Study On Stiffness Characteristics Of Taut Mooring System And Its Influence On Motions Of Spar Platform

Deepwater Truss Spar platforms have bee widely used in recent years due totheir exellent motion performances. Since the platforms always work in deep water,taut mooring systems are considered a better way to locate them. Studying on thenonlinear restoration forces of mooring system, and the natural perfomances as wellas the responses of spar platforms coupling with mooring lines, all these are of greatimportance in designing the platforems.The natural characteristics and motion response charecteristics were invesgatedin six directions for deepwater Truss Spar platform with three points mooring andnine polyester cables in the regular wave. An empirical equation was carried out tosimulate the static stiffness of one single polyester rope which varies due to the strainof the rope, and considering of its material nonlinearity as well as its geometricnonlinearity, its equivalent restoration force changing with offsets can be calculatedwith the newly derived equations of equilibrium. Then, the restoration force-offsetrelations of mooring system consisting of9mooring lines can be calculated,futhermore, the sensitivity tests of influent factors on restoration stiffness were carriedout. Considering coupling between a spar and its mooring lines by using quasi staticanalysis, the coupled equations for surge, sway, heave, roll, pitch and yaw motionswere established. Natural properties can be figured by mathematical methods, and theeffects of mooring system on spar can be seen. Meanwhile, a hydrodynamic modelwas built in SESAM to compute the wave loads on the spar in regular waves, and thenthe dynamic responses of the platform can be examined when the circular wavefrequency is close to the natural frequency of the spar.It shows that the restoration force of mooring system is linear when smallmotions appear, but seems to become non-linear with larger displacement. The naturalfrequencies of the spar platform could change under different mooring conditions,especially in horizontal direction. And the effects on motion of each direction seem tobe different. Compared with the single platform in waves, the amplitudes of sparmotions could be smaller with mooring system, but the pitch angle mutation causedby resonance in heave direction cannot be avoided. What’s more, the natural period insurge direction is different from that in sway direction due to this kind of mooring system mentioned in this paper, thus it will lead to the asymmetric motions when theplatform meets the regular wave in different angles.