An offshore whirlwind
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An offshore whirlwind

28 Feb 2012 (Last Updated July 29th, 2020 21:49)

High wind speeds and ample space are driving wind operators offshore, but a gust of challenges await them. Sarah Blackman learns how operators and their suppliers are learning to cope with tough weather conditions, environmental hazards and high installation costs.

An offshore whirlwind

When the UK Government cut the ribbon on the £1bn Walney wind farm off from the Cumbrian coast in February this year, 2012, it marked the opening of the world’s largest wind farm. But how long will it hold on to its crown?

Walney, with its 102 turbines and a generation capacity of 367MW, is a stark contrast to the modest (40MW) offshore wind farms that were being built in northern European waters ten years ago, but bigger things are yet to come.

Hot on the heels of this mega project are Germany’s 400MW Bard Offshore 1 being constructed off the isle of Borkum, the 500MW Greater Gabbard project currently being developed by Scottish and Southern Energy and RWE npower in UK waters and the 630MW London Array under construction in the outer Thames Estuary – all of which are expected to open in 2012.

Offshore wind farms make up less than 10% of the world’s wind power market, with onshore projects still dominating the sector, but ample space for turbines and the opportunity for greater energy production due to higher wind speeds offshore are driving more and more wind power companies out to sea.

With high wind speeds, however, come tough conditions and greater logistical, installation and operational challenges, as well as a strong need for extremely durable technologies.

Power companies should expect to be put to the test when developing wind farms offshore, but how can they expect to pass?

Importance of environmental planning

“With high wind speeds, however, come tough conditions and greater logistical, installation and operational challenges.”

It is easy to see the appeal of building wind power projects offshore simply because the world’s landscape is becoming more densely populated, and as more onshore wind projects take up more space, the likelihood of these being challenged by not-in-my-backyard lawsuits is looking increasingly probable.

But, it’s marine wildlife communities, not the presence of human residences, which raise most obstacles and wind operators must plan ahead to manage this environmental challenge.

UK-based Vattenfall has worked on a number of wind power projects, both onshore and out to sea, including the 150MW Ormonde wind farm off the coast of Barrow-in-Furness in the Irish Sea.

For this project, the company carried out an extensive survey one year prior to construction and during construction – when invasive piling operations took place – to monitor the presence of birds, fish and sea bed creatures in the area.

Ormonde project manager Matthew Green said: “We had full-time marine mammal observers in boats who made sure animals weren’t in the area before we started each and every pile.”

Monitoring also continued five years after commissioning Ormonde’s 30 turbines to determine whether these populations re-established themselves once Vattenfall moved off site.

“We were restricted until the first of May before we could start operations due to fish hoarding seasons,” said Green. “You need to allow fish populations to establish before you do anything that could cause a problem for fishing companies in future years.

“It is a challenge, but in the industry we are in, it doesn’t make sense for us to compromise the environment during the construction of these projects, we need to make sure the environmental side is properly considered.”

Installation and logistical challenges

The main benefit of building wind projects away from the shoreline is higher wind speeds, which equate to an increase in rates of power generation.

“So, in the name of increasing the reliability of turbines, suppliers such as GE Energy and Siemens are beginning to abandon their gearboxes and deploy direct drives.”

GE Power and Water offshore product manager Vincent Schellings explained: “In inland Germany you would have wind speeds of about 6.5 metres a second, but if you go offshore the average wind speed is typically 9.5 metres a second.”

With high wind speeds, however, come tough conditions for turbine installations and operations, which tend to rely on steady ground or a smooth sea state to be successful.

“If the wind speed is too high it’s physically impossible to install the wind turbine and offshore you have those conditions more frequently than you do onshore,” Schellings said. “The aim, therefore, is to minimise the time that is required to install the wind turbine.”

The cost to install a wind farm offshore is also considerably higher than it is to install a wind farm on land, due to the need for robust foundations to support turbines, which – due to high wind speeds – tend to be larger, with longer blades, than those used onshore. Offshore wind operators must also rely on specialist and expensive vessels to lift and transport this heavy equipment.

Logistical constraints when moving equipment out to sea, however, aren’t as severe as they are when transporting units on land, according to Schellings. “Onshore units have to be shipped to a lot of different sites all across the world in mountains and terrains, and you need to pass bridges and highways. With offshore, if you have the manufacturing facility in the port, you just drop the units on the vessel and ship them out.”

The biggest challenge that power companies face when placing wind farms in the ocean is laying and burying electrical cables. Statistics show that 80% of insurance claims on offshore wind farms relate to cable faults, which occur quite regularly according to Murray Haynes, vice president of the renewable / cleantech power team at Marsh, a UK insurance broker.

“Marsh did an analysis six months ago and saw that we got more than £70million worth of claims for UK offshore wind farms over about three years. This captures about seven or eight projects. More than half of these [claims] were down to cable losses.”

Green explains why some wind operators damage their cables: “When you are laying the cable over, say, 42km, the seabed varies enormously and it is very difficult to maintain a consistent protection level for the cable as you install. Also, if you were to get a storm and the vessel has to leave site, you have to cut the cable and come back. It’s a very costly operation to rejoin the cable.”

Adapting technologies and tweaking designs

“When the UK Government cut the ribbon on the £1bn Walney wind farm in February this year, 2012, it marked the opening of the world’s largest wind farm.”

In order to cope with wind farm installation challenges, manufacturers are beginning to tweak their designs so that offshore turbines can be developed and commissioned on land.

But, wind turbine operators can’t avoid the open waters when it comes to maintaining their offshore farms, so what happens when a unit trips?

“One of our goals is to the make our units as reliable as possible,” said Schellings. “We’ve basically used the experience we have in more than 17,000 onshore turbines that run at very high availability levels and tried to adapt lessons learned into our offshore design.”

So, in the name of increasing the reliability of turbines, suppliers such as GE Energy and Siemens are beginning to abandon their gearboxes – which, due to their numerous moving parts, are subject to extreme stress when faced with wind turbulence – and deploy direct drives.

Schellings continued: “The fewer moving parts you have, the lower the probability that the part will fail. A direct drive is basically a generator with just a couple of moving parts and with that we simply build in a lot of robustness.”

In September 2004 – shortly before its acquisition by Siemens – Bonus Energy erected its 3.6MW direct drive wind turbine prototype at a coastal test site at Jutland, Denmark. This SWT-3.6-107 model has since become one of the company’s two top-selling products.

GE Power and Water’s direct-drive design was first installed on the coast of Norway in 2005 and its 4.1-113 turbine in the Swedish port of Gothenburg in 2011.

Offshore wind – despite its cost implications and installation challenges – has become increasingly popular over the years and Green believes this is because more and more countries are recognising this renewable source to be a critical part of the energy mix.

“No one is claiming that it will be the ultimate solution to all energy problems – obviously it’s dependent on wind,” Green explained. “But it is critical and it is an available technology which can be made use of, and I don’t think there is a country in Europe with a good wind resource which is turning its back on offshore wind.”