Structural Assessment Of Existing Platforms
Authors: Kallaby Joseph , Lee Griff , Crawford Carter , Light Larry , DoIan Dan , Chen J.H. 
Published: October 2008

This paper presents guidelines for structural and foundation assessment of existing, jacket type, pile founded offshore platform structures. The paper underlines the intent of the Assessment Guidelines and discusses the need for them. The essential information required for conducting assessment is then listed, followed by the structural assessment and finally foundation assessment. Structural assessment presents the three proposed levels of evaluation: Screening, Design, and Ultimate Strength. Numerous references are listed at the end of the paper to assist the reader to obtaining more detailed information that could not be covered in this paper.

BACKGROUND

The Gulf of Mexico is the birthplace of the offshore platform in the early forties. By now there are over 4,000 such platforms. Their assessment has always been carried out, but only on an ad hoc basis as the need requires. Recently, the American Petroleum Institute (API) recognized the need for a uniform, coherent, consistent, integrated and defensible guidelines for assessment of existing platform.

In the fall of 1992, API charged Task Group 92-5 to draft Section 17, as a supplement to API RP2A-WSD for circulation to the industry, regulatory and interested parties for comment. All further reference to RP2A is the WSD guidelines (Ref. 56). The draft was completed and issued in November 1993 (Ref. 30). Seven work groups addressed different aspects of the assessment process. Work Group 4 addressed structural assessment and Work Group 5 the foundations. Work Group 4 consisted of five separate task units to address each of the platform categorization, data gathering, screening level, design level and ultimate strength level. Each task unit consisted of several members, whose leader is a co-author of this paper, along with the leader of Work Group 5.

This paper is based on, and is a tribute to their efforts.

UNDERLYING INTENT OF THE ASSESSMENT GUIDELINES

It is not uncommon to read either too much or too little into a guideline. It is helpful to have a clear understanding of the underlying intent of Draft Section 17 guidelines, outlined as follows:It is not required that a structure be analyzed for ultimate strength. If it passes the designated Design Level environmental loading, no further analysis is required. If it passes the Screening Level, no further analysis is required.If a structure does not pass the Design Level, it can be analyzed for the Ultimate Strength Level environmental loads using elastic analysis, with safety factors removed. If the structure passes, strength and stability, since permanent deformation is allowed. In this analysis, members and joints unload, as limited by their structural capacity, calling on other framing to muster their reserve strength to avoid collapse. If collapse is forecast, the structure must be strengthened, its use changed or it should be abandoned.It is just as permissible, if the engineer so elects, to bypass the Screening and Design Levels, and perform only Ultimate Strength Level assessments. If the structure passes, no further action is required. For seismic regions, for example, Design Level analysis is neither suggested nor required, while Ultimate Strength is.