Your questions answered: The London Array

The team behind the world’s largest wind farm reveals some of the key challenges in the project.


The London Array is the world’s largest offshore wind farm and began operation earlier this year. Located in the outer Thames Estuary, the 100km2 installation of 175 3.6MW turbines has a combined generating capacity of 630MW – and that’s just phase 1.

We asked the team behind the Array to answer your questions on the engineering challenges involved in such an ambitious project, and how much it all costs.

To what extent was/is the London Array built as a “project” and to what extent as a “production line” i.e. a continually expanding deployment of turbines effectively manufactured en masse to reduce costs?


London Array was conceived and developed by the three partners – DONG Energy, E.On and Masdar – as an individual project with all the different elements (turbines, array and export cables, offshore sub-stations and onshore substation) designed to interlink with each other in a bespoke package. The expertise that the partners brought from previous renewables projects meant that whilst the build was not part of a “production line” of offshore wind farms, the installation methods used to build the project have been refined from past experience in constructing offshore wind farms.

Some elements of the project were more bespoke than others, largely because of the varying geography of the site, which has water depths ranging from intertidal to 25m. Consequently, each foundation monopile, for example, was manufactured according to the particular needs of its site. The array cables were also individually cut and installed. The turbines, on the other hand, are standard models. The 175 Siemens V6 120 WTGs chosen for use at London Array are the most popular Siemens turbine deployed offshore in Northern Europe.

With more and more energy supplies being provided in very large-scale offshore environments, away from the protective environment of our mainland services, what security measures – both process and physical – are provided to ensure reliable supply of the proposed volume of electricity?


We take our responsibility regarding health, safety, security and the environment very seriously and it’s at the heart of operations at the wind park. With regard to the physical security of the assets we remotely monitor the site with camera surveillance and intruder alarms. Our high voltage cables are buried below the sea bed and we have a cable protection system, which protects the cables as they enter the turbines. The onshore substation at Cleve Hill is in a secure compound to protect the electrical assets that receive the power from the wind park. We also have a number of management processes, working in conjunction with the appropriate authorities, to ensure the security of the assets.

What is the maintenance strategy in the case of a turbine break-down and what measures have been taken to ensure the best possible output with minimal maintenance?

We have a team of 70 technicians, including a breakdown crew, based at our operation and maintenance base at Ramsgate, working on the maintenance of the wind farm. In the event of unexpected problems, crew transfer vessels to take repair crews to the turbines can generally be available at very short notice.

Good maintenance is key to minimising turbine break-down and it is something that we are always looking to improve upon. Currently, the maintenance programme is time-based but we are exploring whether a risk-based condition monitoring programme might be more effective.

As well as having the right programme in place, you also need the right people. We spend a lot of time developing the skills of our technicians, most of whom have been employed from the local area. The project is also developing the skills that it will need in the future by investing in apprenticeships.


What measures are in place to assist shipping traffic in avoiding the Array?

The first thing to be aware of is that one important reason why the London Array site was selected for wind farm development is that it lies outside the main shipping arteries in the Thames Estuary. Although the wind farm straddles the Knock Deep Channel, the two sandbanks that form much of the site, Kentish Knock and Long Sand, deter most vessels from entering the site as they tend to go around the sandbanks rather than through them. But, there is shipping in the area and we are very aware of our responsibility to other sea users.

Two sandbanks deter most vessels from entering the site

During the course of the project – and its development began in 2001 – we have developed close relationships with Trinity House, the Port of Ramsgate and the Port of London Authority. This has helped inform the measures we have put in place to help boats pass safely through London Array. These include navigation lights on the turbines and a 24/7 manned marine control room.

We also publish regular Notices to Mariners that are distributed to, among others, local fisheries and yacht clubs, Port of London Authority, Maritime and Coastguard Agency, Ramsgate & Thanet Port Control, UK Hydrographic Office, Trinity House and Ramsgate Lifeboat Services as well as a whole host of individuals and interested parties who have asked to be included on the distribution list. During construction these notices were issued weekly. Now that the site is largely quiet they tend to go out monthly.


How much of a risk are turbine fires? What fire-protection systems have you installed inside the nacelles?

Fires can occur in wind turbines as in other offshore installations, and specialist systems are required to deal with the particular conditions within a turbine. Siemens has developed a certified fire-safety concept for offshore wind turbines that uses its latest innovations in fire detection. This includes reliable fire detectors with the highest immunity to false alarms available on the market. Thanks to modular design and robust cabinets for all core components, this ensures high flexibility, easy installation and reliability in harsh environments, such as offshore. These systems can also be remotely monitored and serviced.

How long are the turbines designed to last and could their lifetime be extended or could they be replaced with like-for-like units? Could the Array exist indefinitely?

The lifetime of the turbines is approximately 24 years. It is possible that the turbine’s lifetime could be extended with refurbishments at appropriate intervals, and it is also possible to replace the turbines with new ones. This would need to be done within the consenting conditions that apply to London Array Phase 1. London Array has been developed under a 50-year lease for the site from the Crown Estate – which gives it a finite lifetime – and is not designed to be there indefinitely.

How much energy (in MWh) do you expect to generate annually? What is your expected load factor?

We expect a load factor of c.40%, giving output of c.2,200,000MWh – enough to meet the electricity needs of around 500,000 households.

How does the capital cost per MWh of predicted annual energy output compare to new nuclear and gas plants of the equivalent size?


Phase 1 of London Array cost £1.9bn to build, has a 630MW capacity and is expected to produce around 2,200,000MWh of electricity a year. Wind is currently a more expensive form of energy generation than traditional sources largely because it is a newer form of power plant, but the government has set a target of reducing the cost of offshore wind to £100/MWh.

One of the partners, DONG Energy, has also set a target of reducing the cost of its offshore wind projects to €100/MWh for projects sanctioned in 2020 and has put a number of plans into action in order to achieve that aim. Once constructed, offshore wind farms have relatively low operating costs and produce no waste products.

We’re often told the cost of renewable energy is falling but by how much do you expect to see costs per turbine drop?

That’s not really a question that is pertinent to London Array: firstly because the turbines themselves were ordered as a complete package and not purchased individually; secondly, because Phase 1 of the project is now complete and turbine installation has finished. All that work was carried out in under 12 months with the first turbine being installed in January 2012 and the last in December that year. Going forward, we are aiming to reduce ongoing operational costs by developing a best in class maintenance strategy. This includes moving from a time-based deterministic plan to a risk-based condition monitoring plan.


How much money would the scheme lose each year in the absence of any subsidies and if it had to pay for the transmission lines and the backup generation needed when the wind is not blowing?

There are a number of issues here to clarify. With regards transmission, London Array pays transmission charges to use the national transmission system in the same way as any other generator. For the offshore transmission assets, London Array paid for the assets (offshore substations, export cables etc) needed to take the power generated by the wind farm to the onshore substation, for onward transmission into the national grid. These offshore transmission assets have now been sold as required by regulations under the OFTO (Offshore Transmission Owner) regime and London Array currently pays a fee to use them.

Wind is currently a more expensive form of energy generation because it is newer

As an emerging technology, offshore wind is currently more expensive than conventional forms of power generation, but most companies involved in the offshore wind sector are working hard to reduce costs to make the technology more competitive. Government support mechanisms such as the Renewable Obligation play an important role in helping developers make the significant commitment required to invest in a multi-billion pound project such as London Array.

The UK also has a target to reduce carbon dioxide emissions by 34% by 2020 and a legal-binding commitment of generating 15% of all power from renewable sources by 2020, so offshore wind has a key role to play in the UK’s energy mix. Wind farms such as London Array have a key role to play in helping the country achieve those targets. Without incentives, it is unlikely that these offshore developments would proceed.


The analysts’ view

We asked two energy analysts for their views on the costs of offshore wind relative to other electricity sources, trends in wind turbine prices and to what degree offshore wind is artificially supported by subsidies.

Angus Crone, Bloomberg New Energy Finance

Our levelised cost of electricity model currently has offshore wind at $221 per MWh, onshore wind at $83, PV solar at $128, nuclear on $101 and combined cycle gas turbines (CCGT) at $70. Note that these are global averages and are levelised costs, not simply capital costs. They also exclude the costs of CO2 emissions in the case of CCGT gas.

Only about 45-50% of offshore wind capital costs are the turbine. The rest consists of foundations, cabling, installation etc, and there is probably even more scope for cost reductions there. Our estimate is that levelised costs of electricity for offshore wind will come down by 22% between 2012 and 2020. There will be further significant cost reductions beyond that.

We think costs will peak in the next couple of years and then fall

Offshore wind costs have already gone up in the last few years because of the move to deep water. We think costs will peak in the next couple of years and then fall, as improvements in installation knowhow, foundations and turbine technology outweigh the extra costs of going to deeper and deeper water.

Offshore wind projects would simply not be built at the moment without support provided by the roc scheme, or after 2017 the contracts for difference feed-in tariff scheme. However the government hopes that by supporting offshore wind now, it will make possible a big reduction in costs over the next 15 years and also help create a world-leading industry in the UK.

Marianne Boust, IHS Energy

Renewable costs in general in the UK are very high because of the long permitting process and the uncertainty over the future support policy (i.e. the contract for difference scheme). IHS analysis suggests that new offshore wind projects is at least twice the price of new build CCGT or new nuclear. RWE was just quoted saying that  £155/MWh was not enough to guarantee the viability of their offshore wind projects. In comparison wholesale power prices are below the £60/MWh mark.

Note that London Array is cheaper than new projects because the capital costs have increased in the recent years. Most projects were developed near-shore but now projects are going further away and at deeper site. The supply chain (both equipment and people skills) struggle to keep up with the project’s development, hence costs are rising. Also, there has been more unplanned maintenance and the overall risk profile of offshore projects for banks has increased over time.

We’re seeing costs rising because of the greater complexity of projects

Looking ahead, the UK government expects that the price of offshore wind will decrease below £100/MWh in order to reduce the consumer bill. This is highly questionable given that we’re seeing costs rising because of the greater complexity of projects as sites are further away from the shore, and because of the immaturity of the supply chain. IHS expects very little cost reduction until 2015/16 and estimates that the required price for offshore wind in the UK could reach £120/MWh by 2020 as bottlenecks in the supply chain disappear and players streamline their installation process.

Consequently offshore wind is not mature enough to be bankable without subsidies, be it direct incentives or soft loans via the Green Investment Bank. It’s hard to estimate the potential loss operators would make without government support but you can have an indication if you compare the draft strike price with current wholesale power prices.