API RP 2 SK - Station Keeping - An Emerging Practice
Authors: Petruska David Joseph , Castille Craig Thomas , Colby Craig , Stiff John James , Stone Barbara , Wisch David J. 
Published: October 2008

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).