Flexible Pipe

Due to the recent development of deep water fields, the mechanical loads in flexible risers have been significantly increased and very high strength wires are often needed (SMUTS ≥ 1200 MPa). Moreover, the development of deep water fields containing H2S leads to consider some H2S content during the design phase. Steel wires are not in direct contact with the conveyed fluid, but are located in the annular space which is a specific medium for corrosion less severe than the bore. The new very high strength steel presented in this paper is compatible with annulus environment containing H2S and is very beneficial for designing deep water risers.

The use of steel risers for deepwater application is not always feasible, due to its high weight. Fiber reinforced composite materials offer an interesting alternative due to its several advantages, such as high specific strength and stiffness, high corrosion resistance, low thermal conductivity, good structural damping, and high fatigue resistance. Thus, the use of composite risers is an interesting alternative to deepwater oil fields. The design of composite risers requires the consideration of different load conditions and lamination schemes. Therefore, the design of a composite riser joint is more complex than conventional risers since strength and stiffness of these components depend on the number of layers and the thickness and orientation of each layer. This work presents a methodology for design of Composite Catenary Risers (CCR) based on the procedure used for design of conventional risers. An optimization technique is applied to the preliminary design of composite riser jo...

Flexible risers that for deep-water production are designed with strong collapse capacity. However, this collapse capacity is susceptible to pipe curvature in the flooded annulus condition. For the curved riser sections within the touchdown zone, significant reduction of collapse capacity can occur once the external sheaths are worn out, resulting in the so-called 'wet collapse'. Therefore, the collapse capacity of flexible risers should be well-determined during design stage. Mostly, curved collapse analyses are performed through costly numerical simulations since there is no well-established analytical models. This study focuses on the wet collapse mechanism of curved flexible risers, aiming to pave the way for the development of analytical solutions. 3D full FE models were adopted to gain insight into the wet collapse mechanism of curved risers, which had been verified by the data from public literature. The investigation indicates that the deformed cross-sectional shape of the carcass is the dominant factor of the curvature effect.

A flexible riser is a flexible pipe that transports materials between seafloor and topside structures. As oil and gas production heads to water depths greater than 3000 meters, huge hydrostatic pressure may cause the collapse failure of flexible risers. Generally, the collapse strength of a flexible riser is designed by considering the effects of initial imperfections, e.g., ovality of the carcass, and radial gap between the carcass/liner and pressure armor. These two imperfections may cause a significant reduction in the collapse strength of flexible risers under the flooded annulus condition. However, there are few analytical models available in the public literature that could take those factors into account. In this paper, an analytical approach is presented to predict the critical collapse pressure of the flexible risers with initial imperfections. The analytical results were compared with the numerical simulation, which showed reasonably good agreement.

Purpose. To propose an engineering approach for evaluating surface corrosion fatigue of flexible pipes of coiled tubing equipment, on the basis of the generalized data about corrosion fatigue behaviour of structural steels and analysis of the existing methods and approaches for assessment of these phenomena. Methods. We applied the original method of corrosion fatigue study which takes into account the parameters of electrochemical dissolution of cyclically deformed surface in conditions of its deformation, as well as the known methods of fracture mechanics, corrosion science, electrochemistry and materials science. Findings. The stages of initial corrosion damaging and surface corrosion fatigue nucleation in structural steels were considered. The criterion ratio for evaluating the period of surface corrosion fatigue crack nucleation that relates the number of loading cycles before crack initiation, the corrosion current, the constants of electrochemical dissolution of metal surface to the cyclic loading parameters is derived. This criterion allowed to obtain and substantiate the experimental formula for predicting the number of loading cycles prior to the surface corrosion fatigue crack nucleation. Originality. The suggested new model describes the surface fatigue crack nucleation as a result of interaction between corrosion and operational cyclic loadings of the coiled tubing equipment flexible pipes. Practical implications. The proposed relationship for calculating macrocrack nucleation period is applicable for engineering assessment of serviceability and fracture risk of flexible pipes of coiled tubing equipment under assigned operating conditions.

Purpose. To propose an engineering approach for evaluating surface corrosion fatigue of flexible pipes of coiled tubing equipment, on the basis of the generalized data about corrosion fatigue behaviour of structural steels and analysis of the existing methods and approaches for assessment of these phenomena. Methods. We applied the original method of corrosion fatigue study which takes into account the parameters of electrochemical dissolution of cyclically deformed surface in conditions of its deformation, as well as the known methods of fracture mechanics, corrosion science, electrochemistry and materials science. Findings. The stages of initial corrosion damaging and surface corrosion fatigue nucleation in structural steels were considered. The criterion ratio for evaluating the period of surface corrosion fatigue crack nucleation that relates the number of loading cycles before crack initiation, the corrosion current, the constants of electrochemical dissolution of metal surface to the cyclic loading parameters is derived. This criterion allowed to obtain and substantiate the experimental formula for predicting the number of loading cycles prior to the surface corrosion fatigue crack nucleation. Originality. The suggested new model describes the surface fatigue crack nucleation as a result of interaction between corrosion and operational cyclic loadings of the coiled tubing equipment flexible pipes. Practical implications. The proposed relationship for calculating macrocrack nucleation period is applicable for engineering assessment of serviceability and fracture risk of flexible pipes of coiled tubing equipment under assigned operating conditions.

Deep water oil and gas exploitation in Brazil remarkably increased the utilization of flexible pipes in conjunction with floating production systems. In Campos Basin nowadays oil and gas transfer, water and gas injection and well control and monitoring are carried out almost entirely by flexible pipes, including risers, flowlines and umbilicals. Periodic inspections have detected a considerable incidence of damage in the top section of risers, which may affect their structural integrity and eventually induce different failure mechanisms. These include mostly external sheath damage, corrosion and/or fatigue-induced damage to the tensile armours and torsional instability. These damages are generally originated during installation or, more frequently, during operation due to contact with another riser or the platform structure. In order to mitigate the progression of these damages, besides periodic inspections, repair techniques were developed for both emerged and submerged riser sections. Apart from the inspection program, surface monitoring procedures, such as nitrogen injection, pressure monitoring and flow measurements in the annular space are being implemented, for a continuous flexible riser integrity assessment. This paper describes and evaluates these techniques, as well as reports the results obtained from field experience.

A personal viewpoint is presented of the problems and unknowns relating to the materials aspects of flexible riser pipes, and possible solutions/indications for future research given. Through a brief description of the functions, materials and construction of flexible risers, the pipe terminations are highlighted as the major problem. A discussion of safety, testing and inspection reveals the major unknowns to be the performance of polymeric materials under extreme service conditions, the effects of long-term erosion/corrosion and the performance of pipes under compression. Three types of termination are then considered in detail.

This paper addresses friction models used for evaluating dynamic stresses in non-bonded flexible risers. A review of the most commonly used methods to model friction between layers in such structures was performed. Four models for calculating friction under dynamic contact pressure and constant shear interaction stick stiffness conditions were then formulated to enable stress calculation of cross-sections exposed to variable tension and bending loads. The friction models were implemented into a computer code used for numerical studies. A sensitivity study was carried out to determine the optimum shear interaction stick stiffness parameter, K 0 , with respect to representing the plane surfaces remain plane assumption. This was followed by an investigation of the performance of the developed models with respect to the tendon axial force next to the outermost fibre position. The proposed friction models were also verified against full scale tests in terms of bending moment-curvature data. As the test data indicated less bending stiffness in the stick domain than that can be obtained by the plane surfaces remain plane assumption, a method to estimate the parameter K 0 was proposed, which accounted for the shear deformation of the plastic layers. The initial strain concept was further applied to deal with the significant hysteresis observed in the test data for low internal pressures pointing the way forward with respect to dealing with this effect in stress analysis. Furthermore, a friction model comparison study was carried out for investigating the static friction effect on the axial force at the outermost fibre position and the bending moment-curvature relationship.

Even though hydraulic collars are largely used to install flexible risers, neither the loads imposed by this equipment nor the response of the risers to these loads have been previously studied. Hence, this paper presents a three-dimensional nonlinear finite element (FE) model to predict the response of flexible risers to loads imposed by hydraulic collars and also provides a set of equations to predict these loads. The FE model relies on an analogy between helical tendons and orthotropic shells to simulate the inner carcass and the pressure armour of flexible risers. Shell elements are used to represent the polymeric layers and three-dimensional beam elements simulate the wires of the tensile armours. Material, geometric and contact nonlinearities are addressed. Contact interactions between the layers of the riser are handled by surface to surface contact elements with a contact detection algorithm based on the pinball technique and contact forces evaluated with the augmented Lagrangian method. A 9.5 flexible riser is analyzed and the numerical results are compared to those from the experimental tests. The agreement between all results indicates that the proposed FE model is an efficient approach to predict the response of flexible risers to loads imposed by hydraulic collars and, moreover, may be used to analyze the response of such structures to other types of loads.

Interlocked armor layers of unbonded flexible risers may crush when risers are being launched. In order to predict the behavior of interlocked armor layers, they are usually simplified as rings with geometric and contact nonlinearity ignored in the open-literature. However, the equivalent thickness of the interlocked armor layer has not been addressed yet. In the present paper, a geometric coefficient g is introduced to the equivalent stiffness method, and a linear relationship between g and geometric parameters of interlocked armor layers is validated by analytical and finite element models. Radial stiffness and equivalent thickness of interlocked armor layers are compared with experiments and different equivalent methods, which show that the present method has a higher accuracy. Furthermore, hoop stress distribution of interlocked armor layer under crushing is predicted, which indicates the interlocked armor layer can be divided into two compression and two expansion zones by four symmetrically distributed singular points.

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This paper proposes a finite element (FE) modal approach to determine the critical loads and buckling mode shapes associated with the lateral buckling of the tensile armors in flexible pipes. This approach is based on pre-curved and twisted beam finite elements that are restrained to slide on the surface of a torus. The influence of the pipe's bending is addressed by assuming that the pre-buckling configuration of the tensile armors are geodesics or loxodromes of a torus and, moreover, friction effects are approximated using elastic springs. The proposed FE approach is employed in the classical buckling analyses of different tensile armors of several flexible pipes that have been previously studied in the public literature. The conducted analyses indicated that the buckling modes are sinusoidal and the pre-buckling configuration of the tensile armors in bent pipes affects their lateral buckling load. Moreover, friction between layers increases the lateral buckling resistance of the armor. Finally, comparisons with experimental and numerical results available in the literature indicates that the proposed approach is promising.

The instability of the tensile armor wire of flexible pipes is a failure mode associated with deep and ultra-deep water applications. Real compressive forces acting on the pipe are necessary to trigger this process. The loss of stability may be divided into two distinct processes, according to the main direction of the wire's displacement: radial or lateral instability. This study aims at proposing a numerical tool for predicting lateral and radial critical buckling loads for the tensile armor wires of flexible pipes. A simple finite element model, based on springs and beams elements, was developed in ABAQUS® to deal with this problem in an efficient and reliable manner. A parametric study was conducted concerning the behavior of the critical load when the laying angle, the initial curvature and the total pipe length are varied. The results were consistent with previously published literature data and analytical expressions, proving its applicability to pipe engineering projects. It also has the advantage of approaching the problem three-dimensionally, which allows further modelling modifications, such as including friction effects.

Bonded flexible marine hoses are frequently used in offshore offloading operations. The proper performance of these hoses throughout the service life is important. Many studies have been developed considering the global dynamic behavior of such hoses; however, only a few considered its local structural behavior. The present work aims to better understand the local mechanics of these hoses by proposing both a numerical and an analytical model. The first approach is based on a finite element model, which is constituted of continuum and REBAR finite elements. The first type of elements aims to represent the elastomeric matrix, whereas the second is introduced in order to properly represent both the polymeric and the metallic reinforcements. The second approach is based on an adaptation of a well-known analytical model for the axisymmetric behavior of bonded and unbonded flexible pipes. The present paper analyzes a 20 00 bonded flexible marine hose subjected to different axisymmetric loads. The stiffness of the hose and the stresses and strains in each of its components are computed using both models and compared to reference values found in the literature. In a general manner, the results agree well, thus indicating that both models are promising tools for the design of these structures.

This article studies the structural response of a 6.0" flexible pipe under pure tension considering intact and damaged conditions. In the damaged condition, several wires of the tensile armor layers are assumed to be broken. A three-dimensional nonlinear finite element (FE) model devoted to analyze the local mechanical response of flexible pipes is employed in this study. This model is capable of representing each tensile armor wire and, therefore, localized defects, including total rupture, may be adequately represented. Results from experimental tests validate the FE predictions and indicate a reduction in the axial stiffness of the pipe, a non-uniform redistribution of forces among the remaining intact wires of the damaged tensile armor layers and high stress concentrations in the wires near the broken ones. Moreover, the FE model indicates that significant normal bending stresses may arise in the pressure armor and inner carcass due to an uneven pressure distribution on these layers. Finally, the results obtained are employed to estimate the pull out capacity of the studied flexible pipe.

This paper focuses on a theoretical approach to access the fatigue life of flexible pipes. This methodology employs functions that convert forces and moments obtained in time-domain global analyses into stresses in their tensile armors. The stresses are then processed by well-known cycle counting methods, andS-Ncurves are used to evaluate the fatigue damage at several points in the pipe’s cross-section. Finally, Palmgren-Miner linear damage hypothesis is assumed in order to calculate the accumulated fatigue damage. A study on the fatigue life of a flexible pipe employing this methodology is presented. The main points addressed in the study are the influence of friction between layers, the effect of the annulus conditions, the importance of evaluating the fatigue life in various points of the pipe’s cross-section, and the effect of mean stresses. The results obtained suggest that the friction between layers and the annulus conditions strongly influences the fatigue life of flexible p...

Even though hydraulic collars are largely used to install flexible risers, neither the loads imposed by this equipment nor the response of the risers to these loads have been previously studied. Hence, this paper presents a three-dimensional nonlinear finite element (FE) model to predict the response of flexible risers to loads imposed by hydraulic collars and also provides a set of equations to predict these loads. The FE model relies on an analogy between helical tendons and orthotropic shells to simulate the inner carcass and the pressure armour of flexible risers. Shell elements are used to represent the polymeric layers and three-dimensional beam elements simulate the wires of the tensile armours. Material, geometric and contact nonlinearities are addressed. Contact interactions between the layers of the riser are handled by surface to surface contact elements with a contact detection algorithm based on the pinball technique and contact forces evaluated with the augmented Lagrangian method. A 9.5 flexible riser is analyzed and the numerical results are compared to those from the experimental tests. The agreement between all results indicates that the proposed FE model is an efficient approach to predict the response of flexible risers to loads imposed by hydraulic collars and, moreover, may be used to analyze the response of such structures to other types of loads.

Bonded flexible marine hoses are frequently used in offshore offloading operations. The proper performance of these hoses throughout the service life is important. Many studies have been developed considering the global dynamic behavior of such hoses; however, only a few considered its local structural behavior. The present work aims to better understand the local mechanics of these hoses by proposing both a numerical and an analytical model. The first approach is based on a finite element model, which is constituted of continuum and REBAR finite elements. The first type of elements aims to represent the elastomeric matrix, whereas the second is introduced in order to properly represent both the polymeric and the metallic reinforcements. The second approach is based on an adaptation of a well-known analytical model for the axisymmetric behavior of bonded and unbonded flexible pipes. The present paper analyzes a 20 00 bonded flexible marine hose subjected to different axisymmetric loads. The stiffness of the hose and the stresses and strains in each of its components are computed using both models and compared to reference values found in the literature. In a general manner, the results agree well, thus indicating that both models are promising tools for the design of these structures.

The instability of the tensile armor wire of flexible pipes is a failure mode associated with deep and ultra-deep water applications. Real compressive forces acting on the pipe are necessary to trigger this process. The loss of stability may be divided into two distinct processes, according to the main direction of the wire's displacement: radial or lateral instability. This study aims at proposing a numerical tool for predicting lateral and radial critical buckling loads for the tensile armor wires of flexible pipes. A simple finite element model, based on springs and beams elements, was developed in ABAQUS® to deal with this problem in an efficient and reliable manner. A parametric study was conducted concerning the behavior of the critical load when the laying angle, the initial curvature and the total pipe length are varied. The results were consistent with previously published literature data and analytical expressions, proving its applicability to pipe engineering projects. It also has the advantage of approaching the problem three-dimensionally, which allows further modelling modifications, such as including friction effects.

Due to compressive loads, offshore flexible pipes may exhibit global and local instabilities and as a consequence structural failures. This work presents an experimental investigation concerning the local structural behavior of a real 2.5-inch flexible pipe under compressive load. Three instabilities events were observed in the experimental test: elephant's foot buckling, birdcaging and wrinkling. Digital image correlation techniques, applied for the first time in this scenario, allowed a clear experimental assessment. Correspondence between axial compressive loads and observed instabilities is discussed. Linear elasticity formulations are used to explain the structural behavior, culminating on an instability diagram for elephant's foot buckling deflagration, based on a von Mises stress yielding criterion. Keywords Flexible pipe Á Structural instability Á Radial buckling Á Birdcaging Á Elephant's foot buckling Á Compressive loads Á Experimental analysis Á Digital image correlation Technical Editor: Celso Kazuyuki Morooka.

Plastic anisotropy is very common to metallic materials. This property may significantly affect the performance of structures. However, the actual orthotropic yield criterion is often replaced with a criterion based on the assumption of normal anisotropy. The present paper aims to reveal the influence of this replacement on the distribution of strains and residual strains in a thin hollow disk under plane stress conditions. The boundary-value problem is intentionally formulated such that it is possible to obtain an exact semi-analytical solution without relaxing the boundary conditions. It is assumed that the disk is loaded by external pressure, followed by elastic unloading. The comparative analysis of the distributions of residual strains shows a significant deviation of the distribution resulting from the solutions based on the assumption of normal anisotropy from the distribution found using the actual orthotropic yield criterion. This finding shows that replacing the actual ort...

ABAQUS is used to create a detailed finite element model for a 10-layer unbonded flexible riser to simulate the riser's mechanical behavior under three load conditions: tension force and internal and external pressure. It presents a technique to create detailed finite element model and to analyze flexible risers. In FEM model, all layers are modeled separately with contact interfaces; interaction between steel trips in certain layers has been considered as well. FEM model considering contact interaction, geometric nonlinearity, and friction has been employed to accurately simulate the structural behavior of riser. The model includes the main features of the riser geometry with very little simplifying assumptions. The model was solved using a fully explicit time-integration scheme implemented in a parallel environment on an eight-processor cluster and 24 G memory computer. There is a very good agreement obtained from numerical and analytical comparisons, which validates the use of numerical model here. The results from the numerical simulation show that the numerical model takes into account various details of the riser. It has been shown that the detailed finite element model can be used to predict riser's mechanics behavior under various load cases and bound conditions.

Deep water oil and gas exploitation in Brazil remarkably increased the utilization of flexible pipes in conjunction with floating production systems. In Campos Basin nowadays oil and gas transfer, water and gas injection and well control and monitoring are carried out almost entirely by flexible pipes, including risers, flowlines and umbilicals. Periodic inspections have detected a considerable incidence of damage in the top section of risers, which may affect their structural integrity and eventually induce different failure mechanisms. These include mostly external sheath damage, corrosion and/or fatigue-induced damage to the tensile armours and torsional instability. These damages are generally originated during installation or, more frequently, during operation due to contact with another riser or the platform structure. In order to mitigate the progression of these damages, besides periodic inspections, repair techniques were developed for both emerged and submerged riser secti...

Flexible risers that for deep-water production are designed with strong collapse capacity. However, this collapse capacity is susceptible to pipe curvature in the flooded annulus condition. For the curved riser sections within the touchdown zone, significant reduction of collapse capacity can occur once the external sheaths are worn out, resulting in the so-called 'wet collapse'. Therefore, the collapse capacity of flexible risers should be well-determined during design stage. Mostly, curved collapse analyses are performed through costly numerical simulations since there is no well-established analytical models . This study focuses on the wet collapse mechanism of curved flexible risers, aiming to pave the way for the development of analytical solutions. 3D full FE models were adopted to gain insight into the wet collapse mechanism of curved risers, which had been verified by the data from public literature. The investigation indicates that the deformed cross-sectional shape of the carcass is the dominant factor of the curvature effect.

Due to compressive loads, offshore flexible pipes may exhibit global and local instabilities and as a consequence structural failures. This work presents an experimental investigation concerning the local structural behavior of a real 2.5-inch flexible pipe under compressive load. Three instabilities events were observed in the experimental test: elephant's foot buckling, birdcaging and wrinkling. Digital image correlation techniques, applied for the first time in this scenario, allowed a clear experimental assessment. Correspondence between axial compressive loads and observed instabilities is discussed. Linear elasticity formulations are used to explain the structural behavior, culminating on an instability diagram for elephant's foot buckling deflagration, based on a von Mises stress yielding criterion. Keywords Flexible pipe Á Structural instability Á Radial buckling Á Birdcaging Á Elephant's foot buckling Á Compressive loads Á Experimental analysis Á Digital image correlation Technical Editor: Celso Kazuyuki Morooka.

Due to compressive loads, offshore flexible pipes may exhibit global and local instabilities and as a consequence structural failures. This work presents an experimental investigation concerning the local structural behavior of a real 2.5-inch flexible pipe under compressive load. Three instabilities events were observed in the experimental test: elephant's foot buckling, birdcaging and wrinkling. Digital image correlation techniques, applied for the first time in this scenario, allowed a clear experimental assessment. Correspondence between axial compressive loads and observed instabilities is discussed. Linear elasticity formulations are used to explain the structural behavior, culminating on an instability diagram for elephant's foot buckling deflagration, based on a von Mises stress yielding criterion. Keywords Flexible pipe Á Structural instability Á Radial buckling Á Birdcaging Á Elephant's foot buckling Á Compressive loads Á Experimental analysis Á Digital image correlation Technical Editor: Celso Kazuyuki Morooka.

Two layer metal plastic structures with range shape cross-section are frequently used [1 3]. The external plastic rust preventive cowers are applied in metal pressure vessels and pipelines. Plastic tubes with the reinforcing external metal layer are widely used [4]. Stresses distribution in multilayer structures subjected to internal pressure has been successfully determined by FEM [2]. However, it is difficult to use when it is necessary to determine how design parameters affect the stress values and strength of the structure. Mechanically inhomogeneous two-layer pipe may be divided into two pipes with different mechanical properties: internal pipe 1 with modulus of elasticity E1 , Poisson’s ratio v1 , limit of elasticity σe1 , power index of material hardening in elastic plastic zone m01 and external pipe 2 with E2 , v2 , σe2 and m02 (Fig. 1). Due to acting internal pressure p radial stretch and contact pressure pc on the surface materials H (hard) and M (mild) appears. When E1 &g...

The high strength to weight ratio, good corrosion resistance, and excellent fatigue property make carbon fiber-reinforced plastics a competitive material solution to replace steel in deepwater riser application. In this work, scaled-down composite riser joints were fabricated using a filament-winding machine. The prototypes comprise several carbon fiber-reinforced plastic layers wound over an aluminum liner. They consist of a middle tubular section and two metal-composite interface end fittings for the transfer of load between joints. A series of mechanical tests, including tension and combined tension-bending loading tests were performed to characterize their structural capacity and evaluate the improvement in performance over a purely metallic mandrel. In addition, finite element analyses incorporating elastic–plastic properties of the metallic liner, interfacial failure, and complex carbon fiber-reinforced plastics failure modes were carried out. The numerical predictions are in ...

The demand for the lifetime extension of flexible pipes is increasing due to the need to extend the lifetime of the existing production fields. There have been many challenges with the lifetime extension of flexible pipes after the end of the initial design service life due to the inherent conservatism with the common analysis approach, safety factors and operation beyond the design limits. A lifetime assessment should be performed on flexible risers for re-qualification during the original design life if the design envelope is exceeded or there is a need for lifetime extension. Hence, a systematic approach for lifetime assessment execution is established to determine the integrity level of the flexible risers and define the recommended actions required, such as mitigations, repairs or monitoring to maintain an acceptable risk for the required extended service life based on consistent methodology. The primary objective of this paper is to present a riser integrity management field-p...

Evaluating the fatigue life of a flexible riser requires several dynamic problems to be solved, considering a consistent representation of all environmental loads imposed to the structure. For each load case the stress field over the distinct concentric layers has to be determined. Accordingly, slip between adjacent layers must be considered as the resulting wear may reduce the section of the armour wires. This paper presents a design-oriented simplified methodology to assess the fatigue life of flexible risers, directly applicable in preliminary and early design stages.

The use of unbonded flexible pipes in the offshore industry is continuously moving towards deeper waters, leading to a demand for flexible pipes with ever-increasing structural capacity. Detailed design tools which capture the true mechanical behavior of the individual pipe layers enable improvement of the cross-sectional pipe design and at the same time enhances the pipe operation reliability. The defining feature of unbonded flexible pipes is the relative movement between the individual layers. The relative movement reduces the internal loading state of the armor layers, leading to improved fatigue performance. The relative sliding between the layers introduces a hysteresis behavior during bending, where the loading condition of the armor wires is dependent on the load history. This thesis aims to provide a general design tool to asses the tensile armor wire loading and the bending hysteresis effect for the dynamic operational conditions of the unbonded flexible pipes. The present...

Flexible riser is a device which transports production fluids between floating vessels and subsea wells. With fewer remaining easy-to-access oil fields nowadays, flexible risers are being required to be installed in a water depth of over 3000m. However, the hydrostatic pressure at such a water depth may cause the collapse of flexible risers, and therefore predicting the critical collapse pressure is of great importance to their design. Riser collapse is a complex phenomenon related to material properties, geometry of the pipe and its overall surface topography and, therefore, makes the prediction of critical pressure challenging. Collapse prediction approaches of flexible risers have been developed for decades, yet a comprehensive review on their predictive capabilities, efficiency and drawbacks is lacking. In this paper, the recent advances on collapse studies of flexible risers are reviewed, which summarizes the methods developed for critical pressure prediction and highlights the related gaps in current research. This review aims to facilitate the current anti-collapse design and be a baseline for future utilization of flexible risers in deeper water expansion.

The paper focuses on wrinkling of lined pipes (sometimes referred to as clad pipes) under bending loading, where a corrosion-resistant thin-walled liner is fitted inside a carbon-steel outer pipe. The problem is solved numerically, using nonlinear finite elements to simulate liner pipe deformation and its interaction with the outer pipe. Stresses and strains are monitored throughout the deformation stage, detecting possible detachment of the liner from the outer pipe and the formation of wrinkles. The wrinkling behavior of elastic and elastic-plastic (steel) lined pipes under bending is examined. The results indicate that the lateral confinement of the liner pipe due to the deformable outer pipe and its interaction with the outer pipe has a decisive influence on the wrinkling behavior of the lined pipe. It is also shown that the behavior is characterized by a first bifurcation in a uniform wrinkling pattern, followed by a secondary bifurcation. The values of corresponding buckling curvature are determined and comparison with available experimental results is conducted in terms of wrinkle height development and the corresponding buckling wavelength. The results of the present research can be used for safer design of lined pipes in pipeline applications.

Flexible pipes are now widely used in oil and gas industry in both riser and flowline applications. Long term performance of the pipes, especially internal pressure sheath ageing is concerned by people. PA material is usually used as an internal pressure sheath for flexible pipe and hydrolysis of PA material is limited when using. This report analysis different ageing critical using for PA ageing in flexible pipe design and give some recommendation for following design.

The combination of ultra-deepwater applications and large pipe diameter requirements presents severe engineering challenges for rigid pipe technology and remains outside current flexible pipe qualification scope. To address those needs, General Electric, with the support of Research Partnership to Secure Energy for America (RPSEA), has recently embarked on a development program to qualify flexible pipe with an internal diameter of greater than seven inches for ultra-deepwater applications. The program is based on a novel hybrid flexible riser technology that will be developed and qualified by a combination of design, analysis of performance, material and subcomponent testing, and finally a field trial. The concept consists of an optimally engineered combination of metallic and composite reinforcing layer technologies. This design approach allows the pipe system properties to be tailored to yield the optimal result for any application conditions. The approach offers performance advantages including reduced risk on critical end fitting technology, reduction of sheath leakage by fusing together the reinforcement and liner, superior matrix chemical resistance by using proven thermoplastic materials, and a reduced layer count leading to easier inspection. This paper will review the conceptual design of the flexible pipe and end terminations, and an overview of the plan for pipe qualification will be discussed. Introduction The relatively recent ultra-deepwater (> 1500 m) discoveries in the Gulf of Mexico, as well as off the coasts of Brazil and Angola, present an enormous opportunity and technical challenge. Specifically, the combination of ultra deepwater and relatively large pipe diameter is outside current flexible pipe qualification scope and imposes severe engineering challenges to both rigid and flexible pipe technologies. For a flexible pipe solution, the combination of greater than 2000 m operation depth and high design pressures (>10,000 psi) will require technical innovations in current flexible pipe technology and rigorous testing to prove to customers that these new technologies are robust and durable. To address those needs, General Electric, with the support of RPSEA, has recently embarked on a development program to qualify flexible pipe with an internal diameter of greater than seven inches for ultra-deepwater applications. The program is based on a novel hybrid flexible riser technology that will be developed and qualified by a combination of design, analysis of performance, material and subcomponent testing and finally, a field trial. The concept consists of an optimally engineered combination of well-understood and mature metallic and composite reinforcing layer technologies. This design approach allows the pipe system properties to be tailored to yield the optimal result for any application conditions. The approach offers potential performance advantages including reduced risk on critical end fitting technology, reduction of sheath leakage by fusing together the reinforcement and liner, superior matrix chemical resistance by using proven thermoplastic materials, and a reduced layer count leading to easier inspection. This paper will review the conceptual design of the flexible pipe and end terminations and an overview of the plan for pipe qualification will be discussed. To begin, an overview of the requirements and conventional pipe technology will be presented. A discussion of the proposed conceptual design will follow with some design and analysis predictions from the first few months of the program.

The primary aim of the present composite development program is to enhance access to deepwater fields in the Gulf of Mexico, Brazil, and West Africa. To accomplish that goal, composite materials are being incorporated in unbonded flexible pipelines to lower mass and enhance the overall system performance to expand the operational design envelope. In addition, the use of composite materials will allow a significant improvement in pipe operating pressure (>70 MPa), pipe operating temperature (>125C) and due to increased CO 2 and H 2 S resistance, will improve sour service performance and lifespan. Composite materials are well known for their low density and high specific strength, stiffness and fatigue performance. These properties are desirable and will certainly enhance pipe performance, but the overall performance of the pipe during all stages of manufacture and deployment must be considered, as well as a conservative approach to introducing these new materials. Some of the key factors that need to be assessed are material failure modes under varied pipe loadings, dynamic interactions and exposure to severe oil field environments. There are several individual standards, specifications and joint industry projects (JIPs) focused on composite pipes that address some of these issues, but there is also a general lack of consensus with regard to testing standards and understanding of the long-term performance. As flexible pipe suppliers, the industry must aim to provide performance assessments and address all key challenges to allow the flexible pipe industry to build confidence in the new and enabling composite pipe technologies. In a previous paper, we presented design concepts and a toolbox approach to construct different composite pipe solutions to meet all the aforementioned performance parameters. The present paper selectively highlights important failure modes and design considerations, demonstrates an understanding of behavior in the matrix and fiber phases, and addresses concerns related to the chemical performance of composite materials. The present paper also highlights and addresses some of the concerns of composite pipes and focuses on areas for future development and testing. These results will support the selection and standardization of analysis tools and testing methods across the industry. Bespoke testing capabilities to address the relevant failure mechanisms and installation strategies for composite pipes will also be discussed.

The primary aim of the present development program is to enhance access to deepwater fields in the Gulf of Mexico, Brazil, and West Africa by reducing system, transportation and installation costs for flexible production pipe technology. To accomplish that goal composite materials are being incorporated in hybrid unbonded flexible pipe structures to enhance their overall system performance and expand the operational design envelope. The use of composite materials enables significant improvements in operating pressures at larger pipe diameters, reduced weight and top tension, and the enhanced resistance to CO 2 and H 2 S drives further improvements in the structural sour service performance and lifespan. GE Global Research and GE Oil & Gas, with the support of Research Partnership to Secure Energy for America (RPSEA), embarked on a development program to qualify flexible pipe with an internal diameter of greater than seven inches for ultra-deepwater applications. The concept consists of an optimally engineered combination of metallic and composite reinforcing layer technologies. This hybrid design approach allows the pipe system properties to be tailored to yield the optimal result for any application conditions. The approach offers performance advantages including reduced risk on critical end fitting technology, continuous reinforcement of the liner eliminating discontinuities and local strains by fusing together the reinforcement and liner, superior matrix chemical resistance by using industry proven thermoplastic materials, and a reduced layer count leading to easier inspection. This paper will provide an overview of the progress made during the collaborative RPSEA program and specifically, will highlight the bespoke testing capabilities developed for the new composite pipe layer. Although there are several individual standards, specifications and Joint Industry Projects (JIP) in relation to composite pipes that address some of the required tests, there remains a general lack of consensus with regard to specific testing standards and understanding of the long term performance, especially considering the variety of composite pipe technologies being developed by the industry. This paper will highlight the results from mid-scale burst testing along with analysis validation, and a bespoke rotating bending fatigue test rig used to assess the bending fatigue of the composite pipe structure. Lastly, progress toward the use of sub-scale tests allied with analysis to more rapidly assess the fatigue life will be discussed.

This paper presents a numerical 3D finite element model to simulate a flexible pipe under crushing-traction condition, which is a typical situation found during its laying operation. This model considers the geometry of some layers from the flexible pipe, responsible of providing the most contribution to its radial strength (e.g., interlocked carcass, internal polymeric layer, pressure armor, and external polymeric layer) and geometry of laying system shoes. It also considers the flexible pipe initial ovalization and the squeezing effect due to the tensile armor layers under traction. A numerical-experimental comparison is presented, in order to show the model validity.

This paper discusses a nonlinear analytical model for flexible pipe crushing analysis, improving analytical models previously published in OMAE2003, 2010 and 2011. Following that path, and still based on the concept of equivalent pipes and rings, the present model discusses a strategy to cope with an elastic-to-plastic analysis. Such a model considers the combined action of squeezing and concentrated loads applied as representing those due to the caterpillar shoes. Experimental results and finite element analysis are used to assess the pertinence of analytical modeling hypotheses.

A new optical measurement approach for minimum bending radius (MBR) tests of flexible pipes is presented. The method is based on an accurate tracking of the elastica through image processing. The new approach shows to be not only appropriate to the standard measurement requirements but also enables a comprehensive understanding of the structural mechanisms embedded in the problem. Samples were made from two distinct flexible pipes, with inner diameters of 2.5 and 4 inches. A progressive set of tests was performed using five types of samples, obtained by dissecting each flexible pipe up to a certain layer. Sample 1 is made up with the internal carcass only. Sample 2 is constructed up to the liner (the internal plastic sheath covering the carcass). Sample 3 is mounted up to the pressure armor. Sample 4 is manufactured up to the external tensile armor. Sample 5 is made from the entire pipe, up to the external polymeric sheath. The experimental methodology is given in detail. The samples are bent through the pulling of a cable linked to the extremities. The traction is measured by a load cell. A special stereoscopic system is used to optically obtain the geometry of the elastic line of the bent sample. From the measured elastica, the curvature is calculated. Results of bending moment at any section may be also calculated from the elastic line. In particular, bending moment vs. curvature, measured at the vertex of the arch, are calculated for each sample. Introduction Flexible Pipes are increasingly used for the transport of oil & gas, due to their flexibility, reliability and high resistance to the several types of loads inherent to this type of application. These loads include high pressures, both internal and external, tensions and curvatures and the flexible pipe resistance to these different loads is obtained by a construction with many component layers, each one responsible for the resistance to a given load (See Figure 2 for the different layers, represented by the different samples used in the test). Since they are slender structures, a significant amount of bending is induced during their operation and must be taken into account in its design. This paper describes an experimental methodology to relate bending moment and imposed curvature, which is specially suited for the validation of a design methodology, since its results allow the developer of such methodology to have insights into details of the mechanical behavior of the tested structure, on a layer-by-layer basis. The test was applied to obtain the maximum curvatures that different layers of two flexible pipes can reach without damage.

Reinforced thermoplastic pipes (RTPs) are considered as prospective alternatives to traditional steel pipes in different offshore oil and gas applications due to their attractive properties. The spoolable versions of these pipes can be efficiently installed by the reel-lay method with relative ease. Nonlinear flexural behaviour of RTPs composed of a liner made of pipe grade polyethylene (PE), aramid fibre reinforced PE layers and a PE outer cover is modelled and analysed using finite-element analysis (FEA) considering the material nonlinearity. The pipe is modelled as a cylindrical shell in Abaqus/Standard. Numerical algorithms reflecting strain-dependent mechanical characteristics of PE are employed to perform simulations. Advantages of the proposed modelling approach are demonstrated with numerical examples. The minimum allowable bend radii of RTPs with different ply angles are determined. The effects of diameter-to-thickness ratios and the material nonlinearity on the spoolability of RTPs have been investigated. It is shown that the spoolability of RTPs can be improved by employing a certain two angle-ply reinforcing layer system, which could reduce the installation costs.

Evaluating the fatigue life of a flexible riser requires several dynamic problems to be solved, considering a consistent representation of all environmental loads imposed to the structure. For each load case the stress field over the distinct concentric layers has to be determined. Accordingly, slip between adjacent layers must be considered as the resulting wear may reduce the section of the armour wires. This paper presents a design-oriented simplified methodology to assess the fatigue life of flexible risers, directly applicable in preliminary and early design stages.

This paper presents a numerical 3D finite element model to simulate a flexible pipe under crushing-traction condition, which is a typical situation found during its laying operation. This model considers the geometry of some layers from the flexible pipe, responsible of providing the most contribution to its radial strength (e.g., interlocked carcass, internal polymeric layer, pressure armor, and external polymeric layer) and geometry of laying system shoes. It also considers the flexible pipe initial ovalization and the squeezing effect due to the tensile armor layers under traction. A numerical-experimental comparison is presented, in order to show the model validity.

The paper presents a theoretical-experimental comparison concerning standard crushing-traction tests of flexible pipe prototypes. The theoretical model for crushing is analytical and based on classic assumptions of equivalent pipes, applying model previously published in OMAE2003, and OMAE2010. Such a model considers the combined action of squeezing, concentrated loads due to the caterpillar shoes as well as the effect of initial ovalization. The experimental measures include a detailed internal geometrical mapping of the deformed carcass, until plastic deformation becomes evident. Discussion is made on the pertinence of modeling hypotheses. Sensitivity analyses, regarding initial ovalization and helical pitch of the pressure armor are also addressed.