Unbonded Flexible Pipe Research Articles

Nonlinear behavior analysis of smooth bore unbonded flexible pipes under radial compression

Smooth bore unbonded flexible pipes are widely used to transport oil and gas in offshore fields. Due to its main application in shallow water, the radial bearing capacity is usually designed to be low for economic reasons. If the compressive load applied by tensioner equipment is too high during installation, the flexible pipe will fail due to excessive deformation. In this paper, the nonlinear behavior of smooth bore flexible pipes passing through tensioner equipment was investigated numerically and experimentally. A three-dimensional numerical model considering the nonlinear behavior of the materials, layer-to-layer contacts, and friction for flexible pipe radial compression analysis was established. A radial compression test was conducted to verify the accuracy of the numerical model; Then the numerical model was used to analyze the radial compressive behavior of each layer, and its load-bearing capacities and failure mechanisms were investigated. Finally, a parametric study was performed on the opening angle and number of tensioner tracks. The results of this study can provide a useful reference for the installation of smooth bore flexible pipes.

Application of a data-driven approach for maximum fatigue damage prediction of an unbonded flexible riser

The fatigue damage prediction is a challenging issue in the design and maintenance phases of unbonded flexible pipes. This arises from the necessity of conducting numerous numerical analyses, encompassing both global performance and local structural assessments, to determine the long-term fatigue damage. To handle the computational-demanding problem for fatigue damage estimation, this work applies an efficient machine learning approach known as the AK-MDAmax approach for maximum fatigue damage estimation of unbonded flexible risers. Kriging surrogate models are employed to predict the short-term distribution of fatigue damage over the sea states. The maximum long-term fatigue damage of the flexible risers is then estimated using the AK-MDAmax approach. The verification process of the AK-MDAmax model unfolds incrementally, testing its feasibility and stability across four different scenarios with varying numbers of considered locations, ranging from 4 to 512. Comparative analyses against numerical results reveal that the AK-MDAmax approach efficiently and accurately estimates maximum long-term fatigue damage across different locations. Remarkably, a 25-fold increase in efficiency is achieved with an error margin of less than 1%. This study demonstrates the substantial potential of the proposed machine learning methodology, offering designers an efficient tool for optimizing flexible risers and mitigating computational costs and time constraints.

Polymer Fatigue Evaluation Methodology for Unbonded Flexible Pipes

Polymer layers have been used in the offshore Oil & Gas industry for many years in unbonded flexible pipes for static and dynamic applications. The evaluation of fatigue performance for polymer grades in such applications is complex and no qualification method is well defined nor standardized at the time this article is written.Though historically no reported polymer fatigue field failures the complexity of unbonded flexible pipes, the conservatism of this industry makes the introduction of new polymer grades in risers for dynamic applications challenging. Development of a polymer fatigue methodology will allow the industry to quantitatively evaluate risks of fatigue failure of liners and thereby be important in introducing new material grades.In this paper, a design fatigue methodology is built based on inputs from a specific project and results from small- and full-scale tests. The pipe curvature ranges, extracted from the project location, are used to obtain an accumulated tensile strain which is then converted into an accumulated tensile stress. The fatigue damage is then determined, using a design curve established from bending or tension-tension fatigue testing performed on dumbbells from pipe samples. Parameters are included to reflect geometrical effects from manufacturing process but also service life conditions with several temperature profiles. This method can be applied for pressure sheaths as well as sealed outer sheaths with relevant inputs related to their operating conditions. The obtained fatigue damage can then be used to validate the use of polymer grades for specific unbonded pipe designs under their service conditions.

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.

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.

Radial compression pressure estimation of carcass layers in unbonded flexible pipes based on neural networks

Radial pressure resistance is a key factor in the design of unbonded flexible pipe carcass layers because it directly affects the safe installation and use of marine flexible pipes. It is challenging to evaluate the performance of radial compression because the carcass layer, which is a large-angle helical winding structure in innermost flexible pipes with a special-shaped cross section, experiences numerous contact frictions under radial pressure. In this study, a radial pressure estimation approach based on neural networks is proposed to address the aforementioned issues. This study explores the nonlinear relationship between the limited structural responses (circumferential strain) and the overall load (radial compression pressure) from the radial compression experimental data and estimates the radial compression pressure of carcass layers. The input variables are the structural strains, and the output variable is the radial pressure. Furthermore, some examples are used to test the generalization of the method, including robustness and understanding, considering the geometric characteristics of the carcass layer. The results demonstrate that the method can accurately predict the radial pressure applied to the carcass layer and provide a theoretical framework for the design of unbonded flexible pipes in deep-sea environments.

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.

Mechanical Characteristics of Fiber-Reinforced Flexible Pipe Subjected to Axial Tensile Load

Fiber-reinforced flexible pipes are subjected to large axial tension loads in deep-water applications, which may result in the excessive deformation of the pipes. Owing to the anisotropy of the composite materials, accurately describing the tensile behavior of these pipes is difficult. Theoretical, numerical, and experimental methods are employed in this study to investigate the mechanical characteristics of a glass fiber-reinforced unbonded flexible pipe under axial tensile loads. Based on the load–strain relationship of each pipe layer, analytical equations considering the effect of anisotropy and radial deformation are first proposed to calculate the axial tensile stiffness of the pipe. A detailed numerical model is established to simulate the tensile behavior of the pipe. A prototype test is performed on a 4500 mm long sample using a tensile testing machine. The leading roles of outer tensile reinforcement layers in axial tensile capacity are illustrated by the strain energy of the pipe layers obtained by the numerical model. Subsequently, a comparison analysis of the mean fiber direction strains of the selected sections are performed between numerical and experimental results, which validates the numerical model. Additionally, the stress distributions of different pipe layers are discussed based on the results of the numerical analysis. Finally, the comparison of axial tensile stiffness results validates the accuracy of the analytical model considering radial deformation. This study proposes effective theoretical and numerical models to predict the tensile behavior of a fiber-reinforced flexible pipe, which provides useful references for the design and structural analysis of these pipes.

Study on the mechanical characteristics of a fibre reinforced flexible pipe under radial compression loads

Fibre reinforced flexible pipes are subjected to radial compression loads caused by the tensioner during pipe laying, which may lead to excessive deformation or even damage of the pipe. In this study, the mechanical characteristics of a glass fibre reinforced unbonded flexible pipe are investigated under radial compression loading. In the theoretical analysis, the hoop reinforcement layer of the pipe is considered equivalent to an orthotropic circular tube. An analytical equation for calculating the radial stiffness of the circular tube per unit length is then derived based on the classical elastic theory of the ring. Radial compression tests are carried out with a universal testing machine and displacement loading within the elastic deformation range is applied to two 500 mm long samples. A 3D numerical model is established to simulate the compression process of the flexible pipe, through which the distribution characteristics of the displacement, strain and stress of the hoop reinforcement layer are also obtained. The load-displacement curves obtained from the tests and numerical model are linearly fitted to calculate the radial stiffness of the pipe. The radial stiffness obtained by the numerical model is very close to that obtained by the analytical method. However, owing to factors such as material defects and initial ovality, the radial stiffness measured experimentally is lower than the analytical result.

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.

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

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

Numerical investigation of liquid-solid erosion in unbonded flexible pipes

Unbonded flexible pipes (UFPs) are widely used in marine engineering for oil transportation. This study aimed to assess the erosion of UFPs, and a simplified method to evaluate the erosion of the interlock carcasses of UFPs was proposed. Three widely used erosion models were used to predict erosion damage based on a user-defined FORTRAN subroutine, and the simulation results were compared. A series of computational fluid dynamics simulations was conducted. The effects of the geometric shape of the interlock carcass, fluid flow velocity, sand size, and sand concentration on the erosion rate were carefully studied. Based on the simulation results, a modified geometry factor for erosion prediction of the interlock carcasses of UFPs was proposed.

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

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

High strength tape layer modeling for analysis of flexible pipe axisymmetric behavior and birdcaging limit

Flexible pipes are critical elements for offshore oil and gas development and determining its response in axisymmetric loadings is of paramount importance. Often the anti-birdcage tape layer is represented by a tubular structure with either isotropic or orthotropic material properties. However, it is recognized that a tubular structure cannot fully replicate the stiffness and strength and deformation mechanisms of helically wound tape layers. This paper presents a mathematical model for the local analysis of unbonded flexible pipes subjected to axisymmetric loadings where three anti-birdcage tape layer modeling strategies are presented including helical tape, orthotropic and isotropic tubes. This model is then verified against existing test data. The deviations caused by the orthotropic and isotropic tube modeling strategies in different loading conditions are discussed. This is followed by parametric studies on the effects of anti-birdcage tape tensile stiffness, laying angle and direction on the flexible pipe mechanical behaviors in tension, axial rotation in both directions, axial compression, as well as birdcaging limit loads. The importance of considering non-uniform tensions in the filaments that constitute the tape is discussed.

Numerical study of strain-based monitoring parameter on steel strip reinforced thermoplastic pipe (SSRTP) under internal pressure

In offshore engineering, the pipe is the critical component for oil and gas transportation. Its integrity directly influences the transport of oil and gas. Steel strip reinforced thermoplastic pipe (SSRTP), as a new type of unbonded flexible pipe, has come into application in the marine resource transportation field because of its good mechanical performance. The severe and complex subsea conditions make SSRTP subjected to principal loading cases. To monitor the working condition of SSRTP, in this article, the possibility of Fiber bragg grating (FBG) monitoring has been discussed to achieve real-time measurement of hoop strain on the sheath under internal pressure. The experimental results are in good agreement with the finite element analysis (FEA) results, which indicates that hoop stain on the sheath can serve as a monitoring parameter and preliminarily realize the health monitoring of SSRTP with the help of FBG technology. Parametric analysis is further conducted to better understand the mechanical performance of SSRTP based on the proposed monitoring parameter. The results indicate that SSRTP takes better advantage of material properties, compared with the steel pipe with the same dimension; the hoop strain on the sheath is slightly decreased by the friction between layers, but is significantly affected by the geometry of SSRTP due to the change of the radial stiffness.

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

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

An Analysis of Carcass Layer Erosion in Un-Bonded Flexible Offshore Pipework

In this paper, a simplified model for erosion in un-bonded flexible pipes caused by the sand entrained in the produced fluid is established. Flow field analysis is performed based on the governing equations of the continuous fluid and the discrete particles. A two-way coupled Eulerian-Lagrangian approach is employed to solve the gas-solid flow in the pipe bend. To eliminate the influence of the length of the straight pipe section on the stability of the flow field in the pipe, the flow field distribution under different lengths is analyzed to determine the optimal straight pipe length. Six commonly used erosion models are adopted to predict the erosion rate. After comparing the prediction results with experimental data, the most accurate Oka model is selected to calculate the effect of the fluid and structure parameters on erosion. Effects of particle parameters and pipe structural parameters on the erosion rate of curved flexible pipes are numerically fitted, and the quantitative description is given.

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

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

Mechanical behavior of metallic strip flexible pipes during reeling operation

Metallic strip flexible pipes (MSFP), a relatively new style of unbonded flexible pipes, are considered as an attractive alternative to traditional submarine pipes. During its reeling operation, it will inevitably confront various complicated loads, which may affect the integrity and safety of MSFP's utilization. In this paper, the tension-extension and moment-curvature relation of MSFP were obtained by laboratorial tests. The mechanical properties were then imported into the global model established in ABAQUS. The finite element model was adopted to predict the deformation and mechanical responses of MSFP during the operation process. Besides, the effects of reeling length, the diameter of the coiling drum, and the pulling force were discussed. The obtained conclusions will provide some references for optimizing MSFP design and preventing possible damage in the reeling operation.

Axial mechanical experiments of unbonded flexible pipes

Axial structural damping behavior induced by internal friction and viscoelastic properties of polymeric layers may have an inevitable influence on the global analysis of flexible pipes. In order to characterize this phenomenon and axial mechanical responses, a full-scale axial tensile experiment on a complex flexible pipe is conducted at room temperature, in which oscillation forces at different frequencies are applied on the sample. The parameters to be identified are axial strains which are measured by three kinds of instrumentations: linear variable differential transformer, strain gauge and camera united particle-tracking technology. The corresponding plots of axial force versus axial elongation exhibit obvious nonlinear hysteretic relationship. Consequently, the loss factor related to the axial structural damping behavior is found, which increases as the oscillation loading frequency grows. The axial strains from the three measurement systems in the mechanical experiment indicate good agreement, as well as the values of the equivalent axial stiffness. The damping generated by polymeric layers is relatively smaller than that caused by friction forces. Therefore, it can be concluded that friction forces maybe dominate the axial structural damping, especially on the conditions of high frequency.