This paper reports results from experiments carried out in a laboratory flume to investigate the process of seabed scour around cylinders of finite length under the action of currents close to the threshold of motion for the bed sediment. The conditions relate to the design of large caisson foundations for offshore oil and gas installations and wind turbines. Results show that depth and extent of scour are much reduced for cylinders that stand less than one diameter above the seabed compared to tests with a surface-piercing cylinder. When truncated cylinders stand on shallow foundations, scour patterns are not sensitive to skirt depth, even when the cylinder has been undermined. For the present tests, maximum scour depth around truncated cylinders in a tidally reversing current was found to be less than 50% of equilibrium scour depth for an equivalent unidirectional flow.
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