Preliminary design of floating offshore structures involves determining structural dimensions able to provide sufficient buoyancy to carry the required topside, at the lowest possible cost, while satisfying various stability, strength, installation, and response requirements. A novel optimization strategy, capable of carrying out the preliminary design of floating offshore structures, is presented in this paper. The genetic algorithm based strategy searches within prescribed parameter limits for the most cost effective design, while ensuring the design conforms to the constraints given. The design of a truss spar is used to illustrate how the strategy can be applied. The topside weight, design wind speed, maximum wave height, etc are input along with constraints such as, maximum draft at floatoff, maximum heel angle, allowable stress in the truss and limits on pitch and heave period and response. Using empirical estimates for hull weights and simplified response calculations, the strategy is then able to rapidly determine parameters such as hull diameter, hard tank depth, length of keel tank, total length and truss leg diameter such that the total cost of the structure is minimized. The strategy allows for the preliminary design phase to be completed in only a few seconds, while providing initial weight and cost estimates.

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