United Kingdom

United Kingdom

Specialist boats or add-on access systems?

As offshore wind moves further out to sea, the smooth and safe transfer of personnel and components to turbines becomes increasingly difficult. We look at the ongoing debate between proponents of specialist boats and those who favour bolt-on access systems

Tank testing of the Autobrow system in three-metre sea states
Tank testing of the Autobrow system in three-metre sea states

At the end of October, UK green investment organisation the Carbon Trust announced the latest round of its funding for a competition aimed at improving safety and cutting the costs of transferring people and cargo to deepwater wind turbines. This Offshore Wind Accelerator (OWA) programme addresses key concerns surrounding the UK’s 25GW Round 3 of offshore wind projects, which will be built from 2015 onwards at locations that may be up to two hours transit time from shore to turbine, and will present engineers with the problem of trying to climb onto turbines and transfer cargo from boats in waves up to three metres high.

The common practice for near-shore turbines of ‘bump and jump’ — that is, to take a small boat and nudge it against the turbine, allowing the engineer to jump on to the ladder at the base of the turbine — becomes far too dangerous in deeper waters. Larger ships with access systems previously used in the oil and gas industry are a safe option. However, with these ships costing tens of thousands of pounds per day to charter, they are not a suitable or sustainable solution, especially given the UK government’s target of cutting the cost of offshore wind by a third.

New concepts

It is in response to these twin challenges that the Carbon Trust launched its OWA competition in 2010, when 13 access designs were supported with research and development funding from the trust and eight offshore wind developers. On 30 October, it was announced that these designs had been whittled down to six that would share further R&D funding of £650,000.

However, five of the six designs the Carbon Trust has chosen to take forward show that there is still no consensus on offshore access methods. There are two transfer systems, three vessels designs and technology that can be added to existing vessels to improve engineers’ access to turbines. The sixth design is a launch and recovery system for rescuing smaller vessels, and as such not focused on accessing turbines.

This lack of unity is a fair indication of the general split in thinking among those operating in the market.

Tidal Transit managing director Leo Hambro, whose firm supplies vessels for some the UK’s far-from-shore wind farms, including SSE’s 500MW Greater Gabbard wind farm off England’s east coast, is not surprisingly a proponent of purpose-built offshore vessels.

"The turbine access systems equate to one-third of the cost of our boats, which we would then have to add to our charge rate," says Hambro.

"If we can design a boat from the ground up to handle the worst conditions, then there’s no need for motion-compensating gangways for small boats. If the boat is not designed for access from the front, then you might need a turbine access system, but you’re adding 2.5 tonnes to the front of a boat not designed for so much weight, so it will affect the ride and handling.

"In that case, you will need to redesign the boat and if you’re going to the trouble of redesigning the vessel to handle the weight, you should design it to handle the waves."

Tidal Transit has two boats — Ginny Louise and Eden Rose — servicing Greater Gabbard. Built by Spanish shipbuilder Mecurio, these offshore wind-specific vessels are 20 metres long and capable of operating in wave heights of up to 2.5 metres. This, claims Hambro, is better than what can be achieved using a bolted-on access system.

"An access system might increase the boat’s ability to transfer crew from 1.5-metre sea conditions to two metres," says Hambro.

"I’m sure they will achieve that on certain vessels — although the system’s weight will affect ride handling — but that two-metre transfer is one that our boat is already signed off to carry out on Greater Gabbard."

Engineers access turbines from the front of Tidal Transit’s boats, which are able to stick on to the turbine ladders. This means the access point does not change.

"We stick to the tubes on the boat landing and stay level — we don’t ride up and down them as other boats do," says Hambro, alluding to the current common practice of ‘bump and jump’.

"It stays for a long time in a safe place. If you are in a small boat on a 1.5-metre day, the boat will be riding up and down nine rungs of a ladder and there’s no way I would get off in those conditions."

However, Hambro does not wish to be completely dismissive of the new transfer technologies being developed. He says: "If they can make them not to cost or weigh so much, they will be on to a very good thing. However, they are systems that will work well for small vessels, but as far as we are concerned our clients are looking for bigger boats with bigger wave handling."

Mark Warren, project director of Autobrow, one of the Carbon Trust’s chosen technologies, says: "There are only two fundamental approaches to improving the safety and operating envelope: reducing the motion of the vessel or compensating for that motion. The Autobrow personnel transfer system takes the second approach. It is aimed at vessels up to 35 metres long and early tests have shown it can reduce vertical motion during transfer by 95% in sea conditions up to three metres."

Jan van der Tempel, CEO of offshore access firm Ampelmann, is open-minded about the potential of some of the technologies under development. He shares Hambro’s doubts about transfer systems designed to bolt on to smaller vessels, as he too sees a trend of ships getting larger.

However, van der Tempel envisages the use of ships between 50 and 60 metres long — big enough to carry Ampelmann’s access systems. He believes that such systems on large ships, which have sufficient stability to bypass the wave-height restrictions of smaller vessels, are the best way forward, from a financial and safety standpoint.

"I think offshore wind should step away from people having to climb ladders on a daily basis," he says. "You can land a person on a platform and they just open the door, rather than having to climb a ladder for 10 metres… but for that you always need a larger vessel."

Systems such as Ampelmann’s are used on ships that are between five and ten times more expensive to hire than vessels such as those supplied by Tidal Transit. However, van der Tempel points out that his firm’s systems are usable in sea conditions of up to 3.5 metres, meaning "90% of the time you can access the turbine".

"Working with one of our systems in high tides is cheaper than your engineers not being able to access a faulty turbine with a smaller vessel," he says. "It’s not about the cost of the unit, it’s about the overall cost of energy of the offshore wind farm."   

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