Effect of a skirted mudmat foundation on local scour around a submerged structure

Abstract

In this paper, local scour around a subsea structure supported by a skirted mudmat foundation is explored experimentally. For simplicity the structure is represented as a submerged circular cylinder founded on a concentric circular mudmat. The experiments are conducted in the clear-water regime and various geometries are considered by varying the height of the submerged cylinder relative to its diameter h1/D1, and the diameter of the mudmat relative to the submerged cylinder D2/D1. The experimental results indicate that the scour process can be categorized into two modes depending on geometry: Mode 1, in which local scour develops immediately at the upstream side of the mudmat; and Mode 2, in which local scour initiates at the downstream side of the mudmat and propagates upstream. For each mode, the rate of scour reduces as the mudmat becomes relatively large (i.e. as D2/D1 increases). This reduction is most significant if the mudmat size is sufficient to swap the mode of scour from Mode 1 to Mode 2. The reduction in scour rate, however, does not coincide with a reduction in maximum scour depth. Instead, the relative scour depth of the superstructure S/D1 increases as the mudmat becomes larger (i.e. as D2/D1increases). This trend is quantitatively explained by considering the effective aspect ratio of the combined mudmat and superstructure. Collectively, the results of the present study provide insight into the inherent scour protection provided by a skirted mudmat foundation.