SCOUR FEATURE
IDENTIFYING SCOUR RISK
Scour is a significant concern for offshore wind developers and operators, and considerable effort is dedicated to scour prevention and monitoring activity. If scour develops, it has the ability to dramatically weaken structural stability, particularly for the common monopile foundation.
situated on unconstrained depths of sandy sediments, scour clearly presents a severe hazard.
EFFECTIVE TECHNIQUES There are numerous innovative and effective techniques employed specifically for the monitoring of scour, which support yearly or twice-yearly detailed monitoring surveys as stipulated by consenting and licensing agreements.
Operators must also plan for major storm events, which can drastically adjust the
free-flying ROV or tracked crawler system is irrelevant; the costs increase dramatically when ROVs are involved.
SIGNIFICANT COST AND CONCERN Considering the volume of in-service and planned subsea cables associated with offshore wind, this is a significant cost. Another issue with mobilising ROVs to site is the class of vessel required to accommodate the ROV and personnel. Tracking a cable from substation to landfall using a vessel based ROV method is problematic, as the size of vessel required limits the ability to track the cable in shallow water.
Another concern is that not all environments are suitable for ROVs, particularly in shallow areas, and close proximity to the seabed cannot always be achieved. For tracked systems, this becomes less of an issue as the route is engineered to accommodate the trenching and jetting burial systems.
TRACKED INSPECTION ROV Osiris Projects invested in a tracked inspection ROV in 2012 specifically to perform cable depth of burial surveys, and are currently considering designing our own bespoke crawler with enhanced capability. Advances in cable tracking technology will undoubtedly bring towed options, but until then tracked ROVs offer a suitable solution.
It is therefore critical to identify the potential for scour risk so that adequate protection can be designed and implemented. Scour is also problematic for buried cables, and as cables account for another major cost it is equally important to understand the nature of the seabed along the export cable routes.
PREVENTION OF SCOUR THROUGHOUT THE LIFE-CYCLE
Osiris Projects play a role in the prevention of scour throughout the life-cycle. Pre- construction, this involves acquiring data on the nature of the site, paying particular attention to characteristics associated with the presence and extent of scour.
They gather data on the nature of the seabed and water-column using a range of survey and oceanographic sensors, which is then evaluated by scour assessment specialists.
DATA ANALYSED
Aspects such as the nature and mobility of shallow sediments, sand waves, mega ripples and wave and current oceanographic data will be of particular interest. For sites underlain by marine clay, scour is rarely a serious issue, but for developments
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seabed morphology. Osiris Projects regularly performs post-construction monitoring surveys, typically acquiring high-resolution bathymetry which is then combined with previous datasets to assess seabed movement.
VOLUME ANALYSIS
The volume analysis technique is relatively simple; overlaying new and previously acquired bathymetry shows the net change to the shape of the seabed. This method allows you to literally see the movement of sandwaves between surveys, so not only can you monitor the extent of sediment movement around structures, but it is also possible to estimate the rate of change and use this to predict the lifespan of any scour protection in place.
CABLE DEPTH MONITORING For cable depth of burial monitoring, the process is not so simple and there are various inherent challenges. The primary restriction is that the industry standard method of tracking the horizontal and vertical position of the cable is greatly limited by range. This means the sensor must be positioned close to the seabed and cannot be towed from a vessel, it must be mobilised to an ROV. Whether this is a traditional
GPS AND DP TECHNOLOGY Secondly, GPS and DP technology has significantly improved since the first offshore wind cables were installed, so the older the cable, the greater allowance should be made for positional inaccuracy. While ongoing and recent monitoring surveys give accurate positional information, conclusions made on the movement of the cable/seabed since installation can never be made with 100% confidence.
SUMMARY
In summary, scour can be exceptionally hazardous, and surveys are commissioned to contractors such as ourselves to understand the nature of the seabed and the potential scour risk, while monitoring surveys give snapshots which over time provide detailed information on the development of scour.
Osiris Projects
www.osirisprojects.co.uk
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