WindTech: Speed equaliser improves tensioning

WORLDWIDE: Nexus, the Latin word for connection, is the name of a new cable-laying vessel from Dutch offshore contractor, Van Oord.

Nexus… Van Oord’s cable laying vessel, with a carousel for circa 40km cable
Nexus… Van Oord’s cable laying vessel, with a carousel for circa 40km cable

It is highly advanced, and brimming with features, the most significant being its speed equaliser that regulates and levels out the cable tensioning, something that protects the cable quality and integrity during installation and unavoidable, weather-related non-operating hours.

The Nexus is built on the basic DOC 7500 Damen Offshore Carrier, a standard multi-functional vessel platform introduced in 2012. This offers a large flat unobstructed rear deck space and a sleek bow and slender hull. This combination of marine design features, according to Damen, enables the vessel to sustain its speed and direction in head seas, also helping to reduce fuel consumption, particularly useful when installing in deep, far-shore sites.

It is a long way from the rather basic cable laying vessel that the company employed in 2007 to install inter-array and export cables at the 120MW Princess Amalia wind farm in the North Sea. That vessel was in fact a standard size barge with only rudimentary provisions for crew and a cable carousel that was temporarily welded to the deck. Staff were brought out every morning by a service vessel and returned to shore at the end of each day.

Bow shape

The Nexus' distinct bow shape diminishes wave-related accelerations and reduces slamming - the up and down motion of the hull from the waves - to very low levels to improve comfort and safety for vessel, crew and cargo.

The power control centre manages a diesel-electric propulsion system of four main engine-generator units (2 x 2,666kW and 2 x 2000kW), each can be engaged individually depending on propulsion and other power needs.

The propulsion system incorporates two (fully rotatable) azimuth drives, plus two bow tunnel thrusters, and a retractable bow thruster for precision manoeuvring. The latter forms an integral part of the Nexus dynamic positioning system, which is class DP2, which allows for enhanced system back up capability should a fault occur.

The hull's front section contains single cabins for a 50-person crew and another 20 two-person cabins for external contractors, a kitchen, restaurant, and recreational and medical facilities.

Van Oord required an extension of the standard DOC 7500 deck length, making it 122.7 metres long. It was built in Romania and delivered to Vlissingen, Netherlands, for installation of the huge 5,000-tonne cable carousel, a 100-tonne main crane, and additional deck equipment.

Jigsaw puzzle

"Deck extension proved necessary to enhance stability especially by compensating for the carousel's huge mass and specific deck location," said Van Oord commercial manager Theo de Lange. "The deck's breadth is 27.45m, and despite the space available each corner seems to be used for fitting something as part of a complex jigsaw puzzle that had to be solved."

Besides the huge carousel, the most significant technological feature is the advanced on-board cable guiding, speed equaliser and tensioning system positioned behind it. "The cable-laying process demands constant cable speed towards the 'static' seabed, yet the offshore environment is highly dynamic," de Lange said, explaining the three elements of the system, all developed in house. The first tensioner device pulls the cable out of the carousel, while a second tensioner pushes the cable towards the seabed. "The third system compensates for inherent cable speed differences," he says.

Cable speed differences are caused by a combination of wave and sea current induced vessel movements and speed variations caused by the carousel itself, according to De Lange, because the cable radius varies from where it is wrapped around the smallest area near the centre to its largest diameter near the carousel inner wall. As far as de Lange is aware, there is no other system available that is using a comparable system.


The carousel's and the cable's mass gives the carousel a very high mass moment of inertia which together makes it turn at almost a fixed rotational speed.

The cable is laid first on the seabed, then a water jetting device using a second vessel creates a ditch, and gravity forces the cable to drop in. This is followed by autonomous closure and burial by sea currents.

Based on a 280mm cable diameter, the carousel can contain cables for about 40km distance. "Nexus' maximum cable-laying speed is one kilometre per hour, but for the moment we aim at a more conservative 0.5km/h," said De Lange. The standard cable laying speed within the industry is 100-700m/hour, depending on sea water depth, cable type and cable processing system including carousel, with minor variables, he adds. "The first major project for the Nexus is the 600MW Gemini offshore wind park built 85km off the Dutch coast, of which the construction phase has commenced already."

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