Subsea cables

When designing a cable route, the engineer will seek a suitable route around any seabed obstructions (natural or man-made) and aim for soft sediment where they can be buried. However in some areas of Scottish waters this is not possible and cables are laid on the seabed surface due to rocky seabed or other reasons: so there are both buried and surface laid cables. Cables may also be protected with additional measures such as rock placement, rock bags or concrete mattresses.

Today, new international telecommunications cables are exclusively fibre optic which carries many thousands of simultaneous transmissions. Domestic inter-island cables have been upgraded in the last few years and there is now a significant fibre optic network domestically in Scottish waters as well as some copper cables still in existence.

The Scottish Government had responsibility for designing and implementing fibre broadband roll-out in Scotland, which led to the Digital Scotland Superfast Broadband (DSSB) programme beginning in 2013. The programme was split into two projects – one focusing on the Highlands and Islands and the other covering the ‘Rest of Scotland’ (RoS).

As part of the deployment, approximately 11,000 km of new fibre and copper cable were installed, including approximately 400 km of new subsea cable (e.g. connecting the Outer Hebrides to the mainland, and the Western Isles). New subsea cable was provided for 20 crossings to the Scottish Isles (Figure a).

Figure a: Openreach Highlands and Islands subsea cable system installed as part of the DSSB programme. Source: BT Group/ Openreach, DSSB, HIE, 2018 (Yardley et al, 2019).

There are three main types of power cable installed:

Power distribution cables link the Scottish mainland to the islands, except Shetland and Fair isle, and interconnect between islands. They vary in voltage from 11 kV to 33 kV and in diameter from 100 to 200 mm. All power distribution cables are alternating current (AC) cables. Power distribution cables are usually surface laid but are buried or protected where required.

Power transmission cables provide bulk transport of power between areas of power generation and centres of demand. They usually operate above 132 kV and utilise either high voltage alternating current (HVAC) or high voltage direct current (HVDC) technology. The cables vary in diameter and configuration, HVAC cables are between 200 & 400 mm in diameter and are usually installed as a single cable containing 3 conductors and a communications cable. HVDC cables are between 130 and 180 mm in diameter. They are usually laid as bundle comprising 2 power cables, one for each pole, and 1 communications cable. Power Transmission cables are usually buried into the seabed, or protected with rock berms or a cable protection system.

Power export cables provide a connection between offshore generation sites such as wind, wave or tidal energy farms to a landfall site. The connections can utilise a variety of voltages and technologies dependent on the volume of power to be exported. The design and installation of these links is generally similar to Power transmission or distribution cables.

The repair of damage to a power cable usually requires the recovery of the damaged section of the cable to a surface vessel. To undertake a repair the cable is cut on the seabed using a diver or remotely operated vehicle. One cut end is recovered to the deck of a suitable vessel, the damaged section is cut away and a new section of cable jointed in-line with the undamaged cable. The vessel then moves to recover the other cut end, while the newly jointed cable is paid out. The damaged section is cut away and the new section is jointed to the undamaged cable. The repaired cable is then laid down on the sea bed. This type of repair normally requires that the extra cable length is approximately 3 times the depth of water to effect the repair. This loop of cable is laid down on the seabed in an Omega or hairpin configuration and maybe jetted into the seabed or protected using rock.

Subsea power cables create a comprehensive electricity network, connecting Scottish Islands, as well as providing potential for connection to the European Super grid project. The grid project aims to provide energy sharing across the continent through interconnectors, and increasing the potential to use renewable energy or import renewable energy from places such as Iceland or Norway which both have abundant hydro or geothermal renewable energy sources.

There is a growing network of cables bringing offshore marine renewable energy (wind, wave and tidal energy) production to landfall in Scotland.

In July 2019, there were 6,006 km of active cable, 5,030 km for telecommunications and 976 kms for power in Scottish seas. There was a 3% (147 km) increase in the length of active telecomm cables (Table a) and a 103% increase (495 km) in the length of active power cables (Table b) between March 2015 and July 2019 (based on equivalent data sets). A further 114 km of power cable are under construction. Locations of cables under construction, proposed, disused and some historic cable infrastructure (where sections may remain on seabed) are shown in data layers on MS Maps NMPi (each 'cables' on NMPi).

Up to date guidelines and information on submarine cables is available from the European Subsea Cables Association web site. The organisation has compiled guidelines on recommended industry practices and cable protection methods (available on request).