Reinforced Thermoplastic Pipe Research Articles

A Comparative Study on the Post-Buckling Behavior of Reinforced Thermoplastic Pipes (RTPs) Under External Pressure Considering Progressive Failure

A Comparative Study on the Post-Buckling Behavior of Reinforced Thermoplastic Pipes (RTPs) Under External Pressure Considering Progressive Failure

Experimental and computational failure analysis of hydrogen embrittled steel cords in a reinforced thermoplastic composite pipe

Reinforced thermoplastic pipes (RTPs) have attracted considerable attention lately in the oil and gas sector as potential alternative to carbon steel pipes. However, some RTP systems have been experiencing failures as a result of deficient design and inadequate operation practices. This study presents a comprehensive assessment of the failure of a steel-reinforced flexible thermoplastic composite layered pipe in a sour environment characterized by hydrogen sulfide (H2S) exposure. Utilizing scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS), the investigation conclusively attributes the failure of the RTP to hydrogen embrittlement of the carbon steel cords and wires in the reinforcement layer. Finite element models at both meso-scale and full RTP scale were developed, incorporating a novel damage initiation and propagation criterion for failure prediction of embrittled and non-embrittled (e.g. ductile) reinforcement steel cords. The numerical models consistently predicted a reduced burst pressures of the hydrogen embrittled case of the RTP, with a close match with field observations. They also predicted failure modes that closely matched the SEM findings, hence highlighting the unsuitability of the existing steel cords in RTPs for use in sour service environments. Additionally, the presence of moisture in the annular space was identified as an exacerbating factor. This investigation provides important insights for the future design and operation of RTPs susceptible to hydrogen embrittlement, paving the way for improved structural integrity and safety measures. Further research avenues may involve modeling hydrogen diffusion in the thermoplastic layers, simulating the movement of moisture in the annulus through the capillary effect, and incorporating fracture toughness degradation for a more thorough understanding of the interplay between mechanical and environmental factors in the hydrogen embrittlement of steel-reinforced RTP composite pipes.

A Method for Predicting the Load Interaction between Reinforced Thermoplastic Pipe and Sandy Soil Based on Model Testing

This study aims to investigate the interaction between reinforced thermoplastic pipes (RTPs) and sandy soil. The mechanical properties of sandy soil in the South China Sea region were determined through shear tests to obtain fundamental data. Subsequently, a specialized experimental setup was designed and assembled to study the pipe–soil interaction, specifically measuring the lateral soil resistance of flexible pipes at varying burial depths. Data analysis revealed the relationship between soil resistance, lateral displacement, and initial burial depth. To simulate the mechanical behavior of the pipe–soil interaction, the coupled Eulerian–Lagrangian (CEL) method was employed for numerical simulations. The research findings indicate that the lateral soil resistance is influenced by the uplift height and accumulation width of the soil ahead of the pipe. Within a lateral displacement range of 0.5 times the pipe diameter (0.5D), the lateral soil resistance rapidly increases, resulting in a soil uplift along the circumferential direction of the pipe. This process not only enhances the load-bearing capacity of the pipe but also increases the accumulated soil resistance, consequently expanding the soil failure zone. Furthermore, the ultimate soil resistance exhibits an increasing trend with an increasing burial depth. Once the pipe reaches a certain burial depth, the uplift height of the soil reaches a critical state. To address the grid distortion caused by soil deformation, numerical simulations based on the CEL method effectively modeled the pipe–soil interaction forces under significant lateral displacements, exhibiting good agreement with the experimental results. This study provides a solution for investigating soil resistance in submarine pipelines, thereby contributing significantly to the design and performance prediction of underwater pipelines.

Analysis of the mechanical properties of the single layered braid reinforced thermoplastic pipe (BRTP) for oil & gas industries

Reinforced thermoplastic pipes (RTPs) are instantly being broadly scrutinized as a feasible surrogate for metallic pipes due to their superior corrosion resistance, low weight, and low maintenance worth, they have three primary layers inner, outer, and reinforcement. Process parameters for manufacturing reinforced layers are critically essential to enhancing the mechanical performance of RTPs. However, limited comparative research on the impact of braiding parameters on RTP’s mechanical characteristics appears to be publicly available. Therefore, this study examines how these variables affect the mechanical characteristics of braid RTP. This new class of braid RTP is constructed of an HDPE liner pipe, over-wrapped with a single braid layer of resin glass fiber. L9 Taguchi orthogonal array design for experiments was used to create specimens. Furthermore, specimens were subjected to tensile and compression to assess their mechanical properties. Epoxy and ±75 were the most promising parameters in achieving the highest number of tensile and compression properties among choices of thermoplastic polymer matrices and braid angles, respectively. ANOVA suggested that both parameters are significant and significantly impact the mechanical properties of the prototype braid RTP. Also, A1B3 is considered the best combination for producing braid RTP with higher mechanical performance; no notable interaction among factors was found.

Numerical and experimental investigation on tensile fatigue performance of reinforced thermoplastic pipes

With the rapid development of offshore drilling for harvesting oil and gas from the deep sea, the fatigue performance of offshore risers is increasingly demanding. This study investigated the tensile fatigue performance of reinforced thermoplastic pipes (RTPs). The theory of three-dimensional (3D) anisotropic elastic mechanics was combined with a strain potential-energy density function to construct the 3D constitutive relation of RTPs. The stiffness degradation law of RTPs was predicted by introducing a modified 3D Hashin failure criterion and the residual stiffness model. A user-defined material (UMAT) subroutine was developed to evaluate the progressive fatigue failure process of RTPs. A uniaxial tensile fatigue experiment of RTPs was designed to validate the numerical model's correctness. The analysis examined the damage failure modes of RTPs under cyclic loading conditions and revealed the stiffness degradation mechanism. Results suggested that the matrix fatigue damage of the reinforced layer of RTPs is the primary cause of the rapid stiffness degradation, which was observed to be faster with increasing cyclic load stress levels.

Buckling collapse of reinforced thermoplastic pipes (RTPs) with initial imperfections subject to external pressure

In the present study, the collapse behavior of RTPs was analyzed by an experimentally-verified numerical method, in which the eccentricity directions, the combinations of eccentricity and initial ovality are considered. Eigenvalue analysis and Riks analysis were combined to simulate the progressive failure of composites during the collapse, which was conducted by a continuum damage model based on Hashin failure criterion and fracture energy dissipation. The stress analysis showed that the hoop stress of composites is much higher than that of isotropic layers both in the elastic and progressive failure phases. Because of this, thick composites always play important roles in bearing external pressure. Except the experimentally observed O-/U-type collapse modes, the C-type collapse mode was observed and simulated for the first time. They all go through the uniform contraction-collapse-large deformation process, and have the same failure modes of composites. However, different from the O-/U-type collapse modes, the cross-sectional deformation of the C-type collapse is not symmetrical and has a moving maximum-displacement point. The collapse pressure decreases linearly as the increase of the eccentricity and the initial ovality. The effect of eccentricity directions can be concluded as: more dispersed eccentricity distribution makes composites contribute more to the pressure resistance.

Experimental and numerical study on the dry/wet burst failure of reinforced thermoplastic pipes

AbstractThis paper investigates the burst failure of reinforced thermoplastic pipes (RTPs) via combined experimental and numerical research. Two completely different burst forms appeared during the experiments. The dry burst form occurs with matrix cracks, fiber breakage and crack of liner and coating, which is a common burst morphology. A new burst morphology, which is never reported in open literature, is observed in the experiments. This burst morphology, defined as wet burst form, presents a long dent in the coating and invisible composite damage. In order to simulate the burst failure of RTPs, a numerical model was proposed, in which element removal criteria and the nonlinear stiffness degradation model based on Hashin‐Yeh failure criterion were employed to conduct progressive failure analysis on 3D composites. It was found that the wet burst form has lower burst pressure and causes larger damaged area, which is more dangerous in engineering. For the damage propagation, progressive failure occurs on middle composites and induces the final burst when the dry burst form occurs. As for the wet burst form, water flows into the middle of RTPs, which causes the delamination, matrix crack and bulge of coating. During the process, fibers play few roles in bearing loads.Highlights The wet burst form of reinforced thermoplastic pipes is firstly observed. Element removal criteria and nonlinear stiffness degradation model were used to simulate the burst failure. The numerical model has a good performance on predicting the burst pressure and damage propagation of composites. The wet burst form has lower burst pressure compared with the dry burst form. Fiber and matrix tensile failure mode are dominant.

Failure mechanisms of reinforced thermoplastic pipe (RTP) with crack defects at longitudinal and circumferential orientations

Reinforced Thermoplastic Pipe (RTP) is a composite pipeline that has received considerable attention in the oil and gas applications due to benefits such as high strength and corrosion resistance. Limitations occurred, however, when the performance of this pipe-type towards various defects is still not fully addressed. Thus, this paper attempts to highlight the response of RTP imposed to crack failures using hydrostatic burst test and validated with numerical simulations under various internal pressures. Two defect orientations were studied, namely longitudinal and circumferential, as measured with respect to the pipeline length. The established Von-Mises stress criterion was used to compare the intact RTP performance, while Stress Intensity Factor (SIF) for the cracked RTP. Parametric studies having different crack dimensions were also investigated, with results showing that the longitudinal crack would be giving higher impact as compared to the circumferential one, even at low operating pressure. Observations on the RTP specimen’s failure were also shared. Results obtained from this study would be beneficial in giving initial understandings to pipeline operators on the RTP behaviors towards crack failures.

Study on the O-/ U-type collapse of reinforced thermoplastic pipes (RTPs) subjected to hydrostatic external pressure: Experimental tests and numerical analysis

Due to the anisotropy and progressive failure of composites, the collapse problem of RTPs is more complex compared with that of traditional steel pipes. To address this problem, the collapse behavior of RTPs subjected to external pressure was studied by conducting collapse tests and numerical simulations. Two different collapse modes including the O-type and U-type mode were observed during the collapse tests. It was the first time to observe the U-type collapse mode of RTPs in open literature. For O-type collapse mode, the cross-section turned into an ellipse. And for U-type collapse mode, it turned heart-shaped. Meanwhile, numerical simulations were conducted by combing eigenvalue analysis and arc-length method, in which the progressive failure of composites was considered by employing a fracture energy degradation model based on the Hashin criterion. To simulate the U-type collapse mode, eccentricity was also introduced. Research results indicated that the critical pressure obtained by tests and numerical simulations agreed well for both O-/U-type collapse modes. The collapse process could be described as “contraction-collapse point-large deformation”. It was found that the progressive failure of composites would reduce the pressure-resisting capacity greatly. For the composites, the shear failure, the matrix and fiber compressive failure were dominant failure modes.

Mechanical Behavior of Cross-Winding Steel-Wire–Reinforced Thermoplastics Pipe under Internal Pressure

Cross-winding steel-wire–reinforced thermoplastic pipes (CSWRTPs) have been widely applied in oilfield gathering environments in China because of their excellent physical properties. In this study, the failure mode, short-term burst pressure, and strain for a CSWRTP with an external diameter of 75 mm and a wall thickness of 10 mm under internal pressure are investigated through experimental measurements conducted with a burst test system and simulations completed with ANSYS software. A new three-dimensional finite-element burst failure model of CSWRTPs is proposed. Simulation results are in good agreement with the measurements. An axial ductile crack was found on the tested pipe with an uneven circumferential expansion deformation. The simulated short-term burst pressure is in a good agreement with the measured value (19.7 MPa) with an error of +8.7%. The growth of the strain is initially zero, followed by a linear growth, rapid growth, and a clifflike drop when the crack is generated. The effective plastic strain within the inner polyethylene liner is greater than that within the outer polyethylene sheath, and the effective stress within the inner steel wire is greater than that within the outer steel wire. The ultimate pressure–bearing capacity decreases linearly as the strength of the steel wire decreases. Therefore, reduction of the ultimate pressure–bearing capacity of CSWRTP results from the decrease of steel wire strength from corrosion when applied in oilfield gathering systems. This finite-element analysis method can provide a reference to optimize the structural design of CSWRTP.

Investigation on the bending behaviors of reinforced thermoplastic pipes using a strain energy equivalence method and a continuum damage numerical model

AbstractThis paper investigates the bending stiffness and progressive failure of reinforced thermoplastic pipes (RTPs) under bending loads, in which a theoretical method based on the strain energy equivalence of multi‐layered anisotropic cylinders, is proposed. The bending stiffness and stress field was derived from the equilibrium between the work done by bending moments and the strain energy. This new method could consider the hybrid of isotropic and anisotropic materials and improve computation efficiency significantly. To verify the theoretical method, Abaqus/Explicit quasi‐static analysis on 3D composite elements was performed, in which the progressive failure was considered by using a VUMAT subroutine. The degradation of composites was implemented by a nonlinear stiffness degradation model based on the Hashin‐Yeh failure criterion. The comparison showed that the theoretical results are slightly conservative as the theoretical method neglects the contribution of interlayer interactions. Meanwhile, it can accurately predict the dominant stresses of each composite lamina. According to numerical simulations, the fiber tensile failure, the matrix tensile and compressive failure are the dominant failure modes of RTPs under bending loads. Furthermore, the effects of the winding angles of fibers on damage propagation and the effects of the initial ovality on the bending stiffness are also discussed.

Study on the tensile properties of reinforced thermoplastic pipes under different internal pressures and temperatures

This work explores the tensile properties of reinforced thermoplastic pipes (RTPs) at different internal pressures and temperatures. A new mechanical model of an RTP comprising of a different number of reinforced layers has been proposed by combining the constitutive relationship between the elastoplastic material and the continuous displacement condition. On this basis, the effect of the various parameters, including the internal pressure, the winding angle, and temperature, on the tensile properties of RTPs has been investigated. In addition, tensile tests at different internal pressures have been carried out to validate the model. The results thus obtained show that the internal pressure does not affect the tensile property of the RTPs during the elastic stage. However, its influence increases significantly after the strain exceeds the elastic limit. Taking the winding angle of 37.5° as a demarcation point, it is found that at winding angles smaller than this value, the total load increases with an increase in temperature, whereas at angles larger than 37.5°, the total load gradually decreases with an increase in temperature. The temperature differences significantly affect the mechanical properties of the RTPs, but this effect is obvious when the inner surface temperature of the RTP exceeds its outer surface temperature.

Theoretical prediction for the torsional stiffness of reinforced thermoplastic pipes (RTPs) based on the strain energy-work equivalence of anisotropic cylinders

Torsional characteristics must be considered in the mechanical analysis of offshore pipelines. However, there is no specific theoretical model for RTPs to predict the torsional behavior due to the multilayered helically winding laminates. To address the problem, a theoretical model using the energy method is constructed, in which the governing equation is based on the strain energy-work equivalence of helically anisotropic hollow cylinders instead of homogenous structures in previous studies on unbonded flexible risers. The strain energy of RTPs is firstly obtained by the stress analysis of helically anisotropic elements, while the work is derived by the torsion-deformation relationship. It gives a concise theoretical expression of torsional stiffness of RTPs for the first time and greatly improves the computation efficiency. Another improvement is that it could consider the coexistence of the isotropic and orthotropic materials due to the symmetry of isotropic materials. To verify the proposed model, quasi-static torsion tests and numerical simulations were conducted on two RTP specimens. The proposed model could give accurate torsional stiffness predictions in both twisting directions. Besides, the stress distributions, the effect of thickness-radius ratios and fiber's winding angles were also investigated, in which the applicability of the proposed model was further discussed.

Study on Composite Material Failure of Reinforced Thermoplastic Pipes under External Pressure

This paper examines the mechanical properties of reinforced thermoplastic pipes (RTP) under the influence of external pressure. We used the three-dimensional (3D) thick-walled cylinder theory, combined the 3D Hashin failure criterion with the theory of the evolution of damage in composite materials, considered their nonlinear mechanical behaviors, and introduced changes in the winding angle caused by deformation to formulate a model for the progressive failure analysis of RTP. The model was used to examine the failure sequence and external pressure capacity of RTP, and its correctness was verified by a finite element model. The results show that increasing the winding angle of RTP within a certain range can improve its external pressure capacity. When the winding angle exceeded ±50°, the failure of the RTP under external pressure mainly depended on the fiber fracture of the reinforced layer. With an increase in the ply number of the reinforced layer, the external pressure capacity of the RTP increased nonlinearly and its rate of growth gradually decreased.

Stochastic analysis of failure pressure in reinforced thermoplastic pipes under axial loading and external pressure

Reinforced thermoplastic pipes (RTPs) are flexible composite pipes that are acknowledged as a good alternative to conventional submarine pipes. This study aims at investigating the impact of the uncertainties involved in the production process of the RTPs on the failure pressure under combined loading conditions. The stress distributions of RTPs were analyzed using the theory of the three-dimensional (3D) thick-walled cylinder method, combined with the homogenization method. Based on these results, to evaluate the failure mechanisms of RTPs, the 3D Hashin–Yeh failure criterion and the damage evolution model were adopted to establish a progressive failure model. For analyzing the influence of randomness of the related parameters on the first-ply failure (FPF) pressure as well as the final burst failure (FF) pressure of RTPs, a sufficient number of samples were generated using the Monte-Carlo method. Results of the stochastic analysis were validated using the hydraulic burst test data. The statistical evaluation of the stochastic analysis results was performed by using the Weibull probability density distribution function, and the results showed an increase in the probability of getting a lower failure pressure of RTPs than the average failure pressure (considering the randomness of production parameters).

Simulation Analysis of Limit Operating Specifications for Onshore Spoolable Reinforced Thermoplastic Pipes.

Spoolable reinforced plastic line pipes (RTPs), exhibiting a series of advantages such as good flexibility, few joints, long single length, light weight, easy installation, etc., have been widely used in the onshore oil and gas industry such as oil and gas gathering and transportation, high pressure alcohol injection, water injection, sewage treatment, and other fields. However, due to the lack of clear standard specificationof the limit operating properties for RTPs, three typical failure modes, i.e., tensile, flexure, and torsion, frequently occur in terrain changes, construction operation, and subsequent application, which seriously affects the promotion and use of RTPs. In this paper, the stress distribution of a non-bonded polyester fiber reinforced high-density polyethylene (HDPE) pipe (DN 150, PN 2.5 MPa) was systematically studied by the finite element method (FEM), and then the limit operating values under the axial tensile, coiled bending, and torsion load were determined. The corresponding experiments were conducted to validate the reliability and accuracy of the FEM model. The FEM results showed that the critical strain for axial tensile was 3%, the minimum respooling bend radius was 1016.286 mm, and the limit torsion angle of this RTP was 58.77°, which are very close to the experimental results. These limit values will be useful to establish normative guidelines for field construction and failure prevention of onshore RTP.

Experimental program on the burst capacity of reinforced thermoplastic pipe (RTP) under impact of quasi-static lateral load

Reinforced Thermoplastic Pipe (RTP) is a flexible composite pipe that currently receives fair acceptance in the oil and gas industry, with high potentials to be used as a replacement to the carbon steel (CS) pipes especially in high corrosive environment. As RTP is relatively new products, their designs and qualifications are governed by the industry practice at present in the absence of comprehensive technical design standards. Most literatures to date are mainly dedicated to the intact RTPs, while the response of the pipe towards various defects have not been fully understood. Quite often, the assessments are made by referring to the design standards of the CS pipes, which are not entirely suitable due to the flexible characteristics of the composite pipe itself. Thus, this paper aims at assessing the performance of a 4″ RTP when exposed to lateral loads from external interferences such as the anchor dropped. Herein, the damaged to the RTP pipe was made in the form of a dent indented by a cylindrical conical nose under the quasi-static indentation procedures carried out experimentally. The damaged or dented pipe was later tested for its burst capacity using a burst testing facility specially fabricated for composite pipes. During the experiments, two types of failure modes were observed, namely single unit and ply-by-ply failures. Outcomes obtained from the experiments were used to describe the relationships between three important parameters, namely lateral load, indentation depth and burst pressure using polynomial regression equations. These simplified equations may be adopted directly in pipeline assessments and operations of RTP pipes exposed to dents. Moreover, outcomes from this paper will be beneficial in qualifying further RTP as a new free-corrosion risk transportation system for the oil and gas industry.

A combined experimental and numerical simulation approach for burst pressure analysis of fiber-reinforced thermoplastic pipes

Reinforced thermoplastic pipes (RTP) are widely used in the offshore oil and gas production industry for their excellent flexibility, corrosion resistance and internal pressure resistance. RTPs inevitably experience extreme environmental loads including internal pressure and tensile load in the procedures of installation and operation, which influence the structural integrity and safety. Therefore, reinforced layers of RTP are designed to bear internal pressure and tensile load. Herein, the mechanical behavior of fiber-reinforced thermoplastic pipe that consisting of aramid fiber braid reinforced layers is investigated by combination of numerical simulation and experimental methods. The full-scale internal pressure bursting experiments of 1-inch aramid fiber RTPs were carried out to study the burst pressure and failure behavior. A finite element model of aramid fiber RTP is established using ABAQUS, which considers the material nonlinearities as well as frictional interactions between layers. Based on the finite element model which validated by the experimental results, the mechanical behavior of fiber reinforced thermoplastic pipe subjected to internal pressure and tensile load is investigated in detail. The current study compares the influence of braided angle of fiber-reinforced layers, friction coefficient between each layer and load conditions on the mechanical behavior of RTPs. The experiment results reveal that the bursting pressure of 1-inch aramid fiber RTPs is about 76.1 MPa. The failure of aramid fiber RTP under internal pressure is dictated by leakage due to the fracture of fiber-reinforced layers and the fracture of the internal layer at the end fitting. It is worth noting that the braiding angle renders an obvious influence on mechanical properties of fiber-reinforced RTPs, whereas the mechanical behavior remains insensitive to friction coefficient between layers. Moreover, the synergistic influence of internal pressure and tensile load significantly affects the mechanical behavior of fiber-reinforced RTPs. The current study shall serve as a reference for the design and application of fiber-reinforced thermoplastic pipes in marine oil and gas production.

Burst capacity of reinforced thermoplastic pipes based on progressive failure criterion

This paper examines the burst capacity of reinforced thermoplastic pipes (RTPs) under the effect of internal pressure. Used the theory of the three-dimensional (3D) thick-walled cylinder, combined the 3D Hashin failure criterion with the theory of the evolution of damage in composite materials, considered the nonlinear stress-strain relationship of materials, and introduced changes in the winding angle caused by deformation to formulate a analytical model for the progressive failure analysis of RTPs. The model was used to examine the failure sequence and burst capacity of the RTPs, and analytical results agree well with the experimental ones. The results show that the ply number of the reinforced layer had a more significant effect on the burst capacity of the RTPs than the winding angle. Their burst capacity first increased and then decreased with increasing winding angle. When the winding angle was ±60°, the burst pressure reached its peak. With an increase in the ply number of the reinforced layers, the burst pressure of the RTPs increased nonlinearly and its rate of growth gradually decreased. Once a certain ply number in the reinforced layer had been reached, interlaminar separation was more likely to occur in the RTPs before they burst.

Predicting failure pressure of reinforced thermoplastic pipes based on theoretical analysis and experiment

Because they transport high-temperature and high-pressure oil and gas, the pressure-carrying capacity of reinforced thermoplastic pipes (RTPs) needs to be high. This study develops two analytical models to estimate deformation and stresses in RTPs with any number of layers under internal pressure based on their nonlinear mechanical behaviors. The Hashin–Yeh failure criterion and the damage evolution model of composite materials are combined, and changes due to deformation are introduced to the winding angle to analyze the progressive failure of RTPs. The modeling covered stress analysis, failure evaluation, and material degradation. By comparing their results with those of the finite element model (FEM) and experiments, the advantages and disadvantages of the two theoretical models are determined. The stress distribution and failure pressure calculated by them were in good agreement with those obtained by the FEM and the experiments. However, the accuracy of calculation of the proposed model (model-Ⅰ) was clearly better than that of homogenization model (model-Ⅱ), and its results of failure pressure and the failure sequence were more accurate. Model-I was thus used to obtain the optimal range for the winding angle of the fiber’s, and to discuss the influence of diameter-to-thickness (D/t) ratios on the first-ply failure (FPF) pressure and burst pressure.