Hull/Mooring/Riser Coupled Spar Motion Analysis: Sensitivity against Methodological/Environmental/Empirical Parameters
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2012 by the American Society of Civil Engineers. All Rights Reserved. For nonlinear hull/mooring/riser coupled dynamic analyses of a spar, a pseudo time-domain approach is developed and used. The first-order wave-frequency and second-order differencefrequency wave loads and other hydrodynamic coefficients for the hull are calculated in the frequency domain using a second-order diffraction/radiation 3D panel program. The wave forces on the hull are then converted to time-domain signals using two-term Volterra series model. At each time step, the forces on slender members are directly calculated from Morisons formula at the instantaneous body position. The responses of hull and slender members are then simultaneously solved in a combined matrix at each time step. The case-study simulations were conducted for a classic spar designed for 6000-ft. (1829-m) water depth with 14 chain-polyesterchain mooring lines and 23 buoyancy-can-supported vertical risers in 100-year non-parallel wind, wave, and current conditions. The fully coupled hull-mooring-riser numerical results are compared with uncoupled quasi-static and semi-coupled dynamic analysis results, in which the mooring lines and risers are replaced by a set of massless springs. A series of sensitivity studies are then carried out against various analytical/environmental parameters to better understand the role of each parameter and the underlying physics. In all cases considered, the wave-frequency components remain almost the same, while the slowly varying motions change case by case. Of critical note, the equivalent static wind-force modeling may lead to significant underestimation of slowly varying surge/sway and pitch/roll responses in the absence of dynamic wind loading. The importance of the effects of buoyancy cans on spar pitch motions is also addressed.
