Lateral Pipe-soil Interaction Research Articles

Numerical Method of Lateral Pipe-Soil Interaction and Sensitivity Analysis Investigation

The pipe-soil interaction force is a crucial load that ensures the on-bottom stability of submarine pipelines and acts as a vital input parameter for the design of controlled pipeline buckling. It is necessary to precise prediction of soil lateral resistance and sensitivity to environmental parameters. This study developed a numerical simulation technique based on the coupled Euler-Lagrange (CEL) method to systematically analyse the deformation of the pipeline and the soil-pipeline forces on the seabed foundation during service. To validate the numerical method for large lateral displacement of pipe-soil interaction, an experimental approach employing parameter variation was utilized to conduct sensitivity analysis on the environmental parameters. The key parameters affecting soil-pipeline interaction were determined through an analysis of relative variation factors. The research findings demonstrate a satisfactory agreement, with an amplitude difference range of within 15%, between the numerical simulation technique using the CEL method and the experimental results. This method resolves the issue of non-convergence due to grid distortion under conditions of large displacement, thereby enhancing computational accuracy. Sensitivity analysis shows that the relative variation factors for soil friction angle, initial burial depth, and soil density exceed 50%, which is notably higher than the elastic modulus, friction coefficient, and Poisson's ratio of the soil. The results of this study enable the prediction of lateral forces in pipe-soil interaction and identification of environmentally sensitive parameters. This study establishes a foundation for calculating pipeline buckling strength in complex environments and holds paramount importance for the design and construction of pipelines to ensure on-bottom stability.

Micromechanical Analysis of Lateral Pipe–Soil Interaction Instability on Sloping Sandy Seabeds

The micromechanical mechanism of pipe instability under lateral force actions on sloping sandy seabeds is unclear. This study investigated the effects of slope angle and instability direction (upslope or downslope) on pipe–soil interaction instability for freely laid and anti-rolling pipes using coupled discrete element method and finite element method (DEM–FEM) simulations. The numerical results were analyzed at both macro- and microscales and compared with the experimental results. The findings revealed that the ultimate drag force on anti-rolling pipes increased with slope angle and was significantly larger than that on freely laid pipes for both downslope and upslope instabilities. Additionally, the rotation-induced upward traction force was proved to be the essential reason for the smaller soil deformation around freely laid pipes. Moreover, the shape differences in the motion trajectories of pipes were successfully explained by variations in the soil supporting force distributions under different slope conditions. Additionally, synchronous movement between the pipe and adjacent particles was identified as the underlying mechanism for the reduced particle collision and shear wear on pipe surfaces under a high interface coefficient. Furthermore, an investigation of particle-scale behaviors revealed conclusive mechanistic patterns of pipe–soil interaction instability under different slope conditions. This study could be useful for the design of pipelines in marine pipeline engineering.

Experimental study on vertical–lateral coupling pipe–soil interaction characteristics and speed amplification effect of marine pipelines

ABSTRACT Under actual marine environment, the steel catenary pipeline’s touchdown zone frequently interacts with the seabed. The existing understanding of vertical–lateral coupling effect and speed amplification effect is insufficient at present. In this paper, the mechanism tests for the vertical and lateral soil resistance of small-scale pipes are carried out in both dry soil and saturated soil environments. The pipe–soil interaction presents quite different characteristics in dry soil and saturated soil due to the soil liquefaction effect. The pipe movement speed affects the soil reaction force significantly, especially in the high-speed range. During the vertical penetration of the pipe, soil shear strength follows a tri-linear relation. The lateral pipe–soil interaction could be classified into three phases i.e. fast growth, fast descent and stable fluctuation. The vertical–lateral coupling has a significant influence on pipe–soil interaction, presenting that the penetration depth greatly affects the lateral soil resistance.

Lateral buckling of subsea pipelines triggered by dual sleepers

Under high temperature and high pressure (HT/HP) conditions, unburied subsea pipelines may buckle horizontally on the seabed in an uncontrolled manner, which may lead to the failure of the pipeline. So buckle initiators, such as sleepers, will be employed to trigger buckles at planned locations. In this study, mathematical models are proposed to simulate lateral buckling of subsea pipelines triggered by dual sleepers with a gap between them. Analytical solutions of the vertical and lateral deformation for the pipeline with dual sleepers are derived first. Comparisons between the results obtained in this study and in the literature are presented and discussed. Then, the influence of the dual sleepers on the buckled configuration and the post-buckling behaviour is analysed. A parametric analysis about the minimum critical temperature difference, the displacement amplitude and the maximum stress is discussed in detail. Finally, the influence of nonlinear lateral pipe-soil interaction on the post-buckling behaviour is discussed. The results show that the formula derived in this study can simulate both single sleeper and dual sleepers to trigger lateral buckles. When dual sleepers are employed, the minimum critical temperature difference and the maximum stress are reduced compared to single sleeper, which will decrease further through increasing the spacing between the dual sleepers, increasing the sleeper height or decreasing the sleeper friction coefficient. The sleeper friction coefficient must be controlled strictly.

Analytical and numerical study on lateral buckling of imperfect subsea pipelines with nonlinear lateral pipe-soil interaction model

The nonlinear pipe-soil interaction model and the initial imperfections are predominantly factors that affect the lateral buckling behaviour of subsea pipelines with a high-temperature. In this study, mathematical models, taking these two factors into account, are proposed to simulate lateral buckling of subsea pipelines. Analytical solutions are derived from the assumption of rigid-plastic pipe-soil interaction, while numerical results are obtained when considering nonlinear pipe-soil interaction. The analytical and numerical results have an excellent agreement except for the negligible discrepancy in the pre-buckling state. The discrepancy is induced by the difference of mobilisation distance in rigid-plastic and elastic-plastic pipe-soil interaction models. After the analysis of snap-through phenomenon, the influence of breakout resistance, amplitude and half-wavelength of imperfection on the post-buckling behaviour is studied. Finally, the upper and lower bound critical temperature differences are discussed as well. The results show that the hysteresis cycle between pre-buckling and post-buckling states may appear under cyclic thermal loading. The snap-through phenomenon and the hysteresis cycle can take place more easily for large breakout resistance, small amplitude and large wavelength of imperfection. In the post-buckling state, the maximum stress uplifts with increasing breakout resistance or amplitude of imperfection, while it reduces with escalating wavelength of imperfection.

Physical model tests of lateral pipe–soil interaction including the pipe trajectory in sand

Under the conditions of HT-HP, the subsea pipelines transmitting oil and gas in deep sea water often experience lateral buckling due to the accumulation of axial force. The pipe–soil interaction is one of the most important factors during the lateral buckling. In order to understand the pipe–soil interaction deeply, a series of tests about the pipe–soil interaction with different initial pipe embedments and weights were conducted in sand. A PVC pipe segment with an outer diameter of 16 cm is set as the object of this study. The soil resistance and the trajectories of the pipe segment under different conditions were measured during the test process. According to the test data, a formula for predicting the maximal lateral soil resistance was proposed with the help of R software. Moreover, another formula for calculating the lateral displacement reaching the maximal lateral soil resistance was also proposed. The trajectories of the pipe segment under different conditions were set as models, which greatly simplified the calculation of the track of the pipe segment and presented a good way to predict the movement of pipes.

The effect of backfilling stiffness on lateral response of the shallowly trenched-backfilled pipelines in clay

Offshore pipelines need to be trenched and backfilled in shallow waters for physical protection against the environmental, operational, and accidental loads. These buried pipelines may encounter lateral pipe-soil interaction in various incidents such as ground movements, landslides, ice gouging, and dragging by embedment anchors and fishing gears. Using the pre-dredged material for backfilling is a common and cost-effective solution to fill the trenches. During the construction process and being exposed to the environmental loads, the backfill material is significantly remolded and becomes much softer than the native ground. The different stiffness of backfill and native ground along with trench configuration may significantly affect the internal soil deformations, failure mechanisms, and consequently the lateral soil resistance against the laterally displaced pipeline. However, this important and less-explored aspect is neglected by existing pipeline design standards. In this study, centrifuge tests, particle image velocimetry (PIV), and large deformation numerical analysis were conducted to investigate the effect of different backfilling stiffness on lateral pipeline-backfill-trench interaction and the failure mechanisms around a pipeline shallowly buried in clay. The study showed that ignoring the trenching/backfilling effects may overestimate the ultimate lateral soil resistance for large displacements and underestimate it in small to medium displacements.

New Method to Evaluate the Buried Pipeline– Sandy Soil Interaction Subjected to Strike Slip Faulting

ABSTRACT The aim of this study is to provide a new method to assess the interaction between sandy soil and buried pipeline in the presence of strike slip faulting. To that end, two full-scale experiments were carried out on steel and polyethylene pipes. Using ABAQUS and an optimization algorithm, the stiffness of the equivalent transverse spring of the soil was calculated so that the deformation of the pipe would have the highest agreement with the experimental results. The results showed that the specifications of the equivalent pipe-soil spring are highly dependent upon the material of the pipe, a fact which has not been considered in design codes. The initial stiffness and the maximum pipe-soil interaction force for both the steel and polyethylene pipes were calculated and compared to the criteria presented by ASCE and ALA standards. The maximum pipe-soil interaction force in steel pipe test was obtained to be 70% higher than ASCE Guideline given value while in HDPE pipe test it was about 80% lower than code’s value. This fact demonstrates the importance of pipe material on lateral pipe-soil interaction.

Effects of variability in lateral pipe-soil interaction and pipe initial out-of-straightness on controlled lateral buckling of pre-deformed pipeline

The state-of-the-art method for controlling temperature-induced global lateral buckling of a subsea pipeline is to engineer deliberate buckles at widely spaced locations. These buckles are engineered either by the installation process – i.e. ‘snake-laying’ or by installing subsea structures known as buckle initiators at each intended buckle location. The pre-deformed pipeline is new alternative method that involves continuously pre-deforming the pipeline prior to installation onto the seabed. This pre-deformation causes a significant reduction in axial stiffness and therefore significantly increases the buckle initiation temperature. It also allows thermal expansion to be accommodated throughout the pipe length via expansion of the pre-deformed curvatures, rather than being concentrated at specific buckle locations. This paper presents the influence of two of the variabilities in a pre-deformed pipeline design on the buckling performance: the initial out-of-straightness and the lateral pipe-soil interaction. The results show that the concept of a pre-deformed pipeline is robust and the success of the scheme is not affected by these two uncertainties. The pre-deformed pipeline is shown to be a self-governing system where the maximum strain is self-limited at any location. Pipeline pre-deformation is therefore proven to be a cost effective, safe and valuable tool for controlling pipeline lateral buckling.

Limit analysis and finite element evaluation of lateral pipe–soil interaction resistance

The lateral breakout soil resistance of pipelines supported on undrained cohesive soils determined from limit analysis and finite element methods are compared for a linearly increasing soil strength profile. The limit analysis solution is based on an upper-bound technique. The finite element solutions are developed from coupled Eulerian–Lagrangian simulations. The capacities determined by the two methodologies are in close agreement for the perfectly plastic soil conditions. The relative ease with which the limit analysis solutions are obtained allows rapid investigation of the implications of uncertainty of the soil shear strength profile and pipe embedment via Monte Carlo simulations. Monte Carlo simulations illustrate the implications of correlated random variables describing the shear strength profile and pipe embedment.

Modelling the embedment process during offshore pipe-laying on fine-grained soils

Subsea pipelines are becoming an increasingly significant element of offshore hydrocarbon developments as exploration moves into deep-water environments further from shore. During the lay process, pipelines are subject to small amplitude vertical and horizontal oscillations, driven by the sea state and lay vessel motions. Centrifuge model tests have been used to simulate these small-amplitude lay effects, with varying degrees of idealization relative to the real lay process. In the soft soils found in deep water, pipe embedment can exceed a diameter or more, thus significantly affecting the lateral pipe–soil interaction, axial resistance, and thermal insulation. In this paper, results from centrifuge model tests are used to calibrate a model for calculating the dynamic embedment of a subsea pipeline. The model uses elements of plasticity theory to capture the effects of combined vertical and horizontal loading, and incorporates the softening of the surrounding soil as it is remoulded due to the pipeline motions. Influences from the lay rate, lay geometry, and sea state are included in the calculation process. The model is compared with observed as-laid pipeline embedment data from field surveys at three different offshore sites. Using site-specific soil parameters obtained from in situ testing and idealized pipe loads and motions to represent the load and displacement patterns during offshore pipe-laying, respectively, the model is shown to capture well the final as-laid embedment measured in the field surveys.

Discussion of “Lateral Pipe-Soil Interaction in Sand with Reference to Scale Effect” by P. J. Guo and D. F. E. Stolle

The discusser states that in addition to the authors' update of extant guidelines on the resistance to lateral movements of pipes buried in sand, the scale effects should be examined more closely. In particular, the discusser questions how the sand grains used in finite-element analysis could be called 'purely frictional material,' as these grains can be deformed and crushed. The discusser also notes that the sand grain properties include both cohesion c and Young's modulus E, which have dimensions of stress. In applying dimensional analysis (Buckingham's II theorem) to the test results, the discusser notes that the size effect addressed by the authors is really the effect of changing c and E at both the same time and by the same factor. With no firm conclusion as to what proportion of the influence is shared by c versus E, the discusser suggests a more appropriate title for the paper, should cohesion c be shown to have greater influence. The proposed new title would be Lateral Pipe-Soil Interaction in Sand with Reference to the Effect of Cohesion.

Closure to “Lateral Pipe-Soil Interaction in Sand with Reference to Scale Effect” by P. J. Guo and D. F. E. Stolle

The authors note that their research describes the effects of pipe (or size effect) as well as the differences between a full-scale and a reduced scale (or model effect), particularly under 1 g conditions. They further state that pressure sensitivity affects perfectly rigid grains; even those that interact by frictional forces with an intergrain friction coefficient working independently of the magnitude of the normal force. For that reason, the authors do not see particle crushing as inducing stress-level dependency in the normal stress range of the soil-pipe interaction presented in their research; however, they do concur that particle crushing under high confining pressures may contribute to stress-level dependency. As to the effect changes in cohesion c and elastic modulus E by the same factor, the authors state that their paper does not consider this. They add that in physical modeling, the scale effect cannot be viewed as identical to the effect owing to a variation in the cohesion c and the elastic modulus E by a factor.

Lateral Pipe–Soil Interaction in Sand with Reference to Scale Effect

This paper investigates the pipe.soil interaction for pipes buried in cohesionless soil when subjected to lateral ground movement. A review and reappraisal of the literature is conducted to identify some inconsistencies and shortcomings of previous studies. The effects of model scale, stress level (via burial depth), burial depth ratio, and soil properties are systematically addressed through finite element analyses. The study concludes that the effects of pipe size and burial depth must be taken into account to properly estimate the maximum pipe.soil interaction forces induced by lateral ground movement. A unique equation considering scale and the effect of burial depth is proposed to determine the maximum pipe-soil interaction forces at various burial depth ratios.