Monopile Scour Mitigation

Wind turbine monopiles can be susceptible to a type of erosion called scour. Especially at sites with tidal currents, a significant section of the soil around a monopile can be removed due to the effect of the foundation on the local flow pattern and velocities.

The main issues associated with scour damage are:

- Reduction of foundation stability
- Weakening cables, placing stress on connections and riders

We have strong experience with scour mitigation projects in marine applications. We utilise this expertise to design dedicated offshore wind scour mitigation solutions. An example is our single lift scour collar which uses the same geotextile backing used in our Coastmat product. Another example can be found in our RipRap grouting solution, originally applied to coastal stabilisation projects but also very well suited for monopile scour mitigation.

FAQ’s

What is scour?

Scour is the displacement of sand, soil or clay around a structure induced by vessel wakes, currents, waves and/or tides. Typically the introduction of a hard structure onto the seabed causes flow amplification and vortex eddys. This may be a pipeline, jacket, mono pile or manifold structure. Vortices and flow amplifications tend to transport sediments away from the structure resulting in scour.

How can monopile scour be prevented?

Scour is typically mitigated by installing a protective skirt around the structure.

In the case of a monopile this traditionally takes the form of a rock mound installed around the pile. Attempts have been made using rock filter bags and geotextile sand containers. These both have the disadvantage of introducing plastic into the marine environment and cannot be readily decommissioned. They are also unstable in high velocity applications.

Subcon’s scour collar consists of a geoetxile filter layer bonded to the underside of an articulated concrete block collar.

In what cases would a frond mattress be deployed?

The buoyant fronds in frond mats dissipate the wave and current energy close to the seabed.

The lower velocities have a lower transport capacity as a result, therefore collecting sediment from its surroundings.

What are cost effective scour protection solutions for monopiles?

Subcon has developed the Single Lift Scour Collar that is used for the scour protection of wind turbine foundations.

This geotectile backed, concrete block mattress provides high performance scour protection for monopiles that significantly decrease the time taken to install scour protection when compared to dumping rock or placing rock filter bags. This solution is ideal for applications where sourcing local rock is not viable or the stabilty requirements exceed that of what rock is able to meet.

The reduced height of the Scour Collar compared to equivalent rock also reduces the risk of secondary scour on the lee side of the monopile.