AbstracbThe results of an experimental investigation of thermally induced parallel edge cracks in a half-plane consisting of brittle material are reported. A free edge of a glass plate heated to a uniform temperature, was brought in contact with a liquid bath cooled by dry ice. The thermal contraction of the boundary layer produced interacting tension cracks. When no initial cracks existed, a large amount of potential energy was released in the formation of a few cracks which extended in a dynamic manner beyond the thermal layer. To prevent this type of cracking, initial cracks were formed before thermally inducing crack growth. This resulted in stable crack growth which qualitatively agreed well with the previously developed theory. The quantitative comparison was also reasonably good.

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The Mad Dog Floating Production System (FPS) will be the first truss spar to use polyester for a permanent mooring system. The breaking strength of the polyester ropes is also the largest ever-made being in the 2000 mT range. As such, prototype testing to validate the breaking strength capacity of the spliced ropes was important along with gathering performance data of the ropes to be used in the mooring design and global performance analyses of the FPS. Since this is the first spar to use a polyester mooring, and since loop currents typically govern the mooring of a spar in the Gulf of Mexico, a better understanding of the "static drift stiffness" or extension of the rope was required. Thus far, polyester moorings have been predominately used by Petrobras in the Campos Basin for semi-submersible FPS and Floating Production, Storage and Offloading (FPSO) units and thus only the dynamic and drift stiffness has been important. Much data is available for these two stiffness, but very little on the static drift stiffness was available. Consequently, a model had to be developed along with procedures to test the ropes to derive this stiffness. This paper will discuss the prototype test plan, which basically follows API RP 2SM but with several deviations, in particular to obtain more dynamic stiffness and static drift stiffness (extension) data over a range of mean loads, load ranges and rate of loading. In addition, axial tensioncompression fatigue testing was conducted explicitly to the mean load, range of loading and number of cycles expected to occur to the mooring while in-service to confirm this is not a problem since tension did fall below 5% of MBL. Finally a stiffness model will be presented that can be used for mooring design / global performance analyses for FPS using a polyester mooring system. Information presented in this paper will help designers of polyester mooring systems and also should impact the future revision of API RP 2SM. Introduction Petrobras has designed and installed numerous polyester mooring systems to semi-submersibles FPS and Floating Production, Storage and Offloading (FPSO) systems (Costa, 2001). However, to date, polyester has not been used in a permanent mooring system outside of the Campos Basin. BP Exploration & Production Inc. ("BP") and the Mad Dog project partners changed that when the taut-leg polyester mooring system was installed on the truss spar in early 2004. The Mad Dog project was facing a significant hurdle in trying to keep development cost down so the company and the partners, BHP Billiton Petroleum (Deepwater) Inc. ("BHP Billiton"), and Union Oil Company of California ("Unocal"), could sanction the project. In order to control cost, it was important that the hull be fabricated and transported to the Gulf of Mexico as a single piece. The size and weight of the hull was already challenging the capabilities of the worldâ??s heavy lift vessels and in addition, payload was increasing to meet topsides requirements. Thus the project team investigated using a taut leg polyester mooring system.

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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.