Spar platforms with cylindrical shape and constant cross section area may experience resonant heave motions in sea states with long peak periods, which are probably excessive for riser integrity due to its low damping and relatively low natural heave period. Changes to hull shape and cross-section that produce more benign heave behaviour were discussed by some researchers in the past. The heave resonant response may be reduced by several means: (1) increasing the damping of the system, (2) the natural heave period kept outside the range of the wave energy, and (3) further reducing the linear heave excitation forces. Conventional offshore hydrodynamic analysis considers a structure in waves, and evaluates the linear and nonlinear loads using potential theory. Viscous damping, which is expected to play crucial role in the resonant response, is an empirical input to the analysis, and is not explicitly calculated. In this paper, the coupled responses of the configurations of several alternative hull shapes of the spar are investigated in frequency domain. An iterative post-processing procedure is applied to incorporate nonlinear viscous effects into the potential solutions. Compared to the classical spar, the spars with the alternative hull shapes are all found to effectively shift the peak heave RAOs outside the range of the wave frequencies in the ocean, and consequently reduce the resonant heave motions through all the above three means. The calculations also reveal that the resonant heave motions are further suppressed by the viscous effects.
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