Upheaval Buckling Of Pipelines Research Articles

Analytical solution of pipeline upheaval buckling considering bi-linear axial pipe-soil interaction model

Under thermal loading, upheaval buckling of subsea pipelines occurs when the axial compressive force exceeds the critical buckling load. In order to accurately predict the upheaval buckling behaviour of subsea pipelines, the axial pipe-soil resistance should be considered more precisely, rather than traditionally treated as rigid-plastic in prior analytical researches on pipeline upheaval buckling. Consequently, this study integrates a bi-linear axial pipe-soil resistance model into the mathematical framework of upheaval buckling. This mathematical model incorporates the von-Kármán type of geometrical nonlinearity and the Euler-Bernoulli beam theory. The research examines typical upheaval buckling behaviour and investigates the influence of axial mobilization distance and ultimate resistance on pipeline upheaval buckling behaviour. The results reveal that incorporating bi-linear axial pipe-soil resistance, in contrast to rigid-plastic resistance, leads the pipeline more susceptible to buckling. Displacement amplitudes increase with the axial mobilization distance during the post-buckling stage. Notably, a larger axial mobilization distance exerts a stronger influence on pipeline buckling. Moreover, the critical buckling temperature exhibits an almost linear negative correlation with axial mobilization distance and a positive correlation with axial ultimate resistance. Additionally, greater axial ultimate resistance magnifies the impact of axial mobilization distance. Therefore, in pipeline buckling design, it is advisable to consider a more sophisticated axial pipe-soil model to accurately account for these complexities.

Influence of Wave-Induced Uplift Forces on Upheaval Buckling of Pipelines Buried in Sandy Seabeds

This study focuses on the buckling of pipelines in shallow waters subjected to surface gravity waves. The wave-induced uplift forces on pipelines buried in sandy seabeds are investigated using Biot's consolidation model. Empathetic imperfection model proposed by Taylor and Tran (1994, “Experimental and Theoretical Studies in Subsea Pipeline Buckling," Mar. Struct., 9(2), pp. 211–257.) is used for the study. Thereafter, buckling analyses are performed on the pipeline with the combined temperature and the wave-induced loads. The differences in the critical buckling temperatures for the pipe with consideration of wave loads are analyzed within a range of sea states. The influence of wave loads is found significant for low burial depth ratios.

Probability of upheaval buckling for subsea pipeline considering uncertainty factors

ABSTRACTFor buried subsea pipelines, upheaval buckling may occur under high pressure and high temperature service conditions. Pipelay initial imperfections and pipe–soil interactions have significant impacts on the upheaval buckling behaviours of pipelines. For a subsea pipeline designed to be installed, these two factors could be seen as random variables along the route and the approach to evaluate the upheaval buckling probability of this subsea pipeline under uncertainty conditions is proposed in this paper. Probabilistic evaluations for initial imperfections which would induce pipeline upheaval buckling and the influences from the pipelay vessels are included in this approach. Furthermore, the method of reliability evaluation can also be employed for evaluating the probabilities of lateral buckling for high temperature pipelines, or the snake-lay anti-buckling technique.

Upheaval buckling of pipelines due to internal pressure: A geometrically nonlinear finite element analysis

This work presents an analysis of upheaval buckling of pipelines triggered by internal pressure. It discusses the existing relationship between internal pressure and equivalent compressive axial force in the buckling context. The main focus is on the relative influence of prop imperfections and soil friction coefficients in critical load prediction and post-buckling configuration. To perform the analyses, numerical models are developed using geometrically-exact finite element of beams, undergoing large displacements and finite rotations. Contact between the pipeline and the soil is also included in the models. As a result, the work shows the equivalence of applying the internal pressure as a distributed load dependent on pipe curvature and as a follower compressive axial force, both in terms of critical load and post-buckling configuration. Varying prop imperfections and soil friction coefficients, it is concluded that the first parameter has more influence in critical load prediction than the last one. The same occurs in terms of post-buckling configuration: for the same increase of internal pressure from critical load, the imperfections have more influence in the post-buckling displacements than the friction between the pipeline and the soil.

Upheaval buckling of surface-laid offshore pipeline

Offshore pipelines operating under high pressure and temperature are subjected to upheaval buckling. Pipeline behaviour in upheaval buckling depends on a number of factors including the shape of pipeline imperfection, installation stresses, loading types, seabed sediment behaviour and the flexural stiffness of the pipe. Current method of predicting upheaval buckling is based on simplified shapes of pipeline imperfection developed for idealized seabed conditions. To account for the effect of internal pressure, the pressure load is represented using an equivalent temperature. However, the applicability of these idealizations on the prediction of upheaval buckling has not been well-investigated. In this paper, the three-dimensional finite element modelling technique is used to investigate the applicability of idealized shapes and their effects on the upheaval buckling of pipeline for a seabed condition at offshore Newfoundland in Canada. The finite element model is then used to conduct a parametric study to investigate the effects of installation stress, loading types, seabed parameters and the flexural stiffness of the pipe. Finally, a design chart is developed to determine the optimum height of seabed features to manage pipeline stability against upheaval buckling under different temperature and pressure loadings.

A New Holistic Approach for Subsea Pipeline Upheaval Buckling Design

For a trenched and buried pipeline, the propensity to upheaval buckling (UHB) is a major design concern. Predictive UHB design is typically required at the outset to determine both trenching and backfilling requirements. Additional rockdump schedule can be established by analyzing post pipelay out of straightness (OOS) survey data incorporating appropriate safety factors based on a structural reliability analysis (SRA). The normal approach is to examine the as-laid pipeline imperfection survey statistics and data accuracy. The structural reliability analysis and load factor calculation are typically performed a priori based on the assumed initial imperfections using the universal design curve methodology. A new pseudo-energy method for UHB and OOS is proposed and discussed in this paper based on the variational principle and modal analysis. The approach takes into account the effects of varying effective axial force, trench imperfections, and vertical uplift resistance, by combining both axial friction and lateral resistance methods into a unified model. A new concept, effective uplift resistance and associated load, is also introduced to deal with nonuniform backfill cover. Adjacent imperfections and backfill profiles are considered in detail. A finite element (FE) model is developed to consist of three-noded quadratic pipe elements using abaqus Ver 6.12, and iterations of FE analyses are performed to demonstrate the tangible benefits of the approach specifically for UHB OOS design in relation to target trenching and backfilling, leading to improved reliability and potential cost saving in UHB OOS design and rockdump installation.

Uplift tests on full-scale pipe segment in lumpy soft clay backfill

Upheaval buckling of pipelines caused by thermal- and pressure-induced loading is an important issue in pipeline design. The uplift capacity of pipelines is determined by the pipe–soil interaction during pipeline upheaval in soil. Pipelines to be installed in soft clay are usually placed into trenches and then backfilled. In this paper, a set of test devices were developed and a series of full-scale model tests were carried out on a pipe segment buried in lumpy soft clay backfill, including backfilling tests, load-controlled uplift tests, and a displacement-controlled test. Eight total pressure transducers were embedded in the wall of the pipe segment to measure soil pressures on the pipe segment, and five linear variable differential displacement transducers (LVDTs) were arranged to record the vertical displacement of the pipe segment and the surface of the soft clay ground. The stabilizing force keeping the pipe segment in place during the backfilling process was found to fit a nearly linear relationship with the dimensionless undrained shear strength of soft clay. The variation of soil pressures on the pipe segment during uplift loading was significantly affected by the buried depth of the pipe segment and the undrained shear strength of the soil. For all present load-controlled tests in lumpy soft clay backfill, the test ultimate uplift resistances were only about 19%–81% of the results calculated by the Det Norske Veritas approach. Mainly due to the voids’ compression, shearing and strain softening of lumpy soft clay backfill, the difference between initial and stable displacements in a loading step for a load-controlled test or initial and stable loads in a displacement step for a displacement-controlled test is remarkable. The limits of uplift resistances are recommended for the instant and sustaining behaviors of the pipe segment, respectively.

Numerical and Experimental Study of Uplift Mobilization of Buried Pipelines in Sands

Offshore oil and gas pipelines are commonly buried in seabed to provide environmental stability, thermal insulation and mechanical protection. These pipelines are frequently subjected to high thermal and pressure loadings that induce pipeline upheaval buckling (UHB). The uplift resistance provided by the cover soil increases with pipeline upward mobilization and reaches its peak at peak mobilization. This peak mobilization is fundamental for safe UHB designs. This paper highlights that the design guidelines underestimate the peak mobilization. The paper presents full-scale uplift results and finite-element parametric studies in which loose and dense cover soils of up to 3 m (soil cover height to diameter ratio up to 15) and pipeline diameters of 114 and 200 mm were investigated. Results from full-scale and finite-element modeling show that the peak pipe mobilization can be much greater than that suggested by current guidelines, and it is a function of the soil cover height to diameter ratio and soil relative density. A new relationship is proposed to predict peak mobilization from the soil cover height to diameter ratio for a given soil state.

Upheaval buckling of a pipeline with prop imperfection on a plastic soft seabed

Conventionally a pipeline resting on a prop tends to buckle upward under thermal loads, which is called upheaval pipeline buckling. To analyze the phenomenon, most previous studies have modeled the pipeline as a long heavy beam resting on a rigid seabed. However, the seabed may be very soft and uneven; consequently, the pipeline may penetrate into the seabed under thermal compressive forces and change its buckling mode. This study is concerned with the effect of seabed resistance on upheaval buckling of pipelines with prop imperfection. We find that the seabed resistance exerts a remarkable influence on the phenomenon of upheaval pipeline buckling.

Engineering measures for preventing upheaval buckling of buried submarine pipelines

In-service hydrocarbons must be transported at high temperature and high pressure to ease the flow and to prevent the solidification of the wax fraction. The high temperature and high pressure will induce the additional stress in the pipeline, which results in the upheaval buckling of the pipeline. If such expansion is resisted, e.g., by the frictional effects of the foundation soil over a kilometer or of a pipeline, the compressive axial stress will be set up in the pipe-wall. When the stress exceeds the constraint of the foundation soil on the pipeline, suddenly-deforming will occur to release the internal stress, similar to the sudden deformation of the strut due to stability problems. The upheaval buckling may jeopardize the structural integrity of the pipeline. Therefore, effective engineering measures against this phenomenon play an important role in the submarine pipeline design. In terms of the pipeline installation and protection measures commonly used in Bohai Gulf, three engineering measures are investigated in great details. An analytical method is introduced and developed to consider the protection effect of the anti-upheaval buckling of the pipeline. The analysis results show that the amplitude of the initial imperfection has a great effect on the pipeline thermal upheaval buckling. Both trenching and burial and discrete dumping are effective techniques in preventing the pipeline from buckling. The initial imperfection and operation conditions of the pipelines determine the covered depth and the number of layers of the protection measures.

Vertical upheaval buckling of submarine buried heated pipelines with initial imperfection

In this paper, a theoretical solution of vertical buckling is proposed with regard to the typical initial imperfection cases of submarine pipelines. Analytical tools are applied to predicting the occurrence and consequence of inservice buckling of a buried heated pipeline in Bohai Gulf. An evaluation is performed to ensure the pipeline structural integrity during operation under loading conditions. Different protection measures are proposed and their validities are analyzed. Analyses show that for the same magnitude of initial imperfection, the upheaval buckling of pipeline with isolated prop model is the most likely to occur. The empathetic model represents a special sub-case of continuous prop model, and the calculated buckle temperature is between the first stage and the second stage of post-upheaval buckling of continuous prop model. And the larger the initial imperfection, the less the axial force required for the upheaval buckling. Meanwhile, it can be seen that a peak point appears on the curves of temperature difference against buckling amplitude for small initial imperfection. Besides, trenching-burial is one kind of protection measures preventing the pipeline from thermal upheaval. The covered depth-to-diameter ratio depends on the design conditions and subsoil properties. For the given pipeline in this paper, the covered depth-to-diameter ratio is recommended to be 5.

A Reliability-Based Calibration Study for Upheaval Buckling of Pipelines

Abstract The present paper describes a reliability-based design procedure against upheaval buckling of rock or soil-covered pipelines. The failure mode considered is “snap-through” buckling. The study is performed using state-of-the-art design methodologies, including an assessment of all known uncertainties related to the load and capacity, measurements, surveys, and confidence in the applied models. A response surface technique is applied within the level III reliability analysis. Target safety levels are discussed for both SLS and ULS conditions, and a case-specific reliability-based calibration study is performed in order to derive a consistent design format.

Asymmetric effects of prop imperfections on the upheaval buckling of pipelines

Submarine pipelines often carry products which are much hotter than the surrounding seawater. The potential thermal expansion is restrained by friction between the pipeline and the seabed, causing the development of large compressive axial forces in the line, which can lead to buckling of the pipeline. This paper takes a fresh look at the vertical buckling of a pipeline encountering a point irregularity on an otherwise perfectly flat seabed, the so-called ‘prop case’. Some approximations and assumptions in earlier work in this area are reexamined and their effects are calculated. Most importantly, the assumption that buckling is symmetric about the prop is tested. Asymmetric results are found, at a lower critical temperature than the symmetric mode, a fact which may have significant implications for design.