Abstract

Offshore pipelines are often buried by ploughing or water-jetting in order for physical protection and thermal insulation. Upheaval buckling (UHB) is a common design issue for buried offshore pipelines when the out-of-straightness of the pipeline, combined with the high axial compressive forces induced by the extreme operating conditions, causes the pipeline to buckle upwards. The soil cover backfilled on top of the buried pipeline serves to provide resistance to UHB . The required pipe upward movement, or mobilisation, to achieve the desired uplift resistance from the soil cover is a vital design parameter, in that the structural integrity of the pipeline under operating conditions relies upon it. Currently, the DNV -RP-F110 is the most commonly used code by practicing engineers worldwide to address UHB in design. However, its provisions on uplift resistance, particularly on the mobilisation side, can be misleading. The main areas that have been investigated in this research are: mobilisation distance, uplift resistance at low covers (cover:diameter ratio, H/D, < 1), and scaling laws in centrifuge modelling. The talk presents the results from 34 physical model tests on pipeline uplift resistance in cohesionless soils, including 8 full-scale tests in saturated loose sand, 10 full-scale tests in dry gravel, and 16 centrifuge tests in saturated dense/loose sand. Cover heights in the full scale tests are up to 0.35m in sand and 0.6m in gravel. The pipe-soil interaction mechanisms, obtained from displacement- and strain-based PIV data, will be shown for both the full-scale and centrifuge tests. Comparison between centrifuge data and full-scale data, i.e. modelling of physical models, will also be presented to illustrate the limitation of centrifuge scaling laws on mobilisation.