Nonlinear Coupled Dynamic Analysis Of Truss Spar In Waves

Spar platform is the main equipment used for deep water oil and gas exploitation in the world and many researchers have focused on discussing the motion performance of Spar platform and have made large progresses. Truss Spar is the most widely used platform in spar family. Its heave and pitch motion have a big effect on its working in specified sea areas safe or not. Due to the coupled effect of heave and pitch motion, the unstable motion may occur when the platform encounters the long period waves. In this paper, I will study the heave-pitch coupled motion of Truss Spar in regular and irregular waves. Judge in what kind of condition the internal resonance will occur. And by calculating the effects of different parameters on motion, give some useful suggestions in design.The heave-pitch coupled motion equations both in regular and irregular waves are established. The wave forces and added mass coefficients are calculated by AQWA-LINE based on diffraction and Morison theory. Damping coefficients are got from model test. The motion equations are solved by4th order Runge-Kutta method in MATLAB software and results of heave and pitch motion responses are obtained. The feasibility of the numerical method is confirmed by comparing with model test results. In addition, the influence of wave parameters, center of gravity location are discussed by using bifurcation diagrams. The approximate method is used to calculate the motion response and make the comparison.When the motions are calculated in irregular waves, given the wave spectrum, random wave forces can be obtained. The random motions can be got by solving the motion equations. The influence of wave parameters, center of gravity location and mooring systems are also discussed. Then the pitch motion probability density distribution is studied by Gauss-Legendre path integral method based on nonlinear stochastic dynamics. At last, the effect of external excitation and parametric excitation strength on probability of density distribution is analyzed.The results show that when the wave frequency is close to heave resonant frequency, at the same time, it’s close to the2times of the pitch resonant frequency, the inner resonant may occur and will induce the unstable pitch motion. Wave height, wave frequency and center of gravity location have a great influence on inner resonant occurring or not. The pitch motion responses have a relationship with significant wave height and wave peak period in irregular waves and the inner resonant may occur.