The London Array – the world’s largest operational offshore wind farm – has been online for two years, and is now moving into a ‘steady-state’ phase of operation. Tildy Bayar visited the site to find out what’s involved.

Substation and turbines at the London Array

Credit: Tildy Bayar

The 630 MW London Array – the world’s largest operational offshore wind farm, covering an area of 100 km2 – has been online since 2012, and fully operational since 2013. First proposed in 2001 as a joint venture between TXU Europe, Farm Energy, ABB, Powergen and Shell, it is currently jointly owned by E.ON (which holds a 30 per cent share), Dong Energy (25 per cent), Masdar (20 per cent, acquired in 2008) and Canadian pension fund Caisse de dépôt et placement du Québec (25 per cent from 2014, when it acquired half of Dong Energy’s holding).

A second phase was initially planned. While consent was originally granted for up to 1 GW, according to plans the total capacity would have come to around 870 MW, and London Array says that, given known constraints, around 830 MW would have been the likely outcome. The company says phase two was eventually cancelled due to a combination of factors, including the presence of overwintering red-throated diver birds. Ensuring continued protection for the birds would have required an environmental impact assessment that promised to take at least three years to complete.

Since phase 1’s completion its turbines have become a home for peregrine falcons, and shellfish also “seem to be attracted” to the foundations, said Mike O’Hare, London Array’s outgoing general manager, “so we have quite the ecosystem”. Bronagh Byrne, then-environment and consent manager, also reported having seen grey seals “basking on sandbanks near the wind farm, with construction going on in the background”. She added that “it was great to know we weren’t having a detrimental effect on our neighbours”.

Another factor was technical challenges. O’Hare said: “The Phase 2 site is technically more challenging than Phase 1 with a large area in very shallow water. Additional issues include a difficult and longer route for the export cables and an exclusion zone for aggregate operations.”

The wind farm was awarded a lease by The Crown Estate in 2003, as part of the UK’s Round 2 offshore wind development programme. In 2005 it became the first Round 2 project to apply for planning consent, which was given in 2006 for the offshore works although the initial planning application for the project’s onshore substation was rejected by the local council. In 2007, after a public inquiry and a contest for alternative designs for the substation that would minimize its visual impact, consent was granted.

Work began onshore in 2009 and offshore in 2011, with the first foundation monopile installed early in that year. Two offshore substations followed, also in 2011, and installation of the array cables connecting the turbines to each other and the export cables connecting them to shore began in 2011 and was completed in late 2012. In October 2012 the first turbine began producing power, and in December the final turbine was installed. Full operation was achieved in April 2013.

At the height of construction there were 1000 people working on the wind farm, using 60 vessels. There are currently 90 permanent employees – 70 technicians and 20 support staff.

The wind farm’s 175 3.6-MW Siemens turbines produce 2.2 TWh to 2.3 TWh per year. According to O’Hare, no other wind farm has produced more than 2 TWh in a calendar year. He says the project has exceeded production expectations, and that there are “just a handful of days” per year when the turbines aren’t turning.

Incoming general manager Jonathan Duffy explains how his team calculates the wind farm’s power production: “We use the capacity factor – the amount of generation if all the turbines worked 100 per cent of the time – versus the actual production,” he says. “Over a year it’s around 40 per cent, but it varies month by month – in winter it’s more like 50 to 60 per cent.” He adds that “we believe London Array is the world’s best-performing 3.6 MW [turbine] wind farm”.

One challenge in designing the wind farm was the need to place the turbines so they were not in each other’s shadow in order to avoid so-called deep array losses, where some turbines block others so that, by the time the wind gets to the ones behind, some of the energy has already been extracted. A cost-benefit analysis had to be performed, which would consider the cost of cabling and how far apart the turbines needed to be.

For phase 1 of the London Array, the energy loss is “in the region of 10 per cent”, Duffy says, while phase 2, if built, “would have had greater deep array losses” because it would have been in the shadow of Phase 1 with the prevailing wind from the southwest. “All large wind farms have this problem,” he notes, adding that new concepts to deal with deep array losses in future include optimizing the shape of the wind farm with respect to the prevailing wind where other constraints such as planning and water depth allow.

Keeping the turbines turning

Keeping the turbines turning is a full-time job involving 90 permanent on-site staff and multiple considerations. London Array makes the point that safety is always the highest priority, with Miles Wakefield, balance of plant manager, saying: “It’s hazardous work. Workers lift heavy equipment in changing weather patterns and, if an accident happens, they are a long way from medical treatment.

Maintenance workers atop a turbine

Credit: Tildy Bayar

“There is a ‘no-blame’ policy for people to speak up if they spot a problem,” he adds. “Safety is the main concern.”

If the significant waves are over 1.5 metres high (which can give a maximum wave height of approximately 2.4 metres), he explains, the work boats won’t ‘push on’ (dock) at the turbines; if there is lightning within 25 km then the Offshore Co-ordinators will “immediately pull everyone off the turbine”. In the Offshore Co-ordinator’s office there is a computer screen showing the real-time location of each worker who is offshore, so if a situation arises then everyone’s location is known and they can be accounted for. The working day is long, with technicians working 12-hour shifts “to maximize the ‘time on turbine'”, says Wakefield, with five to six crew boats normally in the field at one time.

Neill Austen, a crew boat skipper, explains that his work schedule is “two weeks on, two weeks off” and that he usually transports 12 workers at a time on the hour-long journey out to the turbines, dropping them off in teams of two or three workers per turbine. Generally service teams will stay in one location all day, but the teams carrying out warranty works may go to three or four locations.

Last year, O’Hare says, the amount of planned vs unplanned maintenance was “about 50/50”. Routine maintenance, he says, “is critical and in some ways not that complicated: tightening bolts, changing filters”, but there are some “more specialist” tasks such as vibration monitoring. In terms of inspection, he adds, 20 turbine foundations are inspected per year.

He notes that there has been “very little in the way of major issues with the wind farm” and that there have been just three major component repairs in its two years of operation. In 2013 a gearbox (weighing 43 tonnes) was swapped out; more recently, in May a main bearing exchange was accomplished – the problem having been “picked up by vibration monitoring,” O’Hare says.

The team also had to swap out a transformer that had developed a fault, putting a turbine out of action for three months. “Finding a suitable vessel [to carry the transformer] takes time,” O’Hare said, as does the problem analysis: “We’re undertaking a root cause analysis to understand why the fault happened.”

According to O’Hare, on-site risk management measures include optical-fibre distributed temperature sensing on the wind farm’s export and array cables. This is standard equipment for export cables, but O’Hare says London Array is the only wind farm that also has it on array cables.

The technology “looks out for cold spots,” he explains: “The cable gets cold when it gets toward the [water’s] surface, and this would suggest that the cable may not be buried anymore”. In addition, the wind farm uses bathymetric surveys to check if there is any scour development, or erosion of the seabed surface around the turbine base.

In the event of a cable failure, the setup ensures that power can be switched between array cables, and contingency procedures are in place for the transformers and export cables.

Construction challenges

O’Hare outlined some of the challenges his team faced during the construction period. For example, the 175 monopile foundations, which anchor the turbines to the seabed, had to be individually constructed due to varying water depths and seabed conditions. “Each monopile was unique,” O’Hare says.

Also, although it is standard practice to bury the power export cables underneath the seabed, the London Array’s four export cables had to cross the Kentish Flats export cable and the BritNed interconnector “because the other cables were there first”, said O’Hare – so while London Array’s cables are buried under the seabed for most of the route, they do come to the surface with rock berms for protection at the crossing points. Each cable is over 50 km in length and weighs 4500 tonnes.

The initial design featuring a cylindrical grouted connection between the monopiles and transition pieces, an industry standard, was subsequently found by a joint industry project to be “not as effective as was originally thought,” Duffy said, “so late in our design process we adapted and put in a conical design here. We were one of the first wind farms to do this.” He added that “while it slowed things down at the beginning of construction, it will have a positive effect on the integrity of the monopile structures.”

Some of the construction vessels used for the London Array were newly built for the project on spec, O’Hare says. The main construction vessel, used to transport large, heavy parts and materials out to the turbines, arrived later than expected and had a deeper draft (how much the bottom of the ship is below sea level) than planned, he noted – and with much of the construction taking place in relatively shallow waters, this was a problem.

Turbine climbing safety demonstration

Credit: Tildy Bayar

“One of the most difficult things was installing the array cables – there were times when little work could be done,” he added, as some of the turbines are above sea level at low tide and any bad weather had a disproportionate effect. At the peak of construction there were 60 vessels in the water, carrying materials and about 1000 workers. According to O’Hare, there were “5000 or so total [workers] over the construction period”.

And the issues weren’t only offshore. The 150,000 V to 400,000 V onshore substation took three years to complete after planning permission for the original design was refused. Once the winning new design was adopted, there was a commissioning issue with the cables connecting to the National Grid substation which, according to O’Hare, “delayed the initial energization of the turbines by six months”. However, the team was “determined to finish on schedule” and set what O’Hare believes was a new record for commissioning offshore turbines of 10 to 15 per week.

There were also commissioning issues with the offshore substation’s 33 kV bus ducts involving partial electrical discharge; “we had to take them apart and put them back together,” O’Hare said. He noted curiously that, during the construction process, “the heavy lifting and offshore work that should have been more difficult generally went well, while the more conventional electrical aspects caused difficulties.”

Human error also played a small part in slowing down the process. Each 60-metre turbine blade features a lightning protection system that includes eight metal receptors – four on each side. After these were first installed (before the turbines were assembled on-site), it was found that there was higher resistance than might have been necessary for safe operation over the lifetime of the project, as resin had been applied over the metal conductors. According to O’Hare, rope access technicians were used to remedy the receptors on a number of the turbines – “a minor thing to do onshore, but a big deal when working offshore,” he said.

Moving ahead

Duffy comes to London Array with over 10 years of offshore wind experience, including setting up the operations and maintenance infrastructure for the Greater Gabbard offshore wind farm, where he was generation manager. As London Array moves from the early operation stage into a “steady-state” phase, Duffy plans to “take a more systematic view” of operation and maintenance, “look for operational efficiencies and long-term asset protection”, and “focus on risk management and managing asset integrity”. He will also be “a fresh pair of eyes” on the quality of ongoing maintenance.

For example, he says: “You need the right inspection techniques to deal with the shifting seabed and subsea structures which can produce scour, and you need adequate corrosion protection so there isn’t degradation ahead of the design curve.” Currently, says O’Hare, 20 turbine foundations are inspected per year, but this is planned to increase.

In addition, Duffy plans to implement yield improvement initiatives of various kinds. One such initiative that has already yielded results is Siemens’ “lean servicing” strategy, which has cut maintenance teams from four to three persons and streamlined the process by which they work for improved efficiency.

Upgrades are also in the works, with several already installed before the turbines were placed. Given the length of the project’s development, O’Hare says, “the technology often moves on and comes up with new things while you’re still in the development phase”. Early improvements included modifications to the turbine blades from Siemens to improve their aerodynamic qualities, such as ‘DinoShells’- shovel-shaped additions which capture more energy at lower wind speeds; ‘DinoTails’ – shaped like a Stegosaurus’s back plates; and vortex generators, installed at the root of the blade.

Søren Larsen, project manager during construction, perhaps put it best in terms of the wind farm’s future impact. “The experience we’ve had, in terms of what works and doesn’t work, will help to create the next generation of wind farms in the UK and around the world,” he says.

O’Hare agrees. “London Array has shown we can deliver colossal offshore wind farms on time and on budget, cementing the UK’s position as a world leader in offshore wind, and we need to keep building on that,” he says.