The omni-directional fenders will be fine. The stiffness of fenders between Spar and WEC for allowing the relative motions along the spar axial direction could be set to ensure that the natural frequencies of the relative roll/pitch motions bweteen the spar and WEC is higher than the interested frequencies of the responses. For example, these natural frequencies are higher than the wave spectrum's finishing frequency (beyond it the wave energy is less than 0.5% of the total wave energy). The size could be slightly larger than the gap between the spar and WEC. The damping and friction coeeficients should be decided by the properties of your model.
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