Soil-Pipeline Interaction at the Seabed

Chi Yin Cheuk, Cambridge University
Geotechnical Engineering Group


Trenching and backfilling is a common method to avoid thermal buckling of oil pipelines. The high construction cost means that it is important to predict the minimum required burial depth to generate sufficient uplift resistance. Current design methods are, however, highly empirical. Improved design efficiency would arise from a better understanding of the uplift mechanism of a buried pipe. A more efficient solution to relieve the axial stress created by thermal expansion is to leave the pipe unburied and control the formation of lateral buckles along the flowlines. This costeffective design concept requires a thorough understanding of the interaction between the soil and the partially embedded pipeline under lateral movement. Research has been carried out to look into these two design methods for tackling thermal buckling in pipelines.

Pipe uplift behaviour in clay was studied by a series of centrifuge tests. Natural clay collected from the Gulf of Mexico was broken into small lumps to simulate the soil cover created by the trenching and backfilling process. The uplift resistance of a buried pipe was measured at different pulling velocities and the time allowed for the lumpy soil cover to consolidate was also varied. The suction force generated underneath the pipe, which varied with pulling speed, was found to be a significant fraction of the overall uplift resistance.

To establish the failure mechanism during pipe uplift in sand, a series of model pipe uplift tests was conducted in a transparent-sided plane-strain calibration chamber. A new image analysis technique was used to track the soil movement without recourse to intrusive target markers. From an initial embedment of 3 diameters, the pipe was extracted vertically under displacement control. Tests were conducted in uniform silica sands with grain sizes varying over an order of magnitude and at different densities. Results showed that peak uplift resistance was mobilised associated with a distributed shear zone. This was followed by the formation of shear bands as a result of shear strain localisation, giving rise to the strain softening behaviour. Particle size effects were found to be minimal for the chosen grain sizes and cover depth ratio.

A series of full-scale tests on short pipe sections was conducted to study the lateral interaction between seabed soil and on-bottom pipelines. Kaolin clay and a natural seabed clay collected from the Gulf of Guinea were consolidated to the required shear strength by vacuum pressure. The lateral force exerting on a partially embedded pipe was measured during cyclic horizontal movements of up to 7 diameters, while free vertical movement was permitted and recorded. A 1/15th scaled centrifuge test package was also developed. Good agreement was observed between the results obtained from different scales. The results suggested that there are three key phases in cyclic pipe motion; the breakout phase, the steady accretion phase and the residual berm resistance phase. Design equations are proposed based on the experimental results.

Keywords: Pipeline, seabed, sand, clay, uplift, shear band, particle size