Deep linking: work begins on a record-breaking subsea cabling project

The installation of a subsea interconnect connecting Majorca with Spain is raising some huge engineering challenges.

On a damp January morning in Majorca, the mighty cable vessel Skagerrak roared past rows of luxury yachts lining the bay of Santa Ponsa. It had made its way down from Norway, where it was meeting precious cargo shipped from the distant shores of Japan. The cargo had taken several months to get there, travelling through the pirate-infested Gulf of Aden and the Malacca Strait.

Tight ship: Skagerrak is due to lay cable from santa pnsa to the coast of Sagunto

There was a huge sigh of relief when news came that the cargo had arrived safely. ’They are tricky waters to navigate through,’ said Bjørn Ladegård, manager of installation services at French cable manufacturer Nexans. ’We had to watch out for pirates… We hired two guard boats that followed us until [we got to] a defined secure zone and we took certain preventative measures on the ship. We couldn’t risk it getting damaged.’

On board was a 237km piece of subsea power cable, weighing 6,850 tonnes and costing several hundreds of thousands of pounds. It will form part of a 400MW, 250kV DC interconnector between the Balearic Islands and the Spanish mainland. As a two-way connection, it will also pave the way for the future development of renewable energy on the islands, allowing power to be transferred back to the mainland. It was, as Ladegård said, ’not your ordinary cable’.

In fact, once installed it will be the longest ever single piece of cable used for a subsea interconnection. It will also be the second deepest in the world, reaching 1,485m below sea level, and will make up the first HVDC subsea connection in Spain. The connection will consist of two 250kV DC cables and one return cable, jointly manufactured by Nexans and Prysmian as part of a €375m (£316m) contract awarded by Red ElŽctrica.

Initially, the link will provide around 25 per cent of the electricity consumed on the islands. Laying of the first cable began in mid-January and Skagerrak was being prepared for the second leg, where it will lay the cable from Santa Ponsa in Majorca to the coast of Sagunto near Valencia. As one of only two vessels able to do the job, it was under a strict deadline to get the cables under water before holidaymakers take over the Spanish coasts.

’Today we’ll do the pull-in,’ Ladegård told The Engineer. ’It’s the most common way to get our interconnectors onto shore. We’ll position the ship around 500m off the coast and float the cable end in. In Majorca, we have the extra challenge of getting the cable into a horizontal pipe located 6-7m below ground in order to reduce beach disturbance. Once the pull-in is complete, the laying of the remaining cable across to Sagunto can begin.’

The process will take around three weeks and will require tracking of the cable’s position and tension. A remotely operated vehicle known as Captrack will provide precise touchdown position data using a combination of depth sensors, altimeters and inclinometers. The system will be attached to the cable on rollers with the added ability to steer it in deep waters. A team of scuba divers will also be on hand to accurately lay the cable on the shallow parts of the connection.

Weighing 6,850 tonnes, the 237km section of cable is the longest section of cable ever use for a subsea interconnection
Weighing 6,850 tonnes, the 237km section of cable is the longest section of cable ever use for a subsea interconnection

But protecting the cable during installation is just as crucial as maintaining an accurate position. Around 284km of cable will be buried 1m below the seabed using jetting operations, while a further 23km will be protected by creating trenches in the rock. ’A lot of this is done using the Capjet trenching system, which uses specialised swords with waterjet to break up soft-medium hard rock,’ said Ladegård. ’The weight of the cable allows it to drop into place once the rock is eroded and the material then falls back on top to bury it beneath the seabed.’

A similar system, known as Spider, has been developed by Nexans to provide a large-volume dredging capability in deep waters. While Capjet trenches cables into the seabed and covers them as it travels along, Spider flattens out the seabed using water jetting and suction to remove large amounts of soil. This is done with a dredging head on an extendable arm that can be fitted with cutting and intervention tools. It is reserved for use in difficult conditions and where the route is unable to avoid uneven areas.

Divers began laying the first subsea cable in the middle of January 2011
Divers began laying the first subsea cable in the middle of January 2011

Juan Prieto Monterrubio, project manager at Red ElŽctrica, said that technologies such as this will be in demand as subsea cable installations move to deeper waters. ’The market for submarine cables is now very different to what it was in the early 1990s,’ he added. ’Back then, there were very few projects, but nowadays eight factories in the world are working close to capacity. They are busy with all these interconnections between islands, but also with interconnection for offshore wind farms to the continents.’

Monterrubio believes the world of power generation is changing. As new and smaller forms of energy generation rise in popularity, connections with the power grid will become vital. Islands are increasingly seen as an attractive option for renewable energy technologies. The only way to connect many of them is to use underwater power cables snaking through rivers, lakes and the sea to create a hidden network that will ultimately help to reduce the world’s carbon emissions.

Many environmentalists have reacted with cautious enthusiasm. Anything that will help to increase the viability of greener energy is a positive move. But some remain concerned about subsea cables disturbing the balance of fauna and flora on the seabed. In addition, they fear that the cables could stir up industrial chemicals resting on the seabed.

Nexans is keen to stress that, with this and other projects, the environment is high on its list of priorities. What’s clear is that the trend towards longer interconnections will increase. And if environmentalists and engineers can maintain a dialogue, projects such as this could help to tap into the world’s renewable energy potential.

Skagerrak has been converted to carry out larger-scale cable work

Last year, Skagerrak underwent an £8m two-month upgrade at the Cammell Laird Shiprepairers and Shipbuilders dockyard in Birkenhead.

The conversion increased Skagerrak’s capability to carry out even larger-scale power cable and umbilical installation projects and also extended its service life while at sea.

The major part of the upgrade involved the addition of a 12.5m pre-fabricated hull section that increased the ship’s overall length to 112.25m.

An additional accommodation module was also installed, taking the total number of single cabins on board to 60, together with a new work deck, with cable-handling equipment, that has increased on the on-deck storage capacity to around 2,000m2 from 900m2.

Overall, the ship’s deadweight rose from 7,886 tonnes to 9,373 tonnes.