RMS-9: Second-Order Load and Response Statistics for Tension-Leg Platforms

Vertical motions of a tension leg platform are studied. Contributions from both first- and second-order sum-frequency loads are included. The various physical assumptions underlying the fire forcing mechanisms and structural properties are reviewed. Statistics of platform motions and accelerations are obtained. Moment statistics arc found through eigenvalue analysis, which recasts the second-order response as a quadratic sum of independent normal random variables. Rates of extremes and fa­tigue damage are then estimated analytically from a non-Gaussian (Hermite) random process model. Tether tensions and platform motions show clear effects of force nonlinearity, both on response variance and higher moments. These effects are found to be most pro­nounced on acceleration responses1 which amplify sum-frequency contributions. Nonlinear effects are also greater for smaller-period seastates. They are reduced somewhat as the damping decreases; however, responses may remain significantly non-­Gaussian in low-period seastates even with damping values as low as 0.005. In the companion report (RMS-10), these results are weighted over a North Sea scattergram to find long-term extreme and fatigue statistics.