Analysis Of Nonlinear Motion Behavior Of Coupled Heave-pitch Motion For The Deep Sea Spar Platform

A Spar platform is a kind of new type designed for drilling, production, storage and offloading. Due to its structural characteristic, the typical heave natural frequency is approximately twice as the pitch natural frequency. So, when Spar platform is exposed to long period wave environment or ground swell environment, the heave motion makes the pitch motion unstable and thereby the heave-pitch coupled instability motions are induced. For the safety of the platform and overall comprehension and grasping the motion characters under complicated environmental loads, it is important to study the nonlinear dynamic characters of the platform, considering the coupled relation between heave and pitch motion. The main task and the conclusions are as follow:The domestic and overseas development of the research on deep sea platforms is summarized synthetically, especially the development of the Spar platform. The main problem of dynamic characters for the Spar platform is analyzed and reviewed, and the research direction is determined based on the above analysis.Considering the effect of wave surface, the nonlinear coupled heave-pitch parametric equations of classical Spar platform are established, and the calculation program is designed by Matlab software. The numerical simulation shows that, without considering the effect of wave surface always results in the underestimate of vertical motion, while the influence on the pitch motion is small.The steady-state solutions of coupled heave and pitch motion on the condition of internal resonance and combination resonance are resolved applying method of multiple scales and at the same time, it is verified by using numerical simulation. The phenomenon of energy saturation of heave motion and energy transmission from heave to pitch motion will appear when internal resonance occurs. The deviation between heave natural frequency and doubled pitch natural frequency and the one between wave frequency and heave natural frequency jointly decides the critical wave amplitude and stable heave amplitude. The heave and pitch motion will get unstable when combination resonance occurs. The critical wave amplitude of bifurcation phenomenon will become larger when the deviation between heave natural frequency and doubled pitch natural frequency augments. The combination resonance will not be induced if the deviation between wave frequency and heave natural frequency goes beyond a certain range.Stability diagram for damped Mathieu's equation is obtained through method of Hill's infinite determinant. The occurrence process of Mathieu instability is analyzed using numerical simulation. The results of calculation reveal that, the main reason of Mathieu instability motion is large heave amplitude. Not only meeting the frequency relation but also lame heave amplitude is needed to incite the pitch instability motion.The influence of damping; mooring system and heave plate to coupled heave and pitch motion is studied with the help of AQWA soft and numerical simulation. The results show that heave damping, which plays a much larger part than bitch damping, would inhibit the occurrence of pitch unstable motion through suppressing the heave motion. The results also show that, a complete Truss Spar system will not come up with Mathieu instability, albeit energy saturation of heave motion and energy transmission from heave to pitch motion is still occurred. The hydrodynamics of the Truss Spar platform with the heave plate's structural parameters variation is studied also.At last, the limitations of this paper and future research directions are pointed out.