Scour near Offshore Monopiles, Jacket-Type and Caisson-Type Structures

Scour near various offshore structures (monopile, caisson foundation and jacket structure) was studied by performing laboratory flume tests and numerical solutions with a semi-empirical model (SEDSCOUR) and a sophisticated 2DV model (SUSTIM2DV). The laboratory test results show that the maximum free scour depth around a monopile without bed protection is slightly higher than the pile diameter. The maximum scour consisting of pile scour and global scour around an open jacket structure standing on four piles is much lower than the scour near the other structures (monopile and caisson). The maximum scour depth along a circular caisson foundation is found to be related to the base diameter of the structure. The main cause of the scour near these types of structures is the increase in the velocity along the flanks of the structure. Six cases have been used for validation: two laboratory cases (A and B) and four field cases (C, D, E and F). The measured scour values of the new physical model tests with the monopile and the open jacket structure presented in this paper are in reasonably good agreement with other laboratory and field scour data from the literature. The semi-empirical SEDSCOUR model proposed in this paper can be used for the reliable prediction of free scour and global scour near monopiles and jacket structures in a sandy bed (even with a small percentage of mud, up to 30%). The maximum scour depth along a large-scale caisson structure is more difficult to predict because the scour depth depends on the precise geometry and dimensions of the structure and the prevailing flow and sediment conditions. A detailed 2DV model with a fine horizontal grid (2 m) along a stream tube following the contour of the caisson is explored for scour predictions. The 2DV model simulates the flow and sediment transport at 50 to 100 points over the depth along the stream tube and can be run on a time-scale of 1 year.