浮力筒对自由站立式刚性主管轴向振动的影响

摘要

The complex marine environment easily leads to periodic changes in buoyancy of buoyancy tank on freestanding riser system, resulting in axial vibration of rigid riser.When the buoyancy tank frequency is the same or close to the inherent frequency of the rigid riser, system resonance will occur and endanger its operation safety.In view of this, the axial vibration model of the rigid riser has been established to calculate the natural frequency of the axial vibration by numerical method.Taking the five kinds of designed conditions of the buoyancy tank as case study, the changes of the each order of natural frequency with the buoyancy tank conditions have been studied.The dynamic load amplification coefficients of the five conditions have been calculated by steady state response analysis.The results show that the steady-state amplitude of the rigid riser is proportional to the buoyancy of the buoyancy tank, and is inversely proportional to the density, cross-sectional area and total length of the rigid riser.The dynamic load amplification coefficient at the top of the rigid riser is dependent on the buoyant tank equivalent mass, rigid riser density, cross-sectional area, total length and frequency ratio.When the buoyancy tank is two-tanks damaged or center tube damage and one tank damage occurs at the same time, the dynamic load amplification coefficient of the system would be greatly increased.Extending the buoyancy tank water depth should be considered to reduce the vertical environmental load and prolong the vertical movement cycle.The equivalent weight of buoyancy tank should be minimized, and the system`s natural frequency should be increased.The study results could help to improve the mechanical properties analysis of the buoyancy tank and the rigid riser, providing useful references for the design of the freestanding riser system.%复杂的海洋环境易导致自由站立式立管系统浮力筒浮力周期性变化,造成刚性主管轴向振动.当浮力筒运动频率与刚性主管固有频率重合或接近时,系统会发生共振,危及其作业安全.鉴于此,建立刚性主管轴向振动力学模型,运用数值方法求解出刚性主管轴向振动的固有频率.以浮力筒现有的5种设计工况为例,探究了各阶固有频率随浮力筒状态的变化情况;通过稳态响应分析,计算出5种工况下的动载荷放大系数.研究结果表明:刚性主管的稳态振幅与浮力筒浮力成正比,与刚性主管密度、横截面面积和总长度成反比;刚性主管顶部的动载荷放大系数与浮力筒的等效质量、刚性主管的密度、横截面面积、总长度及频率比有关,当浮力筒出现2舱破损和中心管及1舱同时破损时,系统的动载荷放大系数明显偏大;应考虑适当增加浮力筒设计水深,以减小垂向环境载荷作用,延长浮力筒垂向运动周期;尽量减小浮力筒的等效质量,增大系统的固有频率.研究结果有助于完善浮力筒和刚性主管的力学性能分析,为自由站立式立管的系统设计提供有益参考.