Part II Pipeline Design Chapter 14 Seismic Design 14.1 Introduction
Oil and gas pipeline routes often pass through large geographical areas, from the supply point to the end-user, end-user, crossing crossing seismic-acti seismic-active ve areas. Earthquake dam age to oil and gas pipelines ca cause significant financial loss, including secondary losses resulting in service interruption, fires fires,, explosions, and environmental contamination. contamination. Exam ples of such such catastrophes 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 1995 and the 2003 Algeria Earthquake. Earthquake. A general conclusion conclusion drawn drawn om a review review of many earthquake earthquake ev ents shows that, for buried steel pipelines, the direct effect of seismic ground wave on the integrity of long long and straight pipeline s is generally not significant. significant. here there is perm anent ground deformatio deformationn due to soil failure, failure, there may be a severe influence influence upon pipeline integrity. integrity. For unburied pipelines, both seismic ground wave and permanent ground deformation can cause severe severe damage to p ipelines, depending depending on the pipeline pipeline g eometry and connected structures. structures. Dam age to pipeline pipeline systems systems during an earthquake, whether onshore or offshore, offshore, can arise om the traveling ground waves and permanent ground deformation due to soil failures. The primary soil failures are: •
Faulting;
•
Landslides;
•
Liquefaction;
•
Different Differential ial Settlemen t;
•
Ground-cracks.
Seismic ground ground waves produce strains strains in in buried buried pipelines. How ever, because there there are littl littl or no inertia effects from dynamic excitation, the strains tend to be small and often are well within the yield yield rupture threshold threshold of the the pipeline material. The direct effec effectt of seismic wave 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 deformation or buried buried pipes and unburied p ipes, and and seismic seismic ground wav es for unburied pipes are required for designing pipeline systems.
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Offshore pipelines are normally buried for stability and mechanical protection; otherwise they are laid on the seabed. T his Chapter will: •
Ad dress available seismic design code s and standards for offshore pipelines;
•
Discuss a general design and analysis methodology for fault crossing and seismic ground wave;
•
Present design and analysis exam ples using a static model for buried pipe, subjected to permanent ground deformations due to the foundation failure and a time history dynamic m odel for unburied pipelines subjected to seismic ground w aves.
14.2 Pipeline Seismic Design Guidelines
The American Society of Civil Engineers, ASCE (1984) collected some published systematical 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 onshore-buried pipelines, and also force-deformation curves of the pipe-soil interactions for pipelines buried in both clay and sand. ASCE (2001, 2002) has 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, shall not exceed 80% of SMYS of the pipe, but this specification does not provide guidance for the design method. Det Norske Veritas (DNV) in the code of "Submarine Pipeline Systems" classifies the earthquake load into accidental or an environmental load depending on the probability of earthquake occurrence. It also does not provide an earthquake design m ethod for offshore p ipelines. The current Design Code and Guidelines for pipeline systems basically specifies the loads for analysis and the acceptable stress/strain levels for the system design. For buried pipelines, the parameters of interest are the displacement, stress and strain under the imposed permanent ground deformation due to foundation failure. Although the mechanism of the seismic foundation failure varies for different types, a pipeline response model can be generated with only minor modifications. For the unburied pipeline, earthquake design mo tions are typicall presented in the form of seismic time history ground motion or a design response spectrum, which is based upon the estimated ground waves and characteristics of the ground structure.
14.2.1 Seismic Design M ethodology
Several seismic analysis approaches for pipeline design were developed to predict the pipeline behavior in response to differential ground mo vem ents. Two main structural response models are considered: 1. Static Model for Buried Pipelines, subjected to fault crossing due to soil failure. 2. Dynamic Analysis Model for Unburied Pipelines, subjected to ground wave load.
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Chapter 14 Seismic Design 14.2.1.1 Static An alysis of Fault C rossing
Fault crossing is one of the major hazards to offshore pipelines, whether buried or unburied. Numerous investigations have been carried out for fault crossing with different soil mo vemen ts. The ability of a pipeline to deform n 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. 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) summarized a simple equation relating the maximum displacement at ground surface to the earthquake surface-wave magnitude as: og Z =- 6. 35 + 0.93M,
^^
where, is the maximum surface displacement in meters and is the earthquake surfacewave magnitude. The earthquake magnitude is one of the design criteriae based on the 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 n earthquake surface displacement data, which the equation is based on Two typical analytical methods under certain assumptions were suggested for the fault crossing analysis, New mark-Hall (1975) and Kennedy et al (1977). Kennedy and others extended the ideas of Newmark and Hall and incorporated some 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 stifftiess is also considered. O'R ourke and Liu (1999) reported that the K ennedy mo del 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-Hall and Kennedy schemes. It must be emphasized that both schemes are only valid for pipe under tension, since this condition may not be guaranteed under other various combined m odes of fault m ovement. Due to the largely non-linear nature of the problem, a finite element analysis (FEA) is the most general tool for pipeline fault crossing d esign. Non-linear finite element modeling allows accurate determination of pipeline stress/strain at various locations along the pipeline route with a w ide range of param eters. The pipe-soil interaction can be modeled as discrete springs n three dim ensions. 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 equ ilibrium nodal position of the pipe, bending m oment, axial force, strains and stresses. The next section explains a detailed example of finite element analysis for the fault crossing using ABA QUS software.
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14.2.1.2 Ground Wave Analysis
Both permanent ground deformation and seismic ground wave can cause severe damages to unburied pipelines and connected equipments. There are three basic methods available for analyzing the responses of a structure subjected to seismic ground w ave, 1. Static An alysis; Response Spectra Analysis; Time H istory An alysis. In general, a static analysis is sufficient for the long-term response of a structure to applied loads. How ever, if the duration of the applied load is short, such as in the case of an earthquake event, a time h istory dynamic analysis is required. Static Analysis
The pipeline is divided into individual spans or into a series of segments. Static seismic loads are considered to be in direct proportion to the w eight of pipe segme nts. 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 is corresponding to 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
n response spectra analysis, the ground m otion vs. frequ enc y method is used. The maximum acceleration or a given frequ ency and damping is determined based on seismic m aps and soil characteristics. The higher the damping, the lower its acceleration will be. 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 is 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 respo nse at all times. In time history analysis, the seismic time history ground motions (displacement, velocity or acceleration as a ftmction of time) of seismic ground waves in three directions are applied to a finite element model of 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.
Chapter 14 Seismic Design
22
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 is the selected program to develop finite element models of ground soil, pipelines and subsea manifold connection because of its capability to accurately simulate solid objects, pipes, elbows, material and geometric non-linearities, and interactions between soil and pipelines. ABAQUS 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 G as Pipeline Systems.
14.2.2 Seismic Level of Design
Two design levels are normally adopted for the design criteria: 1. Contingency design earthquake (CD E), an Probable design earthquake (PD E). The CDE represents a higher-level earthquake, established on the basis of a geo-seismic evaluation with a typical return period of 200 to 1000 years for pipelines. The 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, the 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.
14.2.3 Analysis Examples
To explore the seismic responses of offshore pipeline systems, two study examples are presented here: Static response of a 42-inch buried pipeline to permanent ground deformations where the pipeline is fully buried under the natural seabed. 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.
14.2.3.1 Buried Pipeline Responses for a Fault C rossing
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°.
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ANCHOR POINT INITIAL PIPELINE POSITION ANCHOR POINT
-UNANCHORED
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+
UNANCHORED LENG TH-
/-INITIAL GROUND POSITION A GROUND POSITION _^^^TER_EARTHOUAKE
INITIAL PIPELINE POSITION
ANCHOR POINT-'
ELEVATION
Figure 14.1 Buried pipeline under
fault crossing.
Figure 14.1 shows a sketch of 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, with set as 1. m. A static analysis of buried pipeline was analyzed by using ABAQUS, the Finite Element 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. Non-linear pipeline-soil interactions in axial, lateral, and vertical directions are modeled with pipe-soil interaction elements and soil characteristics in ft-Xt,fp-yp and fq-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 (ft is 36.6 kN/m, and corresponding maximum deformation, Xt) is 0.004 m. The maximum lateral interaction force per unit length (fp is 175.4 kN/m, and corresponding maximum deformation, yp) is 0.08 m. The maximum upward interaction force per unit length fq is 38.0 kN/m and corresponding maximum deformation zq is 0.044 m. The maximum downward interaction force per unit length fq is 1450 kN /m and corresponding maximum deformatio is 0.13 m. Figure 14.2 shows the displacements of the pipeline in y and z directions under the fault crossing. The corresponding stress distribution at the bottom wall along the pipeline is shown in Figure 14.3. The maximum stress exceeds 80% of SMYS of the pipe, which is within ASME criteria. Therefore, the designed buried pipeline is not suitable for the seismic level which can cause inputted fault distances.
Chapter 14 Seismic Design
22
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Figure 14.3 Stress distributions at the bottom wall along the pipeline. 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 dep th, when other parameters are the same. decrease the damage the pipeline, the possible area the seismic fault cross, the pipeline should not be buried. 14.2.3.2 Responses of Unburied Pipelines for
Ground Wave
A seismic dynamic analysis was performed, using ABAQUS, for an offshore pipeline system. This analysis consisted of two 42 " OD x 0.875" WT (API X65 pipelines) and a 300 metric ton subsea manifold, as shown in Figure 14.5. The pipelines contained oil t a specific gravity of 0.8 with an internal pressure of 60 psi. A settlement of 0. m for the subsea m anifold due to sand liquefaction in the earthquake, is considered.
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Figure 14.5 Offshore pipeline system, with a subsea manifold.
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Figure 14.6 Seismic groun d motions: E-W, N-S and vertical accelerations.
A 10-second seismic event was used in the dynamic analysis. Figure 14.6 shows the acceleration time history in the E-W, N -S and Vertical directions. The maximum accelerations are 0.34g, 0.26g and 0.25g for E-W, N-S and vertical directions, respectively. In the ABAQUS model, the subsea manifold was modeled as a solid box. The straight and curved pipeline sections were modeled as 3D beam elements and elbow elements, respectively. The seabed was modeled as a rigid surface with frictions in both longitudinal an lateral directions. The pipeline-soil interaction was modeled by a linear contact pressure relationship. The accelerations in three directions were applied to the seabed. As shown in Figure 14.7, the maximum Von Mises stress of 191.9 MPa (27.8 ksi) occurs at the spools. Figure 14.8 shows the time history of the maximum Von ises stress in the pipelines.
Chapter 14 Seismic Design
22
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The maximum Von Mises stresses in tlie 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.
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Part II Pipeline Design
22 14.3 Conclusions
This seismic design and analysis methodology as presented here was developed for offshore pipeline design. It has been successfully applied in seismic analyses of buried pipelines under fault crossing and unburied pipelines with a subsea manifold by 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. As discussed, the seismic analysis within this technical note is intended for assistance in developing seismic analysis and design guidelines for offshore p ipelines.
14.4 References 1. Bai, Q., Zeng, W., and Tao, L. (2004), "Seismic Analysis of Offshore Pipeline Systems", Offshore, V ol. 64, No . 10, 2004, pp.100-104. 2. ASCE , (1984), "Guidelines fo th Seismic Design of Oil and Gas Pipeline Systems". 3. ASCE , (200 1), "G uideline fo
Design of Buried Steel Pipe".
4. ASCE , (2002), "Seismic Design and Retrofit of Piping Systems".
5. ASM E B 31.4, (1998), "Pipeline Transportation System for Liquid Hydrocarbons an Liquids". 6. Bai, Y., (20 03), "Marine Structural Design", Elsevier. 7. Bonilla, M . G., (1982 ), "Evaluation of Potential Surface Faulting and other Tectonic Deformation", Open File Report 82-732, U.S. Geological Survey. 8. DN V-O S-FIOIDNV, (2000), "Submarine Pipeline Systems", Det Norske Veritas. 9. Kennedy, R. P., Chow, A. W., and Williamson, R. A., (1977), "Fau t ovement Effects on Buried Oil Pipeline", Journal of the Transportation Engineering D ivision , ASCE, V o. 103, TE5, pp. 61 7-633. lO.Newmark, N. M. and Hall, W. J., (1975), "Pipeline Design to Resist Large Fault Displacements", Proc. US National Conference on Earthquake Engineering, Ann Arbor, Michigan. 11. O'Rourke, M. J. and Liu, X., (1999), "Response of Buried Pipelines Subject to Earthquake Effects", Monograph No.3, Multidisciplinary Center for Earthquake Engineering Research.
