Method for damage mitigation of buried pipelines

Abstract

The invention is related to a method for designing and constructing fault crossing buried pipelines with mitigation against damages due to seismic displacements. According to the invention, a pipeline is designed such that a minimum free length is determined for which the pipeline would not deform more than a predefined maximum allowable deformation for an expected value of offset distance of the ground along the fault at the crossing point. Construction of a new pipeline and modification of an existing pipeline are also disclosed. A pipeline obtained according to the invention comprises a clearance around the pipeline for a distance of minimum free length and a clamp at a distance from the crossing determined according to a predefined portion of the minimum free length.

Description

[0001] DESCRIPTION

[0002] METHOD FOR DAMAGE MITIGATION OF BURIED PIPELINES

[0003] Technical Field

[0004] The invention is related to a method for designing and constructing fault crossing buried pipelines with mitigation against damages due to seismic displacements.

[0005] Prior Art

[0006] Pipelines may be required to pass seismically active areas and cross active faults. Such pipelines will be deformed following permanent ground deformation and therefore are under the risk of damages which can result in heavy environmental, social and economic costs. In case of pipelines crossing strike-slip faults, part of the pipeline is displaced laterally, inducing a strain concentration on the pipeline which may result in failure, and thus in spillage and service disruptions.

[0007] Interactions of pipelines and faults are classified as large strain problems. The permanent ground deformation (PGD) is the main damage source of segmented and continuous pipelines. The fault movement is applied through the soil behind the pipeline with respect to fault movement direction as passive soil pressure. During fault movement, the pipeline at both sides of the fault trace experiences two passive soil pressures with different directions. With increasing fault displacement, the deformation of the pipeline is localized over a short length of the pipeline at the vicinity of the fault trace. As a result, the local buckling, tensile strain and localization limit damage states will be reached.

[0008] In the state of the art, various trench filings, pipeline materials and structures like culverts are employed in order to provide protection for pipelines. However such precautions may not be sufficient for pipelines crossing faults. Seismic phenomena can produce significant localized ground deformations at such crossing points, damaging the pipeline through mechanisms such as shell-mode buckling, beam-mode buckling, pure tensile rupture, flexural failure and section ovalization.

[0009] In order to avoid such damage, a tensile strain limit of 3% has been adopted by the EN 1998- 4 provisions for seismic-fault induced action on buried steel pipelines. Also an ovalization limit of 15% is conventionally adopted to ensure continued operation. Although some mitigation methods have been proposed or applied in practice, their applicability and costs are still limiting their use. As the result, application of the effective methods has been limited to certain high budget projects with huge maintenance costs.

[0010] The document numbered US10106948B2 discloses a system in which deformable elements are provided along the pipeline. These deformable elements allow pipes to deform in a predefined manner, distributing the deformations along their length.

[0011] The document numbered JP2003119765A discloses a method for improving ground characteristics using blocks dispersed in the ground. The block allow shear deformation to be distributed over a larger volume.

[0012] The document numbered CN108240526A discloses a structure to provide seismic protection for pipelines. In an embodiment disclosed in the document, multiple culverts are arranged over an embankment covering the pipeline. It is mentioned that in case of a seismic displacement along the fault, the multiple culverts can be displaced without affecting the pipeline.

[0013] The document title Development of ‘Steel pipe for crossing fault (SPF)’using buckling pattern for water pipelines (Hasegawa, N., Nagamine, H., & Imai, T. (2014). JFE GIHO, 31, 61-65.) discloses a method for providing a predetermined buckling pattern to accommodate the deformations due to seismic events.

[0014] Objects and Brief Description of the Invention

[0015] An object of the invention is to provide a method for damage mitigation of buried pipelines crossing an active fault.

[0016] Another object of the invention is to provide a method for designing a pipeline crossing an active fault with damage mitigation.

[0017] Another object of the invention is to provide a method for modifying an existing pipeline crossing an active fault such that the modification allows damage mitigation.

[0018] According to the invention, through the clearance of the soil affecting the pipeline during fault displacement, a pipeline is designed such that a minimum free length is determined for which the pipeline would not deform more than predefined limit damage states for an expected value of offset amount of the fault at the crossing point. In an embodiment of the invention, a new pipeline is designed such that a clearance around the pipeline is provided for a distance of minimum free length, and a clamp is provided at a distance from the crossing determined according to a predefined portion of the minimum free length. In another embodiment of the invention, an existing pipeline is modified such that a clearance around the pipeline is provided for a distance of minimum free length and a clamp is provided at a distance from the crossing determined as a predefined fraction of the minimum free length.

[0019] According to the invention, a pipeline crossing a fault can be protected against damage during permanent ground deformation due to an earthquake or due to aseismic creep along the fault. A pipeline constructed or modified according to the invention will be protected against damage by employing the limit damage states information corresponding to a plurality of deformation modes and the relevant rates of deformation below which failure of the pipeline through mechanisms such as local buckling is avoided. By employing the limit damage states, the minimum free length of the pipeline is determined to avoid concentration of strain at or near the crossing point.

[0020] Detailed Description of the Invention

[0021] The method implemented according to the objects of the invention described with the use of the attached figures.

[0022] Figure 1 is a schematic drawing of a pipeline according to the prior art, crossing a strikeslip fault before (a) and after (b) a fault movement.

[0023] Figure 2 is a schematic drawing of a pipeline according to the invention, crossing a strikeslip fault before (a) and after (b) a fault movement.

[0024] Figure 3 is a schematic drawing of a pipeline according to the invention.

[0025] Figure 4 is a schematic cross section of the pipeline of Figure 3.

[0026] Figure 5 is a schematic drawing of a clamp according to the invention.

[0027] Figure 6 is a representation of the response of pipelines according to the prior art at different fault movements of 5, 10, 20, 30, 40, 50, 60, 70 and 80 cm in order from left to right.

[0028] Figure 7 is a representation of the response of pipelines according to the invention at different fault movements of 5, 10, 30, 50, 70, 90, 110, 130 and 150 cm in order from top to bottom. For simplicity, the clamp is shown only on the top and bottom pipelines. The distance marked on the bottom pipeline is equal to 24 m, corresponding to the free length employed for the bottom pipeline.

[0029] The features in the figures have been numbered and corresponding numbers are defined below.

[0030] 1. Pipeline

[0031] 2. Trench

[0032] 3. Clamp

[0033] 4. Filler

[0034] 5. First clearance

[0035] 6. Second clearance

[0036] 7. Retaining wall

[0037] 8. Covering element

[0038] 9. Sliding pad

[0039] 10. Block

[0040] 11. Hole

[0041] 12. Collar

[0042] F. Fault

[0043] E. Soil

[0044] The method for designing a pipeline (1) crossing an active fault (F) at a crossing point according to the invention, involves providing a first clearance (5) on a first side of the fault (F) and a second clearance (6) on a second side of the fault (F) along the pipeline (1) at the vicinity of the crossing point, such that at least a part of the pipeline (1) inside the first clearance (5) and the second clearance (6) is not fixed to the ground. The method essentially comprises the steps receiving soil (E) specification data, receiving pipeline (1) specification data, receiving fault (F) specification data including an expected value of offset distance of the ground along the fault (F) at the crossing point, and the angle between the fault (F) and the pipeline (1) at the crossing point, receiving limit damage states of the pipeline (1) at the crossing point, assigning one side of the fault (F) trace as the first side and the other as the second side and expressing the fault (F) movement as the movement of the first side with respect to the second side, assigning an initial free length, assigning an initial ratio as the ratio of the first clearance (5) length and the second clearance (6) length to the free length such that the sum of the first clearance (5) length and the second clearance (6) length is equal to the free length, providing a fixing region that transfers the movement of the first side of the fault (F) to the pipeline (1), positioned on the first side at a distance of first clearance (5) length from the crossing,

[0045] - verifying the sufficiency of the initial free length of the pipeline (1) based on the limit damage states and the expected offset distance along the fault (F), if the response for any deformation mode exceeds the relevant limit damage state, increasing the assigned free length and verifying the deformation using the new free length until a sufficient free length is found.

[0046] The data regarding limit damage states can include the limits for different deformation modes such as tensile strain limit and ovalization limit. These limits can be determined according to the standards and regulations regarding the pipeline (1) or correspond to the maximum allowable deformation that can sustain safe operating conditions of the pipeline (1) for a specific time interval, preferably the designed lifetime of the pipeline (1).

[0047] The expected offset distance along the fault (F) may be expressed as a lateral distance along the fault (F) trace or as a distance in two or three dimensions on the fault (F) plane.

[0048] Receiving data can be performed by retrieving the relevant data from a database, retrieving the relevant data from as an output of a simulation and / or retrieving the relevant data from a designer’s input through an input device. The method in general can be implemented by the use of a computer system.

[0049] The retrieved soil specification data can be compiled from geotechnical reports. The retrieved pipeline (1) data includes material, diameter, strength parameters and moduli and can be compiled from project requirements and the relevant standards and regulations.

[0050] The initial free length can be retrieved from a table of free lengths predefined corresponding to a pipeline (1) specification or retrieved from the designer’s input. The initial free length is preferably equal to 25 times the diameter of the pipeline (1). However, the initial free length can also be assigned to be equal to zero, at the expense of significant computational cost.

[0051] After verifying a free length satisfying the limit damage states for the expected offset, the free length can be optimized by the step if the responses for all deformation modes are below the limit damage states, decreasing the assigned free length and verifying the deformation using the new free length.

[0052] The initial ratio of the first clearance (5) length and the second clearance (6) length to the free length can be predefined or retrieved from the designer’s input. The initial ratio of the first clearance (5) length and the second clearance (6) length to the free length is preferably predefined as 0.60 and 0.40 respectively. This ratio can also be varied in order to determine the free length and / or to minimize the free length.

[0053] The fixing region acts on the pipeline (1) through soil (E) pressure to fix the pipeline (1) to the first side of the fault (F) at a distance of first clearance (5) length.

[0054] In a preferred embodiment of the invention, the fixing region comprises a clamp (3) that locally transfers the movement of the first side of the fault (F) to the pipeline (1). By confining the fixing region to a single point, the clamp (3) allows better control over the transfer of movement to the pipeline (1) and therefore over the deformation response of the pipeline (1).

[0055] A second fixing region may also be provided, positioned on the second side at a distance of second clearance (6) length from the crossing. The second fixing region may comprise a second clamp (3), in a similar manner to the fixing region.

[0056] According to a preferred embodiment of the invention, the free length of the pipeline (1) is verified by numerically determining the deformation of the pipeline (1) for a movement of the first side with respect to the second side for the expected offset distance along the fault (F) and comparing the response of the pipeline (1) with the limit damage states. However, analytical methods may also be applied instead of or together with the numerical simulations. In practice, the determined deformation usually is not exact and is an estimate within acceptable engineering tolerances. The use of the proper soil (E) - pipeline (1) interaction, soil (E) constitutive model and nonlinear analysis method able to be applied for the large deformation problems will increase the certainty of the design. Any analysis method including finite element, finite difference, boundary element etc. can be selected by the designer.

[0057] During the simulation, the modelled pipeline (1) undergoes gradually increasing fault (F) movement until the expected offset is reached. At the end of the simulation, the response of the pipeline (1) (including stresses, strains and cross-section change) are obtained and evaluated.

[0058] The invention also allows the compensation of uncertainties such as those in the exact fault (F) trace location. This is achieved by incorporating the uncertainty in the fault (F) specification data, by incorporating the uncertainty in the limit damage states, by increasing the free length by a factor depending on the uncertainty and / or by increasing the free length by a factor retrieved from the designer’s input based on engineering judgment.

[0059] A pipeline (1) crossing an active fault (F) at a crossing point can be constructed according to the design obtained according to the invention. Instead of constructing a new pipeline (1), an existing pipeline (1) crossing an active fault (F) at a crossing point can be modified according to the design obtained according to the invention. In order to modify an existing pipeline (1), according to the preferred embodiment of the invention, sections of the pipeline

[0060] (1) corresponding to the first clearance (5) and the second clearance (6) are unearthed and a clamp (3) is placed around the pipeline (1).

[0061] The pipeline (1) is buried along most of its length. This is achieved by excavating a trench

[0062] (2) along the planned path of the pipeline (1) into which the pipeline (1) is laid. The trench (2) is then filled with a filler (4) except at the first clearance (5) and the second clearance (6). The first clearance (5) and the second clearance (6) may be formed continuously. However according to a preferred embodiment of the invention, the first clearance (5) and the second clearance (6) are formed on the sides of the pipeline (1) opposite the fault (F) offset direction. Filler (4) can consist of soil (E) or other filling material.

[0063] The first clearance (5) and the second clearance (6) can be formed using a retaining wall (7) to protect the trench (2) wall by retaining the soil (E) and resisting the soil (E) pressure. Covering elements (8) can also be used to provide cover for the unburied portions of the pipeline (1). The use of covering elements (8) also allow surface human and animal traffic over the pipeline (1). Maintenance of the first clearance (5) and second clearance (6), if necessary, can be performed simply by temporarily removing the covering elements (8). The retaining walls (7) and the covering elements (8) can be made of timber or any other suitable material.

[0064] Preferably, a sliding pad (9) is also provided on the bottom surface of the trench (2) inside the first clearance (5) and the second clearance (6) to minimize the friction on the pipeline (1) during the relative motion of the pipeline (1) with respect to the trench (2). The sliding pad (9) can be made of timber or any other suitable material.

[0065] The clamp (3) may be selected from various elements and materials by the designer in accordance with the local geotechnical conditions and the accessible materials and budget, with the condition that the clamp (3) locally fixes the pipeline (1) to the ground with sufficient strength for the expected seismic conditions. The clamp (3) can also include a foundation structure.

[0066] The clamp (3) preferably comprises a concrete block (10) with a hole (11) through which the pipeline (1) passes. A collar (12) surrounding the pipeline (1) is also provided inside the hole (11). The collar (12) is preferably made of rubber or another elastomeric material. The collar (12) allows the pipeline (1) to be fitted to the hole (11) while also preventing the pipeline (1) to be damaged by the hole during the relative motion of the pipeline (1) with respect to the clamp (3).

[0067] The clamp (3) functions by transferring the offset due to the fault (F) to the pipeline (1) at a specific point which is determined by the first clearance (5) length. The first clearance (5), the second clearance (6) and the clamp (3) allow the pipeline (1) to deform only under the influence of the clamp (3) and therefore according to the transferred offset.

[0068] When a pipeline (1) obtained according to the invention is subjected to an offset along the fault (F), the pipeline (1) is deformed within the limit damage states under the influence of the clamp (3). During deformation, the first clearance (5) and the second clearance (6) prevent the pipeline (1) to be influenced by the passive pressure due to the shifting soil (E). The sliding pad (9) prevents the pipeline (1) to be influenced by friction and allow the movement of the pipeline (1) in a plane defined by the displacement vector along the fault (F) and the normal of the fault (F) plane at the crossing point. In case of the fault (F) being a strike-slip kind, the offset will be in the lateral direction and the pipeline (1) will be deformed in a horizontal plane. While the effect of the invention is most easily model for a strike-slip fault (F), throw and limited heave along the fault (F) plane can also be taken into account and accommodated.

[0069] In order to evaluate the efficiency of the method according to the invention, the behaviour of a pipeline (1) crossing a strike-slip fault (F) was investigated through numerical modelling and comparison for pipelines (1) according to the prior art and the invention. The comparison is performed for clayey soil (E) and a crossing angle of 90° which represents the most critical situation. The numerical modelling is performed by a fully nonlinear method based on explicit finite difference scheme. In this formulation, the incremental displacements of the continua are added to the coordinates so that the grid moves and deforms with the material that it represents. The behaviour of the soil (E) is modelled by a proper elasto-plastic constitutive model with a hardening and non-associated flow rule. The interaction between the shell-type structural element representing the pipeline (1) is implemented through an advanced scheme considering both normal-directed compressive / tensile and shear-directed frictional interaction as well as sliding and separation at large strain conditions. The pipeline (1) is modelled as a continuous API 5L X65 steel pipeline (1) with an outer diameter of 914.4 mm and diameter to wall thicknesses (D / t) of 96.

[0070] The deformation of the pipeline (1) according to the invention for a fault (F) movement of 150 cm is distributed over a pipeline (1) length of 24 m. This indicates the elimination of the local buckling damage mode for this critical crossing angle of 90° through the removal of the deformation concentration over short length. The location of the concrete clamp (3) and the length of the passive force-free sections provided by the first clearance (5) and second clearance (6) are determined for a tensile strain performance limit of 3%. The results show the minor effect of the cross-section ovalization limit.

[0071] For this example, a clamp (3) of cubic shape with dimensions of 4^4x4 m has been used. The design requirement of the clamp (3) is that its movement is similar with that of the fault (F). To do that, the clamp (3) is buried and covered with compacted soil at three sides. The confinement and its weight will assure its movement to be similar with the surrounding soil (E). In cases of uncertainty, this goal can be attained by nailing the clamp (3) to the soil (E) by piles. Also, although the main damage source of the pipeline (1) crossing a strike-slip fault (F) is permanent ground deformation, the transient loads may damage the pipeline (1) connection with the clamp (3). In order to avoid such damage, the rubber collar (12) is used. Another aspect of the invention is the free movement of the pipeline (1) along the passiveforce free length. Since the soil (E) mass in front of the pipeline (1) moves with the fault (F), it will not be obstacle for pipeline (1) movement. The only unwanted restriction is due to the friction between pipeline (1) and trench (2) base. To minimize this restriction, a sliding pad (9) is used.

[0072] The visualized result of the numerical modelling for pipelines according to prior art and pipelines (1) according to the invention are presented in Figures 6 and 7, respectively. As can be seen in these figures, the pipelines according to the prior art display concentrated deformations while the pipelines (1) according to the invention display deformations distributed along the free length.

[0073] Besides providing an effective solution for mitigating damage of a pipeline (1) due to fault (F) movement, the invention provides a simple, cheap and quickly constructable solution for both new and existing pipelines.

Claims

CLAIMS1. A method for designing a pipeline (1) crossing an active fault (F) at a crossing point wherein a first clearance (5) on a first side of the fault (F) and a second clearance (6) on a second side of the fault (F) along the pipeline (1) at the vicinity of the crossing point are provided, such that at least a part of the pipeline (1) inside the first clearance (5) and the second clearance (6) is not fixed to the ground; characterized by comprising the steps receiving soil (E) specification data, receiving pipeline (1) specification data, receiving fault (F) specification data including an expected value of offset distance of the ground along the fault (F) at the crossing point, and the angle between the fault (F) and the pipeline (1) at the crossing point, receiving limit damage states of the pipeline (1) at the crossing point, assigning one side of the fault (F) trace as the first side and the other as the second side and expressing the fault (F) movement as the movement of the first side with respect to the second side, assigning an initial free length, assigning an initial ratio as the ratio of the first clearance (5) length and the second clearance (6) length to the free length such that the sum of the first clearance (5) length and the second clearance (6) length is equal to the free length, providing a fixing region that transfers the movement of the first side of the fault (F) to the pipeline (1), positioned on the first side at a distance of first clearance (5) length from the crossing,- verifying the sufficiency of the initial free length of the pipeline (1) based on the limit damage states and the expected offset distance along the fault (F), if the response for any deformation mode exceeds the relevant limit damage state, increasing the assigned free length and verifying the deformation using the new free length until a sufficient free length is found.

2. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized in that the free length of the pipeline (1) is verified by numerical oranalytical determination of the deformation of the pipeline (1) for a movement of the first side with respect to the second side for the expected offset distance along the fault (F) and comparing the response of the pipeline (1) with the limit damage states.

3. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized by further comprising the step if the responses for all deformation modes are below the limit damage states, decreasing the assigned free length and verifying the deformation using the new free length to optimize the free length.

4. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized in that the initial free length is equal to 25 times the diameter of the pipeline (1).

5. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized in that the initial ratio of the first clearance (5) length and the second clearance (6) length to the free length is predefined as 0.60 and 0.40, respectively.

6. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized in that the ratio of the first clearance (5) length and the second clearance (6) length to the free length is varied.

7. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 for the compensation of an uncertainty characterized by incorporating the uncertainty in the fault (F) specification data, incorporating the uncertainty in the limit damage states and / or increasing the free length by a factor depending on the uncertainty.

8. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized in that a second fixing region is provided, positioned on the second side at a distance of second clearance (6) length from the crossing.

9. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 or 8 characterized in that the fixing region comprises a clamp (3) that locally transfers the movement of the first side of the fault (F) to the pipeline (1).

10. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized in that the first clearance (5) and the second clearance (6) are continuous.

11. A method for designing a pipeline (1) crossing an active fault (F) according to claim 1 characterized in that the first clearance (5) and the second clearance (6) are on the sides of the pipeline (1) opposite the fault (F) offset direction.

12. A method of constructing a pipeline (1) crossing an active fault (F) at a crossing point by excavating a trench (2) along the planned path of the pipeline (1), laying the pipeline (1) into the trench (2), filling the trench (2) with a filler (4) characterized in that the trench (2) is not filled with the filler (4) at a first clearance (5) and a second clearance (6), and that a fixing region is provided on a first side of the fault (F) at a first clearance (5) length from the crossing point, wherein the first clearance (5) and the second clearance (6) are formed according to any of claims 1-11.

13. A method of constructing a pipeline (1) crossing an active fault (F) according to claim 12 characterized in that the first clearance (5) and the second clearance (6) are formed by retaining walls (7) protecting the trench (2) wall.

14. A method of constructing a pipeline (1) crossing an active fault (F) according to claim 13 characterized in that covering elements (8) covering the unburied portions of the pipeline (1) are provided.

15. A method of constructing a pipeline (1) crossing an active fault (F) according to claim 12 characterized in that a sliding pad (9) minimizing the friction on the pipeline (1) is provided.

16. A method of constructing a pipeline (1) crossing an active fault (F) according to claim 12 characterized in that the fixing region comprises a clamp (3).

17. A method of modifying a pipeline (1) crossing an active fault (F) at a crossing point by unearthing a first clearance (5) and a second clearance (6) characterized in that a fixing region is provided on a first side of the fault (F) at a first clearance (5) length from the crossing point, and that the first clearance (5) and the second clearance (6) are formed according to any of claims 1-11.

18. A method of modifying a pipeline (1) crossing an active fault (F) according to claim 17 characterized in that the first clearance (5) and the second clearance (6) are formed by retaining walls (7) protecting the trench (2) wall.

19. A method of modifying a pipeline (1) crossing an active fault (F) according to claim 18 characterized in that covering elements (8) covering the unburied portions of the pipeline (1) are provided.

20. A method of modifying a pipeline (1) crossing an active fault (F) according to claim 17 characterized in that a sliding pad (9) minimizing the friction on the pipeline (1) is provided.

21. A method of modifying a pipeline (1) crossing an active fault (F) according to claim 17 characterized in that the fixing region comprises a clamp (3).

22. A pipeline (1) crossing an active fault (F) at a crossing point, positioned in a trench (2) characterized by comprising a filler (4) filling the trench (2) except at a first clearance (5) and a second clearance (6), and a fixing region positioned on a first side of the fault (F) at a first clearance (5) length from the crossing point wherein the first clearance (5) and the second clearance (6) have been formed according to any of claims 1-11.

23. A pipeline (1) crossing an active fault (F) according to claim 22 characterized by further comprising retaining walls (7) protecting the trench (2) walls of the first clearance (5) and the second clearance (6).

24. A pipeline (1) crossing an active fault (F) according to claim 23 characterized by further comprising covering elements (8) covering the unburied portions of the pipeline (1).

25. A pipeline (1) crossing an active fault (F) according to claim 22 characterized by further comprising a sliding pad (9) minimizing the friction on the pipeline (1).

26. A pipeline (1) crossing an active fault (F) according to claim 22 characterized by the fixing region comprising a clamp (3) having a concrete block (10) with a hole (11) through which the pipeline (1) passes.

27. A pipeline (1) crossing an active fault (F) according to claim 26 characterized by the clamp (3) further comprising a collar (12) surrounding the pipeline (1) inside the hole (H).

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