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.     

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。     

Axisymmetric viscoelastic response of flexible pipes in time domain

The challenges for determining the mechanical behavior of flexible pipes mainly arise from highly non-linear geometrical and material properties and complex contact interaction conditions between and within layers components. This paper develops an innovative model to investigate the linear viscoelastic behavior of flexible pipes under axisymmetric loads in time domain. The model is derived from an equivalent linear elastic axisymmetric model by invoking the elastic-viscoelastic correspondence principle. Analytical formulations that describe the behavior of the metallic helical layers based on a combination of differential geometry concepts and Clebsch–Kirchhoff equilibrium equations for initially curved slender elastic rods are presented. The elastic response of the homogenous polymeric cylindrical layers is also presented. The assemblage of both types of governing algebraic equations that approximate analytical solutions for force and moment distributions, deformations in each layer, as well as contact pressure between near layers, taking time-dependent characteristics of polymeric layers into account are provided and it is clear that the relationship between axial force and elongation is non-linear and encompasses a hysteretic response. Besides, the creep behavior in axial direction can also be found. Some insights into the differences in the behavior for several loading conditions are discussed by considering variable frequencies.     

An empirical model for flexible pipe armor wire lateral buckling failure load

The current scenario of oil production in ultra deep waters challenges the design of flexible pipes since they have to withstand high pressures, temperatures and dynamic loads during operation. Amongst the known failure mechanisms in flexible pipes, armor wire lateral buckling is of uttermost importance as there is no undisputable model for its prediction. In this work, compressive failure load for tensile armor lateral buckling is calculated for a sample of 29 flexible pipes based on the resolution of a numerical model that considers force and moment equilibrium conditions and geometric compatibility relations from differential geometry concepts. The obtained results are fitted on an equation via symbolic regression. Thus, a simple empirical model to predict compressive failure load is proposed as a function of the radius of tensile armor, wire lay angle and torsional, normal and binormal bending stiffness of the wire cross section. The model assumes the absence of frictional distributed forces in the wire and curvature of the flexible pipe, therefore it provides conservative results. For the 29 flexible pipe sample, the mean absolute percentage error of the empirical results in view of the numerical results was 0.6%.     

柔性管道骨架层压溃失效机理和安全评价方法研究进展

非黏结柔性管道作为深水油气开发领域的关键设备,是连接海底井口和海洋平台的主要纽带。海洋油气开发水深已经超过了3 000 m,高静水压力是深水软管设计和安全评价的主要挑战,明确深水柔性管道压溃失效机理,并准确地预测柔性管道的压溃压力可以为柔性管道结构设计和安全评价提供依据,柔性管道的压溃压力与骨架层的几何形状以及骨架层之间复杂的相互作用有关,准确高效地预测压溃压力变得极具挑战性。针对柔性管道失效机理研究和压溃压力预测方法进行了总结,指出当前柔性管道压溃失效分析中薄弱的环节,为我国非黏结柔性管道的结构设计和安全评价提供参考。     

玻纤增强柔性管等效简化模型研究

玻纤增强柔性管作为一种新型海底油气输送管道,具有比强度高、柔度大和抗腐蚀性强等特点,因此在深海油气开发中具有非常广阔的应用前景。玻纤增强柔性管主要由内衬层、增强层和外保护层组成,其中增强层的等效模拟是玻纤增强柔性管设计成功与否的关键。根据玻纤增强柔性管的结构特征和材料特性,选取了四种不同的等效简化模型,对比研究了玻纤增强柔性管在轴向拉压荷载、弯曲荷载以及内压荷载作用下的力学性能。将不同简化模型的计算结果与相应的试验数据进行对比,进行等效模型的优选。研究结果表明,在内压载荷和弯曲载荷作用下,基于Halpin-Tsai模型数值结果与试验结果最为接近。在轴向载荷作用下,采用分离式模型或回形模型计算精度更高,若材料达到屈服状态时,则建议采用分离式模型进行模拟。     

Analytical and numerical modeling for flexible pipes

The unbonded flexible pipe of eight layers,in which all the layers except the carcass layer are assumed to have isotropic properties,has been analyzed.Specifically,the carcass layer shows the orthotropic characteristics.The effective elastic moduli of the carcass layer have been developed in terms of the influence of deformation to stiffness.With consideration of the effective elastic moduli,the structure can be properly analyzed.Also the relative movements of tendons and relative displacements of wires in helical armour layer have been investigated.A three-dimensional nonlinear finite element model has been presented to predict the response of flexible pipes under axial force and torque.Further,the friction and contact of interlayer have been considered.Comparison between the finite element model and experimental results obtained in literature has been given and discussed,which might provide practical and technical support for the application of unbonded flexible pipes.     

考虑轴向荷载和外压影响的非黏结柔性管道骨架层失效分析

非黏结柔性管道骨架层由不锈钢材料互锁缠绕而成,主要用于抵抗外压防止软管发生压溃失效.骨架层失效是柔性软管故障的最大风险,骨架层失效模式主要分为压溃失效和拉伸失效.采用非线性有限元方法并且考虑几何大变形、材料非线性、接触和摩擦等非线性效应的影响,对"S"型骨架层结构建立三维有限元模型,进行了单外压作用下的压溃失效分析和单一轴向拉力作用下的拉伸失效分析,而后开展轴向拉力和外压载荷组合作用下骨架层压溃失效分析.结果表明轴向拉力会一定程度上降低骨架层压溃承载力,相关结论可以为柔性软管结构设计和完整性评价提供参考.     

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.     

Three-dimensional Lump-Mass formulation of a catenary riser with bending, torsion and irregular seabed interaction effect

A three-dimensional Lump-Mass formulation of a catenary riser, capable of handling irregular seabed interaction, with bending and torsional stiffness is presented in this paper. This formulation uses only three degrees of translational freedom and one independent torque variable for each computational node. The generality of the present formulation permits static and dynamic analyses of a wide range of offshore-related slender structure systems such as mooring cables, rigid and flexible risers as well as submarine pipelines. Four sets of results are presented for (i) a hanging catenary, (ii) as (i) but subjected to end torsion, (iii) a wire, chain and spring buoy mooring and (iv) a steel catenary riser on an irregular seabed.     

Behavior of Vertical Pressure Imposed on the Bottom of a Trench

A series of model tests were performed to investigate the behavior of vertical pressure imposed on the bottom of a trench. The tests were conducted in two phases to generate soil arching due to soil deformation: (1) the backfilling phase and (2) the lowering-bottom phase. First, the backfilling phase was performed by filling sand into the trench, in which soil deformation was developed due to selfcompaction by the weight of the sand. The lowering-bottom phase was then conducted by lowering the bottom plate of the trench to induce additional soil deformation. The behavior of the vertical pressure acting on the bottom of the trench during the backfilling phase can simulate that of the vertical pressure imposed on the bottom of buried rigid pipes. The behaviors of buried flexible pipes could be observed during the lowering-bottom phase, in which both the minimum and the maximum vertical pressures could be obtained according to the induced soil deformation. A novel method for predicting the vertical pressures in each phase was proposed. The agreements between the predictions by the proposed method and the experimental results were demonstrated.     

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.     

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.     

Validity and limitation of analytical models for the bending stress of a helical wire in unbonded flexible pipes

Unbonded flexible pipe is one of the important pieces of equipment in floating production systems for transport of oil and gas between floaters and subsea wells located in deep water. To assure safety over a long-term service period, analysis of fatigue behavior under alternative bending is a key requirement. An analytical model for prediction of bending behavior is essential for understanding the mechanism of the local stress distribution in the helical wires of the tension armor layers under alternative curvatures and rapid estimation of the service life of flexible pipes for designers in engineering practice. In this paper, seven analytical models available in the literature are selected and summarized. Although the experimental results reported in the literature are limited, a three-dimensional finite element model is developed for investigation of the seven models, and the validity and limitations of these models for different structural parameters of helical wire are discussed, i.e., the width-thickness ratio of the wire section and helical angle. The analytical model based on spring theory resulted in a satisfactory estimation of bending stress for most cases and is recommended as a tool for the basic design of the helical armor wire structures in flexible pipes.     

地震条件下海底双层油气管线的工作理论、强度标准和损伤评价

我国渤海湾是地震多发地区 ,该地区的海底输油管线随时都面临地震的威胁。地震成为渤海输油管线必要的设计条件 ,地震载荷是在役管线安全风险评估必须包括的环境载荷。本文综述了国内外在管线抗震设计及相关研究、双层油气管线的工作理论、强度标准与损伤评判等方面的研究工作 ,对我国制定地震条件下海底管线的评估计算方法、管线的抗震设计准则以及地震灾害的防治与风险分析具有一定的参考价值     

Design and Strength Analysis of A Spherical Connector for Lifting Subsystem in Deep Sea Mining System

1 .IntroductionThe lifting systemis composed of pipes andtheir connectors .It is not onlythe lifting channel formanganese nodule betweenthe miningship andintermediate bin,but alsothe supporting massfor deepsea miningequipment and cables .Sothe reliability…     

细长输流管内外流耦合振动特性研究

管内流动会影响输流管的振动响应,目前关于输流弹性管涡激振动方面的研究较少。基于计算流体力学(CFD)方法,开展内外流对细长输流弹性管振动特性影响的研究。首先在不考虑内流的情况下将弹性管涡激振动数值预报结果与模型试验数据进行对比,验证了数值方法的可靠性。再者考虑内外流耦合作用情况下,对不同内流流速下细长输流弹性管振动位移时—空分布、顺流向最大平均偏移、振动轨迹、内部横向涡的形成与分布等进行了对比分析。结果发现,与外流流速相比,内流流速的增加虽然难以改变弹性管的主振模态,但对沿管体的振动强度影响显著。顺流向最大偏移处管体运动轨迹发生明显的变形和跳跃。在剪切外流和均匀内流对弹性管的联合作用下,沿管跨方向模态间能量转换频繁,伴随着间歇性出现或消失的沿弹性管传播的行波组分,这主要归因于复杂的双重流固耦合系统(外流—管体,内流—管体)。在内流以附加质量力、离心力和科氏力形式的激励下,弹性管内二次流现象明显。在振动过程中,内部横向涡沿管壁生成、脱落并逐渐散布于整个横截面。     

Effect of thickness variation on collapse pressure of seamless pipes

The present paper is concerned with determining the effect of thickness variation on collapse pressure of seamless pipes. There exists extensive information on the effects external pressure has on pipelines. A substantial amount of literature deals with results of analysis, numerical modelling and testing of pipes those are gradually subject to external pressure. This knowledge has been incorporated into standards which are intended for designing linepipe operating in deepwater locations. One such example is the DNV-OS-101 (DNV, 2007) Submarine Pipeline Systems standard. This code only addresses pipes with a nominally uniform wall thickness making it inadequate for designing seamless pipe which is prone to thickness variation arising from the manufacturing process. The DNV (2007) code effectively ignores the problem as it simply provides a vague statement regarding a conservative value of wall thickness that should be used in the design calculations.The present paper uses finite element analysis (FAE) modelling to predict the external pressure acting on the pipes. The present paper explains the manner in which FEA models will be used to study the effect wall thickness variation will have on the pressure at which the pipe will collapse. An improvement to the DNV (2007) code is then suggested by interpreting the results of the FEA models. The case study that follows provides insight on the savings the improved method is projected to offer.     

Structural Design and Protective Methods for the 100 kW Shoreline Wave Power Station

1 .IntroductionThe 1 0 0kWshorelinewavepowerstationofChinaisanOscillatingWaterColumn (OWC)wavepowerdeviceintegratedtoanelectricgrid .ItislocatedattheZhelangTown ( 2 2°39’Nand 1 1 5°34’E) ,ShanweiCityofGuangdongProvince .Theresearchanddevelopmentofthe 1 0 0kWOWCshore linewavepowerstationissupportedbyChina′sNationalKeyTechnologiesR&DPrograminthe 9thfive yearplan .TheconstructionofthestationstartedinJune 1 998andwascompletedinFebruary2 0 0 1 .Uptonow ,ithasbeenoperatingfor2years …     

Shallow plumbing systems inferred from spatial analysis of pockmark arrays

This study describes and analyses an extraordinary array of pockmarks at the modern seabed of the Lower Congo Basin (offshore Angola), in order to understand the fluid migration routes and shallow plumbing system of the area. The 3D seismic visualization of feeding conduits (pipes) allowed the identification of the source interval for the fluids expelled during pockmark formation. Spatial statistics are used to show the relationship between the underlying (polarised) polygonal fault (PPFs) patterns and seabed pockmarks distributions. Our results show PPFs control the linear arrangement of pockmarks and feeder pipes along fault strike, but faults do not act as conduits. Spatial statistics also revealed pockmark occurrence is not considered to be random, especially at short distances to nearest neighbours (<200 m) where anti-clustering distributions suggest the presence of an exclusion zone around each pockmark in which no other pockmark will form. The results of this study are relevant for the understanding of shallow fluid plumbing systems in offshore settings, with implications on our current knowledge of overall fluid flow systems in hydrocarbon-rich continental margins.