Armor Wires Research Articles

Analytical model for tensile armors lateral deflections and buckling in flexible pipes

The present paper addresses the mechanical behavior of tensile armor wires in a flexible pipe subjected to bending and longitudinal loads. An analytical model is formulated to evaluate the equilibrium state of a single armor wire on a frictionless torus surface by solving a consistent system of six nonlinear differential equations through the application of a perturbation technique. Cases studies are performed where the armor wire's responses to both tensile and compressive loads are discussed and compared with the results obtained from a numerical program. Under the circumstance of axial compression, two potential lateral buckling modes, global and periodic, are uncovered and their corresponding buckling limits are identified by establishing eigenvalue problems. While the global buckling limits are found conservative compared with the test data available in the literature, good agreements are observed for the estimated buckling limits regarding the periodic buckling mode.

Tensile Armor Wires Submitted to Slow Strain Rate Tests in a Corrosive Environment and Cathodic Protection: a Comparison Between Two Different Microstructures

Tensile armor in flexible pipes consists of two or more layers of steel wires. Damage to the outer sheath may cause ingress of seawater in the annular space and thus corrosion of the armor wires. This work focused on the susceptibility of these wires to stress corrosion cracking (SCC) and hydrogen embrittlement (HE) using a slow strain rate test (SSRT) under a bending load in an environment that contains chlorides. The behavior of two different microstructures was compared: martensite and pearlite plus ferrite. Furthermore, the materials were mechanically and metallurgically characterized. The results indicate that martensitic steel is more sensitive to both hydrogen embrittlement and stress corrosion cracking than pearlitic-ferritic steel.

Failure Analysis of Metallic Armoured Electro-Optic Mechanical Cables in Underwater Towed Sonar Systems

In towed sonar systems generally, the array is towed behind a ship using a combination of electro-optic mechanical (EOM) tow cables - negatively buoyant metallic armoured Heavy Cable and neutrally buoyant fibre armoured Light Cable- the former for depth control and latter for depth maintenance. Any failure in these cables can affect the performance of the sensor array and can even result in loss of the towed system to sea. During a series of towed array trials, it was observed that the metallic armoured cable had lost its circularity and became flattened during deployment and retrieval from ship. This may be due to the extreme radial forces acting on the cable while passing through a) rollers located at winch spooler and b) main fairlead comprising of 5 rollers located at aft end of ship, which causes permanent changes in cable’s internal configuration. To verify the cause and study the effects of such flattening on functionality and life of the cable, cyclic load testing studies were carried out after simulating the actual deployment-retrieval conditions as that in the ship. Both indigenously developed and imported cable samples were chosen as test samples for performance comparison. During the tests, it was observed that severe flattening of armour and core cable resulting in failure of electrical & optical lines had occurred in indigenous cable samples. The imported cable samples showed no flatness or failure of lines. Based on the results, steps were taken to improve the radial load bearing capacity of the indigenous cable. Modifications were made in a) core cable sheath material b) core cable sheath thickness & c) armour wire material and an improved indigenous cable was developed, which has withstood all the above tests and is now ready to be inducted to the system for sea trials.

A New Multiconductor Cell Three-Dimension Matrix-Based Analysis Applied to a Three-Core Armoured Cable

In this paper, a procedure based on three-dimension matrices is presented: it is the generalization of multiconductor cell analysis to any transmission line where the positions of all the active and passive conductors may change with continuity along their length. A meaningful example of this is given by a three-core single lead-screened armoured cable where cores and armour wires are wound helically with different lay lengths. Therefore, this generalization is presented with reference to these cables but it can be applied to any transmission line. The proposed matrix procedures can be implemented in any standard computer by means of a mathematical software (e.g., MATLAB). The computational complexity of this novel method is shown in comparison with the finite element method commercial software FLUX 3D, which takes 70 h for meshing and solving a 3-m long model. On the contrary, the present matrix algorithms take from minute fractions to maximum 2 min (depending on subconductor number) to obtain results having negligible differences with respect to FEM FLUX 3D ones if paramagnetic materials are involved and differences up to maximum 7% with ferromagnetic ones (which imply, for submarine cable installations, a current rating computation difference lower than 3%).

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.

On the extensional-torsional response of a flexible pipe with damaged tensile armor wires

This work investigates the extensional-torsional response of a flexible pipe with one up to ten wires broken in its outer tensile armor. Experimental tests were conducted, and the results obtained are compared to those from a finite element (FE) and a simplified analytical model. In these tests, the pipe was firstly subjected to pure tension and, then, to clockwise and anticlockwise torsion. The experimental results indicate a decrease in the stiffness of the pipe with the increasing number of broken wires as well as a significant impact on its extensional-torsional balance. High stress concentrations in the wires close to those damaged were observed. Furthermore, local changes in the curvature of the pressure armor and the inner carcass induced bending strains not only in these layers, but also in the inner tensile armor wires. The FE model captured these effects and agreed well with the experimental measurements, while the analytical model predicted the overall response of the pipe, but local effects could not be evaluated.

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

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

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

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

Technology Focus: Offshore Facilities (February 2018)

Technology Focus Welcome to the Offshore Facilities feature in this month’s JPT. The three papers featured in this section were selected out of 141 collected covering various elements of offshore-facilities design, construction, installation, operation, and inspection. The first paper covers the design, fabrication, transportation, installation, and startup of a floating production, storage, and offloading (FPSO) vessel in an ultradeepwater location in the Gulf of Mexico. The vessel is connected to production and an export gas pipeline with a disconnectable buoy, which allows the FPSO vessel to sail off location in the event of a hurricane. The paper highlights the design features of the installation and some of the project-execution challenges. The second paper describes a technology-development effort to monitor the mechanical integrity of flexible risers in offshore use through a technique called Monitoring Based on Optical Fiber Attached Directly on Armor Wires, or MODA. This is an interesting approach to assess the integrity of the tensile armor at the top section of flexible risers, and the authors summarize years of work in developing the required hardware and data processing. The final paper covers field work performed to ascertain the functional capacity limits of existing equipment on a deepwater platform in excess of the original nameplate limits. It is fairly typical of the industry that, sometime after process facilities are installed, an opportunity to increase production is presented and the engineer must divine the absolute limits of the equipment. This paper presents a nice narrative of one company’s work to answer this question. I hope you enjoy reading these papers. Recommended additional reading at OnePetro: www.onepetro.org. OTC 27526 General Design of Lean MEG Storing in the Jacket Legs on Liwan Gas Field of South China Sea by Zhi Xia, China National Offshore Oil Corporation, et al. OTC 27719 Fixed Platforms at Aging Oil Fields—Feasibility Study for Reuse to Wind Farms by J.C. Barros, Genesis Oil and Gas, et al. OTC 27742 Enabling Materials and Corrosion Technologies for Optimizing Offshore Developments by Eric J. Wright, ExxonMobil Production Company

Optical Sensors Monitor Vulnerable Top Sections of Flexible Risers

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 28080, “Monitoring Flexible Risers With Optical Sensors—Operational Experience and Future Perspectives,” by B.M. Santiago, S.R. Morikawa, W. Carrara, A. Kravetz, and J.A. Martins, Petrobras, prepared for the 2017 Offshore Technology Conference Brasil, Rio de Janeiro, 24–26 October. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission. The integrity management of flexible pipes used in subsea production plays a major role in maximizing the availability of the production systems while minimizing safety and environmental risks. One of the key areas for integrity management is the riser top section, where high tensions and curvatures result in high stresses and may lead to fatigue issues. A monitoring technique known as Monitoring Based on Optical Fiber Attached Directly on Armor Wires, or MODA, has been developed to assess the integrity of the tensile armor at the top section of flexible risers. Introduction A flexible pipe is composed of several layers, each having a specific function (Fig. 1). The carcass is a metallic layer designed to withstand external compression forces in the radial direction. The internal pressure sheath is a polymeric layer used to contain the fluid inside the pipe. The pressure armor is a metallic layer designed to withstand the internal pressure. The tensile armors are made of metallic wires designed to resist axial forces acting on the pipe. In risers, the tensile armors are responsible for keeping the flexible pipe suspended from the floating production unit (FPU). The outer sheath wraps the other layers, protecting them from the external environment. The region inside the internal pressure sheath is called the bore, and the region between the pressure barrier and the outer sheath is known as the annulus space. The riser top section is subjected to high tensions because of the weight of the riser and to cyclic tensions and curvatures because of FPU movement, which results in high constant stresses and significant cyclic stresses in the tensile armors. This combination may lead to fatigue issues, especially in a flooded-annulus condition, which severely hinders the pipe capacity to withstand cyclic loads. The MODA monitoring technology has been developed to better assess the integrity of the tensile armor at the top section of flexible risers. MODA Development History. At its beginning, the MODA concept was only retrofitted to risers already in service. To install the MODA sensors, it was necessary to remove a portion of the outer sheath, attach the optic-fiber sensors to the wires, repair the outer sheath, and then provide an optical path to an equipment room at the FPU where the MODA equipment was to be installed. Substantial resources were needed in each MODA installation, and only the most critical risers were considered for monitoring.

Особенности определения механических параметров оптического кабеля, прокладываемого в земле

The article presents the modern design of Type OGD optical communication cable manufactured in line with its technical specifications and used by service providers for laying in the ground. The cable design is described, its main characteristics are given, and the predominant field of its application is indicated. The optical cable has an outer sheath and a core containing optical fibers which constitute the basis of any communication cable because it is exactly the optical fiber located in the core through which the signals are transmitted. The cable contains also a central dielectric strength member around which the optical modules and insulating cords are twisted. Each optical module contains from 2 to 24 optical fibers. The cable inner and outer sheaths are made of polyethylene. The inner sheath is superimposed with an armor made of one or two layers of galvanized steel wires or a layer of dielectric rods. Two design versions of Type OGD cable with an armor made of soft and hard galvanized steel wire are considered. The deformation force causing the same elongation for a cable with the armor made of hard wire is a factor of two higher than it is for a cable with the armor made of soft wire; i.e., the hard-wire armor makes the cable almost two times stronger than that with the soft-wire armor. The number of optical fibers, the size and weight of Type OGD cable withstanding the tensile force equal to 20 kN, and the sizes of hard and soft armor wire are given. The mechanical properties of cables of the considered designs were investigated. The test was carried out using the Type RRK-EK2 tensile and crushing installation. The results from measurements of signal power loss in an optical fiber depending on the value of tensile and crushing force acting on the cable are presented.

An experimental study on the key fretting variables for flexible marine risers

This paper presents an experimental investigation into the effects of contact conformity, contact pressure and displacement amplitude on gross-slip fretting behaviour for grease-lubricated cylinder-on-flat contacts in the context of flexible marine riser pressure armour wire, and compares behaviour with that observed in unlubricated conditions. Characterisation of friction and wear is critical to fretting fatigue life prediction in flexible risers since friction directly controls trailing-edge fretting stresses and hence fatigue crack initiation, on the one hand, and on the other hand, directly affects wear via relative tangential slip (displacement). Wear can have a beneficial or detrimental effect on fatigue crack initiation and propagation, depending on relative slip and slip regime. For the grease-lubricated conditions, the behaviour is determined by whether grease can be retained in the contact (as opposed to being extruded out). Retention (or replenishment) of grease in the contact results in low rates of wear and low coefficients of friction; these conditions are favoured by fretting displacements above a critical value, by low contact conformity, and by low applied loads.

An empirical model for flexible pipe armor wire lateral buckling failure load

The current scenario of oil production in ultra deep waters challenges the design of flexible pipes since they have to withstand high pressures, temperatures and dynamic loads during operation. Amongst the known failure mechanisms in flexible pipes, armor wire lateral buckling is of uttermost importance as there is no undisputable model for its prediction. In this work, compressive failure load for tensile armor lateral buckling is calculated for a sample of 29 flexible pipes based on the resolution of a numerical model that considers force and moment equilibrium conditions and geometric compatibility relations from differential geometry concepts. The obtained results are fitted on an equation via symbolic regression. Thus, a simple empirical model to predict compressive failure load is proposed as a function of the radius of tensile armor, wire lay angle and torsional, normal and binormal bending stiffness of the wire cross section. The model assumes the absence of frictional distributed forces in the wire and curvature of the flexible pipe, therefore it provides conservative results. For the 29 flexible pipe sample, the mean absolute percentage error of the empirical results in view of the numerical results was 0.6%.

Hybrid composite tensile armour wires in flexible risers: A multi-scale model

Traditional carbon-steel armour wires pose limitations (e.g. long spans, weight reduction, corrosion and fatigue) for flexible risers to operate in demanding and deeper water environments. In this context, an alternative to carbon-steel tensile armour wires is proposed recently by the authors (Gautam et al., 2016), comprising of hexagonally packed polymer composite rods with uni-directional fibres and an over-braided (i.e. bi-axial braid with high performance fibres) sleeve. These hybrid composite wires offer opportunities to tailor their mechanical properties by varying the geometrical (e.g. rod diameter, packing) and processing parameters (e.g. material selection, braid pattern) involved. In order to understand the mechanical behaviour of these hybrid composite armour wires, this paper presents a multi-scale model developed by using a combined analytical-computational approach. The multi-scale model is developed to predict the torsional and flexural behaviour of the hybrid composite wires; and the role of over-braid structural parameters, pre-tension and internal friction are investigated. The behaviour of the multiscale model is found to be in good agreement with the experimentally observed behaviour. After validating the multi-scale model with the experimental data available for specific configurations, parametric studies are conducted on the torsional and flexural behaviour of the hybrid composite wires to study the role of internal friction between un-bonded components and the braid tow tension in the over-braided sleeves.

A quasi-linear method for frictional model in helical layers of bent flexible risers

This paper deals with the behaviour of two helical layers of bent flexible risers. An analytical model with frictional effects is developed to summarize the mechanical behaviour of helical armour wires. To ensure a unified process of calculation, all formulas are simplified to build up a system of quasi-linear partial differential equations. The numerical solution is determined by finite difference method. Appropriate results are shown in the paper. The geometrical quantities, slips and stresses are matched with other analytical results and found valid.

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

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

A combined wear-fatigue design methodology for fretting in the pressure armour layer of flexible marine risers

This paper presents a combined experimental and computational methodology for fretting wear-fatigue prediction of pressure armour wire in flexible marine risers. Fretting wear, friction and fatigue parameters of pressure armour material have been characterised experimentally. A combined fretting wear-fatigue finite element model has been developed using an adaptive meshing technique and the effect of bending-induced tangential slip has been characterised. It has been shown that a surface damage parameter combined with a multiaxial fatigue parameter can accurately predict the beneficial effect of fretting wear on fatigue predictions. This provides a computationally efficient design tool for fretting in the pressure armour layer of flexible marine risers.

Recovery length of high strength tapes in damaged flexible pipes

The application of filaments wound around damaged high strength tapes (HSTs) in flexible pipes may be one of the simplest methods to reestablish the initial restraint capacity for the armor wires. This paper aims at evaluating the effectiveness of such method observing the recovery length of filaments and investigating the mechanical behavior of both perfect and damaged HSTs. Analytical formulations that depict the restraint ability of undamaged HSTs are derived from a combination of Clebsch–Kirchhoff equilibrium equations and a modified non-linear friction law in terms of normal stress. As a base case, the restraint ability induced by the interaction between original damaged HSTs and filaments is obtained as well as the required recovery length. The solution is presented in non-dimensional form and it is shown that the repair method is feasible. Besides, some important parameters affecting the recovery length are investigated. It is found that the application of tension force at both ends of filament is the most effective way to reduce the recovery length. It is also shown that external pressure has insignificant effect if tension forces at ends are large. In addition, a case study shows that the recovery length grows almost linearly with the increasing radius of tape.

Radial Buckling of Tensile Armor Wires in Subsea Flexible Pipe—Numerical Assessment of Key Factors

Flexible pipes can be used as risers, jumpers, and flowlines that may be subject to axial forces and out-of-plane bending motion due to operational and environmental loading conditions. The tensile armor wires provide axial stiffness to resist these loads. Antibirdcaging tape is used to provide circumferential support and prevent a loss of stability for the tension armor wires, in the radial direction. The antibirdcaging tape may be damaged where a condition known as “wet annulus” occurs that may result in the radial buckling (i.e., birdcaging mechanism) of the tensile armor wires. A three-dimensional continuum finite element (FE) model of a 4 in. flexible pipe is developed using abaqus/implicit software package. As a verification case, the radial buckling response is compared with similar but limited experimental work available in the public domain. The modeling procedures represent an improvement over past studies through the increased number of layers and elements to model contact interactions and failure mechanisms. A limited parameter study highlighted the importance of key factors influencing the radial buckling mechanism that includes external pressure, internal pressure, and damage, related to the percentage of wet annulus. The importance of radial contact pressure and shear stress between layers was also identified. The outcomes may be used to improve guidance in the engineering analysis and design of flexible pipelines and to support the improvement of recommended practices.

Analytical and numerical models to predict the behavior of unbonded flexible risers under torsion

This paper presents analytical and numerical models to predict the behavior of unbonded flexible risers under torsion. The analytical model takes local bending and torsion of tensile armor wires into consideration, and equilibrium equations of forces and displacements of layers are deduced. The numerical model includes lay angle, cross-sectional profiles of carcass, pressure armor layer and contact between layers. Abaqus/Explicit quasi-static simulation and mass scaling are adopted to avoid convergence problem and excessive computation time caused by geometric and contact nonlinearities. Results show that local bending and torsion of helical strips may have great influence on torsional stiffness, but stress related to bending and torsion is negligible; the presentation of anti-friction tapes may have great influence both on torsional stiffness and stress; hysteresis of torsion-twist relationship under cyclic loading is obtained by numerical model, which cannot be predicted by analytical model because of the ignorance of friction between layers.