API RP 2SK 4th Edition - An Updated Stationkeeping Standard for the Global Offshore Environment
Authors: Shu Hongbo , Yao Aifeng , Ma Kai-Tung , Ma Wei , Miller Jonathan 
Published: April 2018

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: Design limit states are revised to be aligned with the latest ISO 19900 DIS (2018) [10].New normative guidelines for mooring design Robustness Check have been added.Design criteria for MODU moorings has been restructured, encouraging risk-based design criteria selection (QRA). Drag anchor design FOS has also been updated.Provides specific guidance for Response Based Analysis method.Developed new informative guidance on how to conduct chain OPB/IPB and Tension-Tension fatigue analysis.Initiated design criteria for wire rope bend-over sheaves.Initiated new design considerations against mooring line trenching on sea floor.Updated corrosion allowance ranges, selecting criteria based on latest industry data and findings.Updated guidance on how to include vortex induced motions (VIM) effects in mooring design.Added new recommendations to include mooring system life cycle considerations (e.g. installation) in design stage.Thruster-assisted mooring (TAM) design criteria in the North Sea region updated per UK OIS 2013 [18].Regional guide for cyclonic region aligned with APPEA MODU Mooring Guide (2016) [9].

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.