In September of this year four subsea power cables were laid from the Ness of Quoys, in northern Scotland, out into the extreme tidal flows of the Pentland Firth. The cables had to be accurately laid in order to make best use of the shielding provided by the natural crags and gullies in the seabed. Spring tides generate 10 knots of flow and therefore this shielding is vital to the longevity of the power cables. The cables are part of the MeyGen tidal energy project, the UK’s flagship marine energy project, with plans to install 398MW of tidal energy.
The team were using the Siem Daya, a DP 2 construction vessel, as the platform for the works. The vessel was selected for its excellent Dynamic Positioning performance, but even with this capability it was vital that the cable laying was synchronised with the tidal cycle. The ebb flow is weaker than the flood and so all cable laying was to be carried out during the ebb. The vessel was set up to deploy the cable over the starboard side, allowing the vessel to crab along the cable route with its bow headed into the flow. The plan required the lay to start at the end of the flood tide in order to use the slack tide at the end of the ebb to deploy the ROV and spot the lay of the cable termination. If the operation were to become out of sync with the tidal cycle there was a risk that the vessel would either be caught mid channel with its stern into the flow, or the final lay down would have to be executed without the ROV. Either of these scenarios would put the cable in jeopardy.
In order to mitigate this risk the team deployed a met-ocean data buoy, specially designed for tidal environments, into the channel. The buoy was fitted with a Teledyne ADCP and a UHF radio link which enables it to stream live tidal flow data over 15km. The system is called DataFish. During the operation the on-board camera took some fantastic photos that provided a unique record for this ground-breaking project.