Case Study: When offshore installation schedules go awry

UK: RES has created a new offshore cable-pull installation method. Project manager Kevin Todd tells David Weston how it works.

Custom design: the scaffold allows the cable to hang off vertically, in the orange pipes. preventing the cables from bending too much

How would you perform cable pull-in up a foundation's external J-tube if the tower had already been installed? That was the question posed to RES engineers in their work on the construction of Vattenfall's 49.5MW Kentish Flats extension off the coast of Kent, south England.

RES was supporting Bohlen & Doyen, hired as the cable-installation contractor by project developer Vattenfall on its site seven kilometres off the Kent coast in the Thames estuary, comprising 15 MHI-Vestas V112-3.3MW turbines.

Bohlen & Doyen's timetable for the project had been set so that cable pull-in could be completed before the turbine towers were installed, but there was a risk that the cable vessel would not arrive in time. So, UK-based RES was asked to design a way of performing the cable pull-in operation in the event of the towers being installed first. This would prevent the need for cranes, which could cause damage to the towers, and avoid delaying tower installation.

Scaffold building

RES designed a scaffold to be used on the turbine's transition piece to help perform the operation, and a cable-pulling quadrant that could be hauled onto the deck of a transition piece using just an electric winch, attached to a davit crane.

This quadrant was designed to accommodate the eight-metre overpull required, plus the 4.5 tonnes of cable and its minimum bend radius.

"People don't often think about cable handling once they have got it clipped on to the monopile," says RES senior project manager Kevin Todd. "We took that into consideration to make sure that once the cable was on the deck, the minimum bend radius was not exceeded. That is why we made the scaffold.

"The idea of the scaffold was that it kept the cable conformed," he added, explaining that this protected it from extending the minimum bend ratio, and gave access to staff to strip the cables back and set it into a permanent hang-off.

"The cable has to be vertical to install the hang-off correctly. In this case, because there would have been no support above the hang-off, we had to make sure the cable was vertical coming out, otherwise the hang-off would have been impossible," says Todd.

In the end, the scaffold was not needed at the Kentish Flats extension, which was fully completed at the start of December. But the cable company is considering using the scaffold for future developments. It is suitable for foundations of the same design - those with a similar-sized deck on the transition piece and an external J-tube.

"They said they would use it at other projects, but also that it would be useful if a cable needs to be replaced at Kentish Flats," says Todd. "It would be the tool to use, because it has been designed to be used at that project.

"It could work on other wind farms as long as they have access. Quite often, the cable comes up inside the monopile, and that becomes difficult. But there are quite a few wind projects where the J-tube comes up the outside the monopile," Todd explains.

Well spent

Although the scaffold has not yet been used in a real-life situation, the time and money used to design the solution was well spent, believes Todd. Vattenfall said if they did not have a back up, they would probably need it, he adds.

"I think it's just good practice to make sure you have the major items covered off. If it hadn't have been planned for, then it would have taken weeks and weeks to get an accurate design, to test it and to get it built," Todd says. "You can question the cost of it, but vessels have lots of back-up pieces of equipment that hardly ever get used but which you have to have."