The world’s first commercial scale tidal energy system has been installed by Marine Current Turbines in Strangford Lough, Northern Ireland, where the tidal race reaches speeds of four metres per second. When connected to the National Grid, the 1.2 megawatt system will generate enough electricity to supply 1,000 homes.

Strangford Lough is an internationally renowned wetland and inter-tidal environment. It is a designated Special Area of Conservation, an Area of Outstanding Natural Beauty, a Ramsar Site, a National Nature Reserve, a Special Protection Area, a Marine Nature Reserve and a Site of Special Scientific Interest. The Lough supports 75,000 migrating waders and wildfowl, colonies of breeding terns, common and grey seals, otters, porpoises, basking sharks, conger eels, octopods, oysters, mussels and, very unusually, a giant leather backed turtle was recently observed.

Selecting an installation technique

This raised the question ‘how can a high voltage power export cable, from the onshore sub-station to the foot of the turbine structure be safely installed and maintained, without disturbing the wildlife or impacting on the coastline?’ The solution was to use HDD techniques to bore through the bedrock, using LongboreTT’s patented Pipe Pushing Technology to install the cable duct without requiring any marine support.

Marine Current Turbines first contacted LongboreTT in 2005 to discuss the possibility of using HDD to install its export cable. A comprehensive environmental monitoring programme by Royal Haskoning, Queens University Belfast and St Andrews University Sea Mammals Research Unit had been underway in the area since 2003.

Having reviewed the site topography and seismic marine surveys and having sourced all available geological data, LongboreTT conducted a Feasibility Study, developing a bore profile and modelling the stresses that would be imposed on the cable duct, and subsequently the high voltage cable itself, during such an installation operation. Risk Assessments and Environmental Impact Assessments were produced, reviewing access routes, vehicle movements and noise mitigation, as well as developing method statements to minimise the overall impact of the operation and reduce the risks associated with offshore construction. A design team was put together with Marine Current Turbines, the company’s electrical contractors and the marine construction team to develop a robust installation method statement that would gain Food and Environment Protection Act (FEPA) licence approval.

Following visits to ongoing drilling operations, presentations and negotiation with the Northern Ireland Environment & Heritage Service, the Maritime & Coastguard Agency, environmental NGO’s and the local community, the FEPA License was awarded in February 2007.

LongboreTT was awarded the installation contract and acted as Principal Contractor, according to Construction Design & Management Regulations 2007, for the works.

As Strangford Lough has been populated since mesolithic times an archaeological survey was carried out prior to the establishment of a rig base pad. LongboreTT’s American Augers DD-140 drilling rig, with a closed-loop fluids recycling system attached, was set up on a purpose built operations pad inlaid with pond liner material to protect the ground from any potential fluid spills. The top soil stripped from the site was used to form an acoustic barrier between the drilling operation and the foreshore, to minimise any disturbance to the seals during the works.

Installation procedure

Drilling commenced with a 250 mm diameter Tungsten Carbide Insert drill bit on the end of a field-adjustable downhole drilling motor. The pilot bore was tracked by a real time wireline guidance system. The target tolerance at the exit point was critical because not only did the bore have to exit within a four metre wide ellipse on the seabed, it also had to rise at an angle of less than eight degrees to minimise excessive over bend for the subsequent cable pull. It also required a precise azimuth heading so that the cable could be pulled through a ‘J-Tube’ conductor and up into the leg of the turbine structure.

The 434 metre long bore was drilled in vertically bedded Silurian sandstones and mudstones at an average Rate of Progression of 11.7 kilometres per hour. To ensure that any suspended solids were flushed out to sea, exit onto the seabed was timed to coincide with a falling tide. Just before punch out, the bentonite based drilling fluid was circulated out of the bore and replaced completely with freshwater.

As the tidal currents offshore made it impossible to float the cable duct out to sea for a conventional pipe pull, LongboreTT used its patented Pipe Pushing Technology to thrust the duct into the bored hole. Divers could operate in the 24 metre water depth for just 15 minutes at a time, at ‘slack water’ as the tide turned, and as such were only used to confirm that the drilling assembly had hit the target within the required tolerance, to remove rubble from the open hole and to confirm that the duct had been successfully installed. A Simrad acoustic transducer/responder was used to locate and verify the exit point.

Three critical targets were set and met for the project, delivery, budget and incidents. The cable duct had to be installed prior to the arrival of the turbine installation barge. As a proof of concept project, very tight cost controls were imposed on the operation. LongboreTT had the experience and confidence to conduct the works on a fixed price lump sum contract. Finally the sensitivity of the project meant that environmental and health and safety issues were paramount. No incidents were incurred during the works and the operation passed numerous HS&E Audits with flying colours.

Conclusion

The project clearly demonstrated the environmental benefits of using directional drilling techniques to install the cable crossing beneath one of the most sensitive marine environments in the world. In addition, the power cable is protected from the tidal currents as it has been installed 15 metres below the seabed within a solid bedrock horizon. The use of LongboreTT’s Pipe Pushing Technology enabled the duct to be installed without the use of marine support and divers, minimising safety risks and reducing potential cost and programme overruns due to inclement weather and inhospitable sea conditions.

John Ritchie of LongboreTT said, “The successful completion of the project confirmed the open communication and teamwork established between all parties involved in the work itself, environmental bodies, the local community and government agencies, and was only achieved with the positive involvement of all stakeholders in the supply chain.

“LongboreTT continues to build on the relationship developed with Marine Current Turbines, and other marine energy developers, to apply similar solutions to their grid connection problems worldwide. Marine Renewable Energy Systems are helping to save the world’s resources and form part of the solution to the planet’s energy supply problems. HDD can make a significant contribution to these objectives,” said Mr Ritchie.

The Marine Turbine Cable project was awarded the ISTT No-Dig Project of the Year 2008 at the Moscow Conference in June. Dec Downey, Chairman of the ISTT said “The meticulous attention to detail which accompanied every phase of the project, and Longbore’s contribution to the team approach, ensured that the fixed price project was delivered on time and to exacting standards.”