Search Results (9)

Unbonded flexible risers are key components in offshore oil and gas production, enabling safe hydrocarbon transport under extreme deepwater conditions. The pressure armour (PA) layer is the primary metallic layer, which is designed to resist internal pressure; however, its complex helical geometry poses significant modelling challenges. This study presents a comprehensive numerical and analytical investigation of the burst behaviour of the PA layer under internal pressure. Three FE modelling strategies are developed and compared with theoretical predictions: a simplified axisymmetric 2D model, a fully detailed 3D helical model, and an efficient periodic boundary condition (PBC) model. The proposed PBC-based approach offers a practical balance between accuracy and efficiency, providing an effective tool for parametric analysis and design optimisation of flexible risers. Full article

Unbonded flexible risers, which can experience large bending deformation, are key equipment in advancing deep-sea exploration for marine resources. However, the riser experiences coupled loading effects from ocean environment. This results in complex response characteristics, leading to potential damage or even destruction. By presenting an analytical–numerical framework, this study uncovers the mechanism underlying the coupled failure of the pressure- and tensile-armor layers, furnishes a new tension–pressure coupled failure boundary for the ultimate-limit-state design of deep-water risers, and supplies the corresponding theoretical verification. Firstly, based on the axisymmetric load assumption, a theoretical model is proposed based on principle of functionality; afterwards, the failure model is defined by considering the material elastoplasticity. Secondly, a full-layered numerical model with detailed geometric properties is established; meanwhile, a simplified 7-layer model without a carcass layer is constructed for comparison. Finally, after verified through experimental data and interactive verification of theoretical and numerical methods, the simplified numerical model is proved to have calculation accuracy and validity. The characteristics are studied by the proposed methods. The comparison results show that the pre-applied internal pressure has limited influence on the axial stiffness of unbonded flexible rise. The initial axial tension would enhance the anti-burst failure ability of unbonded flexible riser, the failure pressure increases by 35% when the tensile force is 500 kN. Full article

Flexible pipes can be subjected to extreme bending load during installation and in situ operations, as well as high pressure from oil and gas field production fluids. Although field experience shows that the unlocking of the pressure armour layer is a rare failure mode, its consequences are potentially catastrophic. To investigate the unlocking mechanism of the pressure armour layer, a 3D nonlinear finite element model is developed, which contains three layers: a pressure armour layer with a “Zeta” shape cross-section, a polymer layer, and an analytical cylindrical surface representing the radial support from the carcass. The analysis is carried out with the ABAQUS Dynamic Explicit solver using a semi-automatic mass scaling factor. The study focuses on the possibility of pressure armour unlocking due to bending load, where the effects of geometric features of the pressure armour layer and internal pressure value on unlocking are investigated by varying corresponding parameters. The influence of these variables are discussed at the end. Ultimately, in conjunction with the numerical analysis results, critical reflections on the shortcomings of the model and suggestions for improvement are presented based on the numerical analysis results. Full article

Unbonded flexible risers have been widely used in the field of offshore engineering in recent years due to their excellent performance in extreme dynamic marine environments, structural compliance, low installation cost, and low quality. And, the internal pressure capacity of unbonded flexible risers is an important indicator of the mechanical performance of unbonded flexible risers. Based on a 2.5-inch, 8-layer typical unbonded flexible riser model, this paper examines the burst failure of the pressure armor layer. Firstly, the balance equation of each separate cylindrical layer and helical layer is derived by functional principle, and then the overall theoretical modeling of an unbonded flexible riser under axisymmetric loads is established by additionally considering the geometric relation between adjacent layers. Secondly, fully considering the complex cross-sectional geometric characteristics and the interlayer’s contact with the unbonded flexible riser, a simplified numerical 7-layer model is established by Abaqus, and the material with elastic-plastic properties is conferred. Finally, the validity of the proposed theoretical and numerical methods is verified through the axisymmetric behavior of the test data. Then the burst failure of the pressure armor layer is analyzed based on the material. At an internal pressure of 42 MPa, the pressure armor layer reached its yield stress of 300 MPa, with the entire cross-section yielding between 42 MPa and 42.5 MPa. Additionally, the effect of the friction coefficient is examined. Full article

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. Full article

Unbonded flexible risers consist of several helical and cylindrical layers, which can undergo large bending deformation and can be installed in different configurations to adapt to harsh marine environments; thus, they can be applied to transport oil and gas resources from ultra-deep waters (UDW). Due to their special geometric characteristics, they can ensure sufficient axial tensile stiffness while having small bending stiffness, which can undergo large deflection bending deformation. In recent years, the development of unbonded flexible risers has been moving in an intelligent, integrated direction. This paper presents a review of unbonded flexible risers. Firstly, the form and properties of each interlayer of an unbonded flexible riser are introduced, as well as the corresponding performance and configuration characteristics. In recent years, the development of unbonded flexible risers has been evolving, and the development of machine learning on unbonded flexible risers is discussed. Finally, with emphasis on exploring the design characteristics and working principles, three new types of unbonded flexible risers, an integrated production bundle, an unbonded flexible riser with an anti-H₂S layer, and an unbonded flexible riser with a composite armor layer, are presented. The research results show that: (1) the analytical methods of cross-sectional properties of unbonded flexible risers are solved based on ideal assumptions, and the computational accuracy needs to be improved. (2) Numerical methods have evolved from equivalent simplified models to models that account for detailed geometric properties. (3) Compared with ordinary steel risers, the unbonded flexible riser is more suitable for deep-sea resource development, and the structure of each layer can be designed according to the requirements of the actual environment. Full article

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. Full article

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. Full article

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. Full article