Dynamic response of offshore platforms to extreme waves including fluid-structure interaction

Abstract

Basic considerations for dynamic response analyses of offshore platforms under extreme wave loadings are discussed and the main difficulties of the problem are pointed out. These difficulties arise from the random nature of the loading, the nonlinearity of the drag dominated wave forces and the dependence of such forces on the response of the structure. A time domain solution is recommended, using kinematics of a random sea state for the wave force model along with a relative velocity formulation to account for fluid-structure interaction. The proposed solution is based on approximating structural velocities in Morison's equation with their values at the previous time step. This simplification linearizes the equations of motion and permits analyses of detailed structural models by modal techniques. A limited comparison with results from a more accurate solution appears to justify this approximation. As an example, a structure with 1428 degrees of freedom is subjected to a random wave segment and its responses with and without relative velocities are compared. It is shown that relative velocity effects can be well approximated with an appropriate increase in modal damping and therefore analyses neglecting relative motion are justified, provided that the correct increase of viscous damping is specified. The method of solution proposed here can be utilized to establish this increase in the early stages of design.