Mark Jacobson, a civil and environmental engineering professor at Stanford (pictured), has developed a very complex computer model to better examine air pollution, wind power, weather, climate, and hurricanes. “We found that when wind turbines are...

By Jason Deign

Additional reporting by K. Steiner-Dicks

All eyes were on Britain’s offshore wind farms as storms threw 10-metre surges at the UK coastline this winter. Production plummeted as the farms shut down. At one point on the night of December 18, total wind out-turn dropped to around half the forecast value.

Even so, the turbines stood up. There was no repeat of the incident two years earlier at the onshore Ardrossan wind farm in North Ayrshire, Scotland, when a wind-battered 100-metre machine burst into flames following a fault in the pitch control and braking systems.

Footage of the Ardrossan incident made headlines and circulated widely on the Internet. However, it was a sight that is not likely to be seen at sea, despite the adversity of weather conditions offshore.

Improvements in technology are increasingly helping the machines to cope, says Aris Karcanias, renewable energy unit managing director at FTI Consulting.

“In general offshore wind farms would start with a design basis where all conditions are well defined and understood,” he points out. “On that basis, a suitable turbine is selected and it will hopefully never see extreme weather conditions beyond its level of tolerance.”

It is impossible to know that this will never happen for sure, however, so it is important that turbine designs can withstand beyond-tolerance conditions. Operations and maintenance (O&M) plays a significant part in this.

Monopile foundations

Wind farm resilience has come a long way from the industry’s first forays offshore, when storms were said to account for between three quarters and four fifths of failures and heavy seas played a part in weakening the grouted connections on monopile foundations.

Dealing with this problem is an ongoing challenge for some wind farm operators, says Matt Rowland, an underwriter at the renewable energy insurer GCube. “It would have to be an additional cost rather than a standard O&M cost,” he says. “It is a big issue.”

Nevertheless, improvements to the design specification issued by the certification body DNV GL have hopefully resolved the grout problem in newer projects.

Modern turbine designs, too, are theoretically able to take pretty much anything a harsh marine environment can throw at them. In the UK storms this winter, papers told of winds reaching 100 miles per hour, equal to about 45 metres per second (mps).

But this is still way below the 70 mps tolerance of IEC Ia and Ib high-wind class turbine designs, or even IIIb low-wind machines built for up to 52.5 mps. Furthermore, offshore machines are programmed to react to high winds in a way that will minimise the potential for damage.

Most will cut out at about 25 mps and stay headed into the wind with the blades feathered to minimise resistance. Providing everything is in working order then this should be enough for the storm to pass over the machine.

O&M schedules

However, if something goes wrong then the turbine could spin out of control. Hence, says Karcanias, it is important to follow O&M schedules to the letter, making sure oil changes are up to date and pitch and gear systems are in full working order.

It also pays to check on your wind farm meteorology and lidar systems to ensure you have accurate data on weather fronts.

The fact that most modern turbines are equipped with sensors and remote-controlled mechanisms can help in determining the status of machines during and after extreme weather events.

“The more you can remote control your machine, the safer you are,” observes Ivo Arnús Montsalvatge, UK business development director for the onshore turbine maker Norvento.

And in terms of safety, one of the benefits of offshore operations is that when heavy weather hits a wind farm the O&M teams responsible for it are likely to be miles away, at port. So at least if a turbine breaks it is unlikely to injure anyone.

Proceed with caution

That does not remove the need for O&M personnel to proceed with caution upon entering a wind farm after an extreme weather event. When temperature drops, for example, “there might be a problem with things like ice,” says a source at one offshore wind consultancy.

“I have heard of problems where ice has fallen on boats.”

Karcanias says the first step following a bout of extremely bad weather should be a visual inspection of the turbines. This can be carried out from either a boat or a helicopter, by the personnel responsible for upkeep. “There’s always a natural risk,” Karcanias observes.

That risk, for now, seems to have been minimised through a combination of technology development and O&M best practice. But it will need to be taken into account as the industry moves into new territories where really strong winds blow.

The last decade has seen at least seven category 5 hurricanes, with wind speeds above 70 mps, in the Atlantic alone. Arguably, climate change and other factors are contributing to extreme weather, so being extra vigilant with O&M schedules and health and safety procedures, especially during stormy seasons, is an undisputed task to preserve your assets.

Offshore turbines curb hurricane damage

A new study published on Nature.com suggests that hurricanes are causing increasing damage to many coastal regions worldwide and that offshore wind turbines can mitigate hurricane damage. The study uses an advanced climate–weather computer model that correctly treats the energy extraction of wind turbines finding that large turbine arrays (300+ GW installed capacity) may diminish peak near-surface hurricane wind speeds by 25–41 m s−1 (56–92 mph) and storm surge by 6–79%.

According to the study, benefits occur whether turbine arrays are placed immediately upstream of a city or along an expanse of coastline. The reduction in wind speed due to large arrays increases the probability of survival of even present turbine designs.

The net cost of turbine arrays (capital plus operation cost less cost reduction from electricity generation and from health, climate, and hurricane damage avoidance) is estimated to be less than today’s fossil fuel electricity generation net cost in these regions and less than the net cost of sea walls used solely to avoid storm surge damage, said the report.

A peak at this Stanford University video explains the essence of the study’s findings: http://youtu.be/M7uRtxl8j2U

The Nature Climate Change published study suggests that a large array of wind turbines installed offshore could have slowed down the wind speeds and reduced the storm surge of some of the most devastating hurricanes in US history.

Dual benefits of offshore arrays

One of the people behind the study, Mark Jacobson, a civil and environmental engineering professor at Stanford, has developed a very complex computer model to better examine air pollution, wind power, weather, climate, and hurricanes.

Working with University of Delaware researchers Cristina Archer and Willett Kempton, Jacobson modelled three big, damaging storms: Katrina, Isaac, and Sandy.

“We found that when wind turbines are present, they slow down the outer rotation winds of a hurricane,” said Jacobson. “This feeds back to decrease wave height, which reduces movement of air toward the center of the hurricane, increasing the central pressure, which in turn slows the winds of the entire hurricane and dissipates it faster.”

In the computer model, by the time Hurricane Katrina reached land, its simulated wind speeds had decreased by 36-44 meters per second (between 80 and 98 mph) and the storm surge had decreased by up to 79 percent, Stanford stated in a news release regarding the report findings.

For Hurricane Sandy, the model projected a wind speed reduction by 35-39 meters per second (between 78 and 87 mph) and as much as 34 percent decrease in storm surge.

Political resistance in the US

Jacobson acknowledges that, in the United States, there has been political resistance to installing a few hundred offshore wind turbines, let alone tens of thousands. But he believes there are two financial incentives that could motivate such a change.

One is the reduction of hurricane damage cost. Damage from severe hurricanes, caused by high winds and storm surge-related flooding, can run into the billions of dollars. Hurricane Sandy, for instance, caused roughly $82bn in damage across three states.

Second, Jacobson said, the wind turbines would pay for themselves in the long term by generating normal electricity while at the same time reducing air pollution and global warming, and providing energy stability.

"The turbines will also reduce damage if a hurricane comes through," Jacobson said. "These factors, each on their own, reduce the cost to society of offshore turbines and should be sufficient to motivate their development."

Sea walls v. offshore arrays

An alternative plan for protecting coastal cities involves building massive seawalls. Jacobson said that while these might stop a storm surge, they would not impact wind speed substantially. The cost for these, too, is significant, with estimates running between $10bn and $40bn per installation.

Current turbines, according to the report, can withstand wind speeds of up to 112 mph, which is in the range of a category 2 to 3 hurricane. Jacobson’s study suggests that the presence of massive turbine arrays will likely prevent hurricane winds from reaching those speeds.