New concepts use ramps, cradles or docks to capture and lift the craft in a controlled manner

By Andrew Williams, UK correspondent

Round 3 wind farms will be further offshore than current sites and a widely held view is that future O&M activities will likely be launched from motherships with smaller daughter-craft shuttling to turbines to undertake O&M activities.

Recovering a vessel onto a mothership in high sea states is difficult, given the dynamic environment and extreme weather conditions, explains Jan Matthiesen, Offshore Renewable Technology Acceleration Manager at the Carbon Trust.

"New designs and innovations are needed to ensure that these challenges can be overcome, for example, by providing shelter for recovering vessels or by using innovative launch and recovery systems," he says.

Safety paramount

With mother vessels positioned far offshore, HSE considerations are vitally important. Matthiesen argues that the safety of personnel offshore must be the first consideration for any activity and stresses that new innovative launch and recover systems need to ensure that operations can be undertaken safely.

Recovery in high-sea conditions is a challenge due to the relative motions between the mother ship and the daughter craft. Derek Clarke, Joint Managing Director at Divex explains that 'traditional' davit systems require the daughter craft to manoeuvre under the lift wires and hold station while they are connected. During this critical time the lifting hooks swing precariously around at head level, with obvious risks.

Another challenge is the potential for impulse, or snatch, loads occurring within the lifting system. As the relative motions can behave unpredictably it can lead to very rapid increases in loads as slack wires can suddenly become taut - dramatically increasing the loads well above the static weight to be lifted.

"The latest concepts avoid lifting completely by using ramps, cradles or docks to capture the craft and then lift the now constrained craft in a controlled manner," says Clarke.

Cost drivers

The time it takes to launch and recover vessels will have a huge impact on costs, as it is likely to be an often-repeated procedure. The size of Round 3 wind farms will be larger than those currently in operation, with many hundreds of turbines, meaning that several service vessels will be needed to operate and maintain them.

According to Matthiesen, the 'weather window' to undertake O&M activities may be short and if it takes several hours to launch service vessels valuable time will be lost.

"Because these vessels need to be launched and recovered every day, the time it takes to do so will have a huge impact on speed of O&M activities, which will ultimately impact overall energy production," he says.

Clarke's view is that the size of the daughter craft and the required modifications to the mother ship also represent additional costs. He explains that the Divex solution currently under development, and one of the three Launch and Recovery systems shortlisted under the Carbon Trust Offshore Wind Accelerator Programme, will be based on a trade-off between size and engineering complexity and will ultimately help to define the relative costs.

The cost benefit will come from being able to access the turbines for more days of the year to carry out maintenance and repair, thus increasing their availability to produce energy," he says.

Innovative solutions

There are a number of innovations that will ensure safe launch and recovery from service vessels. However, Matthiesen believes that mother ships must also be innovative as it is expected that they will stay offshore for long time periods.

"They need to be self-sufficient, provide a stable platform to operate from and be comfortable for personnel who live on the vessel," he says.

Clarke explains that an internal dock, as typically used by military amphibious support ships, is a proven way to address the problem but comes at a considerable cost as the mother ship must be sufficiently large to accommodate a dock.

The objective is to develop systems that can deploy from smaller mother ships while still being able to achieve the Carbon Trust's '3 metre high seas' criteria.

"The risk of collision is considerable and the larger the vessel the greater the collision energy, hence the desire to reduce the size of vessels operating within the park," he says.

The challenges relating to such launch and recovery systems are many and varied and as the Accelerator Programme selection process nears its conclusion, offshore wind operators await the result with much interest.

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