The paper discusses results from a detailed stochastic dynamic fatigue analysis of a new deepwater platform concept designed for North Sea conditions. The platform concept, developed by Norsk Hydro a.s., is a fixed steel structure in a water depth of 335 m and with a deck load capacity of 35000 tonnes. The platform substructure, shown in Fig. 1, is a tripod trusswork design consisting of a central triangle tower connected to three inclined legs at elevation 45 m below the still water level. The fundamental structural natural period is in the range of 4 to 5 s, indicating significant dynamic amplification of the quasi-static wave-induced response, and fatigue will be an important design consideration. Parameters governing the fatigue life of the structure are identified and the fatigue sensitivity to these parameters is investigated. The results indicate that fatigue is important for structural elements in and above the intersection area between the main tower and the inclined legs. Important parameters governing the fatigue life are found to be the damping value, geometric stress effects, dynamic amplification, structural geometry at waterline, wave load modeling and method used for linearization of nonlinear wave forces.

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