SCOUR FEATURE
REMOTE MONITORING SYSTEMS SUPPORT SCOUR PREVENTION
In collaboration with University of Southampton, Kongsberg Maritime’s subsea experts in the UK and sonar specialists in Canada have designed an innovative solution to remotely monitor the development of scour at the base of offshore wind turbines.
OFFSHORE SCOUR MONITORING SYSTEM
The Offshore Scour Monitoring System, in the context of marine renewables provides long term monitoring of the seabed in the immediate vicinity of turbine foundations.
DUAL AXIS SCANNING SONAR Using the Dual Axis Scanning Sonar (DAS) to continuously monitor the seabed around the turbine, the system logs all the data and transmits this data to shore via the telemetry infrastructure, cabled or wireless, installed in the turbine.
“The system can accurately visualise the scouring at the base of the turbine, showing engineers to what level the supporting capability of the seabed is
lost. Sometimes the pit is several metres deep and can drastically alter the support and vibration characteristics of the turbine so it’s vital that a clear view of what is happening on the seabed is readily available,” explains Peter Ward, Underwater Acoustics and Marine Impacts Specialist at Kongsberg Maritime in the UK.
CONTINUOUS MONITORING The continuous monitoring capability of the system provides valuable information regarding the dynamics of scour formation and sediment backfill. When the tidal flow direction changes, the scoured out sediment may refill the pit, but being unconsolidated, it will have little or no load bearing capability.
This information, which is not available for the less frequent multibeam scour surveys, provides the opportunity for quick remedial action before the scour becomes critical to the monitored structure.
PAYING DIVIDENDS
This is where hydro acoustic monitoring really pays dividends according to Ward:
BI-FLEX CONCRETE MATTRESSES PROACTIVE METHOD
Where cohesionless soils makes up the seabed this material can be subject to scour caused by changes in water particle velocity.
For offshore wind farm structures scour is caused by the installation of piles, hub structures and any objects placed on the seabed that alter the water velocity.
VULNERABILITY
Large diameter objects are vulnerable to significant scour and thus it is large monopiles that need consideration for scour prevention. Although clay soils do not scour, where there is a thin veneer there is a possibility of the material being removed by abrasion leaving underlying sand subject to scour.
PROVEN SYSTEM
Concrete flexible mattresses are now a proven system of scour prevention. Mattresses are designed to be self-stable and prevent the underlying soil eroding and by being flexible any initiation of scour around the mattress edges allows the edge blocks to fall into the scour depression and halts any further erosion.
A more proactive method of scour prevention is to incorporate fronds onto the concrete mattresses. They are laid with the fronds kept under a safe net until the total array has been installed and then the nets are released to allow the fronds to form a covering of this ‘artificial seaweed.’ Fronds work by reducing the velocity of the water locally and in doing so those particles that cannot be held in suspension drop down. The viscous drag created by the buoyant fronds causes this reduction in current velocity.
Over a period of time, and this can be within a matter of days or a few weeks, a sediment
Kongsberg Maritime
www.km.kongsberg.com
embankment builds up to a height of up to 1.1m. Thus not only is scour prevented but a reinforcing embankment is formed.
CABLE PROTECTION
The same approach can be used for protection of the cables, a line of fronded mattresses can be laid where burial is not viable, and thus permits an embankment to be formed giving both stability and impact protection.
Pipeshield has been involved in many offshore oil and gas projects for scour prevention and the following offshore wind farm projects…
• Lincs OWF • Gwynt-y-Mor • Egmond Wind Park • Dan Tysk • Scroby Sands • Lynn Inner Dowsing • Thannet
Pipeshield
www.pipeshield.com Click to view more info
“Our system not only monitors the depth of the seabed around each wind turbine, which is important information in itself, but also provides insight regarding the dynamic changes of the underlying seabed structure. This translates into lower intervention costs for maintaining the structure within the design parameters and avoids reductions in the life span of the wind turbine associated with unstable foundations and vibration induced structural fatigue.”
www.windenergynetwork.co.uk
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