Comparative Risk Assessment of Alternative TLP Systems: Structure and Foundation Aspects

Abstract

Offshore operators can choose from a wide variety of alternative production systems including those based on fixed platforms, tension leg platforms (TLPs), compliant towers, floating production systems, and remote subsea systems. These systems involve varying degrees of technical innovation and technological risk. Life cycle costs, including initial costs and operating costs over the life of the project, need to be carefully considered to define systems that will minimize cost and maximize return on investment. The life cycle includes all stages of the project, including design, construction, operation, and decommissioning. To address decision-making issues in the selection of alternative systems, eighteen sponsoring organizations under the leadership of Amoco helped define the Methodologies for Comparison of Alternative Production Systems (MCAPS) project. Conducted during a two-year period (1988-1990), this project developed and illustrated an engineering procedure to assist the process of making rational comparisons among design alternatives for offshore production systems. The example developed to illustrate application of MCAPS was a TLP concept designed for Gulf of Mexico conditions. Four TLP alternatives were analyzed and compared; these involved three production riser alternatives and two tendon system alternatives. This paper addresses the structural and foundation aspects of the two tendon system alternatives. A companion paper (Stahl et al., 1991) addresses the production riser alternative, the alternative system economic evaluations, and the comparison processes. The structure aspects addressed in this paper include both first-phase coarse qualitative and quantitative evaluations, and second-phase detailed quantitative evaluations. The detailed quantitative evaluation is illustrated with structural damage in extreme conditions, including potential overloading of the hull, tendons, and the foundation piles. The paper concludes that MCAPS is not a simple process. It is an intensive interdisciplinary process requiring good teamwork and extensive familiarity with the system being analyzed. The judgmental elements of the process are considerable. The potential benefits of the process include: an improved understanding, qualitatively and quantitatively of risk mechanisms; identification and mitigation of hazards; improved cost-effectiveness of designs; and improved safety. The specific risk numbers generated are less important than the design improvement insights that are developed; these numbers can only serve as a guide to focus attention on those aspects of the project which generate the most risk and to indicate where cost-effective risk reduction measures can be implemented.