Development of a Methodology for Safety Assessment of Existing Steel Jacket Offshore Platforms

Abstract

ABSTRACT This paper describes a methodology for assessment of the safety of existing steel jacket offshore platforms. The methodology is based on a four-level evaluation process. The first two levels consist of screening of the platforms into one of three categories: "fit for service", "unfit for service", and "marginal", based on qualitative evaluation and simplified quantitative analyses. The higher levels of assessment require detailed computer analyses. Capacity of the platform and potential consequences associated with the loss of serviceability of the platform are key parameters in this methodology. The capacity characteristics are expressed through a Reserve Strength Ratio (RSR = Ultimate Capacity / Minimum Reference Force). The consequences are expressed through evaluations of life, property, resources, productivity, and environmental implications associated with the proposed platform operations. A study is currently (1990) underway to present this methodology in the form of a computerized knowledge-based expert system. The organization of the prototype expert system is presented. INIRODUCTION Steel platforms have been in use for more than 40 years in the Gulf of Mexico and other offshore areas around the world. At present, there are more than 4,000 offshore platforms in the United States. In the Gulf of Mexico alone in 1987, there were 3,080 platforms, of which 73-percent were in water depths up to 100 ft. and 27-percent were in water depth more than 100 ft. [1]. The structural configuration of these platforms ranges from single-well caissons to multi-leg jackets. In 1987, there were 1,969 wellhead platforms, 355 tender-assisted platforms, and 756 self-contained drilling & production platforms in the Gulf of Mexico [1]. The wellhead platforms are single well caissons or multi-well 4-legged jacket platforms. The tender assisted platforms are usually 4-legged jackets and the self-contained platforms are mostly 8-legged jackets. The design criteria of these platforms has changed significantly over the past 40 years. Many of the early platforms were designed for 25-year return period waves, whereas, since about 1969, the majority of platforms have been designed for 100-year return period waves. The American Petroleum Institute (API) first introduced recommended practices for offshore platforms (API-RP-2A) in 1969, to provide consistency in design, fabrication, and installation of offshore platforms. API has periodically updated the recommendations, and the 18th edition [2] now forms the general standard for design of Gulf of Mexico platforms The present day Minerals Management Service(MMS), introduced the Outer Continental Shelf (OCS) Order No. 8 in 1979 [3]. More recently, the MMS introduced updated requirements, which largely follow the API guidelines with some additional requirements [4]. Present OCS requirements define the need for periodic assessment of existing platforms for their continued use (Clause 250.142 a)