Numerical Simulation Of Heave-pitch Coupled Nonlinear Motions For Spar Platform

Spar platform is used for drilling, production, storage and offloading. Due to the deep draft and relatively smaller water-plane area, the structure is insensitive to wave loads. Spar platform normally exhibits excellent motion characteristics even in high sea state. However, when Spar platform is exposed to long period wave environment especially swell environment, for some critical wave period, the heave motion makes the pith motion unstable, and heave-pitch coupled instability motions are induced. The main objective of this study is to find the critical wave period and explain the heave- pitch coupled instability motions with numerical program and MOSES.The heave-pitch coupled nonlinear equations for the classical Spar platform in regular waves are established, considering nonlinear restoring moment. And hydrodynamic loads acting on the Spar platform can be obtained from first-order diffraction and radiation theory.Under the two cases below: 1) the heave natural period is approximately half the pitch natural period; 2) the incident wave period is near to the heave natural period. The heave response increases drastically at resonance and also large coupled pitch response is triggered. The underlying mechanism may be understood in the manner that large heave motion at resonance makes metacentric height negative and kinetic energy transfers from heave to pith mode. It is also shown that for a critical wave period (about 20s), which is not equal the heave natural period, the envelope of heave motion makes pitch motion unstable. This mutual interaction largely amplifies both pitch and heave motions. This kind of heave-pitch instability proved in my study is more important than the previously dangerous phenomenon.The results in this paper prove that the metacentric height and damping play an important role in heave-pitch coupled nonlinear instability motion.We employ MOSES to investigate my previous simulation, and we focus on the effect of mooring lines on the Spar platform. Also, the wave loads are computed through MOSES. In contrast to the wave excited forces which are obtained using Matlab7.1 numerical program, their results agree very well.