Damaged or poorly installed subsea power cables can result in significant costs and revenue losses for offshore wind farm operators. Wind Energy Update speaks to Global Marine System’s Ian Gaitch, Director of Global Marine Energy, to identify a cable planning and installation strategy that can mitigate unforeseen costs.

By Rikki Stancich in Paris

Having laid its first submarine telegraph cable 160 years ago, Global Marine Systems is well versed with cable installation and maintenance issues and how these can translate into unforeseen costs down the line.

According to Ian Gaitch, Director Global Marine Energy, an increasing number of offshore wind farms operators are grappling with problems relating to poorly installed subsea power cables.

“A lot of the work on the market lately has been remedial work. There are a number of sites around Europe where cables have not been installed as they should have been,” he says.

Wind Energy Update’s Rikki Stancich speaks to Ian Gaitch to learn about the benefits of bringing cable contractors to the table earlier rather than later; the pitfalls to be avoided during the site planning process; and strategies for reducing subsea cable installation and maintenance costs.

Wind Energy Update: Would you say that cable maintenance strategy should be a key consideration for offshore wind farm operators?

Ian Gaitch: Yes. Under Ofgem’s Offshore Transmission Operators Regime (OFTO), each bidder is being asked to submit details of operation and maintenance strategy and Global Marine Systems is working closely with a number of bidders to develop such plans.

Wind Energy Update: Is post-installation cable maintenance included in the installation contract and if not, is this a service that you offer to offshore wind farm operators?

Ian Gaitch: No, it isn’t usually included in the contract and yes we do offer this service. However, there hasn’t been a lot of interest to date.

The common belief is that power cables are more protected than telecommunications cables because they are buried at a greater depth.

The fact is that there is a still a risk – say if a large vessel drops its anchor on the cable.

A lot of the work on the market lately has been remedial work. There are a number of sites around Europe where cables have not been installed as they should have been.

In any case, as the number of offshore wind farms increase exponentially, demand for maintenance and repair services will likely increase.

WindEnergyUpdate: From a cabling perspective, what would you say are ideal offshore wind farm site conditions? What factors would you advise developers to consider?

Ian Gaitch: There is never a perfect location. The very nature of wind farms means the turbines will be located in an exposed position, so the conditions will always be rough. 

Ideally, the turbines would be positioned in deeper waters of 20-30 meters. This way, you could use modern technology and remotely operated vehicles (ROVs), which means you can get away from using divers, which are often weather restricted.

You would want to locate the wind farm not too far offshore because you have a lot of people working offshore. It the wind farm is too far out, you would probably need to provide offshore accommodation and the transport time getting on and off-site would take up the better part of the day. Both these factors are costly. So ideally, you would want to locate the wind farm fairly close to shore.

Good seabed conditions are ideal – clay or sand as opposed to rock or chalk, which would need to be drilled into to lay the cable. It costs a lot more to drill into rock to lay a cable.

The cable landing is another consideration. You would be looking for a good stretch of shoreline, in proximity of a beach. Recently we have worked on projects where you cannot get within 3-4 kilometers of the shore and we’ve had to route around to land the cable onshore, which is a more expensive way of doing things.

WindEnergyUpdate: Global Marine Systems has been involved in some of the larger offshore wind projects to date. At what stage of the project are the cable installation companies generally brought in to a project?

Ian Gaith: Usually, the cable installation companies are among the later sub-contractors to be brought into play – probably because it is felt that we have the resources readily available, whereas other contractors, such as turbine suppliers, are often in short supply.

WindEnergyUpdate: Would there be a strategic advantage in bringing the cabling company into the project at an earlier stage?

Ian Gaith: There would be an advantage in bringing cable contractors in earlier, particularly for the kind of large-scale projects included in the Crown Estate’s Round 3.

It would be more beneficial to developers to have a long-term agreement in place with a given contractor for large-scale wind far projects spread across four years, instead of a series of contracts signed with those four years.

There are definitely economies of scale that could be achieved by taking this approach.

Wind Energy Update: In the event of cable damage, how quickly could a power cable be repaired?

Ian Gaitch: Global Marine Services have a fleet of specialist vessels strategically positioned around the globe to respond to damaged telecommunications cables.

Currently, we have nothing like that in place to service damaged power cables for offshore energy. However, we aim to establish a similar set up in coming years.

As things stand, if an offshore power cable is damaged we need to go to the market and source the appropriate vessel, crew and team to carry out the damage repair work.

It could take anything up to three months to repair a damaged cable, which is a long and expensive downtime from the operator’s perspective.

If we had the specialist rapid response vessel and teams in place, it would take more like 4 weeks to carry out the repair.

Wind Energy Update: Global Marine Systems very recently won the contract to install four offshore export cables and install and bury 175 array cables for the first 630MW phase of the London Array offshore wind farm in the Thames Estuary. What technical challenges does this project represent?

Ian Gaitch: There are a number of technical challenges, particularly with the smaller inter-array cable installations. The problem is that, while two-thirds of the turbines are in deeper waters (which is fine), one third is in very shallow water – around a 2-3 meter water depth.

Shallow water presents more problems in the working environment due to changing sand levels and waves breaking on the sand banks.

The scale of the London Array project also presents the logistical challenge of ensuring that all the pieces are of the chain are tuned into the work plan, to ensure the continuity and flow of work.

A small break in one part of the chain can have significant repercussions throughout the rest of the work chain. All the contractors and sub-contractors will need to work very closely to ensure that operations are as smooth as possible.

WindEnergyUpdate: There will be a lot of power running through the export cables. What are the risks – and consequences - of cable damage both during and after the cable laying?

Ian Gaitch: We will be installing from onshore to offshore, simultaneously plowing the cable trench and laying the cable. By taking this approach we minimize the risk of any third-party damage during the cable laying operations (such as from an anchor dropped by a fishing vessel).

Possibly the biggest risk is presented by the fact that the cables will be very long – 45 km in length - which means we need to find a sufficient fair weather window in which to start and complete the cable laying.

If we were caught out by a storm, we would have to cut the cable and go back onshore and then wait for the right weather conditions in which to complete the job, which would cost a lot of time and money.

Wind Energy Update: What percentage of Global Marine Services revenues come from laying offshore power cables for wind farms?

Ian Gaitch: Currently around 25-30 percent of revenue generated is from offshore power. We are looking to increase this to around 50 percent over the next few years.

To respond to this article, please write to:

Rikki Stancich: rstancich@gmail.com