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.     

Non-symmetric cyclic bending of helical wires in flexible pipes

Unbonded flexible pipes have superior fatigue performance as the internal armor layers are allowed to move relative to each other, leading to reduced structural loading. The main interactions between the internal layers are the contact forces and the frictional forces. Frictional interaction leads to a complex non-linear response of unbonded flexible pipes making prediction of cyclic bending fatigue a demanding task. Nevertheless, detailed understanding of local armor wire stresses and the related fatigue phenomena is of paramount importance as unbonded flexible risers are often operated close to their mechanical limits. This paper presents a method for calculating the tensile armor wire loading and the hysteresis effect on flexible pipes when subjected to tension, and non-symmetric cyclic bending. The effect of non-symmetric cyclic bending with different tensile armor lay angles, and frictional conditions are studied. The analysis uses an efficient repeated unit cell finite element model, allowing the analysis to be performed on a desktop computer. The study shows that the tensile armor wires gradually translate towards the compression side of the pipe bending plane, when the unbonded flexible pipe is subjected to combined tension and non-symmetric cyclic bending. In the analysis, cyclic bending is applied until steady-state in the bending response is achieved over a full bending cycle. The global bending response of the flexible pipe and the tensile armor wire loading conditions for fully stabilized non-symmetric cyclic bending become symmetric around the frictionless state for the mean cyclic bending curvature. An approximate analytic model for the tensile armor in the stabilized cycle, based on symmetric bending about the geodesic curve corresponding to the mean pipe curvature, is proposed.     

Parametric interpolation method for probabilistic fatigue analysis of steel risers

Slender marine structures such as mooring lines and risers are susceptible to failures due to stress variations coming from environmental actions. Wave, wind and current are random phenomena, and consequently the most adequate methodology to estimate the fatigue damage accumulation on these structures is the probabilistic fatigue analysis. In practice, the estimation of fatigue life requires the numerical simulation of a huge number of loading cases to compute the multi-dimensional integral of the total fatigue damage.This paper presents an efficient approach to compute the long-term fatigue damage of marine structures. The proposed method needs only a few number of numerical simulations to estimate the structure fatigue life. It uses a parametric interpolation procedure to evaluate the fatigue damage for any individual short-term condition (sea state) required in the calculation of the multi-dimensional integral. In this way, the total number of short-term structural analyzes is considerably reduced.The effectiveness and accuracy of the proposed method is compared to the full direct integration by means of two comprehensive examples. The first studied case is an idealized theoretical model of a single-degree-of-freedom (SDOF) system under wave loading, and the second one is a Steel Catenary Riser (SCR) connected to a FPSO (Floating, Production, Storage and Offloading floating unit).     

Multiaxial Fatigue Analysis on Reeled Deepwater Steel Catenary Risers with Girth Weld Defects

In the present study, we simulated the reel-lay installation process of deepwater steel catenary risers（SCRs） using the finite element method and proposed multiaxial fatigue analysis for reeled SCRs. The reel-lay method is one of the most efficient and economical pipeline installation methods. However, material properties of reeled risers may change, especially in the weld zone, which can affect the fatigue performance. Applying finite element analysis（FEA）, we simulated an installation load history through the reel, aligner, and straightener and analyzed the property variations. The impact of weld defects during the installation process, lack of penetration and lack of fusion, was also discussed. Based on the FEA results, we used the Brown-Miller criterion combined with the critical plane approach to predict the fatigue life of reeled and non-reeled models. The results indicated that a weld defect has a significant influence on the material properties of a riser, and the reel-lay method can significantly reduce the fatigue life of SCRs. The analysis conclusion can help designers understand the mechanical performance of welds during reel-lay installation.     

Multiaxial Fatigue Analysis on Reeled Deepwater Steel Catenary Risers with Girth Weld Defects

In the present study, we simulated the reel-lay installation process of deepwater steel catenary risers (SCRs) using the finite element method and proposed multiaxial fatigue analysis for reeled SCRs. The reel-lay method is one of the most efficient and economical pipeline installation methods. However, material properties of reeled risers may change, especially in the weld zone, which can affect the fatigue performance. Applying finite element analysis (FEA), we simulated an installation load history through the reel, aligner, and straightener and analyzed the property variations. The impact of weld defects during the installation process, lack of penetration and lack of fusion, was also discussed. Based on the FEA results, we used the Brown-Miller criterion combined with the critical plane approach to predict the fatigue life of reeled and non-reeled models. The results indicated that a weld defect has a significant influence on the material properties of a riser, and the reel-lay method can significantly reduce the fatigue life of SCRs. The analysis conclusion can help designers understand the mechanical performance of welds during reel-lay installation.     

考虑涡激运动下海洋柔性立管的疲劳损伤分析

基于ABAQUS/AQUA模块,当Spar平台发生涡激运动时,对与平台相连接的柔性立管进行时域动态响应分析,得到了不同折合速度下柔性立管的应力响应曲线.对立管触地区域和悬垂段进行了细化的分析,并利用S-N曲线和Miner线性累积损伤理论对平台涡激运动引起的柔性立管疲劳损伤进行了计算.研究结果表明:平台涡激运动对立管触地区域和悬垂段的影响较大,这两部分最容易在长时间的工作中产生疲劳损伤.特别是平台运动到近点时,立管触地区域受到的应力急剧增加,这个区域是立管工作的危险区域,应该特别注意.该研究结论可以作为立管结构设计和海底铺设时的参考,通过增加立管触地区域的结构强度和优化铺设方式,从而来提高立管的疲劳寿命和系统的安全性.     

On the response of flexible risers to loads imposed by hydraulic collars

Even though hydraulic collars are largely used to install flexible risers, neither the loads imposed by this equipment nor the response of the risers to these loads have been previously studied. Hence, this paper presents a three-dimensional nonlinear finite element (FE) model to predict the response of flexible risers to loads imposed by hydraulic collars and also provides a set of equations to predict these loads. The FE model relies on an analogy between helical tendons and orthotropic shells to simulate the inner carcass and the pressure armour of flexible risers. Shell elements are used to represent the polymeric layers and three-dimensional beam elements simulate the wires of the tensile armours. Material, geometric and contact nonlinearities are addressed. Contact interactions between the layers of the riser are handled by surface to surface contact elements with a contact detection algorithm based on the pinball technique and contact forces evaluated with the augmented Lagrangian method. A 9.5″ flexible riser is analyzed and the numerical results are compared to those from the experimental tests. The agreement between all results indicates that the proposed FE model is an efficient approach to predict the response of flexible risers to loads imposed by hydraulic collars and, moreover, may be used to analyze the response of such structures to other types of loads.     

Behavior of unbonded flexible risers subject to axial tension

Owing to nonlinear contact problems with slip and friction, a lot of limiting assumptions are made when developing analytical models to simulate the behavior of an unbonded flexible riser. Meanwhile, in order to avoid convergence problems and excessive calculating time associated with running the detailed finite element （FE） model of an unbonded flexible riser, interlocked carcass and zeta layers with complicated cross section shapes are replaced by simple geometrical shapes （e.g. hollow cylindrical shell） with equivalent orthotropic materials. But the simplified model does not imply the stresses equivalence of these two layers. To solve these problems, based on ABAQUS/Explicit, a numerical method that is suitable for the detailed FE model is proposed. In consideration of interaction among all component layers, the axial stiffness of an eight-layer unbonded flexible riser subjected to axial tension is predicted. Compared with analytical and experimental results, it is shown that the proposed numerical method not only has high accuracy but also can substantially reduce the calculating time. In addition, the impact of the lay angle of helical tendons on axial stiffness is discussed.     

Analytical and Numerical Models to Predict the Behavior of Unbonded Flexible Risers Under Torsion

This paper presents analytical and numerical models to predict the behavior of unbonded flexible risers under torsion. The analytical model takes local bending and torsion of tensile armor wires into consideration, and equilibrium equations of forces and displacements of layers are deduced. The numerical model includes lay angle, cross-sectional profiles of carcass, pressure armor layer and contact between layers. Abaqus/Explicit quasi-static simulation and mass scaling are adopted to avoid convergence problem and excessive computation time caused by geometric and contact nonlinearities. Results show that local bending and torsion of helical strips may have great influence on torsional stiffness, but stress related to bending and torsion is negligible; the presentation of anti-friction tapes may have great influence both on torsional stiffness and stress; hysteresis of torsion-twist relationship under cyclic loading is obtained by numerical model, which cannot be predicted by analytical model because of the ignorance of friction between layers.     

非黏结柔性管道抗拉铠装层腐蚀机理与安全评价研究进展

非黏结柔性管道是海洋油气开发的重要装备,在海水环境中,抗拉铠装层发生腐蚀会对安全运行产生严重威胁,因此明确抗拉铠装层腐蚀机理、对柔性管道的检测和对抗拉铠装层的腐蚀评估十分重要。抗拉铠装层腐蚀与环空的气体环境和水环境有关,采用合适的检测方法和设备检测环空,可以帮助判断其腐蚀情况,并为腐蚀疲劳寿命评估和应对策略制定提供参考。针对抗拉铠装层腐蚀机理、非黏结柔性管道检测、腐蚀疲劳寿命评估和腐蚀应对措施进行了总结,指出机理研究和检测评估方法的不足,为国内柔性管道抗拉铠装层腐蚀研究和完整性管理提供参考。     

Nonlinear bend stiffener analysis using a simple formulation and finite element method

Flexible marine risers are commonly used in deepwater floating systems.Bend stiffeners are designed to protect flexible risers against excessive bending at the connection with the hull.The structure is usually analyzed as a cantilever beam subjected to an inclined point load.As deflections are large and the bend stiffener material exhibits nonlinear stress-strain characteristics,geometric and material nonlinearities are important considerations.A new approach has been developed to solve this nonlinear problem.Its main advantage is its simplicity;in fact the present method can be easily implemented on a spreadsheet.Finite element analysis using ABAQUS is performed to validate the method.Solid elements are used for the bend stiffener and flexible pipe.To simulate the near inextensibility of flexible risers,a simple and original idea of using truss elements is proposed.Through a set of validation studies,the present method is found to be in a good agreement with the finite element analysis.Further,parametric studies are performed by using both methods to identify the key parameters and phenomena that are most critical in design.The most important finding is that the common practice of neglecting the internal steel sleeve in the bend stiffener analysis is non-conservative and therefore needs to be reassessed.     

Non-parametric prediction of the long-term fatigue damage for an instrumented top-tensioned riser

Marine risers are susceptible to sustained vortex-induced vibration (VIV) because of their slenderness and light damping. Commonly used tools for analyzing VIV and the associated fatigue damage are based on the finite element method and rely on simplifying assumptions on the riser's physical model, the flow conditions, and characteristics of the response. In order to assess the influence of VIV and to ensure the integrity of the riser, field monitoring campaigns are often undertaken wherein data loggers such as strain sensors and/or accelerometers are installed on such risers. Given the recorded riser's dynamic response, empirical techniques can be used in VIV-related fatigue estimation. These empirical techniques make direct use of the measurements and are intrinsically dependent on the actual current profiles. Damage estimation can be undertaken for the different current profiles encountered and can account explicitly even for complex riser response characteristics. With a significant amount of data, “short-term” fatigue damage probability distributions, conditional on current, can be established. If the relative frequency of different current types is known from a separate metocean study, the short-term fatigue damage distributions can be combined with the current distributions to yield an integrated “long-term” fatigue damage model, which then can be used to predict the long-term cumulative fatigue damage for the instrumented riser. Non-parametric statistical techniques (that do not assume a specific function for the underlying distribution as parametric techniques do) are employed to describe the short-term fatigue damage data. In this study, data from the Norwegian Deepwater Programme (NDP) model riser experiments are used to demonstrate the effectiveness of empirical procedures and non-parametric statistics applied to field measurements to predict long-term fatigue damage, life, and probability of fatigue failure.     

Modeling touchdown point oscillation and its relationship with fatigue response of steel catenary risers

The riser-seabed interaction resulting in a trench formed in the touchdown zone (TDZ) of steel catenary risers (SCR) has a significant influence on accumulated fatigue damage. Several studies have used different trench modeling approaches to investigate the trench effect on fatigue. However, contradictory observations have been reported with no coherent agreement on the beneficial or detrimental effect of the trench on fatigue. In this study, the significance of trench geometry in fatigue damage evaluation was investigated. Using analytical and numerical approaches, a meaningful relationship was observed between the trench slope in different zones and the peak fatigue damage. A new set of rules was proposed for the qualitative assessment of the overall trend of trench effect on the variation of fatigue damage. The proposed assessment rules were validated by performing comprehensive numerical fatigue analysis. A comparison with samples of published experimental and numerical studies was also completed. It was observed that depending on the direction of the low-frequency vessel excursions, the peak fatigue damage may increase towards the near offsets and decrease towards the far vessel offset. This implied that the case dependency of the trench effect on fatigue response in different geographical locations with various environmental loads was a potential source for the contradictory results reported in previously published studies.     

张力腿平台水下丛式井口圆形布局方法研究

张力腿平台(TLP)水下丛式井口布局设计关系到立管服役期间的安全性,是含有丛式井口平台设计过程中考虑的主要因素。根据TLP立管安装、钻井以及防碰等要求,考虑尾流效应对丛式立管的影响,研究水下丛式井口布局设计准则,对丛式井口—立管系统进行碰撞分析以确定水下井口间距阈值,提出水下丛式井口圆形布局方法和相应的布局设计流程。通过算例对计算方法进行了具体运用。研究表明:与目前采用的"等边三角形网格"方法相比,水下丛式井口圆形布局方法可允许水下整体基盘安装位置误差达到水下井口间距阈值的20%,立管的最大倾角可达到0.5°,安装作业窗口增加1倍以上。本方法对水下整体基盘安装位置误差的容错能力强,能有效扩大丛式立管钻井和安装作业窗口,提高丛式立管作业安全性,研究结果可为TLP丛式井口的设计提供参考。     

国外深水钢悬链线立管研究发展现状

介绍国外在新型深水立管系统--钢悬链线立管关键技术方面的研究发展现状,论述浮体一、二阶运动对钢悬链线立管疲劳寿命的影响、浮体升沉运动对钢悬链线立管触地点疲劳寿命的影响;钢悬链线立管与海底相互作用机制的实验研究及结果;钢悬链线立管涡致振动与疲劳的研究现状.并简要论述钢悬链线立管触地点问题的研究结论.     

Practical stochastic fatigue analysis of offshore platforms

A method for stochastic fatigue analysis of offshore platforms is presented. The method accounts for the nonlinearity in the drag loading term, and for the systematic deviation from a Gaussian process for a platform response. The method is based on an assumed form of the stress response process at a hot spot. A number of full scale measurements for elements in fixed platforms and for marine risers show very good agreement with this assumed form. A conventional spectral fatigue approach with stochastic linearization of the drag term in the basis for the new method, which uses results from linearizations in two or more sea states. The method is easily implemented in standard computer programs presently used within the offshore oil industry for fatigue analysis.     

Review of flexible risers and articulated storage systems

The anchorage system for mid-ocean loading or production consists of an articulated tower for mooring the tanker. Flexible risers are also essential components of the anchorage system. The present paper provides a state-of-the-art review on articulated storage systems and flexible risers, giving theoretical background for the development of computer software for the static analysis of flexible risers.In the state-of-the-art review for flexible risers, various analysis techniques for elastic lines and flexible risers under self-weight, current and wave forces are presented. The dynamic response of the flexible riser, including vortex-induced oscillations, is also outlined.The literature concerning the articulated tower and tanker is relatively scarce. Available works related only to dynamic responses of articulated towers. The combined response of tower and tanker is only studied by Chakrabarti and Cotter [(1978), Analysis of a tower-tanker system. In Proceedings of the 10th Offshore Technology Conference, OTC 3202, pp. 1301–1310] in a limited sense. The review of these works is summarised relevant to this paper.In the end, the static analysis of the flexible riser under its self-weight and current is presented using a finite difference approach. The problem essentially involves geometrical non-linearity, which is tackled with the help of an iterative solution based on modified Newton-Raphson technique. The theoretical formulation presented is being used to develop the computer software for the static analysis of the flexible risers.     

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.     

钢悬链式立管管土耦合作用的吸力效应研究

钢悬链式立管（steel catenary riser,简称SCR）的流线段敷设在海床上,在浮体运动和环境荷载作用下管线作拔出海床的上升运动时,软质海床的黏性性质将阻碍管线的拔出而表现出吸力效应。吸力的大小与管线的拔出速度和管土循环作用次数、土的重塑时间等相关。基于现有试验数据拟合得到吸力数值模型,用于改进立管动力分析程序,研究立管拔出速度和管径等对触地区吸力分布、动态响应和疲劳寿命的影响。结果表明：海床土吸力对立管触地区应力特别是弯曲应力的影响较大,二者的变化趋势相似;管径的影响主要体现在贯入深度与管径的相对大小,同一贯入速度下,管径越小则相对贯入深度越大,拔出位移与吸力也会越大,反之则越小;立管的拔出速度是影响海床吸力最大值和拔出位移的主要因素,土吸力和拔出位移随拔出速度的增大而增大,导致触地区的疲劳损伤加剧。因此,探究管土耦合作用的吸力效应及其对SCR触地区疲劳损伤的影响,可为SCR与复杂海床相互作用及工程应用提供重要参考。     

层间摩擦系数对非黏结柔性管抗拉铠装层轴压屈曲失效模式的影响研究

在柔性管运输、铺设安装和服役过程中,由于其抗拉铠装层钢带为柔性细长螺旋结构,当外部载荷超过临界值时,抗拉铠装层钢带容易发生屈曲失效。考虑非线性材料、几何大变形、层间接触和摩擦等非线性效应的影响,运用ABAQUS有限元软件建立了63.5 mm典型非黏结柔性管8层完整结构模型,研究了在抗拉铠装层不同层间摩擦系数的情况下,非黏结柔性管在轴向压缩载荷作用下的力学性能和屈曲失效模式。研究结果表明,层间摩擦系数对非黏结柔性管的抗压刚度和抗拉铠装层屈曲失效模式有较大影响,层间摩擦系数以0.1和0.2为分界点,抗压刚度和屈曲失效模式均呈现出“三段式”变化规律。研究成果可为海洋非黏结柔性管的结构设计和屈曲失效评估提供技术参考。