Nonlinear Numerical Analysis of Vortex-Induced Vibration of A Three-Dimensional Deepwater Steep Wave Riser with Large Deformation Features

The cross-flow(CF)vortex-induced vibration(VIV)of a deepwater steep wave riser(SWR)subjected to uniform or shear flow loads is investigated numerically.The model is based on a three-dimensional(3D)nonlinear elastic rod theory coupled with a wake oscillator model.In this numerical simulation,the nonlinear motion equations of the riser with large deformation features are established in a global coordinate system to avoid the transformation between global and local coordinate systems,and are discretized with the time-domain finite element method(FEM).A wakeoscillator model is employed to study the vortex shedding,and the lift force generated by the wake flow is described in a van der Pol equation.A Newmark-βiterative scheme is used to solve their coupling equation for the VIV response of the SWR.The developed model is validated against the existing experimental results for the VIV response of the top-tension riser(TTR).Then,the numerical simulations are executed to determine VIV characteristics of the SWR.The effects of both flow velocity and the spanwise length of the flow field on the drag coefficient in the inline(IL)direction and the lift coefficient in the CF direction are investigated systematically.The results indicate that compared with TTR,the low frequency and multi-modal vibration are the main components of the SWR due to the large deformation and flexible characteristics.For shear flow,the multi-frequency resonance dominates the VIV response of the SWR,especially at the hang-off segment.

Nonlinear Dynamic Analysis and Fatigue Study of Steep Wave Risers Under Irregular Loads

As a reliable alternative option for traditional steel catenary risers(SCRs),steep wave risers(SWRs)have been widely applied to deepwater oil and gas production.However,the nonlinear dynamic analysis of SWRs is more complicated than that of traditional SCRs due to their special configuration and significant geometric nonlinearity.Moreover,SWRs are highly susceptible to fatigue failure under the combined excitation of irregular waves and top floater motions(TFMs).In this study,considering irregular waves and TFMs,a numerical SWR model with an internal flow is constructed based on the slender rod model and finite element method.The Newmark-βmethod is adopted to solve the dynamic behavior of SWR.Moreover,the Palmgren-Miner rule,a specified S-N curve,and rainflow counting method are applied to estimate the fatigue damage.An efficient numerical computation procedure,i.e.,DRSWR,is programmed with MATLAB in this study.Calculation results are compared with those of OrcaFlex to verify the accuracy of the DRSWR.The nonlinear dynamic response and fatigue damage of an SWR under the combined excitation of irregular waves and TFMs are obtained,and a comprehensive parametric analysis is then conducted.The analysis results show that the buoyancy section undergoes the highest level of stress and fatigue damage under the combined excitation of irregular waves and TFMs.An internal flow with high velocity and high density produces a high level of fatigue damage.The buoyancy factor and length of the buoyancy section should be set moderately to reconcile the reduction of the top tension with increased fatigue life.These results are expected to provide some reference significance for the engineering design of SWR.

Time-Domain Nonlinear Wave-Current Interaction with A Steep Wave Riser Considering Internal Flow Effect

The nonlinear dynamic response induced by the wave-current interaction on a deepwater steep wave riser(SWR)is numerically investigated based on a three-dimensional(3 D)time-domain finite element method(FEM).The governing equation considering internal flow is established in the global coordinate system.The whole SWR consists of three segments:the decline segment,buoyancy segment and hang-off segment,in which the buoyancy segment is wrapped by several buoyancy modules in the middle section,leading to the arch bend and sag bend.A Newmark-β iterative scheme is adopted for the accurate analysis to solve the governing equation and update the dynamic response at each time step.The proposed method is verified through the published results for the dynamic response of steel catenary riser(SCR)and static configuration of steel lazy wave riser(SLWR).Simulations are executed to study the influence of wave height,current velocity/direction,internal flow density/velocity and top-end pressure on the tension,configuration and bending moment of the SWR.The results indicate that the influence of the current on the configuration and mechanical behavior of the SWR is greater than that of the wave,especially in the middle section.With increasing current velocity,the suspending height of the middle section drops,meanwhile,its bending moment decreases accordingly,but the tension increases significantly.For a fixed external load,the increasing internal flow density induces the amplification of the tension at the hang-off segment and the mitigation at the decline segment,while the opposite trend occurs at the bending moment.

Nonlinear Analysis of Bidirectional Vortex-Induced Vibration of A Deepwater Steep Wave Riser Subjected to Oblique Currents

An improved three-dimensional(3D)time-domain couple model is established in this paper to simulate the bidirectional vortex-induced vibration(VIV)of a deepwater steep wave riser(SWR)subjected to oblique currents.In this model,the nonlinear motion equations of the riser are established in the global coordinate system based on the slender rod theory with the finite element method.Van der Pol equations are used to describe the lift forces induced by the x-and y-direction current components,respectively.The coupled equations at each time step are solved by a Newmark-βiterative scheme for the SWR VIV.The present model is verified by comparison with the published experimental results for a top-tension riser.Then,a series of simulations are executed to determine the influences of the oblique angle/velocity of the current,different top-end positions and the length of the buoyancy segment on the VIV displacement,oscillating frequency as well as hydrodynamic coefficients of the SWR.The results demonstrate that there exists a coupled resonant VIV corresponding to x-direction and y-direction,respectively.However,the effective frequency is almost identical between the vibrations at the hang-off segment along x and y directions.The addition of the buoyancy modules in the middle of the SWR has a beneficial impact on the lift force of three segments and simultaneously limits the VIV response,especially at the decline segment and the hang-off segments.Additionally,the incident current direction significantly affects the motion trajectory of the SWR which mainly includes the fusiform and rectangle shapes.

基于悬链线理论的Lazy-wave和Steep-Wave立管形态确定方法

作为改良型的钢质悬链线立管(Steel Catenary Riser,SCR),懒波型立管(Lazy Wave Riser, LWR)和陡波型立管(Steep-Wave Riser,SWR)在深水油气开发中的应用越来越广泛。LWR和SWR的立管形态在其设计中至关重要,并且在海流作用下,LWR和SWR的立管形态会发生相当大的变化。文章将悬链线理论推广到深水LWR和SWR简化了立管形态确定方法,实现了工程设计初期快速确定立管形态的需求,减少了工程设计工作量。

陡波形立管涡激振动疲劳损伤参数敏感性分析

基于柔性杆理论和尾流振子模型计算陡波形立管的涡激振动响应;综合使用S-N曲线法、雨流计数法、Palmgren-Miner线性累积疲劳理论对立管涡激振动导致的疲劳损伤进行计算分析。并以MATLAB为平台编写相应计算程序,将本文计算得到的静力分析结果、固有频率和疲劳损伤分别与专业海工计算软件OrcaFlex和已发表文献进行对比验证。进一步对陡波形立管在涡激振动下的疲劳损伤进行参数敏感性分析,结果表明:浮子段长度、浮力因子、弹性模量、海流速度、波浪高度对陡波形立管疲劳损伤均有较大影响,有望为陡波形立管的实际工程设计提供参考。

随机荷载下陡波形立管的非线性动力分析

对陡波形立管在随机荷载作用下的非线性动力响应进行了数值分析。考虑内部流体,基于柔性杆理论建立了陡波形立管(SWR)的数值模型,采用Newmark-β法求解立管动力响应,利用MATLAB软件编写相应计算程序DRSWR,将DRSWR的计算结果与OrcaFlex进行对比验证,然后对陡波形立管在随机波浪、顶部浮体随机运动共同激励下的应力响应进行了参数敏感性分析。结果表明:浮子段应力水平、应力变化幅值最高;应力极值点为拱弯点和垂弯点;大波高、低周期的波浪会导致立管整体应力水平急剧升高;平均静偏移增大导致立管应力极值减小;大幅高频的慢漂运动会使悬挂点应力显著增大;高密度、高流速内部流体会使陡波形立管处于高应力状态。

陡波型柔性立管浮力块参数优化及应用

浮力块分布和外径等参数直接影响柔性立管在环境载荷作用下的动力响应,对某深水柔性立管进行陡波型布置,开展时域内的整体动态有限元分析,得到浮力块分布和外径对柔性立管动力响应的影响规律,表明浮力块参数是陡波型柔性立管整体设计的一个重要因素。立管动力响应是弯曲加强器的设计载荷,根据上述规律,构造具有不同浮力块参数的柔性立管模型,进行多工况动态分析,通过对比弯曲加强器设计载荷对(T,θ)的散点图,得到弯曲加强器设计载荷优化方法及最优设计载荷。

基于NSGA-Ⅱ算法的陡波型柔性立管优化设计方法

在柔性立管中部添加浮力块可使其形成陡波构型,合理的浮力块布置对陡波型柔性立管整体性能具有重要意义。论文以陡波型柔性立管悬挂点张力和锚固点张力为优化目标,以立管最大有效张力和最小弯曲半径为约束条件,构建了多目标优化模型。针对我国某海域应用于浮式生产储油装置的陡波型柔性立管系统,选取立管动态响应最大的4种典型工况开展动态时域模拟,研究了浮力块参数对立管悬挂点张力和锚固点张力的影响。综合考虑4种工况的极值结果,基于NSGA-Ⅱ算法开展了浮力块布置多目标优化设计,得到了关于悬挂点和锚固点最大张力的Pareto前沿。根据不同工程要求可在Pareto前沿中选择一个解作为最优解。

柔性立管浮筒数量多目标优化设计

以某海况的陡波线型设计为研究对象,以水面浮体的漂移域、管道线型最大曲率和经济性为设计指标,建立柔性立管整体线型的数值分析模型,开展这3项指标对于浮筒设计数量的灵敏度分析。提取上述指标结果,进行归一化处理,以漂移域最小、线型曲率最大、经济成本最低为优化目标,进行具有陡波线型特征的柔性立管系统浮筒数量的多目标优化设计,给出最优设计方案。

浮筒段刚度对浅水柔性立管力学行为的影响分析

随着柔性管道的发展,附件装备的刚度对柔性立管线型的影响逐渐成为研究的热点。基于8英寸(1英寸≈2.54cm)立管,考虑浮筒与柔性管道耦合作用下的非线性刚度,建立浅水陡波形立管分析模型,利用OrcaFlex软件对柔性立管的张力和曲率进行时域分析,并与不考虑浮筒耦合的模型结果进行对比。结果表明浮筒的弯曲刚度对立管线型最大张力结果的影响很小,但对最大曲率的影响比较显著。考虑浮筒弯曲刚度的立管线型分析得到的结果更为准确。