close to 350 tons.
To lower the center of gravity of the combined system lots of weight is required down low. This is accomplished through ballasting. Anything that is cheap, heavy, and plentiful will work as ballast: seawater, concrete, even Bud Light.
A platform that constantly bounces up and down is a great Moon Bounce, but isn’t super useful for supporting a wind turbine assembly. Heave, surge, pitching, and yawing, or more generally the dynamic stability, needs to be controlled. Each design has its own unique way of accomplishing this.
To reduce the heave motion on a barge design, a simple device called a heave plate is used. Heave plates essentially increase the drag of the system. When an object accelerates through the water, it moves the water that directly surrounds it as well. Heave plates are specially designed to move lots of water in a rather turbulent way.
When the water flows around the plate, vortices are created that dissipate energy. The inertia of the large amount of turbulent water is called added mass. The added mass slows the response to waves and results in a positive damping motion on the system, reducing its bounciness.
What about controlling the other motions? Let’s consider a situation in which a strong wind acts on a barge type system. The force of the wind will rock the turbine and platform in the direction of the wind; this is called pitching. The platform and turbine are now leaning with the wind, reducing performance and stability.
The turbine and platform need to rock in the opposite direction, back into the wind. One way of getting the turbine back to it’s intended upright position is to use the thrust of the turbine blades themselves, just like a prop plane. The blades can be angled relative to the wind using a blade pitch controller. In high winds the blade pitch angle is decreased which increases the thrust of the blades and causes the turbine to accelerate into the wind.
As the turbine accelerates into the wind a different controller kicks-in. Because the turbine is now experiencing an increased apparent wind as it moves forward, the rotor speed increases. So as not to destroy the generator, the rotor speed controller sends a command signal to slow down. The blade pitch angle is adjusted once more, reducing thrust, and the rig rocks back with the wind.
As the wind speed fluctuates one can imagine this constant back and forth motion going on indefinitely like a metronome. In some instances these controllers may actually lead to a condition called negative damping. When this happens the controllers are actually overcorrecting, causing the pitching motion to be amplified, creating an unstable condition for the platform.
To reduce the potential for negative damping a fine-tuning ballast controller is used. By moving weight among different tanks strategically located around the platform, its level can be
advantage of winds in areas that were once off limits due to water depth. Although currently more expensive than traditional offshore wind, with a little innovation this cost difference will narrow. Don’t be surprised to see more of these projects in our offshore waters in the very near future.
WindFloat. Source: Principle Power
March 2017 - Technology
SEVENSEAS - 103