Invisible link

Submarine power cables and connecting equipment are central to marine energy

By Morand Fachot

Gathering marine energy is a complex operation, yet it is only one part of the overall equation. The other less visible yet equally crucial part consists in transferring power from marine energy converters to the grid and end users. Many of the systems and parts needed for this are already available and deployed in offshore oil and gas installations and offshore wind turbines. International Standards developed by various IEC TCs (Technical Committees) and SCs (Subcommittees) support this deployment.

Submarine power cables may include optic fibre cables for data monitoring (Photo: ZTT) Submarine power cables may include optic fibre cables for data monitoring (Photo: ZTT)

Pioneering role of IEC first president

Cables are as central to offshore-shore power transmission as they are on land. However, laying and maintaining cables under water presents special challenges, even though the techniques for doing so have been in use for over 150 years. They were used initially to transmit telegraph signals between Britain and France in the 1850s.

Work by the IEC first President, William Thomson, Lord Kelvin, was essential in the successful introduction of submarine telegraph cables. Thomson was knighted in recognition of his work on the transatlantic telegraphic cable project. Today, fibre optic submarine communications cables carrying voice, data and other electronic signals link all continents.

Submarine power cables were introduced much later than their telegraphic counterparts, owing to the far more complex technical issues involved, but were still heavily reliant on technologies developed for telegraphic cables.

Mature technology

Submarine power cable technology is mature now, it has benefitted from developments in land power cables and electric cables for the marine and offshore industry.

Knowledge acquired in the submarine communications cable sector, notably in terms of insulation and armouring for greater abrasion, corrosion and mechanical resistance, has also benefited submarine power cables. For instance, both use XLPE (cross-linked polyethylene) and EPR (ethylene propylene rubber) insulation which offer high chemical resistance to oil and solvents, excellent tensile strength and high abrasion resistance.

The laying of submarine power cables has improved substantially in recent decades with cable manufacturers able to deliver length of up to 160 km in one piece, and with cable-laying vessels that use satellite navigation systems, ROVs (remotely-operated vehicles) and ROTs (remotely-operated tools) to ensure a more precise installation.

Submarine power cables are being installed around the world; sometimes over hundreds of kilometres are laid, connecting islands to mainland grids, providing power to offshore oil and gas installations and transmitting power from offshore renewable energy installations to the mainland.

Both XLPE AC (alternating current) and HVDC (high-voltage, direct current) submarine cables are used for offshore wind farms. Submarine power cables often include fibre optic cables for data communication and monitoring of installations.

IEC power cable Standards

International Standards for power cables and cables for ships and mobile and fixed offshore units are developed by IEC TC 20 and SC 18A, respectively.

Although IEC International Standards do not specifically cover submarine power cables, and sometimes even explicitly exclude them, as in the case of IEC 60502, major submarine power cable manufacturers like ABB or Nexans rely heavily on IEC International Standards to manufacture high quality products. Mentions such as "the continuous current ratings are calculated according to IEC 60287 series of Standards" (ABB XLPE Submarine Cable Systems), or "Maximum value to IEC 60228" and "Calculated in accordance to IEC publications 60287" (Nexans Submarine Power Cables) are present throughout their marketing literature as evidence of compliance with internationally-recognized Standards.

Other IEC International Standards concerning test methods and requirements for power cables, such as IEC 60840 and IEC 62067, may apply also to submarine power cables following possible "modifications to the standard tests" or the development of "special test conditions". .

The last links

Submarine cables are not the only element in the offshore-shore power chain. Power collected by single, or arrays of, renewable energy converters must be distributed to the power grid onshore.

To enable this, the submarine power industry has developed subsea hubs, or UTUs (underwater termination units). These units, which can be interconnected, are placed on the seabed. They integrate power input from one or several devices and transfer the renewable energy onshore.

Underwater connectors are essential to link cables together or to link to renewable energy equipment, to hubs and to the power grid. They can be mated (i.e. connected) in dry or wet environments (dry-mate / wet-mate connectors). Several subsea electrical equipment manufacturers have developed both types for the renewable industry. Most of these devices, such as the MacArtney 11kV (7.6MW) Wet Mate Connector, comply with IEC International Standards.

This piece of equipment is marketed by its manufacturer as an "IEC 60502-4 compliant connector solution specifically designed to meet the needs of the marine renewables industry". It "eliminates the need to bring cables to the surface by wet mating and de-mating cable infrastructure, wave energy converters (WECs), tidal energy converters (TECs) and floating wind turbines".

IEC 60502-4 has been prepared by IEC TC 20: Electric cables.

Overall the industry represents a significant and expanding market when also taking into account associated services and equipment such as supply and cable-laying vessels, ROVs and ROTs.

Fast-growing submarine cable market

The demand for submarine cables has been growing significantly for a number of years and is likely to continue doing so in the future. According to an August 2012 report from the Pike Research consultancy company, the installation of "an additional 6 800 kilometres of high-voltage submarine cables in almost 70 new projects around the world over the next 6 years (…) is nearly double the level of construction during the previous five-year period."

Cable manufacturers are ramping up production to meet the demand of a USD 3,9 billion market growing at 7-8% a year. Nexans, a major power cable maker, was the latest manufacturer to announce, on 16 July 2013, the construction of a new plant to produce submarine power cables.

Key drivers for the subsea cable market are the needs of the offshore renewable energy sector (wind, marine), the power requirements of offshore oil and gas installations that receive electricity from shore-based grids rather than producing it onsite, and international power connections between countries.

International Standards for cables, associated connectors and other necessary systems prepared by IEC TCs will ensure the cable industry will be able to meet demand.

Submarine power cables may include optic fibre cables for data monitoring (Photo: ZTT) Submarine power cables may include optic fibre cables for data monitoring (Photo: ZTT)
Cable-laying vessels are in use across seas and oceans (Photo: ABB Group) Cable-laying vessels are in use across seas and oceans (Photo: ABB Group)
MacArtney 11kV wet mate connector allows underwater connection of power cables (Photo: MacArtney Underwater Technology Group) MacArtney 11kV wet mate connector allows underwater connection of power cables (Photo: MacArtney Underwater Technology Group)