The munitions clearing company and the Lower Saxony munitions disposal service issued the release for the last section of the Riffgat route on September 13.
The route was planned by the wind farm operator and approved by the state of Lower Saxony; transmission system operator TenneT was forced to take over the ready-planned and localised route, which ran alongside a munitions dump area, without the possibility of changes. In contrast to the first investigation carried out by the wind farm operator, the second investigation commissioned by TenneT showed that the submarine cable route was contaminated by considerably more munition remnants.
On behalf of TenneT, the route was then cleared of the unforeseeably large quantity of old munitions over a period of 18 months at high cost. During the operation, TenneT did everything possible to accelerate the difficult and complex clearance. In order to achieve this, remotely-operated underwater vehicles, submarine magnets and other devices were used in addition to divers during the round-the-clock munition remnant clearance operation, even under bad weather conditions. A total of almost 30 tonnes of munition was retrieved and disposed of before the cable route could be assessed as usable and harmless. Now there are no obstacles preventing the submarine cable laying operation.
At the beginning of the week, the penultimate submarine cable section was wound onto the cable laying ship BoDo Installer in Emden. In the middle of this week a further route section was released, and preflushing of the route already started yesterday on this section. The pre-flushing operation will take around two days. Immediately after this, the cable already on board will be wound out precisely into the centre of this trench. Once work has finished on this section, the unit will return to Emden to load the last cable section. The necessary submarine cable has already been stored in TenneT’s cable store in Emden over the last few months.
The flushing plough fitted to the ship has a permanent connection with the carrier ship and is lowered down to the seabed. The cable is then safely and precisely wound out to the required laying depth on the seabed inside the unit. The flushing plough is fitted with nozzles through which water flows at high pressure in order to loosen the seabed floor ahead. The length of the flushing plough can be adapted to differing water depths by adding or removing sections.
The unit is then positioned using four positioning and one main pulling anchor which are placed prior to the operation. It is necessary to fit casing collars to connect the laid cables together. This work will subsequently be carried out by a separate unit.
TenneT continues to predict completion of the connection at the beginning of 2014 due to the continued unpredictability of the weather.
After alpha ventus, this project is the second offshore grid link completed by TenneT using three-phase technology. This link consists of an 80 kilometre long, 155 kilovolt connection, linking the Emden/ Borssum substation to the 220 kV grid. The present substation will be expanded for this purpose. Riffgat is located within Lower Saxony’s twelve-sea-mile zone approx. 15 kilometres to the north-west of the east Frisian North Sea island of Borkum. This means that the wind farm is located near the coast, 50 kilometres from the mainland similar to alpha ventus, so that the link can be realised most efficiently in technical as well as economic terms using a separate three-phase connection. An additional 30 kilometres of overland cable are required in addition to nearly 50 kilometres of submarine cable.
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Press release, September 18, 2013; Image: EWE