Offshore pipelines are often buried to provide (i) thermal insulation, (ii) protection from anchor dragging or trawling and (iii) protection from scour or hydrodynamic loading. Compressive forces in the pipeline associated with transportation of hot fluids tend to cause the pipeline to buckle, which is resisted by the overburden stress from the burial material. Quantifying both the magnitude of the resistance to buckling and the pipeline movement to mobilise this resistance are important for design. Much of the effort in this area has focused on understanding this behaviour for upheaval buckling, where the pipeline moves vertically -less attention has been given to the scenario where the pipeline movement is lateral or inclined. This paper reports results from a program of tests in which pipeline sections buried in trapezoidal rock berms were pulled out at different load inclinations. The tests considered a 0.2 m diameter pipe buried up to 1.2 m deep in rock, with variations in the (rock) cover widths, cover heights and load inclinations. Results show that the resistance during inclined pullout is significantly greater than during vertical pullout. The resistance is also enhanced by higher and wider rock cover. The results allow part of a failure envelope, in vertical-horizontal load space to be assessed, quantifying this influence of pullout direction.
Conleth D. O'Loughlin, David J. White, Alasdair J. Maconochie and G. J. Yun
Keywords: offshore pipelines, model testing, centrifuge, rock berms, load inclination
Publisher: DFI