SEVENSEAS Marine Conservation & Travel Issue 22, March 2017 - Page 101

There are three general designs for floating offshore wind: the spar-buoy, tension-legged platform (TLP), and barge or semi-submersible type. There are other designs out there, but they exist mostly on bar napkins. Before we go into detail, take a look at the below image and make sure you understand the degrees of freedom for a floating offshore wind turbine.

A spar buoy looks like a submerged telephone pole. This design uses lots of ballast (deadweight, more on this later) hung low on a single long cylindrical tank (in one pilot project this tank was over 100 meters long). With the weight hung so low, the combined system has a very low of center of gravity making it quite stable and resistant to pitch and roll motion. Its deep draft (distance below the waterline) also helps to resist heave motion. It is connected to the seafloor with slack mooring lines.

A TLP relies on mooring line tension to provide stability. Taught mooring cables are strung from the seafloor to a submerged, buoyant platform upon which rests the turbine assembly. The tensioned cables absorb the loads generated by the turbine and ocean forces.

A barge design uses multiple, inter-connected, floating tanks to form a wide and relatively shallow draft support structure. Ballast is distributed among the different tanks as needed in different conditions to provide the necessary stability (more on this later). It is also connected to the seafloor using slack mooring lines, but using fewer than the spar-buoy design.

So what’s actually out there producing power? The spar buoy and barge type have seen the most advancement. One example is the Hywind Demo project, deployed by the oil and gas firm Statoil in 2009. It is a spar design in the North Sea in waters 220 meters deep and supporting a 2.3 MW turbine. At $71 million dollars, almost seven times the cost of a traditional offshore wind turbine installation, it was not a cheap project; however, this kind of price tag is common for one-off designs.

WindFloat is a patented barge type design built by Principle Power. In 2011 they deployed their first 2 MW WindFloat1 pilot project off the coast of Portugal. The WindFloat1 just finished up five years of successful testing last year, experiencing waves up to 17 meters high and winds in excess of 60 knots. This project came in at an approximate cost of $20 million, but expected costs for future projects will be significantly less.

An upside down wind turbine isn’t very useful to anyone, so stability control is critical. To keep the platform and turbine from capsizing there are a number of controllers on board. Platform trim, rotor speed, platform pitch, blade pitch, and platform yaw angle are just some of the variables that are monitored and controlled. What makes this tricky is that almost all of these controllers directly or indirectly affect one another, creating quite a bit of feedback.

Static stability refers to how the platform sits in the water with no external forces acting on it. I’ll spare you the naval architecture lesson, but suffice it to say that a lower center of gravity generally equates to better stability. Unfortunately the wind turbine assembly is essentially a giant cantilevered beam sticking-up high into the air. For reference, the combined weight of the turbine and nacelle (called the towerhead mass) for a 6 MW system weighs 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 controlled. It’s really just a fancy system of counterweights. These control systems add complexity and cost, but they are needed, unless you think sinking offshore wind has a bright future ahead of it.

The future for floating offshore wind looks bright. Hywind is set to begin construction on the world’s first floating offshore wind farm in the UK later this year. It will be a 30 MW farm located in water depths of approximately 110 meters. Predictably, the estimated cost for the farm (on a per MW basis) has dropped nearly 70% from their pilot project that was installed in 2009.

Principle Power is also staying busy. They have projects to develop 24 MW wind farms in Portugal and France, and a smaller demonstration project in Japan. Most impressive is a massive 765 MW wind farm planned for the coast of California in 2022. Organized by Trident Winds, this project will be using the Hywind and WindFloat designs.

Floating offshore wind allows developers to take 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.

Wind turbine supports and foundations by depth. Source: BOEM

March 2017 - Technology