Sinking of armour layer around a vertical cylinder exposed to waves and current

• Sinking of a scour protection adjacent to a monopile exposed to waves and current.
• Design diagrams for prediction of the sinking.
• Description of the mechanisms causing the sinking.
• Design diagram for predicting the equilibrium sediment level around the pile.

The mechanisms of the sinking of a scour protection adjacent to a monopile are described in this paper, together with the determination of the equilibrium sinking depth in various wave and combined wave and current conditions based on physical model tests.Sinking of the rocks may ultimately lead to failure of the scour protection. It may cause exposure and free-span of cables, and possibly change the natural frequency of the wind turbine in an unfavourable manner. For these reasons it is important to consider the possible effects of sinking in the scour protection design, and to understand the mechanisms that could lead to unacceptable sinking of the scour protection.The study showed that the sinking is controlled by two mechanisms: removal of sediment adjacent to the pile (destabilizing) and infilling of sediment into the scour protection from the surrounding seabed (stabilizing). The latter mechanism is found to be the strongest, but it might take some time to fill the pores of the scour protection with sediment and during the time delay considerable sinking might take place. This means that the larger the scour protection, the larger the sinking will be (for a given KC-number smaller than approximately 15). The magnitude of the sinking of the scour protection adjacent to a monopile exposed to waves, and combined waves and current, was found to be similar to the current case.