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.     

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.     

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.     

缩减弹性模量有限元法计算加肋轴对称组合壳的极限载荷

将缩减弹性模量的思想融入基于轴对称壳单元的有限元分析,建立缩减弹性模量有限元法,计算加肋轴对称组合壳的极限载荷。建立壳单元弹性状态、局部屈服状态和截面屈服状态的判断条件;提出弹性模量调整策略和组合壳极限状态的判断方法,实现对加肋轴对称组合壳的塑性极限分析。编制了计算程序,算例表明该方法计算时间省,计算精度较高。     

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.     

弹性管阻抗测量标准样件的设计研究

本文通过阻抗测试平台对试件阻抗限值的要求,以及弹性管标准样件机械阻抗和声阻抗的解析表达式及相关因素对阻抗影响的参数化研究,提出了标准样件具体设计的建议.所给出的计算公式和结果可作为实际阻抗测量的比较依据.     

Pressure effects on the static response of offshore tubular connections

The paper focuses on the influence of external hydrostatic pressure on the ultimate capacity of uniplanar X and T welded tubular connections under axial and bending loads. Nonlinear finite element tools, calibrated through available experimental data, are employed to simulate the tubular joints. The numerical results are presented in the form of load-displacement (or moment-rotation) equilibrium paths for different pressure levels, as well as in the form of pressure-load interaction diagrams. It is shown that external pressure causes structural instability, and has significant effects on both the ultimate load and the deformation capacity of the joint. The effects of external pressure on an X joint under balanced axial load are demonstrated and explained qualitatively using a simplified elastic–plastic mechanical model, which describes joint behavior through closed-form expressions.     

Dynamic loading history and collapse analysis of the pipe during deepwater S-lay operation

In the deepwater S-lay operations depending on the required stinger radius and rollers configuration, relatively large plastic deformation is induced when the pipe passes over the stinger, under the combined loadings of bending, axial tension, roller reaction force and the pipelay vessel motion. The resulting plastic deformation does not vanish after the pipe leaves the stinger. It accumulates until the pipe reaches the seabed. The inherited residual deformation might reduce the collapse capacity of the pipe under the external pressure loading. The present paper investigates the dynamic loading history of the pipe during the S-lay operation based on a test-verified finite element model, and then calculates the residual plastic deformation of the pipe cross-section after the pipe reaches the seabed. Finally, the nonlinear collapse analysis is implemented based on the modified RIKS method to evaluate the capacity of the installation-induced deformed pipe. The results confirm that the deepwater S-lay operation will lead to obvious plastic deformation of the pipe, which decreases the pipe collapse capacity to some extent.     

The development of studying flexible pipe bend reinforced by Kevlar fibers

The flexible pipe bend can not only reduce the structural vibration and fluid noise in pipeline, but also realize the flexible connection of a horizontal line and a vertical line and compensate the displacement of three dimensions produced by the shock or vibration of pipeline in the special situations. Up to now, little attention has been paid to study the flexible pipe bend applied in the pipeline of medium or high pressure, because no appropriate framework materials can be used to reinforce it which must endure the burst pressure higher than 10 MPa. The investigation shows that it is possible to produce the flexible pipe bend of medium or high pressure if such fibers with high performance as Kevlar fibers are used to be its reinforced materials. However, its structural designing theory, manufacturing technology and measuring techniques aren‘t yet perfect and systematic, which leads to the instability of the performance of products. Furthermore, few references about its research can be seen. Therefore, it is necessary to systematically and thoroughly develop the structural designing theory, manufacture technology and measuring techniques of flexible pipe bend.     

Dynamic analysis of risers using a novel multilayered pipe beam element model

In this paper a recently proposed formulation for the multilayered pipe beam element is extended to dynamic analysis of risers. Derivations of hydrostatic and hydrodynamic loadings due to internal and external fluid acting on each element layer are presented. Mass and damping matrices, associated to each element layer, are properly derived by adding their respective contributions to the expression of the virtual work due to external loading. The finite element implementation allows for the numerical representation of either bonded or unbonded multilayered risers, including small slip effects between layers. A number of numerical examples have been carried out and the results show the accuracy and efficiency of the new element formulation, even in large scale riser analysis. Moreover, we establish a few benchmarks using multilayered pipes and risers.     

考虑土体剪胀特性的土质边坡稳定性极限分析

剪胀特性是土体的重要特性之一,然而,经典的塑性力学理论高估了土体的剪胀性质,从而使土体强度背离实际情况。通过等效内摩擦角的方法,对原有的土体抗剪强度进行修正,反映出土体剪胀性对土体强度的影响。通过构建对数螺旋形式的破坏机构,推导出相应的内能耗散和外力功率表达式,结合极限分析理论和强度折减方法,建立边坡安全系数的求解方法。结果表明,该方法的计算结果是合理的。随着剪胀角的不断增大,边坡安全系数增大的速度变慢,这种变化趋势是非线性的。针对不同的土质特性,提出相应的剪胀角取值建议。     

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.     

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.     

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.     

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.     

Experimental and numerical modelling of ductile crack propagation in large-scale shell structures

This paper presents a combined experimental–numerical procedure for development and calibration of macroscopic crack propagation criteria in large-scale shell structures. A novel experimental set-up is described in which a mode-I crack can be driven 400 mm through a 20(+) mm thick plate under fully plastic and controlled conditions. The test specimen can be deformed either in combined in-plane bending and extension or in pure extension. Experimental results are described for 5 and 10 mm thick aluminium and steel plates. By performing an inverse finite-element analysis of the experimental results where the simulated crack growth is forced to correspond to the experimental observations, empirical criteria for ductile crack propagation emerge very clearly. Using the experiments with edge crack specimens (ECS) in combined in-plane bending and extension, crack propagation criteria are developed for steel and aluminium plates, mainly as curves showing the critical element deformation versus the shell element size. These derived crack propagation criteria are then validated against a separate set of experiments considering centre crack specimens (CCS) which have a different crack-tip constraint. The applicability of the often-used equivalent strain criterion is discussed versus a more rationally based criterion which takes into account the stress tri-axiality. A large-scale grounding experiment is also simulated showing very good agreement with measurements. The performance of the proposed model is in general good and it is believed that the presented results and experimental–numerical calibration procedure can be of use in practical finite-element simulations of collision and grounding events with the use of shell elements. As discussed, the paper provides a clean framework for further development of macroscopic crack propagation criteria in large-scale plate structures.     

船体复合材料帽型加筋板渐进破坏分析

复合材料加筋板复杂的破坏过程与失效形式增加了应用难度。以面外均布载荷与面内压缩联合作用下的夹芯复合材料帽型加筋板为研究对象,在通过试验结果验证非线性有限元方法准确性的基础上,基于复合材料的hashin准则与胶层界面的最大应力准则展开渐进破坏分析,讨论了极限载荷、破坏模式及失效机理。采用有限元子模型技术对中段破坏区域建模,基于Shokrieh-Hashin准则进行局部破坏分析,讨论蒙皮的铺层损伤规律。结果表明:加筋板呈整体一阶屈曲破坏,极限载荷为316.8 kN;壁板下蒙皮的纤维压缩失效是导致加筋板整体破坏的原因;各铺层的面内失效规律基本相同,蒙皮的纤维压缩失效从外层逐渐向内层扩展,且扩展速率逐渐降低。     

液、塑限联合测定法的研究

液、塑限联合测定法在其他国家已经得到广泛应用,国内尚未正式推广。但作为一种能最大限度消除人为因素影响的试验方法,有着很好的应用前景。本研究主要目的是分析不同性质土的液、塑限联合测定法与人工手搓法试验的对比结果,找出其相应的试验规律,并为该项试验方法在国内得到广泛应用提供参考。     

功率流隔振效率评价指标及其与振级落差的关系

鉴于功率流方法在隔振分析中的日益受到重视,以及以传递功率流直接作为隔振系统的设计指标存在的缺陷,因此有必要借助经典隔振理论的传递率及插入损失概念导出功率流传递率(插入损失);通过建立柔性基础隔振系统的动力学模型,应用解析分析、数值模拟和实验测试方法对功率流传递率的频谱特性进行了分析研究,对功率流传递率作为隔振设计效率指标的有效性进行了论证;针对功率流及插入损失不易测量的问题,对功率流传递率与振级落差之间的数值对应关系进行了分析比较,包括解析分析、数值模拟和实验验证。     

Structural response of a flexible pipe with damaged tensile armor wires under pure tension

This article studies the structural response of a 6.0” flexible pipe under pure tension considering intact and damaged conditions. In the damaged condition, several wires of the tensile armor layers are assumed to be broken. A three-dimensional nonlinear finite element (FE) model devoted to analyze the local mechanical response of flexible pipes is employed in this study. This model is capable of representing each tensile armor wire and, therefore, localized defects, including total rupture, may be adequately represented. Results from experimental tests validate the FE predictions and indicate a reduction in the axial stiffness of the pipe, a non-uniform redistribution of forces among the remaining intact wires of the damaged tensile armor layers and high stress concentrations in the wires near the broken ones. Moreover, the FE model indicates that significant normal bending stresses may arise in the pressure armor and inner carcass due to an uneven pressure distribution on these layers. Finally, the results obtained are employed to estimate the pull out capacity of the studied flexible pipe.