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FPSO储油轮与半潜式平台波浪载荷三维计算方法研究
3-D Computational Method of Wave Loads on FPSO Tankers and Semi-submersible Platforms
【作者】 张海彬;
【导师】 任慧龙;
【作者基本信息】 哈尔滨工程大学 , 船舶与海洋结构物设计制造, 2004, 博士
【摘要】 在海洋工程技术研究领域,结构物的波浪外载荷计算一直是一个热点方向。本文的主要目的是基于三维水动力理论,研究并提出一套适合FPSO储油轮和半潜式平台的,从浮体湿表面网格自动划分、线性波浪载荷计算、非线性波浪载荷计算到结构强度直接分析的完整解决方案。为此,文中着重在下列几个方面进行了探讨。 首先,采用累加弦长的三次参数样条函数与解析方法相结合的方式,提出了一种可广泛应用于船舶、FPSO储油轮以及半潜式平台的湿表面网格自动生成方法。为配合非线性方法以及应用扩展的边界积分方法去除不规则频率对网格的特殊要求,提出了在上述网格生成方法基础上对网格进一步处理的可行方案。 其次,基于三维线性势流理论,在频域内建立了求解低速航行和系泊浮体运动和波浪载荷响应的理论模型,并采用扩展的边界积分方法消除不规则频率现象。应用悬链线理论计算系泊系统作用于浮体的线性化恢复力,并将其表示为刚度系数和锚链上端位移的函数,进而以刚度系数的形式计入浮体运动方程。对几类船舶和半潜式平台进行了广泛的计算,并与模型试验结果进行了对比分析。针对FPSO储油轮,讨论了锚泊系统对运动和载荷的影响。 再次,研究并提出了一种预报浮体非线性运动和波浪载荷的三维时历模拟方法。考虑瞬时物面变化引起的各种非线性因素,在时域内建立浮体的非线性运动方程并求解。其中,入射波力和静恢复力按瞬时物面条件计算,辐射力和绕射力按浮体在瞬时平均湿表面为平衡位置做简谐振荡运动来计算。在此基础上,为进一步提高计算效率,将绕射和辐射问题在静水平衡位置求解,而仅考虑占主导地位的入射波力和静恢复力的非线性因素。分别按上述两个方案编制了计算程序,并对集装箱船和FPSO储油轮的非线性流体力、非线性运动和剖面载荷进行了计算分析,并探讨了各种非线性因素的影响。 进而,应用上述三维非线性方法计算浮体首部垂向相对位移和相对速度,将其用于发生底部砰击和甲板上浪的判据中。在底部和外张砰击计算中结合哈尔滨工程大学博士学位论文现有的方法,采用二维剖面与三维水动力网格匹配的方式计算底部和外张砰击力。在上浪砰击计算中,分析了溃坝理论和洪水波理论在预报浮体上浪压力和流体高度分布的差别,提出了计及波浪超过首部干舷高度变化影响的改进模型。将砰击及上浪载荷计入时域中浮体的非线性运动方程,并考察了其对浮体运动和载荷的影响。 在上述工作的基础上,对FPSO储油轮和半潜式平台的设计载荷计算方法进行了研究。应用谱分析方法研究了 FPSO储油轮的设计载荷,并通过实际计算与油轮的规范值进行了对比。将三维非线性方法引入确定半潜式平台设计载荷的设计波方法之中,并探讨了其与线性预报结果的差别。结合前面提出的网格生成方法,将浮体湿表面脉动压力以控制点坐标为参数构造成累加弦长的三次参数样条函数压力场,从而为利用有限元方法对浮体结构进行直接强度分析时的载荷施加提供了方便。最后,本文对一FPSO储油轮舱段结构和半潜式平台整体结构进行了直接强度计算。关键词:网格自动生成;三维水动力理论;非线性波浪载荷;砰击;上浪;FPSO储油轮;半潜式平台
【Abstract】 The wave load computation is one of the most vital research fields in offshore engineering. The major purpose of this thesis is to present a full solution scheme of auto-meshing of body wet surface, linear wave load computation, nonlinear wave load computation and direct strength analysis for structures of FPSO tankers and semi-submersible platforms. For this reason, the following problems are emphatically investigated.Firstly, the method of accumulative chord length cubic parameter spline function combined with analytic expression is adopted to generate the mesh of body wet surface, which can be widely applied to ships, FPSO tankers, and semi-submersible platforms. On the basis of above, a further treatment for mesh is proposed in order to meet the needs of nonlinear hydrodynamic computation and the removal of irregular frequency effects with extended boundary integral equation method.Secondly, based on the three dimensional potential flow theory, the model for the solution of motion and wave load responses of lowly sailing or moored floating body is established in frequency domain, in which irregular frequency effects are removed with the extended boundary integral equation method being used. The linearized restoring forces acting on the floating body by the mooring system are calculated according to the catenary theory, which are expressed as the function of linear stiffness coefficients and the displacements of the upper ends of mooring chains. Large amount of computations are carried out for ships and semi-submersible platforms, and the comparisons between calculation results and model testing results are conducted. For FPSO tankers, the effects of mooring system on motion and wave load responses are discussed.Thirdly, a kind of three-dimensional nonlinear time history simulation method of floating body motions and wave loads is presented. With the nonlinear factors due to the instantaneous position variation of body surface being considered, the nonlinear motion equations are established and solved in time domain, in which the incident wave forces and hydrostatic restoring forces are calculated according to the instantaneous body surface condition and thediffraction forces and radiation forces are obtained on the assumption that the body is oscillating in harmonic mode at the instantaneous mean wet surface. For improving the calculation efficiency, only the dominant nonlinear factors of incident wave forces and hydrostatic restoring forces are considered, and the diffraction and radiation problems are solved according to the still balanced’ position, by which a kind of practical method with enough accuracy and efficiency is presented. Two programs have been developed, and the nonlinear fluid forces, nonlinear motions and nonlinear wave loads of container ship and FPSO tankers are computed, and the contribution of each nonlinear factor is evaluated.Furthermore, the above-mentioned nonlinear method is adopted to predict the relative motion and velocity of the bow, which are used in the judgment of the happening of slamming and green water. The bottom slamming forces and bow flare slamming forces are computed by matching the two-dimensional section and the three-dimensional hydrodynamic computation mesh. For green water loads, the differences of green water pressure and water height distribution between dam breaking model and flood wave model are analyzed, and an improved model is presented, which can include the effects of the change of relative water height over freeboard. Then, the slamming loads and green water loads are considered in the equations of nonlinear body motions.Based on the above work, the study on design wave loads of FPSO tankers and semi-submersible platforms is carried out. The design wave loads of FPSO tankers are investigated with the spectrum analysis method, and compared with those regulated in oil tanker rules. The three dimensional nonlinear method is introduced in the design wave analysis of semi-submersible platforms and the differences between the nonlinear method and linear method
【Key words】 Auto-generating of mesh; Three-dimensional hydrodynamic theory; Nonlinear wave loads; Slamming; Green water; FPSO tankers; Semi-submersible platforms;