A hard life under water

From Billdal to Donsö, 17 kilometres of subsea cable has been laid on the seabed. It will provide the southern archipelago of Gothenburg with safe electricity. This work was given a high priority as the old cable was in poor condition.

It's an unusually beautiful day at the end of February. The sun is warm, only small ripples can be seen on the water surface. Joakim Lindberg looks out towards the islands off Billdal, just south of Gothenburg.

Despite the beautiful weather, he is a little unhappy as he stands on his barge a few hundred metres out to sea. He is eager to work.

–  Something got stuck in the pipe that will take the cable out to sea under the seabed. It´s such a nice day without wind, and you want to be at sea laying cable, not stand here and wait, he says.

The wind is the biggest problem when it comes to laying subsea cable - at least at the dimensions involved in this work. The cable is 90 millimetres in diameter with a heavy steel reinforcement that will sink it and ensure it lies still on the bottom. It weighs nine kilograms per metre. A standard drum with one thousand metres of 24kV AXCLTV cable from Nexans including the drum weighs 11,000 kilos.

 –  If it is blowing for more than nine seconds, we don't go out. Sometimes we stay ashore even in calmer weather, if it has been blowing earlier and there are swells, says Joakim Lindberg from subcontractor Dyk och Sjöentreprenad that conducts the actual installation.

Ellevio will make its electricity network more reliable on the Onsala peninsula - Askim, Billdal and Hovås - and on the islands in the southern archipelago of Gothenburg. This means among other things, that 145 kilometres of high-voltage cable will be laid - 41 kilometres of which is subsea cable. The work is expected to be completed in 2020. The cabling to the archipelago was given the highest priority. The plan was to start work in March this year but in January a 3.6 kilometre long cable trench had already been dug from the Billdal switchyard to the shore of Killingholmen.

–  The old cable to the islands was not a real subsea cable and required frequent repairs. Once water has entered a cable, the problems often continue, says Jörgen Blomqvist, tender engineer at Kraftringen Service which has a turnkey contract for the work.

From switchgear near the shore two subsea cables were pulled out to sea, the first 250 metres in pipes laid in a horizontally drilled hole under the seabed. One at a time, the two 8.5 kilometre long cables were laid in parallel on the seabed and then pulled out to Donsö, which is one of the largest islands in the archipelago with almost 1,500 inhabitants.

Joakim Lindberg transported three pontoons from home base in Uppsala and connected them to an 11 x 7.5 metre barge with a bearing capacity of 70 tonnes. When laying cables the barge is pulled by a tugboat at a speed of 0.5–1 knots and one thousand metres of cable on a drum is laid on the seabed in 40-60 minutes. When a drum is empty, the barge is towed to Fiskebäck where a new drum is lifted on board. Just over an hour later, they are back where they left off.

A new and time-consuming phase then begins - the splicing of the cables - a job performed by specially trained fitters from Kraftringen Service.

–  Splicing takes 2 people 4-5 hours. It takes time to open the cables and shrink hoses must be heated so that the joint corresponds to the original condition and tensile strength of the cable, says Jörgen Blomqvist.

This means that during winter months there is only time to place one drum a day. It is only possible to roll out two drums during during the long days of summer.

When the cables reached Donsö, Joakim Lindberg put on his wetsuit and swam all the way from Killingholmen to Donsö to ensure that the cable was where it should be. Underwater inspection of the cables took 2.5 working days for each cable laid, with the inspection work being slowed by the cables being a bit deeper under the sea than 17 metres. A diver can only stay at that depth for 60 minutes before having to resurface, often for one to three hours.

-–  Above all, I check that the cable is not lying on any sharp stones or edges that mean holes can be worn into the jacket over time. I then try to move the cable a bit or we will pull a closable protective tube over it. I also film some sensitive spots , he says.

It can be good documentation if something happens. The new subsea cable from Nexans is expected to last for 50 years thanks to the steel wire reinforcement. But there are no guarantees for subsea cable.

–  It's not possible to predict abrasion in the sea as there are places where the cables are never lying still. But it often takes several years before an fault occurs. The film footage can then be of assistance knowing where to start looking, says Jörgen Blomqvist at Kraftringen.

The first power cable was put into operation in March, when it was connected to the existing network on Donsö. The other will be commissioned after summer. When the work is completed in 2020, another 24 kilometres of subsea cable will provide secure electricity to the twenty or so islands that make up Gothenburg's southern archipelago, which has around 4,600 inhabitants.