Review of Cabling Techniques and Environmental Effects Applicable

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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. 78
Cable types and installation techniques It is probable that the subsea cables would be left buried and notified as being disused or out of service. The reason being that to de-bury the installed subsea cables is likely to result in a significant disturbance to the seabed. If cables are only buried to a shallow depth in sandy seabed, an under-runner can be used to de-bury the cable. This device is put on the cable and as the name suggests ‘under runs’ the cable while being towed from a line from a host vessel. This procedure, however will not work for deeper buried cables as is commonly associated with offshore wind farms (1.0m and deeper), especially when the cables are effectively buried in clays, gravels, chalk etc. Therefore, de-burial would involve the use of significant subsea plant (as yet not commercially available to the market on a significant scale) using aggressive methods such as cutting large open trenches to access the buried cables. Certain sections of cable would be removed for a decommissioned offshore wind farm. This would include both the beach section of cable down to the low water point and the sections of cables close to the offshore WTGs before full depth of burial is achieved away from the J-tubes on the structures. It is difficult to predict if cables would be fully removed at any disused cable or pipeline crossing. The crossing point is likely to consist of concrete mattresses or similar. Over the course of time this construction is likely to have formed an artificial reef; to remove the crossing construction would therefore have a negative impact on the subsea environment local to the crossing point. 79
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REVIEW OF CABLING TECHNIQUES AND EN
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Review of Cabling Techniques and En
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Abbreviations AIS Automatic Identif
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TTS Temporary Threshold Shift TWA T
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3 Cable types and 18 installation t
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6 Good practice measures This secti
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7 Gaps in understanding Research fo
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References Introduction Sinden, G.
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Appendix A: Standards and codes of
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Printed in UK on recycled paper. De
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