Sensitive sediment

Thursday, 17 March, 2016 - 12:30
The vibration sword was used in July 2014 for the DolWin2 project. (Photo: Bohlen & Doyen)
The vibration sword was used in July 2014 for the DolWin2 project. (Photo: Bohlen & Doyen)

Laying cables across tidal flats is always an act of aggression against the sensitive ecosystem. The offshore wind power industry is working on modern installation methods to minimize the negative effects as much as possible.

Imagine an Airbus plane shaving off the roof of your home with its wings as you are sitting down for your dinner. This is what it must feel like for crabs, molluscs and sea worms when the surface of the mud flat is shorn off as if with a razor blade during the laying of cables. The sensitive plant life is equally devastated.

Some damage is unfortunately inevitable when connecting German offshore wind farms to the grid by laying cables through the "Wattenmeer" tidal flats, which are a UNESCO World Heritage Site. It is, of course, vital to limit the intervention as much as possible and thus to minimize the resulting damage to flora and fauna.

Some of the people working to ensure this are the ten fulltime rangers who work for the administration of the National Park in Wilhelmshaven. One of these is Onno K. Gent. As a native of East Frisia, he loves the Wattenmeer. One could therefore imagine that he would hate the offshore wind farms, but that is not the case at all. Perhaps this is explained by the fact that he has direct personal experience of the efforts undertaken by the offshore industry to minimize the damage arising from the expansion of its projects and has seen the progress it has already made.

Infamous premiere

The heavy construction machines crawl slowly across the muddy ground on their large tracks. Some of them secure holding anchors that will be used by a laying barge to pull itself through the intertidal zone. A trencher digs a deep groove into the mud and the cable is lowered into it. Once the large machines have moved away, the cable is invisible. The mud flat, however, is torn by deep track marks.

Whilst the grid connection for the first German offshore wind farm, Alpha Ventus, tore up 880,000 m² of tidal flat, the connection for the offshore converter DolWin 1, to which the neighbouring wind farm Borkum Riffgrund 1 is attached, only affected 30,000 m2. This is mainly due to new installation methods, but also due to the increased sensitivity to the problem in the industry following the connection of Alpha Ventus. That German premiere was anything but glorious.

One of the problems was that the holding anchors, each weighing several tonnes and positioned laterally to the cable trench, were dragged several metres through the sediment. Another was that the heavy chains attached to these anchors sheared off the top layer of the mud flats like razor blades, as described above. This led to further damage over a wide area of the sensitive ecosystem, which was heavily damaged anyway by the construction equipment during cable laying. "The damage incurred during the laying of the cable led to the formation of a new tidal channel", reports the ranger Gent. 

It was clear that subsequent projects had to do a much better job of protecting the environment. For BorWin1 and DolWin 1, TenneT decided to use a vibration plough. The transmission grid operator in charge of the grid connection has now gone even further: "whilst we did use a vibration plough on the tidal flats for the DolWin 1 project, in recent years we have been using the vibration sword", reports Cornelia Junge, Corporate Communications at TenneT.

Ploughshares to swords

The installation method using the vibration sword was developed by Bohlen & Doyen in 2012. With the vibration plough the cable is laid at high tide and vibrated into the exposed mud at low tide; the vibration sword combines both steps in one. The big advantage is that most of the heavy work is done at high tide, limiting the damage done to the tidal flat's surface.

The technology also provides an advantage when crossing navigation channels. These cannot be crossed with a plough. Instead, on each side of the channel the system needs to be changed at great effort, from vibration plough to jet sled and back again. "Especially in such areas, the vibration sword saves time and also enormous effort", says Junge.

The transportation of the material – the cable and the technical equipment also need to be transported over the mud flats – also usually takes place at high tide, using barges with a low draft. "The only time we still need to use open construction methods is when working in very shallow areas", Junge explains.

The laying of cables across the tidal flats remains a damaging procedure with long-term negative effects on the sensitive ecosystem, if only due to the presence of the required heavy machinery. Further technical innovations are therefore necessary to better protect the Wattenmeer World Heritage Site.

Katharina Garus

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