SCOUR FEATURE
SCOUR THE CHALLENGE
Scour around offshore wind turbines and particularly over cables can be a significant problem. It is caused by the water speeding up around or over an object, which then gouges out a hole which gets progressively deeper.
CABLES
With cables, once the scour hole is deep enough, it will cause the cable to be suspended in a ‘washing line’ effect, resulting in the cable developing micro
REDUCING RISK
Offshore energy generators work hard to manage and reduce risk in all parts of their operation: from crew transfers to gearbox failures, from blade repair to monitoring processes. Every aspect of the operation is constantly reviewed to identify ways of reducing both risk to the infrastructure and risk to staff. Scour is one of those potential areas of risk, and one that the companies spend considerable time monitoring, investigating and remediating.
The tyres work by adding friction to the seabed which in turn causes the passing current to decrease in speed. This causes sand sediments
to fall and become trapped within the tyre spaces (see Fig 1).
The tyres then act as a reinforcement layer level with the seabed and thus prevents further scouring. If excess sediment lays on top of the tyres, the water flowing over them will lift this excess off, creating a self- levelling skin (see Fig 2).
SEA TRIALS
Scour Prevention has developed the system over the past three years and undertaken a number of tests. In mid- 2012 it undertook sea trials with E.ON at Scroby Sands off the Norfolk coast in an area with high tidal flows and a particularly challenging environment. With mean water depth of 6m, five mats were laid in a row, marked with anchors and buoys, and left in situ for several weeks.
The mats were surveyed every week and after 7 months, after the worst winter storms for years they hadn’t moved at all, proving they’d stabilised the seabed area where they were laid. This system is now available commercially for other energy companies and offshore installers to use in their wind farms.
cracks, causing electricity to ‘leak’ and the field to be shut down with generation income lost whilst the cable is replaced.
STRUCTURES
This can be particularly acute at the wind turbine itself where scour can expose the j-tube and the emerging cable. It can cause structural integrity issues for monopiles by exposing the foundation and reducing its overall stability. This can cause excessive vibration of the turbine as it turns with the wind and in some cases can cause early fatigue of major components.
SCOUR PREVENTION SYSTEMS LTD – INNOVATIVE APPROACH Scour Prevention Systems, based at OrbisEnergy in Lowestoft, have developed a patented device to remediate and eliminate scour around structures and over cables.
The system is a sedimentary entrapment device laid over the cable. Research has shown that the ideal shape for the individual nodes is that of a tyre. As worn tyres are used, this has huge potential for turning a difficult to dispose of waste product into a useful new one.
DEVELOPING THE OFFSHORE WIND SUPPLY CHAIN PROGRAMME Scour Prevention has now secured funding from the Technology Strategy Board’s ‘Developing the Offshore Wind Supply Chain Programme’ to undertake offshore trials of its tyre-mat solution around the base of an offshore turbine on a wind farm off the Norfolk coast with severe scour.
This latest trial is to demonstrate that the tyre-mats will stabilise the seabed around wind turbine monopile foundations thereby remediating and/or preventing scour holes. After being laid the mats will be monitored for one full lunar month to verify their performance in the field.
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www.windenergynetwork.co.uk
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