Study On The Nonlinear Stochastic Dynamic Response Characteristics Of A Truss Spar

Spar platform has been one of the important deep-sea oil production platforms byvirtue of its excellent motion performance and low cost since its inception. Thedynamic response characteristics and stability of the Spar platform are the mainconsiderations of the design. In this paper the heave-pitch coupling motioncharacteristics of the Truss Spar are studied in random waves, the hydrodynamicperformance of heave plate is also calculated. The main contents and conclusions areas follows:(1) The development of the research for deep-sea Spar platform is summarizedsynthetically. The main problem and objective of this study are determined, whichpointed out the direction for the subsequent research.(2) The wave loads acting on the hull of the Truss Spar platform are calculatedaccording to the theory of diffraction. The transfer function between the random waveforces on the hull platform and the wave spectrum are obtained, as well as the transferfunction between wave forces on heave plate and the wave spectrum. The randomwave forces on platform are numerically simulated by the linear wave superpositionmethod according to the Longuet-Higgins random wave model.(3) Considering the static stability and the change of the displacement volume,including the influences of higher order nonlinear terms and the instantaneous wavesurface, the nonlinear coupled heave-pitch motion are established in the stochasticwaves. The dynamic responses of heave-pitch coupling motion for the platform areinvestigated. It is found that, when the characteristic frequency of stochastic wave isclose to the heave natural frequency or the sum frequency of heave and pitch, thelarge amplitude pitch motion is induced under the random parametric-forcedexcitation, when the significant wave height reaches a certain value. Since theexistence of parametrically excited nonlinear term, pitch motion is no longer a forcedoscillation, it becomes a1/2sub-harmonic resonance.It is observed that the pitch motion will be reduced when increasing the heavedamping or increasing the pitch damping, but heave motion will be not the same. Theamplitude of heave motion is increasing with the increase of pitch damping. That is, increasing the pitch damping will prevent the energy of heave mode to transfer to thepitch mode, and it can weaken the coupling relationship between two modes.Therefore, the most effective means of controlling the pitch instability is to increasethe heave and pitch damping. The number and structure form of heave plate, as wellas the pitch of helix side plate and board must be reasonable.(4) The pitch motion of Truss Spar platform is studied using the stochasticdynamics theory. Completely average Ito stochastic differential equations are obtainedby the stochastic averaging method. It is observed that, larger pitch amplitudeprobability increases with the increasing of significant wave height; larger pitchamplitude probability decreases with the increase of the damping.(5) The coupled dynamic response for the Truss Spar platform-mooring systemis calculated under random waves. It is observed that, the heave motion amplitude isreduced when considering the mooring system. Even if the significant wave height islarger, the magnitude of pitch is also greatly reduced. As the increasing of significantwave height, pitch frequency component is not one time of the characteristicfrequency, but1/2times, which is equal to the natural frequency of the pitch.(6) The hydrodynamic performances of heave plate with different forms of theedges have been studied in this paper applying CFD method and the secondarydevelopment of software Fluent. It is found that there are some relationships betweenhydrodynamic performances and the plate structure form. The results show that: Thethickness and the edge form have significant effects on the hydrodynamic coefficients,and the heave plate after tapered performs better in hydrodynamic performance.