Abstract

A probabilistic approach is proposed to derive the design wave loads on bilge keel structures in irregular seas. An equivalent drag coefficient is introduced to account for both drag and inertia contributions in the wave loads. For each irregular sea state, the most probable maximum (MPM) value of the wave load is expressed in terms of the equivalent drag coefficient and the MPM value of the relative flow velocity. A generalized Keulegan-Carpenter (KC) number is also introduced in terms of the MPM value of the relative flow velocity and the associated mean up-crossing period of the relative velocity, which can be directly obtained from 3D diffraction analysis. The KC-dependent equivalent drag coefficient is determined by fitting a modified hyperbolic relation to the values derived based on the total wave load measurement from the model tests in irregular sea states of a Floating Production Storage and Offloading (FPSO) system design. The proposed new approach involves the application of an equivalent drag coefficient for each irregular sea state specified in the metocean report for FPSO design. The wave load formula is directly applicable for calculating short-term extreme wave loads for ultimate strength design. A 2-parameter Weibull distribution is found to fit well the measured short-term wave load distribution. The long-term probabilistic load distribution for fatigue design can be developed by summing the short-term distribution of each sea state weighted by the probability of occurrence. Thus, efficient, frequency-domain wave load calculations can be performed for both ultimate and fatigue state designs of the bilge keels. A practical application is presented of the proposed approach to a deep water Brazilian FPSO project.

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