Effects of Slip-Stick on Top Tensioned Riser Fatigue

In a Truss Spar system, top tensioned risers are tensioned, and centralized, inside a buoyancy can stem that extends all the way to the keel. This stem and buoyancy can contact the hull by guides at the buoyancy can, at the deck stopper, at the heave plates and at the keel where the riser exits the hull and descends to the seafloor wellhead. The buoyancy cans are in constant contact with the hull because of a preloaded contact guides located on each can. The buoyancy cans and their guides are designed to let the riser stroke with respect to the platform, however the friction forces at the guides and keel joint cause the riser to slip-stick. Thus the riser tension fluctuates. When this tension is large enough to overcome the friction force, and the relative velocity between the riser and the hull is non-zero, the riser slides. Otherwise it sticks. These tension cycles caused by slip-stick makes a significant contribution to the overall fatigue damage. While the riser is stuck it acts as a tendon on the Spar affecting its resonant periods and motions. While the riser is sliding it introduces a Coulomb type damping to the motion. The tension variations are the same at each point along the riser below the keel and the fatigue damage must be added to that caused by hull motions and riser VIV. Coupled time domain analyses that include the inertial, damping and stiffness properties of both the hull and the riser are very complex and time consuming. The amount of computation based on such models is not always practical in a design optimization phase. More approximate uncoupled methods, which produce adequately conservative results, can be applied for design purposes. This paper compares and discusses the results of coupled and uncoupled models to compute the damage in the risers due to slip-stick. The two models are based on identical finite-element models of the riser system. The main difference is how the motion input to each model is computed. In the coupled model, the effects of the riser on the motions are integrated in to the time-domain solution of the motion. In the uncoupled model, motions that are computed assuming only horizontal restoring forces from the risers are imposed on the contacts between the riser and the hull. In general, the coupling effects showed that the risers dampen motion in surge, sway, roll, pitch and yaw but have an insignificant effect on the natural periods. The heave comparison showed a reduction in heave due to damping from the risers sliding as well as from the constraints of the risers while they are stuck. Results of the coupled analysis showed lower fatigue damage than those predicted by the uncoupled model and these results are used to show the level of conservatism introduced by the uncoupled model.