Offshore Platform Reliability Acceptance Criteria

Abstract

Summary Development of assessment, inspection, and maintenance (AIM) programs for requalification of existing offshore platforms requires programs for requalification of existing offshore platforms requires definition of criteria to evaluate the adequacy and acceptability of the programs. This paper proposes an approach to judge fitness-forepurpose of programs. This paper proposes an approach to judge fitness-forepurpose of existing platforms and their proposed AIM programs. Introduction Today, about 3,500 major fixed platforms are installed on U.S. continental shelves. About 3,000 of these structures are in the Gulf of Mexico, and roughly one-third are more than 20 years old. Most of the older platforms are located in water depths less than 100 ft {30.5 m]. Development of Gulf of Mexico offshore oil and gas reserves began in the late 1940's with the installation of platforms in 20 ft [6.1 m] of water. Since then, platforms have been installed in depths to 1,350 ft [412 m]. During the past 50 years, design, construction, and operation technologies and practices have changed significantly. For example, platforms are now designed for wave forces that are a factor of two to four times those of first-generation platforms. Tubular joints have seen comparable changes. Many early-generation platforms are faced with technological obsolescence in addition to strength degradation resulting from aging. Many of the existing structures, particularly the newer ones, have been well designed and maintained. Inspections of the older structures, however, have disclosed a wide variety of defects and damages ranging from improperly welded joints to underdriven piles and from cracked joints to extensive corrosion. Platforms owners, operators, and regulators are faced with the task of extending the operating lives and ranges of many of these older platforms. To do this, AIM programs that will qualify structures for continued service and indicate those that need to be re-moved from service are necessary. This paper develops criteria to determine the suitability for service of existing platforms. Approaches During the last three decades, a variety of approaches have been developed, explored, and applied to define tolerable risks associated with offshore drilling and production platforms. Recently, risk-decision approaches have been expanded to full-scope (structure, equipment, personnel, and operations) and life-cycle (concept, de-sign, construction, operations, and decommissioning) evaluations. Risk-decision evaluation approaches have been explored and applied in the Norwegian and U.K. sectors of the North Sea, and very recently, full-scope, life-cycle risk-decision evaluation approaches have been applied in studies of innovative deep-water drilling and production platforms. Two fundamental approaches to defining desirable, acceptable, or tolerable risk levels have evolved from this experience. The first approach, utility evaluation, is based on an assessment of the positive and negative utilities associated with risks. The utilities can be measured in several ways. Measurement of costs and benefits in monetary terms is one common example of this approach. The objective of this approach is frequently called "total cost minimization. "The second approach, experience evaluation, is based on an experience and calibrations with standards of practice. The general premise of this approach is that society, the profession, and industry over am and through experience define tolerable risks-i.e., acceptable combinations of likelihoods and consequences. These two approaches are complementary. No one approach is best for all purposes. They both should be used to determine risk acceptability. Emphasis should be placed on the processes for reaching decisions on tolerable risk levels and on the processes that will facilitate communications concerning potential consequences and likelihoods.