static DP operations, as well as short cycle times between turbines. These results are the basis of the performance guarantees that Da men and its clients provide to the end-users.”


A


s the trend continues for wind farms going further offshore into deeper, more challenging waters


with increased wave heights, it is recog- nised that there needs to be a rethink when it comes to carrying out offshore mainte- nance. Operations & Maintenance activities are clearly more difficult to perform safely and comfortably in this new environment. Different maintenance strategies are neces- sary. Until now, the near shore wind farms have largely been served by converted Platform Supply Vessels from the oil & gas sector and Crew Transfer Vessels, which travel to the farms on a daily basis.


Coupled with this, most O&M engineers are not from a seafaring background so they are having to adapt to ‘life at sea’ for much longer periods – perhaps up to a month at a time. Therefore, comfort levels become even more vital if the industry is to retain its technicians and engineers.


But Damen, the Dutch shipyard group which has 32 shipbuilding and repair yards world- wide, has anticipated offshore wind’s move further offshore and developed its own dedicated vessel – the Damen Service Off- shore Vessel (SOV) for the maintenance of far shore, offshore wind turbines.


Safest turbine access The Damen SOV, equipped with a Dynamic Positioning (DP) system and a walk-to-work telescopic gangway, is designed to give the safest access to wind turbines, in combination with the highest living standards at sea.


Damen expects the vessel to maximise working time and guarantee fast and safe access to turbines. In the worst-case sce-


nario, which is the Central North Sea, the vessel is still capable of gaining access to turbines up to 80% of the time. The vessel is capable of providing access up to 3.1 m Hs. Additionally, it is expected to help improve staff retention rates. The SOV is able to remain at sea for periods of up to a month and it is estimated that the new ves- sel contributes to a 25% reduction in O&M costs over the lifetime of a large wind farm.


Representing more than two years of Research & Development, the new design underwent exhaustive seakeeping tests at MARIN’s Off- shore Basin to ensure the SOV’s Dynamic Positioning capabilities during the turbine approach. Additionally, MARIN was asked to assess the power management system, in- cluding the DP system and gangway through numerical simulations using its in-house time domain simulation software aNySIM.


Peter Robert, Damen Manager Business Development & Market Intelligence, explains: “I believe this is the first time such compre- hensive tests have ever been done. We tested the SOV in waves, wind and in DP mode. The main reason for the tests was verification but the tests were also very important because we invited potential customers to attend.”


Optimised turbine approach He out- lines how the model tests will help Damen optimise the turbine approach strategy. “The DP model tests gave Damen the possibility to test the combined system performance of the ship and the vendor-specific, DP control algorithm. The tests showed that the com- bination of mostly common technology in a novel vessel design and a novel vendor approach for controlling the ship via the DP system, gives a high performance both in


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He points out that at the moment no model or full-scale verification had been performed with this unique approach towards offshore wind maintenance, so it was not possible to verify the performance of Damen’s Walk2Work design in a highly dynamic operation. “It is common knowledge that a DP system is not really apt for rapidly changing environments but this vessel is designed for frequently switching between station keeping and transiting and thus, continuously changing position and heading with respect to wind, waves and current. We set up a fully opera- tional DP system; slowed down, sped up, stayed in position at the turbine and moved out again.”


The first-of-its-kind scale model test saw the vessel pitted against North Sea wind, wave, swell and current simulations. The target was a 30-minute cycle, during which the vessel deploys the gangway and then transits in ‘AutoTrack’ mode for several hun- dred metres at speeds up to 6 knots and above, to the following turbine. This is without having to wait for an ideal weather window or having to rebuild the DP model at every turbine.


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