Offshore pipelines are often placed in the seabed by ‘‘jetting’’ the soil to soften it and allow pipeline penetration. In this paper, it is shown that this has implications for the resistance of the soil to upwards pipeline movements caused by pipeline temperature changes (upheaval buckling). To investigate this, a series of centrifuge model tests have been conducted to investigate the uplift capacity and the loaddisplacement behaviour of pipelines buried in recently liquefied clay. Both short and long-term loadings are considered. The undrained uplift capacities were seen to be lower than the drained capacity so that it was the undrained failure conditions that was critical. Recently liquefied clay may still be consolidating when pipelines are commissioned and are subject to upheaval loads. In such cases, the effective stresses in the soil (and thus the shear strengths) at the expected switch-on time have to be calculated in order to calculate the uplift capacity of the pipeline. A simple method is presented which predicts uplift capacity from the average degree of consolidation of the backfill. Good comparison between the experimental results and the simple prediction method suggests that such a method may be appropriate for initial pipeline design.
As the North Sea has matured as a producing region, pipeline activity has moved from large diameter trunk pipelines to smaller "in-field" flowlines. These are often buried in the seabed which provides protection from fishing activity and provides additional thermal insulation. The move to small diameter pipelines has prompted the use of reel-lay techniques, requiring thicker walled pipe to prevent buckling during the bending and straightening process. As a consequence of this new approach upheaval buckling is promoted by the elevated temperatures (which lead to thermal expansion) and the high degree of lateral and axial soil restraint.
