Designed with wind in mind

EUROPE: The industry asked for a farshore service operation vessel that improved accessibility and reduced seasickness. Damen's solution was to build it from scratch.

by Peter Robert 29 April 2016

Long and slender… The changed hull shape contributes to ability to provide turbine access in waves up to 3.1 metres high

As wind farms move further offshore, operations and maintenance (O&M)activities become more difficult to perform safely and comfortably.

Yet, if the industry is to retain its technicians, improving working conditions at sea is vital and project operators are having to reconsider the type of vessels best able to serve this new breed of wind farm.

Projections for the next five to eight years indicate that more than 30 service operations vessels (SOV) will be needed in the North Sea and around Europe.

By starting with a blank sheet of paper, we knew we could develop a complete package, enhancing performance, safety and on-board comfort. So Damen began to design a purpose-built wind-farm SOV from scratch, able to meet the needs of the North Sea wind industry and projects around 50 kilometres from shore.

Cost benefit

As well as the vessel construction, we were able to look at working conditions and workflow. Significant changes in the draft of the final vessel, interior layout and advanced dynamic positioning (DP) have improved cost efficiency enough to outweigh the cost of development.

Discussions with offshore-wind developers, operators and maintenance firms showed the main concerns about moving to farshore sites to be the ability of engineers to carry out timely, safe work in a comfortable working environment.

Specifically, they wanted to keep optimal turbine availability and retain experienced staff, who in many cases have little seafaring experience and would need to adapt to longer periods offshore.

The vessel design represents more than two years of research and development, involving a thorough series of tests at the Maritime Research Institute Netherlands. We were able to do exhaustive seakeeping tests on the dynamic positioning (DP) capabilities to ensure the system lives up to expectations.

In another first, rather than waiting for an actual vessel to be built to test it, a scale-model test pitted the SOV against simulations of North Sea wind, wave, swell and current.

The most fundamental change that we made was to the hull, which, at 90-metres, is longer than a conventional PSV, and the bow has been lowered by 1.5 metres to create a V-shape.

Together, this substantially reduces slamming and ensures that the bow thrusters remain in the water, optimising DP performance.

Another key changes was to move the superstructure to midship to achieve a slender aft. This reduces the wind-induced moment, lowering the required bow thruster power, which in turn makes stern to weather operations more comfortable.

We retained the standard PSV monohull design to minimise accelerations caused by roll motions, with the expectation that the SOV will be on DP 80% of the time at speeds between 0 and 5 knots.

The main propulsion system is diesel-electric, which powers twin azimuth thrusters. A symmetrical wind profile and a four-split main switchboard to distribute the power mean the vessel requires less installed power than a conventional PSV.

It also divides the generator sets efficiently, and in the event of a failure only 25% of switchboards would be out of action, rather than half.

Transfer

The new vessel can stay at sea for up to a month, accommodating 40-45 maintenance personnel and 15-20 mechanics, with workshop and storage facilities optimally positioned for efficient working.

Crucially, its walk-to-work functionality can provide fast and safe access to turbines in up to 3.1-metre wave height, exceeding the industry standard of 2.5 metre. This increases turbine availability, with an estimated 25% drop in O&M costs over the lifetime of a large wind farm.

The motion-compensated access gangway system mounted on the port side, uses software that is aligned with the vessel's DP system to accommodate gangway motion limits.

Having gathered data on the DP system, we were able to feed this back to the supplier, who adapted it to suit the vessel. As a result, the gangway can be deployed and the vessel moved in automated GPS "auto track" mode for several hundred metres at speeds of up to 6 knots to the next turbine, without having to wait for an ideal-weather window or rebuild the DP model at every turbine. Helicopter access and a daughter craft offer alternative transfer methods.

Lower costs

The end result is a vessel that can be used at a lower cost for up to 80% of the time, and is suitable for central North Sea conditions where new projects are planned. For areas where most offshore wind farms are currently located, the SOV has a 95% operability window.

By creating conditions that will reduce seasickness and improve working conditions, we hope this will help encourage an engineer to stay in the job. Not only is this vital for an industry in need of more expertise, it will also avoid the costs of staff replacement and ensure better continuity for project work.

Having planned the first model for stock, Damen has already received its first order from Bibby Marine Services and the first vessel will roll off the production line next summer as the Bibby WaveMaster 1.

Peter Robert, is business development and market intelligence manager for Damen Shipyards. He is speaking at the �u�X��ʘ�� Vessels & Access Forum on 24-26 May.