Jan 2015
Electricity Transmission
Offshore Cable Repair Vessel and Universal Joint
Jan 2015
Jul 2018
Sean Kelly (Partner)
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Network Innovation Competition
The amount of renewable generation in general and offshore wind in particular is growing rapidly. National Grid’s latest Future Energy Scenarios, issued in July 2014, show 3.9GW of offshore wind in service already, rising to as much as 31GW by 2030. Even in the scenario with the lowest offshore wind build the amount in service doubles by 2030.

Large offshore wind developments require high voltage (132kV and above) export cables. To date just under 30 such high voltage export cables have been installed, with each connecting an average of 150MW of offshore wind. The total length of wind farm HV export cable installed to date is 920km, but could reach almost 8,000km by 2030.

In contrast to onshore transmission systems, offshore transmission does not provide full redundancy. This means that even where an offshore wind farm is connected by multiple cables, the failure of any cable will reduce the amount of power that can be exported. Should this failure occur the wind farm owner will be exposed to the bulk of the financial loss resulting from being unable to export all of their potential output.

For windfarms that are connected by a single cable the situation any cable failure will mean that no power at all can be exported. Furthermore the loss of the single cable will deprive the wind turbines of the auxiliary power they need for the heaters, dehumifiers, oil circulation, etc needed to prevent equipment deterioration.

Wind farm designers have attempted to reduce the impact of prolonged offshore cable repairs by using multiple smaller-capacity cables, but this increases the cost of connecting wind farms substantially: for instance the use of two cables in place of a single double-capacity cable may increase the installed cost of cabling by 80%.

This project will seek to:

i) Convert an existing telecom-cable repair vessel so that it can also repair OFTO power cables which are much larger and heavier than telecom cables and usually require non-coiling storage. This cable vessel would immediately become available to all transmission owners through the Atlantic Cable Maintenance Agreement (ACMA). This would mean access to a vessel that is permanently on standby to undertake repairs at a price that is a small fraction of current offshore cable repair prices.

ii) Manufacture and test a new “universal” cable jointing system which will allow dissimilar sections of OFTO-type subsea cable to be jointed together.

iii) Train and qualify jointers to repair cables on board the repair vessel referred to in (i) above using the joint referred to in (ii) above.

It is envisaged that the project would have started by Q2 2015, with the project complete by Q2 2018 at the latest.

In order to ensure value for money for consumers, we have agreed a contractual structure with our project partner (GMSL) which ensures that they cannot profit unreasonably from the funding provided by electricity customers. This structure includes protections for consumers that would apply should it, for any reason, become impossible to use the vessel and joint through the ACMA.