无粘结柔性管抗拉伸层结构分析

柔性管抗拉伸层是复杂的空间螺旋线结构,其结构响应分析对柔性管疲劳分析、强度分析和屈曲分析有重要作用。文章基于曲梁理论,应用斜驶螺旋线假设和测地线假设两种空间曲线公式,以空间细长杆理论及胡克定律本构方程为基础,采用格林应变张量与第二Kirchoff应力张量度量,对深海无粘结柔性管抗拉伸层螺旋形钢缆结构平衡方程进行了推导,编写了分析程序。利用该程序,分析了抗拉伸层钢缆在轴对称载荷下和弯矩作用下的曲率变化和结构响应;同时利用三维直梁有限元模型与曲梁有限元模型建立数值模型,将程序结果与数值模拟模型结果进行了对比,证明了结果可行性。该结果可为柔性管抗拉伸层结构设计提供快速的预估计方法。     

A finite element model for flexible pipe armor wire instability

The constructive disposition of metallic and plastic layers confers flexible pipes with high and low axial stiffness respectively when tensile and compressive loads are applied. Under certain conditions typically found during deepwater installation or operation, flexible pipes may be subjected to high axial compression, sometimes accompanied by bending. If not properly designed, the structure may not be able to withstand this loading and fails. From practical experience observed offshore and in laboratory tests two principal mechanisms, which will be discussed in this paper, have been identified regarding the configuration of the armor wires. When the pipe fails by compression the armor wires may exhibit localized lateral or radial deflections, consequently permanent damage is observed in the armor wires with a sudden reduction of the structure’s axial stiffness. The pressure armor may also unlock, thus causing potential fluid leakage.In this work a finite element model is developed to estimate the critical instability load and failure modes. An axi-symmetric model is constructed employing a complex combination of beam and spring elements. For each armor layer only one wire needs to be modeled, hence the computational cost is minimized without compromising the phenomenon characterization. A parametric case study is performed for a typical flexible pipe structure, where the friction coefficient between the wire armors and the external pressure are varied, and the critical instability loads and failure modes are obtained and results are discussed.     

A finite element methodology for birdcaging analysis of flexible pipes with damaged outer layers

Flexible pipes are commonly exposed to damages on the outer layers due to abrasion with seafloor or improper installation and operation, which may render them vulnerable to birdcaging failures. This paper presents a finite element model for the residual axial compressive strength evaluation of a flexible pipe with local damage on the outer layers. The elastoplastic nonlinearity of tensile armour steel layers and hyperelasticity of polymeric outer sheath are taken into account. This model is verified against existing test data. Parametric studies are then performed by varying the damage size in either the pipe axial or circumferential directions. The flexible pipe axial resistance, deformations, as well as the tensile armour wires layers stress states near the damaged section under different damage and axial compression conditions are discussed. The case studies show that damage on the outer layer, especially the anti-birdcage tape layer, is highly detrimental to flexible pipe residual strength against axial compression. The present results and discussions are instructive in understanding the flexible pipe birdcaging mechanism.     

非粘合柔性管破段强度解析分析（英文）

This paper presents an analytical scheme for predicting the collapse strength of a flexible pipe, which considers the structural interaction between relevant layers. The analytical results were compared with a FEA model and a number of test data, and showed reasonably good agreement. The theoretical analysis showed that the pressure armor layer enhanced the strength of the carcass against buckling, though the barrier weakened this effect. The collapse strength of pipe was influenced by many factors such as the inner radius of the pipe, the thickness of the layers and the mechanical properties of the materials. For example, an increase in the thickness of the barrier will increase contact pressure and in turn reduce the critical pressure.     

压力载荷下非粘结柔性立管应变影响因素分析

基于Abaqus/Explicit准静态和质量放大方法研究了一类典型非粘结柔性立管在压力载荷作用下应变响应特性,对影响立管整体轴向延伸率和绕轴向扭转角度的因素进行了分析。数值模型计入金属层实际截面形状、铺设角度以及几何、接触、材料非线性。计算结果表明：数值解与理论值吻合较好;立管端部边界条件对轴向延伸率影响不大但对绕轴向扭转角度影响较大;抗压铠装层为径向压力主要受力构件,其铺设角度虽然对压溃性能不大,但在应变分析中不可忽略;拉伸铠装层铺设角度对应变影响同样较大。文中数值方法可弥补理论方法限定在小位移、小变形范围,无法计入层间摩擦、材料非线性及初始制造椭圆率等缺陷。     

Axisymmetric structural behaviours of composite tensile armoured flexible pipes

To address the weight and corrosion challenge in deep-water, replacing the steel tensile armour in flexible pipes with composite materials is an alternative conceptual approach. An axisymmetric structural responses model is built for this novel composite armoured flexible pipe, with interlayer gaps that may occur in the unbonded structure considered through an iterative algorithm. The tensile strength of steel and composite armoured pipes are predicted based on different constitutive relations of steel and composite. Essential quantities are obtained, such as tensile stiffness, deformations of each layer and interlayer gaps or contact pressures. Considering the helix form of carcass and pressure armour, a finite element model is established for the verification of the theoretical model. Case study shows that the tensile stiffness of flexible pipe is overestimated with the interlayer gap ignored. Compared with steel armoured flexible pipe, the composite armoured pipe, whose tensile stiffness decreases less as external pressure increases, meanwhile has higher values of tensile ultimate strength and torsion stiffness. Some suggestions about fiber types and volume fraction for composite tensile strips are given to ensure good performance of axial tensile strength and stiffness.     

An investigation on flexible pipes birdcaging triggering

An investigation on triggering mechanisms for the birdcaging failure mode of flexible pipes, used in offshore oil and gas production, is carried out. From previous experimental observations, a conjecture is made: the local axisymmetric instability of the external plastic layer, caused by the high radial loading which is internally applied due to the helical armor wires tendency to expand when the pipe is subjected to compression, would be this trigger. A simple instability onset criterion for the external plastic layer, namely, the polymer intrinsic yielding limit stress, is proposed and assessed, analytically, numerically and experimentally for HDPE tubes. Then, previous birdcaging experimental observations are investigated further, focusing on the flexible pipe external plastic layer, to assess the proposed criterion. Strong evidences of validity are obtained.     

Buckle propagation of damaged SHCC sandwich pipes: Experimental tests and numerical simulation

Sandwich pipe (SP) combining high-strength performance and thermal insulation has been considered an effective solution for oil and gas transportation in ultra-deepwater. Strain hardening cementitious composite (SHCC) is well known for its capacity to withstand both tensile load and external hydrostatic pressure. The sandwich pipe considered in the research is constituted of concentric steel pipes with SHCC annular layer. In the present research work, the SHCC was manufactured, and full scale sandwich pipes were assembled. Intact and damaged specimens were submitted to controlled external pressure in a hyperbaric chamber to obtain the collapse and propagation pressures, respectively. Modeling and simulation of the buckle propagation of the SPs were correlated with the experimental results. The results show that sandwich pipe with SHCC core has an excellent structural strength under high external pressure in both intact and damaged conditions. Moreover, the results also show that the interaction between the annular and the inner/outer pipes provides a significant contribution to the buckling resistance under propagation pressure.     

Experimental study on the interlayer friction and wear mechanism between armor wires of umbilicals

Friction and wear behavior between armor wire layers of umbilical plays an important role in its mechanical properties, in particular its fatigue life. The present paper addresses friction tests to investigate the interlayer sliding friction and wear mechanism of armor wires in an umbilical. A series of friction tests were carried out to study the evolution of coefficient of friction (COF) and wear between armor wire layers. The results show that the COF increases with increasing number of reciprocating sliding cycles in dry friction condition. A dual stable trend was found in the evolution process of interlayer friction and wear of armor wires. A friction model was proposed to describe this trend by dividing the evolution process into three stages: the initial stable stage, the transition stage, and the latter stable stage. The mechanism of the dual stable trend was revealed, which was mainly caused by wear process of nylon fiber tape, and effected by dry/wet wear condition and wear coefficient. The effect of the contact condition, sliding amplitude and number of nylon fiber tape on COF was studied. The COF of the latter stable stage increases with increasing sliding amplitude and number of layers of nylon fiber tape. The finding in this paper provides a valuable approach for a more accurate fatigue life prediction of an umbilical.     

Local structural analysis of flexible pipes subjected to traction,torsion and pressure loads

Helically armored cables or pipes find a wide range of applications as structural members in engineering. An example of this is the increasing use of flexible pipes in the oil offshore production. Although keeping a geometrical similarity with other helically armored structures such as wire ropes and ACSR conductors, and borrowing from them a useful methodology for the structural analysis, some care must be taken in order not to indiscriminately use an approach which was not thought for a flexible pipe: internal and external pressures, for instance, are a great concern in the analysis of flexible pipes, but obviously not for wire ropes. This work aims at giving some additional contribution to the structural response of flexible pipes when subjected to axisymmetric loads, including the effect of both internal and external pressure in pipe displacements. Derivation of linear operators, relating the stress-resultants to their related displacements or deformations in each of the layers of the pipe, as well as the process of deriving an analogous linear operator to represent the behavior of the pipe as a whole, are clearly presented, highlighting interesting mathematical aspects and their associated physical meaning. A numerical case study of a 2.5″ flexible pipe subjected to traction and internal pressure is also presented and discussed.     

Bending behavior modeling of unbonded flexible pipes considering tangential compliance of interlayer contact interfaces and shear deformations

Rigorous analytical formulations are given to describe the gross slip initiation and progression in tensile armor layers of unbonded flexible pipes. Then two mechanisms are thought to contribute to the decrease of layer stiffness before gross slip begins. The first one considers the micro-slip occurred at the interlayer contact interfaces. The relative displacement between an armor wire and the underlying layer is determined according to the theory of contact mechanics. Shear deformations of the supporting plastic layer are taken into account in the other mechanism where plane sections no longer remain plane. The results of bending moment-curvature relationship from the presented models are compared with the available test data and good correlations are found. The shear model is seen to describe the slip transition better than other models do.     

Application of analytical model in the prediction of dynamic responses and fatigue damage of flexible risers: Part I – Improvement of analytical model considering shear deformation and varying tension effects

Flexible risers have been widely utilized for the transfer of oil and gas products from a well to production units. The components of flexible risers, unlike steel risers, experience complex contact phenomena during bending. The contact between helical wires and adjacent layers especially causes a significant level of bending nonlinearity, making it hard to estimate the structural responses. Accordingly, a large-scale dynamic analysis of flexible risers usually involves an analytical model that predicts the bending moment and axial stress of helical wires based on theoretical approaches. The analytical model consists of an axis-symmetrical model and a bending model. Among them, the bending model plays a critical role in the prediction of the bending responses of flexible risers. The conventional bending models usually neglect the shear deformation of internal layers and continuity of sliding force, which leads to a significant error of analysis. Furthermore, the previous bending models assume that the contact pressure on helical wires is constant during bending. In real operating conditions, however, most flexible risers experience a considerable change of tension that governs the slip of helical wires. Hence, the current study presents a new dynamic analysis method for flexible risers. The suggested analytical model improves the bending model based on an accurate estimation of the internal strain field considering the shear deformation and continuous sliding force. Also, this study proposes a stiffness update method to reflect the effect of varying tension in the dynamic analysis. The presented method updates the bending property of flexible risers considering the continuous change of the contact pressure from varying tension. For the validation of suggested method, the current study carries out numerical simulations with a pure bending and varying tension for the internal diameter 7 inches flexible risers. It is identified that the suggested analytical model provides accurate analysis results. Moreover, it is found that the effect of varying tension gives a significant impact on the bending behavior of flexible risers by changing the slip condition of helical wires. Part I of this series of papers describes the detailed formulation method for the analytical model and with some verification examples. The suggested analytical model is expanded to the large-scale dynamic analysis in Part II for the investigation of the effect of shear deformation and varying tension.     

A finite element model for unbonded flexible pipe under combined axisymmetric and bending loads

Flexible pipes are key equipment for offshore oil and gas production systems, conveying fluids between the platform and subsea wells. The structural arrangement of unbonded flexible pipes is quite complex, encompassing several layers with polymeric, metallic and textile materials. Different topologies and a large amount of intricate nonlinear contact interactions between and within their components, especially because of the relative stick-slip mechanism during bending, makes numerical analysis challenging. This paper presents an alternative three-dimensional nonlinear finite element model that describes the response of flexible pipes subjected to combined axisymmetric and bending loads. To simulate the response of a flexible pipe under axial tension or compression combined with uniform curvature, an equivalent thermal loading is employed on the external sheath, which is modelled as an orthotropic thermal expansion material with temperature-independent mechanical properties. To assess the feasibility of the proposed model, the bending moment versus curvature of the finite element solution is compared with experimental results obtained in literature and good agreements are found between them. Detailed finite element results such as contact pressures, armour wire slip displacements and friction, normal and transverse bending stresses are also shown and compared with available analytical models.     

Experimental and finite element analysis of fatigue strength for 300 mm2 copper power conductor

The fatigue strength of a 300 mm² stranded copper conductor was investigated experimentally and by finite element (FE) analysis. An analytical model was also developed and validated. Wires taken from the outer layer of the conductor were fatigue tested in tension–tension loading and compared with similar data for wires taken from a 95 mm² conductor. The wire cross section was deformed due to the compacting process that was applied during fabrication. When corrected for stress concentrations due to the deformation the data for the two sets of wire fell within the same scatter-band. Full scale testing was carried out in a specially designed rig with constant tensile load and reversed displacement controlled bending with a fixed curvature variation. The loading is a simulation of the loading of a power cable hanging from a floating vessel through a bellmouth. Conductors were tested in two states; dry and lubricated. A finite element model was established for the copper conductor. The model was formulated by a combination of elastic beam and beam-contact elements that included the effects of friction. The effect of local bending due to contact forces was included in the model. Two contact conditions were investigated; the point (trellis) contact between adjacent layers of wire and the inline contact within each layer and between the first layer (centre wire) and the second layer. The FE model was validated by a calibration test of a full scale conductor, and by sensitivity studies varying the size and the number of elements of the model. Fatigue analysis of the conductor was carried out, based on the S–N curve for individual wires. Taking into account the effects of friction and local bending, agreement was obtained between predicted and experimental fatigue strength of the conductor, for the FE model as well as the analytical model.     

Predicting the wet collapse pressure for flexible risers with initial ovalization and gap: An analytical solution

As offshore hydrocarbon production moves towards ultra-deep water, flexible risers have to withstand the huge hydro-static pressure without collapse. They are designed with strong collapse capacities, allowing them to operate under the condition where their annuli are flooded by the seawater. However, initial imperfections can weaken the collapse capacity under such a flooded condition, triggering the so-called “wet collapse”. Two common initial imperfections, the carcass ovality and the radial gap between the carcass and pressure armor, would reduce the collapse strength of flexible risers significantly. Mostly, collapse analyses are performed through numerical simulations, which are less feasible for the design stage of flexible risers comparing with analytical models. To date, there are few analytical models available in public literature to predict the wet collapse pressure of flexible risers accounting for initial ovality and gap. To meet this demand, an analytical model is established in this paper to address these issues. This model is developed as a spring-supported arch, solving the collapse pressure with stability theories of ring and arched structures. This analytical model is verified by numerical simulations, which gives prediction results that correlate well with the numerical ones.     

海底光缆制造中的优化

在计算双层铠装海底光缆拉伸强度的基础上，建立了海底光缆结构参数的优化模型，对于海底光缆的设计制造有一定的指导意义。海底光缆铠装钢丝层之间的滑动对于海底光缆的拉伸强度有重要影响，分析这种滑动产生的原因，提出控制滑动的一些方法，并建立相应的结构参数优化模型。     

Application of analytical model in the prediction of dynamic responses and fatigue damage of flexible risers: Part II – Dynamic analysis of flexible risers in large-scale domain using a direct moment correction method

In recent years, the dynamic responses of flexible risers have been the focus of many researchers. Most flexible risers undergo a substantial level of irregular motion from environmental loadings, which involves a continuous slip of helical wires. The slip of helical wires especially leads to a hysteretic effect by reducing the bending stiffness, making it hard to predict the dynamic responses of flexible risers. The current study, as an extension to Part I, presents a new large-scale dynamic analysis method for flexible risers. The suggested method creates a large-scale model for the dynamic analysis that considers a geometric and bending nonlinearity of flexible risers. The kinematics of each beam element is formulated based on a Green-Lagrangian strain and the interaction with the seabed, providing a realistic analysis of flexible risers. In particular, the current study introduces a direct moment correction method that modifies the internal force vector using an improved analytical model. The improved analytical model is assigned at each node of the large-scale model and estimates an accurate bending hysteresis curve considering the effect of shear deformation and varying tension. The suggested method corrects the bending moment and shear force of all beam elements based on the bending hysteresis curves obtained from the improved analytical model, by which a complex bending behavior of flexible risers is reflected in a large-scale domain. As a result, this study achieves a more accurate prediction of the dynamic responses and fatigue damage of flexible risers. A new dynamic analysis program, called OPFLEX, is developed herein based on the suggested analysis method. Using the developed program, the current study conducts several numerical investigations to identify the effect of the shear deformation and varying tension. Consequently, it is confirmed that the shear deformation of internal layers reduces the fatigue damage of helical wires by delaying the increase of internal stress. It is also identified that the effect of varying tension deteriorates the fatigue life of flexible risers through a continuous change of contact pressure during bending.     

深水导管架桩腿制造误差与抗屈曲特性研究

文章通过典型算例详细阐述了具有制造误差的深水导管架桩腿屈曲分析方法,将线性扰动加载的特征值届曲预测算法和描述几何非线性屈曲过程的改进的Riks算法应用于导管架桩腿的详细设计校核,对比研究了模态扰动模拟制造误差和直接模拟制造误差模型的分析结果,并对典型桩腿结构在静水外压下的弹塑性屈曲响应特性进行了研究.分析结果表明,随着制造误差的增加,导管架桩腿抗静外压的临界屈曲载荷在下降,而且制造误差大的桩腿的极限承载能力较低.     

RFID wireless system for detection of water in the annulus of a flexible pipe

Corrosive environments are responsible for the highest degree of degradation and failure in marine structures. The presence of sea water in marine structures such as flexible pipes can cause a significant reduction in their operational life, especially when associated with permeated gases, which could lead to corrosion related failure mechanisms such as corrosion-fatigue and hydrogen cracking. The ingress of sea water into flexible pipes can occur either due to ruptures in their external polymeric sheath or to permeation of condensed water from the pipe bore. This event since flooding of the so-called annular space of flexible pipes is the trigger for all knows corrosion assisted failure modes, it is clear that a system that is able to reliably detect the presence of water in the structure is highly desirable. This work will describe a radio frequency identification (RFID) system designed for this purpose; it relies on the measurement of shifts in the resonance frequency of specially-designed tags which would be inserted within the layers of the flexible pipe during manufacturing. This paper shows the design and validation process of these tags and also of a reader which is meant to be scanned along the outside surface of the pipe by a remotely-operated vehicle (ROV). The study was performed through a finite element analysis and a test in which the tags were inserted within a full-scale mock-up of a flexible riser, which was then flooded with synthetic seawater. Results show that the shift in response due to sea water is clearly identifiable and distinguishable from other effects.     

Burst pressure of steel reinforced flexible pipe

The burst pressure of steel reinforced flexible pipe (SRFP) considering plasticity is investigated through experimental, theoretical and numerical methods. The results obtained from the aforementioned methods are in good agreement with each other, which illustrates the accuracy and reliability of the proposed theoretical and numerical models. The mechanical responses of PE layers and the steel strips are studied in detail, and the rationality of the strain uniformity assumption for the steel strip's cross section in the theoretical model is confirmed from its von Mises stress variation along the width at different points in FEM. Some influential parameters of SRFP on the burst pressure are also investigated in order to guide its cross-section design. The theoretical model and the FEM proposed in this paper can not only give an estimation to the safety and reliability of the pipe when it is subjected to internal pressure, but can also provide some reference for improving and optimizing its cross-section design.