Featured image for The Big Lift: why floating offshore wind could be more cost-effective than fixed-bottom
Thought Leadership / October 5, 2023

The Big Lift: why floating offshore wind could be more cost-effective than fixed-bottom

By Adrian Griffiths


Even after many years working in the engineering world, I still find something fascinating about large structures being craned into position. Watching a crane gently lift a 15MW nacelle that weighs around 800 tons and gently placing it on top of a 120 m tower is, for me at least, exhilarating—even when I am only watching the video!

Today there are some enormous land-based cranes. Who can fail to be impressed by Big Carl, the ring crane currently operating at Hinkley Point, supporting the build of the new nuclear power station there. But it is marine contractors that seem to have mastered the really big lifts, with jack-ups and floating cranes at a size and lift capacity that is truly awe-inspiring. They are incredible pieces of equipment and understandably come with a price tag to match. As floating offshore wind (FLOW) seeks to achieve a levelized cost of energy (LCOE) that competes with fixed-bottom wind installations, the cost of the big lift, placing the nacelles onto the towers, is an aspect where it may have some advantages.

When designing a lean manufacturing system, it is important to determine which piece of equipment will provide the drum beat for the entire system, the TAKT time, the process that will determine the overall output. Everything upstream and downstream of it should have a throughput capability greater than it. There should be a buffer before it, that should be full when everything is working, and one after it, that is normally empty. This arrangement means that when problems occur elsewhere in the factory, the rate-limiting equipment can keep using material from its upstream buffer, putting its output into the downstream buffer. When the issue is resolved, all other systems can “sprint” to return the buffers to their normal levels. If the rate-limiting asset is stopped, that time is lost forever, and profits are eroded. Financially, it usually makes sense that this rate-limiting piece of production equipment is the most expensive asset in the system, and it makes sense that the offshore wind industry views the crane as the pace setter of the manufacturing system.

In the case of fixed bottom wind turbines, there is no option to having this expensive asset, the crane, spending most of its time moving from one point to another, setting up, packing up, waiting for weather windows, etc., rather than adding value and lifting a nacelle. This is where FLOW has a distinct advantage. Firstly, the ring crane, while still costly, is less expensive than when it is an integral part of a vessel. Then, the floating platforms and WTGs, which can be made in multiple locations, can be lined up ahead of the crane, the upstream buffer, and the lift conducted in the relative calm of a port, making sure that the crane is always adding value, lifting nacelles, and as efficiently as possible. The completed units from the “integration port” can be towed to the wind farm using much smaller, cheaper, and more available tugs.

Putting these generators into service is a major feat of engineering. Maintaining them afterward is also a big challenge. Once again, FLOW could offer an advantage over fixed bottom. In the case of the latter, if a WTG experiences a major parts failure, an available sea crane must be located and sent to the unit in the next suitable weather window. Given the cost of these vessels, there are not many of them in the world, so aligning the weather and the vessel to affect the repair could mean that the generator is unproductive for quite some time. By contrast, deploying a tug to bring the whole WTG and floater unit back to port where the ring crane is should mean the generator is back up and running much more quickly and cost-effectively. There are, of course, several significant provisors to this assertion. Disconnecting the unit from the mooring system and the inter-array cables must be slick. The unit must be stable when being towed and at a reasonable speed, so even short weather windows can be used. After the repair is completed, hooking it back up in the field must also be quick and without the need for costly specialist assets.

Gazelle’s innovative platform is a good example of this. Its lightweight, buoyant design facilitates stable towing, even up to Beaufort 7   wave and wind conditions. The platform can be connected to the pre-laid mooring system in a few hours without the need for ROVs. It may even be possible to disconnect and re-hook up using just one tug.

Innovations like this point to the LCOE advantages of FLOW compared to Fixed, keeping CAPEX and OPEX costs low and generators running more of the time.

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