Upheaval Buckling Research Articles

Centrifuge modelling of upheaval buckling in sand

When operating at high temperature and internal pressure, initial imperfections in a cold laid pipeline can lead, through expansion and an increase in axial stress, to imperfection· growth and upheaval buckling. Efficient design against upheaval buckling requires accurate prediction of soil uplift resistance. However, current design methods are based on empirical uplift factors chosen from soil classification data, and do not capture the strong influence of backfill density on uplift resistance. A series of tests are reported, in which the pipe uplift is modelled in a 0.8 m diameter mini-drum centrifuge. A key advantage of the mini-drum centrifuge is that uplift tests can be carried out on small soil samples taken from boreholes along the pipeline length. Pipe uplift can be considered as a routine laboratory test. The results demonstrate the strong influence of backfill density on peak uplift resistance. Observation of the failure mechanism suggests that existing design methods based on vertical slip planes are unrepresentative of the failure. Instead, a new solution is presented, which more closely represents the observed mechanism, and provides excellent correlation with the measured data of peak uplift resistance across a range of backfill densities. Design curves are presented, which require element test results as input parameters. However, upheaval buckling is an unusual example in which centrifuge models, standing alone, provide the most efficient design tool.

A simplified model of upheaval thermal buckling of subsea pipelines

That very close relationships exist between the thermal buckling of components such as continuously welded rail-tracks, concrete road pavements or subsea and buried pipelines, and the simple buckling of columns has long been recognized. It is surprising, therefore, that greater use has not been made of this relationship to provide both simplified design analysis and a more direct physical interpretation of behaviour. In the following the “classical” Martinet analysis is first reinterpreted and reproduced in terms of the buckling of the simple clamped column. This approach is then extended to provide an alternative conceptual understanding of the mechanics for, and a more direct and simplified analysis of, the upheaval buckling of systems containing initially imperfect geometries. It is suggested that the simple, closed form, solutions for both initial lift-off and the maximum upheaval buckling loads make this alternative approach particularly suited to future design to prevent potentially damaging upheaval buckling.

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.

An investigation into upheaval buckling of buried pipelines—II. Theory and analysis of experimental observations

This paper and its companion describe an experimental study of some aspects of upheaval buckling of buried pipelines by means of a small-scale model apparatus. Several theories of upheaval buckling, in which the foundation is modelled as a rigid base, are reviewed. Then a theory of railway-track buckling due to Tvergaard and Needleman is described: key features are the modelling of the soil by a nonlinear force-displacement relationship, and buckling by growth and localization of a periodic initial mode. An alternative, approximate investigation of the same situation leads to a simple formula for the axial load at which the localization of buckling occurs. This formula involves the flexural rigidity of the pipe, the amplitude of an initial periodic imperfection and the “plateau” soil resistance: the detailed form of the soil-resistance as a function of displacement appears to be immaterial. Experimental observations are presented on the growth of both horizontal and vertical displacement in the pipe as the axial load is increased; and it is shown that the observed buckling loads agree satisfactorily with the theoretical analysis.

An investigation into upheaval buckling of buried pipelines—I. Experimental apparatus and some observations

This paper and its companion describe an experimental study of some aspects of the upheaval buckling of buried pipelines by means of a small-scale model apparatus. After a discussion of relevant literature, the experimental apparatus is described in detail: it involves a thin-walled steel pipe of diameter 6 mm and length 5 m, buried in a bed of artificial soil. The horizontal and vertical profiles of the pipe are measured by an electrical remote-sensing device. Axial load is imparted to the pipe by a screw arrangement and/or by interior oil pressure; and transverse horizontal loading may be applied through a string and pulley. The arrangements for assaying the resistance of the soil to transverse motion of the pipe are described. Results are presented for several tests involving both axial and transverse loading, and cyclic axial loading. The paper concludes with a short discussion.

Upheaval Buckling of a Mechanism under External Pressure and Compressive Loading

A mechanical or structural element often buckles when it is compressed beyond a certain amount. Constraints may raise the value of the critical compressive load. In some cases a constraint may completely prevent the occurrence of buckling. This situation is demonstrated here by a simple two-bar model that rests on a rigid foundation and is subjected to pressure and compressive loading. If the system is ‘perfect’, buckling is not possible. However, if the foundation is not flat, or if the load is applied eccentrically, a sudden jump in deflection may occur as the load is increased.

Prop-imperfection subsea pipeline buckling

In-service buckling of subsea pipelines can occur due to the introduction of axial compressive forces caused by the constrained expansions set up by thermal and internal pressure actions. Proposed herein is a mathematical model relating to a pipeline, the otherwise horizontal and straight idealised lie of which is interrupted by an encounter with an isolated prop or point irregularity. The overbend produced can serve, in the presence of enhanced topologies involving trenching, burial, discrete or continuous, and fixed anchor points, to trigger vertical or upheaval buckling of the pipeline under in-service conditions. The results of a series of case studies are contrasted with data appertaining to alternative models available in the literature: experimental support is additionally noted. By questioning the implicit stress-free-when-straight assumption present in these alternative models, it is considered that a consistent, imperfection-prone isolated prop formulation is hereby provided, suitable for design application.

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.

Design Criteria for Upheaval Creep of Buried Sub-Sea Pipelines

A new criterion is presented for the design against upheaval creep of buried hot marine pipelines. Observations have substantiated that the gradual upheaval of an “imperfect” buried pipe can take place when subjected to variable temperature and pressure loading. The imperfection amplitudes can thus grow until at a certain stage the overburden is insufficient to prevent upheaval (snap) buckling of the pipe. This paper shows that the “classical” upheaval buckling analysis is not applicable when designing against upheaval creep failure of an “imperfect” pipe. A new design procedure is established which determines the uplift resistance required to keep the upward movement of the “imperfect” pipe below critical values, thus preventing a progressive upheaval failure. The various aspects to be considered during the design and installation of a pipeline are highlighted, resulting in requirements for acceptable out-of-straightness of the pipe.

Axisymmetric Upheaval Buckling of a Heavy Sheet

A large elastic sheet with a significant self-weight subject to biaxial in-plane compression while lying on a rigid horizontal plane may buckle. The buckles, circular areas which lift away from the rigid subgrade, are essentially a plate phenomenon, an axisymmetric analogue to the widely discussed upheaval buckling of beams. It is found that plate upheaval is relatively insensitive to subgrade friction, a fact which may simplify design against such buckling.

Two-dimensional upheaval buckling of a heavy sheet

A large elastic sheet with a significant self-weight subject to biaxial in-plane compression while lying on a rigid horizontal plane may buckle. The buckles, circular areas which lift away from the rigid subgrade, are essentially a plate (two-dimensional) phenomenon, analogous to the widely discussed one-dimensional upheaval buckling of beams. It is shown that plate upheaval is relatively insensitive to subgrade friction, a fact which may simplify design against such buckling.