Tensile Armor Research Articles

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.

Numerical Study on Flexible Pipe End Fitting Progressive Failure Behavior Based on Cohesive Zone Model

Flexible pipes are extensively used to connect seabed and floating production systems for the development of deep-water oil and gas. In the top connection area, end fitting (EF) is the connector between the flexible pipe and floating platform, as a critical component for structural failure. To address this issue, a combined numerical and experimental prediction method is proposed in this paper to investigate the failure behavior of flexible pipes EF considering tensile armor and epoxy resin debonding. In order to analyze the stress distribution of the tensile armor and the damage state of the bonding interface as the tensile load increases, a finite element model of the EF anchorage system is established based on the cohesive zone model (CZM). Additionally, the effects of the epoxy resin shear strength (ss) and the steel wire yield strength (ys) on the structural load-bearing capacity are discussed in detail. The results indicate that wire strength and interface bonding have a substantial effect on the anchorage system’s failure behavior, and the low-strength wire anchorage system has a three-stage failure behavior with wire yielding as the predominant failure mode, while the high-strength wire anchorage system has a two-stage failure behavior with interface debonding as the predominant failure mode.

Comparative strength and stability analysis of conventional and lighter composite flexible risers in ultra-deep water subsea environment

The hybrid flexible risers have a multi-layered structure and use thermoplastic composite for the pressure and tensile armour. In contrast, a conventional flexible riser uses heavier carbon steel as armour which significantly contributes to its weight. For shallow-water applications, the conventional risers are widely used in offshore oil and gas industry due to their corrosion resistance properties and low transportation costs. However, the weight of conventional risers is a key limitation in ultra-deep-water applications. This shortcoming can be addressed by including a lightweight carbon fibre reinforced polymer (CFRP) composite as one of the individual layers. The use of CFRP reduces the effective tension at the hang off point which is a key limitation in extending the range of flexible risers. Here, the dynamic stability and functional load interactions of both risers (viz: a thermoplastic CFRP riser and a conventional flexible riser) at a water depth of 3000 m were studied. A global analysis was performed considering the onerous 1000-year hurricane wave with 100-year currents. The investigation considered ±150 m vessel offsets, three vessel headings (viz: 135, 180, 225°) and three vessel draughts (ballasted, empty, loaded). Additionally, a numerical model with a variable bending stiffness was used to capture the orthotropic material behaviour of a flexible riser. Results showed that the buoyancy requirement and effective tension were 2.1 times greater and 2% higher for the conventional riser compared to its composite counterpart. The most onerous case for a conventional riser was at zero offset whereas for its composite counterpart was at –150 m along the length of a riser. It was observed that the heavier masses of a conventional riser aid in aligning the weight vector with the upward direction of the buoyancy force. Contrarily, the composite risers undergo large displacements leading to misalignments and instability. Furthermore, the observed bending radius of the flexible riser was found to be within the allowable minimum bend radius at the hog bend location.

Tensile failure behavior of unbonded flexible riser with damaged tensile armor wires

Unbonded flexible risers (UFRs) connect surface production platforms to subsea production systems, and are therefore vital for tasks such as the transportation of oil and gas and the injection of water. The tensile armor layer is the main load layer of UFRs. Once this layer is partially broken, the safety of offshore oil and gas gathering and transportation is seriously threatened. In this study, an 8-layer UFR model is developed to calculate the effects of damaged inner and outer armor wires and boundary conditions on the tensile stiffness, ultimate load capacity, and stress distribution and load ratio of the inner and outer tensile armor layers. Broken inner tensile armor wires are found to have a greater impact on the tensile properties and produce the same torsional buckling failure mode as observed in the field. The influence of the pressure load on the ultimate load capacity of the UFRs is analyzed, and the results demonstrate that the outermost pressure increases the interlayer action between the tensile armor layers and the adjacent layers, thereby reducing the torsional angle of the UFRs under the ultimate tension. Additionally, the innermost pressure is found to have a negligible effect on the ultimate load.

Influence of structural parameters of unbonded flexible pipes on bending performance

Unbonded flexible pipes have complex structural forms and numerous structural parameters. There are different degrees of contact and slip between the structural layers under bending loads, which brings challenges to the structural design and bending characteristics analysis of flexible pipes. In this paper, according to the law of energy conservation and deformation geometry theory, the mechanical equilibrium equation of each layer of unbonded flexible riser under bending load is established and the contact condition between layers is introduced in the first step. According to the slip state of the spiral strip, the response of the flexible riser under bending load was divided into three stages: no slip, partial slip and complete slip. The expressions of bending moment and bending stiffness were derived. The influence of structural parameters of tensile armor layer of unbonded flexible pipes on its bending performance under bending load is studied by using MATLAB software to compile the corresponding calculation program. The results show that the increase of friction coefficient between layers of tensile armor will improve the bending resistance of flexible pipes. The increase of helix angle will reduce the bending stiffness of flexible pipes in the initial stage. The increase of the number of spiral strips will increase the bending stiffness of flexible pipes in the initial stage. In the process of structural design of flexible pipes, the influence of the above structural parameters on the bending characteristics of flexible pipes should be taken into account.

Impact of damaged external polymeric layer on axial compressive response of flexible pipes

A fully three-dimensional finite element model was developed in order to study the axial compressive response of flexible pipes. The model is parameterized and features five layers, with emphasis on the layers responsible for reacting to axial loads. Tensile armor instability may occur in the form of lateral buckling and radial buckling, also known as “birdcaging”, which is influenced by several parameters, including defects on the layers that are supposed to restrain and protect the tensile armor, more specifically, the external polymeric layer. By being the outermost layer, the external sheath is responsible for keeping the pipe watertight and it is the most likely to sustain damage from launching, collision from vessels, anchors or ice structures and even damage caused by sea creatures. This article focuses on the effect of damage on the external polymeric layer on the tensile armor instability failure modes and uses a finite element model to simulate that phenomenon. For that purpose, the outermost layer was deliberately damaged with cuts on a key position. The cut dimension is parameterized for this analysis and the results will show that, if the flexible pipe is equipped with a high strength layer and the differential pressure is not taken into account, the external polymeric layer will not play an effective part on protecting the tensile armor layers from instability. However, it may influence the triggering of the instability on a specific position of the pipe.

Friction coefficient influence in a flexible pipe: A macroelement model

Flexible pipes are structures responsible for several operations in offshore playing a vital role and hold immense importance in oil extraction.Modelling such pipes is not trivial, due to its layered structure. The helical wounded wires layers, which are responsible for handling axial loads, present a challenge in modelling in special. Another important factor for consideration are layer interactions, since they play a major role in pipe behavior.To obtain the pipe response under various loads, the authors proposed the macroelement model: a finite element model in which elements take profit of the problem geometrical characteristics, simplifying the meshing process and contact treatment, while reducing memory use and processing time.The present article is an excerpt of this research line and focuses on the understating of the frictional behavior in layer interactions by studying a flexible pipe. The model considers all the previous developed elements to simulate a four-layered flexible pipe, consisting of two tensile armor layers and two cylindrical ones. The loading considered is pressure combined with either tension or compression and the effects of the friction coefficient are investigated. Resultsare compared to classical finite element for validation, showing excellent agreement, differences in performance and efficiency are highlighted.

On the pressure–torsion response of a flexible pipe with section ovalization

Ovalization, as an important factor affecting the performance of flexible pipes, is unavoidable in the manufacturing process. In this paper, a model of a flexible pipe with initial ovalization is established numerically. The numerical model is verified by a case analysis, and the influence of ovalization on the torsional response of a flexible pipe is discussed. It is found that the ovalization always has a negative effect on the torsional stiffness of the flexible pipe. Then, the effective numerical model is used to calculate the torsional response of a flexible pipe with initial ovalization under dry and wet annulus pressure. It is found that the dry annulus pressure improves the torsional stiffness of the flexible pipe by advancing the interaction between the layers and hindering the radial expansion of the outer tensile armor layer. However, the dry annulus pressure also increases the stress of the tensile armor layers, which reduces the withstanding capacity of the flexible pipe. Especially with the increase of ovalization, the loading capacity decreases more obviously. The wet annulus pressure has little effect on the torsional response of the flexible pipe, but it extends the risk of radial collapse of the carcass layer.

Corrosion of tensile wires covered with PA11 layers in simulated annulus environments at low CO2 pressure

AbstractHigh‐strength carbon steel wires covered with polymer layers were exposed to different test environments simulating the conditions in the annulus of flexible pipes at low CO2 pressure and flooded with seawater. The polymer layers mimicked the antiwear tapes which are placed in between the steel layers of the tensile armor of the pipes. Specimens without the polymer layer were exposed to the same test environments for comparison and allowed identifying how the presence of the polymer layer influenced the corrosion mechanism. In some of the experiments, oxygen was introduced into the gas mixture to simulate fresh seawater entering the annulus after a breach in the outer sheath. Surface analysis of the corroded specimens after removal of corrosion scales was carried out by scanning electron microscopy, optical profilometry, and confocal microscopy. The polymer layers were observed to have a considerable effect on the corrosion morphology.

A novel helix contact model for predicting hysteretic behavior of unbonded flexible pipes

This paper presents novel equivalent layered contact numerical and analytical models to predict the bending hysteretic behavior of unbonded flexible pipes. A double helix contact beam model, instead of surface-to-surface contact algorithm, is proposed to simulate normal contact and tangential friction behavior between tensile armor tendons and supporting layers. The spatial beam distance is evaluated based on Hertz contact theory. The proposed model is validated through comparison with bending moment-curvature relations obtained from experimental data, a full layered finite element model, and existing analytical models considering or not shear interactions. The shear stiffness is determined by accounting steel armor layers as boundary conditions. The initial contact pressure is introduced into analytical models by accounting residual stress in cylindrical layers and residual strain in helical layers. The bending stresses and relative slip displacements of tensile armor tendons are cross validated between the proposed model and available data. The results show that the proposed model could give consistent bending hysteretic consequences, while shorten simulation time due to significantly reduction of total elements. Sensitivity studies including definition of helix contact beams, equivalent material properties of inner core layer and tangential stick stiffness between layers are performed to validate feasibility of the proposed model.

Full-scale experimental and numerical analyses of a flexible riser under combined tension-bending loading

The flexible riser top connection with floating production units presents additional tensile armour integrity assessment uncertainties associated to the bend stiffener contact interaction that leads to a non-uniform curvature and contact pressure distribution in a region of large dynamic forces and moments. In this paper, a full-scale 6″ flexible riser and bend stiffener bending-tension experimental campaign is carried out in a horizontal rig. The external tensile armour wires are instrumented with strain gauges for axial strain measurement in several riser cross-sections inside and outside the bend stiffener region. The rig assembly allows the riser rotation in order to measure strains at different circumferential angular positions. The bend stiffener polyurethane hyperelastic mechanical response is obtained by uniaxial tensile tests performed at room temperature. A nonlinear finite element model (FEM), including the riser/bend stiffener contact interaction and interlayer friction mechanisms, is employed to numerically investigate the mechanical behaviour of the flexible riser subjected to the experimental loading conditions. The FEM axial strains on the tensile armour wires are compared with the measured results for pure tension and tension-bending loads. Under axisymmetric loading a relevant experimental axial strain dispersion is observed when compared to the average values. Under tension-bending loading, the interlayer friction coefficient influences on the contact pressure and curvature distribution are initially assessed with the numerical model. The strain gauges located in the cross-sections with highest values of contact pressure and curvature are selected for a detailed numerical-experimental strains comparison. In addition, a parametric study is conducted to calibrate the friction coefficient that yields the minimum mean squared error for all measured data. Generally, good correlations are found between the experimental and numerical results.

Assesment of electrochemical machining-induced pitting geometry on fatigue performance of flexible pipes’ tensile armor wires

In the extraction and production of oil and gas in sea environments, multiple components and systems are used, among them, flexible dynamic pipes, responsible for the transport of fluids. Because they are exposed to mechanical stress and corrosive conditions, it is important to assess the fatigue performance of the flexible pipe's components when there are signs of corrosion on their surfaces. Especially, the steel wires that bear the mechanical load during a riser's operation, are often vulnerable components to failure derived from the combination of corrosion and oscillating loads. The present work has the objective to assess the effects of corrosion pits, characterized by their dimensions, quantities and location, in the fatigue life of flexible pipes' tension wires. To make possible the correlation of the pits' parameters with their fatigue performance, the electrical discharge machining process was used, allowing a greater control of pits' development when compared to natural corrosion. The Taguchi method was employed to reduce the total amount of parameter combinations to be tested and to ascertain the effects of each parameter in the fatigue performance. Some pits obtained by electrical discharge machining didn't have the determined dimensions, especially the narrower and deeper pits. Fatigue tests were conducted in tensile armor wire specimens to assess the number of cycles to failure at pre-defined load levels. The fatigue testing results of electrical discharge machining corroded specimens suggest a more significant effect of the pits' diameter in reducing the fatigue performance, when compared to the controlled parameters of depth, quantity, location and aspect ratio. The maximum cycles reduction was around 73%, for wider pits, located in the planar face of the specimen, and the minimum reduction, around 40%, for narrower pits, located at the edge of the specimen.

A global-local approach for dynamic stress evaluation of lazy wave flexible risers subjected to random wave and vessel motion

ABSTRACT With the increasingly severe marine environment, the safety reliability of unbonded flexible risers under service conditions is closely related to its dynamic stress response. In this paper, a systematic approach for riser dynamic stress evaluation subjected to random wave and vessel motion is developed to explore the influencing factors. This methodology proposes a global model to acquire the riser global dynamic response under random wave and vessel motion, considering the effects of nonlinear bend stiffener (BS) constraint and nonlinear bending behaviour of flexible pipes. Then, an axisymmetric load model and bending load model are employed to convert the forces and curvatures obtained from the global model into the dynamic stresses of the tensile armour layers. The findings show that BS and riser top angle can affect the dynamic curvature for the riser top-end region, however, has little influence on the effective tension and bending curvature for the other regions.

INTEGRITY ANALYSIS OF FLEXIBLE PIPES TENSILE ARMOUR WIRES USING NON-DESTRUCTIVE METHOD OF INVERSE MAGNETOSTRICTION: A METHOD COMPARISON

Flexible pipes have been used for decades for conveying produced fluids from oil wells to floating production, for storing and offloading units, and for flowing injection fluids inside wells under the seabed. Over the years, with an increase in oil demand globally, the exploration of new and deeper oil reservoirs has become a reality. With greater water depths, the top tension of the risers has increased significantly, as well as the pressure from the water column on the pipe structure, demanding the application of bigger pipes and the use of stronger materials. Pre-salt reservoirs on the Brazilian basins are rich in carbon dioxide (CO2) and hydrogen sulfide (H2S), which are great contributors to the premature fragilization of armour wires on the top section of the flexible risers and are capable of compromising the integrity of the structure. The wire fragilization and possible breakages cannot be avoided once the riser is connected to the floating unit, but they can be monitored, to avoid a complete riser structural failure. Several systems have been developed to monitor tensile armour wires integrity, such as visual monitoring, which visually detects torsion on the structure, acoustic waves, and magnetic collar systems, which have failed to provide reliable results. As the armour wires provide axial resistance to the structures, Fiber Bragg grating is being used in modern flexibles to monitor armour wire deformation and has been delivering reliable results. Another method, the inverse magnetostriction, is being tested using an anisotropic magneto sensor to detect wire deformation and generate a 2D stress map of the measured area. In this article, two tests are performed using the same equipment, but in different specimen. One test was performed on the wires without being applied on the flexible structure. The other test was performed on a window opened on a 6” nominal bore flexible pipe, with the objective of identifying if the method is reliable in detecting armour wire failures on flexible risers. This method could read stresses variations on the armour wires, but a larger sensor could penetrate deeper on the structure and provide a sharper stress map.

Flexible riser tensile armour stress assessment in the bend stiffener region

The flexible riser within the bend stiffener in the top connection is a structurally vulnerable region for extreme and fatigue loads as it is subjected to large dynamic tensile forces and curvatures. This paper presents a nonlinear finite element model (FEM) for the flexible riser with bend stiffener taking into account interlayer friction and riser-bend stiffener contact interaction subjected to combined axisymmetric and bending loads. A case study with typical extreme and fatigue load cases is carried out to assess the curvature variation and bend stiffener contact pressure influence in the riser tensile armours stresses. For the extreme load case, detailed finite element results such as contact pressures and friction, normal and transverse bending stresses are compared with a uniform curvature FEM and available analytical models. A fatigue load case is also performed, where the stress amplitudes and cumulative fatigue damages for the internal tensile armour wires are compared with a uniform curvature FEM and analytical models. From the case study results under extreme loading, it can be observed that the model with bend stiffener interaction predicts larger stresses in comparison to the uniform curvature FEM and analytical models. Under typical fatigue loading conditions, significant differences in the cumulative damage are observed between the models, highlighting the importance of an accurate modeling for lifetime integrity assessment in a region of highly variable riser curvature.

Finite element analysis of flexible pipes under compression: A study on the damaged high strength tape and its effects on tensile armor instability

To study the compressive behavior of flexible pipes and tensile armor instability, a nonlinear Finite Element model was developed. This fully tridimensional model recreates a five layer unbonded flexible pipe. When the pipe is submitted to compressive loads, tensile armor instability may occur in the form of lateral buckling or radial buckling, also known as “birdcaging”. This complex failure mode may be influenced by several parameters, including defects on the layers that are supposed to restrain and protect the tensile armor, more specifically, the high strength tape. This layer is usually made of high a performance textile material that is not directly exposed to the environment. However, damage may be sustained when the outermost layer is subjected to deep cuts and breached. This article focuses on the effects of a damaged high strength tape and its implications on the tensile armor instability failure modes. The study shows that damage in the high strength tape may compromise the cohesion between the layers, significantly reducing the critical instability load and the flexible pipe's stiffness, as well as altering the yielding for the layers.

Development of a Robot for In-service Radiography Inspection of Subsea Flexible Risers

The extreme operational environmental conditions and aging conditions of subsea structures pose a risk to their structural integrity and is critical to their safety. Nondestructive testing is essential to identify defects developing within the structure, allowing repair in a timely manner to mitigate against failures that cause damage to the environment and pose a hazard to human operators. However, to be cost effective, inspections must be carried out without taking the risers out of service. This poses significant safety risks if undertaken manually. This paper presents the development of an automated inspection system for flexible risers that are used to connect wellheads on the seafloor to the offshore production and storage facility. Due to the complex structure of risers, radiography is considered as the best technique to inspect multiple layers of the risers. However, radiography inspection, in turn, requires a robotic system for in-situ inspection with higher payload capacity, precise movement of source and detector which is able to withstand an extreme operational environment. The deployment of a radiography inspection system has been achieved by developing a customized subsea robotic system called RiserSure that can provide precise scanning motion of a gamma ray source and digital detector moving in alignment. The prototype has been tested on a flexible riser during shallow water sea trials with the system placed around a riser by a remotely operated vehicle. The results from the trials show that the internal inner and outer tensile armor layer and defects in the riser can be successfully imaged in real operational conditions.

A full layered numerical model for predicting hysteretic behavior of unbonded flexible pipes considering initial contact pressure

This paper presents a method to predict the bending hysteretic behavior of unbonded flexible pipes using full layered numerical general finite element models with an implicit solver. A predefined stress fields method is proposed to simulate the initial contact pressure effect introduced by the fabrication process. Detailed finite element modeling techniques are illustrated based on system balance principles of a cantilever beam system. The models can capture the interactions (normal contact and tangential friction behavior) between and among layers adequately, thus giving reasonable stress predictions of tensile armor tendons in both longitudinal and transverse (radial and lateral) directions. The results show that the proposed method through prescribing a fixed initial hoop stress value in the outer sheath could give consistent bending moment-curvature relations for all test pressure levels and corresponding numerical results. The stress-curvature relations reveal that the tensile armor tendons slide in an interval between the loxodromic and geodesic curves as proved in the open literature. In addition, sensitivity studies including element types, boundary conditions, material properties, normal contact stiffness and friction coefficients are performed to validate the feasibility of the proposed models. Moreover, if plastic layers are modeled by solid elements, the friction moment will be larger than that of using shell elements due to Poisson’s effect on pipe elongation.

Alternative analytical and finite element models for unbonded flexible pipes under axisymmetric loads

The calculation of stress components in tensile armour wires of flexible pipes subjected to combined tension and bending is challenging given the intrinsic geometrical and physical nonlinearities. Mechanical loading simulation in finite element models yields heterogeneous curvature distribution, directly impacting stress evaluation. Addition of two auxiliary thermal layers internally and externally subjected to radial and longitudinal thermal expansions simulates equivalent axisymmetric loading, achieving relatively uniform curvature distribution. Calculation of equivalent thermal loads and expansion coefficients involves time consuming trial and error processes by running several finite element models. Therefore, a theoretical formulation that takes into consideration thermal sheaths with orthotropic thermal expansion coefficients and respective thermal loads is developed. A three-dimensional finite element model is also developed for direct application of axisymmetric mechanical or equivalent thermal loadings. A 6″ flexible pipe is employed in a case study to compare the finite element model results within themselves and with the analytical mechanical and thermal models. The analytical results are shown to be rigorously identical. The finite element models present small differences when compared to the analytical results allowing combined application of thermal and bending loading that results in constant curvature away from the boundaries.

Tensile Response of a Flexible Pipe With an Incomplete Tensile Armor Layer

Abstract Unbonded flexible pipes are widely utilized in the exploitation of offshore oil and gas resources. They are connected to two of the most critical types of system: floating production platforms and underwater production systems. However, if some tensile armor wires are substituted by cables or broken, the tensile armor layer will be incomplete, which seriously reduces the safety and reliability of the flexible pipe. In the present study, models of a flexible pipe with a complete tensile layer and with the tensile layer partially missing were established. The error for the tensile stiffness obtained by the finite element model of an intact flexible pipe was only 1% compared with experimental results. Because the load borne by the inner tensile armor layer is larger under tension than that borne by the outer tensile armor layer, the loss of inner tensile armor wires has a greater impact on the tensile properties. The maximum axial elongation of the flexible pipe increases with the number of missing inner tensile armor wires as a cubic polynomial. If the distribution of the missing armor wires is too dense, a stress concentration and local bending may occur, which will reduce the tensile strength of the flexible pipe.