the possibilities for additions ormodifications to existing MODUs, which in many cases have already been extensively modified from the original design, arelimited due to physical space and other constraints. A few recent new-build MODUs and major MODU conversion projectshave capacity of mooring with up to 16 lines, but the complexity and cost of deploying 16 lines compared to 12 (along withthe inherently greater risk associated with increased complexity) is such that 16 lines systems have subsequently beendeployed on a very limited number of locations.

In addition to the MODU upgrades, many operators have elected to augment MODU mooring systems with polyester linesand higher capacity anchors on certain higher risk locations. The potential improvements these bring were highlighted inOTC paper, Improved Mooring MODU Design Codes for Hurricane Season [7]. Experience from both the 2006 and 2007hurricane seasons was that augmented systems were deployed on 30% to 50% of GoM MODU mooring deployments at anygiven time. Polyester lines and high capacity anchors inventory levels in GoM have reportedly increased five-fold from pre-2005 levels. Still during 2007, options were sometimes limited due to occasional inventory shortages. A more significantimpact was the limited availability of high capacity anchor installation vessels, which are in tight demand.


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A major change in post-Rita operations is how hurricane risk is now scrutinized and planned for with well drillingprogram development. Lower risk locations are often prioritized during peak hurricane season. Higher risk locations areplanned such that they can be completed prior to the onset of the peak of hurricane season. In addition, many MODUs,especially in the mid-water depth capacity range, have moved out of the GoM with the award of international drillingcontracts. The few remaining MODUs with non-upgraded systems are limited to locations that have appropriate risk levels.All these actions contribute to lower risk profile by avoiding risk to the extent that it is prudent.

With the rollout of the new API RP 2SK Commentary in 2008, the MODU industry is transitioning towards a continuous

improvement mode with regard to minimizing hurricane mooring risk. Longer term equipment upgrades are still ongoing.For example, older components can be replaced by newer, higher capacity components during replacement cycles if theMODU can support such changes. A few rigs that ordered long lead-time equipment should be upgraded in time for the 2008hurricane season. Research and development related to MODU mooring systems, especially in the area of anchor design andanalysis, should continue. Anchors that can better resist out-of-plane loading, or possibly cause less damage if unseated, areunder development. Unfortunately, all these actions combined do not guarantee that a MODU will not suffer mooring failureif another large storm occurs, but certainly overall mooring reliability and risk levels have improved.

2SK Work Group Future PlansThe assessment method in the new RP 2SK Commentary provides flexibility for assessing MODUs to assist in

determining the best and safest place to operate during the hurricane season. The Commentary focuses on the protection forinfrastructure while also providing a framework for regulatory agencies approval of drilling permits. It was also developedwith a large and diverse Task Group and still reached a solid consensus.

The Task Group still has some work to complete. As part of future development of API RP 2SK, the Task Group willcontinue to monitor how the Commentary is working and evaluate results should a hurricane develop and encounter any ofthe moored MODUs or permanent floating production facilities. The next scheduled update of RP 2SK to the 4thEdition isplanned for 2010. At that time, the Commentary will be incorporated into RP 2SK along with any learnings, experience orother changes that serve to improve the design of stationkeeping systems for all floating platforms. Feedback from users ofthe document is always encouraged.

AcknowledgementsThe authors would like to thank API and Sub Committee 2 for permission to publish this paper. In addition, they would liketo thank all the Task Group members who helped in developing the new Commentary. Without their volunteer work anddedication these updates would not have been possible.

DisclaimerThe information presented in this paper does not necessarily represent the views of the companies employing the authors ofthis paper. Information in this paper is not meant to be a substitute for reference to the respective API publications. At thetime of writing this paper, the 2SK Commentary was in draft version for balloting. Prior to publication, some changes mayoccur.

References1. API RP 2SK, Design and Analysis of Station Keeping Systems for Floating Structures, 3rdEdition, October 2005.2. API RP 2SK, Commentary, Gulf of Mexico MODU Mooring Practice for Hurricane Season, 20083. API RP 95F, Interim Guidance for Gulf of Mexico MODU Mooring Practice - 2006 Hurricane Season, 1 stEdition, May

2006.4. API RP 95F, Interim Guidance for Gulf of Mexico MODU Mooring Practice - 2007 Hurricane Season, 2 ndEdition, April

2007.5. API RP 2INT MET, Interim Guidance on Hurricane Conditions in the Gulf of Mexico, April 2007.6. API RP 2I, In-Service Inspection of Mooring Hardware for Floating Structures, 2nd Edition, 2087.

7. Petruska, Dave, Stone, Barbara, Kwan, Tom, Zimmerman, Evan, Devlin, Paul, Wishahy, Momen, OConnor, Pat,Improved Moored MODU Design Codes for Hurricane Season, OTC 18900, 2007.

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