OTC-29024-MS API RP 2SK 4th Edition - An Updated Stationkeeping Standard for the Global Offshore Environment Hongbo Shu, Insight Offshore Consulting, LLC, Chevron Energy Technology Company; Aifeng Yao, Kai-Tung Ma, and Wei Ma, Chevron Energy Technology Company; Jonathan Miller, Miller Technical Consulting Copyright 2018, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2018. 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 been reviewed 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, its officers, 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 to reproduce 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. Abstract The American Petroleum Institute Recommended Practice on Design and Analysis of Stationkeeping Systems for Floating Structures (API RP 2SK) provides guidance on the best practices for floating structure stationkeeping system engineering so they will remain safely within their designed excursion limits. By so doing, the engineer ensures the safety of the floating structures and the integrity of riser systems connected to the exploratory and/or producing wells. This API standard provides the comprehensive guideline for designing stationkeeping systems and has been cited in well over 100 technical papers worldwide. API RP 2SK is referenced by Code of Federal Regulations Title 46 (46 CFR Part 250) for oil and gas related operations in the US Outer Continental Shelf. Since its 3rd edition was published in 2005, the industry has collected significant data and learnings from offshore mooring operations, especially from the experience in the hurricane seasons in 2004, 2005, and 2008. Although much of these learnings have been captured in the Appendices (e.g. Appendices H, CH and K, May 2008), it is difficult for end users to quickly understand and implement the requirements of the standard when they are interspersed with informative recommendations in a document that is over 250 pages long. To be consistent with the recent formatting style of international standards, this 4th edition of API RP 2SK has sorted all requirements into the "normative" section at the front, while recommendations are organized in the corresponding "informative" part in the Annex. In addition, regional requirements or considerations have also been included in annexes to make this API standard globally applicable. On the technical front, the following updates are included in this edition of API RP 2SK: 1. Design limit states are revised to be aligned with the latest ISO 19900 DIS (2018) . 2. New normative guidelines for mooring design Robustness Check have been added. 3. Design criteria for MODU moorings has been restructured, encouraging risk-based design criteria selection (QRA). Drag anchor design FOS has also been updated. 4. Provides specific guidance for Response Based Analysis method. 5. Developed new informative guidance on how to conduct chain OPB/IPB and Tension-Tension fatigue analysis. 6. Initiated design criteria for wire rope bend-over sheaves. 2 OTC-29024-MS 7. Initiated new design considerations against mooring line trenching on sea floor. 8. Updated corrosion allowance ranges, selecting criteria based on latest industry data and findings. 9. Updated guidance on how to include vortex induced motions (VIM) effects in mooring design. 10.Added new recommendations to include mooring system life cycle considerations (e.g. installation) in design stage. 11.Thruster-assisted mooring (TAM) design criteria in the North Sea region updated per UK OIS 2013 . 12.Regional guide for cyclonic region aligned with APPEA MODU Mooring Guide (2016) . This paper introduces the newly updated API RP 2SK, highlights the updated or new requirements and guidance notes, compares the structure of the 3rd and 4th editions, and API/ISO stationkeeping standards, helps users get familiar with the new edition of API RP 2SK to be published in 2018. Introduction API RP 2SK is used by the offshore industry worldwide for stationkeeping design of offshore floating drilling and production platforms as well as other floating offshore installations such as construction vessels, floating hotels, floating wind power platforms. It has also served as the base document for a few industry standards, such as APPEA guideline for MODU mooring in Australia  and ISO 19901-7. The current structure of the API standards related to technical topics of stationkeeping design is illustrated in Figure 1 below. Figure 1—The current structure of API standards related to key areas of stationkeeping systems (as of 2017). For historical reasons, the specifications or guidelines for some of the mooring components (such as anchors or underwater-disconnectable mooring connectors) are included in the appendices of the API RP 2SK. The testing guideline and specification for fiber mooring ropes are included as part of the API RP 2SM . In the ISO side, ISO 19901-7:2013  combines the design analysis guideline for both steel and fiber rope mooring systems which were derived from API RP 2SK 3rd edition  and API RP 2SM 1st edition, but adds a standalone specification (ISO 18692: 2007) for fiber mooring ropes. API and ISO jointly published standards of metocean criteria , and geotechnical design . However, the joint development work is currently on hold. In 2017, ISO published its 1st edition of mooring chain specification – ISO 20438 . In early 2018, ISO 19900 DIS was released which sets the general requirement and frame work for other standards on offshore structures to follow. Figure 2 shows the current structure of ISO stationkeeping standards in comparison with API's in Figure 1. OTC-29024-MS 3 Figure 2—The current structure of ISO standards related to key areas of stationkeeping systems (as of 2017). Ideally, a design analysis standard should only include the requirement (Normative or Informative) for designing a system, which will then be supported by a suite of component specifications and testing requirements for the system, rather than mixing the component specifications and testing standards in a design standard. The benefits of separating the design requirements from the component's specification/ testing requirement include allowing the different content to be updated in a timely manner as new technical advances become known and widely accepted in the industry, as well as adding clarity to the natural boundaries among system-vs-components and different components in the system. This philosophy was supported by the industry communities on both API and ISO sides, but due to time and resource limitations, the work of restructuring the API stationkeeping standards in accordance with the above philosophy was deferred to the future. The feedback from the industry after using the 3rd edition of API RP 2SK since 2005 includes that although the document contains a lot of important and useful information, it is difficult for a user, especially a new engineer, to quickly figure out what are mandatory and what are optional requirements when it comes to designing a stationkeeping system. In the 4th edition of API RP2SK, all mandatory requirements are included in the Normative Part while most of the guidance notes, optional, or regional requirements are in the informative part of the standard. Only limited informative materials are included in the normative to keep the continuity in discussion and to give users proper reference. This document structure provides a clear distinction between what is a mandatory requirement, and what are optional considerations for the design of either a permanent or a mobile stationkeeping system, which is consistent with new API standard guideline and ISO standard structure. During this restructuring, the editing panel also moved and updated the illustrative materials such as new design guidelines and analysis examples into the Informative part of the standard to preserve the educational value of the standard for new engineers. As it became widely accepted by the industry since its introduction in 2008, the risk-based design criteria selection for both mobile and permanent mooring systems found in Appendix K has been elevated to the normative requirements. In this edition of the RP2SK, more specific guidance notes have been provided to clarify the minimum design criteria and requirements . It is also recognized that the mooring integrity should be managed during the life cycle of the mooring system from its design stage. In the early stages of revising RP2SK, the topic of mooring integrity management (MIM) was given a separate section of its own. Later, the topic of MIM was elevated into its own separate recommended practice  . Therefore, MIM requirement at design stage is removed from this edition of RP2SK, and users are referred to the API RP 2MIM to be published in 2018. On another note, API RP2I (2008) needs to be updated to include inspection guidelines for permanent moorings. Recent publications   noted that the failure rate of mooring lines is higher than design expectations for other offshore structure components. The response from the industry tends to increase the design factor of safety and/or design criteria in ULS. These measures all led to using stronger and larger mooring components. However, the mooring line failure rate has remained stubbornly unchanged since 1990s. The industry gradually realized that just using larger components isn't the right solution for reducing the mooring line failure rate. The reliability of a stationkeeping system can only be improved when the quality assurance 4 OTC-29024-MS is ensured consistently in every step of the way from design to installation. This edition of the API RP 2SK has been updated with clear guidance for the users/stakeholders to understand how to carry out the design to meet the intent, and to test the sensitivity of their design's performance to the perturbations in the design conditions. This edition of API RP 2SK also initiated coverage and added new guidance notes on several important technical developments in the offshore industry since 2008. For example, robustness design check, mooring chain out of plane/in-plane bending (OPB/IPB) fatigue design, steel wire over the sheave bending fatigue design, the impact of vortex-induced motions of a floating structure on mooring design considerations, updated corrosion and wear/abrasion allowance based on the local water conditions, and mooring system design considerations against taut-line trenching on the seafloor. In the following sections, several noteworthy technical updates will be highlighted and discussed in more detail. Design Limit States To give more consistent requirement for designing offshore structures, both API and ISO organizations are changing the standards structure to establish the design frame work and general requirement for oil & gas industry's floating offshore structures in a hierarchically elevated standard, such as ISO 19900 DIS:2018, whereas the sub-system design standard (such as the RP2SK) will be aligned to provide supporting and more specific/detailed guidance on how to carry out the design of a sub-system (such as the mooring system) of the floating offshore structure. To that end, this edition of API RP2SK also restructured design considerations into four Limit States to be aligned with those of ISO 19900 DIS:2018 , and to be consistent with ISO 19901-7 DIS:2017, see Table 1 below: Table 1—Comparison of the limit states for ISO 19900 DIS (2018), API RP 2SK 4th ed. and ISO 19901-7 DIS (2017). In the 4th edition of RP2SK, the ULS for mooring system design includes both intact and one-line damaged (redundancy) design conditions. The SLS includes operational design situations, which will define the operational limit conditions for drilling, production and offloading, as well as operational considerations when any mooring line is broken or any thruster has failed. In ISO 19900 DIS, ALS for offshore structures address complete loss of integrity of the structure, or of a vital part of the structure, such as loss of stationkeeping (free drifting). Therefore, there is no ALS normative design requirement for stationkeeping in this standard except for that requirement on disconnectable FPSO systems. Design Robustness Check In the Appendix K of API RP2SK 3rd edition  , the robustness check was presented in the form of weak point analysis. The objective was to determine the probable failure mode of the mooring system, to aid the risk assessment and mitigation strategies at the design stage. In the offshore industry, it is now a common practice to check critical mooring design to a longer return period than that specified in ULS requirements OTC-29024-MS 5 (i.e. a "robustness check"). In ISO 19900 DIS (2018) , the robustness check for offshore structures has been elevated to a normative requirement. In the 4th edition of RP2SK, the robustness check is broadly defined as analyzing a stationkeeping system's survivability in exposure to deviations of identifiable critical design conditions/parameters. The goal is to have inherent safe design and to prevent catastrophic system failure in the event of any single point of failure. More specifically, robustness checks should examine a stationkeeping system's survivability in exposure to greater loading in a sea state higher than that of ULS; or in unfavorable heading combinations of winds, waves, or currents; or in the event of floating structure damage such as accidental flooding of a compartment, exposure to greater stress due to higher than anticipated corrosion/wear rates or pitting; exposure to abnormal fatigue mechanism (such as an unusually strong loop current event in early service life of a Spar mooring system). Checking the survival of stationkeeping system against these conceivable scenarios is consistent with ISO 19900 DIS normative requirement for robustness check of an offshore structure design. Furthermore, during mooring system robustness check, the foundation structure of the on-board mooring hardware (fairleads, chain jacks, winches, and chain stoppers) should be verified to be able to meet the structural design criteria of API RP2A  against the corresponding greater loadings in the robustness check conditions. It is recommended that permanent moorings (especially for manned floating platforms) be designed with sufficient reserve capacity to withstand the higher design load in the perturbation of ULS sea states without the complete loss of a stationkeeping system, therefore the floating platform connected to it. Return periods specified for robustness check design situations should be selected at least one order of magnitude greater than that used for ULS to show the system's response variation. This is consistent with the recommendation in Clause 5 of ISO 19900 DIS , where the return period for ALS is recommended from 1,000 to 10,000 years. The selection of return period for robustness check and the associated design acceptance criteria shall be the responsibility of the operator of the floating structure, but the minimum safety factor for line tension should not be less than 1.0. For disconnectable mooring systems, if the joint probability of occurrence of the accidental/abnormal environmental event combined with the failure to disconnect is less than 10-4 per annum, an ALS design check is not required. This is consistent with ISO19901-7  . Therefore, robustness check for such a load case is not required either. Summary of Unified OPB/IPB/TT Fatigue Design Methodology and Guidance Out-of-plane bending (OPB) fatigue of the mooring chain is a newly discovered fatigue mechanism that can significantly reduce the service life of a mooring line  . Although the three OPB induced failures   all appeared to be the consequence of improper design of geometric restriction on the chain links near the top connections, the industry still doesn't have any widely accepted or effective screening criteria to eliminate the OPB fatigue from any mooring system design considerations. Therefore, OPB and its associated In-Plane-Bending (IPB) fatigue should be addressed in the design of a mooring system unless it can be shown that OPB/IPB fatigue is not a controlling design factor for the system under consideration. Chain OPB fatigue analysis is similar to analysis and design of a structural component. The global analysis used for OPB fatigue analysis is typically performed in time domain. In addition to the general requirements for global analysis in the RP2SK, the numerical model of the floater and mooring lines needs to be able to capture the relative angle between the mooring line and the floater to which it is attached, both in the vertical plane and in the horizontal plane. The input parameters for the OPB analysis can be obtained from full scale testing or from FEA of the chain links, considering the chain link geometry, the mechanical properties of the bar material and the magnitude of the proof load that will be applied during manufacturing. Furthermore, the interaction between the chain links and the top connection as well as the response of the 6 OTC-29024-MS top connection to rotation need to be considered. If an articulated top connection is used, care should be taken to model the proper break out moment. Typically, a dual axial articulated top connection is better than a single axial design against OPB fatigue. OPB/IPB fatigue damage can be calculated following the hotspot method. For this purpose, the B1 SN curves in Table 2 (DNV RP C203 ) are recommended. Other SN curves may be used by the designer provided that the SN curve is developed using sufficient amount of full scale OPB fatigue test data. OPB fatigue safety factor is a function of the SN curve used in estimation of fatigue damage. Preliminarily, the following factors of safety are recommended: for SN curves with slope of 3.0 the OPB factor of safety is 3.0, and for SN curves with a slope larger than 3.0 the OPB factor of safety is 5.0. Table 2—B1 SN Curves In the traditional mooring fatigue analysis, tension-tension (TT) fatigue is analyzed based on tension loading diagram and compared against the TN curve. Since tension load and bending load are related, and the tensiontension fatigue can be easily converted to be analyzed in stress domain, it is, therefore, proposed that the chain fatigue analysis can be combined into a "unified fatigue analysis" and compared against a select SN curve for estimating total fatigue damage. Toward that end, the timeseries of tension, the primary and secondary bending moment components are used to calculate the nominal tensile, OPB, and IPB stress components in the affected links using the moments of area of the chain link. In the case of free corrosion, the corroded chain diameter is used for calculation of chain moments of area. The total stress at OPB hotspots are then calculated by applying the appropriate stress concentration factors on the nominal stress components. In combining the stress components, attention must be given to the phasing between the stress components. There are four OPB hotspots on a chain link that could have different combinations of OPB and IPB. Specifically, the total stress time series at each OPB hotspot can be estimated from Note that the hot spots of tension-tension fatigue are different from those of OPB hotspots, as shown in Figure 3. However, the total stress at the TT hotspots can be assembled similarly provided that the corresponding SCF for each stress component at the hot spot being analyzed is determined beforehand. Finally, rain flow cycle counting is applied on the total stress timeseries to develop the stress range histogram of each sea state. The long-term stress range histogram is developed from the stress range histogram of each sea state and the corresponding probability of occurrence of the sea state. The total fatigue life of the corresponding mooring chain links can then be estimated. OTC-29024-MS 7 Figure 3—Hot spot locations are distinctly different for OPB loading (left) and TT loading (right) Revised Risk-Based Design Criteria for MODU Mooring Mobile or temporary moorings stay at one location for a short term (typically a year or less), compared to tens of years for permanent moorings on production facilities. Despite the exposure time to the environment is relatively shorter, the annual failure rate of mobile mooring lines has been seen to be on the order of 10-2 which is about one order of magnitude higher than that of a permanent mooring line . The root cause analysis for these mobile mooring line failures indicated that improving reliability of MODU moorings may be achieved through a better design and better handling of mooring hardware. Aiming to improve the reliability of temporary moorings such as those used by MODUs (Mobile Offshore Drilling Units), the new RP2SK streamlined the design criteria for a mobile mooring. There appears to be a lack of clear guidance on designing a mobile mooring system to a proper return period in Appendix K in the previous version (revision 3). The gap is prominent especially for moorings in tropical cyclone regions, also known as hurricane or typhoon areas. Previous version does not have a clear guidance on what return period shall be used as a minimum to account for the risk associated with proximity/distance and failure consequence. Revision 4 of RP2SK updated the design criteria by providing a clearer guidance. It recommends minimum return periods to be used, and specifies when a quantitative risk assessment (QRA) should be performed to justify the design criteria for any region with tropical cyclones. The guidance on selecting design criteria uses Appendix K of the 3rd edition API RP2SK as a basis, and added some refinement and improvement to aid the users. There are two key variables that need to be assessed to determine the return period for a MODU mooring at a specific site : • • Distance of Proximity – the distance from the MODU under consideration to any other surface and subsea infrastructure. Production throughput of nearby facilities – the production rates (oil and gas) of the nearby surface production facility or flow rates of pipelines that could be in the path of a dragging mooring line. This approach provides a rational basis for QRA/criteria selection, and maintains consistency with Appendix K of the 3rd edition RP2SK. Additionally, the revision extends its application from Gulf of Mexico (GOM) to any regions worldwide that are subject to tropical cyclones; whereas Appendix K was only applicable to GOM. Guidance on performing a QRA is also provided, and the aspects on how to produce trustworthy results are addressed. 8 OTC-29024-MS Update to the Corrosion and Wear Allowance of Mooring Chain Protection against corrosion and wear of permanent mooring chain is usually provided by increasing the chain diameter based on the targeted service years. Figure 4 includes examples of more severe than expected splash zone corrosion in tropical environment that resulted in premature replacement of top chains. Revision 4 of RP2SK has incorporated latest findings on corrosion and wear from recent joint industry researches, such as SCORCH and DeepStar MAC JIPs    . Based on those findings, it recommends that the diameter increase shall be determined by a site-specific assessment dependent upon several parameters, e.g. level of dissolved nitrogen, water temperature, water salinity, level of dissolved oxygen, splash zone action, and water particle velocity. Figure 4—Examples of severe splash zone corrosion prompts the need to update corrosion allowance Typical values of corrosion and wear allowance recommended in the 4th edition. of the API RP 2SK are preliminarily specified as follows: • • • 0.4 mm to 1.0 mm per year of the design service life, for those parts of a mooring line in the splash zone or zone of hard-bottom sea floor contact, 0.1 mm to 0.4 mm per year of the design service life, for the rest of the mooring line. If there is evidence that corrosion rates experienced exceed 1.0 mm per year (site specific data), more conservative corrosion allowances should be considered. It should be noted that the corrosion and wear allowance recommended for permanent mooring chain is intended for defending the long-term general corrosion (aka uniform corrosion). Field observations of used mooring chains have indicated that the general material loss experienced by individual steel chain links could vary along the body of a link, such as more severe metal loss at the contact zone with significant interlink motions compared to that at the straight section of the link. In addition, extensive local pitting often in the form of elliptical pits have been founded on chain links near the sea surface, especially from the mooring systems installed in the tropical waters. The pit depth growth rate could significantly outpace the general corrosion and wear allowance of the links at the same water depth. However, the current knowledge and experience in the mooring industry is not sufficient enough to develop more specific guidance on local OTC-29024-MS 9 pitting allowance for offshore mooring chains. As such, mooring designers and offshore facility operators should exercise their discretion when it comes to the selection of chain corrosion and wear allowance and the long-term mooring integrity management for a specific stationkeeping system under development. The prior experience in the rate of corrosion or wear from the existing mooring installations in the same region should provide the best guidance on how to set proper corrosion/wear allowance for new designs unless new counter measures with proven results are deployed. VIM considerations in mooring system design Floating structures comprised of cylindrical structural members such as spars and multi-column platforms (i.e. semi-submersibles and TLPs) can be susceptible to Vortex Induced Motions (VIM) when exposed to ocean currents. VIM was first reported for classical spars some 15-20 years ago, leading to the development of Appendixes H and CH in the 3rd revision. Since then VIM has been also reported on several semisubmersible production platforms in the GOM. VIM of the host structure is cyclic by nature and may contribute to excessive fatigue damage of the mooring system. Model testing has been the primary tool for VIM prediction at the design stage. The confidence in model test results, however, relies on how scaling effects, mooring system stiffness, external damping contribution from mooring lines and risers, current profiles and turbulence effects, and hull appurtenances are understood and properly incorporated in the model test. Most of the VIM model tests to date were performed in a towing tank using a scaled model with a four-point linear spring system to simulate the mooring and riser stiffness. This practice, while simple in the model testing design and execution, has been confirmed as a plausible cause for the discrepancy between model scale VIM test data and field measurements for some deep draft semis. It has been shown by the field measurements, model tests and CFD studies that the mooring- and riser-induced damping significantly reduces the VIM amplitude as well as the range of velocities over which lock-in occurs  . The importance of including mooring and riser damping in a VIM model test and potential means of inclusion are discussed in the latest revision. With the technological advancements in computational power, the use of Computation Fluid Dynamics (CFD) for VIM design and analysis has become a viable option to predict VIM performance. High-fidelity CFD has been used to produce accurate VIM predictions matching the corresponding model test predictions and/or field measurements for several deep draft semi-submersibles  . CFD analysis can account for effects such as scale, mooring and riser damping, current profile, non-linear mooring system stiffness etc. that are typically ignored or cannot be easily modeled in a tow tank test. Additionally, it can provide a detailed picture of the overall flow around the floating structure that is otherwise difficult to measure in a model scale experimental setup. CFD and model testing are not mutually exclusive. A CFD study can be conducted to predict the VIM behavior and to provide design curves, after the CFD model is validated by a tow tank test. Guidance on how to use CFD to predict VIM has been incorporated into Revision 4 of API RP 2SK. Design with project life-cycle in mind The design capability of a mooring system is only as good as what can be achieved in the installation. Before a mooring system design is finalized, the designer should conduct a constructability review of the mooring system with the key stakeholders from fabrication, transportation, installation, site geotechnical survey, and project management. This design review cycle would allow the mooring designer to gather and verify all construction related restrictions for the mooring system, and to adjust the design to meet the design goals within the site-specific restrictions. The designer and the stakeholders together should also define and agree on the acceptable dimensional tolerances in the mooring component fabrication and system installation. The final design analysis cycle should verify that the cumulative effect of the dimensional tolerance will not cause problem for the system installation or hook-up, or to cause the mooring system to fail to meet any 10 OTC-29024-MS design targets such as extreme loading limit, platform offset limit, or fatigue performance. This edition of the RP2SK provides guidance on the following areas: 1. 2. 3. 4. Mooring system installation considerations during design stage, Mooring line test loading requirement, Post installation as-built survey and establishment of as-built capacity, In service mooring integrity monitoring. The industry trend has been evolving in the direction of integrating requirement of installation, monitoring, maintenance and replacement/removal in the early stage of design. When the industry common practice in this direction becomes clear, API RP2SK shall be revised again to include more specific (normative) guidelines based on industry's best practice. Concluding remarks API RP 2SK is the industry's minimum standard for design and analysis of a floating structure's stationkeeping system. It is referenced by the US regulatory requirement for offshore structures (CFR 46). it is fitting for this standard to bring the stationkeeping system's life-cycle reliability considerations to the attention of the floating system designers and other stakeholders (project management, RCS and regulators) alike at the earliest stage of an offshore project. This is because that the proper design of the safety critical stationkeeping system is a key step in delivering a safe and reliable floating structure after concept selection. The industry experience shows that the surest route to improve stationkeeping system's reliability is to carry out all work in consistent quality for all steps of implementing such a system, from system design/ manufacturing/handling/integration, to system installation/hook-up/maintenance. This edition of the API RP2SK has been updated with that goal in mind. The many changes implemented in the 4th edition RP2SK have made it largely consistent with the industry's high-level standards and parallel standards. The efforts from both API and ISO editing panels had resolved major discrepancies in the API RP2SK and ISO 19901-7 in their new editions. Future efforts should be devoted towards combining the two stationkeeping standards into one or perhaps jointly develop the next generation code. It is understood that there are still higher-level issues to be resolved before the standards can be jointly developed. Meantime, the users of API RP2SK will finally have an updated standard since 2005. The 4th edition of API RP2SK included updated guidance notes on many important technical topics related to the design of stationkeeping systems, and initiated coverage for regional requirement such as the Atlantic North seas, cyclonic environment, and ocean conditions where extreme low temperature and ice-loading must be considered. As the industry accumulates more data from field experience and improves design practices, some of these informative guidance notes can be revised and elevated into the normative sections to help the designers achieve the goal of ensuring more robust stationkeeping system for a floating structure in any environment worldwide. Acknowledgements The authors would like to acknowledge the support of their respective companies and industry colleagues in producing this paper. We wish to acknowledge the contributions and comments made by Aimin Wang of Exmar, Pedro Vargas of Chevron, Yongyan Wu of Aker Solutions, David Petruska and Prudence Chen of BP, Amir Izadparast of SOFEC, Caspar Hyel of Shell, Jun Zou and Arun Antony of Houston Offshore Engineering, Sue Wang of ABS, Srinivas Vishnubhotla of DNV, David Smith of ExxonMobil, Tom Kwan of Kwan Engineering Services. The views expressed herein include a collation of facts and opinions provided by the authors and therefore do not reflect the opinions of any one author or his company. OTC-29024-MS 11 Nomenclature API DIS FOS ISO JIP MAC MODU RCS QRA SCORCH SCF TAM VIM American Petroleum Institute Draft International Standard Factor of Safety International Organization for Standardization Join Industry Project Mooring Against Corrosion (a DeepStar JIP) Mobile Offshore Drilling Unit Recognized Class Societies Quantitative Risk Analysis Seawater Corrosion of Steel Wire Rope and Chain (JIP) Stress Concentration Factor Thruster Assisted Mooring Vortex Induced Motions References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. API RP 2SK, "Design and Analysis of Stationkeeping Systems for Floating Structures", 3rd Edition, October 2005, Addendum May 2008. API RP 2SM, "Recommended Practice for Design, Manufacturing, and Maintenance of Synthetic Fiber Ropes for Offshore Mooring", 2nd Edition, August 2014. API RP 2I, "Recommended Practice for In-Service Inspection of Mooring Hardware for Floating Structures", 3rd ed., April 2008. API RP 2GEO, "Geotechnical and Foundation Design Considerations", 1st ed. 2011. API RP 2MET, "Derivation of Metocean Design and Operating Conditions", 1st ed. 2014. API Spec 2F, "Specification for Mooring Chain", 6th ed. 1997. API Spec 9A, "Specification for Wire Rope", 26th ed. 2011. API RP 2MIM, "Recommended Practice for Mooring Integrity Management", 1st ed. to be published in 2018. APPEA, "MODU Mooring in Australia Tropical Waters Guideline", Australian Petroleum Production & Exploration Association Ltd., Rev. 0, Dec. 2016. ISO 19900, "Petroleum and natural gas industries — General requirements for offshore structures", DIS (Draft International Standard), 3rd Edition, to be published in 2018. ISO 19901-7, "Petroleum and natural gas industries - Specific requirements for offshore structures - Part 7: Stationkeeping systems for floating offshore structures and mobile offshore units", 2nd ed., May 1, 2013. P. Jean, K. Goessens, D. L'Hostis, "Failure of Chains by Bending on Deepwater Mooring Systems", OTC-17238-PP, 2005. Fontaine, E., Potts, A.E., Ma, K.T., Arredondo, A., and Melchers, R.E., "SCORCH JIP: Examination and Testing of Severely-Corroded Mooring Chains from West Africa", Proc. Offshore Technology Conference, OTC 23012, May 2012. J. Rosen, A. Potts, E. Fontaine, K. Ma, R. Chaplin, W. Storesund, "SCORCH JIP - Feedback from Field Recovered Mooring Wire Ropes", OTC 25282, Offshore Technology Conference, May 2014. ABS Consulting, "Gulf of Mexico MODU Mooring Reliability JIP", Managed by ABS Consulting, 2005. 12 OTC-29024-MS 16. K. Ma, A. Duggal, P. Smedley, D. L'Hostis, and H. Shu, "A Historical Review on Integrity Issues of Permanent Mooring Systems", OTC 24025, Offshore Technology Conference, May 2013. 17. P. Smedley, & D. Petruska, "Comparison of Global Design Requirements and Failure Rates for Industry Long Term Mooring Systems", Proceedings of the Offshore Structural Reliability Conference, Houston, TX, September 2014. 18. UK HSE Executive, Offshore Information Sheet, Nov. 2013. 19. K. Ma, R. Garrity, K. Longridge, H. Shu, and A. Yao, and T. Kwan, "Improving Reliability of MODU Mooring Systems through Better Design Standards and Practices", OTC 27697, OTC Conference, May 2017. 20. A. Izadparast, C. Heyl, K. MA, P. Vargas, and J. Zou, "Guidance for Assessing Out-Of-Plane Bending Fatigue on Chain Used in Permanent Mooring Systems", Proceedings of the 23rd Offshore Symposium, Society of Naval Architects and Marine Engineers (SNAME), Houston, February 2018. 21. E. Fontaine, J. Rosen, A. Potts, K. Ma, R. Melchers, "SCORCH JIP - Feedback on MIC and Pitting Corrosion from Field Recovered Mooring Chain Links", OTC 25234, OTC Conference, May 2014. 22. D. Witt, K. Ma, T. Lee, C. Gaylarde, S. Celikkol, Z. Makama, I. Beech, "Field Studies of Microbiologically Influenced Corrosion of Mooring Chains", OTC 27142. OTC Conference, May 2016. 23. C. Carra, T. Lee, K. Ma, A. Phadke, D. Laskowski, G. Kusinski, "Towards API RP 2MIM DeepStar Guidelines for Risk Based Mooring Integrity Management", Deepwater Offshore Technology, Oct. 2015. 24. Irani, M., Jennings, T., Geyer, J., Krueger, E., "Some Aspects of Vortex Induced Motions of a Multi-Column Floater", Proceedings 34th International Conference on Ocean, Offshore and Arctic Engineering, St. John's, Newfoundland, Canada, 2015. 25. Sterenborg, J., Koop, A., de Wilde, J., Vinayan, V., Antony, A., Halkyard, J., "Model Test Investigation of the Influence of Damping on the Vortex Induced Motions of Deep Draft Semi-Submersibles Using a Novel Active Damping Device", Proceedings 35th International Conference on Ocean, Offshore and Arctic Engineering, Busan, South Korea, 2016. 26. Kim, S. J., Spernjak, D., Holmes, S., Vinayan, V., Antony, A., "Vortex-Induced Motion of Floating Structures: CFD Sensitivity Considerations of Turbulence Model and Mesh Refinement", Proceedings 34th International Conference on Ocean, Offshore and Arctic Engineering, St. John, Canada, 2015. 27. Wu, G., Ma, W., Kramer, M., Kim, J., Jang, H., O'Sullivan, J., "Vortex Induced Motions of a Column Stabilized Floater, Part II: CFD Benchmark and Prediction", DOT-2014. 28. ISO 19901-7, "Petroleum and natural gas industries - Specific requirements for offshore structures - Part 7: Stationkeeping systems for floating offshore structures and mobile offshore units", DIS, 3rd ed., 2017. 29. ISO 18692, "Fiber ropes for offshore stationkeeping – Polyester", 1st Edition, 2007 30. ISO 20438, "Ships and Marine Technology – Offshore mooring chains", 1st Edition, 2017. 31. ISO 10425, "Steel wire ropes for the petroleum and natural gas industries—Minimum requirements and terms for acceptance", 2nd Edition, 2003. This is the same as API SPEC 9A, 25th edition, 2004. 32. API RP2A, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms—Working Stress Design", 21st Edition, 2000, Errata and supplement 2005. 33. DNV RP-C203, "Fatigue Design of Offshore Steel Structures", 2011.