TFN Scour Prevention & Remediation
Scour of marine structures is a common problem, affecting many industry sectors including renewable energy, oil & gas, utilities and infrastructure. Piles, pipelines, cables etc are all prone to scour in certain conditions. Wind turbine monopiles, like any other structure protruding from the seabed in a moving tide, are subject to scour. The scouring of the seabed around the pile develops a hole or depression with sloping sides around the base of the turbine. This localised reduction in bed level presents the following risks:
· Exposure of ‘J’ Tube bell mouth and cables
· Increased length of exposed pile from the seabed
· Decreased depth of penetration into the seabed
· Reduced pile stability & vibration frequency
For remediation of existing scour, the TFN’s can be installed in single or multiple layers around the base of the turbine which reinstates naturally occurring seabed sediment in the area immediately round the monopile.
On sites subject to general seabed lowering, due to natural sediment dynamics, TFN’s can be utilised to stabilise the scour level at the base of the turbine foundation, within the existing scour hole and prevent further scouring which would occur during the general seabed lowering that the site is subject to. The stability of the foundation can then be maintained as the natural seabed level across the site is subject to cyclic decrease and increase. This natural cycle of seabed level rising and falling can occur over a few days, usually during storm events, or slowly over a period of years. Where prolonged periods of general seabed lowering occur, the TFN’s will maintain foundation stability and prevent further scour which would usually occur in line with the reduction of surrounding bed levels. The deepening of scour holes, caused by general bed level reduction has proven to cause serious problems on a number of OWF sites with turbines having to be decommissioned as a result. The Use of TFN’s in such circumstances will extend the operational life of turbines affected.
On new developments, where seabed and tidal conditions will promote scour, TFN’s can be used to control the level of this scouring following the installation of the foundation. If the level of scour can be controlled, reducing the risk, savings can be made to foundation design and construction methodology:
If scour can be maintained within the design specification, then contingency measures in foundation design can be reduced, with significant cost reduction.
· As TFN’s would be installed following foundation installation, the extensive upfront investment in pre-installed scour prevention systems such as rocks or sand bags, can be mitigated.
· With the use of TFN’s, piling operations would not be hindered by pre-installed scour protection and cable installation and subsequent protection is simplified offering savings in both time and costs.
· Scour development can be monitored on new build sites and TFN’s only installed to foundations where required, making significant cost savings.