The efficiency of spar vortex induced motion (VIM) suppression strakes has been found to be dependent upon the height of the strakes, the number of strakes in the helix, the pitch of the helix and the extent of strake coverage. As spars have increased in diameter, the height of the VIM suppression strakes has grown to maintain strake efficiency. This has introduced new challenges to the structural design of the strakes as a result of the increased hydrodynamic pressure loads on the strakes and the effect of global bending of the spar hull. The connection of the strakes to the hard tank outer shell often has high stress concentration, which in combination with the cyclic hydrodynamic loading, tends to result in fatigue considerations governing the design of these connections. As a result, finite element models, both global and local, are required to perform detailed stress analysis for the structural strength and fatigue assessment of the strakes and surrounding hard tank structures. This paper will use a generic, large diameter truss spar as an example to outline the methodology and key elements used for strength and fatigue analyses of the strakes. In particular, an efficient unit load method will be presented, which has been used to de-couple the finite element stress analysis and the actual hydrodynamic pressure load analysis for fatigue damage calculation. This method incorporates a time domain global analysis of spar hull global bending loads, and a frequency domain analysis of wave induced hydrodynamic pressure loads on the strakes. Final fatigue damage is calculated based on combined principal stresses using the spectral method. Analysis results and findings are presented for both the in-place condition and the wet tow condition. By applying this procedure, Technip has been able to repeatedly design robust and reliable VIM suppression strakes in a cost efficient process, within the constraints of tight fabrication schedules.

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