The majority of floating wind structures, possibly 95%, are moored in a fixed position and heading using multiple mooring lines.
The structure and mooring lines have to be designed to accommodate the weather loadings from every direction. Personnel access has to be provided on all sides.
The moorings are generally individually attached to the structure on the surface at deck level. Individual pull-in winches are used at each mooring point.
Connection, and disconnection, of the mooring system involves the support of an anchor handling vessel. This is a complex process that may take a number of days depending on the weather and the configuration of the mooring system.
This is an expensive design as the structure and mooring system has to be very large to accommodate the loadings from all sides. Connection and disconnection will also be a costly exercise.
The Spintral system uses a submerged buoy mooring system that enables a weathervaning structure to be used.
When not in use, the submerged buoy would float at a position 20 – 30 m below the surface.
The floating structure would then be towed to the field and positioned above the buoy. The buoy would then be pulled upwards into the mooring turret by a single winch and then be mechanically locked into the turret. The electrical power transmission cables would then be connected.
In order to enable the high electrical power generated to be transferred from the wind turbine to the dynamic umbilical, a Continuity Connector is used in the mooring system. This avoids the requirement to use a slip ring and provides a continuous cable transmission system.
Connection and disconnection of the floating structure would be a very quick operation and not be very weather dependent.
The cost of the mooring system and the installation operations on a 10 MW system would be 15% of the expenditure, approximately £10.27m.
The use of a submerged buoy system should reduce the mooring system cost and also the number of structure pull-in winches required.
This system will also reduce the number of vessels required for connection / disconnection of the structure. The operation will also be much quicker and safer.
The main feature of this system is that the electrical power cable is protected within a steel riser pipe.
One of the main problems experienced by the offshore wind industry is failure of electrical cables and connectors during installation and operation.
There is a risk that the dynamic cable between the seabed and the floating structure could also suffer damage.
By installing the cable within the riser pipe then it will be protected at all times. The pipe will also be filled with inhibitor fluids for additional protection.
The riser pipe will be supported by mooring chains and buoyancy in a hybrid single line assembly. Hybrid Single Line Mooring Brochure
When not in use, the hybrid mooring line termination pullhead would float at a position 20 – 30 m below the surface.
The floating structure would then be towed to the field and positioned above the termination pullhead. The pullhead would then be pulled upwards into the mooring turret by a single winch and then be mechanically locked into the turret. The electrical power transmission cables would then be connected.
In order to enable the high electrical power generated to be transferred from the wind turbine to the dynamic umbilical, the power cable is allowed to spiral rotate within the steel riser. This avoids the requirement to use a slip ring and provides a continuous cable transmission system.
One of the main barriers to using weathervaning floating structures for offshore renewable energy applications has been the requirement to be able to continually transmit high MW electrical power from the rotating structure to the fixed dynamic umbilical.
The component normally used for rotary electrical transmission connections is a Slip Ring. This is a mechanical device that uses contacting brushes to transfer power from the rotating connection to the static connection. These components are available for small power capacities, however, have not been developed for multi MW operations. It may not be possible to develop a Slip Ring for this type of duty due to the power and heat involved.
In general, Slip Rings are expensive, unreliable and are difficult to maintain in the field.
The lack of a suitably rated rotary connector may be the reason that the majority of the floating device developers have opted to use an expensive multi moored structure rather than a weathervaning structure.
The Spintral system uses a novel design of continuity connector to continuously transfer the power. This uses a continuous cable/connector system which does not have any rotary connection.
This avoids the requirement to use a slip ring and provides a continuous cable transmission system.
The mooring system and dynamic umbilical are integrated into a termination head that is locked off in the turret and supported by a slew bearing.
The dynamic umbilical is then connected directly into a series of cable spools mounted in the turret. The cables on each spool are then routed over catenary management sheaves to the connection point for wind turbine.
As the structure rotates clockwise about the turret, the cable is spooled from the coils onto the cable management system and forms a deeper catenary.
As the structure rotates anti-clockwise about the turret, the cable is spooled onto the coils from the cable management system and forms a shallower catenary.
The cable can rotate +/- 30 rotations before it has to be mechanically reset back to the starting position.
The Continuity Connector is an enabling technology that will, for the first time, allow a weathervaning floating structure to be used for a high power floating renewable energy application.