3D modelling of the upheaval buckling of offshore pipelines

Researchers: Dr Byron Byrne, Dr Chris Martin, Jens Schupp

Sponsors: EPSRC

Submarine pipelines are used to transport oil and gas between offshore facilities and also to facilities based on land. The pipelines are often laid in trenches, that are subsequently backfilled, so that they are protected from damage. The oil and gas that is pumped through the pipe is usually much hotter than the ambient water temperature and will cause the pipe to expand. Occasionally the expansion of the pipe can become localised and a buckle occurs. The only resistance to the pipe as it buckles is that provided by the surrounding soil. As the backfill material over the pipe is weaker than the in-situ material beneath the pipe it will tend to move upwards to the seabed surface and so be prone to damage. This is a well known area of concern for offshore pipeline designers and is usually called 'upheaval buckling'. Calculation methods have been devised to estimate the soil resistance to the 'upheaval buckle'. However these are all based on two dimensional tests ('plane strain' tests). These tests usually consist of lifting sections of pipe upward through a soil sample to determine the resistance of the pipe to vertical movement. A second, less well publicised, area of concern is that of pipeline floatation. This occurs when the seabed sediments, used for backfilling of the trench, are sufficiently unstable that large sections of pipe 'float' upwards as the oil or gas is pumped through the pipe. This might be because of a change in the pipe characteristics, such as buoyancy, or it might develop from an instability such as an upheaval buckle leading to an 'unzipping' of a length of pipe. Research on pipeline floatation is relatively sparse and the identity of soil conditions where floatation might occur is not clear. This research project will seek to address two fundamental issues:
(i) To conduct tests in three dimensions to assess the effect of carrying out true model scale tests compared to the traditional two-dimensional tests. The pipelines will be trenched and backfilled within a soil sample before the upheaval buckling behaviour is induced. The results will be compared to theoretical calculations of pipeline behaviour.
(ii) To investigate mechanisms of and the conditions necessary for pipeline floatation and to assess the relationship, if any, with the upheaval buckling mechanism.