Ride the wave: vessels for wind turbine maintenance

A sea craft using supercar suspension could be the solution to maintaining offshore wind turbines.

It’s probably fair to say that wind turbines have become one of the most divisive forms of renewable energy available in the UK.

But whichever side of the fence you sit on, from a purely technical point of view it’s difficult to deny that the wind sector presents some unique and interesting engineering challenges.

For onshore turbines, engineers have risen to this quite impressively, demonstrating an ability to effectively transfer knowledge and skills from other sectors to solve issues such as torque handling with innovative gearless generators, for example.

Softening the blow: the craft’s pods adapt to the water’s undulating surface, minimising bumps

With offshore, though, there is a whole new set of challenges to tackle and not just from a scale point of view.

Even the most robust turbines will be subject to routine maintenance and unscheduled downtime. So if the planned next-generation offshore mega-farms are going to be cost effective, engineers will need to get to them in potentially rough sea conditions or the turbines will sit idle and lose money (see panel).

For this reason, the UK Carbon Trust – through its industry-backed Wind Accelerator Programme – launched a competition this summer in order to find technologies that might help achieve this. The potential solutions detailed in the entries submitted so far are varied, but one thing they have in common is the need for some kind of vessel that can cope with large waves.

Continuing the tradition in the renewable sector of transferring technologies from other sectors, one of the potential solutions has its roots in the automotive industry.

Nauti-Craft is an Australian company headed by inventor and engineer Chris Heyring. While Nauti-Craft is focused on marine applications, it draws experience and ideas from a previous company co-founded by Heyring called Kinetic, which builds innovative suspension systems for high-performance cars.

These were used by Citroen to win the World Rally Championship in 2003, 2004 and 2005 and by Mitsubishi to win the Paris Dakar campaign in 2004 and 2005 – until, as Heyring puts it, ’they were banned for being too competitive’. The suspension systems are now fitted as standard to the current Toyota Landcrusier and Nissan Patrol off-roaders, as well as the McLaren MP4-12C supercar.

“It’s a bit of a circus trick managing seas of this size – it’s a David and Goliath thing”

Chris Heyring, Nauti-Craft

The key to the Kinetic system is that it adjusts the front and rear stabilisers using a set of interconnected hydraulic cylinders.

’What it basically does is redistribute the weight as evenly as possible between all the wheels, all the time,’ Heyring said.

’With a normal vehicle, because it’s got progressive springs, when you go over a sleeping policeman diagonally you get two wheels that take the majority of the load, while the other two opposite wheels don’t carry much weight and tend to spin. With our system, that doesn’t happen – it’s the same all the way round. It’s quite freaky.’

Heyring, a boat enthusiast prone to sea sickness, had actually been working on suspension for boats since the early 1990s but, after selling off his stake in Kinetic a couple of years ago, saw the potential to develop this further.

’The land and sea can look similar to fast- moving vehicles – in fact, at speed, water becomes hard, like the surface of a desert. The best way to move fast and economically is to ski over the top,’ Heyring said.

This, however, required a radical re-thinking of the architecture of conventional boats. Their initial designs involved the use of multiple hulls or pods, which support the main deck while moving independently through a ’passive-reactive interconnected suspension system’.

In technical terms, the system decouples roll, pitch, warp and heave inputs to maintain the deck at a level altitude.

’The Nauti-Craft’s suspension system helps to spread the vessel’s load equally between pods so, no matter whether on top of a crest or down in a trough, the pods accommodate to the ever-changing undulating water surface, minimising the crashing and bashing,’ Heyring said.

The potential benefits are increased passenger comfort, reduced structural loading, improved fuel efficiency and greater speed. When Nauti-Craft found out about the Carbon Trust’s project, it had already conducted some preliminary work with military marine vehicles and immediately saw the potential.

Stability: Nauti-Craft’s suspension enables it to dock with the base of offshore turbines

’The soldiers get bashed to buggery going through rough seas for a couple of hours. They wear kidney belts, but they still get lumbar and neck injuries and, on a really long mission, some of them end up peeing blood; it’s no joke. You’ve got to provide more comfort and safety,’ Heyring said. ’So with the wind farm business, you’ve got to be aware of what you’re putting your workers through.’

Nauti-Craft has now built and tested an 8m-long ’4-play’ prototype boat with four hulls. It plans to build a larger 30m supply vessel, using a catamaran-type platform but with aspects of the same suspension system, to accommodate the volume of equipment needed for the wind application.

’What we found with the prototype was that you could actually sit there with a glass of champagne and you didn’t even have to hold on, whereas in the standard boat you jump straight up in the air and crash back down,’ Heyring added.

Perhaps one of the most promising aspects of the whole concept, from the perspective of the wind sector, is the ability of boats equipped with Nauti-Craft’s suspension to dock with the base of offshore turbines and remain stable.

This would require a miniature ’harbour’, resembling a two-pronged tuning fork, fixed perpendicular to the turbine shaft. The boat would slot into this and the deck would essentially become part of the turbine, remaining entirely fixed while the hulls below would be moving in unison with waves, absorbing any impact from them. Maintenance engineers would simply walk across from the boat to the turbine base, carrying the necessary equipment and tools. Nauti-Craft’s 4-play is one of 13 designs shortlisted by the Carbon Trust and will receive some financial help from the Wind Accelerator Programme. However, only a couple of them will be selected for further development and commercialisation.

But, regardless of the outcome, Heyring is confident that Nauti-Craft vessels will find a place in other marine applications, including for recreational, commercial, military and passenger markets.

’It is really a bit of a circus trick managing seas of this size – it’s a David and Goliath thing. Once we can prove our technology here, putting it onto a luxury gin palace to go in and out of Monaco harbour is going to be a piece of cake. Even some of the military applications don’t have to unload people in waves the size of a small house.’

Fixing turbines in storms

Providing efficient maintenance for wind turbines is a top priority.

Each turbine in an offshore wind farm requires an average of four visits a year for routine maintenance, as well as unexpected downtime to replace components. Although this may not sound like much, if you consider a wind farm with upwards of 500 turbines, maintenance engineers will be busy all year round.

During downtime, the turbines are in an idle state, in which the blades are turned out of the wind and the brakes are applied – essentially, they just sit and wait to be fixed, which costs money.

Maintenance vessels are then launched from shore; that is provided waves don’t exceed around 1.5m in height. This rarely happens with current farms, which are typically less than 25km off shore, but the next generation could be as far as 300km out, with thousands of turbines.

The shortlisted entries for the Carbon Trust’s competition essentially fall into two broad categories. Launch and recovery systems will likely be fitted to permanent bases or motherships, for launching and recovering daughter craft from the sea.

Meanwhile, the transfer systems will take personnel and equipment from vessel to turbine, potentially with motion-compensation mechanisms. The overarching goal is to make transfers possible for a minimum of 300 days a year in 3m-high waves, thereby increasing revenues by as much as £3bn.