The Effect of Tether Damage on Tension Leg Platform Dynamics

Abstract

The design of a tension leg platform (TLP) provides for tethers of equal stiffness at each corner. Damage to the tethers results in a loss of stiffness and consequently a change in the static equilibrium configurations and dynamic response of the platform and also affects the tether dynamics. Computer algorithms for the linear and nonlinear analysis of a tension leg platform subjected to regular waves have been developed assuming that the platform is modeled as a rigid body. In the linear case the tethers are modeled as one-degree of freedom springs whereas in the nonlinear study the tethers are modeled as a network of beam elements having bending and axial stiffness and also geometric stiffness because of the large displacements assumed. It is assumed that the hydrodynamic forces on the assembled structure equal the sum of the hydrodynamic forces on the component bodies while their interaction is ignored. A parametric study of the platform motion responses and tension variations has been presented for cases involving the loss of tether stiffness. Motion responses of the platform and the tension at each leg have been computed for a regular wave traveling from aft to forward for cases involving intact tethers and with a certain amount of stiffness lost, in one instance at both aft tethers, and in another case at only the aft starboard tether. A regular wave of 10.0-m height and 8.0-s period in 160-m water depth has been used for calculation purposes. The total mean pretension is assumed to remain constant. It is seen that the lateral motions of the platform (such as surge) are not affected by the loss of tether stiffness while the motions in the vertical plane (such as heave and pitch) increase with reduction in stiffness. In all cases the dynamic tension amplitude increases with loss of tether stiffness. For a given configuration, the nonlinear analysis yielded larger dynamic tension amplitudes than those obtained from linear analysis because of the inclusion of bending stiffness in the tethers and also because large displacements were taken into account.