Review of Cabling Techniques and Environmental Effects Applicable

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind
Farm Industry – Technical Report
scars on the seabed, the persistence of which will depend on the local seabed
conditions. In coarse sands and gravels the grappling hooks will penetrate
to a depth of up to 1m and infilling of the resulting scar will occur almost
instantaneously (EMU Ltd, 2004). In coarser grounds, the grappling scars will be
more persistent; cobbles will be displaced and overturned.
The data from the burial assessment survey is then used to generate a burial
protection index (BPI) for the complete cable route. The BPI takes into account all of
the potential hazards to the cable system and assesses the results from all survey
data, including the BAS survey. The BPI is then used to design the type of armouring
along the cable route, i.e. rock armour, double armour or single armour and to derive
a burial depth which will take into account the safety of the cable system whilst
ensuring an environmentally friendly and cost effective installation.
By assessing the BPI correctly for the complete route of the cable, projects
can expect routes with a varying burial depth requirement. This may result
in burial depths varying from 0.5m to 2.0m along the route. The subsea
telecommunications industry (and the cable installation contractors working
within this industry) have established that it is far better to only bury cables
to the minimum depth required rather than comply with a global target burial
for the complete system length. A significant number of the cable installation
contractors who work in the offshore wind farm industry have experience with
the subsea telecommunications industry and they are lobbying to have a transfer
of technology so that the lessons learnt can be applied for offshore wind farms
in UK waters. The argument which states why try to embed a cable down to
3.0m if the cable is safe and has been buried with an acceptable burial method
to 1.0m is compelling and deserving of serious consideration.
There is a real possibility that BAS surveys may become part of the survey regime
for offshore wind farms. Therefore, the environmental impacts of a BAS survey
may need to be considered as part of future consenting requirements. However,
as the BAS tool will generally be a scaled down version of the primary burial
tool intended for the actual burial works, there are no different environmental
effects other than those already described for the primary burial tools. The only
difference is that the BAS tool will make a first pass along the route in advance
of the primary burial tool completing the installation of the subsea cable.
3.11 Decommissioning
The Energy Act has not yet provided any clear guidance on the legislation related
to the decommissioning of offshore wind farms. However, it is almost certain that
all of the offshore structures would have to be removed to the seabed (partial
removal). The nacelle and towers would be removed in reverse operation to
construction using a heavy life vessel. Cables would be disconnected after being
isolated offshore and pulled out of the J-tubes.
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