Experimental study of local scour around submerged cylinders supported by a skirted mudmat in steady current

In this paper, local scour around a cylindrical subsea structure supported by a skirted mudmat foundation is explored experimentally. The experiments consider various geometries defined in terms of the height of the subsea structure relative to its diameter h₁/D₁, and the diameter of the mudmat relative to the structure D₂/D₁. The experimental results indicate that the scour development process can be categorized into two modes depending on h₁/D₁ and D₂/D₁. In one mode local scour develops immediately at the upstream side of the mudmat, whilst in the second mode scour initiates at the downstream side of the mudmat and propagates to the upstream side. For both modes, the rate of scour development reduces as the mudmat becomes relatively larger (i.e. as D₂/D₁ increases). However this reduction does not coincide with a reduction in maximum scour depth. Instead, it is shown that the relative scour depth of the superstructure (S₀/D₁) actually increases as the mudmat becomes larger (i.e. as D₂/D₁ increases). This trend of increasing scour depth and reducing scour rate with the introduction of a larger mudmat is observed for all structure aspects ratios; however, for small h₁/D₁ the scour depth is the smallest and the scour rate is the slowest. Collectively, the results of the present study provide some insight into the scour protection provided by a skirted mudmat foundation.