In this paper, the second-order mean and double-frequency wave loads on the ISSC TLP in regular waves are investigated. The second-order wave loads on the compliant (massless-spring-supported) TLP are compared with those on the stationary TLP in order to see the effect of body motions. A higher-order boundary element method (HODEM) with free-surface Green function is used for the computation of the first- and second-order hydrodynamic loading. In particular, the capability of HODEM in calculating the second spatial derivatives of the first-order potential on the body surface IS demonstrated. The numerical results for the stationary ISSC TLP are compared with Matsui et al"s (1992) experimental results, and reasonably good agreement is observed.
Most of deepwater compliant offshore platforms such as tension-leg platforms (TLP) are designed so that their natural frequencies are away from typical wave frequencies in order to avoid unacceptably large wave-induced motions. Therefore, in order to predict those resonant high- and low-frequency responses in a reliable manner, designers need to compute with reasonable accuracy the second-order sum- and difference-frequency wave loads as well as damping. In monochromatic waves, the second-order force consists of two parts: mean and double-frequency excitations. The second order mean forces can be obtained entirely from the first-order results, hence the relevant modules can straightforwardly be extended from the first-order diffraction/radiation programs. On the other hand, the second-order double-frequency forces need the computation of the second-order potential, which requires substantial human effort and computational time. The second-order double-frequency wave loads on the full geometry of a stationary TLP were calculated in Kim (1991), Lee et al. (1991), and Molin and Chen (1990) using the constant panel method (CPM). Chau (1989) used a higher-order boundary element method to compute double-frequency forces on four columns of a stationary TLP.