Numerical Prediction of Spar Motions Considering Top Tension Riser Stiffness

Spar motions are often predicted without explicitly considering the stiffness effect of the buoyancy can of top tension risers (TTRs). This may result from the inability of most motion prediction programs to take this effect into consideration. The forces exerted by the TTR buoyancy can on the spar provide a net beneficial righting moment. Hence, neglecting the effect of TTR stiffness could produce excessive predicted spar motions, particularly maximum pitch angle. The derived spar configurations based on such predicted motions are likely to be conservative and not optimal. This paper proposes a simple “Linearized P-Delta” model to account for the effect of TTR stiffness on spar motions. The predicted motions using the proposed model are compared with the model test results for a production truss spar in the Gulf of Mexico. The comparison results indicate that the proposed model is quite rigorous and correlates reasonably well with the model test results.