Armor Wires Research Articles

Analysis of the bending performance of unbonded flexible risers

ABSTRACT As the key equipment for offshore oil and gas production systems, the unbonded flexible riser (UFR) is subjected to extensive interlayer and intralayer non-linear contact and slip that make it difficult to analyse its bending performance. In this paper, by combining the theoretical model of bending of the UFR with the finite element model, the influence of the number, width, external pressure and coefficient of friction of tensile armour wires on its bending performance was studied. The results showed that the non-slip bending stiffness of the UFR decreased by 3.20 and 23.35 kN·m2 every time one of the outer tensile armour wires was removed and the width of the wires was reduced by 0.3 mm, respectively. The external pressure and coefficient of friction did not influence its bending stiffness in the non-slip and complete slip stages, and were linearly related to the curvature of the critical slip.

Thermo-Chemo-Mechanical Modeling of Residual Stress in Unidirectional Carbon Fiber-Reinforced Polymers during Manufacture.

The application of carbon fiber-reinforced composite materials in marine engineering is growing steadily. The mechanical properties of unbonded flexible risers using composite tensile armor wire are highly valued. However, the curing process generates a certain amount of internal residual stress. We present a detailed analysis of epoxy resin laminates to assess the impact of thermal, chemical, and mechanical effects on the curing stress and strain. An empirical model that correlates temperature and degree of cure was developed to precisely fit the elastic modulus data of the curing resin. The chemical kinetics of the epoxy resin system was characterized using differential scanning calorimetry (DSC), while the tensile relaxation modulus was determined through a dynamic mechanical analysis. The viscoelastic model was calibrated using the elastic modulus data of the cured resin combining temperature and degree of the curing (thermochemical kinetics) responses. Based on the principle of time-temperature superposition, the displacement factor and relaxation behavior of the material were also accurately captured by employing the same principle of time-temperature superposition. Utilizing the empirical model for degree of cure and modulus, we predicted micro-curing-induced strains in cured composite materials, which were then validated with experimental observations.

Analysis of crystallographic orientation and crack behavior in armor wire with spheroidized pearlite: Simulation and modeling with linear elastic fracture mechanics

This study investigates crack propagation in a pearlitic armor flexible riser subjected to CO2-rich marine environments, employing Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD), and Finite Element Modeling (FEM). A gradient in mechanical properties across the material's depth reveals a balance between surface strength and inner shear stress resistance, transitioning from γ-fiber and (001) texture at the surface to {110〈uvw〉 texture deeper. Our study revealed that Mode I (KI) fracture predominantly influences the failure, emphasizing the critical role of tensile stress in crack resistance. Analysis shows that crack growth is most susceptible at the midsection pit (z/a = 0), with pit size significantly affecting stress intensity. The study highlights the importance of material processing and microstructural orientation in determining mechanical properties, contributing to the development of robust materials for challenging marine applications.

Carbon Dioxide Corrosion Mechanisms: Historical Development and Key Parameters of CO2-H2O Systems

The recent failures in flexible pipes have motivated the exhaustive research of corrosion mechanisms on high-strength carbon steel armor wires that are the main structural compounds of those structures that mostly operate in seawater environments in the presence of carbon dioxide (CO2) and confined spaces. Recently, the literature reported discoveries about electrolyte properties (as Fe+2(aq)/HCO3-(aq.) ratio) and supersaturation, near neutral pH inside the confined space, multiphase reactions of contaminants present in CO2 gas, the formation and dissolution mechanism of FeCO3 film, and interaction of CO2 gas impurities with the corrosion scale. Therefore, this review is aimed at presenting an up-to-date narrative of the CO2 corrosion phenomenon in carbon steel, connecting background fundamentals with current data studies.

Influence of metallurgical factors on the hydrogen induced cracking of carbon steel wires in H2S-containing environments

Flexible pipes armor wires used in offshore oil and gas fields can corrode if water and acid gases, such as H2S, build up in the annulus. Such an environment may provide the conditions for hydrogen-induced cracking (HIC) depending on the corrosive solution composition and the metallurgical characteristics of the armor wire, such as microstructure and microhardness. The critical zones of HIC incidence were investigated for tensile and pressure armor wires with carbon content of 0.7 and 0.3 wt% C, respectively. Results were obtained through immersion testing and hydrogen permeation measurements in near neutral substitute ocean water (solution A) and standard acid solution (solution B) at ambient temperature bubbled with a gas mixture (1 % H2S + 99 % N2). Scanning electron microscope (SEM) and computed tomography (CT) evidenced the microstructure and microhardness features associated to crack morphology and location. According to the results, the increase of carbon content in cold drawn steels promoted the accumulation of hydrogen mainly in regions of high microhardness, inducing a high density of cracks in a stepwise cracking (SWC) propagation. This indicates that the driving force for HIC was associated with metallurgical factors, such as microhardness, the presence of hydrogen trapping sites of high energy, dislocations and ferrite/cementite morphology and distribution, occurring even in the absence of non-metallic inclusions. Additionally, environmental conditions, such as the low pH found in solution B, increased the criticality of HIC. Both materials presented a crack sensitivity ratio (CSR) that grew over time in solution B, but no cracks were observed in the pressure armor wire tested in solution A. Hydrogen permeation curves were delayed for both tested armor wires in solution A due to its near neutral pH characteristic. Hydrogen diffusivity (Deff) of the tensile armor wire was lower than the pressure armor wire, indicating the presence of a higher density of hydrogen trapping sites in a harder and elongated pearlite.

Influence of coiling behavior on axial stress in steel wires of submarine power cables: A numerical study

Coiling processes that are commonly employed to lay submarine power cables to static tanks on ships cause twisting and bending deformations of the cables, potentially affecting the integrity and safety. This study investigates the mechanical behavior of submarine power cables subjected to combined torsional and bending loads using analytical models and finite element method-based software. The objective of the numerical analysis was to evaluate compressive stresses generated within the armor wires of submarine power cables. Additionally, the investigation was extended to assess the potential risk of wire buckling, i.e., bird-caging. The influence of wire pitch length and tank diameter on the magnitude of compressive stress was examined, and the findings revealed a decrease in the likelihood of wire buckling with longer pitch lengths and larger tank diameters. The results of this study will offer valuable insights into the cross-sectional design and the mechanical behavior of submarine power cables subjected to coiling operation.

A 2D FEM Model for Impedance and Loss Calculation of Armored Three-Core Cables With Inclusion of 3D Pitching Effects

A method is introduced for impedance and loss calculation of three-core power cables with steel armoring, based on 2D finite element method (FEM) computations. The pitching effect of the cores and armor wires is taken into account by 1) enforcing identical net current on the wires, and by 2) introducing a fictitious non-conductive material between the wires having a complex-valued permeability. The permeability is obtained by considering the effect of the pitching on the total energy in a volume slab around each wire, which involves the solving of a small 2D FEM problem in an optimization loop. The method permits to calculate the positive-sequence and zero-sequence impedance on cables, induced sheath currents, and losses in cores, sheaths, and armor. The method is validated against published results using a full 3D FEM model. The calculations are fast, requiring only a few seconds of computation time. The procedure is simplified by use of pre-calculated look-up tables for the fictitious material permeability value.

Assessing the influence of cyclic bending on pearlitic wire's microstructural evolution: a simulation of spiraling and armoring processing effects

Flexible pipelines are widely used offshore for oil and gas transportation and are subjected to several complex stress modes during service. Stress tensors cause different alterations in the macro and microstructure of the material. Therefore, studies involving the relationship between mechanical behavior and the crystallographic orientation of tensile armor wires need deep investigation. This work investigated the role of complex plastic deformation during the spiraling tensile armor on the microstructure and crystallographic texture of pearlitic steel with 0.74 wt%C. A heat-treated sample was prepared as a starting point. Further specimens were subjected to cold bending to investigate the strain accumulation during spiraling and armoring processing and then compared to an industrial twisted tensile armor sample. The microstructural and texture evolution was characterized by scanning electron microscopy, X-ray diffraction, and backscattered electron diffraction. Specimens were submitted to the Vickers microhardness and tensile tests to evaluate the mechanical properties. Results revealed that the bending deformation causes a reduction of (001)//ND fiber and increases the mechanical resistance of the material, which may prevent the onset and crack propagation. (111)//ND fiber increased while (110)//ND decreased, leading to stability in the slip systems. The formation of shear band structural changes resulted from the increase of (110)//ND with armoring and a decrease of (111)//ND fibers. Bending strains enhance the desirable texture components while diminishing the undesirable ones. Deformations to which the materials were subjected provide less mobility of dislocations, which causes an increase in mechanical strength. Little change in texture and microstructure was also observed.

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.

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.

Fully Coupled Computation of Losses in Metallic Sheaths and Armor of AC Submarine Cables

The problem of computing in an efficient and accurate way the armor losses in three-core AC submarine cables has been of great interest to the offshore cable community for some years. Indeed, efficiency of the computation method is crucial to allow comprehensive parametric analysis as might be needed in the design study. In (Giussani <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> 2021), an algorithm based on integral equations for performing such calculations with an accuracy comparable (within acceptable tolerances) to state-of-the-art Finite Element Method (FEM) approaches was presented. In the present study, we extend the above mentioned algorithm to account for the mutual inductive coupling between armor wires and metallic screens, which are often present in actual cables structure. We show that the most notable properties of the original algorithm are preserved in this coupled formulation. In particular we discuss how exploiting the cable structure periodicity and symmetry leads to huge computational cost reductions. The new extended simulation tool is applied to a parametric study to assess how geometry and material properties influence losses and to assess in particular the impact of the metallic sheaths.

Hole-Drilling Method for Deep Residual Stress Measure in Tensile Armour Wires

Hole-Drilling Method for Deep Residual Stress Measure in Tensile Armour Wires

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.

CO2-SCC in Flexible Pipe Carbon Steel Armor Wires

Carbon dioxide-induced stress corrosion cracking (CO2-SCC) is an environmentally assisted corrosion cracking phenomenon that has recently been identified as a new failure mode in flexible pipe armor wires. The phenomenon has been observed to take place notably in severe CO2 environments and is a cause of great concern to the flexible pipe industry. As its detection, a diverse and extensive testing program has been established to develop an understanding of the phenomenon and define safe application limits for carbon steel wires to prevent the initiation of CO2-SCC. Several different testing methodologies have been explored and small-scale laboratory testing has played an instrumental role in this overall effort. This paper focuses on the results from three different small-scale testing methodologies and the impact of different parameters such as CO2 fugacity, temperature, and confinement that play a crucial role in the initiation of CO2-SCC. Furthermore, careful prominence has been given to the test setup and methodology that has been rigorously developed over the last few years. With this developed protocol in place, CO2-SCC has been effectively reproduced on all wire grades in a small-scale testing environment. Results have also shown that for a CO2 fugacity greater than 15 bar and applied stress at 100% of the actual yield strength, all existing wire grades are susceptible to CO2-SCC thus creating significant limitations to flexible pipe design with respect to this new failure phenomenon.

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.

Experimental validation of ultra-shortened 3D finite element electromagnetic modeling of three-core armored cables at power frequency

Due to recent advances, the numerical analysis of submarine three-core armored cables can nowadays be developed through the finite element method (FEM) in a small slice of the cable. This strongly reduces the computational burden and simulation time. However, the performance of this ultra-shortened 3D-FEM model is still to be fully assessed with experimental measurements. This paper focuses on this validation for an extensive variety of situations through the experimental measurements available in the specialized literature for up to 10 actual cables. In particular, it deals not only with relevant calculations at power frequency, like the series resistance and inductive reactance or the induced sheath current, but also with other aspects never analyzed before through 3D-FEM simulations, such as the zero sequence impedance, the magnetic field distribution around the power cable, as well as side effects due to the nonlinear properties of the armor wires. All this considering different armoring and sheath bonding configurations. Results show a very good agreement between measured and computed values, presenting the ultra-shortened 3D-FEM model as a suitable tool for the analysis and design of three-core armored cables, and opening the possibility to reduce the need of extensive experimental tests in the design stage of new cables.

Hydrogen uptake and diffusivity in steel armor wires with different chemical composition, carbide distribution, grain size, and degree of deformation

AbstractIn this study, six flexible pipe steel armor wires used in oil and gas transportation are characterized, and their hydrogen diffusion coefficients and hydrogen uptakes are measured using an electrochemical hydrogen permeation technique. The wires have ferritic–pearlitic microstructures with round, lamellar, or partially lamellar carbides and the shape and orientation of the grains indicate that the microstructures were plastically deformed to different degrees. It was assumed that hydrogen was transported through the ferrite, so the presence of cementite in the steel armor wires leads to longer hydrogen diffusion paths through the ferrite, which was quantified with a tortuosity factor. After compensating for tortuosity, the normalized steady‐state flux shows a tendency to increase as the grain size decreases. The effective diffusion coefficients increase with a decrease of the ferrite–cementite interface area, suggesting trapping on the ferrite–cementite interfaces. The uptake of diffusible hydrogen was lowest for the least plastically deformed materials and about twice as high for the more plastically deformed materials.

Experimental study on the interlayer friction and wear mechanism between armor wires of umbilicals

Friction and wear behavior between armor wire layers of umbilical plays an important role in its mechanical properties, in particular its fatigue life. The present paper addresses friction tests to investigate the interlayer sliding friction and wear mechanism of armor wires in an umbilical. A series of friction tests were carried out to study the evolution of coefficient of friction (COF) and wear between armor wire layers. The results show that the COF increases with increasing number of reciprocating sliding cycles in dry friction condition. A dual stable trend was found in the evolution process of interlayer friction and wear of armor wires. A friction model was proposed to describe this trend by dividing the evolution process into three stages: the initial stable stage, the transition stage, and the latter stable stage. The mechanism of the dual stable trend was revealed, which was mainly caused by wear process of nylon fiber tape, and effected by dry/wet wear condition and wear coefficient. The effect of the contact condition, sliding amplitude and number of nylon fiber tape on COF was studied. The COF of the latter stable stage increases with increasing sliding amplitude and number of layers of nylon fiber tape. The finding in this paper provides a valuable approach for a more accurate fatigue life prediction of an umbilical.