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Collaborating Authors
Stiff, John James
Abstract During the development of ISO 19905-1 [ref. 1] from Society of Naval Architectsand Marine Engineers (SNAME) publication T&R Bulletin 5-5A [ref. 2], someof the metocean actions assessment methodologies have changed. This paperdescribes two areas of change, and one area where the traditional 5-5A methodshave remained the same, but has been calibrated against the ISO metoceandocumented methodologies contained in ISO 19901-1 [ref. 3]. ISO 19901-1 is oneof the " top level" ISO documents in the 19900 series of standards that allother standards within the series are required to follow. The three main areasaddressed within this paper are: Wave Spreading: It is well-established that seastates are generallyshort-crested and therefore produce lower overall actions than the equivalentlong-crested regular wave seastate. Extensive work has been undertaken duringthe development of 19905 to demonstrate factors that can reasonably be used toapproximate these effects. The factors included in 19905-1 are jack-up specificas they account for leg spacing and are more appropriate than theapproximations for single piles included in 19901 1. Vertical Wind Profile: ISO 19901-1 sets out a logarithmic relationship betweenwind speed and height/averaging period, suggesting that this is more accuratethan the traditional power law profile. The environmental action clauses of19905 permit the use of power law wind profiles. A calibration of this methodis included in the 19905-2 Technical Report [ref. 4]. Intrinsic/Apparent wave period: ISO 19901-1 defines the difference between theintrinsic and apparent wave periods, and states how they should each be usedwhen calculating the wave actions on an offshore structure. The paper describesa method for modeling a random seastate to account for the fact that theactions are calculated using the intrinsic wave periods and that the dynamicsare calculated using the periodicity of the apparent wave. A simplified SDOFdynamic amplification approach is presented based on the apparent waveperiod. Wave Spreading SNAME [ref. 2] determines quasi-static wave loads from an applicablehigher-order wave theory such as Stoke's 5th or an appropriate order of StreamFunction and hydrodynamic coefficients that are considered realistic e.g. adrag coefficient of 1.0 for a typically rough tubular. To account for theeffects of wave spreading in short-crested seas, SNAME recommends that "Fordeterministic/regular wave force calculations it is appropriate to apply akinematics reduction factor of 0.86 in order to obtain realistic forceestimates (see Commentary). This factor may be considered to implicitly accountfor spreading and also the conservatism of deterministic/regular wavekinematics traditionally accomplished by adjusting the hydrodynamicproperties." SNAME further recommends that the factor be applied by means of a reduced waveheight. This recommendation was largely because the software available in theearly 1990's, when SNAME was being developed, generally did not allow for theinclusion of kinematics reduction factors. The factor recommended in SNAME wasdetermined to provide a good match to the results of prior practice e.g. withfull wave height but drag coefficients in the range 0.65 to 0.70 for a (rough)tubular; it also provided a good match to the results from random seas with arealistic rough tubular drag coefficient of 1.0 and crest height increased toapproximate that of higher order wave theory (as is recommended in both SNAMEand ISO 19905-1).
Abstract This keynote paper describes the place, significance and development of the ISO19905-series standards (Mobile Offshore Units) within the 19900-series ofoffshore structures standards. It provides an overview of both the new standardfor the site-specific assessment of jack-ups, ISO 19905-1 [ref. 1], and thesupporting technical report, ISO/TR 19905-2 [ref. 2]. The Technical Reportcontains background information and a detailed example "go-by" calculation, afeature previously found to be extremely useful. The paper is one of seven in aseries outlining the key technical aspects of the development of ISO 19905-1and their significance in relation to the SNAME T&RB 5-5A source document[ref. 3]. The planned ISO 19905 3, site-specific assessment of floating units, will be described, but not discussed in any detail. In 1996, ISO Work Group 7, part of Sub-Committee 7, was formed and mandated bythe parent ISO Technical Committee, ISO/TC67, to develop the jack-up siteassessment standard using SNAME T&RB 5-5A as the basis. While thedevelopment of 5-5A, first published in 1994, was rigorous, conscientious, andbased on state of the art knowledge, the published document contained much thatwas new, controversial and perceived by some to be too conservative. Consequently the IADC and others sponsored many projects to advance the processand accuracy of site assessment. Some of this work was incorporated intorevisions of 5-5A, but after 19905-1 started to be developed, much was takendirectly into that document, bypassing 5-5A. This paper provides an overview ofthe projects undertaken and the process used to develop ISOs 19905-1 and19905-2. Other papers in the session give the detailed background to thechanges from 5-5A. The approach chosen in ISO 19905-1, although sometimes different from that inSNAME and other relevant ISO 19900 series standards, has been developed toembrace new technology whilst providing a site assessment methodology forjack-ups that is robust, calibrated against the best-practice application of5-5A, and allowing for improved technology/computing capability. The Jack-Up site-assessment ISO will be increasingly used throughout the worldand it is vital that potential users are confident that the standard has asound provenance. Introduction This paper is the first of a series of complementary papers describingtechnical aspects of the development of ISO 19905-1 in relation to the SNAMET&RB 5-5A source document. The other papers are:23342: Dowdy et al., " Environmental Actions in the New ISO for theSite-Specific Assessment of Mobile Jack-Up Units", 23040: Tan et al., " Structural Modelling and Response Analysis in the New ISOStandard for the Site-Specific Assessment of Mobile Jack-Up Units" 23521: Wong et al., " Foundation Modelling and Assessment in the New ISOStandard for the Site-Specific Assessment of Mobile Jack-Up Units" 23071: Frieze et al., " Structural Acceptance Criteria in the New ISO Standardfor the Site- Specific Assessment of Mobile Jack-Up Units" 23337: Kudsk et al., " Long-Term Applications in the New ISO Standard for theSite-Specific Assessment of Mobile Jack-Up Units" 22949: Stiff et al., " Benchmarking of the New ISO Standard for theSite-Specific Assessment of Mobile Jack-Up Units"
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Structural Modeling and Response Analysis in the New ISO Standard for the Site Specific Assessment of Mobile Jack-Up Units
Tan, Pao-Lin (American Bureau of Shipping) | Mobbs, Brad (LeTourneau Technologies) | Perry, Michael John (Keppel Offshore & Marine) | Stiff, John James (ABS Consulting) | Stock, Douglas J. (Digital Structures)
Abstract There are two ISO documents addressing the site-specific assessment of mobileoffshore jack-up units, ISO 19905-1 and ISO 19905-2 [ref. 1 and 2]. 19905-1 isthe main ISO standard while 19905-2 is a Technical Report addressing additionalinformation relevant to the assessment of jack-ups, but not critical to theanalysis (collectively referred to as 19905 in this paper). Both of thesedocuments have been developed from the SNAME T&R Bulletin 5-5A [ref. 3](herein referred to as 5-5A). During the development of 19905 from 5-5A therewas major restructuring of certain sections, particularly in the area ofresponse assessment. In addition, there were changes in both required andacceptable methods of assessment. This paper explains the most significant ofthe changes that come under the mandate of ISO TC67/SC7/WG7 Panel 3, whichdeveloped the Clauses in 19905 on Structural Modelling and Response Analysis, and provides an indication of their impact on site assessment. Other companionpapers in this OTC session on Site-Specific Assessment of Mobile Jack-Up Unitsaddress how other parts of 19905 differ from 5-5A. The approach chosen for the ISO standard is somewhat different from that in5-5A, and it (19905) should produce a more complete and consistent assessmentof jack-up operability. The jack-up site-assessment ISO standard will beincreasingly used throughout the world, and it is vital that users understandhow the methodologies originated, and how to interpret the document. Introduction The major changes affecting site assessment, introduced by Panel 3, aredescribed and include:Additional assessment load cases Fd, for extreme storm event and earthquakehave been added to 19905, and the formulation of the assessment load case forincluding action factors has also changed to be generally applicable for theoriginal assessment load cases in 5-5A and the additional assessment load casesin 19905. The Drag-Inertia method is a popular time domain method for estimating thecontribution of dynamics to the response of a jack-up. The method had, however, been subject to some criticism as it produced results which were, in somecases, less conservative than other methods. The Drag-Inertia method is easy toimplement and produces values that are stable; both qualities are desirable fora site assessment. During the development of 19905, a study was performed whichsuggested factoring the Dynamic Amplification Factors (DAFs) produced by theDrag-Inertia method as a practical way to bring the DAFs in line with the meanDAF results produced by the Winterstein method. The simple factor is a functionof period ratio, Tn/Tp. As 19905 was developed, a potential un-conservatism came to light forjack-ups operating close to resonance. In order to control this, and to ensurethe reasonably possible action combinations are assessed, additional actioncombinations have been suggested for jack-ups operating close to resonance. Additionally, there are new restrictions on the use of the Single Degree ofFreedom (SDOF) method. As computers become faster it is possible to include increasing levels ofnonlinearity into analyses. It is now possible to undertake a " one-stage" analysis of a jack-up in which the nonlinearities of the foundation, thestructure and the loading are included within a time domain assessment. Thetime histories of the responses are then used to develop the most probablemaximum extreme (MPME) values that are used to check performance. This paperdescribes the analysis, and the proposed method for developing the partial loadfactors.
- Reservoir Description and Dynamics > Reservoir Simulation (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Siting, assessment of hazards (0.81)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Platform design (0.68)
Abstract This paper describes how the structural acceptance criteria in ISO 19905-1[ref. 1] were developed from those of SNAME T&R Bulletin 5-5A (5-5A) [ref.2]. T&R Bulletin 5-5A has all the acceptance criteria, includingstructural, geotechnical, on-bottom stability, and all the dimensional checksin a single section. This was not consistent with the structure of thedeveloping ISO, so a new clause was created for the structural criteria. Thegeotechnical checks were included with other geotechnical calculations and astand-alone clause developed that listed all of the acceptance checks that hadto be undertaken. In effect, a single clause identified what needed to bechecked, and the details of the checks were moved to the relevant clause wherethe detailed calculations were enumerated. At the same time, the structuralcriteria were reformulated, in part for consistency with the provisions ofother ISO 19900-series documents (particularly ISO 19902 Fixed steel structures[ref. 3]), but also to ensure a more complete assessment was undertaken. Introduction T&R Bulletin 5-5A was first published in 1994, and up until that point hadnot been extensively used in practice - it was a brand new document that hadbeen developed over the preceding six years by the Jack-Up Joint Industry Study(JU-JIS). In 1997, when there was a move to take 5-5A and develop it into anISO jack-up site-specific standard, advantage was taken of the opportunity toimprove the layout of the document, to update it to incorporate changes in thebase documents that had been used in its original development (e.g. AISC LRFD[ref. 4]), and to incorporate lessons learnt from the first few years of itsuse. It was also obvious that this ISO standard would have to be compatiblewith the other standards in the 19900 series (of which ISO 19905-1 is part), and particularly the Fixed offshore structures standard, ISO 19902. Early discussion took place as to the overall structure of the AcceptanceCriteria that should be included in the standard. The intent of 5-5A was thatit was a Load and Resistance Factor Design (LRFD) based assessment procedure, but it became clear early on in the use of 5-5A that it could be misinterpretedto have the load factors applied as โload effectโ factors, after the responseanalysis. For a linear system there is no difference, but for a non-linearjack-up system, the differences can be significant. The ISO was required to bein a Partial Factor format but there was no requirement that it be an LRFDstandard. Hence, early on, there were discussions concerning the merits ofconverting the load factors to factors on the input parameters, such asmetocean factors. This was relatively quickly dismissed; however, there was anobvious need to clear up any possible misunderstanding that the assessmentshould follow a LRFD based approach. The discussions about the type of codechecks are beyond the scope of this paper, but it is important to set the stageso that there is an understanding of the amount of work needed to convert 5 5Ato the ISO. It is also important to realize that, whereas 5-5A had all theAcceptance Criteria in a single section, the new ISO would have the generalacceptance criteria in one clause, and the structural member checks in aseparate clause.
Abstract The paper presents the logic behind, and the results of, a series of comparisonanalyses between a number of jack-up site-specific assessments using both thenew ISO 19905-1 on the site-specific assessment of mobile jack-up units [Ref.1] and the Society of Naval Architects and Marine Engineers Technical &Research Bulletin 5-5A [Ref. 2]. The study shows that the differences in theresults of analyses between those undertaken using 5-5A and ISO 19905-1 aregenerally small, although one area of potentially larger difference, which wasintentionally excluded from the comparison, (the normally lower kinematicsreduction factor in 19905) could lead to lower wave loads in 19905calculations. ISO 19905 is largely based on 5-5A, however, the ISO development incorporatedsome major changes, including:Enhancements to calculation methods Significant changes in the geotechnical calculations, including the method ofcalculating acceptable bearing capacity, incorporation of deep penetrationcases, distinction between backfill and backflow, etc. Changes to enhance compatibility with other ISO documents in the 19900series, particularly ISO 19902 addressing Fixed Offshore Structures [Ref. 3](e.g. the structural code checks for tubular members) Introduction of additional structural member checks Re-structuring of 5-5A into a more logical ISO format Introduction of "benign" changes required for ISO compatibility (e.g.introduction of apparent and intrinsic wave periods), although some proved tobe less benign than others Additional loadcases for jack-ups operating close to resonance Given so many changes, there was concern that the ISO may not be useable (e.g.there would be gaps, inconsistencies, and serious transposition errors), andeven if an analysis could be undertaken using it, the analysis results would besignificantly different from a similar assessment using 5-5A. To ensure the compatibility and acceptability of ISO assessments results, a twopart benchmarking process was undertaken: Part 1 Contract a single consultant to ensure the complete ISO could be used toproduce an answer. In effect, could an analysts start an analysis using thedocument and arrive at a solution - regardless of whether the solution wascorrect. This work resulted in some important changes. Part 2 Contract four consultants to assess four jack-up designs, and comparethe results to a comparable series of assessments based on SNAME Bulletin 5-5A. In most cases each jack-up was assessed by two consultants with the analysesaligned at specific points. Alignment allowed differences in ISO interpretationto be identified, and ensured that results did not diverge as they progressed.A detailed sample calculation "Go-By" was also produced.
Abstract This paper presents a probabilistic study of various Gulf of Mexico (GoM) MODU mooring systems that was performed as part of the MODU Mooring JIP (Joint Industry Project). The intent was to determine the relative changes in reliability between different types of MODUs, with different mooring systems, in different water depths, designed for different hurricane conditions and located at different regions in the Gulf. In addition, the effects of assuming reduced mooring line strength were investigated. Twenty cases were selected which cover four different classes of MODUs, four different types of mooring systems, three different numbers of mooring lines, as well as three different water depths. These base cases were chosen so that trends or differences between different hull shapes and mooring systems could be identified. These twenty base cases are further expanded into sixty cases according to location in the Gulf as well as the return period used to design the mooring system. The results indicate that on average the " notional?? (i.e., relative) annual failure probability of mooring line failure decreases by one order of magnitude when increasing the MODU mooring system design return period from 10-yr to 50-yr. Two orders of magnitude decrease are observed if the design return period is increased from 10-yr to 100-yr. In addition to the above findings, reliability sensitivity to four different GoM Metocean regions is also provided. Redundancy of the mooring system for 8, 12 and 16-lines is also examined in terms of the additional reliability gained by the extra lines. Introduction The probabilistic reliability approach was performed using an updated procedure that was originally developed in prior work by DeepStar [1]. The results can be used by operators, drilling contractors, API work groups and regulators alike to understand, on a comparative basis, the performance of various MODUs and mooring systems. A base set of 20 separate MODU and associated mooring cases were evaluated as shown in the first twenty cases of the matrix in Table 1. These selected twenty base cases cover four MODUs (or MODU classes), four types of mooring systems, three numbers of mooring lines and three water depths. The footnotes of Table 1 provide definitions of these cases. These base cases were chosen so that trends or differences between different hull shapes and mooring systems can be identified. This base set of cases was then further defined into a larger set of 60 according to location in the Gulf as well as the return period used to design the mooring system. The Gulf locations were defined as the Central, West Central and West according to MODU JIP metocean study [5] and the latest API guideline [6], and the design return periods were defined as 10, 25, 50 and 100 year. The remainder of Table 1 shows the resulting matrix of cases studied. The full set of 60 cases was initially evaluated for reliability using 100% CBS (Catalogue Break Strength) for the mooring line. The entire set of 60 was then re-evaluated using 80% CBS to determine the sensitivity of the results to lower than expected mooring line strength. For the 8-10 mooring line cases, the first and second line failures were determined. The second line failure was used to represent the mooring system failure. For the 12 and 16 line cases, the third line failure was also determined, with the third line failure representing the mooring system failure. This allowed a comparison of the effect of mooring line redundancy in terms of the first line failure, versus the mooring system failure.
Abstract The MODU Mooring Strength and Reliability Joint Industry Project (JIP) was a massive industry effort that involved all sectors of the MODU mooring industry, including drilling contractors, operators, equipment suppliers, consultants, and regulators. The project was originally conceived in January 2005 in response to Hurricane Ivan, and became a fully funded project by November 2005 after Hurricanes Katrina and Rita. This paper describes the background to the JIP, how it evolved as requirements changed, and industry expectations. It includes a synopsis of the major reports/deliverable, particularly those not specifically covered by other papers in this OTC special session on MODU risk. It will also describe some of the effects that the study has had on the way MODUs operate and are regulated within the Gulf of Mexico, and how industry is changing its mode of operation to adapt to the new requirements that were, in part, brought about because of the JIP. JIP Background Immediately after Hurricane Ivan industry leaders under the direction of Alan Verret (Offshore Operators Committee - OOC), Sandi Fury (Chevron) and Tim Sampson (API) convened the Hurricane Response Committee with representatives from Gulf of Mexico Operators, Drilling Contractors, OOC, API and IADC to review storm damage, share information on preparation, response & lessons learned as well as begin the process of preparing an industry response for regulators. During these meetings it was apparent that Operators felt they were faced with risks from MODU excursions in terms of potential pipeline and platform damage while the some Drilling Contractors believed Ivan was a rare event and did not justify an immediate rush into upgrades. The industry prepared their response and delivered it to the MMS and USCG in New Orleans. Overall the response was well received; however there was some skepticism that industry was taking the MODU loss of station keeping seriously. Following that meeting the regulators official response was "No MODUs Adrift" and "No Risk". The icing on the cake was found in a Noble Denton post mortem report on Hurricane Andrew [ref. 1] which validated the regulators skepticism by predicting more MODUs losing station when hurricane activity increased. As a result, the OOC Drilling Technical Subcommittee (DTSC) discussed the current situation with OOC Leadership and decided more action was necessary due to the magnitude of deepwater production, increased subsea infrastructure, projected growth for deepwater areas, political sensitivity, and the reduction in available insurance in the GOM. In addition, Drilling Contractors had legitimate questions which needed to be answered in order to determine the effect a given upgrade would have on a specific MODU. Thus the OOC DTSC formed an ad hoc " Industry Planning Committee for GOMEX Mooring Reliability?? in January 2005 when the Committee first formulated the idea of the MODU Mooring Strength and Reliability JIP. The subsequent events of Hurricane Katrina and Rita continued to energize the Committee and provided additional data on which to base its observations and recommendations. The Committee, under the auspices of the Offshore Operators' Committee and chaired by Craig Castille of Dominion E&P Inc.1, was comprised of interested representatives from both drilling contractors and operators. There were a number of issues that needed resolution, each requiring careful thought and planning in order to reach the desired goal. In June 2005 the Committee sent out requests for proposals to qualified contractors. The proposal request contained a number of subjects, but was intentionally broad in scope, thereby allowing the prospective contractors to complete the details of how they foresaw the study developing. It is important to realize that at this stage industry was still thinking of Hurricane Ivan in terms of an unusual and remote event: it was vital to react to the event, but the expectation was that there would not be another equivalent storm in the near future. Figure 1 shows an outline of the Industry response and timeline to the hurricanes.
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- North America > United States > Louisiana > Orleans Parish > New Orleans (0.24)
- Energy > Oil & Gas > Upstream (1.00)
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Abstract After the hurricanes in the GoM that resulted in several semi-submersible MODUs breaking adrift, one task of the MODU Mooring Strength and Reliability JIP was to develop a comparison between the risks associated with MODUs drilling within the Gulf of Mexico and other industries, including other aspects of the offshore oil industry. The paper reviews the baseline risks associated with moored MODUs in Hurricane season, and extrapolates to include the calculated potential risks of a drifting MODU causing major damage to critical infrastructure. It compares these historical and calculated risks, on a consistent basis, to the risks associated with offshore fixed platforms, offshore collisions, blowout events, pipelines, dams, and trains. It will then show how the industry enhancements in MODU mooring systems (brought about since the 2005 hurricane season) lower the operating risks and contrasts them with other industries and other sectors of the offshore industry. The perception after the 2005 hurricane season was that semi-submersible MODUs were operating at too high a risk in hurricane season and that mooring design codes should be severely tightened since " Rigs Adrift are not Acceptable?? (Chris Oynes, MMS, 26-Jun-05). The paper, which is based on a JIP report [ref. 1], shows that the enhancements in mooring systems and codes brought about by industry significantly lower the mooring failure1 risks which are now similar to tolerability levels in other industries. Whitman (Farmer) Diagram: a two dimensional view One of the complexities of a comparative risk assessment is determining how the derived data should be presented so that it conveys the correct message both easily and honestly. One common method is to use a Risk Assessment Matrix (RAM) which has color-coded blocks of differing " Risk?? (defined as the product of probability of an event with its consequence). An example RAM is given in Figure 1. The difficulty with using a RAM for the JIP comparative risk assessment (CRA) is that it is hard to show the ranges of risk associated with any event: it is not a simple case of one probability and one consequence; there are ranges of both. Hence a more flexible graphical representation was needed. Another way of presenting comparative risk data is in the form of a diagram used by Robert Whitman in 1984 [ref 2], but probably developed by Reg Farmer in 1967 [ref. 3]. This diagram plots the probability of an event on the ordinate vs. the consequences on the abscissa (see Figure 2). (For reasons of simplicity and familiarity in the offshore industry, this paper will refer to this type of diagram as a Whitman Diagram, even though this may be an imprecise attribution.) As can be seen, it allows for a range of probability and consequence to be associated with any individual risk. It can be extremely effective in giving a visual image of the comparative risks, but must be constructed with enormous care, or can lead to misrepresentations and incorrect conclusions. The comparison of the consequences is relatively straightforward, although combining different consequences on the same diagram can cause confusion, but it is the probabilities that are the major challenge.
Abstract A number of hurricanes have passed through the Gulf of Mexico resulting in MODUs parting moorings or collapsing, but none have come close to the devastation caused by Hurricanes Katrina and Rita which came through the Gulf of Mexico in August and September 2005. 20 semi-submersible MODUs suffered some level of significant mooring impairment, and 8 jack-ups were declared total losses (with many more suffering additional settlement and other damage). While this was a severe outcome for the MODU fleet, which had previously experienced relatively few failures1 due to storm overload, over 100 platforms were destroyed, and the hurricanes caused disasters and major regional devastation on land. This paper reviews the level of damage to the MODU fleet caused by the two hurricanes. Much can be learned about the likelihood and consequences of future failures by studying past experience. Information on the location of where mooring component parted helps assess the consequences of those mooring lines parting, and the site details for the jack-ups help point towards the drivers for their survival. These can then be put into the perspective of other structures of the industry. Semisubmersible MODUs in the Gulf of Mexico have changed their general usage since development of the API RP2SK [ref.1] to which their moorings are designed for individual locations. The consequences of MODUs drifting in an extreme hurricane are potentially much greater than in the past because of the recent development of the deep water, highly productive, platforms and their associated extensive subsea infrastructure. Damage to this highly productive infrastructure is detrimental to both the nation and industry. In the aftermath of these hurricanes, which followed Hurricanes Lili, and Ivan, the industry re-examined and upgraded the standards for siting MODUs. An industry driven JIP was launched on moored semi-submersible MODUs which chronicled the incidents and provided guidance for the change in standards. The MMS commissioned a study on the jack-up failures and successes and industry developed a new standard for jack-ups, API RP 95J. Together with MMS-NTLs, these documents provide guidance for siting MODUs to avoid future multiple losses, minimize consequences of failure, and increase the probability of survival.
- South America > Atlantic Basin (0.99)
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Abstract Hurricanes in the past five years have provided significant information regarding performance of moored vessels in the Gulf of Mexico in particular for the Mobile Offshore Drilling Units (MODU). Starting with Hurricane Lili in 2002 and ending with Hurricanes Ivan, Katrina and Rita in 2004 and 2005, the result was twenty-three MODUs having suffered either complete mooring failures or partial mooring failure with the vast majority of these occurring in the 2004 and 2005 hurricane season. After the 2005 hurricane season, the API RP 2SK, Design and Analysis of Station Keeping Systems for Floating Structures [1], Task Group was reactivated to explicitly address the moored MODU issue with the group made up of rig owners, operators, contractors and regulators. A summary of the API RP 2SK Commentary, Gulf of Mexico MODU Mooring Practice for Hurricane Season [2], and evolution from API RP 2SK, 3rd Edition, to API RP 95F, Interim Guidance for Gulf of Mexico (GoM) MODU Mooring Practice - 2006 and 2007 Hurricane Season [3, 4] to the Commentary will be presented. New requirements, adjusted return periods, introduction of formal risk assessment processes and direct linkage to inspection of mooring components are now incorporated. A large Joint Industry Project was also formed that focused on gathering data, assessing MODU performance, calibrating observed performance to code expectations and addressing some analysis 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 moored MODUs for the GoM during hurricane season. Improvements the MODU owners have made to the fleet will also be discussed along with options that can help mitigate the risk of a MODU failing and doing damage to infrastructure during a hurricane. Introduction MODUs in the Gulf of Mexico are a critical part of the infrastructure required for exploration and development of oil and gas in 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 to better understand the causes of moored MODU failures and learn from them. This knowledge has resulted in changes to the industry standards. API RP 2SK 3rd Edition establishes a minimum acceptable design code for mooring systems. RP 2SK requires a mobile mooring, 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 are provided, however, on what is considered " away from?? and " in the vicinity of.?? Some examples are given for operations in the vicinity of other structures. RP 2SK also permits using a risk analysis to determine the design return period but in no case shall 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 and carving 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 of MODUs working in the deep water has increased along with more infrastructure thus increasing the probability of failure and also possible consequences of failure (damage).
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