Improvement of mechanical behavior of buried pipelines subjected to strike-slip faulting using textured pipeline

Abstract

The present study investigates the mechanical behavior of a new generation of buried pipelines, dubbed the textured pipeline, which is subjected to strike-slip faulting. In conventional cylindrical pipelines, the axial and bending stresses brought about in their walls as a result of fault movement, lead to local buckling, which is construed as one of the major reasons contributing to pipeline failure. The present study has assessed 3-D numerical models of two kinds of buried textured pipelines, with 6 and 12 peripheral triangular facets, subjected to a strike-slip faulting normal to the axis of the pipelines, with and without internal pressure, with the two kinds of X65 and X80 steel, and with different diameter-to-thickness ratios. The results indicate that, because of specific geometry of this pipeline shell which is characterized by having lower axial stiffness and higher bending stiffness, compared to conventional cylindrical pipeline, they are considerably resistant to local buckling. The results of this study can be conceived of as a first step toward comprehensive seismic studies on this generation of pipelines which aim at replacing the conventional cylindrical pipelines with textured ones in areas subjected to fault movement.

Keywords

buried pipeline / textured pipeline / local buckling

Introduction

In today’s world, buried pipeline systems are construed as the most influential means to transport oil and gas. The rapid development that the oil and gas industry has witnessed necessitates the construction of thousands of kilometers of pipelines across the world. Given that these pipelines are lengthy, it is highly probable that they cross active faults (on land and in the sea); accordingly, they are in constant exposure to various geotechnical hazards. The characteristic distinguishing this kind of pipeline from other structures constructed on ground is that the inertial forces exerted by the pipe’s weight and its content are not very important. The most important seismic damages occurring in pipelines are caused by wave propagation, permanent ground deformation (PGD), or landslides [¹]. However, the most critical damage to continuous buried pipelines is permanent ground deformation, which leads to a significant increase in axial strains and this increase is attributed to great axial deformations resulting from bending and stretching in the walls of pipes [²,³]. This phenomenon causes fracture in the pipe wall which is investigated by several researchers [⁴–⁶], or leads to local buckling that is the main object of this paper. The bulk of research devoted to minimizing the damages caused by an earthquake suggest the followings: using materials with high strength or ductility [⁷–⁹], using joints with high tensile or pressure capacity (especially at the intersection of pipe and fault) [¹⁰,¹¹], implementing methods for damping the earth agitations transferred to pipelines, investigating the best route for the transfer of pipelines [¹²,¹³], and employing composite materials known as GRP to reduce the elasticity modulus leading to more appropriate wave propagation [¹⁴]. The present study has investigated a new method for reducing the hazardous effects of strike-slip faults on continuous buried pipelines. In this method, a new generation of buried pipelines with a new geometry, recognized as the Pseudo-Cylindrical Concave Polyhedral Pipeline, or briefly referred to as the textured pipeline, is proposed. Due to lower axial stiffness than conventional cylindrical pipes, this generation of pipelines has higher strain capacity in the axial direction and demonstrates considerable potential against tensile and bending strains caused by fault movement. In this research, the main concept is replacing the conventional cylindrical pipe by a textured pipe with the same material, diameter, and wall thickness. Indeed, the textured pipe is cylindrical in the global sense with a faceted wall instead of a smooth wall. Figure 1 depicts the outline of conventional cylindrical pipelines and the texture pipelines with 6 and 12 peripheral facets (N = 6 and 12). The idea of deploying this kind of shell was put forward in 1969 by Miura, who suggested that the post-buckling pattern of a circular cylindrical shell which is under axial pressure can be adopted as a new basis for designing shell structures dubbed “Pseudo-Cylindrical Concave Polyhedral shells” [¹⁵]. Since being proposed, the polyhedral shells, due to their favorable stiffness characteristics, are being put into different uses such as autonomous underwater vehicle [¹⁶,¹⁷]. Recently, Albermani et al. scrutinized the behavior of offshore pipelines, which were made of this kind of shell and were under hydrostatic pressure, and concluded that, compared to conventional cylindrical pipelines, this generation of pipelines is more resistant to propagation buckling, which is a destructive phenomenon for under-pressure pipelines [¹⁸,¹⁹]. Indeed, the pipe stiffness in diagonal direction has been always the main concern of the researchers [²⁰,²¹]. Albermani et al. have also expounded on the geometrical and mechanical characteristics of these kinds of pipelines. In the same line of research, Guo et al. deploying the three-dimensional printing technology, embarked on constructing textured pipelines with hyper-elastic materials [²²]. Then, by conducting experimental and numerical studies on them, the authors proved that, unlike textured pipelines made of elasto-plastic materials which, compared to conventional cylindrical pipelines, increase propagation buckling capacity significantly, in these kinds of pipelines, local buckling increases only 8%, which is rather insignificant. Furthermore, although manufacturing of such pipe was seemingly impossible and may cost a lot until now, however, drawing on 3D printed molds, they proposed a new method for constructing pipes with textured geometry. The present paper focuses on the mechanical behavior of these kinds of pipelines toward fault movement; however, issues concerned with manufacturing, possible flow losses and induced stresses due to the geometry of the pipe are not within the scope of this paper. To put it differently, the present study attempts to compare the mechanical behavior of buried textured pipelines subjected to fault movement with the behavior of conventional cylindrical pipelines.

Numerical modeling

The structural response of two kinds of textured steel pipelines, with 6 and 12 peripheral facets (N = 6 and 12), under pure axial and lateral displacement and fault movement is examined numerically using advanced computational tools. The models are analyzed simply based on certain input parameters such as perpendicular fault movement to pipeline axis, internal pressure, and various pipeline materials. For a more comprehensive analysis, some other uncertain input parameters should be taken into account which requires sensitivity analysis proposed by some researchers [²³–²⁵] that is out of the scope of this paper. Advanced finite element program ABAQUS [²⁶] is applied to simulate the mechanical behavior of the textured steel pipeline (buried and unburied pipelines), the surrounding soil and their interaction. Also, the nonlinear geometry of the soil and the pipe and their inelastic material behavior is taken into consideration. For numerically analyzing the more complicated models specially with curved surfaces, it is recommended to employ isogeometric analysis which is a powerful method in this regard [²⁷,²⁸].

A prolonged prismatic model is considered (Fig. 2), where the pipeline is buried in the soil. Figures 2(a) and 2(b) represent the corresponding finite element mesh for the soil, and Figs. 2(c) and 2(d) for the two kind of textured steel pipes (N = 6 and 12). Four-node reduced-integration shell elements (S4R) are applied for modeling the pipeline, and eight-node reduced-integration “brick” elements (C3D8R) are employed to model the surrounding soil. Sadowski and Rotter [²⁹] proved that, by deploying this element for modeling pipelines, answers obtained for issues concerned with bending are acceptable. The number of elements and nodes is about 599018 and 436388, respectively, for textured pipe with N = 12, 490248, and 273942 for textured pipe with N = 6, 54132, and 57272 for ordinary pipe and also, the numerical analyses for all models were converged with the total iteration less than 10. The top surface stands for the soil surface, and the burial depth is chosen equal to about 2 pipe diameters, which is in agreement with pipeline engineering practice [³⁰].

To validate the findings and compare them with the behavior of conventional cylindrical pipelines, we employed soil block and pipe dimension of Vazouras et al.’s study [³¹], in which the adequacy of dimensions for whole model are analyzed in their study. Moreover, the analyzing method used in this paper is adopted from their research and clearly the new contribution in this research is the modeling and studying the mechanical behavior of new generation of pipelines.

For buried cases in this study, the soil block dimensions in directions x, y, and z are considered as 60, 5, and 10 times the pipe diameter, respectively. The analysis is performed in two steps: first, gravity loading is exerted and, afterwards, fault movement is applied. The vertical boundary nodes of the primary block and the end nodes of the steel pipeline stay constant in the horizontal z direction and, a uniform displacement is exerted in the external nodes of the second block and the end nodes of the pipeline in the horizontal z direction, due to fault movement. A narrow transverse zone of width w is placed between two blocks of soil which is considered in several numerical studies of fault-foundation interaction [³²,³³]. Thus, the discontinuity at the proximity of the fault could be removed, which sometimes leads to numerical problems. Furthermore, Bransby et al. presented that this consideration may be in agreement with a more realistic presentation of the fault movement mechanism [³⁴]. The soil-pipeline model after a seismic fault displacement in the z direction is illustrated in Fig. 3.

Elastic-plastic behavior is employed for the material of the pipelines and soil. To represent the mechanical behavior of the steel pipe material, a large-strain J2 flow (von Mises) plasticity model with isotropic hardening is applied. Moreover, Mechanical behavior of soil material is presented through an elastic-perfectly plastic Mohr-Coulomb model, described by cohesion c, friction angle ϕ, elastic modulus E, and Poisson’ratio ν. In this research, the dilation angle ψ is presumed to be equal to zero. For interface friction between the steel pipe surface and the soil block, a contact algorithm is considered with a real friction coefficient μ. In this study, μ is considered to be equal to 0.3. Furthermore, the separation of the pipe and the soil block is allowed during analyzing and the geometry of the soil at the contact surface with pipe is completely conformed to the geometry of the textured pipelines (Fig. 2(b)).

Numerical results

Throughout the study, all the investigations are conducted on two kinds of textured pipelines with 6 and 12 peripheral facets (N = 6 and 12). Different diameter-to-thickness ratios are considered for textured steel pipelines in this research. For all models in this study, the outer pipe diameter, D, is considered equal to 914.4 mm (36 in.), and the pipe wall thickness, t, varies from 6.35 mm (1/4 in.) to 19.05 mm (3/4 in.), thus, the D/t values are between 48 and 144. The soil block has dimensions 60 m × 5 m × 10 m in directions x, z, y, respectively.

In Section 3.1, textured-steel pipelines with API 5L X65 [³⁵] steel material and thickness equal to 12.7 mm (0.5 in.) are modeled under axial and lateral displacement. In Section 3.2, the behavior of the buried pipeline is examined under cohesive soil conditions using appropriate values of soil parameters c, Φ, and E. The same models with internal pressure are analyzed in Section 3.3. Subsequently, in Section 3.4, X65 pipelines with different values of the diameter-to-thickness ratio D/t are analyzed, to identify the influence of the diameter-to-thickness ratio on the structural response. Finally, high-strength steel X80 pipeline is analyzed under fault imposed displacements in Section 3.5.

Textured pipeline under axial and lateral displacement

As it was touched upon at the outset of the paper, fault-imposed deformations are considered as the most critical damage to continuous buried pipelines. At first, by performing some simple numerical modeling, textured pipelines, are placed under pure axial and lateral displacement, Fig. 4, and their mechanical behavior is compared with conventional cylindrical pipelines’ behavior to examine whether the different geometry of the walls of these pipelines leads to an improvement in their mechanical behavior under axial and bending stresses.

Relatively thick-walled X65 textured pipelines with N = 6 and 12 are considered, with diameter and thickness equal to 914.4 mm (36 in.) and 12.7 mm (0.5 in.), respectively, so that D/t = 72. The API 5L X65 steel material, with a nominal stress-engineering strain curve depicted in Fig. 5(a) and Table 1, is a typical steel material in oil and gas industry. The yield stress σ_y is equal to 450 MPa (65 ksi) followed by a plastic plateau up to 3% strain and a strain-hardening pattern with a hardening modulus of E_s/300, where E_s is Young’s modulus of the steel material.

Figures 6(a) and 6(b), show the axial force diagram vs displacement for conventional cylindrical and textured pipes with N = 6 and 12 and steel X65 for two models with lengths equal to 20 and 60 m (buckling factor of 62 and 188, respectively) which are placed on roller supports and under pure axial displacement. For analyzing the models, an initial imperfection is exerted on the middle of the pipes in the form of lateral displacement equal to one thousandth of pipe length.

As it can be seen, in conventional cylindrical pipe with L= 20 and 60 m, axial force and displacement increase linearly and after reaching a certain value, they undergo a sharp drop which is indicative of a local buckling in the pipe wall. The same trend can be observed in Fig. 6(a) for short length textured pipe with N = 12; however, the differences reside in the facts that the textured pipe has less axial stiffness, and undergoes local buckling at greater lateral displacements. As the number of peripheral facets of short textured pipe decreases (N = 6), the axial stiffness decreases considerably; accordingly, at a certain axial displacement, the axial force is significantly less than the other two pipes. Moreover, in this textured pipe, no considerable drop is observed in the axial force, indicating the lack of local buckling. As Fig. 6(b) reveals, which is related to the long pipes with the length of 60 m, textured pipes display no sensitivity to local buckling. Accordingly, it can be concluded that the buckling brought about in the pipes has transformed from local buckling in conventional cylindrical pipe to global buckling in textured pipes and such optimal performance is expected from textured pipes.

Figure 7 represents the maximum axial strain in the compression side of pipes with the length of 60 m and under different lateral displacements, imposed on the middle of the pipeline perpendicular to its axis. Pipes are placed on pinned supports with distance of 15 and 45 m from the ends.

As it can be observed, when the displacement in the middle of conventional cylindrical pipe and textured pipe with N = 12 reaches 60 and 80 cm, respectively, axial strain begins to increase rapidly and reaches the value of 0.013 in compression side of wall which represent the onset of local buckling in pipes. However, for the displacement of 100 cm, such sudden strain-rate change is not observed in the wall of textured pipe with N = 6, indicating and corroborating the resistance of textured pipe against the occurrence of local buckling. Moreover, at a specific value of lateral displacement, textured pipes experience considerably smaller strains and at the folding of the pipelines’ walls which are more controllable.

As the research findings demonstrate, with axial and lateral displacements imposed on them, textured pipes, compared with conventional cylindrical pipes, perform better against local buckling and, by bearing the strain uniformly throughout their length, experience less stress in their walls. The more the number of peripheral facets (N) of these pipes increases, the more their cross sections resemble a circle; hence, their performance is similar to the performance of conventional cylindrical pipes. Accordingly, it can be concluded that, by considering other factors which are effective in designing these pipelines, an optimum N can be obtained in which buried pipelines subjected to fault movement demonstrate a considerably better mechanical performance, compared to conventional cylindrical pipelines; besides, in this way, we can significantly decrease the possibility of the occurrence of local buckling, which is conceived of as one of the most destructive factors for pipelines.

To conduct a more detailed analysis of the mechanical behavior of these pipelines toward fault movement, real models of buried textured pipelines were investigated under different conditions.

Moderately thick X65 textured steel pipeline in cohesive soils without internal pressure

The X65 steel textured pipelines are presumed to be buried in a cohesive soil and to be crossing a fault zone having a width w equal to 0.33 m. Its internal pressure p is equal to zero, but it will be increased in the next sections of the study. A soft clay is used in the models, which under “undrained” loading conditions has a friction angle ϕ = 0°, cohesion c= 50 kPa, Young’s modulus E= 25 MPa, and Poisson’s ratio ν = 0.5 (Table 1).

For model validation, given that there were no experimental models for buried textured pipelines, first, a nonlinear static analysis was performed on conventional cylindrical pipeline which were buried in soil. By imposing a lateral displacement of 1 m, a local buckling is observed in the pipeline at the distance of 5.4 m from the fault (Fig. 3). In the study undertaken by Vazouras et al., this value is obtained to be 5.45 that shows the results are in reasonable agreement [³¹]. Furthermore, according to the results obtained from the analysis of models in the elastic range and according to conclusions drawn from analytical relations proposed by Vazouras et al. for determining the maximum axial and bending strain caused by the deformation of strike-slip fault, represented in Fig. 8, we can argue that results are acceptable. In this case, because the nonlinear geometry is considered for analyzing the model, the gained numerical strains are higher than the analytical ones. Analytical equations for the maximum bending ({{\displaystyle ϵ}}_b) and axial strain({{\displaystyle ϵ}}_m)are as follows:

(1){{\displaystyle ϵ}}_b={\displaystyle \frac{{\mathrm{\pi }}^{\mathrm{2}}}{\mathrm{4}}}\left({\displaystyle \frac{d}{L}}\right)\left({\displaystyle \frac{D}{L}}\right),

(2){{\displaystyle ϵ}}_m={\displaystyle \frac{d^{\mathrm{2}}{\mathrm{\pi }}^{\mathrm{2}}}{\mathrm{16}{L}^{\mathrm{2}}}},

where d is the incremental fault offset, D is the diameter of the pipe and L is the unanchored (critical) length of the pipe.

Accordingly, in these models, by replacing the conventional cylindrical pipelines with textured pipelines, the obtained results are expected to be reliable.

Figure 9 shows the maximum axial strain in compression side of wall for conventional cylindrical and textured pipelines with N = 6 and 12 for lateral displacements up to 100 cm. The internal pressure is equal to zero in this section. It is worth mentioning that for the textured pipelines, the maximum axial strain is shown for the points similar to point A on the surface of the pipeline (Figs. 10(b) and 10(c)), and therefore, the results would be developed for fewer points as the N decreases (Figs. 9(b) and 9(c)). Similar trends in diagrams are observed for other similar points on the surface of the pipelines.

Results concerned with the effect of strike-slip fault on these three kinds of pipes suggest that, in a displacement of about 70 cm, in both conventional cylindrical and textured pipelines with N = 12, the axial strain in the compression side of pipeline wall is about 0.01. However, the difference resides in the fact that, in conventional cylindrical pipeline, this value is achieved by a sudden increase in the strain in a short length of pipeline; while, in textured pipeline, the strain increment occurs more slowly and in a larger length of the pipeline. This value of fault displacement which leads to a sudden increase in strain in conventional cylindrical pipeline can be called the critical displacement (d_cr).

After this stage, with the steady increase in displacement up to d= 100 cm, strains in textured pipeline are still uniform; while, in conventional cylindrical pipeline, they increase very rapidly. The observed rapid increase in strain in conventional cylindrical pipeline is indicative of a substantial decrease in the stiffness of pipelines’ wall and the occurrence of local buckling. Steady increase in strain in textured pipeline can be attributed to less axial stiffness and the spring-like state of the pipeline wall, which due to its geometry, prevents strain concentration in one point.

Furthermore, unlike conventional cylindrical pipeline, in textured pipeline, the maximum strains occur in a controlled manner and at the folding of the pipelines’ wall and this can minimize the damages occurring to pipeline. In Miura study [¹⁵], this finding was confirmed to hold true for circular cylindrical shell under axial pressure. Furthermore, it can be observed that, up to a displacement of 100 cm, textured pipeline with N = 6 experience uniform and considerably small axial strains and no sudden increase can be observed. Accordingly, it can be argued that the walls of the textured pipelines, due to having undulations on their surface, are characterized by having greater bending stiffness than the conventional cylindrical pipeline, and the more the number of N increases, the more the cross section of the pipeline resembles a circle and consequently, walls demonstrate more sensitivity to local buckling. With an increase in fault displacement, which leads to an increase in the axial compressive strain, the depth of undulations on pipelines’ walls increases, leading to an increase in the second moment of inertia of walls’ surface and, consequently, to an increase in the bending stiffness in shell and a decrease in the rate of axial strains. This finding is substantiated by the study conducted by Khalilpasha and Albermani [¹⁹]. As the figure shows, in a displacement of 100 cm, considerably small strains occur in textured pipeline.

Furthermore, having the preceding remarks in mind, we can conclude that, by decreasing the number of N, which leads to a considerable decrease in axial strains, concerns regarding failure in the geometric folding of the pipelines’ walls due to strain concentration, alleviate. Although, this stress concentration (depicted in Fig. 10) is considered in all numerical models in this study, it is not desirable at the pipelines’ wall. It is worth mentioning that, as the number of peripheral facets decrease (N = 6), the maximum axial strain approaches the fault location. Figure 10 shows the location of maximum axial strain in the three pipelines at the displacement of 100 cm.

According to Fig. 11, depicting the maximum axial tensile strain for all three types of pipelines, we can argue that textured pipelines, due to having less axial stiffness, experience higher axial strains than conventional cylindrical pipeline and both of textured pipelines, in their tensile side, exhibit a rather similar behavior. Furthermore, in fault displacements up to 100 cm, axial strain for all three types of pipelines is considerably less than the strain which can lead to a rapture in the walls of the pipelines and, in different references, this value for conventional cylindrical pipelines with X65 steel is considered to be 0.04 [³⁶,³⁷].

It should be noted that, for textured pipelines, this value can be obtained using numerical and experimental studies; however, it is not within the scope of the present study.

Moderately thick X65 textured steel pipeline in cohesive soils with internal Pressure

The effects of internal pressure on the mechanical behavior of buried pipelines are investigated in this section. By applying a safety (reduction) factor equal to 0.72 [³⁸,³⁹], the maximum operating pressure P_max of pipeline is expressed by the following term:

(3){{\displaystyle \mathit{P}}}_{\max }=\mathrm{0.72}\times \left(\mathrm{2}{{\displaystyle \mathit{\sigma }}}_y\times \mathit{t}/\mathit{D}\right),

that is equal to 9 MPa (90 barr) for X65 steel pipelines. Numerical results represented in Fig. 12 are related to pipelines with internal pressure of 50 barr, which is equal to 56% of the maximum operating pressure presented in the Eq. (3).

As it can be observed, in conventional cylindrical pipeline, the critical displacement of the fault (d_cr) has decreased from 70 to 57 cm; while, this internal pressure did not exert a significant effect on the axial compressive strains of textured pipelines, which is considerably less than the axial strains of conventional cylindrical pipeline. Textured pipeline with N = 6, likewise, in this state, exhibit no sensitivity to local buckling and compressive strain value for a displacement of 100 cm is achieved to be about 0.02. Accordingly, in line with the results obtained from the study undertaken by Khalilpasha and Albermani [¹⁹], we can conclude that, due to the undulations existing on the wall of textured pipelines, they are highly resistant to internal and external pressures exerted on their body. Figure 13 shows the location of maximum axial strain in the three kinds of pipelines, for the fault displacement of 100 cm.

Diagrams in Fig. 14 depicting the values of strain in the tensile side of pipelines wall, indicate that, in the critical displacement, strains in the tensile side of conventional cylindrical pipeline has undergone a significant increase; however, such an increase is not observed in textured cylindrical pipelines with N = 6 and 12.

Effects of the diameter-to thickness ratio

To scrutinize the effects of the diameter-to-thickness ratio, results are gained for 36 in. diameter X65 steel pipeline with thickness varying from 1/4 to 3/4 in., related to D/t values between 48 and 144, for ordinary and textured pipelines with N = 6 and 12, buried in cohesive soil. The numerical results are summarized in Table 2.

According to the obtained results, we can conclude that, in conventional cylindrical pipeline, as the value of D/t increases, the value of critical displacement (d_cr) decreases substantially. This finding suggests that low-thickness pipelines are considerably more sensitive to local buckling and experience failure at small displacements. The results obtained for textured pipelines indicate that, except for textured pipelines with N = 12 and thicknesses of 1/4 and 1/2 in., for other models, the value of critical displacement could not be calculated, suggesting that textured pipelines do not exhibit considerable sensitivity to local buckling and, in them, deformations appears as global buckling. Thus, using these kinds of pipelines with optimum characteristics, can pave the way for preventing the destructive effects of local buckling.

Structural behavior of high-strength X80 steel textured pipelines

The mechanical behavior of buried high-strength steel (API X80) textured pipelines under fault movement is also analyzed, using the numerical tools represented in the previous sections. The nominal uniaxial tensile stress-strain relationship of the X80 material is shown in Fig. 5 and Table 1. The material curve with a yield stress of 550 MPa and a plastic plateau up to a strain of 1.48% illustrate a seamless steel pipe material. Results are gained for 36 in. diameter X80 steel textured pipelines with D/t ratios between 48 and 144. The numerical results are summarized in Table 3.

In line with the results obtained for the X65 pipelines, for high-strength X80 steel pipelines, it is likewise observed that as the thickness of the pipeline decreases, the value of d_cr decreases considerably. Furthermore, textured pipelines exhibit no sensitivity to local buckling.

For conventional cylindrical pipeline with D/t = 48 and thickness of 2/3 in., likewise, because no local buckling is observed, the value of critical displacement could not be calculated. It can be attributed to the considerable tensile deformation in the pipeline wall which has affected the mechanical behavior of this thick walled pipeline.

In general, it can be concluded that, in fault-imposed deformations, if the pipeline wall shows resistance to local buckling up to a certain value of displacement, beyond that value, with an increase in the lateral displacement, which leads to an increase in tensile strains in the pipeline, no local buckling is observed in the pipeline, even with high values of displacements, and pipeline failure resulting from wall rapture due to tensile stresses.

Conclusions

Using advanced finite element simulation tools, the mechanical behavior of buried textured steel pipelines subjected to strike-slip faulting was examined and a comparison was drawn between their behavior and conventional cylindrical pipelines’ behavior.

At first, using simple models, the hypothesis regarding the improvement of the mechanical behavior of textured pipelines (with focusing on the formation of local buckling) which are under pure axial pressure and pure bending was examined. The findings revealed that, compared to conventional cylindrical pipeline, textured pipelines exhibit less sensitivity to local buckling, and global buckling is the dominant type of buckling in these pipelines.

To ensure the obtained results, numerical models of textured pipelines which were buried in soil and subjected to strike-slip faulting were assessed under different conditions and the results were compared with the results obtained from conventional cylindrical pipeline. To put it otherwise, by focusing on pipeline failure due to local buckling or rapture, structural responses of pipelines buried in a kind of clay with and without internal pressure were investigated. Also, the effects of different ratios of diameter to thickness and types of pipelines (steel X65 and X80) on the mechanical behavior of these pipelines were examined.

Numerical results reveal that, unlike conventional cylindrical pipeline, especially thin-walled one, which display high sensitivity to local buckling, textured pipelines display considerably less sensitivity to it and, as the number of the peripheral facets (N) of these pipelines decreases, this sensitivity decreases more such that, in large fault displacements, no local buckling is observed in their walls. This can be attributed to the specific geometry of these pipelines which, due to having undulations on their shell, their axial stiffness is decreased and leads to an increase in the strain capacity at the axial direction of the pipeline. Moreover, as the fault displacement increases, the depth of undulations on wall increases and due to this increment, the second moment of inertia of the surface increases which leads to a higher bending stiffness of wall and, consequently, strengthen the resistance of wall against local buckling. Besides, deformations in the wall of textured pipelines are controlled and occur at the folding of the shell. Although, stress concentration is observed at the folding of the shell that is not desirable, however, it is considered in all numerical models in this study.

The results obtained from models with different diameter-to-thickness ratios and X65 and X80 steel pipelines demonstrate that, by decreasing the D/t ratio and using high-strength steel, wall resistance to local buckling increases and pipeline deformation due to large fault displacement, transforms from local buckling to global buckling and this desirable behavior is more obvious in textured pipelines.

Drawing on what was explicated above, we can conclude that, using textured pipelines can considerably increase the resistance of pipelines’ walls to local buckling, which is construed as one of the most important factors in pipeline failure against fault movement. In fact, by considering other factors which are effective on the behavior of these kinds of pipelines, we can obtain the optimum characteristics of these pipelines and replace conventional cylindrical pipelines with them in areas which are subjected to faulting.

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