A Systematic Optimization Process for Deepwater Platforms

ABSTRACT In the last decades several deep water fixed platforms have been installed in the Gulf of Mexico. With the Mississippi Canyon 109 platform (1030' depth) a systematic approach was successfully developed to optimize the design of the structure. The optimization resulted in a cost reduction in the order of 20%. This platform was installed in the summer of 1991. The paper describes the optimization process and the results. The optimization is in terms of final installed cost, rather than just weight or other simplistic parameters. The approach emphasizes fabrication and installation aspects. It focuses on selecting and designing the optimum concept and architecture, rather than optimizing and analyzing a pre-selected concept. This method has now been used for the design of the Viosca Knoll 989 structure (1285' depth) with similar success, and for the conceptual design of a 3000' water depth tower. INTRODUCTION The objective of optimization is to design for the minimum final installed cost. A platform may be light, have excellent in-place performance but be very expensive to build; or it may be economical to fabricate but expenSive to install. Therefore, a genuine cost optimization exercise can only be achieved when the costs of materials, fabrication, and installation are considered. Strictly speaking maintenance and even removal cost should be included. However, with the exception of cathodic protection, these items seem to have little impact on optimization. This understanding sets the stage to define the principles for a successful optimization. PRINCIPLES OF OPTIMIZATION The first principle of a successful optimization is a direct corollary of designing for total installed cost. It is that the optimization must consider the development in its entirety. Simple optimization studies limited to one aspect such as weight, or installation time are unlikely to lead to the overall lowest platform cost. The second principle for optimization is to distinguish between design and analysis. The developments of computer power and analytical techniques have provided tremendous analytical capability. However, even though design software does exist, computers are poor in designing in an architectural sense. Computers analyze and can compute a local optimum for maximum utilization of a given member, they can not, or are at best poor at, selecting the best architectural configuration for major offshore structures. This is because the optimum architectural design involves many decisions which are based in a practical knowledge of the fabrication and installation process and which can not be effectively incorporated in a piece of software. The third principle for a successful optimization is to select for the design team, a small group of flexible high caliber individuals. By including in the team several individuals it is possible to include strong experience in design, fabrication, installation, and analytical techniques. It is also essential that these individuals are flexible in their approach. They should not be wedded to preconceived ideas, be prepared to look at everything, and be ready to accept the findings of the optimization work. The fourth and final principle of optimization is that optimization efforts need to be planned.