A Review Of Operations And Mitigation Methods For Offshore Platforms
Authors: Turner J.W. , Wisch D.J. , Guynes S.J. 
Published: October 2008

API-RP2A "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms" will be updated with a new section "Assessment of Existing Platforms to Demonstrate Fitness For Purpose," which is currently in "draft" status. This paper addresses specific mitigation and operation actions that can be used to bring a platform structure into compliance with these guidelines. These actions can generally be categorized as:reduction of loading,increasing of strength, and/orreduction of consequences.

Introduction

The American Petroleum Institute (API) formed a Task Group (TG) in 1992 to develop procedures to assess existing offshore platforms to demonstrate Fitness-For-Purpose (FFP). As part of this TG, a sub group was formed to develop recommended practices related to operations and mitigation methods that could be employed on platforms that would allow them to meet FFP guidelines. The intent of this paper is to provide background information and a detailed list of references that will assist engineers in developing platform specific actions. This paper is referenced from the draft version of the assessment guidelines (Ref. 1) of API-RP2A and is offered as additional commentary by the members of the API sub-group.

The approaches described here are a compilation of methodologies that range from the very simple and obvious to those which may require significant engineering and construction efforts. Many of the references will guide the practicing engineer to historical projects that faced the issues of required upgrades and/or mitigation of consequences. The specific tasks listed benefit one or more of the following broad categories:reduction of loading,increasing of strength, and/orreduction of consequences.

Where appropriate for each of the actions listed, a general description of the benefits, possible negative consequences, interactions, and relevant references are given.

Reduction of Loading

Reduction of Vertical Gravity Loading:

A prime example that can be accounted for in the assessment of existing platforms is to accurately determine actual deck weights. Most platforms, when they are designed, use area live loading procedures to account for undetermined facility designs. Once installed, the actual equipment loading may be substantially less than what was used in the original structural design. Additionally, the structure may have been designed for a platform rig, but now accommodates cantilever jack-up rig drilling operations, which do not impose any direct gravity loading on the platform.

In many instances, revised upper bounds of deck payload can be established for future operations which can be subsequently used in a platform assessment. As an example, a specific jacket may have been originally designed for a 21,000 kip deck payload, but henceforth could be limited to only 13,000 kips. Appropriate operational procedures would then be necessary to ensure that the revised deck payload criteria are observed. Other related jacket vertical load reductions can come from a partial removal of unneeded deck structures. Conversely, jacket vertical loading can also be reduced by increasing platform buoyancy. Obviously, any net increases in load over the original design must be accounted for in the assessment process.