Gravity current-submarine structure interaction: Hazard analysis via high-resolution simulations

As the offshore industry moves towards deeper ocean environments, submarine structures become increasingly exposed to less understood hazards, among them gravity and turbidity currents. With this motivation in mind, the interaction of gravity currents with submerged cylinders mounted at or above a non-erodible bed is investigated by means of Navier-Stokes simulations. To the author's knowledge, the present is the first in-depth numerical study of such interaction. Emphasis is placed on the drag and lift forces exerted on the cylinder, the wall shear stress near the cylinder, and the reduction of the front speed of the oncoming gravity current.;The drag increases towards a maximum when the current impacts the cylinder, and eventually reaches a mean quasisteady value. Maximum loads are crucial information for the design of submarine structures. Thus, an estimate is obtained for the maximum drag, by noticing that this drag results from an imbalance of hydrostatic pressure forces, and the deceleration of dense fluid flow as it encounters the cylinder. Some time after the current meets the cylinder, the drag and lift undergo sustained fluctuations for certain parameters. These fluctuations result from Karman vortex shedding. The wall shear stress near the cylinder is related to the scour near pipelines mounted on erodible beds. The magnitude of the wall shear stress is largest when the current impacts the cylinder. Consequently, the most aggressive scour is expected at impact. This is the key difference between the scour in gravity current flows and constant density flows. The reduction of the front speed of the oncoming gravity current by protective barriers is predicted with a simplified theoretical model, based on the shallow water equations.