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 A small diameter pilot hole is drilled under directional control along a predetermined path using a mud-motor or jet bit on the end of the pilot string. The pilot string is drilled up to 80 metres in length, before the washover pipe is advanced in rotary mode until it is approximately 30 metres behind the drill bit. Alternate pilot string and drilling operations take place until the exit point is reached. The smaller pilot string is then removed. If necessary, pre-reaming operations are carried out with pull back pipe added behind the reamer to enlarge the drilled hole to a size suitable for accepting the duct pipe. A pull-back pipe is connected to a ‘cleaning’ reamer which in turn connects to a swivel joint, (to prevent pipe rotation) that is attached to a pipeline towhead. The drill rig is then used to pull the duct pipe into the pre-drilled hole. Drilling fluid is used to lubricate the process which consists of water and clay minerals which will remain in the annulus and protect the pipe. Cable pulling and jointing After the drilling procedure has installed a duct for the cable path, the cable can be pulled up through the duct and connected in a joint transition pit. For an offshore wind farm, the export cables would be laid up to the exit point of the duct on the beach and connected to the pulling line which would have been pre-installed through the duct created by the directional drilling. The line would then be used to pull the cable through the duct and a mobile winch unit is required to provide the pull force. When the cable end reaches the joint transition pit it is strain connected (to safeguard the future integrity of the cable joint) to a fixed point in the joint transition pit and the cable joint is made between the subsea cable and the land section of cable. Site restoration Following the completion of the drilling and cable pulling operations the land site would be restored and the joint transition pit would be secured with locks on the entry point to the pit with a security fence around the joint transition pit. Access to the joint transition pit is retained for the operational life of an offshore wind farm to secure a maintenance capability on the joint. 3.8.7 CABLE BURIAL IN MOBILE SEABEDS The burial of subsea cables in areas where the seabed sediments are mobile has always posed a significant technical challenge to the cable installation industry to ensure that the cables remain fully protected for their design operational life. If a subsea cable is buried along the longitudinal line of a mobile seabed area, such as a sandwave area, there is a high probability that sections of the cable may 66
Cable types and installation techniques become exposed as a consequence of moving seabed sediments. Experience gained from the post installation surveys of subsea telecommunications cables has shown that sections of cable which may have been left surface laid during the initial installation have subsequently been re-buried by the movement of sandwaves, whilst at the same time previously buried sections of cable have been exposed. In order to mitigate the effects of mobile sediments, deeper cable burial is often specified. This solution may work where the amplitude of the sandwaves are relatively low (typical 1m to 1.5m). However in areas where the amplitudes are much higher, deeper burial will not be possible on a practical basis. Furthermore, if deeper burial is attempted in these areas there would be a potential for a significant impact on the local environment. To successfully mitigate the effects of mobile sediment seabeds, the only effective method is to undertake accurate and detailed surveys and to carefully plan the cable route to avoid any potential problems. The survey data should provide detailed bathymetric, geophysical and geotechnical information and the results of this survey should then be reviewed against historical chart data to ascertain any evidence of the mobility of the seabed in the area where the cables are planned to be buried. With all survey and historical data to hand, a detailed cable route risk assessment can be completed. This will identify the optimum route for the cables and may well involve a complete route diversion to avoid mobile sediments such as sandbanks etc. The subsea cables can also be routed to follow the troughs of any major sandwaves or sandbanks such that any movement of sands would result in mobile sediments being deposited over the cables as opposed to exposing them. However, future accessibility of the cables for any potential repair or maintenance activities needs to be considered if such a policy is adopted. 3.8.8 CROSSING NAVIGATION CHANNELS When the planned cable routing for the export cables requires a crossing of either an existing navigable channel or a proposed route for a navigable channel, it is necessary to install the cables to a suitable reference elevation to protect the cables from any future planned dredging operations along the line of the channel. There are two options which are commonly employed. Option 1 – the pre-excavation method The pre-excavation method requires an initial pre-excavation across the navigable channel before the export cables are installed. Export cables would then be buried using a subsea cable plough (or possibly a trenching vehicle) which will pass over the recently pre-excavated channel, during the cable lay procedure. It is anticipated that the cable crossing corridor across the channel 67
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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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Page 18 and 19: Review of Cabling Techniques and En
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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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