An approach to estimate the optimal depth of burial of crude oil pipelines for different marine conditions in Kuwait

Abstract

Crude oil export is the main business in Kuwait. To transfer the crude oil from land to the ships moored in deeper marine water, submarine pipelines are used. Kuwait is also planning to increase the export to 4.0 million barrel/day in the future and hence need more export terminals, which in turn need more submarine pipelines. One of the challenging problems here is selection of optimum burial depth of the submarine pipeline for the prevailing design condition and soil type. The optimum burial depth of submarine pipeline is the depth at which the pipeline will be stable during the design environmental conditions. It depends on the reduction of wave forces due to burial in the seafloor. Also, the wave forces on the pipeline at any depth of burial depend on the wave characters, hydraulic properties of the sea bed soil and all other marine environmental parameters. Physical modeling is used as tool for assessing the wave forces on the pipeline model for a wide range of wave conditions, for different burial depths and for four types of cohesion-less soil types from Kuwaiti coastal area, covering hydraulic conductivity in the range of 0.286 to 1.84 mm/s. It is found that for all the four soil types, the horizontal wave force reduces with increase in depth of burial, whereas the vertical force generally increases for half buried condition, mainly due to the significant change in the magnitude as well as the phase lag between the hydrodynamic water pressures on the top and bottom of the pipe. Among the soils, well graded soil is found to be good for half burial of pipeline, since the least vertical force is experienced for this soil type. On the other hand, uniformly graded and low hydraulic conductivity soil (like Al-Koot soil with hydraulic conductivity of 0.286 mm/s) attracts the maximum vertical force for half burial case. But, such soil type is found to be good for full burial or further increase of burial, since it is found to attract less vertical wave force, when compared to the soils with high hydraulic conductivity (1.84 mm/s from Shuaiba coast). From the detailed investigations and case study analysis, the minimum safe burial depth for a 1.0 m steel pipe is 1.5 m to 2.0 m for transporting crude oil in Kuwaiti marine environment. The results of this study can be used to select the minimum safe burial depth in a range of cohesionless soils and for a range of marine wave conditions.