A flexible-segment-model-based dynamics calculation method for free hanging marine risers in re-entry

In re-entry,the drilling riser hanging to the holding vessel takes on a free hanging state,waiting to be moved from the initial random position to the wellhead.For the re-entry,dynamics calculation is often done to predict the riser motion or evaluate the structural safety.A dynamics calculation method based on Flexible Segment Model(FSM) is proposed for free hanging marine risers.In FSM,a riser is discretized into a series of flexible segments.For each flexible segment,its deflection feature and external forces are analyzed independently.For the whole riser,the nonlinear governing equations are listed according to the moment equilibrium at nodes.For the solution of the nonlinear equations,a linearization iteration scheme is provided in the paper.Owing to its flexibility,each segment can match a long part of the riser body,which enables that good results can be obtained even with a small number of segments.Moreover,the linearization iteration scheme can avoid widely used Newton-Rapson iteration scheme in which the calculation stability is influenced by the initial points.The FSM-based dynamics calculation is timesaving and stable,so suitable for the shape prediction or real-time control of free hanging marine risers.     

Enhanced multi-layer fatigue-analysis approach for unbonded flexible risers

This paper proposes an enhanced approach for evaluating the fatigue life of each metallic layer of unbonded flexible risers. Owing to the complex structure of unbonded flexible risers and the nonlinearity of the system, particularly in the critical touchdown zone, the traditional method is insufficient for accurately evaluating the fatigue life of these risers. The main challenge lies in the transposition from global to local analyses, which is a key stage for the fatigue analysis of flexible pipes owing to their complex structure. The new enhanced approach derives a multi-layer stress-decomposition method to meet this challenge. In this study, a numerical model validated experimentally is used to demonstrate the accuracy of the stress-decomposition method. And a numerical case is studied to validate the proposed approach. The results demonstrate that the multi-layer stress-decomposition method is accurate, and the fatigue lives of the metallic layers predicted by the enhanced multi-layer analysis approach are rational. The proposed fatigue-analysis approach provides a practical and reasonable method for predicting fatigue life in the design of unbonded flexible risers.     

三根附属控制杆对海洋立管涡激振动抑制作用实验研究

海洋立管的涡激振动会严重影响立管结构的使用寿命.通过室内水槽实验研究在立管模型周围等分布置三根附属控制杆来减小立管涡激振动响应的新型抑制措施.实验中观测了0.24 m/s、0.31 m/s、0.37 m/s以及0.44 m/s四种均匀流和两个极限来流方向下的涡激振动抑制效果.实验结果表明:三根附属控制杆抑制措施可明显降低立管模型的横向振动幅值,但对主管的振动频率改变不大;同时,这一抑制措施对来流方向有较强的适应性,避免了以往单根控制杆在流向发生改变时可能加剧立管涡激振动的弊端.     

抑制涡激振动的螺旋列板设计参数研究

基于水池模型实验结果和工程设计经验,结合国内外试验数据,着重分析用于抑制海洋立管涡激振动的螺旋列板几何参数(鳍高和螺距)及覆盖率对立管涡激振动的影响;并对水动力直径和水动力系数的选取对预报涡激振动的影响进行了分析,进而提出了适合于海洋立管工程应用的螺旋列板几何和设计参数选取的建议,为螺旋列板工程应用、海洋立管强度和疲劳设计提供参考。     

Dynamic response of marine risers by single wave and spectral analysis methods

This paper uses the Galerkin method in the solution of the marine riser differential equation and compares the dynamic bending stresses in a tension-leg-platform riser calculated by the linearised single wave and linearised spectral analysis methods.The results show that it is possible to make some rational assessment of approximate peak values of bending stress in the spectral method.The analysis methods are applicable to any riser system subject to horizontal exciting forces due to fluid and vessel motion.     

Nonlinear dynamic analysis of deep water risers

The paper presents a theoretical and numerical approach to the dynamical behaviour of risers in deep water which takes into account two types of nonlinearity; that due to viscous drag forces and that due to the large displacements of the riser when submitted to strong axial loads. As the second nonlinearity may have a significant influence upon the behaviour of risers in deep water, a method for automatically updating the structural geometry during the dynamic analysis is given. A computer programme has been written for this purpose.     

Large deformation three-dimensional static analysis of deep water marine risers

The problem of static three-dimensional, nonlinear, large deformation response of a marine riser is formulated within small strain theory and solved numerically. This type of analysis is necessary, for the new generation of drilling and production risers. The mathematical model takes properly into account the effects of internal and external pressure and the complete nonlinear boundary conditions, without linearizing the follower forces. The extensibility or inextensibility condition is used as the constitutive relation in the tangential direction. Torsion and bending are coupled. The external load and the boundary conditions are deformation dependent. A solution method is developed based on an incremental finite element algorithm, which involves a prediction-correction scheme. In the correction phase deformation dependent quantities are updated. The extensibility or inextensibility condition is used to reduce the degrees of freedom of the system. The numerical results of the developed computer code compare very well with available semi-analytical and numerical solutions. Three numerical applications are used to demonstrate the importance of large deformation, nonlinear and three-dimensional analyses.     

超深水钻井作业隔水管顶张力确定方法

对于深水和超深水钻井作业,确定钻井隔水管系统顶张力是钻前设计非常重要的工作。研究三种隔水管系统顶张力确定方法,分别是理论方法、基于隔水管系统底部残余张力方法和基于下放隔水管系统的最大钩载方法。算例计算与超深水钻井实践对比表明,在相同的隔水管系统配置下,三种方法计算结果都接近于实际钻井作业时的顶张力设定值。但研究认为,基于下放最大钩载的顶张力计算方法简单实用,推荐作为优选方法。     

Hybrid frequency–time domain models for dynamic response analysis of marine structures

Time-domain models of marine structures based on frequency domain data are usually built upon the Cummins equation. This type of model is a vector integro-differential equation which involves convolution terms. These convolution terms are not convenient for analysis and design of motion control systems. In addition, these models are not efficient with respect to simulation time, and ease of implementation in standard simulation packages. For these reasons, different methods have been proposed in the literature as approximate alternative representations of the convolutions. Because the convolution is a linear operation, different approaches can be followed to obtain an approximately equivalent linear system in the form of either transfer function or state-space models. This process involves the use of system identification, and several options are available depending on how the identification problem is posed. This raises the question whether one method is better than the others. This paper therefore has three objectives. The first objective is to revisit some of the methods for replacing the convolutions, which have been reported in different areas of analysis of marine systems: hydrodynamics, wave energy conversion, and motion control systems. The second objective is to compare the different methods in terms of complexity and performance. For this purpose, a model for the response in the vertical plane of a modern containership is considered. The third objective is to describe the implementation of the resulting model in the standard simulation environment Matlab/Simulink.     

A variational formulation for three-dimensional analysis of extensible marine riser transporting fluid

This paper presents a model formulation for static and dynamic analysis of three-dimensional extensible marine riser transporting fluid. A variational model formulation is developed based on the principle of virtual work-energy and the extensible elastica theory. The virtual work-energy functional is composed of the virtual strain energy due to axial stretching, bending, and torsion and the virtual work done by the external and internal fluid. The governing dynamic equilibrium equations are derived in the Cartesian coordinate. The finite element method is used to obtain the numerical solutions. The numerical examples are provided to demonstrate interesting effects of fluid transportation and axial deformation on large displacements and dynamic properties of the three-dimensional extensible marine riser.     

Hydroelastic response of marine risers subjected to internal slug-flow

It is the purpose of this study to investigate the dynamic behaviour of catenary pipelines for marine applications, assuming the combined effect of harmonic motions imposed at the top, and the internal slug-flow. The analysis is based on the assumption of a steady slug-flow inside the pipe that results in a relatively simplified model for the formulation of the internal flow. The slug-flow model is described using several assumptions and empirical correlations which attempt to reveal the ill-understood and concealed properties of the slug-flow. The pipeline dynamics are investigated in the two dimensional space omitting the out-of-plane vibrations. The system of differential equations is generic and accounts for the steady effect of the internal liquid as is conveyed through the structure.The two models, those of the internal slug-flow and the pipeline’s dynamical model, are properly combined through the internal flow terms of the dynamic equilibrium system. The solution provided is achieved using a frequency domain technique which is applied to the linearized governing set. The effect of the slug-flow is assessed through comparative computations with and without internal flow effects. The conclusions are drawn having the structure excited under axial and normal motions paying particular attention to the variation of the dynamic components along the complete length of the pipeline.     

Dynamic Characteristics of Marine Risers Conveying Fluid

The lateral vibration differential equation for a marine riser conveying fluid is derived by useof the small deflection theory.and the effect of internal flow velocity and top tension on the natural fre-quency of the riser is studied by use of FEM.At the same time,the preliminary relationship between thenatural frequency and riser span under different internal flow velocities is obtained,the effect of riser sup-ports on the vibration frequency is computed.It is found that the natural frequency of the marine riser in-creases with the increase of top tension.however decreases with the increase of internal flow velocity.In ad-dition,the frequency decreases drastically with the increase of riser span.     

Numerical and experimental analysis of the static behavior of highly deformed risers

This paper models a slender, flexible structure used as a drill string or riser in the offshore oil and gas industry that connects the well-head with a floating control vessel. These systems are used in deep-water drilling applications and present considerable design challenges due to their extreme flexibility and susceptibility to buckling and vibration. Two typical configurations are used (Bai and Bai, 2005), with a common feature involving the attachment of a buoy designed to relieve some of the axial forces acting on the riser, especially at the attachment points. Previous work by the authors studied numerical results of small-amplitude vibrations and two other equilibrium configurations using parameter values that closely resemble the full-scale application (Santillan et al., 2008). Here, two new configurations are considered, and experiments are designed and conducted to verify these equilibrium results.     

海洋柔性立管弯曲加强器参数敏感性分析

针对海洋立管弯曲加强器所受环境载荷与功能载荷的特点,基于有限元法对某海洋柔性立管弯曲加强器结构进行数值分析。通过有限元分析软件建立了弯曲加强器与柔性立管组合等效模型,重点讨论了弯曲加强器的设计参数对其自身防弯性能的影响,根据弯曲加强器的曲率及柔性立管的应力分布,对弯曲加强器结构进行优化设计,得到结论可供设计参考。     

基于凝集质量法的海洋缆索动力学建模与仿真技术

海洋缆索广泛用于海洋工程领域,海上作业过程中海缆在波浪、流等动力因素作用下容易产生显著的动力响应。为保证海上作业的安全,需要对海缆的动力响应进行有效的分析和研究。首先介绍了缆索的通用弹性动力学方程,然后给出了用凝集质量法求解此方程的模型,算法采用四阶龙格库塔法,利用Matlab仿真计算了水下拖缆回转的形态,经验证,结果有效。由此可得到凝集质量法在解决缆索动力问题中,也是一个很好的选择。     

内孤立波与深海立管相互作用数值模拟

提出了一种对内孤立波与深海立管相互作用耦合数值模拟方法。流场采用内孤立波数值水槽方法进行模拟,结构响应采用基于薄壳理论的有限元方法进行计算,采用一种将流场和结构响应数据进行实时传输的方法,实现了流体与固体之间的耦合数值模拟。对内孤立波作用下某长径比为1 200的深海立管载荷及其动力响应特性进行了数值模拟与分析。结果表明内孤立波不仅会对深海立管产生突发性剪切载荷作用,而且还会使立管产生大幅度变形响应现象,因此在深海立管设计与应用中,内孤立波的影响是不可忽视的。研究表明,该方法为研究内孤立波作用下深海立管动力特性及其工程预报相关问题提供了一种有效的手段。     

海洋纤维增强热塑性立管极限承载拉力预测方法

海洋新型纤维增强热塑性立管因其可盘卷、耐腐蚀、耐疲劳和轻质化等优点,在深水油气开发中应用前景十分广阔。热塑性立管具有复合材料的各向异性、受力耦合效应及复杂的本构关系,且承受浮体运动和复杂海洋环境载荷,其失效模式尚未明确。针对轴对称载荷作用下纤维增强热塑性立管极限承载力问题,进行热塑性管稳态热传导和热应力的理论推导,求解了稳态温度和应力分布,首次给出了在任意温度载荷作用下管体径向位移的解析解,并直接求解其径向、轴向、环向和剪切应力。采用各向同性层Von Mises和各向异性层最大应力（Max Stress）准则或Tsai-Hill准则判定热塑性管的失效,基于应力分布、失效准则和二分法计算了热塑性管的极限载荷。温度载荷、纤维铺设角度和径厚比对管道的应力分布影响显著。不同温度载荷会改变失效指数沿径向的变化趋势,增大轴向拉力将增大热塑性管的失效指数,选用不同的失效准则在管体失效判定上存在一定的差异。热塑性管温度越低、纤维铺设角越小及径厚比越大,管道对轴向拉伸载荷的承载能力越强。     

正弦激励下零浮力海洋拖缆动力特性与实验研究

海洋地震拖缆是海洋油气资源勘探中的关键设备,为研究海洋地震拖缆的动力特性及其在不同频率正弦激励干扰下的动力响应,建立了零浮力拖缆的数学模型,设计了一套完整的实验方案,通过弹性测力机构放大张力信号以排除环境干扰,测量了不同拖曳速度与激振频率条件下拖缆首部的张力变化及缆上位移响应情况,初步分析了正弦激励作用下零浮力拖缆的动力学响应。弹性测力机构具有良好的线性度与可重复性,拖缆受到的水阻力与拖曳速度的平方成正比,数值计算与实验结果对比验证了数学模型的正确性,初步分析了正弦激励作用下零浮力拖缆的动力学响应。     

考虑温度应力下深海输油立管动力特性及响应研究

考虑深海输油立管管内外温差产生的温度应力以及管内流体流动和管外海洋环境荷载的共同作用,建立了深海输油立管振动微分方程,采用有限元法对输油立管动力特性及动力响应进行求解,并将考虑温度应力及不考虑温度应力的立管动力特性及在波浪作用下的立管动力响应进行了对比。结果表明,对于动力特性,温度应力的存在会使得立管自振频率降低;对于动力响应,会使得波浪作用下动力响应幅值增强而频率略有降低,易引起立管的疲劳破坏。     

Enhanced Multi-Layer Fatigue-Analysis Approach for Unbonded Flexible Risers

This paper proposes an enhanced approach for evaluating the fatigue life of each metallic layer of unbonded flexible risers. Owing to the complex structure of unbonded flexible risers and the nonlinearity of the system, particularly in the critical touchdown zone, the traditional method is insufficient for accurately evaluating the fatigue life of these risers. The main challenge lies in the transposition from global to local analyses, which is a key stage for the fatigue analysis of flexible pipes owing to their complex structure. The new enhanced approach derives a multi-layer stress-decomposition method to meet this challenge. In this study, a numerical model validated experimentally is used to demonstrate the accuracy of the stress-decomposition method. And a numerical case is studied to validate the proposed approach. The results demonstrate that the multi-layer stress-decomposition method is accurate, and the fatigue lives of the metallic layers predicted by the enhanced multi-layer analysis approach are rational. The proposed fatigue-analysis approach provides a practical and reasonable method for predicting fatigue life in the design of unbonded flexible risers.