Damen has unveiled its solution to what is predicted to be major demand for vessels to install ground tackle following increasing global interest in the floating offshore windfarm market.
Offshore wind has come a long way since the first installations in European waters some 20 years ago.
At the time, 3MW turbines with monopile foundations in relatively shallow waters were the norm. The economy of scale from today’s 12-15MW turbines with 17-18MW models in development has promoted the development of floating offshore turbines, allowing projects in waters considered too deep for monopile foundations.
Floating offshore wind demands a different and more complex approach to installation over monopiles, however, involving typically three anchors and ground tackle for each floating turbine.
Following the theme of economy of scale, Damen’s answer is a very large and powerful vessel named FLOW-SV, capable of installing the nine anchors and mooring chain required for three floating turbines. This saves time on transits and takes offshore support vessels in a new direction and to a new level.
Anchor-laying requirements
Laying anchors (or suction piles) for offshore structures is well established within the offshore energy industry, but the scale of this requirement has resulted in a vessel 150m in length and 32m beam.
Damen describes it as ‘a big ship’, adding: “The size and weight of anchors and chains needed for installing offshore floating wind turbines is so big that a larger vessel is needed than any previous anchor handling vessel.”
The size of vessel required became clear when the amount of gear to moor three floating turbines in a base case water depth of 100m was considered.
Five kilometres of chain (chain links 152mm diameter); 4.5km of fibre rope (112mm diameter); nine anchors each weighing 15 tons and almost 100 clump weights of 10 tons each; D shackles and tensioners as well as steel cable all add up to a vessel capable of loading in excess of 4,000 tons. Suction piles can also be accommodated to secure ground tackle.
For anchor laying, the chain is guided to large winches at the forward end of the large open aft deck, from where it leads to the stern and is deployed over board.
An active heave-compensated 250t crane is installed on the starboard aft deck, with two cargo rail cranes provided for handling deck gear.
A triple-drum anchor winch can deploy 600 tons of pulling force. Two work class ROVs can be deployed through a moonpool for inspection of the anchors, and FLOW-SV will include methanol as a fuel source.
Damen says that a “unique” feature is the combination of bollard pull and added pulling force from the bow reaction anchor winch, adding up to 1,000 tons proof load.
The bow anchor is deployed and embedded by reverse bollard pull, the vessel then moving to the spot where the floating turbine’s anchor is to be installed.
After laying out the mooring anchor and chain it is proof loaded, ensuring a secure seabed connection for which the vessel uses its 400 tons of bollard pull. The pulling strength is enhanced by the forward anchor-handling winch, which generates an additional 600 tons: at that moment approximately 1,000 tons of proof load is generated.
Extra thrust
The complex operating profile calls for a capable propulsion arrangement with two fixed-pitch propellers in nozzles and two azimuthing thrusters at the stern providing forward bollard pull. The propellers provide ample reverse bollard pull when the bow anchor is set, the azimuthing thrusters engaged for DP together with a retractable azimuth and tunnel thrusters forward. The azimuth thrusters are used for transit, eliminating the requirement for rudders.
The FLOW-SV concept is being designed with industry experts including Temporary Work Design, First Marine Solutions and Intermoor, with mission equipment specifications provided by Kongsberg and McGregor.
Damen can also supply chains, floaters and tugs, making it a ‘one stop shop’ for floating windfarm installation operations, including assistance with finance.
Summing up, the Dutch shipbuilder says: “Analysis of planned installation of floating wind turbines offshore indicates that this new type of installation vessel is needed.”
The description and specification of the vessel makes this a somewhat modest statement presenting a type of vessel that could transform this method of laying anchors with an eye to mirroring the efficiencies and economy of scale that offshore wind is now bringing to the global energy market.
