As the world rushes to meet looming deadlines for the transition to renewable energy, we will see fundamental changes in offshore construction. In this series of articles, Engineering Matters considers some of the challenges faced, and some innovative physical and digital solutions. In this first of a series of articles looking at energy transition over the coming weeks, we explore the scale of the problem with Peter Richards, chief engineer at Fugro.

The offshore energy sector is seeing a pace of change unknown since the discovery of North Sea gas in the 1960s, or the oil crisis of the 1970s. The discoveries in the North Sea led to the rapid development of offshore fossil fuels  extraction. But, even at its peak, this saw only scores of rigs operating at once.

As the world moves toward renewables, these numbers will be dwarfed by the number of offshore wind turbines. Where the fossil fuel sector required hundreds of rigs, the renewables sector will require thousands of turbines. Each of these must be secured: either with foundations for individual turbines, or with anchors, for floating arrays.

In 2021, there was 14.6GW of installed capacity in the European Union, according to the European Commission. By 2030, that is set to increase by at least 25 times.

But where will all of these new wind turbines—and other renewables, like wave generators—go? We have already picked a great deal of the low hanging fruit. Where turbines can be built with relative ease, they have been built, or soon will be.New developments will often be further offshore, in deeper water, in an area without consistent seabed conditions.

“Some of the earliest turbines were built on breakwaters, close to ports, harbours and existing infrastructure,” says Peter Richards, chief engineer with Fugo. “All the early sites have really been on the continental shelf, at depths of 20 or 20 metres. Today, we’re up to 70 metres of water depth, and we’re now looking at much deeper locations for floating wind.”

Over the next ten years, we will see a fundamental shift in how power is generated. Manufacturers are building wind turbines that push the limits of fabrication yards, of docks, and of lift vessels. Developers are scouring the planet for locations that can support installations of perhaps hundreds of multi-megawatt turbines. 

Wind turbines, whether on land or on sea, have pushed every limit of the engineering sector for the last twenty years. Not long ago, a big wind turbine on land would have a capacity in the hundreds of kilowatts. A big, 2.5 or 3 megawatt turbine could only be built offshore

Today, onshore turbines regularly push past the 3MW mark, and much larger turbines are being built. They stand 100 metres tall. Each blade can be as much as 80 metres long. And nacelles—the bit with the rotor, where the blades attach, and the generating components—can weigh into the hundreds of tonnes.

Pushing the limits

But these onshore turbines are tiny, compared to their offshore counterparts. These can have double digit megawatt capacities. Whole ports are dedicated to their handling. Concrete ‘gravity bases’ are constructed, in parallel, dozens at a time, using fleets of transporters and rail-mounted tower cranes. And they must be placed, in vast installations, around the world’s coasts.

They must sit on stable foundations, or be anchored securely to the seabed. And, as Richards explains, this has become increasingly challenging. While the leap in turbine capacities has reduced the overall number of turbines that must be installed, it has seen them soar in size.

“Twenty years ago, when we were installing two megawatt turbines, there would have been hundreds of thousands of turbines required [to meet current renewable energy targets],” says Richards. 

However, manufacturers are now building turbines five or six times bigger than before. This will cut the number of turbines required, but make their installation on secure foundations much harder.

“The power of a turbine is the square of the diameter of the blades. And that means that you have to install the turbines higher up. And by maintaining them higher up offshore, you get cleaner air, meaning that you don’t get all the disturbances from the interaction of the airflow. 

“It’s less to do with a number of locations and more to do with bigger turbines. And we’re already talking about, by the end of the decade, turbines in the order of 10 or 20 megawatts.”

Each of these turbines will weigh hundreds of tonnes. They must be held perfectly upright, a hundred metres or more above the waves. To achieve this, they require a firm foundation.

“Offshore, there are three principal foundation types,” explains Richards. “Gravity foundations are basically big concrete weights that support the tower structure. And in the early days, those were used in places like the Baltics: they’re very shallow sheltered locations, where you need a certain amount of ground preparation, but there’s no piling involved. 

“Jacket foundations are similar to what you see in oil and gas installations: big lattice structures that are piled at the four corners. 

“But the most prolific and common type of foundation for wind farms over the last twenty years has been monopiles. These are essentially steel tubes that are driven into the seabed. Initially, those were in the order of four metres diameter. Today, we’re currently at eight, nine, ten metres diameter and by the end of the decade, the prediction is that these tubes will be 12 metres and bigger in diameter.”

These monopiles can each weigh over 3,000 tonnes, before the tower, nacelle, and blades are attached. That’s not an easy cargo to transport, or load to lift.

“There are two [logistical and installation] requirements,” says Richards, “There are the requirements for the foundations, that are quite specific in terms of the size of these monopiles and the difficulty in terms of racking them horizontally and then lifting them into the vertical and placing them on the seabed.

“And then there are the requirements for the visible part of the wind turbine, the tower sections, the nacelle, and the blades. Those components are much lighter.

“What you tend to see is the heavy lift vessels—which are used predominantly for installing the foundations, whether they be jackets or monopiles—have lifting capacities in the order of thousands and thousands of tonnes. 

“The turbine installation vessels have more modest lifting capacities, of maybe a few hundred tonnes for tower sections, blades and nacelles. What has increased is the nacelle height above mean sea level. So the industry has had to build vessels with much bigger crane jib lengths on the crane so that they can lift these key components much higher now to erect the turbines.”

Those vessels are in short supply, as are the expert surveying and offshore construction crews needed to support their work. Those challenges, and the growing size of turbines, will require new tools. In Part 2 of this series, we will look at one such tool, which quite literally takes a tunnel boring machine, and turns it upon its cutting head.