Submitted for presentation at the Eighth LIQUEFACTION HAZARD MITIGATION FOR OIL AND GAS PIPELINES

Submitted for presentation at the Eighth U.S. National Conference on Earthquake Engineering (100th Anniversary Conference Commemorating the 1906 San Francisco Earthquake), Earthquake Engineering Research Institute, San Francisco, CA, April 2006, and publication in the conference Proceedings.

Honegger/Nyman/Youd 1 Paper No. 1035

LIQUEFACTION HAZARD MITIGATION FOR OIL AND GAS PIPELINES

D.G. Honegger1, D.J. Nyman2 and T.L. Youd3

ABSTRACT

Potential for liquefaction-induced damage to oil and gas pipelines stems from ground deformation caused by flow failure, lateral spread, buoyant rise, ground settlement and ground oscillation. Practical approaches to assess and mitigate these hazards for pipelines have evolved significantly over the past decade. The mitigation process starts with assessment of liquefaction hazards along a pipeline route using geologic screening criteria. This screening is followed by a review of pipeline alignments to identify crossings posing a significant threat to pipeline integrity. Areas requiring subsurface investigation are identified to complete the assessment of liquefaction-induced hazards. The quantity and quality of available subsurface data are key to considering the relative merits of using empirical approaches or more complex continuum models to establish design ground displacements for evaluating pipeline response. Design strategies for crossingpotential liquefaction hazard zones are based upon empirically determined lateral spread displacements using the limited available geotechnical data for pipeline projects and non-linear finite element analysis of pipeline-soil interaction.

Introduction

Liquefaction poses a hazard to pipelines only when loss of soil shear strength leads to some form of permanent ground displacement or loss of stability. The primary liquefaction-related hazard to pipelines is lateral spread, which induces horizontal displacement of surficial (often competent) soil layers ranging up to several meters thick. These soil layers may slide down gentle slopes or toward a free face (e.g., an incised river channel or shoreline bluff). Lateral spread displacements may extend back as much as 100m or more from river channels and create tensional features such as open fissures at the head (up-slope) of the failure; shear deformation along the margins; and compressional features such as buckling at the toe.

Flow failure is the most catastrophic type of permanent ground deformation caused by liquefaction. Flow failure occurs on steeper slopes (greater than 6% or 3.5º) underlain by loose liquefiable soils. Flow failures are characterized by large lateral displacements (several meters or more) and severe internal disruption of the failure mass. Pipelines within the mobilized soil

1D.G. Honegger Consulting, 2690 Shetland Place, Arroyo Grande, CA 934202D.J. Nyman & Associates, 12337 Jones Road, Suite 232, Houston, TX 770703Professor, Brigham Young University, 368 CB, P.O. Box 24009, Provo, UT 84602


are usually displaced and can be severely damaged by the soil displacement. Identification ofpotential areas of flow failure is based upon the presence of liquefiable deposits resting on slopes greater than 6%. These areas must be avoided by the pipeline alignment unless the pipeline can be located below the depth of potential flow failure. Such avoidance is often achievable for local flow failures that might occur in steep riverbanks where the depth of failure is more likely to lie above the depth of pipeline burial.

Other forms of liquefaction-induced ground deformation affecting buried pipelines include buoyant rise, ground settlement, and enhanced ground oscillation. The type and extent of ground failure depends on site geometry and the depth, thickness, and extent of the liquefied layer. Ground displacements associated with these failure types are typically not a significant risk to new welded steel oil and natural gas transmission pipelines constructed in conformance with current welding practices. Pipelines located within liquefied soil layers can rise or fall if the reduction of strength of the liquefied soil is sufficient to allow positive or negative buoyancy of the pipeline. The tendency for buoyant rise is most common for pipelines installed without special weight coating as the weight of the pipe and contents are generally less than the displaced weight of the liquefied soil. Where negative buoyancy is provided by a concrete coating (e.g., at river crossings, marshes, and areas where the pipeline is laid below the water table), the pipeline will tend to settle if the reduced shear strength of the liquefied soil is not sufficient to provide the necessary bearing capacity.

Ground settlement can arise from volumetric contraction strains of liquefied soil deposits that occur as a result of particulate realignment and ejection of water to the ground surface. Settlements may be as much as 10% of the thickness of the liquefiable layer for loose sands, as much as 5% of the thickness for moderately dense sands, and as much as 1% for dense sands. Uniformly thick layers of homogeneous sediment usually settle rather evenly. Where granular layers vary in thickness or at geologic contacts, differential settlements typically occur.

Liquefaction-induced ground oscillation occurs on level ground as a result of decoupling of the liquefied soil from underlying non-liquefiable strata. This decoupling allows large transient ground motions or ground waves to develop, but permanent displacements are usually small and chaotic. Pipelines in areas of ground oscillation commonly are subjected to several cycles of straining during a liquefaction event.

New, well-constructed, welded steel oil and natural gas transmission pipelines are highly resistant to damage from the relatively small displacements associated with buoyant rise, settlement, and ground oscillation. Because of this high resistance, pipeline assessments based upon maintaining pressure integrity are often focused only on the potential for very large and very concentrated ground displacements associated with flow failures and lateral spread displacement. However, extensive exposure to post-earthquake repair cost can prompt the assessment of pipeline response for less severe liquefaction hazards. For example, the potentialneed to lower long segments of pipeline that would no longer have adequate soil cover following buoyant rise may dictate construction measures to assure pipeline vertical stability.


Challenges – Pipeline Lateral Spread Hazard Assessments

The delineation of lateral spread hazard for assessing pipeline response must include (1) identification of specific areas along the pipeline alignment potentially susceptible to lateral spreading, (2) variations in amounts and directions of potential spread displacements along the pipeline, and (3) the strength properties of the soil surrounding the pipeline within and outside of the zone of ground displacement. Unless detailed soil properties are defined (seldom available for most pipeline projects), rough estimates and considerable judgment are required to definecritical soil parameters. Thus, it is generally appropriate to use approximate empirical procedures and correlations to estimate ground displacements and spring constants for design.

The following discussion highlights some of the more important issues where judgment plays a dominant role in defining lateral spread hazards for pipeline crossings. For discussion purposes, a hypothetical pipeline river crossing configuration is provided in Figure 1. Questions that typically arise in defining the hazard for pipeline assessment are identified by the letter identifiers in Figure 1 (e.g., Q1, Q2, etc.) and referenced in the headings of the following sections.

Variation in Subsurface Conditions (Q1)

It is unusual to have sufficient subsurface soils information to fully characterize types and locations of various substrata along a pipeline alignment. For existing pipelines, there is anadditional limitation that subsurface information must be collected some distance from the pipeline alignment to avoid risks of pipeline damage. It is very common for soil profile information to be initially characterized using subsurface investigations performed for other nearby structures (e.g., bridges, tunnels) or based upon one or two borings on each riverbank. The cost associated with performing extensive subsurface investigations within a waterway are generally prohibitive, necessitating very approximate characterization of the variation of soil layers. Unfortunately, characterization of soils beneath the water surface is often essential fordeveloping reliable estimates of lateral ground displacement.

In addition to subsurface information adjacent to the pipeline at a river crossing, it is desirable to have subsurface information within a 100-m to 200-m wide zone to support judgments of the width and variations of displacements within the spread. However, geotechnical information is normally not obtained for locations outside of the pipeline right-of-way and, hence, is generally unavailable for the assessment of lateral spreads.

Direction of Lateral Spread Displacement (Q2)

In general, it can be assumed that the direction of lateral spread displacement will be perpendicular to a riverbank. However, variations of 10º to 15º from perpendicular commonly occur, and it is unlikely that the pipeline alignment will be perfectly perpendicular to the riverbank over the width of the lateral spread unless the bank has been artificially modified. This condition raises a question as to how the dominant direction of lateral spread displacement should be defined.


Length and Width of Lateral Spread (Q3)

Pipeline river crossings are typically constructed perpendicular to the riverbanks to provide the shortest possible crossing distance and the most favorable orientation of the pipeline relative to the direction of potential ground displacement. Therefore, it is not uncommon for apipeline alignment to change direction within 100m of a riverbank. Such changes of alignments raise issues related to the width of the zone of lateral spread (measured perpendicular to the direction of displacement) that should be considered for segments of the pipeline not alignedperpendicular to the river. Only limited and approximate investigations are available to guidejudgments on lateral spread dimensions even for relatively homogeneous site conditions (e.g., Honegger 1994).

Location of Lateral Spread along Riverbank (Q4)

Where similar soil stratigraphy and bank topography exists along a riverbank, the designer must estimate amounts and distributions of displacements that will affect the pipeline. The location of the lateral spread can be especially important when the pipeline alignment contains bends, valve stations, tie-ins, or other appurtenances in close proximity to the river crossing exposed to lateral spread displacement. Assuming that these appurtenances will be subjected to large displacements is generally the most severe scenario. However, it is generally more appropriate to address the most likely scenario, not simply the worst case.

Distribution of Lateral Spread Displacements within Spread Zone (Q5)

Once the dimensions of the lateral spread are defined, estimates of variation in the ground displacement are needed along the length and width of the spread zone. It is always possible to assume block-like displacement with abrupt boundaries between the spread zone and regions considered to be stationary, similar to that commonly assumed for fault crossings. This is a conservative assumption that may lead to significant overestimates of the risk to the pipeline. It is more reasonable to assume some distance over which the lateral spread displacementsattenuate near spread boundaries. Judgment is required in these instances to select development lengths when site-specific information that could provide some indication is unavailable.

Simultaneous Displacement of Both Riverbanks (Q6)

It is not uncommon for liquefaction susceptibility and the potential for lateral spread displacement to exist on both sides of a river crossing. The issue then arises as to whether or not it is reasonable to consider simultaneous displacements of both banks in the pipeline strain assessments. Where river channels lie in the mid-part of a floodplain, there is likelihood that simultaneous bank displacement can occur. In these instances, subtle variations in soil conditions due to depositional history may lead to different bank displacements. The authors are unaware of any focused studies that address the issue of simultaneous bank displacement. An approach that has been adopted on some projects is to consider the full predicted lateral spread displacement on one riverbank to occur simultaneously with no more than 50% of the predicted displacement on the opposite bank. The sum of the spread displacements from both banks can be taken less than the channel width, since once the channel is “choked” with displaced soil,


further spread action would be arrested. Where the channel is near the edge of an active flood plain, only displacement on the bank nearest to the center of the flood plain need be assumed.

Effects of Pipeline Trench (Q7)

Pipeline river crossings constructed using dredging techniques can result in trenches that extend 10m to 15m on either side of the pipeline, depending upon the trench depth and stability of the trench walls. Other than being placed in a very loose condition, the properties of the backfill are typically unknown. Furthermore, concerns about damaging the pipeline generally preclude testing the backfill material after placement. If the trench is constructed through non-liquefiable material, questions can arise with respect to the potential for liquefaction of the trench backfill. Liquefaction of the backfill could lead to buoyant rise, but lateral soil deformations will likely be small in narrow trenches and will occur above the pipeline. Thus, lateral displacement of trench backfill can generally be ignored.

Overview of Process for Identifying and Mitigating Lateral Spread Hazards

For major new oil and natural gas pipeline projects, assessments of liquefaction hazards along the pipeline route are typically performed using a systematic, multi-phase screening process. This process consists of preliminary screening of the pipeline alignment using geologic criteria, evaluation of site-specific information to estimate liquefaction susceptibility, estimation of lateral spread displacement hazard along pipeline, assessment of pipeline strains, and development of mitigation measures. This process requires logical assumptions with measured conservatism based upon the geologic and geotechnical information available for the site.

Preliminary Screening Assessment

Preliminary screening assessments are most often conducted as “desktop” studies using pipeline alignment maps, geologic maps, stereo aerial photographs, and available geotechnical data from soil borings and test pits along the alignment (normally performed to define pipeline trench excavation requirements and not necessarily liquefaction assessment). In the preliminary screening process, information on surface geology and liquefaction susceptibility criteria provide a means for identifying potential liquefaction hazard areas. Based upon the authors’ experience, inclusion of areas identified by susceptibility ratings of moderate or less are generally not warranted for specialized pipeline design. Adopting the recommendations in Youd and Perkins(1978), preliminary screening is typically focused on saturated soils that have a high to very high liquefaction susceptibility. This generally restricts consideration to granular soils loosely deposited in the past 500 years (e.g., river channel, flood plains, deltaic, esturine, and colluvium soils). Topographic maps and stereo aerial photographs are used primarily in the assessment of slopes and the identification of the margins of flood plains and marsh areas. Pipeline issues of importance in the screening phase are pipeline burial depth and type of construction at waterway crossings (e.g., conventional trench or horizontal directional drilling). A key decision in the preliminary screening process is the determination of the level of liquefaction susceptibility considered to be a significant pipeline hazard. Areas with ground slope less than 0.2% and areasat a distance more than about 50 times a free-face or bank height from a river or shoreline can generally be excluded as having low likelihood for significant lateral spread displacement.


The result from a preliminary screening is a systematic identification of segments in the pipeline alignment that warrant more detailed investigation to define lateral spread hazard. These additional investigations may include site reconnaissance to confirm the local geologic setting, additional geotechnical exploration, and topographic surveys.

Evaluation of Site-Specific Information

Information from site-specific investigations allows definition of subsurface stratigraphy, identification of depth and thickness of the liquefiable layers, determination of post-liquefaction strength of liquefiable soil deposits, and estimation of lateral spread displacement. Ideally, site specific investigations should include several borings for SPT and/or CPT soundings distributed across a water crossing or other potential liquefaction-susceptible zone. The test data from these investigations are used in empirical relationships to estimate lateral displacements and post-liquefaction soil strengths.

The amount of site-specific information is rarely adequate for rigorous determination of ground displacement and soil strengths, especially for long-distance pipelines that cross remote regions with many areas potentially susceptible to liquefaction. Practical constraints, such as, lack of site access, lack of available technical resources, delays in permitting and approval of the pipeline alignments, etc., generally result in limited information. Thus, determining liquefaction susceptibility and estimating lateral spread displacement involve require considerable experience-based judgments.

Estimation of Lateral Spread Displacement Hazard

Unless special circumstances warrant a state-of-the-art assessment of lateral spread displacement, typical practice relies upon empirical methods such as Youd et al. (2002). The primary advantage of empirical methods is that they provide a simple means to estimate lateral displacements with limited data requirements. Input parameters for empirical methods are typically related to descriptors of soil stratigraphy, local topography, and ground motion. Disadvantages of empirical methods include the inability to predict displacements at the pipeline elevation, inability to accommodate complex surface or subsurface features, and implicit reliance on similarity between sites of past lateral spreads and the site in question.

For sites with simple topography and stratigraphy (e.g., constant slope or a single free-face underlain by horizontal soil layers of uniform thicknesses), empirical procedures yield adequate estimates of lateral displacement, but the distribution of displacements at depth must be presumed. Estimating subsurface displacement requires assumptions related to how sliding planes develop within liquefiable soil layers. One of two approximations is generally applied: (1) constant ground displacement equal to the surface displacement is assumed to the bottom of the liquefiable soil layer; or (2) a linear decrease in displacement is assumed between the top and bottom of the liquefiable layer. Each assumption requires judgments regarding the variation in residual shear strength within the liquefiable soil layer (e.g., relatively constant or linear variation with depth) and the uniformity of the liquefiable layer thickness.


Assessment of Pipeline Response

The analysis of strain in a buried pipeline subjected to lateral spread displacements generally requires finite element analysis that accounts for nonlinear behavior of the pipeline and surrounding soil and large displacement effects. Inelastic pipe behavior is simulated by specifying a nonlinear stress-strain curve for the pipeline steel. Typical elements used in determining pipeline response account for nonlinear stress-strain behavior, but not compressive buckling or wrinkling of the pipe wall. Consequently, it is necessary to apply test-based compressive strain criteria that normalize axial strains over a particular length of pipeline encompassing the wrinkled or buckled sections (usually about one to two pipe diameters). The segment of the pipeline used in the analysis model should extend away from the zone of ground displacement to points of virtual soil anchorage. If pipe bends or other discontinuities exist near the zone of displacement, they could generate virtual anchor points that should be included in the analysis. Soil-pipeline interaction can be modeled by discrete nonlinear springs oriented in the axial, horizontal, and vertical directions. The methodology for implementing such analyses is well-established and described in available in industry guidance documents (e.g., Honegger and Nyman 2004).

For rare events such as earthquakes, the most common performance requirement for oil and natural gas pipelines is that pressure integrity be maintained. This performance requirement is based upon the premise that local distortion of the pipe wall may occur, requiring post-earthquake repairs. Pressure integrity strain limits are based upon consideration of the pipe material, wall thickness, welding specifications, welding inspection, minimum detectable weld flaw sizes, and expected weld quality. The establishment of appropriate pipe strain limits requires the collective judgment of pipeline engineers and welding and material specialists.

The assessment of buoyancy effects from liquefaction involves determination of whether or not sufficient resistance to vertical buoyancy forces is provided by the weight of the pipeline and contents and resistance of the surrounding soil to pipeline uplift. This approach differs from the typical pipeline practice of neglecting soil resistance for pipeline installations below the water table. While typical pipeline practice is generally appropriate for long-term buoyancy loadconditions, it is much too conservative for assessing pipeline response under the relatively short-term loading resulting from liquefaction. Accounting for soil resistance requires an estimate of the shear strength of the liquefied soil around the pipeline as well as non-liquefied soil that may be present above the liquefied soil layer.

Mitigation Measures

Lateral spread hazard for pipelines can be mitigated by crossing rivers with alignmentsthat minimize transverse displacement while taking full advantage of tensile capacity of the pipe to withstand displacements parallel to the pipeline, placing the pipeline at sufficient depth to avoid the areas of highest ground deformation, and limiting the length of pipeline exposed to ground displacement. For projects with numerous river crossings, it may be beneficial to develop standard crossing concepts based on bounds of lateral spread displacements determined for all crossings in segments within a region characterized by uniform seismicity. These guidelines should specify the location of the crossing, the crossing angle, the depth of pipeline


burial, the geometry for transitions from deep burial beneath the crossing to nominal burialdepth, and pipeline alignment restrictions on either side of the crossing. Recommendations for preferred crossing alignments can be based upon three parameters as illustrated by the example provided in Figure 2. As noted in Figure 2, recommended crossing configurations for lateral spread hazards will often differ considerably from the crossing alignments that would typically be constructed in the absence of such hazards.

The effects of buoyancy are typically mitigated by employing one or more of the following measures: increasing the weight of the pipeline through the use of concrete coating or set-on weights, increasing the depth of the pipeline to assure that sufficient soil cover remains following buoyant rise, and installing backfill materials with improved liquefaction resistance (e.g., compacted fill or freely draining gravel).

Conclusions

Lateral spread displacements and buoyancy effects are the most significant liquefactionhazards to oil and natural gas pipelines. Current methodology for defining lateral spread hazardto pipelines will often result in reliance on experience and judgment to define ground displacement characteristics, which can lead to considerable uncertainty in assessing pipeline response. Extra design conservatism is warranted in such instances to increase pipeline safety. Careful planning of pipeline alignments through zones of potential lateral spread displacement can substantially reduce this uncertainty by optimizing pipeline alignments to take full advantageof the inherent strength and ductility of the pipe steel and girth welds. The economic impacts of buoyancy effects can be mitigated by providing sufficient resistance to buoyant forces or by modifying the liquefaction susceptibility of the backfill.

References

Honegger, D.G. and D.J. Nyman, 2004. Guidelines for the Design and Assessment of Natural Gas and Liquid Hydrocarbon Pipelines, Report L51927, Pipeline Research Council International, Arlington, VA.

Honegger, D.G., 1994. “Assessing Vulnerability of BC Gas Pipelines to Lateral Spread Hazards,” Proceedings of the Fifth US-Japan Workshop on Earthquake-Resistant Design of Lifeline Facilities and Countermeasures for Soil Liquefaction, edited by T.D. O’Rourke and M. Hamada, Technical Report NCEER-94-0026, State University of New York at Buffalo, NY.

Youd, T.L., C.M. Hansen, and S.F. Bartlett, 2002. Revised Multilinear Regression Equations for Predicting Lateral Spread Displacement, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 128, No. 12, American Society of Civil Engineers, Reston, VA.

Youd, T.L. and D. M. Perkins, 1978. Mapping of Liquefaction Induced Ground Failure Potential, J. Geotechnical Engineering Division, v. 104, no. GT4, American Society of Civil Engineers, Reston, VA, p. 433-446.


Figure 1. Illustration Depicting Typical Issues Related to the Definition of Lateral Spread Hazards for Pipeline Crossings.


1. Pipeline Crossing Location and Alignmenta. The river crossing should be made at locations where the river channel is essentially straight for a

minimum distance of DS and preferably 2DS. b. The pipeline should cross the channel at a perpendicular angle. c. If the river channel meanders such that an essentially straight segment is not available for the

crossing, then the pipeline should be aligned so as to bisect a meander and cross the channel in a direction perpendicular to the tangent of the channel at the point of crossing.

2. Crossing Burial Depth –The pipeline should pass under the river channel in a horizontal plane at an elevation such that the elevation of the top-of-pipe is greater than DL. This crossing depth should be maintained at this elevation for a distance each side of the channel as illustrated in Figures A and B and defined as follows:

a. For essentially straight river channels, maintain a minimum deep-burial distance of DS on either side of the channel banks.

b. At the inside bank of a bend in a curved river channel, a deep burial distance of DS is required beyond the edge of the stream bank or to a point of intersection between the pipeline alignment and a 2DS channel chord (at least DS on either side of the pipeline) perpendicular to the pipeline alignment, whichever is greater.

c. At the outside bank of a bend in a curved or meandering channel, a minimum distance of DS from the channel bank.

3. Adjacent River Channels –The pipeline alignment shall be maintained at a minimum separation distance of SD from the nearest significant river channel (rivers with channel banks 1m or greater in height). If not, the pipeline should be buried with a top-of-pipe elevation greater than DL until a separation of SD is achieved.

4. Pipe Slope Transition –The pipeline shall transition from full river crossing burial depth to standard depth on a slope not to exceed 10 percent (1 vertical : 10 horizontal). Vertical bends at the bottom and top of this transition shall not exceed a standard field bend radius. Free stress bending in lieu of field bends is permitted provided the 10 percent transition slope is not exceeded.

5. Setback Distance –The pipeline shall be extended in a straight plan alignment each side of the river channel for a total distance of SD, or to the edge of the floodplain, whichever is shorter.

Figure 2. Example of General Recommendations to Mitigate the Effect of Lateral Spread Displacement at Pipeline River Crossings.

Submitted for presentation at the Eighth LIQUEFACTION HAZARD MITIGATION FOR OIL AND GAS PIPELINES

Documents

Transcript of Submitted for presentation at the Eighth LIQUEFACTION HAZARD MITIGATION FOR OIL AND GAS PIPELINES