Diving into the world of Floating Wind

There are four main elements of a floating wind unit

The floating foundation provides the buoyancy required for the structure to stay afloat and stability against the wind and waves. Fixed foundations support the mass of the turbine by being embedded in the seabed, which exerts a vertical upwards force that balances the downwards force the weight of the turbine has. Floating units provide this vertical force entirely through the buoyancy of the floating foundation.

The station-keeping system is the mooring lines and anchors. The main purpose of this system is to maintain the position of the floating foundation. The design of the mooring system also helps control motions and loads in the subsea cables. The anchors provide the attachment point for mooring lines to the seabed.

Electricity is delivered from floating turbines back to shore via subsea array and export cables. Unlike seabed-fixed offshore wind where the cables are fixed to the seabed to prevent movement, floating wind uses 'dynamic cables'. This means the cables are stiffer than normal to be able to withstand additional floating motions, but also have appropriate protection to control the movement and loads at joints. Array cables normally follow a catenary or lazy wave configuration.

In the short- to medium-term at least, floating projects will fundamentally use the same turbine technology as seabed-fixed projects. However, an important difference between seabed-fixed and floating wind is the level of interaction between the turbine and the foundation. In particular, the design of the floating foundation is defined by the motions that the turbine can safely withstand (namely inclinations – how far it can tilt, and accelerations - how fast it can move), with the turbine control system being modified to meet these conditions too.

Most of the floating foundation designs on the market can be classified into one of five generic 'types':

This design relies on the differential between its Centre of Gravity (CoG) and its Centre of Buoyancy (CoB) to provide stability. The structure will be configured such that the CoG is considerably below the CoB. This means that if the foundation is tilted, it will act as a pendulum returning to vertical. 

This design is similar to a spar buoy but, instead of being a single structure, it has a buoyant top section and suspended mass underneath so its CoG is beneath its CoB. 

This design gains its stability from its waterplane area (the area of the floating foundation within the waterline). As the structure tilts, some parts are pushed under water while others are lifted, resulting in a restoring moment and overall buoyancy.

This design is similar to a barge, as it gains stability from the waterplane area with three or more separated columns. 

This design relies on the differential and balance between the buoyant upward force exerted by the structure and the downward force exerted by the vertically taut mooring tendons. 

Station-keeping systems can be broadly categorised into four types, with the selection of the best one depending on the water depth of the site, the soil conditions and the design of the floating foundation.

This system is typically installed in water depths of less than 500m and uses chain and/or wire rope lines. A significant length of the mooring line lies on the seabed, when this is lifted off the seabed by floating foundation motions, the suspended weight of it pulls it back into place. This is an example of a restoring force, a force that acts to bring back a system to equilibrium i.e. its original resting state.

This system is typically used in water depths of more than 250m and uses lightweight mooring lines of synthetic or wire rope. With this system, none of the length lies on the seabed and the restoring force is created through the stretching and relaxing of the mooring lines. This configuration has a much smaller seabed 'footprint' (the amount of area a mooring line system takes up on the seabed) than the catenary system. 

This system is a hybrid of the catenary and taut systems, with a small proportion of the mooring line lying on the seabed. It can be used across all water depths and has a smaller seabed 'footprint' compared with the catenary system, but larger than taut system. 

This system is typically used in water depths of more than 120m and only with specific tension-leg platform (TLP) floating foundation designs. This configuration does not permit vertical movement in the floating foundation, and uses mooring tendons that are either steel wire/chain or synthetic fibre ropes. This configuration has the smallest seabed 'footprint'.

Most floating foundation concepts are designed with a mooring system that keeps the foundation in a fixed orientation. With these designs, the alignment of the turbine with the wind direction is achieved with a turbine's standard nacelle yawing system (the mechanism that rotates the turbine 360 degrees around the tower top), in the same way as seabed-fixed. Some floating concepts, however, use a single turret-moored solution in which the unit can 'weather-vane' (rotate) around the single mooring line attachment point. In this case, the alignment of the turbine with the wind direction is largely achieved by rotating the whole platform. The dynamic cable is bought through the centre of the turret to an electrical swivel, to allow uninterrupted connection.