Project

Experimental & Computational Hybrid Assessment of Natural Gas Pipelines Exposed to Seismic Risk

Goal: EXCHANGE-Risk is a H2020 research and mobility project lead by the University of Bristol with the aim to:

- Promote transfer of know how between the academic and industrial sector in Europe, United States and Canada on Seismic risk of natural gas pipelines
- Innovative methods for pre- and post-earthquake pipeline inspection, system monitoring and network resilience
- Intercontinental, multiple-support, hybrid experimentation utilizing experimental and computational facilities among the project partners

Date: 1 January 2016 - 31 December 2019

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Project log

Anastasios Sextos
added a research item
Though often overlooked, the impact of seismic transient ground deformation on natural gas (NG) pipes can be highly adverse. Particularly, pipe elbows may undergo excessive in-plane bending demand and buckling. In this paper, a critical scenario of a pipe coupling two industrial structures typically found in an NG processing plant is studied. High strain and cross-sectional ovalization on the elbows are probable during an earthquake due to the out-of-phase oscillation of the two structures imposing asynchronous displacement demands at the two pipe-ends. A parametric study was first performed to investigate various structure-pipe-structure configurations which increase seismic demands to pipe elbows. Simultaneous mobilisation of divergent oscillation between two supporting structures at the low-frequency range, a lower pipe-structure stiffness ratio, a shorter length of straight pipe segments in the linking pipe element and a higher pipe internal pressure have led to the onset of critical strain demands in pipe elbows.
Anastasios Sextos
added 3 research items
This paper reports on results from a series of 1-g, reduced-scale shake table tests of a 216-m-long portion of an onshore steel gas transmission pipeline embedded in horizontally layered soil. A set of first-order dynamic similitude laws was employed to scale system parameters appropriately. Two sands of different mean grain diameter and bulk density were used to assemble a compound symmetrical test soil consisting of three uniform blocks in a dense-loose-dense configuration. The sand-pipe interface friction coefficients were measured as 0.23 and 0.27. Modulated harmonics and recorded ground motions were applied as table excitation. To monitor the detailed longitudinal strain profiles in the model pipe, bare Fiber Bragg Grating (FBG) cables were deployed. In most cases, the pipe response was predominantly axial while bending became significant at stronger excitations. Strain distributions displayed clear peaks at or near the block interfaces, in accord with numerical predictions, with magnitudes increasing at resonant frequencies and with excitation level. By extension to full scale, peak axial strain amounted to 10−3, a demand half the yield strain, but not negligible given the low in situ soil stiffness contrast and soil-pipe friction.
This paper presents a novel, bridge-dependent approach for quantifying the increase of design quantities due to spatially variable earthquake ground motion (SVEGM). Contrary to the existing methods for multiple support bridge excitation analysis that are either too complicated to be applied by most practitioners or oversimplied (e. g. Eurocode 8, Annex D provisions), this method aims to strike a balance between simplicity, accuracy and computational efficiency. The method deliberately avoids generating support-dependent, acceleration or displacement, asynchronous inputs for the prediction of bridge response. The reasons behind this decision are twofold: (a) first, the uncertainty associated with the generation of asynchronous motion scenarios, as well as the exact soil properties, stratification and topography is high while, (b) the response of a bridge is particularly sensitive to the above due to the large number of natural modes involved. It is therefore prohibitive to address SVEGM effects deterministically in the framework of a design code. Instead, this new method is based on two important and well-documented observations: (a) that SVEGM is typically globally beneficial but locally detri-mental [1], and (b) that the local seismic demand increase is very closely correlated with the excitation of higher modes, which are not normally activated in the case of uniform ground motion [2,3]. Along these lines, a set of static analyses are specified herein to complement the standard, code-based response spectrum analysis. These static analyses apply spatially distributed lateral forces, whose patterns match the shape of potentially excited anti-symmetric modes. The amplitude of those forces is derived as a function of the expected amplification of these modes according to the process initially proposed by Price et al. [4]. Two real bridges with different structural configurations are used as a test-bed to demonstrate the effectiveness of the new method. Comparison of the results with those obtained through rigorous response history analysis using partially correlated, spatially variable, spectrum-compatible input motions [5] shows that, the simplified method presented herein provides a reasonably accurate estimation of the SVEGM impact on the response of the bridges examined at a highly reduced computational cost. This is essentially an elastic method that is found to be simple, yet precise enough to consist an attractive alternative for the design and assessment of long and/or important bridge structures in earthquake-prone regions. Download fulltext: https://authors.elsevier.com/c/1aTYdytxOFGx~
Anastasios Sextos
added a research item
The damage potential of spatially variable seismic ground motion on buried pipelines has long been confirmed by field evidence, but it is still debatable whether transient seismic loads can be truly detrimental to the pipeline integrity. In the absence of systematic scrutiny of the effects of local site conditions on the seismic behaviour of such structures, this study presents a staged approach to numerically investigate the elastic-plastic buckling response of buried steel natural gas pipelines subject to transient differential ground motions arising from strong lateral site inhomogeneities. The first stage involves the study of 2D linear viscoelastic and equivalent-linear site response for the case of two sites and the resulting seismic demand in terms of longitudinal strains for input motions of various intensities and frequency content. The influence of key problem parameters is examined, and the most unfavourable relative ground deformation cases are identified. In the second stage of analysis, the critical in-plane ground displacement field is imposed monotonically on a near-field trench-like 3D continuum soil model encasing a long cylindrical shell model of the pipeline. Next, the performance of the buried pipeline is assessed under axial compression. The impedance contrast between the laterally inhomogeneous soil profiles is shown to govern the amplitude of induced elastic strains, which are maximized for low-frequency excitations. It is also demonstrated that peak axial strains along the pipeline considering equivalent-linear soil behaviour under strong earthquake motion can be as much as two orders of magnitude larger than their linear counterparts, as a result of the severe, spatially variable moduli degradation. It is finally shown that the seismic vibrations of certain inhomogeneous sites can produce appreciable axial stress concentration in the critically affected pipeline segment near the material discontinuity, enough to trigger coupled buckling modes in the plastic range. This behaviour is found to be controlled by pronounced axial force-bending moment interaction and is not accounted for in code-prescribed limit states.
Anastasios Sextos
added a research item
The damaging potential of spatial variability in seismic ground motion on the integrity of buried pipelines is demonstrated in this paper. A numerical analysis methodology is developed first to determine the seismic demand of a typical straight steel natural gas pipeline running through a site composed of two different media with an impedance ratio of 2 and swept by vertically propagating SV-waves. The analysis follows a sub-structured, two-phase approach involving the computation of pipeline input excitation from 2D linear viscoelastic and linear-equivalent seismic site response models and the quasi-static application of the derived critical motion profiles on a near-surface 3D continuum soil model surrounding an extended inelastic shell model of the pipeline. The focus is then placed on identifying the ground and exciting conditions bearing adverse effects on the peak pipeline response. By comparing the pipeline demand in terms of stress and strain to capacity metrics prescribed in present seismic codes, the importance of the local site response is gauged. Results show that low-frequency ground vibrations produce the most unfavorable demand on the pipe for the set of cases examined. More importantly, even though pipeline axial strain demand-to-capacity ratios for elastic local site response under weak excitation imply a large safety margin, pipeline demand can exceed capacity near the site boundary under strong excitations and subsequent nonlinear soil response. Plastic local buckling may also develop in the pipeline under high-intensity input motions, thus highlighting the necessity to account for non-synchronous earthquake ground motion in case of horizontally nonhomogeneous sites.
Anastasios Sextos
added 3 research items
The present work focuses on the response of pipelines traversing slopes under asynchronous acceleration time histories induced by earthquakes. To this end, nonlinear generalized mechanical models are developed and used in order to describe the soil-pipe interaction phenomena. More specifically, two basic types of models are used: the first one comprises two elastoplastic generalized spring elements connected in parallel with a trilinear elastic spring (labeled as sub-models 1 and 3), while the second model comprises three elastoplastic elements, all connected in parallel (labeled as sub-models 2 and 4). The purpose of the current study is twofold, firstly to introduce nonlinear mechanical models that enable a smooth transition between the elastic and plastic phases and at the same time control the damping in the soil-pipe interaction, and secondly to investigate the effect of the soil strength and soil damping on the axial force that develop in pipelines on slopes. Results are presented from a real pipeline subjected to asynchronous accelerations along a submarine slope.
Historically, a number of major catastrophic earthquakes have resulted in a real loss of life and property worldwide. During the last decades, in particular, the overall exposure to seismic risk has increased not only due to the higher population density but also due to the more challenging construction methods and the multilayered connection between the various urban socioeconomic activities. Within this complex built environment, the energy transportation systems, despite their structural simplicity, are seemingly one of the weakest links in terms of seismic safety. Currently, energy production, preservation and safe transportation is one of the top priorities. This demands the need to eliminate the probability of occurrence of a potential seismically induced failure (i.e., related to explosion, fire, leakage etc) that would not only have devastating environmental impact in the affected areas, but could also cause operation disruptions with equally significant socioeconomic consequences throughout Europe. EXCHANGE-Risk is an Intersectoral/International, Research and Innovation transfer scheme between academia and the industry in Europe and North America focusing on mitigating Seismic Risk of buried steel pipeline Networks subjected to ground-imposed permanent and co-seismic deformations. This paper discusses the challenges addressed by EXCHANGE-Risk and the recent advancements made in this topic. It also paves the road for further discussion on the methodologies, experiments and tools that need to be developed to mitigate seismic risk of natural gas pipelines at a European level and beyond.
In context of pipeline design, the soil’s stiffness and ultimate lateral resistance are critical values. Existing approximations for ultimate lateral resistance vary significantly and there are few expressions for the soil stiffness parameters. These parameters are implemented in soil constitutive models for pipeline design under different displacement induced conditions. This research focusses on the experimental investigation of the response of the soil-pipe system to enforced horizontal loading for application to natural gas pipelines. The study examines shallow burial depths (<5.5D), using a section of solid acrylic pipe in dry Leighton Buzzard grade B sand. Plane-strain conditions have been replicated and experiments run to evaluate quantities for soil stiffness, through lab experiments and validated finite element analyses (FEA). Monotonic loading was applied onto the pipe within varied conditions, changing both burial depth and soil density to establish the relationship with soil stiffness. The FEA results were used to ratify such behaviors, establishing an analysis for displacement response and corresponding stiffness parameters. Absolute and relative stiffness values from both analyses were then compared with existing literature and the results reflect the importance of relative soil properties and expected displacements. Results find that horizontal loading within shallow burial depths induce 2D displacements, which could be modelled as vectors rather than horizontal displacements.
Anastasios Sextos
added a research item
The computational demand of the soil-structure interaction (SSI) analysis for the design and assessment of structures, as well as for the evaluation of their life-cycle cost and risk exposure has led the civil engineering community to the development of a variety of methods towards the model order reduction of the coupled soil-structure dynamic system in earthquake regions. Different approaches have been proposed in the past as computationally efficient alternatives to the conventional FEM simulation of the complete soil-structure domain, such as the nonlinear lumped spring, the macroelement method and the substructure partition method. Yet no approach was capable of capturing simultaneously the frequency-dependent dynamic properties along with the nonlinear behavior of the condensed segment of the overall soil-structure system under strong earthquake ground motion, thus generating an imbalance between the modeling refinement achieved for the soil and the structure. To this end, a dual frequency-and intensity-dependent expansion of the Lumped Parameter Modeling method is proposed in the current paper, materialized through a multi-objective algorithm , capable of closely approximating the behavior of the nonlinear dynamic system of the condensed segment. This is essentially the extension of an established methodology, also developed by the authors, in the inelastic domain. The efficiency of the proposed methodology is validated for the case of a bridge foundation system, wherein the seismic response is comparatively assessed for both the proposed method and the detailed finite element model. The above expansion is deemed a computationally efficient and reliable method for simultaneously considering the frequency and amplitude dependence of soil-foundation systems in the framework of nonlinear seismic analysis of SSI systems.
Anastasios Sextos
added a research item
Buried gas pipelines in seismic-prone regions may suffer leaks or breaks as a consequence of an earthquake, especially if the pipeline is subjected to large differential displacements due to geotechnical failures (e.g., landslide, liquefaction). This paper presents a methodology to assess the risk of a gas pipeline infrastructure at regional level in the aftermath of a seismic event. Once earthquake characteristics, such as magnitude and epicentre, are known, seismic intensity measures (IMs), such as peak ground acceleration (PGA) and peak ground velocity (PGV), are estimated at the location of each pipe through a simulation-based procedure. The potential updating from real-time data coming from accelerometric stations is considered. These IMs are then used to study the cascading landslide and liquefaction hazards providing a hybrid empirical-mechanical-based estimation of permanent ground displacements (PGD). With the aid of literature damage and fragility functions, loss figures and damage maps are derived as decision-support tools for network managers and stakeholders. Losses provide a preliminary estimation of repair costs, while damage maps support the prioritisation of inspections in the aftermath of the event. The risk methodology is a novel combination of cutting-edge and consolidated approaches. Firstly, different cross-correlation models between PGA and PGV are included. Secondly, a new three-phase back-to-back geotechnical approach is provided for both landslide and liquefaction, representing (i) the susceptibility, (ii) the triggering, and (iii) the PGD estimation phases. The 1976 Friuli earthquake and the high-pressure gas network of NorthEast Italy are assumed as test-bed scenario for the risk methodology aimed at emphasising pros and cons of the different alternative options investigated.
Anastasios Sextos
added a research item
Evidence from past earthquakes suggests that damage inflicted to buried natural gas (NG) pipelines can cause long service disruptions, leading to unpredictably high socioeconomic losses in unprepared communities. In this review paper, we aim to critically revisit recent progress in the demanding field of seismic analysis, design and resilience assessment of buried steel NG pipelines. For this purpose, the existing literature and code provisions are surveyed and discussed while challenges and gaps are identified from a research, industrial and legislative perspective. It is underscored that, in contrast to common belief, transient ground deformations in non-uniform sites are not necessarily negligible and can induce undesirable deformations in the pipe, overlooked in the present standards of practice. It is further highlighted that the current seismic fragility framework is rich in empirical fragility relations but lacks analytical and experimental foundations that would permit the reliable assessment of the different parameters affecting the expected pipe damage rates. Pipeline network resilience is still in a developing stage, thus only few assessment methodologies are available whereas absent is a holistic approach to support informed decision-making towards the necessary mitigation measures. Nevertheless, there is ground for improvement by adapting existing knowledge from research on other types of lifeline networks, such as transportation networks. All above aspects are discussed and directions for future research are provided.
Anastasios Sextos
added a research item
In the past decades, a number of major earthquakes caused serious damage to natural gas pipeline networks. In most cases, the devastating effects were caused by permanent ground displacement. However, there exist at least two well documented cases (Mexico City and Northridge Earthquakes) where damage were due to seismic wave propagation. Response of buried pipelines is significantly different from that of above-ground structures. However, similarly to bridges or dams, pipelines are also prone to the effects of spatial variability of earthquake ground motion due to their length, which, in some cases, extends beyond national borders. This paper focuses on the effects of asynchronous excitation on the seismic response demand of natural gas pipelines belonging to transmission networks. Parameters examined include time delay due to finite wave propagation velocity and loss of coherency along the pipelines' length, a parameter known to contribute to seismic strains. Impact of local site effects on pipeline response is examined through the use of bedrock-soil surface slope that forms a basin, with impedance ratios varying with depth. Finite element analysis and lumped springs are used to model the interacting soil-pipeline system while excitation input motions are generated through 2D site response analyses. The paper summarizes the effects of various parameters on seismic demand to pipelines. The results indicate that ignoring the wave passage effect, the stress state in the pipeline is roughly symmetric, with the axial strains of the pipeline to be increased over the inclined sides of the basin and to be almost null in the middle. When the wave passage effect is incorporated in the analysis the stress state is no longer symmetric and the location of the maximum strains in the pipeline moves towards the central region of the basin but near to the inclined edge from which the seismic waves are coming. The comparison of the computed axial strains with the respective strains used in conventional design processes showed that in the case of irregular subsurface topographies the conventional may result in unconservative design.
Anastasios Sextos
added a project goal
EXCHANGE-Risk is a H2020 research and mobility project lead by the University of Bristol with the aim to:
- Promote transfer of know how between the academic and industrial sector in Europe, United States and Canada on Seismic risk of natural gas pipelines
- Innovative methods for pre- and post-earthquake pipeline inspection, system monitoring and network resilience
- Intercontinental, multiple-support, hybrid experimentation utilizing experimental and computational facilities among the project partners