Abstract
A parametric study is conducted to investigate the influence of some of the nonlinear effects produced by the variable submergence and the methods used for calculating hydro-dynamic forces, on the dynamic response of Tension Leg Platforms. The nonlinear effects included in the study are the variable added mass and the tension fluctuation in the tethers produced due to the variable buoyancy and the vertical wave forces. In addition, the nonlinearity due to relative-velocity squared drag force is considered. Hydrodynamic forces on the large diameter members of the super structure are obtained by means of modified Morison’s equation using (i) average water particle kinematics across the diameter of the members, (ii) water particle kinematics calculated at the centre of the cross-section, and (iii) water particle kinematics same as (ii) above, but the inertia coefficient is modified to take into account any diffraction effect. The effect of variable submergence on the water particle kinematics is included by both Chakrabarti’s [1] and Hogben’s [2] approaches. The nonlinear equation of motion is solved in time domain by using iterative step by step integration procedure. Two integration algorithms namely. Newmark’s – beta-method and Argyris 3rd order integration scheme have been used. Relative efficiencies of the two integration schemes have been examined in relation to obtaining the steady-state surge response of the T.L.P.
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