With the Spintral system, the heavy generator and gearbox are relocated to the base of the tower or in a machinery room underneath it.
A vertical drive shaft and gearbox arrangement will connect the rotary motion from the blades through to the generator.
On a 10 MW wind turbine the weight of the gearbox and generator could be about 500 Te.
Removing this weight from the top of the tower will reduce the loadings on the tower considerably – by about 25%. The amount of steel used in the tower can therefore be reduced by this amount.
The size of the floating structure, and mooring system, can also be reduced by about 25%.
The whole floating assembly will be much more stable as the 500 Te of weight will act as ballast when located in the machinery room within the floating structure, rather than at the top of the tower, hence the name – Ballast Tower.
In the nacelle, a right-angled gearbox will be provided to convert the horizontal drive from the blade hub into a vertical drive shaft on the tower.
A transmission drive shaft mounted in the tower will then connect the drive from the top of the tower to a second right-angled gearbox in a machinery room below the base of the tower.
The gearbox horizontal drive will then connect to the standard gearbox and generator assembly.
This will enable the blade hub rotation in the nacelle to be transmitted through to the generator unit in the machinery room.
It should be noted that the right-angled gearbox in the machinery room could be fitted with a number of output drive shafts.
For example, 2 of 5 MW generators could be used instead of 1 off 10 MW generator.
This would be a beneficial arrangement in the event that there is a gearbox or generator failure. If the 10 MW unit failed then the wind turbine would have to stop generating. If one of the 5 MW units failed then the wind turbine could still continue to generate at 50% capacity.
In the event that the generator failed and had to be replaced then it would require to be handled from the machinery room onto the back deck of a supporting vessel. This would be a weather dependant operation.
If the failure occurred during the winter months then a suitable weather window would have to be available for this operation.
The use of 5 MW generators would enable the wind turbine to continue to generate at 50% capacity until the replacement operation could be performed.
As much equipment as possible should be relocated from the nacelle down to the machinery room i.e. switch gear, control system etc. The size and weight of the nacelle should be minimised to further improve the stability and also reduce the wind loadings.
The wind turbine accounts for 50% of the costs, therefore, there should be a lot of focus on reducing the costs of the components in the turbine.
Relocating the gearbox and generator will reduce the loadings on the tower by 25%, this would save approximately 200 Te of steel on the tower.
There would, however, be an addition cost due to the right-angled gearboxes and vertical drive shaft.
The size, weight and cost of the nacelle would reduce considerably.
The use of 2 off 5 MW generators instead of one large 10 MW generator may reduce costs.
The equipment used is normally bespoke design as it has to fit into the limited space within the nachelle. Locating the equipment in a large machinery room will provide more space, therefore standard, and more cost effective, off the shelf equipment may be used.
The use of a Ballast Tower will reduce the size of the floating structure by 25%. A smaller mooring system will therefore be required and this will be easier to install. These account for 38% of the costs at £26m. On the basis that these costs can be reduced by 25% then this would save £6.5m on each 10 MW unit.
The Ballast Tower is designed so that any standard HAWT wind turbine can easily be reconfigured for use. The blades, hub and yaw mechanism in the nacelle will remain the same. The gearbox and generator will be relocated to a machinery room in the floating hull.
The Ballast Tower can be used on the Spintral floating structure or on any other floating wind structure.
The Ballast Tower can also be used on onshore and bottom fixed wind turbines. It can be used to retrofit existing turbine installations.