Tensile Armor Layers Research Articles

High-energy computed tomography for inspection and monitoring of flexible riser

Flexible risers have been widely used in the offshore oil and gas industry and have many applications, such as production, gas lifting, or gas and water injection. The structure comprises several metal and polymer layers, which allow it to withstand a sizeable bending curve and high internal pressure. Therefore, guaranteeing the integrity of flexible risers throughout their useful life is extremely important. It requires developing and applying effective inspection and monitoring techniques, particularly emphasizing the tensile armor layers. This work evaluated the ability of the High-Energy Computed Tomography (CT) Technique to detect internal defects such as cracks, breaks, and fissures in the riser layers. A flexible riser sample of approximately 2 m in length and 281 mm in external diameter was scanned using a tomography system consisting of a 6 MeV Betatron and a set of 14-bit A-Si digital detectors. The images were acquired in 360° with angular steps of 2°. After acquisition, the images were reconstructed, and 3D models were built for qualitative and quantitative evaluation. Through the segmentation process, the metal layers of the flexible riser could be analyzed individually. Defects were found in the tensile armor layers, indicating ruptures and cracks and the onset of pitting. The results strongly suggest that computed tomography can be used as a non-destructive technique for inspecting flexible risers, with good efficiency in detecting defects, and that its application can be extended to field tests.

Nonlinear Slippage of Tensile Armor Layers of Unbonded Flexible Riser Subjected to Irregular Loads

The unbonded flexible riser has been increasingly applied in the ocean engineering industry to transport oil and gas resources from the seabed to offshore platforms. The slippage of helical layers, especially the tensile armor layers of unbonded flexible risers, contribute to the nonlinear hysteresis phenomenon, which is a research hotspot and difficulty. In this paper, on the basis of a typical eight-layer unbonded flexible riser model, the nonlinear slippage of a tensile armor layer and the corresponding nonlinear behavior of an unbonded flexible riser subjected to irregular external loads are studied by numerical modeling with detailed cross-sectional properties of the helical layers, and are verified through a theoretical method considering the coupled effect of the external loads on the unbonded flexible riser. Firstly, the balance equation of each layer considering the effect of external loads is established based on functional principles, and the overall theoretical model of the unbonded flexible riser is set up in consideration of the contact between adjacent layers. Secondly, the numerical modeling of each separate layer within the unbonded flexible riser, including the actual geometry of the carcass and pressure armor layer, is established, and solid elements are applied to all the interlayers, thus simulating the nonlinear contact and friction between and within interlayers. Finally, after verification through test data, the behavior of the unbonded flexible riser under the cyclic axial force, torsion, bending moment, and irregular external and internal pressure is studied. The results show that the tensile armor layer can slip under irregular loads. Additionally, some suggestions related to the analysis of unbonded flexible risers under irregular loads are drawn in the end.

Axial Tensile Ultimate Strength of an Unbonded Flexible Riser Based on a Numerical Method.

Unbonded flexible risers consist of several helical and cylindrical layers, which can undergo large bending deformation and can be installed to different configurations to adapt to harsh marine environments, and is a key equipment in transporting oil and gas resources from Ultra Deep Waters (UDWs) to offshore platforms. The helical interlayer of an unbonded flexible riser makes the structural behavior difficult to predict. In this paper, the axial tensile behavior and the axial tensile ultimate strength of an unbonded flexible riser are studied based on a typical 2.5-inch eight-layer unbonded flexible riser model, and verified through a theoretical method considering the contact between adjacent layers. First, the balance equation of separate layers is deduced by a functional principle, and then the overall theoretical model of an unbonded flexible riser is established considering the geometric relationship between adjacent layers. Then, the numerical model considering the detailed geometric properties of an unbonded flexible riser is established to simulate the axial tensile behavior. Finally, after being verified through the experimental results, the axial tensile stiffness and axial tensile strength of an unboned flexible riser considering the elasticity of the tensile armor layer are studied using the proposed two methods. Additionally, the effect of frictional coefficients is conducted. The numerical and theoretical results show good agreement with the test results, and the friction between adjacent layers would increase the axial tensile stiffness of an unbonded flexible riser.

Study on torsional response and ultimate loading of unbonded flexible risers under corrosion defects

During the process of deep-sea oil and gas exploitation, unbonded flexible risers (UFRs) are corroded by CO2 and other media over long periods of time, which causes serious damage to the tensile armor layers and threatens the safety of the UFRs in service. To study the effects of torsional response, ultimate loading, and failure mode on UFRs after corrosion, and to reveal the torsional failure mechanism of the UFR while considering corrosion defects, an 8-layer UFR torsional response model is established in this paper. Local bending of the UFR and lateral buckling failure of the inner tensile armor layer occur under counterclockwise ultimate torsion, whereas strength failure of the inner tensile armor layer occurs under clockwise torsion. When the degree of corrosion is 0.4 mm, the counterclockwise torsional stiffness of thickness corrosion, width corrosion, and combined corrosion decreases by 27.6%, 10.8%, and 28.4%, and the ultimate loading of counterclockwise torsion for thickness corrosion, width corrosion, and combined corrosion decreases by 37%, 13.4%, and 39.3%, respectively. Therefore, the influence of thickness corrosion and combined corrosion on the torsional response of UFRs is worthy of further attention.

Parameter sensitivity analysis of the axial stability for a marine flexible pipe

Marine unbonded flexible pipes serve as the most essential equipment in offshore oil and gas exploration and exploitation. Axial compressive loads during installation or in service in the complex marine environment usually lead to buckling failure. A flexible pipe is a composite structure with multiple functional layers, of which the tensile armor layer plays a key role with regard to the response of the pipe subjected to axial loads. In this paper, a simplified three-dimensional finite element model is developed, focusing on the tensile layer and replacing the carcass layer, pressure sheath layer, and pressure armor layer by a cylindrical rigid body to reduce computational expense. By using this model, the buckling failure modes of the tensile armor layer (in particular the birdcaging phenomenon) are analyzed. Several key parameters that affect the stability of the flexible pipe under axial compression and torsion are emphasized, and their effects on its axial and torsional stiffness are compared and discussed. The results show that both the lay angle of the steel wires and the interlayer friction coefficient have a significant influence on the axial and torsional stiffness of the pipe, whereas the damaged length of the outer sheath has virtually no effect.

Cross-sectional mechanical characteristics and sensitivity analysis of unbonded flexible risers under axial loads

Unbonded flexible risers exhibit complex structures. Different structural layers can withstand axial loads and exhibit different degrees of coupled deformation. Based on the different material properties and structural forms of each layer of an unbonded flexible riser, the structural layers are divided into three types: cylindrical, steel helical, and polymer helical layers. This study establishes a theoretical model of flexible risers under axial loads based on the law of conservation of energy and the geometry of deformation, and deduces theoretical expressions for the axial load and axial stiffness of flexible risers. MATLAB was used to compile calculation programs to calculate the cross-sectional mechanical properties of flexible risers under axial tensile and compressive loads and to compare the calculation results with the experimental results and the results of other researchers to verify the reliability of the theoretical derivation and calculation programs. By further calculating the cross-sectional force distribution of each structural layer of the flexible risers under axial tensile loads, it is clarified that the tensile armor layer is the main component that can withstand axial tensile loads. A sensitivity analysis of the helix angle and number of helical strips of the tensile armor layer on the tensile properties of flexible risers was conducted; the results show that the helix angle had a more obvious influence on the tensile properties of flexible risers. The results of this study can provide a reference for the structural design and optimization of flexible risers.

Study on the influence of bending curvature on the bending characteristics of unbonded flexible pipes

Unbonded flexible pipes are widely used in offshore oil and gas development as key equipment for offshore oil and gas transportation. Their biggest advantages are their flexibility and easy bending, making them more adaptable to the complex marine environment and the movement of offshore floating oil and gas platforms. These platforms also bring challenges to the structural design and analysis of flexible pipes. The mechanical response of each spiral strip layer used to construct the flexible pipe is the most complex response in the bending process. To analyze the bending performance of flexible pipes, this study focuses on the derivation of the explicit relationship between the bending curvature and the minimum critical curvature of a flexible pipe in the partial slip process of a spiral strip layer. An interlayer friction coefficient is introduced in the theoretical derivation, and complete expressions of the bending moment and bending stiffness for the non-slip, partial slip, and complete slip stages of the spiral strip layer are established. The expressions explain the hysteresis phenomenon of the bending characteristics of flexible pipes under a bending load. The results show that the hysteresis present when analyzing the bending moment-curvature relationship and the bending stiffness-curvature relationship of flexible pipes is mainly caused by the nonlinearity of the bending moment-curvature relationship of the tensile armor layer, while the bending moment-curvature relationship of all other layers is linear and the bending stiffness remains constant. The contribution of the carcass layer, pressure armor layer and anti-wear layer to the bending moment and bending stiffness is very small. The minimum critical curvature is the key variable affecting the slippage of the spiral strips of the tensile armor layer. The minimum critical curvature has a linear relationship with the interlayer friction coefficient and a nonlinear relationship with the helix angle.

Numerical Analysis of Mechanical Behaviors of Composite Tensile Armored Flexible Risers in Deep-Sea Oil and Gas

As oil and natural gas production continue to go deeper into the ocean, the flexible riser, as a connection to the surface of the marine oil and gas channel, will confront greater problems in its practical application. Composite materials are being considered to replace steel in the unbonded flexible pipe in order to successfully meet the lightweight and high-strength criteria of ultra-deep-water oil and gas production. The carbon-fiber-reinforced material substitutes the steel of the tensile armor layer with a greater strength-to-weight ratio. However, its performance in deep-water environments is less researched. To investigate the mechanical response of a carbon fiber composite flexible riser in the deep sea, this study establishes the ABAQUS quasi-static analysis model to predict the performance of the pipe. Considering the special constitutive relations of composite materials, the tensile stiffness of steel pipe and carbon fiber-reinforced composite flexible pipe are predicted. The results show that the replacement of steel strips with carbon fiber can provide 85.06% tensile stiffness while reducing the weight by 77.7%. Moreover, carbon-fiber-reinforced strips have a lower radial modulus, which may not be sufficient to cause buckling under axial compression, so the instability of the carbon fiber composite armor layer under axial compression is further studied in this paper; furthermore, the characteristics of axial stiffness are analyzed, and the effects of the friction coefficient and hydrostatic pressure are discussed.

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.

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.

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.

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.

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.

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.

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.

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.