Second-order sum-frequency wave loads on large-volume structures

Abstract

The second-order sum frequency wave loads on the stationary ISSC Tension-Leg Platform (TLP) are obtained by the three-dimensional boundary integral equation method. The numerical results for the full TLP are compared with approximate estimations based on the results for a single column (Kim and Yue5) or four columns only (Chau11), where both or the latter part of the wave interaction between four columns and the influence of pontoons are neglected. Unlike the first-order problem, it is shown that both wave interaction and pontoon effects are generally important to the calculation of second-order sum-frequency wave loads even at relatively high frequencies. Particularly, the vertical forces and pitch moments can be greatly amplified in consequence of the second-order interaction between columns, and the resulting standing wave effects. Therefore, it is not conservative to estimate the springing wave loads on a TLP by such approximation methods despite the substantial savings in computing, time. To support this rather surprising conclusion, the vertical forces are also calculated based on the asymptotic formula of the second-order diffraction potential which is valid at large depths (Newman12).