Developments In The Assessment And Requalification Of Offshore Platforms
Authors: Bea R.G. , Craig M.J.K. 
Published: October 2008

This paper discusses developments in the assessment and requalification of offshore platforms. A four-level assessment approach is proposed that attempts to qualify platforms for service at the simplest level. These developments are illustrated with results from platform requalification experiences in the Gulf of Mexico and offshore California.

INTRODUCTION

There are approximately 6,000 platforms located on the world's Continental Shelves. Approximately one-third of these structures are being called upon for extended service or re-use.

There are no generally accepted approaches to either assessment analyses or requalification procedures. This paper pro tides an engineering approach that is intended to help make assessment and requalification efforts more effective and efficient.

APPROACH

A general approach to the assessment and requalification of offshore platforms is summarized in Fig.1 The initial steps include selection of a platform for reassessment and requalification, performing a condition survey according to API RP 2A guidelines1, evaluating the results, and then developing an inspection, maintenance, and repair program2 that will provide the basis for the assessment and requalification.

The next step is the definition of the loading and capacity characteristics of the of the structure, expressed with the Reserve Strength Ratio (RSR). The RSR is the ratio of the ultimate lateral load capacity of the platform (Ru) to a reference lateral loading (SR):(Mathematical Equation) (Available in full paper)

SR is based on site, platform, and operations specific environmental conditions. The platform loadings are determined according to current API RP 2A guidelines.1

An attempt

Fig. 1- Assessment and requalification approach based on evacuation of platform Reserve Strength Ratio (RSR)(Available in full paper)

Is made to eliminate all 'biases' in the characterization of conditions and the computation of loadings and to develop a best estimate analyses of loadings and structure performance characteristics. The required detail of the environmental conditions and loading analyses increases as the level of detail of platform capacity analyses increases.

Four levels of analyses of increasing detail and difficulty are used to determine the RSR:Level 1- scoring factor analysesLevel 2- simplified limit equilibrium AnalysesLevel 3- modified elastic state-of-the practice AnalysesLevel 4- state-of-the-art nonlinear analyses

The primary objective of this approach is to allow assessment and requalification of platforms with the simplest method possible. There are more stringent Fitness-For-Purpose

(FFP) criteria associated with the simpler methods because of the greater uncertainties associated with these methods and the desire to minimize the likelihood of false positives associated with the simplified methods.

If a platform that is being reassessed fails to pass a screening Level, its performance characteristics must be investigated in ore detail at a higher Level until it either passes or it cannot be requalified.

Level 1 RSR is based on five factors that address the platform capacity and four factors that address the platform environmental and operating loadings:3