Harlingen Vlieland - Crossing

Until recently, Vlieland was the only island in the Wadden Sea that still had to use its own generators for its electricity supply. A new 38-kilometre-long 20 kV cable between Harlingen and Vlieland, with a diameter of 100 millimetres, connects the island to the Dutch national power grid. VBMS (as part of Visser & Smit Hanab) laid this cable along the bottom of the Wadden Sea. The project was commissioned by Nuon Friesland.

Facts and figures

Interconnector cable
20 kV insulated connection cable
Equipment
Pontoons: Stemat 65, Stemat 47 and Stemat 48 / Small burial sledge
Project categories

Technology

In order to have as shallow a draught as possible and avoid running aground, the installation team worked from two pontoons: an installation pontoon and a cable storage pontoon. A specially built, high-pressure, burial sledge system was used to install the cable 2-3 metres deep at the bottom of the Wadden Sea. The cables were delivered on four giant spools, each holding 10 kilometres of cable. Additionally, two landing points had to be built with horizontal directional drilling (HDD). The project also involved crossing four underwater channels and a live gas pipeline.

Tides

The need to allow for tidal conditions meant that the workdays were anything but normal. Every so many hours, the pontoons would become stranded, all the machinery would come to a halt and the only thing the workers could do was wait for the tide to change. The work schedule had to be adjusted to these environmental constraints, which was very tiring for the whole team.

Special measures for the Wadden Sea

Many special conditions had to be taken into account when choosing the cable route. Firstly, the wishes of the various interest groups had to be considered. Subsequently, the problems presented by the tides and currents of the Wadden Sea had to be allowed for, since these can cause cables to shift. As a result, the depth of the cable was increased from the usual 2 metres to 3 metres. In view of the future, and to provide protection against currents and ship anchors, a 100-millimetre-thick, insulated cable was used. This avoided the need for laying a second ‘emergency’ cable. A signal wire was added to assist in locating potential problems.

 

Connection

A substation was built on the mainland, near Harlingen, for the cable connection. On the island, the cable was connected to a substation that distributes electricity to the rest of the island.

Project location

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