Submarine pipelines encounter significant wave forces in shallow coastal waters due to the action of waves. In order to reduce such forces (also to protect the pipe against anchors and dropped objects) they are buried below the seabed. The wave force variation due to burial depends on the engineering characteristics of the sub soil like hydraulic conductivity and porosity, apart from the design environmental conditions. For a given wave condition, in certain type of soil, the wave force can reduce drastically with increased burial and in certain other type of soil, it may not. It is hence essential to understand how the wave forces (both horizontal and vertical) vary while the pipeline is buried in soils of different hydraulic conductivity. The selection of minimum safe burial depth of submarine pipelines mainly depends on the magnitude of wave force on the buried submarine pipeline. The minimum safe burial depth is the depth of burial at which the hydrodynamic forces encountered by the submarine pipelines do not destabilize them during the design environmental condition. The variation of wave forces on buried submarine pipeline is investigated using four different cohesion less soils with hydraulic conductivity varying from 0.286 mm/s to 1.84 mm/s. The physical modeling investigations were carried out for a wide range of random wave conditions (PM spectrum with significant wave heights from 5 to 20 cm and peak period from 1.0 to 3.0 s) and for different depth of burial. The horizontal and vertical hydrodynamic forces on the submarine pipeline were estimated by numerically integrating the measured dynamic pressures around the circumference of the pipe line at 12 points.
It is found from the study that the horizontal force reduces with increase in depth of burial, and is less dependent on the hydraulic conductivity of the soil. Whereas, the vertical wave force varies quite significantly (generally increases up to certain depth of burial and reduces with further increase in depth of burial), mainly due to the significant change in the magnitude and the phase lag between the dynamic pore water pressures. In general, if the hydraulic conductivity is high (order of 1.84 mm/s), then varying the relative burial depth from e/D = 0.5 to 1.5 does not provide appreciable advantage from the vertical force reduction point of view. On the other hand, for a soil with low hydraulic conductivity (order of 0.29 mm/s), changing the depth of burial from e/D = 0.5 to 1.5 reduces the vertical wave force more than 50%, where ‘e’ is the vertical distance between the sea floor and pipeline bottom and ‘D’ is the pipeline diameter. For half buried (or half exposed) condition, the pipeline in the soil with high hydraulic conductivity attracts the least vertical force and attracts high vertical force in the soil with low hydraulic conductivity, due to appreciable Bernoulli effect in low hydraulic conductivity soil. The results of this study will help the submarine pipeline design engineers to select the minimum safe burial depth in a range of cohesion-less soil in a wide range of hydraulic conductivity and random wave conditions.