Nonlinear dynamic analysis of offshore structures
In this thesis consideration is given to a selection of nonlinear dynamic problems in the field of offshore engineering. Hydrodynamic loading on fixed horizontal and vertical tubular members and the dynamic response of articulated towers together with the distribution of shear force and bending moment along the tower are investigated using various wave theories. Effects of nonlinear convective acceleration terms in the calculation of fluid inertia forces and moments are examined and attention is given to integration of wave forces up to the free surface for vertical members. Calculation of fluid loading at the displaced position of the articulated tower and any Mathieu type instabilities that may occur have been considered. The dynamic analysis of a damaged Single Anchor Leg Storage (SALS) system subject to loss of buoyancy in the yoke chamber is studied. The equations of motion of the yoke/riser system are derived assuming large displacements and solved in the time domain. Time histories of the response, variations of the riser tension, velocities of riser top end and the time histories of pivot reactions are given. Natural periods and mode shapes for small displacements of the system are calculated. Two methods of simulating random seas, both represented by a sum of harmonic wave components, are used to simulate second order low frequency (slow drift) force on a tanker in head seas by Pinkster's time domain method. In one method the wave amplitudes are generated randomly from a Rayleigh distribution and in the other they are obtained deterministically via the wave spectrum. Time histories of slow drift force and response together with simulation results with various duration lengths are presented and compared. Estimates of the extreme vessel response and its relation to rms value are compared with the result of a commonly used method of determining peak/rms ratios. The results of these investigations highlight the importance of accurately simulating nonlinear effects in both fixed, floating and compliant offshore structures from the point of view of safe design and operation of such- systems.