Unbonded Flexible Pipe Research Articles

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.

Sealing Performance of the End Fitting of a Marine Unbonded Flexible Pipe under Pressure Penetration

The sealing performance of end fittings is very important for offshore oil and gas pipelines. To investigate the sealing behavior of a ring-shaped wedge seal, global and local numerical models of the ring–pipe interaction have been developed based on the finite-element method. First, the sealing process of the ring under pressure is simulated. Second, a criterion for the penetration of fluid pressure is applied in these models to assess how the sealing capacity changes along the contact surface. Finally, the contact magnitude of interference and the shape of the sawtooth heaves on the sealing ring are predicted and compared. The results show an interesting concentration of von Mises stress in the sealing ring and also that the peak contact pressure appears in the sealing zone. However, the penetration of fluid pressure has obvious effects on the distributions of von Mises stress and contact pressure. The best sealing performance is when the axial displacement of the sealing ring is 1.4 mm and the contact magnitude of interference is 0.3 mm. Given the sawtooth heave of sealing ring, semicircular heave gives the better sealing capacity compared with trapezoidal heave.

Numerical analysis of stress concentration factors on tensile armour wires inside the end-fitting of an axially tensed unbonded flexible pipe

ABSTRACT The present paper examines the case of a metal based unbonded flexible pipe under a number of axial tensile operational loads of varying intensity, expressly focusing on the response of the segments of the pipe’s tensile armour wires that lie inside the pipe’s End-Fitting (EF), with intent to track the development of high stress concentration areas there and assess the stress concentration factors (SCF) involved. Additionally, a parametric study is performed, to identify potential factors that might affect the maximum SCF values measured on these wire segments inside the EF. Based on the parametric study’s findings, an approximate relationship for evaluating the expected maximum SCF on the tensile wire inside the EF is developed. The paper adopts a numerical approach to the problem, based on the development of a two-dimensional finite element model for evaluating the wire’s response to axial tensile operational loads. The model considers both the effects the EF’s assembling procedure and the Factory Acceptance Test (FAT) have on the behavior of wire segments lying inside the EF. An analytical approach is employed as a benchmarking tool of the numerical model’s performance.

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.

Comparison Between Different Finite Element Analyses of Unbonded Flexible Pipe via Different Modeling Patterns

Three kinds of models based on the same flexible pipe with 8 layers have been separately created to investigate the effects of different modeling approaches on numerical simulation results of finite element (FE) models for unbonded flexible pipes. Then the mechanical property of the unbonded flexible pipe under tension, torsion and bending load has been analyzed and compared via ABAQUS software on the basis of three created models. The research shows that different modeling methods of flexible pipes make a great difference in the results. Especially, modeling simplifications of the carcass and pressure armor have a great impact on the accuracy of the results. Model 3, in which the carcass is simulated by spiral isotropic shell and other layers are simulated by solid element, possesses good adaptability, which has been proved by comparing the experiment data and other models. This paper can offer a reference for the FE modeling methods’ selection and mechanical property analysis of unbonded flexible pipe.

A general model to predict torsion and curvature increments of tensile armors in unbonded flexible pipes

An analytical model is proposed to investigate the local torsion and bending behavior of tensile armors in unbonded flexible pipes. The general expressions of torsion and curvature increments, including the effects of axial strain and twisting rotation of tendon cross-section, are firstly derived based on the generalized Frenet–Serret equations. Then the expressions corresponding to the strained helix assumption are obtained, which are further simplified by ignoring the twisting rotation. Afterwards, explicit expressions are given for axisymmetric case and tendon stick and slip states when bending is included. The developed model is finally verified with a finite element simulation. The results show that the model prediction of normal curvature increment is in good agreement with the numerical solution while the changes in torsion and transverse curvature cannot be obtained accurately through simple analytical formulas. Several earlier models are also discussed briefly and are demonstrated to be special cases of the present general model.

Bending tests and cross-sectional analyses of multilayered flexible pipe models

Simplified models of multilayered unbonded flexible pipes for offshore applications were manufactured and put to a series of bending tests where biaxial bending strains of tensile armor wires were directly measured. Focusing on the effects of constraint on the tensile armor wires by outer layers on the bending stiffness of the pipe and the bending stresses of tensile armor wires, four types of pipe models with different specifications of holding bandages, which hold the tensile armor layers from the outside, were prepared. Cross-sectional analyses based on four kinds of methods were also carried out and comparatively examined with the above bending test results, and the applicability of each method was evaluated. As a result, it was found that the normal and binormal bending stresses of the tensile armor wires in the slip region remarkably change depending on the specification of the holding bandages, suggesting that the degree of constraint on the tensile armor wires by the outer layers should be taken into consideration in the cross-sectional analyses.

Tensile strength of the unbonded flexible pipes

Flexible pipelines must be designed considering the extreme situation for which additional components are needed against severe loading and environmental conditions. Tensile armor is employed in case of high tensile forces, which increase with water depth; and massive components used against high internal and external pressures. In this paper, the mechanical behavior of the pipe under pure tension is investigated by both theoretical and numerical analysis, then the contribution of the external pressure to the axial problem is examined using an analytical analysis. As the deformations along the longitudinal direction of the studied pipe highly depend on the radial stiffness of the cross section, the influence of the pressure armor is carefully evaluated in the inception phase, and its radial stiffness is verified by its own numerical results. The plastic behavior for both steel and polymer layers is taken into account by using the secant modulus method. Moreover, this work answers a question raised by producers that need to know when tensile armor is required to reinforce Metallic Strip Flexible Pipes (MSFP). The theoretical model is employed to carry out the comparison between pipes with different configurations, in order to investigate the influencing parameters of the tensile stiffness. The results obtained from the theoretical and the numerical simulations lead to a remarkable confidence in the analytical solution thanks to a relatively small difference between the outcomes.

Mechanical responses of metallic strip flexible pipes subjected to pure torsion

Metallic strip flexible pipes (MSFP), a relatively new style of unbonded flexible pipes, are considered as an attractive alternative to traditional submarine pipes. Pipelines will inevitably confront torsion load which may affect the integrity and safety during their installation and application. Throughout this paper, the mechanical behavior of MSFP subjected to pure torsion is investigated by both experimental and numerical methods. Results from these two methods are essentially in agreement with each other. The finite element method (FEM) can be adopted to predict the failure torque of MSFP under torsion and analyze the detailed stress conditions of each layer. In addition, the effects of boundary conditions, layout manners of steel strips, friction coefficients between contact surfaces, twist directions and tension loads are discussed. The obtained conclusions will benefit cross-section design of MSFP and relative practical engineering.

Mechanical analysis of un-bonded flexible pipe tensile armor under combined loads

Marine un-bonded flexible pipes wrapped with fiber-reinforced polymer are essential in offshore oil and gas exploration because of their high flexibility and reliability. These flexible pipes may be subjected to tensile loads, compressive loads, and torsional loads, causing the armor wires to undergo large displacements that may result in failure. This study develops a nonlinear simplified finite element model to investigate the mechanical behavior of flexible pipe armor wires under various types of load. The stiffness change rule of flexible pipe armor wires is obtained from numerical simulations of the flexible pipe tensile armor layer under combined loads. The results reveal that the flexible pipe stiffness under combined loads varies significantly from that under single loads, and the combination of different load types can either increase or decrease the stiffness. This study provides essential reference data for the further design, fabrication, and application of flexible pipes.

ТЕРМІНОЛОГІЧНІ ОСОБЛИВОСТІ МІЖНАРОДНОГО НАФТОГАЗОВОГО СТАНДАРТУ

Стаття присвячена дослідженню термінологічного поля міжнародних нафтогазових стандартів на прикладі ISO 13628-2. Уточнено поняття «міжнародний нафтогазовий стандарт», «нафтогазовий текст», «нафтогазовий термін». Доведено, що МС ISO 13628-2 як нормативний документ і взірець англомовного фахового тексту виступає як складноорганізована та чітко структурована модель, що відзначається високим рівнем інформативності, оскільки терміни служать одиницями знання про об’єкти, процеси, дії та поняття у нафтогазової промисловості. Виокремлено основні тематичні групи термінів, що грунтуються на поняттєвому взаємозв’язку між терміноодиницями. Описано типові термінологічні моделі, які можуть стати основою для подальшої побудови алгоритму їх перекладання. (This paper describes a case study of some terminological properties of texts known as international petroleum-related standards, with a focus on ISO 13628-2 as a typical sample of specialized language text limited to design and operation of subsea production systems. This standard is analysed as a normative document and a bearer of technical information encoded in terms – special cognitive units serving to transfer and standardize information concerning unbonded flexible pipe systems for subsea and marine applications. The article considers terminology models and proves that terms have a systametic relationship with the petroleum-specific conceptsthey represent. The standard’s terminology field includes units generally found in scientific and technical texts and those forming the core consisting of six sub-domains. Analysis of the corpus of the text indicates various patterns of single word and multi-word terminological units. The most productive morphological way of term formation is adding suffixes. Various patterns of petroleum-specific abbreviations are singled out. Analysis of multi-word terms reveals availability of constituents ranging from two to eight. These are noun phrases, largely limited to those including adjectives and nouns only; occasionally containing prepositions and rarely including verbs, adverbs, or conjunctions. This research might serve as a basis for developing algorythms for traditional and machine-based translation of the terms studied.)

Tension-bending analysis of flexible pipe by a repeated unit cell finite element model

Unbonded flexible pipes consist of multiple subcomponents which interact through frictional contact. A full 3-D finite element analysis of unbonded flexible pipes is computationally expensive, and a more efficient approach for practical engineering purposes is required. This work presents a repeated unit cell (RUC) finite element model for analyzing flexible pipes subjected to combined constant tension and curvature. Periodic boundary conditions reduce the model size by taking advantage of the structural and loading periodicities, and by assuming uniform wire behavior in the armor layer. The RUC model is suitable for resolving the local tensile armor stress distribution and the global pipe response. A flexible pipe is studied with the RUC model for various tension-bending load configurations and the results have been compared with existing analytical models for validation. The study showed strong correlation between the RUC model and the analytical models, with some difference in the wire bending stresses. This difference can to some degree be explained by the difference in the wire kinematics assumptions. It is found that the proposed RUC model is a robust and computationally efficient approach for analyzing flexible pipes.

End Fitting Effect on Stress Evaluation of Tensile Armor Tendons in Unbonded Flexible Pipes Under Axial Tension

This paper deals with the effect of termination restraint due to end fitting on the stress evaluation of tensile armors in unbonded flexible pipes under axial tension. The problem is characterized by one single armoring tendon helically wound on a cylindrical supporting surface subjected to traction. The deviation from the initial helical angle is taken to describe the armor wire path as the pipe is stretched. The integral of this angle change gives the lateral displacement of the wire, which is determined by minimizing the energy functional that consists of the strain energy due to axial strain, local bending and torsion, and the energy dissipated by friction, leading to a variational problem with a variable endpoint. The governing differential equation of the wire lateral displacement, together with the supplementary condition, is derived using the variational method and solved analytically. The developed model is verified with a finite element (FE) simulation. Comparisons between the model predictions and the FE results in terms of the change in helical angle and transverse bending stress show good correlations. The verified model is then applied to study the effects of imposed tension and friction coefficient on the maximum bending stress. The results show that the response to tension is linear, and friction could significantly increase the stress at the end fitting compared with the frictionless case.

Finite Element Investigation on the Tensile Armor Wire Response of Flexible Pipe for Axisymmetric Loading Conditions Using an Implicit Solver

Composite flexible pipe is used in the offshore oil and gas industry for the transport of hydrocarbons, jumpers connecting subsea infrastructure, and risers with surface platforms and facilities. Although the material fabrication costs are high, there are technical advantages with respect to installation and performance envelope (e.g., fatigue). Flexible pipe has a complex, composite section with each layer addressing a specific function (e.g., pressure containment, and axial load). Continuum finite element modeling (FEM) procedures are developed to examine the mechanical response of an unbonded flexible pipe subject to axisymmetric loading conditions. A parameter study examined the effects of: (1) pure torsion, (2) interlayer friction factor, (3) axial tension, and (4) external and internal pressure on the pipe mechanical response. The results demonstrated a coupled global-local mechanism with a bifurcation path for positive angles of twist relative to the tensile armor wire pitch angle. These results indicated that idealized analytical- and structural-based numerical models may be incomplete or may provide an accurate prediction of the pipe mechanical response. The importance of using an implicit solver to predict the bifurcation response and simulate contact mechanics between layers was highlighted.

Dynamic analysis of unbonded flexible pipe with bend stiffener constraint and bending hysteretic behavior

Nowadays the widespread application of unbonded flexible pipes in the offshore engineering requires more and more research on the global dynamic behavior. However, due to its complex internal structure, the relative slip between each layer may occur, which may result in the moment-curvature hysteretic behavior. Therefore, accurate knowledge of unbonded flexible pipe mechanical characteristics and global dynamic behavior is of major importance during the design phase and fatigue analysis. In this paper, the effects of bend stiffener constraint and bending hysteretic behavior are taken account of into the riser global model based on the rod theory. This development permits more realistic riser structural properties to be modeled in the dynamic simulation, and reports the detailed time history loading results. A finite element model by OrcaFlex is developed to demonstrate the validation of the proposed model. The static/dynamic responses of flexible riser in the top-end region and touchdown zone (TDZ) are well investigated. Besides, essential parametric analyses are presented and find that: bend stiffener can optimize both the curvature distribution and amplitude in the top-end region; the top angle mainly alters the curvature distribution; the impact of bending hysteretic behavior on the curvature is of great significance, especially for the TDZ.

Slip and stress of tensile armors in unbonded flexible pipes close to end fitting considering an exponentially decaying curvature distribution

An analytical model is given to investigate the end-fitting effect on slip and stress of tensile armors in unbonded flexible pipes under tension, torsion and varying bending in the absence of friction. An exponentially decaying curvature distribution is assumed to represent controlled curvature over the end region. The deviation from the initial helical angle is taken to describe the armor wire path as the pipe is stretched, twisted and bent, which is determined by minimization of the strain energy functional using the Euler equation. The obtained simultaneous differential equations are numerically solved by transforming them into a boundary value problem. An analytical solution is found by neglecting the twisting rotation of the wire cross-section. The developed model is validated with a finite element simulation and geodesic based analytical expressions for constant curvature and with a current numerical model for varying curvature. Reasonable correlations are observed between the model predictions and the results from other methods. The validated model is then applied to typical flexible pipe designs to find the level and location of the greatest increases in stress. The results show that the end restraint could cause a significant stress increase in the armor wire at the end fitting vicinity and the critical location is close to the extreme fiber position on the compressive side of the pipe for typical cases. The effect of end restraint on layer bending stiffness is also evaluated.

Confined collapse of unbonded multi-layer pipe subjected to external pressure

If a leakage exists on the outer sheath of an unbonded multi-layer pipe, external hydrostatic pressure will be applied on the surface of the internal sheath inducing the possibility of a confined collapse. Also, when the pipe bore is depressurized at a fast rate, the pressure remaining in the pipe wall can cause the similar collapse of the internal sheath. The present paper uses a flexible pipe with multi-layer structure to examine the mechanical response of the internal cylinder confined by outer cylinders under external pressure. The previous related researches mainly focus on the rigid or almost rigid confinement conditions. However, this study emphasizes on structural elastic stability in terms of the different confinement levels and D/t ratios of the internal cylinder. The initial investigation is numerical and employs two-dimensional models to obtain the nonlinear pressure-deformation equilibrium paths using a Riks algorithm. The results are fitted as uniform formulas for the prediction to the critical pressure and corresponding collapse modes. The simplified analytical model followed is developed through the principle of virtual work to compare with the fitting formulas. Experimental tests of an unbonded flexible pipe with simple construction are also conducted to validate and agree with the theoretical estimations.

Large-Scale Erosion Testing of an Unbonded Flexible Pipe

Summary Unbonded flexible pipelines are commonly used in offshore field developments to transport oil and gas to production facilities. Sand is an inevitable byproduct from oil-and-gas production. Sand erosion has become an important concern for both design of new field developments and prolongation of existing oil-and-gas fields, especially for fields with low mixture density and high velocities. Erosion in smooth pipes can be determined with industry-standard erosion-prediction methodologies. However, these models are usually valid for smooth pipes only, with limited information available on erosion in flexible pipes. This paper presents experimental results from a large-scale erosion test of an unbonded flexible pipe. A 9.75-in. inner-diameter (ID) flexible pipe with a bending radius of 2.5 m was exposed to sand and proppant particles at velocities ranging from 30 to 47 m/s. Erosion was determined by performing weight-loss measurements at selected cut-out windows, at 0, 20, 40, 60, and 80° along the outer periphery of the carcass. In addition, microscopy analysis was performed on selected eroded carcass pieces to determine the localized erosion contour of the flexible carcass geometry. Results show that the highest erosion is found at the leading edge of the carcass strip. Experimental results are compared with computational-fluid-dynamics (CFD) simulations and industry-standard erosion-prediction methodologies.

Numerical investigation of friction joint between Basalt Fiber Reinforced Composites and aluminum

Flexible risers are used in the offshore oil industry for exporting hydrocarbons from subsea equipment to floating production and storage vessels. The latest research in unbonded flexible pipes aims to reduce weight by replacing metal components with composite materials. This would result in lighter and stiffer flexible risers, which would be well suited for ultra deep water applications. This paper develops a new finite element model used for evaluating the efficiency of anchoring flat unidirectional fiber reinforced tendons in a mechanical grip. It consists two flat grips with the fiber reinforced tendon in between. The grips are pressed against the composite and the pullout force is ensured through friction. The novelty of the paper is represented by the detailed investigation of the influence between the coefficient of friction and the pullout force. By comparing numerical and experimentally obtained results, it is possible to show the importance of friction decay in the grip. Improper contact between the grips and composite is also taken into account and leads to good agreement between numerical and experimental results. This study shows how to avoid over-estimating the efficiency of such grip by using dry friction in finite element models.

A study on the axial behavior of bonded flexible marine hoses

Bonded flexible marine hoses are frequently used in offshore offloading operations. The proper performance of these hoses throughout the service life is important. Many studies have been developed considering the global dynamic behavior of such hoses; however, only a few considered its local structural behavior. The present work aims to better understand the local mechanics of these hoses by proposing both a numerical and an analytical model. The first approach is based on a finite element model, which is constituted of continuum and REBAR finite elements. The first type of elements aims to represent the elastomeric matrix, whereas the second is introduced in order to properly represent both the polymeric and the metallic reinforcements. The second approach is based on an adaptation of a well-known analytical model for the axisymmetric behavior of bonded and unbonded flexible pipes. The present paper analyzes a 20″ bonded flexible marine hose subjected to different axisymmetric loads. The stiffness of the hose and the stresses and strains in each of its components are computed using both models and compared to reference values found in the literature. In a general manner, the results agree well, thus indicating that both models are promising tools for the design of these structures.