Thesis: Richard Sandford

Doctor of Philosophy, Oriel College, University of Oxford, Trinity Term 2012

Lateral buckling of High-Pressure/High Temperature On-Bottom Pipelines

On-bottom (or unburied) pipelines, which carry hydrocarbons across the seabed, are typically subjected to compressive axial loading arising from restrained thermal- and pressure-induced expansion. This compressive loading usually causes a nominally straight pipeline to buckle into a mode lying predominantly in the plane of the seabed; this response is widely termed lateral buckling. Predicting the response of an on-bottom pipeline to the thermal and pressure-induced axial compressive loading is the primary focus of this thesis.

In assessing whether or not the structural integrity of a pipeline is at risk during lateral buckling (and also the post-buckling regime), finite element analyses are typically carried out. In these analyses, the pipeline is modelled as an assembly of beam elements while the connection between the pipe and the seabed is modelled using a macro-element, which defines the relationship between the loads and displacements of the pipe. In this thesis, the development, calibration and implementation of a macro-element model for use in lateral buckling design is described. The proposed macro-element model accounts for the response during lateral displacement of multiple pipe diameter amplitude (as appropriate to the movement of the crown of a buckle), as well as the reversals in the direction of lateral displacement due to intermittent shut-downs in the operation of the pipe. The model is of the hardening plasticity type and is cast in terms of vertical and horizontal force resultants. Results from numerical analyses (using both finite element limit analysis and the displacement-based incremental finite element method) are used to calibrate the model. Its performance is tested by examining the results of retrospective simulations of experimental tests. Finally, the results of field-representative structural analyses are presented, which demonstrates the suitability of the model for use in design practice.