Tensile Armor Research Articles

Analysis of tensile response of flexible pipe with ovalization under hydrostatic pressure

Flexible pipes play an important role in the transportation of oil and gas from the seabed to floating production units. They are also employed to inject water or gas into offshore wells. In deep-sea oil and gas extraction, flexible pipes must be able to withstand very high tensile and hydrostatic pressures. In this paper, an equivalent method is used to represent the complex carcass layer and the pressure armor layer as a regular cylinder, and then a numerical model of a 2.5" flexible pipe was developed and verified by experiment. A tensile analysis of flexible pipes with different degrees of initial ovalization indicated that ovalization of 0–1% does not affect the tensile rigidity, but increasing the ovalization creates an uneven load and stress concentration on the tensile armor layer. When a flexible pipe is subjected to hydrostatic pressure, the outer pressure increases the tensile rigidity of the flexible pipe more than the annular pressure, but the reduction of the ultimate tensile load is clear. A high annular pressure can cause the collapse of the carcass layer.

Comparative study between crack tip opening displacement techniques by statistical analysis

Standardized tests may not always achieve all their analytical objectives. Particularly to structural integrity, where a reproduction of the practical occurrences of the industry is desired, new techniques are being developed. Therefore, this work aims at evaluating fracture toughness in tensile armour through four measurement methodologies (standard, video, laser and stretch zone (SZ)) to obtain crack tip opening displacement (CTOD), and compares their responses by ANOVA and Duncan’s tests. The findings showed equivalence between the standard and other approaches, thus allowing distinct methods for fracture toughness evaluation.

Quantitative analysis of localized corrosion in tensile armor wire of flexible riser submitted to sweet stress corrosion cracking testing with crevice

Quantitative analysis of localized corrosion in tensile armor wire of flexible riser submitted to sweet stress corrosion cracking testing with crevice

Mechanical Analysis of Flexible Riser with Carbon Fiber Composite Tension Armor

As oil and gas exploration moves to deeper areas of the ocean, the weight of flexible risers becomes an important factor in design. To reduce the weight of flexible risers and ease the load on the offshore platform, this paper present a cylindrical tensile armor layer made of composite materials that can replace the helical tensile armor layer made of carbon steel. The ACP (pre) of the workbench is used to model the composite tension armor. Firstly, the composite lamination of the tensile armor is discussed. Then, considering the progressive damage theory of composite material, the whole flexible riser is analyzed mechanically and compared with the original flexible riser. The weight of the flexible riser decreases by 9.73 kg/m, and the axial tensile stiffness decreases by 17.1%, while the axial tensile strength increases by 130%. At the same time, the flexible riser can meet the design strength requirements of torsion and bending.

Flexible pipe tensile armor monitoring using eddy current technique

ABSTRACT Flexible risers are composite cylindrical structures, which are widely used in offshore oil and gas production facilities. Failure of a flexible riser during operation causes major economic and environmental losses. The improvement of structural integrity programs based on nondestructive techniques and risk-based inspections is therefore utterly important. In this article, the feasibility of using the eddy current technique to identify wire rupture of a riser tensile armor is studied. As an important part of the work, an innovative coil prototype (probe) is developed based on the eddy current technique. To assist the construction of the prototype, numerical simulations using the Finite Element Method are applied to reproduce the physical nondestructive test (NDT) and the results are used as support for determining the parameters of the inspection circuit. The resulting signal in all analyses proved the efficacy and viability of Eddy Current Test to inspect the tensile armors of flexible pipes with eventual ruptures.

Estimation of flexible riser curvature distribution and bend stiffener polyurethane behavior using the Levenberg-Marquardt algorithm in full scale bending-tension tests

Long term full-scale bending-tension tests are traditionally performed to verify the numerically estimated flexible riser lifetime. For a proper tensile armour stress calculation, the riser curvature distribution has to be accurately determined, being highly affected by the bend stiffener polyurethane response. The actual material response may be significantly influenced by the loading rate, environmental temperature and humidity, ageing and self-heating phenomenon, requiring an extensive experimental campaign in addition to advanced constitutive models. In this work, an inverse problem methodology is proposed to decrease riser curvature distribution estimation uncertainties by combining optical configuration measurements, a direct finite element model and the Levenberg-Marquardt algorithm to estimate a representative polyurethane response. A full-scale riser/bend stiffener bending-tension test is conducted and an optical monitoring system is employed to track photoluminescence targets along the system length to estimate its deformed configuration. Five tests are performed and eight targets close to the bend stiffener tip selected for the inverse calculation of a representative bend stiffener hyperelastic response and riser top tension. The remaining target displacements and the tension measured with a load cell are employed for validation. The case study shows an excellent correlation between the numerically calculated deformed configuration and experimental measurements with the verification targets.

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.

Effects of thermal spray aluminium coating on SSC and HIC resistance of high strength steel in a sour environment

Flexible risers are critical equipment for the production of oil and gas in offshore fields around the world. The annulus region may condense acidic water containing H2S exposing the tensile armour steel wires to a sour environment, which may promote sulphide stress cracking (SSC) and/or hydrogen-induced cracking (HIC). This work aims to evaluate through electrochemical and constant displacement tests the effectiveness of thermal spray aluminium (TSA) coating to mitigate SSC and HIC in high strength steel used to manufacture the tensile armour of a flexible riser. Electrochemical tests confirmed the anodic behaviour of the coating against the steel and estimated a service life of 2.5 years. The constant displacement test indicated that the non-coated samples failed by SSC although some cracks were also found in the samples mid-section due to HIC. For Al-coated samples, no signs of SSC and HIC were observed. The TSA coating showed a dual barrier effect, hindering either corrosion of steel or hydrogen up-taking, and neither SSC nor HIC was observed. Samples with a coating defect were cathodically protected and no HIC was observed.

Model for Wireless Magnetoelastic Strain Sensors.

This paper describes a magnetoelastic strain sensor based on the ∆E effect and discusses some materials used in its construction. A polycrystalline Fe–Al–B alloy with good quality magnetoelastic properties was used as the transducer and glued to the test object, either brass plates or rods of SAE 1010 steel. The strain-dependent magnetic field of the transducer changes the operating point of the resonator, a strip of field-annealed Metglas 2826MB3, resulting in a modification of its resonant frequency. A model was developed to simulate the strain-dependent magnetic field acting on the resonator and thus to calculate curves of resonant frequency vs. deformation. With the help of this model, differences in the shape of the frequency vs. strain curve can be understood. For a sensor with resonant frequency of 60.5 kHz glued to a rod of SAE 1010 steel, a total resonant frequency variation ∆f ~7 kHz was observed for a deformation of 1100 ppm. The geometry of this sensor is especially favorable for the remote monitoring of a steel surface, such as the wires of the tensile armor of a marine riser.

Improvement of the bending behavior of a flexible riser: Part ii – hysteretic modeling of bending stiffness in global dynamic analysis

Flexible risers have been widely utilized for the transfer of oil and gas products from the well to the production unit. Continuous interactions of tensile armor layers and anti-friction tapes trigger the slippage of helical wires and a highly nonlinear hysteric behavior in bending. For the precise prediction of the bending behavior of flexible risers, Part I of this series of papers proposed a new analytical model that successfully models the interlayer interaction. In this paper, Part II, the proposed analytical model is extended to the global domain of riser configurations; that is to say, a Multi-Hysteresis Modeling (MHM) method is proposed The proposed method employs accurate and realistic bending hysteresis curves with consideration of shear deformation and axisymmetric loads. Based on the proposed method, three comparative studies are performed to investigate the effects of interlayer interaction. As a result, the interlayer interaction is found to have a considerable impact on the fatigue damage of flexible risers. For a practical global dynamic analysis procedure, this paper also introduces a partial hysteresis modeling (PHM) method, by which only a small number of local regions are modeled with bending hysteresis curves.

Theoretical model for variable helical angle of tensile armour wires in bent flexible pipes

This paper systematically addresses a theoretical model to account for the variation of the initial helical lay angle of the tensile armour wires in flexible pipes during bending condition. Based on differential geometrical discussion and curved beam theory, a non-linear differential equation system which contains the unknown variable helical angle is established. Moreover, different expressions of wires’ strain before and after wires’ slip are included in the paper to underline the discrepancy between scenarios in non-slippage and slippage regions. The variation of the armour wires helical angles, which are usually implicit in previous work, can be obtained numerically, revealing potential application in global bending problem. Case studies compared with results obtained through analytical and numerical models available in the literature as well as parametric analyses corresponding to original helical angle, initial tensile force, friction coefficient and bending curvature are performed to verify the feasibility of the proposed theoretical method.

Improvement of the bending behavior of a flexible riser : Part I – nonlinear bending behavior considering the shear deformation of polymer layers

An un-bonded, multi-layered assembly of heterogeneous materials in the cross-section of flexible risers enables riser systems to withstand a large radius of curvature while preventing structural damage or buckling of many critical areas such as a touch down zone. The contact surfaces between layers exhibit a continuous sliding of metallic components after a certain level of bending, which lowers the bending stiffness of flexible risers. Such interactions are beneficial in order to reduce the stress of tensile wires, but complicate prediction of the limit state and fatigue damage of riser systems in that the sliding of metal layers involves a complex contact phenomenon that is difficult to solve numerically. This series of papers deals with the development of an improved analysis method for flexible risers using theoretical approaches. Part I, as presented in these pages, develops an analytical model that is capable of estimating the cross-sectional stiffness of flexible risers with consideration of external loads and inter-layer interaction. In Part II, the proposed analytical model is applied to a large-scale riser model to achieve an efficient global dynamic analysis of flexible risers.This paper, the first part of a two-paper series, proposes an analytical model that formulates the stress of tensile armor layers through equilibrium equations that take into account the shear and radial deformation of polymer layers. The equilibrium equations are derived under a linear differential equation for each component of flexible risers, so that not only the shear interaction forces but also the contact pressure originating from the bending of layers is considered in the analytical model. The shear interaction forces between two tensile armor layers are formulated through an equivalent shear stiffness by which the shear stiffness of layers is linearly modeled as a series of springs. The proposed model is verified by comparison with the nonlinear bending stiffness from existing bending models and finite element (FE) analysis. For additional verification, the axial stresses of inner and outer tensile armor also are compared. And for further, more in-depth understanding, a case study of various combined load cases is carried out.

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.

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.

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.

A global-local fretting analysis methodology and design study for the pressure armour layer of dynamic flexible marine risers

In this paper, a global-local fretting design methodology for the pressure armour layer of flexible marine risers is outlined. This includes global dynamic riser analysis, geometrical and analytical sub-models and local nub-groove contact finite element analysis. Furthermore, a fretting test rig is developed and utilised to quantify coefficient of friction and wear coefficient under representative nub-groove loading conditions. The combination of the global-local computational methodology and experimental characterisation of pressure armour wire material allows for the development of running condition fretting maps. This identifies design criteria for critical riser global curvatures that are associated with minimum number of cycles to failure. The design methodology presented in this paper is applied to a realistic riser design study, using extreme sea-state loading conditions. In this case study, the predicted pressure armour fretting fatigue lives are found to be in the same range as the plain fatigue lives of the tensile armour layer.

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.

A modal finite element approach to predict the lateral buckling failure of the tensile armors in flexible pipes

This paper proposes a finite element (FE) modal approach to determine the critical loads and buckling mode shapes associated with the lateral buckling of the tensile armors in flexible pipes. This approach is based on pre-curved and twisted beam finite elements that are restrained to slide on the surface of a torus. The influence of the pipe's bending is addressed by assuming that the pre-buckling configuration of the tensile armors are geodesics or loxodromes of a torus and, moreover, friction effects are approximated using elastic springs. The proposed FE approach is employed in the classical buckling analyses of different tensile armors of several flexible pipes that have been previously studied in the public literature. The conducted analyses indicated that the buckling modes are sinusoidal and the pre-buckling configuration of the tensile armors in bent pipes affects their lateral buckling load. Moreover, friction between layers increases the lateral buckling resistance of the armor. Finally, comparisons with experimental and numerical results available in the literature indicates that the proposed approach is promising.

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.