Pipe Soil Research Articles

CFD-DEM modeling of filtration through conventional and conical geotextile filter systems

A significant contributor to retaining wall structural failure occurs due to inadequate drainage in the backfill. A numerical model based on a computational fluid dynamics and discrete element method (CFD-DEM) coupled approach was developed to simulate particle movement in the graded filter zone and piping through the geotextiles. The model was used for conventional as well conical geotextile filter systems that use a series of woven geotextiles filtering a coarse-grained backfill soil. The model results were compared with laboratory results to verify the accuracy. The results indicated that conical filter systems contribute to higher soil piping rates but provide higher permeability than conventional geotextile filtration system counterparts. The model predictions compared with the laboratory measurements indicated that the movement of particles (i.e. suffusion) influenced the soil-geotextile contact zone permeabilities and caused a decrease in system permeabilities. A retention ratio, [Formula: see text], successfully predicted piping rates for different types of woven geotextiles with a percent error range of 13–29%. Overall, the model predictions matched the laboratory results within an order of magnitude or less, indicating the predictive capability of the model.

Integrated Agrogeophysical Approach for Investigating Soil Pipes in Agricultural Fields

Soil erosion is one of the most significant challenges for soil management and agri-food production threatening human habitat and livelihood. Although soil erosion due to surficial processes is well-studied, erosion due to subsurface processes such as internal soil pipes has often been overlooked. Internal soil pipes directly contribute to the total soil loss in agricultural fields and impede agricultural sustainability. Locating, measuring, and mapping internal soil pipes and their networks are vital to assessing the total soil loss in agricultural fields. Their hidden and uncorrelated nature of subsurface occurrences constricts the applicability of manual and remote sensing-based detection techniques. Non-invasive agrogeophysical methods can overcome these limitations with detailed subsurface pictures and high spatial resolution. In this study, the applicability of three agrogeophysical methods including seismic refraction tomography (SRT), electrical resistivity tomography (ERT), and ground-penetrating radar (GPR) was tested at Goodwin Creek, an experimental field site with established internal soil pipes. SRT showed low P and S wave velocities anomalies in soil pipe-affected zones. ERT results indicated the location of soil pipes with high resistivity anomalies. However, both SRT and ERT lack resolution to identify individual soil pipes. GPR diffraction hyperbolas and their apexes however effectively identified individual soil pipes. The agrogeophysical anomalies for soil pipes were compared with the low penetration resistance of the cone penetrologger (CPL) results. Correspondence between low PR in CPL and agrogeophysical anomalies verify the locations of internal soil pipe-affected zones. Moreover, the fragipan layer is identified below the soil pipe-affected zone by all three methods.

Study on the Pipe–soil Interaction under Instability of Slope Based on Flume Experiment

To study the landslide of buried gas pipeline caused by rainfall, based on the flume experiment the laboratory model of rainfall-induced pipe–soil interaction of slope instability was established. By monitoring the cross-section strain of the pipeline under the slope instability induced by rainfall infiltration, the effect of soil on the pipeline is analyzed and calculated. The results show that the pipeline is located at different failure surfaces of the slope, which is affected by slope instability to different degrees. In the slope toe of apertures, ultimate cross-section strain is the largest, calculation of soil force and maximum limit 1515.535 N/m, and that of slope top pipe is 216.169 N/m, attenuation trend from toe to the top. Resultant force directions are distributed in soil and the horizontal angle is 68°. The results can provide reference and technical reserve for the study of pipe-soil interaction of buried natural gas pipelines under the action of landslide and the further improvement of pipeline safety.

Take it to the bank: A numerical examination of the effects of soil pipes on bypass of riparian buffer nitrate removal capacity

Streams are vital landscape ecosystems, and urbanization and modern agriculture have introduced pollutants (including excess nitrate) which threaten these waterways. Riparian zones have been shown to attenuate pollutants, and riparian buffers are commonly implemented to capture these benefits. Nevertheless, preferential flowpaths such as soil pipes have potential to bypass such attenuation. We used a MODFLOW groundwater model with the conduit flow package (CFP), along with the groundwater transport code MT3D-USGS, to simulate flow of water and nitrate through riparian groundwater to a gaining stream. We conducted a numerical sensitivity analysis to examine the impact of soil pipe presence and characteristics, as wells as soil matrix characteristics, on nitrate transport and uptake by denitrification within the saturated zone. We found that in systems with 2.0-m-long soil pipes, low head gradient (∇h less than 0.0035), low hydraulic conductivity (K < 10−4.75 m/s), and high reaction-constant (k > 4 day−1), soil pipes increased riparian bypass of nitrate by several orders of magnitude. Yet soil pipes increased the volumetric flowrate of water across the riparian zone only by up to 3.9 times. This comparatively greater effect on transport occurred because the accelerated advection of nitrate through the soil pipes decreased the residence time available for denitrification. We created two non-dimensional parameters that show when bypass will be important, the riparian bypass potential (ψ) that accounts for key governing factors such as volumetric flowrate and Damköhler number, and riparian predictive index (ζ), which is calculated from easily measurable parameters. Nitrate bypass starts to increase by orders of magnitude above ψ ≈ 2.75 and ζ ≈ 3.4. Our results emphasize the importance of accounting for soil pipes when constructing riparian buffers and predicting their effects on dissolved pollutant transport.

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.

Combining UAV remote sensing and pedological analyses to better understand soil piping erosion

The study and mapping of evidences of soil piping using non-destructive techniques is a key issue in quantitative geomorphology. This paper attempts to combine Aerial mapping systems (Unmanned Aerial Vehicles (UAV)), soil physical and chemical attributes, and near-surface geophysical survey tools (ground penetrating radar and electrical resistivity tomography) to provide a comprehensive understanding of geometric features of soil piping in arid and semi-arid regions. The UAV Mapping System was applied to prepare ortho-photos, topography, analytical hill shading, drainage density, and land use maps of two different piping sites (site 1: rangelands, and site 2: agricultural lands) in Sarakhs plain, Razavi Khorasan Province, northeastern Iran. The physical and chemical soil attributes were analyzed in six soil profiles to check the hypothesis that these soil attributes control the occurrence of piping-related features (i.e., sinkhole, blind gully, gully), and to test if there are any differences in soil properties between the two land use types. The near surface geophysical tools were used to determine the approximate size of soil pipes, and to simulate their internal structure. The results of the derived UAV’s independent variables confirmed that typical soil erosion features related to piping (i.e., sinkhole, blind gully, gully) developed exactly in adjustment with subsurface processes (i.e., drainage density). The quantitative algorithms of pedology revealed significant differences of Na+ and SAR for soil profiles with and without piping erosion, but these soil properties themselves are not enough to explain piping development at the two study sites. The volume of GPR line surveys was 1701.5 m3 in site 1 and 1203 m3 in site 2. The potential distribution of subsurface pipes revealed by applying GPR were more extensive than those deduced from soil surface observations: three-dimensional pipe number density (number of soil pipes per unit soil volume; # m−3) of potential pipes and collapsed cavities simulated with a mean pipe length and pipe depth of 143.4 cm and 88 cm in rangelands and 175.75 cm and 79.14 cm in agricultural lands, respectively. The maximum density of pipes or pipe roof collapses occurred within soil depth boundaries of 0–50 cm at site 1 and 30–100 cm at site 2. The ratio of surface erosion features to subsurface potential piping in rangeland and agricultural lands was ca. 26.1 % and 10.4 %, respectively. The electrical resistivity tomography (ERT) measurements indicated higher resistivity values in piping-prone areas, where pipes were initiated at the bedrock interface. Integrating all data obtained by different techniques in this study allowed to better understand the extent of piping erosion in both surface and subsurface soil horizons.

Stress Shifting Effect of Hydrogen Mixed Natural Gas Pipe Under Seismic Wave

Abstract Hydrogen energy is a kind of clean secondary energy sources. Mixed hydrogen natural gas transportation technology is a new scheme of hydrogen transportation. Using natural gas pipe to transport hydrogen is expected to further promote its application. In order to study the mechanical properties of buried steel pipe under seismic waves, a numerical model is established. The time history and stress distribution of pipe section under seismic wave are analyzed. Effects of seismic intensity, surrounding soil, buried depth and seismic wave type on pipe's mechanical properties are discussed. The results show that the pipe section stress fluctuates under seismic wave, and the stress shifting effect occurs. The maximum stress is located in the directions of 45 deg, 135 deg, 225 deg, and 315 deg. Stress increases with the increasing of seismic intensity, and the stress distribution of pipe section is also changed. Stress responses of the pipe in different soil are different, and the stress distribution of pipe section at the maximum stress time is similar. The deeper the buried depth is, the greater the pipe stress is. Pipe stress is related to the maximum acceleration of the seismic wave and spectrum characteristics. Those results can provide a basis for the design and safety evaluation of mixed hydrogen natural gas pipes.

Drained Axial Pipe-Soil Resistance at Low Confinement Using Tilt Table and Direct Shear Tests

Establishing a reliable value for the axial pipe–soil interface resistance is critical for the design of offshore pipelines. Over the last few years, several independent research groups have modified existing methodologies or devised new testing setups to further develop our understanding of the behavior of the pipe–soil interface at low contact stresses. The most common laboratory experimental methods adopted have been based on either tilt table tests or interface direct shear tests. However, a direct comparison between the results of these two testing methodologies has not been established. This paper aims at designing and fabricating a tilt table and interface direct shear setup capable of accurately estimating the drained interface resistance at low normal stresses, in addition to comparing their results under identical testing conditions. Both setups tested the drained clay–solid interface response by varying the soil composition (high and low plasticity clay), the interface roughness (smooth and rough), and the normal stress range. Results showed that the drained residual failure envelopes for the clays and clay–solid interfaces were nonlinear and could be modeled by a simple power model. The interface response was clearly dependent on the interface roughness, particularly when the roughness was normalized with the mean grain size. In the case of smooth interfaces, identical friction angles were obtained from the tilt table and the interface direct shear, whereas in the case of rough interfaces, the interface direct shear yielded lower friction angle values.

Sonic assessment of physical ageing of plastic pipes

This article explores the potential of vibro-acoustics to detect physical ageing of plastic pipes. For this purpose, two different topics are combined: the ability of vibro-acoustics to estimate the storage modulus of a plastic pipe, and the sensitivity of the estimated storage modulus to changes due to ageing. Concerning the first topic, a vibro-acoustic method was applied to two water-filled HDPE pipes, one surrounded by air and another by sand. The excitation was achieved via an impact hammer and the propagating signal was recorded with the aid of hydrophones. Signal analysis led to the estimation of the axial wavenumber of the propagating axisymmetric fluid-borne wave. This value was used in the dispersion equation for the propagating mode to evaluate the storage modulus of the pipe material for a given experimental setup. Results revealed that the vibro-acoustic method gives consistent and reliable estimations of the storage modulus. Concerning the second topic, samples from two PVC pipes with an age difference of 41 years were subjected to dynamic mechanical analysis to study the behaviour of the storage modulus as a function of frequency. Results showed that it is feasible to distinguish discrepancies in the magnitude of the storage modulus due to ageing, provided that the measurement uncertainty is small. The uncertainty analysis highlighted the parameters that need to be more accurately known in order to lower the overall uncertainty of the estimated storage modulus when the proposed vibro-acoustic method is used. Irrespectively of the medium surrounding the pipe (air or soil), the distance between the points of the recording signals should be sufficiently long to measure the signal phase accurately. It was found that the accurate knowledge of the pipe’s geometry, i.e. the wall thickness and internal radius, was more or equally important for controlling the overall uncertainty than that of the parameters of surrounding soil.

Simulation analysis of the effect of single-chamber double-line pipe jacking through different soil materials on surface uplift and subsidence.

This article is based on the relocation project of the 330 kV overhead line in Xi’an, China. In this paper, the soil settlement under different jacking depths was calculated by using the modified Peck’s formula. Meanwhile, by modeling in ABAQUS, the jacking process of a single-chamber double-line large diameter pipeline under different soil conditions was simulated, and the ground deformation data under the different simulated working conditions were obtained. The results of the two methods were compared with the construction monitoring results, and it was found that the finite element simulation results were closer to the actual results. The control variable method was used in the analysis of the surface soil deformation law to analyze the effect of different soil parameters and pipe jacking depths on surface soil deformation. Finally, the best soil conditions applicable to single-chamber double-line large diameter pipe jacking construction were obtained through comparative analysis. The results show that (1) when using double-line construction, the maximum surface settlement under different soil conditions is located 11–15 m from the centerline of the soil above the pipeline, the minimum settlement location is inside the isolation pile, and with the increase in jacking distance, the settlement at the same section of the surface will gradually decrease and finally produce a small uplift. (2) In the first jacking, the settlement of powder clay is the largest, and the maximum settlement points in the surface section are more distributed. The maximum settlement value is approximately 11.66 mm. The settlement of powder soil is the smallest but produces a certain uplift deformation, and its maximum settlement is more concentrated in the surface section. After the comparison of deformation and soil parameters, loess-like soil is more suitable for single-compartment double-line large diameter pipe jacking construction. (3) When the top pipe burial depth changes, the greater the burial depth is, the smaller the settlement but the greater the lateral influence range. In the soil parameters, the modulus of elasticity only changes 3 MPa, and the settlement change value is approximately 5 mm. By changing the parameters, it can be obtained that the larger the modulus of elasticity of the soil is, the smaller its deformation. The larger the internal friction angle of the soil is, the smaller its deformation, but the maximum value of settlement change is only 1.7 mm, which means that the change in the internal friction angle has little effect on the soil deformation.

Probabilistic Seismic Risk Analysis of Buried Pipelines Due to Permanent Ground Deformation for Victoria, BC

Buried continuous pipelines are prone to failure due to permanent ground deformation as a result of fault rupture. Since the failure mode is dependent on a number of factors, a probabilistic approach is necessary to correctly compute the seismic risk. In this study, a novel method to estimate regional seismic risk to buried continuous pipelines is presented. The seismic risk assessment method is thereafter illustrated for buried gas pipelines in the City of Victoria, British Columbia. The illustrated example considers seismic hazard from the Leech River Valley Fault Zone (LRVFZ). The risk assessment approach considers uncertainties of earthquake rupture, soil properties at the site concerned, geometric properties of pipes and operating conditions. Major improvements in this method over existing comparable studies include the use of stochastic earthquake source modeling and analytical Okada solutions to generate regional ground deformation, probabilistically. Previous studies used regression equations to define probabilistic ground deformations along a fault. Secondly, in the current study, experimentally evaluated 3D shell and continuum pipe–soil finite element models were used to compute pipeline responses. Earlier investigations used simple soil spring–beam element pipe models to evaluate the pipeline response. Finally, the current approach uses the multi-fidelity Gaussian process surrogate model to ensure efficiency and limit required computational resources. The developed multi-fidelity Gaussian process surrogate model was successfully cross-validated with high coefficients of determination of 0.92 and 0.96. A fragility curve was generated based on failure criteria from ALA strain limits. The seismic risks of pipeline failure due to compressive buckling and tensile rupture at the given site considered were computed to be 1.5 percent and 0.6 percent in 50 years, respectively.

Contamination from plastic pipes in small systems: migration and leaching

In recent years, extensive laboratory-scale research has focused on the biofilm formation, leaching, and migration of organic chemical compounds from plastic pipes into drinking water. This research has been conducted on an existing distribution network in three rural villages near the dead ends to study the small diameter pipes associated with stagnant or low flow conditions. The distribution network was constructed more than ten years ago using unplasticized polyvinyl chloride pipes. Samples of water, soil, and aged pipes have been collected and tested in the laboratory. Results showed a heavy biofilm formed on the inner surfaces of the uPVC pipes. The biofilm has resulted in high concentrations of trihalomethanes, total organic carbon, and PAHs in the drinking water. The predominant PAHs were the acenaphthylene followed by the naphthalene, while no PAHs have detected at the source. The SEM images showed deterioration of pipe walls, swelled parts, and pores. The EDX confirms the migration of some elements including O, Cl, Ca, Ti, Mg, Fe and K due to the biodegradation of the pipe material and the Ca/Zn stabilizers. However, PAHs released from the plastic pipes into the flowing water are caused by metabolic activities. In addition, results showed that the surrounding soil has classified as having low organic content. Hence, uPVC pipes need protection or change in the manufacturing processes to reduce their hazards in distribution networks over time.

Analytical solution for the vertical vibration of a pipe pile embedded in poroelastic soil

This paper presents an analytical method to investigate the vertical vibration of a pipe pile in layered poroelastic soils due to a harmonic load. The relationship of general displacement and stress is obtained through a stiffness matrix which is derived by applying analytical layer element method. The pipe pile with a thick wall is governed by vibration theory of a one-dimensional bar. By considering the compatible conditions at the inner-surface and outer-surface of a pipe pile and soil, the vertical vibration performance is obtained. The proposed method is proved correct by comparing with the existing solutions. Parametric studies are carried out to illustrate the influences of wall thickness of the pipe pile, soil permeability and pile-soil stiffness ratio on the vibration characteristics of the pipe pile.

Modelling and analysing failure modes of buried pipelines perpendicularly crossing landslide boundaries

The pipeline response to shallow translational landslides is investigated using semianalytical and numerical methods. The orthogonality of a pipeline and the landslide boundary is considered in this study. In the finite element (FE) analyses, ground movements are simplified as spatially distributed horizontal transverse deformation. The pipe-soil interaction is modelled accounting for large ground deformation, inelastic material behavior, and special contact conditions at the soil–pipe interface. Pipeline failure patterns are summarized based on a series of FE models with varying diameter thickness ratios of the pipe and soil conditions. According to the FE analysis results and the published literature, the existing failure criteria are evaluated and extended to cover new failure modes. The dynamic evolution in the predominant failure modes due to the pipe elongation effects is investigated. A semianalytical solution for the sliding distance to the onset of pipe wall wrinkling is given. In addition, a pipeline failure criterion is proposed based on the relative stiffness. This dimensionless parameter is correlated with failure features, including failure modes, sliding distances to failure initiation, and failure positions. The reliability of the proposed FE model and failure prediction methodology is validated using published results from centrifuge tests and FE analyses. Simple charts for failure prediction in terms of relative stiffness parameters are provided. The proposed approach allows efficient and reliable safety evaluation on X65 steel pipelines subjected to horizontal transverse ground deformation without conducting numerical modelling.

Joint geomorphological and geophysical (electrical resistivity) investigation for the configuration of soil pipe

Soil piping is a complex mechanism of subsurface soil erosion, which results underground conduits (cave/tunnel) of varying dimensions. Soil piping associates with severe consequences, such as land subsidence and land slide. Therefore, the investigation of soil pipe is crucial. However, the study of soil pipe is challenging unless characteristic surficial evidences of the pipe are available. Based on the surficial evidences, soil pipe can be configured with geophysical techniques which in-turn aid in designing precursory measures. Therefore, in the present study, we carried out a combined geomorphological and geophysical investigation to configure the soil pipe at Kinanoor village, Kasaragod, Kerala, India. Based on the vital geomorphological information, we carried out resistivity survey and configured an underground soil pipe of diameter ∼6.5 to 7 m that is seated ∼3 m beneath the surface. This hollow pipe is underlain by the only accessible road of that locality which makes the road vulnerable for transportation. Therefore, a bridge like structure is recommended to construct at the pipe location to stabilize the risk factor. Since the study area is situated on a fringe-slope, the geomorphological investigation points out that the disturbance in natural course of the drainage system and the accumulation of water in the up-slope area due to the man-made activities might act as potential causes for the piping in the area. Therefore, it is suggested not to disturb the natural course of the drainage which may lead to subsidence of the area in future.

Plastics derived from disposable greenhouse plastic films and irrigation pipes in agricultural soils: a case study from Turkey.

Plastics are ubiquitous. It has been used in human activities, from agriculture to packaging, infrastructure, and health. The wide range of usage makes plastics an omnipresent pollutant in the environment. This study investigated the abundance and type of plastics in agricultural soil in the Adana/Karataş region in Turkey, where disposable low-tunnel greenhouse plastic films and irrigation pipes were in use. For this purpose, 1 kg of soil samples from the top 5 cm (from the surface) was taken from 10 different sampling locations. An average of 16.5 ± 2.4 pcs/kg was found in the soil samples. The highest amount of plastics was seen at the Bahçe-4 location with 39.7 ± 12 pcs/kg and the lowest amount of plastics at the Karataş-1 location with 0.7 ± 0.3 pcs/kg. The average size of plastics was found to be 18.2 ± 1.3 mm. The average size of plastics originating from greenhouse cover was 18.9 ± 1.4 mm, and from disposable irrigation pipes was 12.5 ± 3.5 mm. It was determined that 41.9% of extracted plastics were microplastics, 36.3% were mesoplastics, 16.3% were macroplastics, and 5.6% were megaplastics. Results indicated that residual plastics decreased in the soil where used plastics were removed after usage. As a result, it is worth noting that a significant amount of plastics remain in soil due to plastics being used in agricultural areas.

Study and Analysis on Stress Safety Judgment Method of Heavy Vehicle Rolling on Long-Distance Pipeline

With the rapid increasing mileage of China’s long-distance oil and gas pipelines, the intersection construction with other civil engineering, encroachment by invading the pipeline route, and other third-part problems occurred occasionally. Heavy vehicles rolling the serving pipeline have become one of the inevitable safety risks. The field test method of three in-service long oil and gas pipelines under heavy vehicle rolling is elaborated. The pipeline stress and soil pressure during tests are monitored and analyzed. The parametric model of pipe mechanical response under heavy vehicle rolling and stacking load is established, and the quantitative feedback of each parameter on pipeline stress safety response under heavy vehicle rolling is formed. The results show that the influence factors such as vehicle weight, vehicle speed, buried depth of pipeline, horizontal distance and cover plate on the stress state, vertical displacement state, and contact stress of pipe soil interface are highly consistent with the finite element method and test method, which could be a guideline for the safety assessment at rolling site.

Can soil piping impact environment and society? Identifying new research gaps

AbstractSoil piping is a widespread, although often overlooked land degradation process. So far, subsurface soil erosion studies have been focused on the importance of soil piping in hydrological and geomorphological processes, and factors controlling piping processes. Nowadays, the environmental changes being caused by the Anthropocene have clearly demonstrated that society depends on soil more than ever before, so the traditional studies of soil erosion processes need to be redefined. In that sense, geomorphologists face to overcome new piping‐related problems. In this article we identify new possible areas of research: (i) soil pipes and pipe collapses (PCs) as natural hazards, (ii) role of soil piping in carbon cycle, (iii) soil pipes and PCs and their relationships with biodiversity, and (iv) piping‐affected areas as geodiversity sites. Only better recognition of natural hazards driven by soil piping, such as land subsidence and degradation, landslides, flooding and off‐site sediment effects may result in better prevention and control measures in piping‐affected areas. Moreover, in the context of Global Change the role of soil piping in carbon cycle should be raised. Land‐use and land‐cover changes, as well as climate change may affect piping dynamics in different morphoclimatic regions and soil loss due to piping may lead to carbon loss. Soil pipes and PCs are closely interlinked with biodiversity, both positively and negatively. Piping erosion may directly and indirectly destroy vegetation and animals, although in some cases piping erosion may create new habitats and provide favourable conditions for some species. However, soil piping is not only an environmental and societal problem, but it may also contribute to the world geodiversity, which is clearly observed in badland sites. Piping erosion may have a significant impact on environment and society, thus further research with new questions is essential to provide knowledge for sustainable development.

Lateral Buckling of Pipe-in-Pipe Systems under Sleeper-Distributed Buoyancy—A Numerical Investigation

The crude oil in pipelines should remain at high temperature and pressure to satisfy the fluidity requirement of deep-sea oil transportation and consequently lead to the global buckling of pipelines. Uncontrolled global buckling is accompanied by pipeline damage and oil leakage; therefore, active buckling control of pipelines is needed. Pipe-in-pipe (PIP) systems have been widely used in deep-sea oil pipelines because of the protection and insulation characteristics of the outer pipe to the inner pipe. In this study, sleeper-distributed buoyancy is used as an active buckling control method for the global buckling of PIP systems with initial imperfections. The accuracy of this technique is verified by comparing the finite element model of a 3D pipeline with experimental data. The effects of buoyancy density, pipe–soil friction coefficient, initial imperfection, stiffness ratio of inner and outer pipes, and buoyancy unit interval on the global buckling performance are also analyzed. The critical buckling force and lateral displacement of this method are studied using an analytic solution, and the relevant calculation formulas are obtained and verified to provide a basis for its engineering application.

Probabilistic seismic responses and failure analyses of free-spanning subsea pipelines under offshore spatial earthquake motions

A comprehensive literature review in seismic analysis of free-spanning offshore pipelines (FSOPs) reveals that two critical aspects, i.e. the offshore earthquake motions and various uncertainty sources, have been normally ignored in numerous previous studies. Specifically, the offshore pipelines are usually modeled by assuming all the structural parameters to be fixed and are excited using the onshore earthquake motions. Such analytical schemes may result in severe misestimates of seismic response predictions of FSOPs. In this paper, a probabilistic approach is developed for numerically investigating the seismic responses and failure mechanisms of FSOPs subjected to the offshore spatial earthquake motions (Off-SEMs). For this purpose, a buried offshore pipeline of API X65 with a free span of 30 m is selected and the corresponding three-dimensional finite element (3D FE) model is established in the ABAQUS software, where the soil–pipe and water–pipe interactions are modeled using the nonlinear soil springs and added mass and damping methods, respectively. Then, the three-dimensional Off-SEMs are stochastically synthesized by explicitly considering the effects of spatial variability and overlying seawater. Next, the general law of dynamic responses and failure modes of FSOPs subjected to the Off-SEMs is analyzed and summarized by the deterministic dynamic analysis. Defining the earthquake motions, structural modeling and soil properties as uncertain sources, the relative sensitivity of seismic behaviors to uncertain parameters involved in these uncertain sources is identified separately by the sensitivity analysis. Finally, uncertainty analysis is performed to examine and discuss the influence of different types of uncertainty sources on the seismic behaviors of FSOPs. Numerical results indicate that the pipeline diameter has the highest influence on the seismic behaviors of FSOPs, while the Poisson’s ratio of pipeline material has the smallest effect; uncertainties in earthquake motions, structural modeling and soil properties have significant contributions to the dynamic characteristics and seismic behaviors.