Arctic Pipeline with Thaw Settlement Analysis by Finite Element Method

Transcription

1 Proceedings of the Twenty-sixth (2016) International Ocean and Polar Engineering Conference Rhodes, Greece, June 26-July 1, 2016 Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN ; ISSN Arctic Pipeline with Thaw Settlement Analysis by Finite Element Method Kyung Il Kim a, Kyu Jung Yeom a,b, Woo Sik Kim b, Kyu Hwan Oh a, a Department of Materials Science and Engineering, Seoul National University, Gwanak-gu, Seoul, Korea b Gas Research Institute, Korea Gas Corporation Ansan-si, Gyeonggi-do, Korea ABSTRACT The depletion of natural resources has made it important to secure the supply of gas in the arctic region, which has barely been developed. There has been quite some research done in evaluating the impact of permafrost on pipelines. However, the results of the previous research may not be easily applied to mechanical properties which is changed temperature changing. In the arctic region, pipelines are damaged by thaw settlement, with arctic environment. In order to reduce this pipeline damage, it is essential to analyze the mechanical behaviors of the pipelines, taking thaw settlement into consideration, during their design. In this study, we developed an elasto-plastic finite element model of thaw settlement to analyze the stress and displacement of a pipeline in the arctic region. The developed model utilized the soil material properties based on the depth of the thaw settlement using ABAQUS. KEY WORDS: Finite element method; arctic; stress-based; pipeline; thaw settlement INTRODUCTION The transport of petroleum and gas in the arctic region is mostly carried out through pipelines. Currently, large deposits are being developed, along with the construction of pipelines in the biggest petroleum and gas deposits in the arctic region in Russia, and countries around the world are making efforts to transport gas from this extremely cold region. However, because there is still a serious lack of understanding about the arctic region, there is insufficient technical capability for the construction of pipelines there. In particular, a pipeline is required to transport gas from an arctic region such as Alaska or Siberia, and a considerable cost is expected (Alaska Pipeline Project, 2011; Bradshaw, 2010). Thus, it is necessary to minimize the pipeline construction cost. Because the materials and design required for the construction of a pipeline account for a large portion of this cost, many studies are still required on material selection and design. In addition, it is important to construct a pipeline at the minimum cost while still preventing it from being damaged by thaw settlement, which is an important environmental factor in the arctic region (Kim, 2014). Many studies have already been carried out on the ground settlement in a general environment, and studies are actively being carried out on thaw settlement in the arctic area. Kim at al(1998)has carried out stress analysis of buried pipeline, and Lee and Kim(2004) has carried out a study on the effect of ground settlement on the soundness of a buried pipeline. However, the previous studies covered only 2-D modeling and applied the Winkler foundation theory. In addition, because a beam element was applied without applying elasto-plasticity to a pipeline, it was particularly difficult to measure the detailed stress and displacement in relation to the pipeline location in the arctic region. In this region, because the temperature difference in relation to the ground depth is large, and the material properties of the soil change considerably when its temperature drops below 0 C, a modeling and analyzing method that considers these factors is required. In this study, the behavior of soil was analyzed in detail by applying the Mohr-Coulomb theory and taking these factors into account. In addition, the analysis was carried out in the plastic domain for the stress and displacement of the pipeline by applying elasto-plasticity to it, and the stress applied to each position of the pipeline was investigated in detail by utilizing an independent 3-D finite element analysis model. The analysis was performed by inputting different material properties for the soil for different ground temperatures, which depended on the ground depth. Application of Finite Element Method Finite element method (FEM) analysis condition. There are several environmental factors that only appear in the arctic region. Among these, the phenomena that frequently has the largest influence on a buried pipeline are thaw settlement and frost heave. Thaw settlement, which we focus on in this study, refers to the subsidence of a structure that occurs when a large amount of ice melts in an unstable thawing section 1197

2 of a permafrost region. It is caused by a natural change in the temperature condition or the inflow of heat from a structure. Figure of thaw settlement in arctic region is shown in Fig. 1. The methods for installing a pipeline for the transport of natural gas in the arctic region vary depending on the condition of the soil through which the pipeline passes, and include installing it on the ground and covering it with soil, burying it underground, and installing it on a raised stand on the ground. In general, the temperature effect must be analyzed when performing an analysis of soil and piping in combination. To accomplish this, soil at different temperatures was studied in a thermal elasto-plastic analysis. The modeling was carried out in 1/2 scale for the stress distribution and boundary condition, and a C3D8R element (8-node linear brick element) was used. True Stress (MPa) API X70 The analysis used 5,600 and 32,000 elements for the piping and soil, respectively. The previously mentioned elastic-plastic analysis was performed without using the beam element generally used when analyzing soil. The Mohr-coulomb equation was used to take into account the friction and cohesion of the soil (Verruijt, 2004). The modeling was carried out using the commercial program ABAQUS CAE 6.10, and a finite element analysis was carried out using ABAQUS/Standard True Strain (%) Fig. 2. Stress-strain curve of API 5L X70 pipe Table 2. Value of model size Classification value Length of settlement(m) 4, 8, 12, 16 Depth of settlement(m) 0.5 Depth of active layer(m) 3 Depth of pipeline(m) 2 Fig. 1. Schematics of thaw settlement (Mackenzie Gas Project, 2004) Finite element analysis model and material properties of piping. API RL X70 pipe, with a diameter of 762 mm, thickness of 20 mm, length of 3000 mm, and internal pressure of 17 MPa, was used for the pipeline model. The material properties of the piping are listed in Table 1. In addition, the true stress-strain curve prepared by converting the engineering stress-strain curve obtained through an actual tensile test was applied to the finite element analysis, and is shown in Fig. 2. Because this takes into account not only the yield stress of the piping but also plastic deformation, it is known that a pipe fracture can be most accurately simulated using these piping material properties. Finite element analysis model and material properties of piping. In the general environment of the arctic region, the temperature varies depending on the depth of the ground, and, the material properties of the soil greatly vary, particularly at ultra-low temperatures, which were applied to the new model. The soil ground was modeled using an axial length of 30 m, which was the same as the length of the piping, along with a width of 3 m and height of 6 10 m. Among these, in the case of the height, up to 5 m from the bottom was classified as permafrost and the upper 1 5 m was the active layer, which were modeled differently. Among these, the basic model where the height of the active layer was 3 m is shown in Fig. 3. Table 1. Material properties of API 5L X70 pipe Classification Value Density (kg/m 3 ) 7850 Poisson s ratio 0.3 Elastic modulus (GPa) 207 Yield strength (MPa) 530 Ultimate tensile strength (MPa) 626 Fig. 3. Soil model for arctic region 1198

3 In the case of thaw settlement, the modeling analysis was carried out by changing the length and depth of the settlement. In the case of the settlement length, the analysis was carried out by selecting a total of four models, with lengths of 4 m, 8 m, 12 m, and 16 m, while the settlement depth was set as a fixed variable of 0.5 m. (Table 2) When analyzing the effect of the depth, the analysis was carried out by selecting a total of five models for the thaw settlement depth ranging from 0.5 m to 1.5 m in 0.5-m steps. The settlement length was set as a fixed variable of 8 m. The soil in contact with the pipeline was modeled in such a way that the deformation and stress could be analyzed in more detail by allocating more meshes to the soil model. In addition, the ground of the thaw settlement part is weaker than the surrounding ground without settlement. But if the settlement model is empty, it arise a problem with the contact. Because of that problem, the analysis was carried out by applying 1/10 of the density and elastic modulus of the surrounding area. Because the material properties of the soil in the arctic region have large ranges of variation, they differ by area, and the types of soil are also diverse. This study was carried out by applying the material properties of the clay obtained from an actual survey in the arctic region (Wu et al., 2010). The density, elastic modulus, Poisson s ratio, cohesion, and friction angle of the active layer and the permafrost at temperatures of - 20 to 20 were investigated. In relation to the material properties of clay, although the density does not vary with the temperature, the elastic modulus varies greatly, to the extent that the active layer had a value of 200 MPa at -20 while those at -5 and 0 were 50 MPa and 6 MPa, respectively. Moreover, the Poisson s ratio, cohesion, and the friction angle also varied a little depending on the temperature. In this study, the ground and pipeline were differentiated from those used in the existing studies. The effect of the section dividing the active layer and permafrost, a peculiarity of the soil environment in the arctic region, was applied to the analysis. In addition, in the case of the active layer, the temperature appeared to differ with the depth, as shown in Fig. 4, and the material properties showed large differences depending on the temperature. In particular, the values of the elastic modulus and cohesion were much different. To apply these to the analysis, the area was divided in the analysis model, as shown in Fig. 5, and the material properties that corresponded to each soil were applied to each area. To give a suitable boundary condition for the conditions of a long range pipeline, the boundary condition between the pipeline and soil was applied as shown in Fig. 6. At the bottom of the model, the Z direction, X direction, and Y direction all were pinned, and a symmetry condition was used for the Z direction, which is the side part of the model, and the pinning was applied in the X direction. In addition, the boundary condition was set to allow all the parts except the bottom of the model to move freely in the Y direction (the direction of gravity) so that the stress and displacement applied to the pipeline by the soil could be analyzed (Hyuk Lee, 2010). Gravity was also added to the entire model to analyze its effect on the pipeline. Fig. 4. Temperature change due to soil depth (Pissart, 1987) Fig. 5. Different properties of soil depth Fig. 6. Boundary condition of soil model 1199

4 Result of Finite Element Analysis In this study, the deformation and stress distribution applied to the API X70 pipe were analyzed using two variables, the thaw settlement length and thaw settlement depth. When one variable was changed, the other variable was fixed. In addition, to confirm the trend of the stress distribution applied to the pipe, it was checked with the von Mises stress. Effect of thaw settlement length. We examined a model where no thaw settlement took place, and those where the effect of the settlement depended on the length. The modeling was done using four settlement lengths. At this time, the effects of the variable were checked by setting the pipe burial depth to 2 m and the thaw settlement depth to 0.5 m. The models with the different thaw settlement lengths are shown in Fig. 7. The stress applied to the top of the pipe when thaw settlement occurred was expressed using the von Mises stress, and the result depended on the settlement length, as shown in Fig. 8. It can be seen that the largest stress is applied to the center of the pipe, and less stress is applied toward the ends. In addition, the stress becomes equal to the von Mises stress of the 0-m model where no thaw settlement occurred as the point moved toward both ends. It was also confirmed that the stress in the case when no settlement occurred increased compared to the case when the settlement length was 8m. It was shown that stress of MPa was applied to the top part of the pipe when the settlement length was 8 m. However, when the length was longer than 8m, the stress of the upper pipe actually decreased. smaller stress was applied to the center of the pipe compared to the upper part of the pipe. Moreover, stress of bottom pipe(279.3mpa) is larger than stress of upper pipe(277.5mpa). As can be seen by these results, the length of the thaw settlement can be thought to have a effect on the stress applied to the pipe. The conclusion can be drawn that, in general, a longer thaw settlement will allow a larger stress to be applied. This was thought to be because, with a longer settlement length, the force the pipe can withstand becomes weaker, which affects the soil pressure. von Mises(MPa) m 8m 12m 16m Fig. 8. Stress distribution of upper pipe depending on thaw settlement length von Mises(MPa) m 273 8m 12m m Fig. 9. Stress distribution of bottom pipe depending on thaw settlement length Fig. 7. Soil models of different thaw settlement lengths: (a) 4 m, (b) 8 m, (c) 12 m and (d) 16 m In addition, the stress distribution in both the upper and lower parts of the pipe was confirmed, as shown in Fig. 9. The stress distribution in the lower part of the pipe differed with the length of the thaw settlement. The largest stress was applied to both ends of the thaw settlement, and a The displacement of the pipe in relation to the thaw settlement length was confirmed, as shown in Fig. 10. When the thaw settlement became longer, the sagging at the center of the pipe increased. Although the pipe did not show any large deformation until the length of the thaw settlement was 4 m, it showed a deformation with about 3 mm of sagging when the length was 16 m. This is thought to be a very small deformation. Effect of thaw settlement depth. To determine the effect of the thaw settlement length, an analysis was carried out by modeling a total of five cases with thaw settlement depths ranging from 0.5 m to 1.5 m in 0.5-m steps after fixing the length of the thaw settlement to 8 m and the depth 1200

5 of the pipe burial to 2 m. The von Mises stress distribution over the pipe in relation to the change in the depth of the thaw settlement is shown in Fig. 11. There was a small difference depending on the settlement depth. Because the increase at the center of the pipe when the settlement depth increased was almost 2 MPa. displacement(mm) m -4 8m 12m 16m -5 Fig. 10. Stress distribution of upper pipe depending on thaw settlement length von Mises(MPa) m 1m 0.5m Fig. 11. Stress distribution of upper pipe depending on thaw settlement depth The deformation of the pipe also showed a small difference in relation to the settlement depth, as shown in Fig. 12. Although the pipe appeared to have sagged at the center, it held the same position as the depth changed. When analyzed based on the above analysis results, a small difference appeared in the stress and displacement applied to the pipe as a result of the change in the depth of the thaw settlement. Accordingly, the thaw settlement depth was thought to have a small effect on the pipe. displacement(mm) m 1m 0.5m -5 Fig. 12. Displacement of upper pipe depend on thaw settlement depths Conclusion In this study, we presented a new finite element model utilizing the elasto-plastic behavior that suits the environment of the arctic region. Moreover, the FEM model considers the temperature distribution in soil. However, this model does not consider heat transfer between the pipe and soil and within soil. The conclusions drawn on the effect of thaw settlement in the arctic region on the deformation and stress distribution of a buried pipeline are as follows: (1) Effect of thaw settlement length A longer thaw settlement produced a greater stress effect on the pipe. In a soil model where the length of the pipe was 30 m, the stress applied to the pipe when the settlement length was at a maximum of 16 m was higher than that of the model where no settlement occurred, by about 4 MPa. In addition, it was confirmed that the stress distributions applied to the upper and lower parts of the pipe were different. (2) Effect of thaw settlement depth A small difference appeared in the stress and displacement applied to the pipe in relation to the thaw settlement depth. As can be seen from the above results, when thaw settlement occurs, we believe that the focus should be placed on both the length of the thaw settlement and its depth. Moreover, if the effect on the pipe is predicted by applying the material properties of the soil in the specific area through the new finite element model presented in this study, it is expected to be helpful in actually designing the pipeline. ACKNOWLEDGEMENTS This research was supported by a grant (13IFIP-B ) from the Industrial Facilities & Infrastructure Research Program funded by the Ministry of Land, Infrastructure and Transport of the Korean government. REFERENCES Alaska Pipeline Project (2011). Draft Resource Report 1 - Rev 0. Bradshaw, M (2010). A New Energy Age in Pacific Russia: Lessons from the Sakhalin Oil and Gas Projects, Eurasian Geogr and Econ, 1201

6 51(3), Kim, HS, Kim, WS, Bang, IH, Oh, K, and Hong, SH (1998). Analysis of Stresses on Buried Natural Gas Pipeline Subjected to Ground Subsidence, KOSOS, 13(2), Kim, WS (2014). Technology Trend of Energy Pipe, KSME, 54(1), Kim, YJ, Kang, JM, Kim, YS, and Hong, SS (2007). Analysis of Environmental and Geographic Characteristic and Resource Development Condition, Civil Expo, Lee, Hyuk (2010). Finite Element Analysis of Buried Pipeline, Manchester United University. Lee, OS, and Kim, DH (2004). Effect of Ground Subsidence on Reliability of Buried Pipelines, KSPE, 21(1), Mackenzie Gas Project (2004). Application for Approval of the Mackenzie Valley Pipeline Pissart, Albert (1987). Geomorphologie Periglaciaire Textes des Leçons de la Chaire Francqui Belge Verruijt, A (2004). Soil Mechanics. Wen, Z, Sheng, Y, Jin, H, Li, S, Li, G, Niu, Y (2010). Thermal Elastoplastic Computation Model for a Buried Oil Pipeline in Frozen Ground, Cold Reg Sci and Technol, 64, Wu, Y, Sheng, Y, Wang, Y, Jin, H, Chen, W (2010). Stresses and Deformations in a Buried Oil Pipeline Subject to Differential Frost Heave in Permafrost Regions, Cold Reg Sci and Technol, 64,