Seismic Design

18 Seismic Design Chapter Outline 1. Intro Introdu duct ctio ion n 435 435 2. Seis Seismic mic Haza Hazard rdss 436 436 Surfac Surfacee Faulti Faulting ng 436 Land Landsl slid ides es 438 438 Liquef Liquefact actio ion n 438

3. Pipeli Pipeline ne Seis Seismic mic Design Design Guidel Guideline iness

438

Pipeline Pipeli ne Seismi Seismicc Desig Design n 438 Pipeli Pip eline ne Design Design Criter Criteria ia 440

4. Seismic Seismic Design Design Method Methodolo ology gy Static Static Analysi Analysiss of of Fault Fault Crossing Crossing Ground Ground Wave Analys Analysis is 442 Seismi Seismicc Level Level of Design Design 443

5. Analy Analysi siss Exam Exampl ple e

441 441

444 444

Buried Buried Pipelin Pipelinee Responses Responses for a Fault Fault Crossi Crossing ng 444 Responses Responses of of Unburied Unburied Pipelines Pipelines for a Ground Ground Wave 446

6. Mitig Mitigati ation on Method Methodss

447

Modifying Modifying Loading Loading and and Boundary Boundary Condition Conditionss Modifying Pipeline Con �gura gurati tion on 449 449 Modifying Modifying Pipeline Pipeline Route Route Selecti Selection on 449 Improving Improving Emergency Emergency Response Response 449

1.

449

Intr Introd oduc ucti tion on

When When a pipeli pipeline ne system system trave traverse rsess throug through h vario various us seismi seismicc damage damage areas, areas, many many potential damages, such as slope instability at scarp crossing, soil liquefaction, and fault movement, may hit the pipeline system. Different kinds of seismic hazards often impose hazardous geotechnical loads on subsea pipeline systems. In some extreme situations, the loads due to those seismic hazards may be so large that the subsea pipeline system yields and suffers plastic deformation. Damages and disruptions of the subsea pipelines caused by an earthquake may have severe effects on service life, sinc sincee it ma may y lead lead to sign signifi ifica cant nt financ financia iall loss loss due to serv servic icee inter interru rupti ption ons, s, fires fires,, explosions, and environmental contamination. Examples of such catastrophes include the 1964 Alaska Earthquake, the San Fernando Earthquake of 1971, the Guatemala Earthquake in 1976, the 1987 Ecuador Earthquake, the Kobe Earthquake in 1995, and Subsea Pipeline Design, Design, Analysis, Analysis, and Installatio Installation. n. http://dx.doi.org/10.1016/B978-0-12-386888-6.00018-3 http://dx.doi.org/10.1016/B978-0-12-386888-6.00018-3 Copyright  2014 Elsevier Inc. All rights reserved.

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Qiang Bai and Yong Bai

the 2003 Algeria Earthquake. A general conclusion drawn from a review of many earthquake events shows that, for buried steel pipelines, the direct effect of seismic ground waves on the integrity of long, straight pipelines is generally not significant. Where there is permanent ground deformation due to soil failure, there may be a severe influence on pipeline integrity. For unburied pipelines, both seismic ground waves and permanent ground deformation can cause severe damage, depending on the pipeline geometry and connected structures. Seismic ground waves produce strains in buried pipelines. However, because there are little or no inertia effects from dynamic excitation, the strains tend to be small and often are well within the yield rupture threshold of the pipeline material. The direct effect of seismic waves is, therefore, generally not expected to cause rupture or buckling failure to buried pipelines. Nonetheless, seismic waves can cause damage to unburied pipeline systems, especially in the interfacing area, such as in the pipeline transition section from buried to unburied and the pipeline tie-in spool to the subsequent structure. In general, the seismic analyses of the permanent ground deformation for buried pipes and unburied pipes and seismic ground waves for unburied pipes are required for designing pipeline systems. Many subsea pipelines are often buried for stability and mechanical protection in the shallow water area; otherwise, they are laid on the seabed. This chapter l

l

l

l

2.

Addresses available seismic design codes, standards, and design criteria for subsea pipelines. Discusses a general design and analysis methodology for fault crossing and seismic ground waves. Presents design and analysis examples using a static model for buried pipe, subjected to permanent ground deformations due to the foundation failure, and a time history dynamic model for unburied pipelines subjected to seismic ground waves. Summarizes the mitigation methods for subsea pipelines to avoid seismic hazards.

Seismic Hazards

Damage to pipeline systems during an earthquake, whether onshore or offshore, can arise from the traveling ground waves and permanent ground deformation due to soil failures. The primary soil failures are l

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Surface faulting. Landslides. Liquefaction. Differential settlement. Ground cracks. Seismic wave propagation.

Surface Faulting Surface faulting is the earth surface deformation associated with the relative displacement of adjacent parts of the surface crust. Surface fault displacements can

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occur rapidly during an earthquake. In addition, relatively minor displacements may accumulate gradually over many years as seismic creep. Surface fault crossing is one of the major hazards to subsea pipelines, whether buried or unburied. Numerous investigations have been carried out for fault crossing with different soil movements. Surface faulting is an important consideration for buried pipelines, because pipelines crossing fault zones must deform longitudinally and bend to accommodate the ground surface offsets. For subsea pipelines laid on the top of the seafloor, fault movements are generally of little, if any, consequence. However, it is possible that subsea faulting could produce a vertical offset that would cause a spanning pipeline to be elevated above the seafloor and may cause vortexinduced oscillations due to water currents, then cause fatigue damages of the pipeline. Figure 18.1 illustrates the classifications of fault movements, in which the surface faults are classified on the basis of their direction of movement with respect to the ground surface. A strike-slip fault is one in which the predominant component of ground movement is horizontal displacement. Normal-slip and reverse-slip faults are those in which the overlying side moves downward and upward, respectively, in relation to the underlying side of the fault. The amount and type of ground surface displacement is the main factor for designing pipelines to resist permanent ground deformation at fault crossings. Bonilla (1982) [2] summarizes a simple equation relating the maximum displacement at ground surface to the earthquake surface-wave magnitude as log L ¼ 6:35 þ 0:93 M s

[18.1]

where L is the maximum surface displacement in meters and M s is the earthquake surface wave magnitude. The earthquake magnitude is one of the design criteria based on historical seismicity and geological data. Displacement data from the fault of similar earthquakes might be used in selecting a value for designing pipelines, because of a big deviation in earthquake surface displacement data, on which the equation is based. The ability of a pipeline to deform in the plastic range under tension helps prevent rupture at fault crossings. If compression of the pipeline in a fault crossing is unavoidable, the compressive strain should be limited to within the local buckling criteria.

Figure 18.1 Classification of surface fault movement. Source: Honegger and Nyman [1].

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Landslides Landslides are mostly triggered by strong ground shaking during earthquakes, which include rock falls, disrupted soil slides, rock slides, soil slumps, soil block slides, and soil avalanches, as illustrated in Figure 18.2. The potential threat to pipeline performance includes the following parameters: l

l

l

l

The amount of landslide displacement. The depth of the landslide relative to the depth of the pipeline. The type of ground displacement associated with the landslide movement. The direction of landslide movement relative to the pipeline.

An approximate estimate of potential landslide movement can be made based on the existing slope and a general description of the near-surface material. Several commercial software packages are available for the analysis. Stress-deformation analyses have been used to estimate the permanent deformations caused by inertial instabilities. The strain potential and stiffness reduction approaches allow the estimation of permanent deformations from relatively simple analyses.

Liquefaction Liquefaction is the transformation of a saturated noncohesive soil from a solid to a more liquid state as a result of increased pore-water pressure and concomitant loss of shear strength. Liquefaction hazards to pipelines include pipeline flotation and sinking, which require the pipe to be located below the ground water table within a zone of liquefiable soil. Ground settlement occurs when a liquefiable soil layer is beneath a layer of competent or hard soil. If the pipeline is located in the layer of competent soil near the surface, it is subjected to displacement associated with subsidence of the ground. Loss of shear strength gives rise to bearing failures and large deformations in surface structures founded on liquefied soil. Assessing liquefaction potential is based on both peak ground acceleration and earthquake magnitude. Estimating the peak ground acceleration at the site of interest can be performed using probabilistic or deterministic approaches.

3.

Pipeline Seismic Design Guidelines

Pipeline Seismic Design Most loads of seismic hazards on a buried pipeline are due to the ground movements relative to the pipeline. The pipeline is deformed to match the ground movements, and the loads are displacement controlled. Pipelines buried with minimal soil cover or in relatively weak soils and subjected to high axial loads due to the ground movement may experience upheaval buckling. The current design code and guidelines for pipeline systems under displacement-controlled loads are basically strain-based design, in which the acceptable strain levels for the system based on limit-states design are limited (see Chapter 4, “Limit-State Based Design” of this book). The

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Rock fall (disruptive)

Rock topple (disruptive) Bluff line

Main scarp

Toe

e n b a r G S li p s u rf a

Earth slump (coherent)

c e

e i d g r r e s u s e P r

Earth block slide (coherent)

Weathered bedrock soil etc.

Source area Main track Depositional area

Bedrock Earth flow (occurs very slow to rapid and can be coherent to disruptive in nature)

Debris avalanche (disruptive and occurs very rapid to extremely rapid)

Figure 18.2 Various types of landslides. Source: Barnes [3].

limitation of pipeline strains associated with varying levels of performance is an ongoing area of investigation within the pipeline industry. For buried pipelines, the parameters of interest in the seismic design are the displacement and strain under the imposed permanent ground deformation due to foundation failure. The American Society of Civil Engineers, ASCE (1984) [4], collected some published systematic papers in seismic analysis and design as a standard, giving seismic design guidelines for oil and gas pipeline systems. These guidelines provide valuable information on seismic design considerations for pipelines, primarily

440


onshore-buried pipelines, and force-deformation curves of the pipe-soil interactions for pipelines buried in both clay and sand. ASCE (2001, 2002) [5], [6] also developed seismic design guidelines for onshore piping systems and buried pipes but not for petroleum pipelines and offshore pipelines. The American Society of Mechanical Engineers (ASME) states that the limit of calculated stresses due to occasional loads, such as wind or earthquake, should not exceed 80% of SMYS of the pipe, but this specification provides no guidance for the design method [7]. DNV in the code of “Submarine Pipeline Systems” [8] classifies the earthquake load into an accidental or environmental load, depending on the probability of earthquake occurrence. It also does not provide an earthquake design method for offshore pipelines [8]. However, in the selection and specification of a pipeline for a seismic design, the pipeline system may not be adequately addressed in a conventional stress-based design but a strainbased design. Considerable research efforts in the pipeline industry have been directed at understanding the behavior of pipe at high strains, with this effort increasing over the last few years with more focus on strain based design.

Pipeline Design Criteria Longitudinal tensile strains of 3–5% for assessing the ability of pipelines to maintain pressure integrity when subjected to earthquake-generated ground displacement were recommended in ASCE (1984) [4]. The failure strains in the strain-based design are estimated usually by fracture mechanics approaches, advancements have been made from the research and practices about the strain capacities of pipeline. The longitudinal compression strain limit is defined as follows: εcp

¼ 1:76

t

D

and

 4%

[18.2]

Test data from the available papers are plotted in Figure 18.3, with calculated strains from Eq. [18.2]. The tests performed by Mohareb et al. (1994) [9] focused on conditions where the axial load was constant, five of the seven analysis cases utilized a constant axial load. The axial force applied in the pipe tests by Mohareb et al. corresponds to a 45 C temperature differential, a tension force equivalent to the force necessary to counteract axial shortening from the Poisson effect of internal pressure, and a compressive force to counteract the tension produced by the closed-end conditions of the test specimens. Ghodsi et al. (1994) [10] repeated the tests of Mohareb et al. but included a girth weld in the center of the test specimen to assess the impact of a girth weld on the initiation of wrinkling. Zimmerman et al. (1995) [11] carried out pipeline tests to examine postwrinkling behavior, including a relationship for compression strain limits for X70 steel. The testing program carried out by Dorey et al. (2001) [12] concentrated on the investigation of strains associated with the initiation of wrinkling. These test data, compared with Eq. [18.2], were taken past the point of wrinkle formation and development of maximum pipe moment capacity.

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Figure 18.3 Comparison of recommended compression strain limits with test data. Source: Honegger and Nyman [1].

The longitudinal tension strain limit for pressure integrity when evaluating pipeline response to permanent ground deformation is defined as the follows: εtp

4.

 4%

[18.3]

Seismic Design Methodology

Several seismic analysis approaches for pipeline design were developed to predict the pipeline behavior in response to differential ground movements. Two main structural response models are considered: l

l

A static model for buried pipelines subjected to fault crossing due to soil failure. A dynamic analysis model for unburied pipelines subjected to ground wave load.

Static Analysis of Fault Crossing Two typical analytical methods under certain assumptions were suggested for the fault crossing analysis, Newmark and Hall (1975) [13] and Kennedy et al. (1977) [14]. Kennedy and others extended the ideas of Newmark and Hall and incorporated some

442


improvements in the method for evaluation of the maximum axial strain. They considered the effects of lateral interaction in their analyses. The influence of large axial strains on the pipe’s bending stiffness is also considered. O’Rourke and Liu (1999) [15] report that the Kennedy model for strike-slip faulting, which results in axial tension, provides the best match to ABAQUS finite element results, based on an independent comparison of the available analytical approaches. The ASCE guidelines give a detailed description of both the Newmark and Hall and Kennedy et al. schemes. It must be emphasized that both schemes are valid only for pipe under tension, since this condition may not be guaranteed under other various combined modes of fault movement. Due to the largely nonlinear nature of the problem, a finite element analysis (FEA) is the most general tool for pipeline fault crossing design. Nonlinear finite element modeling allows accurate determination of pipeline stress and strain at various locations along the pipeline route with a wide range of parameters. The pipe-soil interaction can be modeled as discrete springs in three dimensions. The pipeline is represented as a sequence of finite straight beam elements supported on the bottom by the bearing springs. The imposed fault movement is then input into the FE model as a static displacement boundary condition. The analysis is performed to determine the equilibrium nodal position of the pipe, bending moment, axial force, strains, and stresses. The next section explains a detailed example of finite element analysis for the fault crossing using the ABAQUS software.

Ground Wave Analysis Both permanent ground deformation and seismic ground wave can cause severe damage to unburied pipelines and connected equipment. Three basic methods are available for analyzing the responses of a structure subjected to seismic ground wave: l

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Static analysis. Response spectra analysis. Time history analysis.

In general, a static analysis is sufficient for the long-term response of a structure to applied loads. However, if the duration of the applied load is short, as in the case of an earthquake, a time history dynamic analysis is required. For the unburied pipeline, earthquake design motions are typically presented in the form of a seismic time history ground motion or a design response spectrum, which is based on the estimated ground waves and characteristics of the ground structure.

Static Analysis The pipeline is divided into individual spans or a series of segments. Static seismic loads are considered to be in direct proportion to the weight of pipe segments. The peak acceleration from the response spectrum is applied as a lateral force distributed along the pipe, and bending stresses and support reactions are calculated. The seismic static coefficients are usually obtained from the seismic “zone,” which corresponds to

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a level of seismic acceleration. Many design software programs can perform static analysis, but these methods are primarily used in building seismic design.

Response Spectra Analysis In response spectra analysis, the ground motion versus frequency method is used. The maximum acceleration for a given frequency and damping is determined based on seismic maps and soil characteristics. The higher the damping, the lower is its acceleration. The responses of displacements (translations and rotations), loads (forces and moments), and stresses at each point for each natural frequency of the system and for each direction are obtained after analysis. The calculated loads, displacements, and stresses of the piping system are typically calculated by taking the square root sum of squares of the response in each of the three directions. The response spectra method is approximate but often a useful, inexpensive method for preliminary design studies.

Time History Analysis This analysis method involves the actual solution of the dynamic equation of motion throughout the duration of the applied load and subsequent system vibration, providing a true simulation of the system response at all times. In time history analysis, the seismic time history ground motions (displacement, velocity, or acceleration as a function of time) of seismic ground waves in three directions are applied to a finite element model of a system to obtain time history excitations of the system, including stresses, strains, and reaction forces. Time history analysis is a more accurate, more computationally intensive method than response spectrum analysis and is best suited to the transient loadings where the profile is known. An example of time history analysis with a finite element model for the ground wave movement with ABAQUS software is detailed in the next section. ABAQUS software is the selected program to develop finite element models of ground soil, pipelines, and the subsea manifold connection because of its capability to accurately simulate solid objects, pipes, elbows, material and geometric nonlinearities, and interactions between soil and pipelines. The ABAQUS software also provides analytical models to describe the pipe-soil interaction. These models describe the elastic and perfectly plastic behavior by defining the force exerted on the pipeline and its displacement. These definitions are suitable for use with sands and clays and can be found in detail in the ASCE guidelines for the seismic design of oil and gas pipeline systems.

Seismic Level of Design Two design levels are normally adopted for the design criteria: l

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Contingency design earthquake (CDE). Probable design earthquake (PDE).

CDE represents a higher-level earthquake, established on the basis of a geoseismic evaluation with a typical return period of 200 to 1000 years for pipelines. The

444


intensity of CDE is taken as the design limits, exceeding causes of pipe failure or at least sufficient damage to cause an interruption of service. On the other hand, PDE is a lower-level earthquake, which assumes only minor damages to the pipeline system without interrupting the service. These events are likely to occur during the life of the pipeline and are therefore incorporated as part of the design environmental load. PDE is usually taken to have a return period of 50 to 100 years.

5.

Analysis Example

To explore the seismic responses of offshore pipeline systems, two study examples are presented [16]: l

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The static response of a 42-inch buried pipeline to permanent ground deformations, where the pipeline is fully buried under the natural seabed. The dynamic response of a 42-inch unburied pipeline system to seismic waves, where the pipeline is laid on the seabed and connected to a subsea manifold.

Buried Pipeline Responses for a Fault Crossing A buried steel pipeline with a 42-inch diameter and a 0.875-inch wall thickness, material of API 5L Grade-X65, contains oil at a specific gravity of 0.8. The pipeline is backfilled with a 3-foot sand depth median, with a density of 120 pounds per cubic foot and a friction angle of 35 . Figure 18.4 illustrates a buried pipeline under a fault crossing due to an earthquake. The fault length in the plan direction is set as 1.2 [m], in the vertical direction, set as 1.0 [m]. A static analysis of buried pipeline was analyzed using the ABAQUS software. Here, the unanchored length varies, depending on the pipeline size and axial pipe-soil interaction force (friction force). The 1000 [m] long pipeline, with both ends fixed, is modeled by using pipe elements in the example. Nonlinear pipeline-soil interactions in the axial, lateral, and vertical directions are modeled with pipe-soil interaction elements and soil characteristics in f t- xt, f p- yp, and f q- zq force-deformation curves. Based on the formulas suggested in the ASCE guidelines, the maximum axial interaction force per unit length at the pipe-soil interface, f t, of 36.6 [kN/m], and corresponding maximum deformation, xt, of 0.004 [m]. The maximum lateral interaction force per unit length f p of 175.4 [kN/m], and the corresponding maximum deformation yp of 0.08 [m]. The maximum upward interaction force per unit length f q of 38.0 [kN/m] and the corresponding maximum deformation zq of 0.044 [m]. The maximum downward interaction force per unit length f q is 1450 [kN/m] and the corresponding maximum deformation zq is 0.13 [m]. Figure 18.5 shows the displacements of the pipeline in the y and z directions under the fault crossing. The corresponding stress distribution at the bottom wall along the pipeline is shown in Figure 18.6. The maximum von Miss stress exceeds 80% of SMYS of the pipe, which does not satisfy the ASME criteria. Therefore, the designed buried pipeline is not suitable for the seismic level that can cause inputted fault distances.

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Y

Anchor point

Initial pipeline position

X

Anchor point

Ground motion Unanchored length

Unanchored length Plan

Unanchored length

Unanchored length Initial ground position

Z

Ground position after earthquake

X Anchor point

Initial pipeline position

Anchor point

Elevation

Figure 18.4 Buried pipeline under a fault crossing. (For color version of this figure, the reader is referred to the online version of this book.)

] 0.2 m [ , z

∆

&

0

y

∆

, s -0.2 n o i t c e r i d-0.4 z

,

y

n i s -0.6 t n e m e c -0.8 a l p s i D

-1

-100

-80

-60

-40

-20

0

20

40

60

80

100

Coordinate of x direction, [m] Displacement in y direction

Displacement in z direction

Figure 18.5 Deformations of pipeline in y and z directions. (For color version of this figure, the reader is referred to the online version of this book.)

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5.0E+08 4.0E+08 ] a P [ , e n i l e p i p g n o l a s e s s e r t

3.0E+08 2.0E+08 1.0E+08 0.0E+00

-1.0E+08

S

-2.0E+08 -3.0E+08 -100

-50

0

50

100

Pipeline axial direction, [m] Axial stress

Shear stress

Figure 18.6 Stress distributions at the bottom wall along the pipeline. (For color version of this figure, the reader is referred to the online version of this book.)

Sensitivity calculations of different buried depths of the pipeline also show that the maximum stress and strain of the pipeline are proportional to the buried depth, when other parameters are the same. To decrease the damage of the pipeline, in the possible area of the seismic fault cross, the pipeline should not be buried.

Responses of Unburied Pipelines for a Ground Wave A seismic dynamic analysis was performed as an example, using the ABAQUS software, for an offshore pipeline system. This pipeline system consists of two 4200 OD  0.87500 wt (API X65 pipelines) and a 300 tonne subsea manifold, as shown in Figure 18.7. The pipelines contained oil at a specific gravity of 0.8 with an internal pressure of 600 [psi]. A settlement of 0.1 [m] for the subsea manifold due to sand liquefaction in the earthquake is considered.

Figure 18.7 Subsea pipeline system, with a subsea manifold.

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A 10-second seismic event was used in the dynamic analysis. Figure 18.8 shows the acceleration time history in the E-W, N-S, and vertical directions. The maximum accelerations are 0.34 g, 0.26 g, and 0.25 g for E-W, N-S, and vertical directions, respectively. In the ABAQUS model, the subsea manifold is modeled as a solid box. The straight and curved pipeline sections are modeled as 3D beam elements and elbow elements, respectively. The seabed is modeled as a rigid surface with frictions in both longitudinal and lateral directions. The pipeline-soil interaction is modeled by a linear contact pressure relationship. The accelerations in three directions are applied to the seabed. As shown in Figure 18.9, the maximum Von Mises stress of 191.9 [MPa] (27.8 [ksi]) occurs at the spools. Figure 18.10 shows the time history of the maximum Von Mises stress in the pipelines. The maximum Von Mises stress in the time history always occurs in the spool areas. The difference of natural frequencies and weights for the subsea manifold and pipelines causes the response difference between subsea manifold and pipelines. Therefore, the maximum stress occurs in the spool areas. The seismic design and analysis methodology presented here was developed for subsea pipeline design. It has been successfully applied in seismic analyses of buried pipelines under fault crossing and unburied pipelines with a subsea manifold using a static analysis and a dynamic time history analysis. The sensitivity analysis results show that the buried depth of buried pipeline and the soil stiffness in the pipeline-soil interaction are the primary factors affecting pipeline stress in an earthquake.

6.

Mitigation Methods

Several mitigation methods improve postearthquake conditions if the pipeline response is found to exceed the acceptance criteria of pipeline seismic design. The mitigation method of a subsea pipeline under seismic load is selected based on the pipeline location, expected failure mode, potential for collateral damage, risk acceptance

) g ( n o i t a r e l e c c A

0.4

0.4

0.4

0.2


0

-0.2

-0.4

0.2


0

-0.2

2

4

6

8

10

Time (s)

(a) E-W acceleration

0

-0.2

-0.4 0

0.2

-0.4

0

2

4

6

8

10

Time (s)

(b) N-S acceleration

0

2

4

6

8

10

Time (s)

(c) Vertical acceleration

Figure 18.8 Seismic ground motions: E-W, N-S and vertical accelerations. (For color version of this figure, the reader is referred to the online version of this book.)

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Figure 18.9 Maximum Von Mises stress in the pipelines and tie-in spools. (For color version of this figure, the reader is referred to the online version of this book.)

philosophy, and estimated mitigation costs. According to the previous subsea pipeline seismic analysis experiences, the following mitigation options can be selected: l

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l

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Modify pipeline loading and boundary conditions. Modify pipeline configuration. Modify pipeline route. Improve emergency response. 200 MaxVM-MPa ) a P180 M ( s s e r t s s e160 s i M n o V . x a140 M

120 2.0

4.0

6.0 8.0 Time (s)

10.0

12.0

Figure 18.10 Time history of maximum von Mises stress. (For color version of this figure, the reader is referred to the online version of this book.)

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Modifying Loading and Boundary Conditions The capacity of a buried pipeline to withstand ground displacements can be improved by minimizing the soil resistance to pipe movements, in which the most common approach is to reduce the strength of the soil surrounding the pipeline or the frictional characteristics of the pipeline. The mitigation methods for a buried pipeline include l

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Place the subsea pipeline on the seabed instead of burying it to reduce the lateral soil resistance. Use smooth, low fiction coatings on the pipeline surface to reduce the axial soil friction resistance. Bury the pipeline in a shallow trench with loose backfill to release high restraints for the buried pipeline sections. However, this has limited applicability, due to other specifications and restraints for the subsea pipeline.

For the unburied pipeline, modifying the boundary condition at the pipe end is a suitable method to reduce the pipeline seismic stress.

Modifying Pipeline Con � guration The modifications also can be made by changing the welding condition, increasing the pipe wall thickness, using a high-strength steel grade for the pipe material, replacing sharp bends and elbows with induction bends or gradual pipeline field bends, and the like to increase the ability of subsea pipeline to resist the ground displacement. The allowable longitudinal compression strain increases with the increase of the pipe wall thickness, as shown in Eq. [18.2]. The bending and axial strength of the pipeline relative to the soil also is improved. Isolation valves, automatic or remotely controlled, may be provided on each side of the zones of ground displacement to mitigate the possible pipeline ruptures.

Modifying Pipeline Route Selection Soil loads on buried pipelines are the result of relative movement between the pipe and the surrounding soil. Select pipeline routing with suitable soil properties, as the site soil properties play an important role in the seismic analysis. In most cases, the pipeline fails due to soil instability or failure (such as land faulting, landslide, settlement, or liquefaction). Therefore, selecting a suitable pipeline route by maximizing the distance from the ground deformation zone to avoid bad geological areas and hazard helps prevent pipeline failure during strong earthquakes.

Improving Emergency Response The normal emergency response procedures are typically inadequate for dealing with postearthquake recovery, because multiple emergencies may be occurring simultaneously. Modifying and planning for the postearthquake response procedures to address the consequences of pipeline damage need to be coordinated with local and regional government authorities, as well as key customers.

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It is not sufficient to simply have an earthquake response plan. Because of the infrequent nature of earthquakes, regular earthquake simulation exercises are necessary to maintain personnel readiness and identify potential planning deficiencies. These exercises should be coordinated with local and regional planning exercises to identify coordination issues and take full advantage of current information on earthquake hazards and other earthquake damage that could risk a rapid response to pipeline damage.

References [1] Honegger DG, Nyman DJ. Guidelines for the seismic design and assessment of natural gas and liquid hydrocarbon pipelines. Contract PR-268-9823. Pipeline Research Council International, Inc, Arroyo Grande, CA; 2005. [2] Bonilla MG. Evaluation of potential surface faulting and other tectonic deformation. Open File Report 82-732. U.S. Geological Survey, Menlo Park, CA; 1982. [3] Varnes DJ DJ. Slope movement types and processes, landslides analysis and control. Special Report 176. Washington, DC: Transportation Research Board, National Academy of Sciences; 1978. [4] ASCE. Guidelines for the seismic design of oil and gas pipeline systems. Washington, DC: American Society of Civil Engineers; 1984. [5] ASCE. Guideline for the design of buried steel pipe. Washington, DC: American Society of Civil Engineers; 2001. [6] ASCE. Seismic Design and retrofit of piping systems. Washington, DC: American Society of Civil Engineers; 2002. [7] ASME. Pipeline transportation systems for liquid hydrocarbons and other liquids. ASME 31.4. New York: American Society of Mechanical Engineers; 2006. [8] DNV. Submarine pipeline systems. DNV-OS-F101. Det Norske Veritas, Norway; 2010. [9] Mohareb ME, Elwi AE, Kulak GL, Murray DW. Deformational behavior of line pipe. Structural Engineering Report 202. Edmonton, AB, Canada: Department of Civil Engineering, University of Alberta, Canada; 1994. [10] Ghodsi YN, Kulak GL, Murray DW. Behavior of girth-welded line pipe. Structural Engineering Report 203. Edmonton, AB, Canada: Department of Civil Engineering, University of Alberta; 1994. [11] Zimmerman TJE, Stephens MJ, DeGreer DD, Chen Q. Compressive strain limits for buried pipelines. In: Proceedings of the 1995 Offshore Mechanics and Arctic Engineering Conference, vol. 5. New York: American Society of Mechanical Engineers; 1995. pp. 365–78. [12] Dorey AB, Cheng JRR, Murray DW. Critical buckling strains for energy pipelines. Structural Engineering Report 237. Edmonton, AB, Canada: Department of Civil Engineering, University of Alberta; 2001. [13] Newmark NM, Hall WJ. Pipeline design to resist large fault displacements. Proc. US National Conference on Earthquake Engineering. Ann Arbor, MI; 1975. [14] Kennedy RP, Chow AW, Williamson RA. Fault movement effects on buried oil pipeline. J Transportation Engineering Division. ASCE 1977;103(TE5):617–33. [15] O’Rourke MJ, Liu X. Response of buried pipelines subject to earthquake effects. Monograph No. 3. Multidisciplinary Center for Earthquake Engineering Research Buffalo, NY; 1999. [16] Bai Q, Zeng W, Tao L. Seismic analysis of offshore pipeline systems. Offshore 2004;64(10):100–4.

Seismic Design

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