Hull/Mooring/Riser Coupled Spar Motion Analysis with Buoyancy-Can Effect
Additional Document Info
Nonlinear hull/mooring/riser coupled dynamic analyses of a classic spar designed for 6000-ft waterdepth are conducted in the time domain. The first-order and second-order 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, 14 chain-polyester-chain mooring lines and 23 buoyancy-can-supported vertical risers, are calculated using the Morison equation. The numerical simulations were conducted for 100-yr Hurricane condition with non-parallel wind, wave, and current. The fully coupled results are compared with those of uncoupled quasi-static analysis and semi-coupled dynamic analysis. In all cases considered, the wave frequency components remain almost the same, while the slowly-varying motions change from one case to another mainly due to mooring/riser damping. Several different riser models are also investigated. The first one modeled risers as an elastic rod extended to the keel and free to slide in vertical direction with constant tension, while restricted in horizontal direction. It is seen that neglecting the portion of risers inside the spar hull results in the over-estimation of the pitch responses. Secondly, a simple new buoyancy-can modeling is tested to overcome the problem. The new modeling treats the riser part above keel as inverted pendulum, so that it can give additional restoring moments in roll and pitch mode. The new riser model significantly reduced the maximum pitch/roll responses. The efficacy of this simple approach is further confirmed by more rigorous modeling with risers extended inside moon-pool.