Tensile Armor Layers Research Articles

Numerical study on the axial-torsional response of an unbonded flexible riser with damaged tensile armor wires

The local mechanical response of a 2.5" unbonded flexible riser with damaged wires is studied by a nonlinear finite element (FE) model. Firstly, the riser was supposed to be under pure tension and pure torsion. Stiffness and force distribution of the intact riser predicted by the analytical and numerical models are compared, and deviations between the two solutions are discussed. Then, two up to eight wires in the outer tensile armor layer are assumed to be damaged. Numerical results indicate that uneven deformations caused by the broken wires result in a clear nonlinear force redistribution among the outer wires. The maximum and minimum forces supported by the remaining intact outer wires are closely related to the number of damaged wires, boundary conditions and the load direction. Finally, the effect of internal frictions on the load recovery of damaged wires is studied by increasing the FE model length and applying the pre-pressure. It was found that damaged wires can progressively recover their load-carrying capacities with the increase of internal frictions. In addition to the boundary effects, axial stress of the damaged wire is linear with the length of the riser, which indicates that results obtained from the short model can be employed to estimate the load recovery characteristics of the long model.

Theoretical and numerical methods to predict the behaviour of unbonded flexible riser with composite armour layers subjected to axial tension

Unbonded flexible risers are widely applied to deep-water oil and gas production field, and their complex cross-section mechanical properties have always been a challenging issue in ocean engineering. Over the recent years, the traditional steel tensile armour layers of unbonded flexible risers are replaced by composite materials, owing to their advantages of lightweight and high strength. In this paper, a theoretical deduction based on energy method and a numerical method in consideration of detailed geometric properties are proposed to analyze the behaviour of the unbonded flexible riser with composite armour layers. Both the theoretical and numerical methods take full consideration of the material orientation of composite, and for the numerical model, two local coordinate systems are established to define the material properties of composite armour layers. Based on a 2.5-inches unbonded flexible riser model with eight-layers, some cases from theoretical and numerical methods are presented for validation. Then the effect of the fiber volume fraction of composite on the axial tensile stiffness, as well as the weight of the unbonded flexible riser, is discussed. Furthermore, characteristics analysis of axial stiffness is carried out and the effect of the boundary condition is discussed as well.

Effect of Axial Compression and Wet Collapse Loads on Torsional Response of Flexible Pipe

Flexible pipes connect the two critical systems of the floating production platform and underwater production, which play an important role in the transportation of oil and gas as well as water injection. However, flexible pipes are often subjected to multiple loads that threaten their stability in the service process. In this study, a model of a flexible pipe containing practical cross section in all layers is established to study the effects of axial compression and wet collapse loads on clockwise and anticlockwise torsional responses. The results of calculation show that the axial compression and radial expansion of the flexible pipe are caused by the axial compression load, which reduces clockwise torsional stiffness. The wet collapse load induces the bending deformation of the flexible pipes in clockwise and anticlockwise torsion, where the bending deformation of clockwise torsion is significantly affected by the change in the Young’s modulus of the tensile armor layers. This study shows that clockwise torsional stiffness is small and exhibits a trend of axial compression. Clockwise torsion has poor stability in combination with axial compression and wet collapse loads, respectively. Therefore, to prevent instability in flexible pipes, clockwise torsion load, or which coupled with axial compression and wet collapse load, should be avoided.

Parameter sensitivity analysis on the buckling failure modes of tensile armor layers of flexible pipe

Unbonded flexible pipes have become widely employed in marine environments with the development of the offshore oil and gas exploration industry. Such pipes are subject to complex loads, which may lead to significant failure. Tensile armor layers are key components of these flexible pipes, and radial buckling failure may occur under high compressive or other combined loads. In this paper, a simplified numerical model of the flexible pipe is developed using the finite element (FE) method, and the main failure modes of the tensile armor layers are simulated. This simplified model is an improvement over those employed in previous studies on tensile armor failure. Several parameters that have important effects on pipe stiffness are investigated. The results show that an increase in the winding angle of the tensile armor wires as well as the damage to the outer sheath of the flexible pipe will decrease the compressive stiffness significantly. The presence of friction between adjacent layers will increase the stiffness to a small extent while the external hydrostatic pressure enhances its axial stiffness greatly and improve the stability of the pipe. These results may be used as a reference in the development and engineering applications of flexible pipes.

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.

Finite Element Analysis of Flexible Pipes Under Compression: Influence of the Friction Coefficient

In order to study the compressive behavior of flexible pipes, a nonlinear finite element model was developed. This fully tridimensional model recreates a five-layer flexible pipe with two tensile armor layers, an external polymeric sheath, an orthotropic high strength tape, and a rigid inner nucleus. The friction coefficient is known as a key parameter in determining the instability response of flexible pipes’ tensile armor. Since the featured model includes all nonlinear frictional contacts between the layers, it has been used to conduct several experiments in order to investigate its influence on the response. This article includes a description of the finite element model itself and a case study where the friction between the layers of the pipe is changed. The procedure of this analysis is described here, along with the results.

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.

Failure characteristic analysis of tensile armour layer of unbonded flexible riser under axial compression

ABSTRACTUnbonded flexible risers have been applied in the harsh environment for several decades, and structural failure sometimes appears. Owing to the large axial compression by installation or to the combined loads of axial compression and external pressure during operation, radial or lateral failure of tensile armour layers can occur, threatening structural safety. In order to study the mechanism of failure on tensile armour layers of unbonded flexible riser subjected to axial compression, a simplified numerical model, based on ABAQUS/Explicit, with tensile armour layers, structural tape and outer sheath is proposed. The nonlinearity of material is taken into consideration in this model and the validity of this model is verified by a theoretical method. Failure characteristics for helical wire under axial compression are studied in consideration of the nonlinear factors including number of tendons for the tensile armour layer, laying angle of the tensile armour layer, thickness of outer sheath and friction coefficient between layers. In addition, the impact of the external pressure on axial ultimate strength is discussed.

A simplified multi-layered finite element model for flexible pipes

A flexible pipe connects offshore platforms to the flowlines and transport gas and oil. It may experience axial compression due to the reversed end cap effect during installation. This can trigger radial buckling or lateral buckling of tensile armor layers. The ultimate strength assessment of the flexible pipe is complicated and time-consuming due to material nonlinearity, large deformation, and nonlinear contact mechanism. These difficulties make the nonlinear analyses difficult to converge. This paper proposes a simplified 5-layered model which can improve the convergence without deteriorating the accuracy. Analytical methods are suggested to determine an equivalent layer to replace inner four layers. In addition, the factor of penetration tolerance (FTOL) of shell element layers needs to reflect the thinning of polymer layers, which makes the axial stiffness equal to the solid element model. Analytical methods are used to determine the factor and a stepwise increasing approach is applied in a numerical analysis.The 5-layered model with the stepwise FTOL application is verified by comparing with 8-layered model, analytical model and experiment results with respect to axial and bending stiffness. The model is used for an ultimate strength assessment, the failure mechanism and the interaction between layers are investigated in detail with incremental loading.

Analytical prediction for lateral buckling of tensile wires in flexible pipes

The development of oil and gas fields in deepwater challenges the design of flexible pipes due to high pressures, temperatures and dynamic loads. Amongst the known failure modes, lateral buckling of tensile armor layers in flexible pipes has attracted extensive interest for its complicated mechanisms. The present paper addresses lateral buckling behavior of tensile armor wires in flexible pipes when armor annulus is flooded. Based on the thin curved beam theory, a single armor wire is modeled on a cylindrical surface without considering the effect of friction and bending through a system of six order nonlinear differential equations. Then, by applying a perturbation technique, analytical solutions are obtained which reveal a series of potential lateral buckling modes. An estimate for the lateral buckling is obtained under conditions given herein. Case studies are performed comparing the proposed model with the results obtained through a numerical program and alternative analytical model available in the literature.

Finite Element Analysis of Flexible Pipes Under Axial Compression: Influence of the Sample Length

In order to study the axial compressive behavior of flexible pipes, a nonlinear tridimensional finite element model was developed. This model recreates a five layer flexible pipe with two tensile armor layers, an external polymeric sheath, an orthotropic high strength tape, and a rigid inner core. Using this model, several studies were conducted to verify the influence of key parameters on the wire instability phenomenon. The pipe sample length can be considered as one of these parameters. This paper includes a detailed description of the finite element model itself and a case study where the length of the pipe is varied. The procedure of this analysis is here described and a case study is presented which shows that the sample length itself has no practical effect on the prebuckling response of the samples and a small effect on the limit force value. The postbuckling response, however, presented high sensitivity to the changes, but its erratic behavior has made impossible to establish a pattern.

Numerical analysis of buckling failure in flexible pipe tensile armor wires

Flexible pipes may be exposed to high axial compression and bending during deep-water installation. As the compression force is mainly sustained by the tensile armor layers, this may result in localized lateral or radial buckling failure in these layers. In this paper, a finite element model was created to evaluate the critical instability load of tensile armor wires under external pressure and compression. The tensile armor wires are modeled by curved beam elements under loxodromic assumptions. Other layers׳ contribution was simplified by spring elements and equivalent beams. The buckling load capacity and associated failure modes are obtained. The results are also compared with the results based on 3D Euler beam elements and results published in the literature. Parametric analyses were further included with respect to external pressure, friction modeling and the influence of initial imperfections.

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.

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.

Imperfection analysis of flexible pipe armor wires in compression and bending

The work presented in this paper is motivated by a specific failure mode known as lateral wire buckling occurring in the tensile armor layers of flexible pipes. The tensile armor is usually constituted by two layers of initially helically wound steel wires with opposite lay directions. During pipe laying in ultra deep waters, a flexible pipe experiences repeated bending cycles and longitudinal compression. These loading conditions are known to impose a danger to the structural integrity of the armoring layers, if the compressive load on the pipe exceeds the total maximum compressive load carrying ability of the wires. This may cause the wires to buckle in the circumferential pipe direction, when these are restrained against radial deformations by adjacent layers.In the present paper, a single armoring wire modeled as a long and slender curved beam embedded in a frictionless cylinder bent into a toroid will be studied in order to gain further understanding of this failure mode. In order to study the compressive behavior, both perfect beams as well as beams with small geometrical imperfections are studied. The mathematical formulation of the problem is based on curved beam equilibrium and allows large deflections to be taken into calculation.

On modelling of lateral buckling failure in flexible pipe tensile armour layers

In the present paper, a mathematical model which is capable of representing the physics of lateral buckling failure in the tensile armour layers of flexible pipes is introduced. Flexible pipes are unbounded composite steel–polymer structures, which are known to be prone to lateral wire buckling when exposed to repeated bending cycles and longitudinal compression, which mainly occurs during pipe laying in ultra-deep waters. On the basis of multiple single wire analyses, the mechanical behaviour of both layers of tensile armour wires can be determined. Since failure in one layer destabilises the torsional equilibrium which is usually maintained between the layers, lateral wire buckling is often associated with a severe pipe twist. This behaviour is discussed and modelled. Results are compared to a pipe model, in which failure is assumed not to cause twist. The buckling modes of the tensile armour wires can be obtained by the presented method.

Theoretical and experimental studies of stresses in flexible pipes

This paper presents one model for predicting stresses from axi-symmetric effects and two alternative formulations for predicting bending stresses in tensile armour layers of non-bonded flexible pipes. The models were developed to comply with the framework of non-linear finite element technology allowing direct implementation into existing codes for flexible riser analysis, all based on corotated kinematics allowing for large displacements and small strains. Experimental studies were further carried out using strain measurements from fibre-optic Braggs to validate the performance of both formulations in terms of bending stresses and fatigue.