Marine Scour

Scour is the phenomenon which you observe if you stand on a beach at the edge of the surf zone. Waves push water up and down the beach; as the water runs past your feet, the sand around them is carried away and a depression, or “scour pit”, forms where you are standing.

More generally, scour is the type of erosion which occurs when an object or structure is placed in the marine environment. Oceanic flows caused by tidal currents and waves are disturbed when they clash with a structure. If the structure is located on a seabed which has loose sediments (sand, mud or gravel), then this flow disturbance can cause erosion in the area immediately surrounding the structure and form a scour pit.

Scour represents a considerable challenge for offshore engineers because it has the potential to compromise stability and cause fatigue to marine structures. This can prove problematic in the high energy environments where wind turbines are often located.

Monopiles are the most common type of foundation used in offshore wind farms. Typically, scouring will form a circular pit around the pile in the shape of an upside-down cone (Figure 1), but their shape has been observed to be more irregular if there are unusual seabed or flow conditions. The depth of the pit is determined by the size of the pile; as a rule of thumb the maximum scour depth will be double the width of the pile.

The scour process becomes more complex for structures with other shapes. For example, scouring occurs if shifting sandbanks expose the cables associated with offshore wind farms. Scour undermines the cables and can leave them dangling freely in the water column, where they are exposed to potentially damaging forces from currents and waves.

Scour protection methods include rock blankets, artificial seaweed frond mats, turbine collars, reticulated concrete blocks, and Scour Prevention System’s tyre mats. The offshore wind industry is experiencing a period of rapid expansion, and the development of effective scour protection is vital to decrease project risks for this fledgling industry.