Offshore pipe clay seabed interaction in axial direction

Offshore pipelines are increasingly being required to operate at high temperature and pressure as more petroleum deposits are explored further away from the shore. These extra-long pipelines are usually placed in deep sea; normally trenching to bury them becomes impractical, and therefore they are placed directly on the seabed, embedding partially only by the self-weight. As the length of pipeline increases along with the operating temperature, the likelihood for the pipeline to expand during heating cycles increases. Therefore, during heating and cooling cycles, the pipeline can experience continued expansion or the behaviour known as “pipeline walking” amounting to metres of expansion. The pipe-soil interaction behaviour could be classified into three dimensions, corresponding to axial, vertical and lateral directions. Commonly, all three of these interaction behaviours are studied in isolation, and the understanding developed combined to predict the overall response of the pipe under combined loadings. In recent years, some efforts are also reported to characterise the combined behaviour of the pipe in two or more directions. In particular, extensive research is undertaken to investigate both the vertical and lateral pipe displacement behaviours. However, the pipe axial interaction phenomenon, which is considered as the prime reason for pipe instability, is yet to be clearly established either in isolation or in combination with the interaction behaviours in the other two directions. Another unique characteristic to offshore pipe embedment is the low effective stresses at the interface. In submerged condition, significant buoyant effect (Bruton et al. 2005) would act on the sealed pipe walls and reduces the effective stresses on the soft seabed soil. As a common practice these petroleum pipes are provided with thick concrete coatings which serve as corrosion barriers. Although such coatings make the pipe heavier, the thickness of the pipe (nearly twice the diameter) counteracts the weight by enhancing buoyancy. Therefore, in deep seabed conditions, the pipe interaction occurs at very low effective contact stresses on soil, thereby providing very low interface shear strengths. As these conditions are not very common to geotechnical applications (e.g., strip footings), the soil behaviour at such low effective stresses has rarely been investigated. Funded by the CSIRO Cluster Pipe Project, the main research objectives of the current study are to interpret the axial interaction behaviour of offshore pipelines to be laid on the seabeds of the North Western Shelf of Australia. A special sophisticated 2D actuator setup (Monash Advanced Pipe System or MAPS) was purpose built to simulate the pipe walking on a model clay seabed. Element pipe testing was conducted for both drained and undrained pipe displacement rates The results led to the characterisation of the governing parameters on the axial pipe-soil interaction behaviour. The problem was also studied by means of extensive numerical studies. The models provided the means to study the soil behaviour that cannot normally be measured by real time experiments.