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Undersea cable laying and the role of trenching tools

Sometimes it works. Sometimes the months of planning, the sweating over the tiny details, the calls – o, the endless calls to distributors and contractors – the contracts, the wee hours when you wake from sleep and wonder, 'Did they sign off that form?', sometimes it goes just the way it should. Or even better. That's what's just happened in the North Sea – and it involved over 70km of undersea cables being laid three weeks ahead of schedule.

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To say Siem Offshore Contractors have been working hard in the German Bight in the North Sea to install 59 inter array grid cables on the Nordsee One offshore windfarm is no understatement. They used the CLV (cable lay vessel) Siem Aimery, the ISV (installation support vessel) Siem Moxie, and the multipurpose Siddis Mariner for the installation of cable protection systems and concrete mattress. “The Siem Duo,” said the firm, “demonstrated their advanced weather operability during this project and particularly with gangway access being completed in up to significant wave heights of 3m from Siem Moxie.”

The field of undersea cables installation is a very busy one for the offshore sector. It is, as The Independent put it, “a fascinating business” made possible by “men and women toiling for long and tedious hours to make (it) possible.”

Specially designed vessels carry the submarine cable on board – in the case of the Siem Aimery a below-deck compartment was developed with two turntables to carry the 79km of aluminium cable. Ships are capable of holding up to 2,000km of cable. When the destination is reached, the cable is painstakingly laid out on a plough on the seabed – obsessively controlled by the cable operator.

There’s a good reason for this. Each cable holds layers of optic fibre and wires, covered in a protective layer to keep the water out. They carry DWDM (Dense Wavelength Division Multiplexing) laser signals at a rate of terabytes per second. Optical repeaters are used to strengthen the signals. But they can be damaged relatively easily. Sharks like to bite them. Back in 1987 the New York Times wrote sharks had “shown an inexplicable taste” for the cables and the UN said “Bites tend to penetrate the cable insulation allowing the power conductor to ground with seawater.” They’re figuring out ways to put an end to this – Google, The Independent says, is “helping build (cables that) feature a kevlar-like protective layer to fend off the toothy sea creatures.”

Costs vary – projects can be as much as $500m and can take up to four weeks. They can be tricky and result in operational downtime if problems arise. But because it’s essential for the growth of global communications networks and is also used for communication between oil platforms and from fields to the shore end, much is spent on solving the problems.

A company two years ago sought to come up with a solution to a number of foreseeable issues: bad weather, strong currents, seabed sand waves and mega ripples. These are common concerns for those working on offshore renewable locations which are found in shallow waters. The result was a 42 tonne sub-sea ‘beast’ – the Hi-Traq. It won Product of the year at the 2014 BEEAs.

William Stephenson, product manager for IHC Engineering Business, said they decided to invest considerable amounts into R&D to devise an innovative solution after being approached by a number of companies who were having difficulties trenching. It has independent track steering enabling three forms of steering; crab, skid and wagon steering. This allows small radius trenching required for inter-array cables as they leave a turbine. “The vehicle was designed from the ground up starting with a blank sheet of paper by looking at industry lessons learned, the causes for recent project delays and by examining the issues faced by existing equipment operating in shallow water locations,” he said. “Well established and best practice approaches were taken during the design phase but the vehicle format is completely new. We have used reliable and robust engineering techniques with standard cylinders and pins.”

There are many other tools used by expert crew. One example comes from Global Marine Solutions which identifies four key machines– the Injector, the Rocksaw Trencher, Burial Sleds and Ploughs. The injector uses patented hydrojet technology for deep burial installation; where the injector can’t go because this operates best in softer soils – it’s time to bring in the Rocksaw Trencher. The company says it is “capable of burying subsea cables to depths of up to 4 metres. Driven by a 2000 HP hydraulic power pack, the rocksaw trencher is able to bury cables in a variety of environments, and in diver depths up to 40 metres.”

Developing improved technology in this field is crucial if the offshore renewable sector is to continue to grow. “Presently the industry as a whole is working towards lowering the LCOE for offshore renewables to make the sustainable energy source more competitive with fossil fuel alternatives,” said Stephenson. “Continual technology advancements are required to make installation, operation and maintenance, and ultimately decommissioning operations less expensive in order to reduce our reliance on fossil fuel powered energy generation.”