Analysis Of Cable Tension And Heave Motion For Spar Platforms

The thesis is focused on analysis of dynamic tension in cables and the coupled effects between Spar platform and the deep sea mooring system, which is referable in design and fabrication of Spar platform.The equations of the three dimensional motion of a marine cable are formulated considering seabed touchdown, fluid drag force, interia force and structure damping. The characteristics of dynamic tesion and the interactions of seabed and the cable are invested. The equations of the hull of Spar platform are formulated according to force relationships. A coupled analysis procedure is formulated in which motions of cable and hull are coupled by matching the forces and displacements of the hull and its mooring system at their connections(fairleads or porches) based on their own equations. Motions and mooring line tensions from simulations using coupled and separated analysis procedures are compared for a specific Spar platform operating in 1018m water depth.It is found that the particles in the lower part of cable have a high motion frequency which is vital in fatigue analysis due to its bounce against seabed when the whole cable is motivated by its top end motion. The cable tension will change along with the cable top end motion. If the cable top end has large frequency or motion amplitude, cable tension will be large and the altertive slack-taut phenomena may emerge. It is also concluded that the hypsography in seabed can effect cable configuration, tension and the movement of hull greatly.At the meantime, It is found that cable fluid resistance can reduce the heave motion of Spar platform. The reduction is 6.6% in 1018m water depth and it will be even more larger as the depth is deeper. Large dynamic tension is emerged during the heave motion of the hull. Increace in cable fluid drag coefficient can reduce the heave free motion of the hull.