American Lifelines Alliance Research Articles

Evaluation of polyethylene pipe–sandy soil interaction subjected to strike-slip faulting

Pipelines are simulated in the form of a continuous beam resting on a collection of linear springs. The relationships put forth by the American Society of Civil Engineers (ASCE) guidelines are commonly used to derive the specifications of the spring elements. Some recent studies have suggested that these relationships are accompanied by a certain degree of inaccuracy. In this study, two full-scale tests were carried out on polyethylene pipes (with diameters of 120.5 and 214 mm) buried in sandy soil. The displacement of the pipe along its length was recorded throughout the entire test. Then, using the Abaqus finite-element software package and an optimisation algorithm developed in the Matlab software, a modelling approach was adopted so that the properties of the equivalent linear springs simulating the soil could be determined. In this way, the displacements obtained from the experiments would have the highest level of congruence with the values derived from the numerical simulations. Using this approach, the initial stiffness and the maximum force resulting from the pipe–soil interaction were computed and compared with the values given by the ASCE and American Lifelines Alliance (ALA) guidelines. The results showed that for a polyethylene pipe at the condition of strike-slip faulting, these values were much smaller than the values put forth by ASCE and ALA.

Development of Fragility Curves for the Underground PVC Water Distribution Pipes in Quezon City and Damage Estimation under a Magnitude 7.2 Scenario Earthquake

Quezon City is traversed by the West Valley Fault System (WVFS) that has the capability of generating a magnitude 7.2 earthquake known as “The Big One”. It has an extensive water distribution network that is very susceptible to damages that will be caused by the ground shaking component of the M 7.2 earthquake. This study focuses on determining the behavior and estimating the damage of underground polyvinyl chloride (PVC) water pipes due to the M 7.2 earthquake by using appropriate empirical repair rates (RR) and developing fragility curves. The appropriate empirical RR equation was determined by comparing the results of selected PVC RR equations and the simulation using line-element modeling. The PGV ranges from 23.10 cm/s to 64.49 cm/s as determined using the Boore and Atkinson (2008) ground motion prediction equation. Using the results from the empirical and simulations methods, the equation by the American Lifelines Alliance (2001) was determined to be the appropriate empirical RR equation for the study area. The expected average repair rate of PVC is 0.05 repairs/km length of pipe or an estimated 84 total PVC pipe repairs in the city. Three fragility curves were generated showing the relationship of PGV and RR which is an important tool in estimating the underground pipe damages with respect to the M 7.2 earthquake and other future earthquakes of similar properties.

Theoretical solution of the vibration response of the buried flexible HDPE pipe under impact load induced by rock blasting

The blasting vibration load can pose great threats to the safety of urban buried pipelines. Winkler model is commonly used to simulate the soil-pipe interaction and calculate the response of the stiff pipe, but the effectiveness of the resulting for the flexible pipe is doubtable. In this study, a theoretical method based on the Winkler model and the Timoshenko beam theory is presented to calculate the effect of blasting load induced by rock blasting on the overlying flexible buried pipe, and the modified soil spring stiffness is provided for accurately calculating the vibration response of flexible high-density polyethylene (HDPE) pipe buried in the silty clay. The proposed theoretical method and modified spring stiffness are tested and calibrated against full-scale experiments and its detailed numerical modeling with ANSYS/LS-DYNA. It is indicated that the analytical method with empirical spring stiffness (from American Lifeline Alliance 2001) overestimates the vibration velocity magnitude of the HDPE pipe subjected to blasting vibration. Reducing the spring stiffness to one-tenth of the original value can provide improved calculation results for flexible HDPE pipe subjected to blasting vibration.

An analytical procedure for modelling pipeline-landslide interaction in gas pipelines

Landslide occurrence is a major consequence of seismic activity in uphill ground conditions. There effects range from minor damage to severe malfunctioning of local infrastructure. Particularly for transport systems such as underground gas transmission pipelines, the nature of landslide phenomenon is usually catastrophic, thereby necessitating a detailed design analysis for ensuring structural integrity. Numerous research studies have addressed these specific considerations, facilitated substantially by the advent of numerical software simulation. However, relevance of theoretical analysis and their application is still considered significant. This research study aims to determine the limiting axial strains of an underground gas pipeline subjected to longitudinal landslide movement. A novel mathematical formulation is first developed, giving due consideration to the pipeline-soil interaction, internal pressure, weight and temperature loads. Numerical simulation study through COMSOL Multiphysics software is then conducted for the same given conditions. Percentage error of around 6% was observed between the results obtained from both approaches. The proposed analytical methodology was further validated through sensitivity analysis involving slope angle variation. For different slope angles, percentage error between software and analytically computed results varied between 4% and 6%. Effects of changes in pipeline material, soil cohesive strengths and temperature gradient on underlying strains and loads contribution profile were also investigated through the sensitivity analysis study. For the achieved results, compliance with American Lifeline Alliance code was ensured and achieved for both tensile and compressive strain domains.

Methodology for failure mode prediction of onshore buried steel pipelines subjected to reverse fault rupture

Oil and gas buried steel pipelines are vulnerable to permanent ground displacements, such as those resulting from tectonic fault activation. The dominant failure mechanism is strongly dependent on the type of faulting. The more complex case is the reverse fault type because the crossing pipeline is significantly compressed and bent and consequently, it may fail due to local buckling, upheaval buckling or tensile weld fracture. Which among those failure modes will be critical, depends on a set of parameters, comprising fault crossing geometry, diameter to thickness ratio (D/t) of the pipeline, pipeline steel grade, and backfill soil properties. An extensive parametric study is carried out, followed by statistical processing of the results in order to formulate simplified statistical models for the prediction of the predominant failure mode according to criteria set by the American Lifelines Alliance and EN 1998-4 standards. The study thus offers the first comprehensive attempt to quantify the qualitative criterion that deeply buried pipes with high D/t ratio tend to buckle locally, while shallowly buried pipes with low D/t ratio tend to buckle globally. Pipe designers may use the provided expressions to predict the predominant failure mode in order to either apply the necessary seismic countermeasures or re-design the pipeline if necessary.

Seismic Fragility Analysis of Tunnels with Different Buried Depths in a Soft Soil

Seismic fragility of an engineering structure is the conditional probability that damage of a structure equals or exceeds a limit state under a specified intensity motion. It represents the seismic performance of structures and the correlation between ground motion and structural damage, playing an indispensable role in structural security assessment. A practical evaluation procedure of acquiring the fragility curves of tunnels in a soft soil has been proposed in this paper. Taking a typical metro tunnel in Shanghai as an example, two-dimensional finite element models of soil-tunnel cross-section were established. The nonlinear characteristics of soil layers were considered by the one-dimensional equivalent linear analysis in the equivalent-linear earthquake site response analyses (EERA) program. The ground motions were selected based on seismic station records. Comparing the analytical fragility curves with the empirical curves derived from American Lifelines Alliance (ALA) and HAZUS shows that the proposed method is reliable and feasible. Further study about the influence of buried depths on the fragility of the tunnel was performed. The results indicate that the failure probability of the tunnel is not monotonically decreasing with the increase of the buried depth for a given peak ground acceleration (PGA.)

Experimental and finite element study of the reverse faulting effects on buried continuous steel gas pipelines

Permanent ground displacement (PGD) caused by surface faulting is considered as one of the most significant hazards affecting buried pipelines. Pipelines crossing reverse-slip faults are subjected to compressive actions (stresses and strains) which can result in buckling of the pipe. In current work, the results obtained from the full-scale laboratory testing and finite element analyses of 4″ (114.3mm) and 6″ (168.3mm) steel gas pipes (without internal pressure) buried inside a split box and subjected to a reverse faulting of 0.6m (pure dip-slip) are presented. These pipes are commonly used in gas distribution lines and networks. The experimental setup, procedure and instrumentation as well as the finite element (FE) modeling of the problem are described in detail. It is observed that the soil failure in the moving part of the split-box occurs along vertical surfaces extending from the sides of the pipe to the ground surface. The experimental results indicate that both pipes exhibit an S-shape deformation with two local buckling sections where the excessive yielding and plastic deformations of the pipes could lead to rupture failure. Both pipes exhibited “diamond-shape” buckled sections. The buckled sections of the pipes in the fixed and moving parts of the split box were unsymmetrical with respect to the fault plane. Using the factor of ovality to measure the pipe cross-section distortion, it is found that the cross-section distortion is more severe for the buckled section of the pipe in the moving part of the split box in comparison to its fixed part. Also, the distance between the buckled sections increases by increasing the pipe diameter, while the distortion of the pipe cross-section increases by increasing the pipe diameter over thickness ratio. Using the FE models that were validated utilizing the experimental results, the maximum equivalent soil–pipe interaction forces and their distribution along the pipes were determined and the results were compared with that of American Lifeline Alliance Guidelines for the Design of Buried Steel Pipe (ALA, 2005) [33]. The obtained maximum bearing force is less than the suggested values by ALA, while the maximum uplift force slightly exceeds those of ALA. The results indicate that for the considered cases, the uplift force is sensitive to the pipe diameter and its relative stiffness, while the ALA (2005, [33]) suggests a constant force for the burial depths considered in this study.

Seismic Reliability Assessment of Water Distribution Networks

A methodology for assessing the seismic risk in a WDN is presented here as obtained from the analysis of network mechanical reliability when pipe failures are due to earthquakes. The mechanical reliability analysis is performed considering WDN topological changes subsequent to multiple failure scenarios; while pipe failure probability law is brought from the American Lifeline Alliance that assumes empirical vulnerability functions derived from data collected during recent earthquakes in the Pacific area. A multi-objective optimization is run to explore the space of contemporary pipe failures which may occur during an earthquake. The method is applied and discussed using C-Town network.

Seismic reliability assessment of urban water networks

SUMMARYWe present a framework for the seismic risk assessment of water supply networks, operating in either normal or abnormal conditions. We propose a methodology for assessing the reliability of water pipe networks combining data of past non‐seismic damage and the vulnerability of the network components against seismic loading. Historical data are obtained using records of damages that occur on a daily basis throughout the network and are processed to produce‘survival curves’, depicting their estimated survival rate over time. The fragility of the network components is assessed using the approach suggested in the American Lifelines Alliance guidelines. The network reliability is assessed using graph theory, whereas the system network reliability is calculated using Monte Carlo simulation. The methodology proposed is demonstrated both on a simple, small‐scale, network and also on a real‐scale district metered area from the water network of the city of Limassol, Cyprus. The proposed approach allows the estimation of the probability that the network fails to provide the desired level of service and allows the prioritization of retrofit interventions and of capacity‐upgrade actions pertaining to existing water pipe networks. Copyright © 2013 John Wiley & Sons, Ltd.

Reliability Assessment of Urban Water Distribution Networks Under Seismic Loads

Presented herein is a methodology for the seismic assessment of the reliability of urban water distribution networks (UWDN) based on general seismic assessment standards, as per the American Lifelines Alliance (ALA) guidelines, and localized historical records of critical risk-of-failure metrics pertaining to the specific UWDN under assessment. The proposed methodology is applicable to UWDN under both normal or abnormal operating conditions (such as intermittent water supply), and the assessment of reliability incorporates data of past non-seismic damage, the vulnerabilities of the network components against seismic loading, and the topology of a UWDN. Historical data obtained using records of pipe burst incidents are processed to produce clustered ‘survival curves’, depicting the pipes’ estimated survival rate over time. The survival curves are then used to localize the generalized fragility values of the network components (primarily pipes), as assessed using the approach suggested by the ALA guidelines. The network reliability is subsequently assessed using Graph Theory (Djikstra’s shortest path algorithm), while the system reliability is calculated using Monte Carlo simulation. The methodology proposed is demonstrated on a simple small-scale network and on a real-scale district metered area (DMA). The proposed approach allows the estimation of the probability that a network fails to provide the desired level of service and allows for the prioritization of retrofit interventions and of capacity-upgrade actions pertaining to existing water pipe networks.

Numerical Analysis of Buried Pipelines with Right Angle Elbow under Wave Propagation

Pipelines are often referred as “lifelines” and this demonstrates that pipelines play an important role in human's life. Due to huge length and wide geographical distribution of pipelines, they are subjected to more seismic hazards. Seismic wave propagations affect the operation of buried pipelines which influence safety of these systems during and after earthquakes. Wave propagation causes out-of-phase displacements through pipe's longitudinal axis. As a result of more gradient and concentration of stresses and strains, damages increase in bend areas. Considering this subjects, it is crucial to study the buried pipelines’ behavior especially in elbow regions. The interaction between soil and pipe has been evaluated by using nonlinear springs as suggested by American Lifeline Alliance which can consider slipping between pipe and soil. In 3D finite element models used in this study, applied wave is distributed spatially as a sine wave over the longitudinal leg of elbow. Results showed that mechanical properties of soil can affect the response of pipe in elbow region. In the presented paper, in addition to using beam model, a beam - shell hybrid model is used to find the pipe response in the elbow region and the results of these two methods are compared.

Consequences of critical infrastructure interdependencies: lessons from the 2004 hurricane season in Florida

This paper summarises a study conducted by Virginia Tech for the American Lifelines Alliance (ALA) in 2005 to document interdependencies and impacts among utilities and transportation systems (lifelines) associated with the 2004 hurricane season in Florida. The greater Orange County, Florida area was chosen as the study area for the investigation, which focused primarily on the experience of owners and operators of electric power systems, water and wastewater utilities, natural gas and petroleum fuel systems, and communications and transportation networks. Key personnel from utility and transportation organisations were interviewed by the study team to help identify lifeline interdependency issues that affected the ability of lifeline owners and operators to provide continued service to customers. Detailed information was collected on interdependent lifeline performance and lessons learned from the impacts of multiple hurricanes that could provide the basis for developing improved guidance for hazard mitigation and lifeline service restoration.

Seismic Fragility Curves for Un-Anchored On-Grade Steel Storage Tanks: Bayesian Approach

In this article, Bayesian approach is used for estimation of seismic fragility of un-anchored on‐grade steel storage tanks based on historical data and American Lifeline Alliance tanks database. The approach properly accounts for epistemic as well as aleatory uncertainties. The point and interval estimates of the fragility are formulated based on structural reliability method that implicitly or explicitly reflects the influence of epistemic uncertainty. The fragility curves developed herein are compared to corresponding relations currently available in the technical literature, the comparison suggest that actual tank performance is better than that predicted in the literature. Finally, application of fragilities in loss estimation methodology is investigated and showing a considerable scatter in the results corresponding to different fragilities.

Hedgehog and Fox Strategies for Reducing Risks in Infrastructure Systems

In this paper, we employ the metaphors of the hedgehog and the fox, as elaborated by Isaiah Berlin and recently employed by Philip Tetlock, in order to contrast two opposite but complementary approaches to infrastructure risk management. We summarize how a combined hedgehog and fox strategy is in evidence in two peer-reviewed monographs recently published by ASCE, as well how additional activities such as those by the American Lifelines Alliance have deployed this combined strategy. We provide plausible reasons why a combined hedgehog and fox strategy can be more successful in reducing risks in civil infrastructure systems than either an exclusively hedgehog or an exclusively fox strategy.