ABSTRACT

An API task group has developed the process for assessment of existing platforms to determine fitness for purpose; this has been released as a draft supplement to API RP 2A. The process is prescriptive, with criteria based upon consequence of failure in terms of life safety and environmental impact. Platforms are assessed according to either design basis check or analysis. Two levels of analysis may be used, with increasing complexity and decreasing conservatism design level and ultimate strength. Design level analysis is similar to that used in new platform design, while ultimate strength analysis attempts to provide an unbiased estimate of platform capacity.

This paper describes the draft API assessment process and associated acceptance criteria. The criteria are based on over forty years of successful operations, field experience, and detailed investigation of platform failures and survivals in past hurricanes, notably hurricane Andrew. The draft process is currently being tested through a joint industry project, with over 20 participating operators and contractor.

BACKGROUND

In 1992, an API task group was established with the objective of developing guidelines for assessment (i.e., demonstrate fitness for purpose) of existing platforms. The impetus for developing assessment guidelines was the evolution of platform design practice over the past forty years, resulting in new platform design standards which are considerably more stringent than those used earlier. Concerns were raised by the Minerals Management Service (MMS) regarding the adequacy of older structures, prompted also by the expiration of initial operating permits and by the occurrence of significant environmental events such as hurricane Juan (1985) and the Loma Prieta earthquake (1989). Hurricane Andrew (1992) provided further justification for establishing assessment guidelines, as well as a large amount of reliable information useful in this regard.

An API task group, chaired by Kris Digre of Shell, was directed to develop a supplement to the API document, "Recommended practice for Planning, Designing and Constructing Fixed Offshore Platforms - Working Stress Design" (RP 2A - WSD) 20th edition. The supplement was to provide guidance on assessment of fried offshore platforms located in US waters, addressing metwean, earthquake, and ice loading. The task group effort was divided among seven supporting work groups:

  1. assessment process,

  2. condition assessment,

  3. loading,

  4. structural,

  5. foundations,

  6. operational/mitigation measures, and

  7. acceptance criteria.

Considerable interaction was required between the groups, particularly in establishing criteria.

The task group completed the work within one year and issued a draft supplement prior to the International Workshop on Reassessment and Re-qualification of Offshore Production Structures held in New Orleans in December of 1993 [1]. Discussion of the draft supplement was a principal topic at the workshop, and the recommended prescriptive assessment procedure was widely endorsed [1].

The purpose of this paper is to describe the assessment process developed by the API task group and contained in the draft supplement to RP 2A, to discuss the evolution of this assessment process, and to provide justification for the specified acceptance criteria. Discussion will be restricted to the metocean assessment process and criteria; ice and earthquake loading assessments are addressed in [2].

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