Variability of spar motion analysis against design methodologies/parameters
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abstract
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 from Morison equation. 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 mooring lines and risers are replaced by a set of massless springs. A comprehensive sensitivity study of the simulation results against various analysis/environment parameters was carried out to better understand the underlying physics and the role of each parameter. In all cases considered, the wave frequency components remain almost the same, while the slowly-varying motions change from one case to another. It is particularly underscored that the equivalent static wind modeling may lead to significant underestimation of slowly varying surge/sway and pitch/roll responses for not including the effect of resonance.
