Anticipating upcoming large-scale floating wind farm installation, Damen has developed the FLOW-SV vessel concept. The Damen FLOW-SV is specially designed to install ground tackles for offshore turbine floaters and will be bigger than any anchor handling vessel before.
The vessel can load the immense lengths of chain needed to install and secure nine anchors or suction piles. Instead of mooring one floating wind turbine in a mission, three can be secured. It also applies sufficient proof loading on the anchors to make the installation more efficient.
‘With the FLOW-SV, we have developed a vessel which covers the entire process of attaching mooring lines,’ explains Wijtze van der Leij, sales manager at Damen. ‘This vessel, combining the supply, installation, securing and inspection of the ground tackles for floating turbines in one vessel, introduces a big step forward towards large scale installation of floating wind farms. The concept reflects our expertise in operational demands and efficiency.’
Van der Leij adds: ‘At the moment, we are still in the concept phase. But we are now entering a stage where we would like to partner up to refine and customise this concept. Together with a launching customer, we aspire to accelerate the floating offshore wind farm installation industry.’
Larger than any anchor handling vessel before
Measuring approximately 150 metres in length and a 32-metre beam, the FLOW-SV is a big ship. The size and weight of anchors and chains needed for offshore floating offshore wind turbines are so big that a larger vessel is needed than any anchor handling vessel before. As the FLOW-SV will be able to take all the materials and equipment needed to install three floaters, the vessel saves time on transits and loading. Secured at all three corners, a floater needs three ground tackles.
When Damen engineers defined the amounts of gear that the FLOW-SV needs to take on board to be able to install three floaters in a base case of 100 metres water depth, their calculations added up to five kilometres of chain (chain links of 152 mm diameter), 4.5 kilometres of fibre rope of 112 mm diameter, nine anchors each weighing 15 tonnes and almost 100 clump weights of 10 tonnes each, D-shackles, tensioners, as well as steel cable.
Getting the gear to the place of operation means the vessel carries a load in excess of 4000 tonnes. Depending on seabed conditions, depth and wind farm operator requirements, anchors can be replaced with suction piles to provide a secure ground tackle.
When deploying the anchors at sea, the chain is guided to large winches on the forward end of the large open aft deck, from where it leads over the deck to the stern where it is deployed into the water. A 250-tonne crane (at 12.5 metres reach) with active heave compensation is installed along the aft deck’s starboard side while two smaller cargo rail cranes are available for handling gear on deck. A triple drum anchor winch can deploy 600 tonnes pulling force.
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1000 tonnes of proof-load
A unique feature of the FLOW-SV is the combination of bollard pull generated by the thrusters and added pulling force from the bow reaction anchor winch. This adds up to 1000 tonnes of proof load.
The FLOW-SV deploys the bow anchor and embeds it by reversed bollard pull. It then moves to the spot where the anchor for the floater needs to be installed. After letting out enough chain length, the anchor for the floater needs to be proof loaded, ensuring a secure seabed connection.
Using the four thrusters, FLOW-SV has 400 tonnes of bollard pull. Pulling strength is enhanced by the forward anchor handling winch, that generates another 600 tonnes. At that moment, approximately 1000 tonnes of proof-load is acting on the turbine floater anchor.
Two fixed propellers in nozzles and two azimuthing thrusters at the stern propel the FLOW-SV and provide forward bollard pull. These propellers in nozzles turn 180 degrees to provide ample reverse bollard pull when the bow anchor is set. The azimuthing thrusters are also engaged for dynamic positioning, together with the retractable azimuthing thruster and tunnel thrusters in the bow section of the vessel.
Sea bottom inspection is needed to determine the best spot to install the anchor. FLOW-SV has two working class ROVs (remote operated vehicles) with two separate control rooms next to a moonpool (an opening to launch submarine vehicles in the middle of the vessel).
After installing the anchors for the turbine floater, the ROVs can be deployed through the moonpool for inspection of the anchor and to see if it has dug in to provide secure ground connection.
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In line with the purpose of installing renewable energy supply, the FLOW-SV is laid out for using the renewable fuel methanol. Tanks and piping are in place according to safety regulations for this future fuel. Six generators feed the electric thrusters. When the floater anchors are being secured and maximum bollard pull is required, the two fixed 5.5 metres propellers in nozzles are engaged.
At transit journeys between harbours and windfarm sites, the azimuthing 4.5-metre propeller size thrusters propel the vessel. This eliminates the need for rudders. All propulsion units are positioned to have the least possible interference with anchor chains.
Analysis of planned installation of floating wind turbines offshore, indicates that this new type of installation vessel is needed. Large anchor handling vessels available today have limited carrying capacity and are booked in long-time charters or in other maritime operations.
Planned wind farm installation would require some 100 vessels of the FLOW-SV type. That is why Damen started the development of FLOW-SV with valuable input from industry experts like Temporary Works Design (TWD), First Marine Solutions (FMS) and Intermoor. Knowledge about critical mission equipment specifications was provided by Kongsberg and McGregor.
Besides developing this new type of vessel, Damen can also supply the chains, floaters, and the vessels for towing.