As part of the construction of offshore wind farms, Boskalis installs rocks as scour protection around monopile foundations on the seabed. Scour occurs when waves, tides, or currents remove sediment from the seabed, thereby risking a weakening of the foundations.
The rock is transported and installed by our subsea rock installation vessels which are fitted with an inclined fall pipe, enabling them to get within a few meters of a foundation and install the rock with great precision. However, the requirement to maintain a minimum distance between the vessel’s fall pipe and the monopile means it is not always possible to install the rock close enough to the monopile. To overcome this problem, Boskalis has developed a water jet system that attaches to the outlet of the fall pipe and enables the rocks to be installed up to one and a half meters closer to the monopile than when relying on gravity alone.
Three jet pumps that are attached to the bottom of the fall pipe serve as an extension of the pipe and carry the rock horizontally into position directly next to the monopile. The water pressure can be adjusted according to water depth to further increase the accuracy of rock placement. Boskalis has successfully secured a patent for the system.
The jet system has been fitted to our Rockpiper installation vessel and during 2025 was used to apply scour protection around 36 foundations across two projects.
“This work demonstrated the value of the fallpipe jet system since prevailing currents made it even more important that the scour protection was installed as close as possible to the monopile foundation,” explained Arjen Gossije, Manager Operations within Boskalis’ Seabed Intervention business unit. “Failure to do so risks
exposing the monopile to scour and weakening of the foundation.” Besides the Rockpiper, the fallpipe jet system will be installed on our new subsea rock installation vessel, Windpiper which has a cargo
carrying capacity of 45,500 tons. The Windpiper is currently being converted from an existing vessel and is expected to undertake its first subsea rock installation project in 2026
