Tether Deformation and Tension Leg Platform Surge

Abstract

Estimates of tension leg platform (TLP) surge presented in the literature are generally based on the assumption that the tethers are straight at all times, i.e., that transverse deformations of tethers due to hydrodynamic and inertial forces may be disregarded. The purpose of this note is to verify the validity of this assumption in the case of a deep water TLP. The equation of motion of the tethers for the platform being considered is a nonlinear partial differential wave equation. The numerical solution of that equation showed that, for an assumed harmonic motion of the platform, the hydrodynamic and inertial forces that cause the tethers to deflect will increase the amplitude of the restoring force on the platform by a factor of about three. However, the resulting platform motion does not differ significantly from that corresponding to the assumption that the tethers are straight.