Upheaval Buckling Research Articles

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.

Perturbation analysis for upheaval buckling of imperfect buried pipelines based on nonlinear pipe-soil interaction

Submarine pipelines are often operated under conditions of high temperature and high pressure, which may induce considerable axial compressive force. If the lateral movement is constrained, for instance, burying pipe or dumping gravel, upheaval buckling may happen. Pipe-soil interaction affects the upheaval buckling behavior of submarine pipelines. However, pipe-soil interaction models currently existing fail to reflect the drop process of uplift resistance when the vertical displacement is over mobilization distance. To solve this problem, a new nonlinear pipe-soil interaction model is presented and the governing differential equation of an imperfect pipeline on soft foundation is deduced. The solution to the governing differential equation is proposed based on nonlinear perturbation expansions. The effect of soil conditions, burial depth and initial imperfections on critical force as well as localization pattern of upheaval buckling are discussed. Results show that the capacity of pipeline against thermal buckling increases with the burial depth or maximum uplift resistance and decreases with the OOS of pipeline or mobilization distance. Critical buckling force is almost unaffected by pipe-soil interaction form, but the post-buckling response depends on pipe-soil interaction forms. The pipe-soil interaction model can be used not only in pre-buckling design but also in post-buckling control.

Strength reduction for upheaval buckling of buried pipes in blocky clay backfill

Offshore pipelines are often buried to protect the pipeline from external loads and upheaval buckling. Models for pipe uplift resistance in clay soils are based predominantly on homogenous backfill conditions. In practice, however, there will be significant soil disturbance during installation. With certain trenching techniques this may produce a backfill more akin to a matrix of lumps of intact soil connected by weaker remoulded interfaces. This research uses centrifuge modeling to assess the resistance provided by a representative lumpy clay backfill that has experienced self-weight consolidation. A model pipe is then uplifted through this model backfill in order to assess the soil uplift resistance. Results show that the uplift resistance in this material is governed strongly by the size of the lumps and, to a lesser extent, by the rate at which displacement occurs. When interpreted in terms of the strength reduction η, that may be used to correct between theoretical and measured uplifts, lower values were derived than those currently used based on intact soils. The value of η is seen to be controlled by a non-dimensional drainage parameter, but may be practically estimated based on an estimate of the size of lumps relative to the pipe diameter.

Upheaval buckling of in-service cased insulated flowline

ABSTRACTAn upheaval buckling prediction approach is proposed for a high-temperature cased insulated flowline (CIF). Based on the tensile cracking and compression crushing analyses of the CIF concrete-weighted coats, the curvature-related nonlinear bending stiffnesses of CIF systems are discussed. A changeable stiffness is included in this buckling prediction approach developed using a transfer matrix method. Due to a relatively low bending stiffness, greater curvature and higher bending stress would be induced in the carrier pipe at field-joint positions in the buckling response of CIF systems, while the shear force change would be induced in the five pipe coats of CIF systems. Using the approach developed in this paper, the field-joint stress of a CIF carrier pipe and the interlayer shear forces of the CIF pipe coats can be obtained along the buckling path of the CIF systems by plotting the relationship curves between the carrier pipe axial forces and the CIF buckle lengths.

Distributed monitoring method for upheaval buckling in subsea pipelines with Brillouin optical time-domain analysis sensors

A method for monitoring the upheaval buckling of subsea buried pipelines by distributed optical fiber sensors is introduced. The proposed method originates from the fact that the bucked pipe has the deterministic patterns of the bending-induced and axial force–induced strains, which can be used to identify the pre- and post-buckling states of the pipe. The challenge for utilizing these features is that the distributed fiber optic sensor can only measure the compound strain created by both bending and axial compression. A monitoring scheme by placing three sensors with [Formula: see text] space around the circumference of the pipeline was proposed in this article. An approach was presented to extract the bending-induced and axial force–induced strains from the longitudinal strains measured by the distributed fiber optic sensors. The experimental programs were then designed to investigate the feasibility of the proposed method. It is demonstrated that the proposed method is possible to detect the occurrence of buckling in the pipe. However, further research is necessary in order to examine the efficiency of the proposed approach under the complex loading patterns at subsea levels.

Buckling and Post-buckling Behavior of Beams With Internal Flexible Joints Resting on Elastic Foundation Modeling Buried Pipelines

The buckling and post-buckling behavior of axially loaded Winkler beams with flexible internal hinges is addressed, aiming to provide a background for the investigation of upheaval buckling of buried pipelines equipped with flexible joints for their protection against activation of reverse seismic faults. In order to acquire qualitative understanding of the interaction between the hinge stiffness and the soil stiffness for different cases, the beams under investigation are considered as either simply-supported or clamped. At first, elastic critical buckling loads and corresponding eigenmodes are numerically obtained using linearized buckling analysis, and eigenmode cross-over is investigated considering soil and hinge rotational stiffness. Geometrically nonlinear analyses with imperfections (GNIA) are then performed, indicating for most cases descending post-buckling paths, thus unstable post-buckling behavior, with the exception of cases of very soft soil. The sensitivity of the response to initial imperfection shape and magnitude is also addressed, to identify their impact on the post-buckling behavior. Beam buckling behavior is moreover examined by considering the beam being surrounded by soil exhibiting different stiffness in the upward and the downward direction. The results are compared to the case of a continuous beam, in order to highlight the impact of internal hinges on the beam overall buckling behavior.

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.

Estimating On-Bottom Stability of Offshore Pipelines in Shallow Waters of the Gulf of Guinea

Pre-installation Stability analyses of pipelines are required to prevent lateral and upheaval buckling in service. In this study, the hydrodynamic forces associated with an offshore pipeline is analyzed, thereby determining limiting steel wall-thickness and submerged weight necessary to prevent collapse and propagation buckling, contain pressure and ensure on-bottom stability. Relevant design equations, Codes and Procedures were integrated to create a comprehensive platform for analyzing lift, drag and inertia forces acting on submerged pipelines. Hence, a user friendly template with multiple design settings has been developed with MathCAD® for on-bottom stability analyses. The analysis tool is based on the absolute lateral stability method in DNV RP F109. A case study of Φ762 mm x 34 km pipeline to be installed Offshore Escravos, Gulf of Guinea is simulated and analyzed using the design tool developed. Pipeline behavior under different environmental and pipeline conditions such as water depth, wave height, steel and concrete thickness were investigated. The results showed that concrete and steel wall thicknesses are the most critical parameters in the on-bottom stability of offshore pipelines. With a determined optimal wall thickness of 20.6mm, concrete thicknesses of 78.796 mm, 61.386 mm, 53.043 mm and 42.58 mm corresponding to 5 m, 10 m, 15 m and 20 m water depths, respectively were obtained. Also, the results showed that for pipes OD > Φ32.5 in (Φ825.5 mm) alternative stability methods may be required as the necessary concrete thickness may exceed allowable limits.

Offshore pipeline performance evaluation by different artificial neural networks approaches

This paper investigates the upheaval buckling behaviour of offshore pipeline buried in clay soil considering the possible variability in soil, operating condition and pipe properties. A 2-D finite element model of the pipeline–soil system was developed in OpenSEES software to model the upheaval buckling. Further, the uncertainty in the controlling variables was modelled using the optimized Latin Hyper Cube (LHC) sampling technique to draw the samples from appropriate probability distribution. Finally, six different models based on artificial neural networks (ANNs) were developed to predict the performance of offshore pipeline using the simulated upheaval buckling displacement. A total number of 500 data were collected from simulation, randomly divided into 350, 75 and 75 datasets, and were used for training, validating and testing the proposed models, respectively. Comparison between results showed that all models are capable to deliver displacement values very close to the simulated ones. To determine the best performance model, several controlling methods were used and finally one of the models was suggested as the best one. An additional analysis was performed for displacements above 30mm where the number of achieved data is limited and scattering in data is observed. Analysis of the results illustrated that the models are reliable for predicting displacement values in upper band (above 30mm) as well.

Modelling of upheaval buckling of offshore pipeline buried in clay soil using genetic programming

Offshore pipeline is generally recognised to be the safest and most economical way to transport oil and gas. These pipelines are operated in elevated temperatures and pressures those are much higher than the ambient conditions. That will causes axial expansion in the pipeline, if such expansion is restrained by soil friction, the compressive force will be built up in the pipe, finally, induces the buried pipeline to buckle in the vertical plane. This paper investigates the effect of uncertainty in soil, operating condition and pipe properties on upheaval buckling behaviour of offshore pipeline buried in clayey soil. To simulate the upheaval buckling, a 2-D finite element model of 500m long pipeline-seabed soil system was developed in OpenSEES using the thermal element. Using the finite element model prediction of upheaval buckling height, a total number of 12 upheaval buckling height prediction models were proposed by using genetic programming with varying levels of complexity and accuracy. To achieve the best performance model, a scoring table was proposed considering several factors including coefficient of determination, sum of errors, difference between training and testing errors, sum of residuals, deviation of predicted results from experimental one and complexity and generality of the models. Finally, the effect of each parameter on upheaval buckling displacement was studied by parametric analysis and the results were compared by simulated ones. On the basis of the results, most of the models developed using genetic programming show very good prediction with the numerical results. The developed model can be used to improve the design and upheaval bucking risk assessment of buried pipeline.

Static and dynamic analysis on upheaval buckling of unburied subsea pipelines

Upheaval buckling is one of the most common problems threatening the safe operation of subsea pipelines, which is trigged by the increasing of temperature and inner pressure. In order to predict the critical buckling temperature and post buckling path of upheaval buckling, ABAQUS is used to build four kinds of numerical models, and they are static and dynamic models both in 2D and 3D. Two analysis procedures which combine the static and dynamic processes are applied to aforementioned models. The results show good agreement with existing test data. For snap upheaval buckling, pipelines have two different buckling modes. Such buckling modes are not found in experiments. In addition, only 3D dynamic model can catch such buckling modes. For bifurcation upheaval buckling, predicted buckling temperatures of those models are all acceptable with an error of 5%.

Laboratory tests and thermal buckling analysis for pipes buried in Bohai soft clay

Upheaval buckling of submarine pipelines occurs due to relative movement of pipeline and surrounding soil and is often triggered by high operational temperature of the pipeline, initial imperfection of the pipeline, or a combination of both. Since buckling can jeopardize the structural integrity of a pipeline, it is a failure mode that should to be taken into account for the design and in-service assessment of trenched and buried offshore pipelines. In this study, a series of vertical (uplift) and axial pullout tests were carried out on model pipe segments buried in soft clay deposit similar to that present in Bohai Gulf, China. Pipe segments with three different diameters (= 30 mm, 50 mm and 80 mm) were buried in different depth-to-diameter ratios ranging from 1 to 8. Based on the results of laboratory tests, nonlinear force–displacement relations are proposed to model soil resistance mobilized during pipeline movement. The proposed nonlinear soil resistance models are employed in finite element analysis of buried pipelines with different amplitudes of initial geometric imperfections. Thermal upheaval buckling behavior of pipelines operating at different temperatures is studied. Results show that the capacity of pipeline against thermal buckling increases with the burial depth and decreases with the amplitude of initial imperfection.

Upheaval buckling solution for submarine pipelines by segmented ditching and hot water flushing

An upheaval buckling solution is proposed using a preheating method combined with constraints from two segmented ditching constructions, which are scheduled before and after preheating. In this solution, some selected pipe segments along the route were curve-laid and preserved resting on the seabed in the first ditching construction, while other straight segments were trenched. The subsequent hot water flushing operation induced the curve-laid segments to buckle on the seabed, and then these prebuckles were laterally constrained by the second ditching operation carried out during preheating. After preheating, the cooling rebound of these prebuckles was constrained by the new trenches, and axial pretension was induced in the pipe wall to offset the axial compression in service and upgrade the thermal stability of the entire pipeline. A detailed design approach is provided in this paper, and corresponding Finite Element Model (FEM) verification is given for an example pipeline.

Numerical study on upheaval buckling of pipe-in-pipe systems with full contact imperfections

Upheaval buckling may occur in high-temperature and high-pressure subsea pipe-in-pipe (PIP) systems. Finite element method and dimensional analysis method are used to investigate the upheaval buckling response and critical buckling force of PIP systems with full-contact imperfections. Results show that imperfections, along with clearance (the gap between the centralizer and outer pipe) and stiffness ratio of the inner and outer pipe, influence buckling response and buckling force. Two empirical formulas that correspond to two typical imperfection profiles are proposed to calculate the critical upheaval buckling force of PIP systems on the basis of dimensional analysis and numerical results. The proposed empirical formulas show good accuracy.

Reliability functions for buried submarine pipelines in clay subjected to upheaval buckling

This paper presents the reliability analyses for studying and quantifying the variation of the reliability index (β) with the main parameters involved during the upheaval buckling of submarine buried pipes caused by high pressure and temperature conditions (HPHT). The analyses correspond to typical geotechnical and operational conditions of pipelines installed in the Bay of Campeche, offshore Mexico. The parameters that mainly control the vertical uplift capacity of pipe-soil system and considered as uncertain are: the undrained shear strength of soils (Su) and the soil cover height (H). Driving vertical loading depends mainly on the levels on temperature and pressure combined with vertical imperfections (δ), therefore those parameters were also considered as uncertain. Model errors for both capacity and driving vertical force were estimated and applied in the analyses.The limit state function was established by comparing the uplift pipe-soil capacity with the vertical driving loading. Parameter uncertainty was covered based on literature reviews, information of the proper conditions for pipelines operating in the Bay of Campeche and numerical analyses.A lower-bound capacity of the pipe-clayey soil system was included in the reliability analysis and is a novelty of this work. Lower-bound capacity is the smallest possible physical limit of the uplift pipe-soil capacity and its effect on the reliability is studied. Furthermore, finite element analyses were executed to evaluate the uplift capacity of the pipe-soil system and determine the analytical model error for estimating the capacity.The reliability indices (β) of the pipe-soil system under upheaval phenomena were evaluated by means of directional simulations. Consequently, the reliability functions β-δ and β-H/D (ratio cover height over pipe diameter) were quantified. It was found that the inclusion of the lower-bound capacity by means of a left-censored distribution possibly increases β in comparison to the estimations using conventional distributions, the maximum increments observed are in the range of 22–34%.

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.

Interaction between Upheaval/Lateral and Propagation Buckling in Subsea Pipelines

Due to high service temperatures and internal pressures in oil and gas pipelines, axial compression forces are induced in the pipe due to seabed friction. Slender trenched pipelines can experience global buckling in the vertical plane (upheaval buckling) while untrenched pipelines buckle in the horizontal plane (lateral buckling). Furthermore, deep subsea pipelines subjected to high external hydrostatics pressures can undergo catastrophic propagation buckling. In this study, the possible interaction between upheaval/lateral buckling and propagation buckling is numerically investigated using finite element analysis. A new concept is proposed for subsea pipelines design that gives higher capacity than conventional pipelines.

Upheaval Buckling Analysis of Buried Offshore Pipelines under High Temperature and High Pressure

Nonlinear finite element upheaval buckling model of buried offshore pipelines under HT/HP is built using ABAQUS. The petroleum is defined as uniform flow; temperature field of offshore pipelines produced in the process of petroleum transportation is obtained by heat transfer analysis; offshore pipelines are buried in trench of sandy seabed, interaction between seabed and offshore pipelines is defined as friction, seabed interaction with offshore pipelines will limit the movement of offshore pipelines; coupled fluid-structure analysis for three phase model of oil-pipe-soil is conducted to obtain stress under HT/HP. Initial imperfection of pipeline is introduced to calculate upheaval buckling of buried offshore pipeline under HT/HP. Through numerical analysis, the axial force of pipelines under HT/HP is obtained and thus resulted in upheaval buckling.

Critical upheaval buckling forces of imperfect pipelines

A buried pipeline may have upheaval buckling when it works in high temperature and high pressure (HT/HP) conditions. The upheaval buckling behavior is sensitive to initial structural imperfections. There have already been some approximation formulas of critical axial forces for some particular shape imperfections. However, these formulas did not take into account of the imperfection out-of-straightness (OOS) as a whole. Based on dimensional analysis and finite element (FE) analysis some brand new formulas are presented for the critical axial forces. These formulas are different from the traditional formulas in form and they include the out-of-straightness directly and integrally. Finally a case study is presented which illustrates the application of these formulas.

Response of Surface Cracks in Tubular Members During Global Buckling and Instability

Beam-columns in compression are subject to potential buckling. If the axial load is combined with a global bending moment, member instability may occur. One situation where this can occur is for pipelines on the seabed, where thermal strains induce significant compression in the pipeline due to constrained expansion. This may lead to upheaval buckling or snaking. In this condition one should also assess how a weld defect in the buckle zone will evolve. If the crack is located on the compressive side of the cross section, the crack remains closed. If the crack is located on the side where beam bending eventually leads to tension, fracture may develop. This has not been studied sufficiently in the past, and is the topic of the present paper. Here the interesting case of initial compression, with closure of the crack, is followed by a transition to tension and opening of the crack when the transverse displacement increases in the post-buckling regime. Simple cases of tubular beam-columns with surface cracks are investigated for cracks growing in a ductile manner. The simulations are based on shell and linespring finite elements.