Coupled response of compliant offshore platforms

A three-dimensional finite element analysis has been used to simulate the coupled static and dynamic behaviour of compliant ocean structures. Nonlinearities which result from large deflections, reduced or zero stiffness in compression, and nonconservative fluid loading are considered. The spatial variation of fluid loading is also addressed. The structures are assumed to be in the Morison regime. Linear wave theory is used and multidirectional seas may be simulated. A variable current profile may be specified and concentrated masses and loads, as well as foundation properties, may be modelled. Updated Lagrangian coordinates and a residual feedback, incremental-iterative, solution is adopted. Viscous relaxation is used to start the static solution of problems with small initial stiffnesses. The dynamic solution is performed in the time domain and uses the Newmark integration scheme. Consistent mass matrices are developed for both beam-column and cable elements. The directionality of the hydrodynamic added mass is accounted for, as is the discontinuity of the mass density for elements which pierce the water surface. Examples are presented of an articulated tower, of a guyed tower, and of a tension leg platform.