Alternative Scour Protection Methods

Arklow Bank is a shallow-water sandbank located 10km off the east coast of Ireland in approximately 3MWD. It is exposed to strong currents, frequently exceeding 2m/s, breaking waves and has a dynamic sand/gravel seabed.

Type25 SSCS frond mats were installed around the 1m-diameter met mast pile foundation standing in 5MWD to prevent scour and provide permanent scour-protection.

The post-installation survey showed the successful application and effectiveness of the mats with a build-up of sediment providing a stable protective sand bank, such that the buoyant fronds 1.25m in length were completely buried and no longer visible.

Specially designed SSCS frond mats were later installed around two of the 6.3m-diameter turbine pile foundations and along associated power cables to rectify scour and provide permanent protection following the failure of rock armour.

Rock armour, installed shortly after pile installation to provide scour-protection, was found during post-installation surveys nine months after installation to have dissipated and sunk below the natural seabed level, exposing the pile and cables and creating scour to a depth of 4m and extending to approximately 20m from the pile edge.

The SSCS frond mats were installed between and around the few remaining rocks to rectify this scour, reinstate the seabed soils and provide long-term protection.

The 2011 Completion Report of the remedial works provided by GE Energy and prepared by Island Maritime Ltd indicated ongoing successful performance of the SSCS frond mats:

• "Initial observations of the effect of … frond mattress installation are positive."
• "Frond mattresses have already begun to bury as currents are slowed by the action of the fronds and sand in suspension is encouraged to deposit on the mattresses."
• "Immediate observations of the effects of frond mattresses around WTG 4 and WTG 5 are very positive."