Nonlinear hull/tendon/riser coupled dynamic analyses of a TLP (tension-leg platform) designed for 3000-ft water depth are conducted in the time domain. The first-order and second-order sum- and difference-frequency wave loads and other hydrodynamic coefficients for the hull are calculated from a second-order diffraction/radiation program, while the forces on slender members, 8 vertical tethers and 8 vertical tensioned risers, are calculated from Morison formula. The numerical simulations were conducted for 100-yr Hurricane condition with non-parallel wind, wave, and current. The results are compared with those of uncoupled quasi-static analysis and semi-coupled dynamic analysis, in which the tethers and risers are replaced by a set of massless nonlinear springs. A comprehensive sensitivity study of the simulation results against various analysis/environment parameters was carded out to better understand the underlying physics and the role of each parameter. It is particularly underscored that the viscous damping from tethers and risers may be significant and the equivalent static wind modeling may lead to underestimation of slowly varying surge/sway responses.
Tension-leg platforms have been increasingly popular as an economic and reliable oil production platform in the deep areas of the Gulf of Mexico. As water depth increases, the portion of the tether and riser mass becomes larger against the hull mass, and the resulting inertia and damping effects from them are expected to be important. In this case, to accurately account for the inertia and damping effects of tethers and risers on the hull motions, hull/tether/riser coupled dynamic analyses need to be employed in time domain. For some systems, the coupling effects may magnify the extreme hull responses, however in most cases, the coupling effects more likely lead to smaller extreme responses due to additional riser/mooting damping, which results in less expensive tether/riser system.
