Author: Bernadette Ballantyne

Partner: Fugro

The offshore wind industry is pushing to meet an enormous 40GW target over the next decade to build new wind turbines around the UK in some of the most difficult construction sites in the world. At the same time the industry is being challenged by government to continue to bring down costs and accelerate construction.

Waters where the new turbines will live are getting further offshore making them more vulnerable to ocean weather systems. The ground conditions are more complicated, construction is more difficult and the turbines are six times larger than they were when the industry first began in 2000. 

The implications of this are massive. And so are the foundations which hold the turbines securely in the seabed. As towers and blades get ever larger the invisible infrastructure that anchors them into the multi-layered geology of the UK’s coastal waters also has to evolve. 

“Up until now throughout the world, all the easy sites have been developed, so that’s all the shallow water nearshore sites,” says Peter Richards, chief engineer for Fugro. Peter is based at Falmouth in Cornwall, where a team of over 150 drilling specialists are working on a new system to transform the way that the wind turbine foundation are delivered.

Transforming foundation construction

Going back to the early days of some of the demonstration projects in the Baltic, monopiles were the typical wind farm foundation. “Back in those days, they will be sort of two or three metres diameter. And then over the years they’ve grown in size and we’re now currently up to installing piles that are 8-9-10 metres in diameter,” says Peter.

Monopiles are exactly as they sound. Single tubes of steel that run deep into the ground, hammered into the seabed with the wind turbine constructed on top. The problem lies in hammering the tube into the ground. There is a limitation on the soil resistance meaning that the pile and the ground can’t take the force of the hammer. This means piles have to be drilled instead of hammered or driven. “Unfortunately, conversely drilling is relatively slow,” says Peter. “And it means that you are increasingly more exposed to those changes in weather, whether they be wind or wave. And that means your installation programme for foundations is very vulnerable. So, the joint objective for drilling offshore is to minimise the weather impact and improve your productivity ultimately, so that you become competitive with driving foundations.”

For the team of experienced marine contractors in Falmouth there were two objectives for the new system; improving productivity and minimising the impact of the weather. “So about two and a half years ago, we were looking at some of the French wind farms with very, very large diameter rock sockets and we and we realised that if we’re only drilling 20cm an hour that the cycle time per foundation was going to be measured in weeks rather than in days. So we knew at that point that we’re there needed to be a significant improvement and drilling rate on these, these larger monopiles.”

To date foundation construction has used a top drive approach where all the energy, whether it’s rotation or impact is supplied from the top of the drill string. And then the drill string is used to transmit that energy to the to the cutting device. “What’s unique about offshore drill strings is that you have to deploy them through the water column,” says Peter. “So unlike land drilling, where you’re in effect drilling a socket into the rock, the drill string is unsupported. Because the drill string is unsupported it means you have to keep it in tension.” 

This is what is unique and limiting about offshore drilling. “It means you have to have what’s referred to as big pendulum, a very, very, very heavy drill string, where you’re holding back that the drill and then just letting a little bit of weight go onto the cutters to do that to do the work.”

And for harder ground this means that heavier and harder cutters, have to be used. “Because this is all done through deadweight cantillege, you get to very, very large heavy drill strings there are different difficult to deploy and operate.”

More efficient drilling

The heavier bits and strings then have to be rotated slowly to maintain stability and this further limits excavation rates. Instead Peter and his team want to bring the more efficient bottom drive process into offshore wind industry, which he says is typical of horizontal drilling where tunnel boring machines place hydraulic jacks behind the face cutter.

“The lessons are already there with tunnel boring machines. Most importantly, the whole tunnel boring machine isn’t rotating. It’s only the faceplate of the tunnel boring machine that’s rotating,” he says.

One of the benefits of that, apart from the fact there’s a lot less friction within the system, is that although the machine may weigh several hundred tonnes, only 30-40 tonnes of that machine is rotating. “So we’re able basically to rotate that face plate at much, much higher speeds with a TBM approach, and this is where the, the terminology VBM comes from vertical boring machine,” says Peter.

The first iteration of Fugro’s new drilling technology for offshore wind is a 3m diameter drill called the VBM3000. “This year, we conducted some on land benchmark testing of old technology versus that the new, at a smaller sort of prototype sort of level and we have been able to achieve in excess of two to three times faster drilling rates in these sort of typical ground condition. And I would say in the softer conditions as much as five or six times faster,” says Peter.

This involved test drilling into concrete which was manufactured at a range of hardnesses to emulate the variety of ground conditions that the drill is going to face. Detailed design began in March 2019 and the first drill is destined for a site in Scotland where seabed conditions will be variable and complex. Each turbine will have a tripod foundation structure consisting of three piles. The engineering, procurement, construction and installation contractor for the foundations is Saipem and they face a challenging deadline – to complete all 54 foundation structures before the end of 2021 with half a degree of verticality and a 50mm tolerance.

Integrated template 

An experienced oil and gas contractor, Saipem is moving into offshore wind and sought out a  drilling sub-contractor that already had experience. “My role initially was to look at various contractors see what they had to offer and recommend which contractor would be best to work with us on a project,” says Chris Armstrong, drilling and template lead for Saipem. “This drill that we’re going with is a new drill, but it’s a combination of existing technology that has been proven. The drill itself looks to be very advantageous for us,” he says.

One of the main attractions for Saipem is that the new drill is able to provide an integrated solution that includes the template structure as well. A template is a 1000t six legged steel structure that sits on the sea bed and acts as a framework for the placement of the piles. Three of its legs contains a circular space for a pile to be drilled through into the ground.

“The functional template is very important to make sure that we can install the drill in the  right location and make sure that the drive can be driven from the templates itself,” says Frank Wong, transportation and installation manager for Saipem. “So it’s very important that interface is the right interface. If somebody designed a template separate to a drill, we end up with a solution that is unlikely to match.” 

Another advantage is that by sitting on the seabed, the template and the drill will not be vulnerable to the weather issues that a top drive vessel mounted system would have to contend with. Placement of the template will come from Saipem’s enormous vessel, the Saipem 7000. “The Saipem 7000 is a semi-submersible crane vessel, it has two 7000 tonne capacity cranes on it, which is typically been used in the oil and gas industry for installing jackets and large top sides,” explains Chris. “It has large deck space which is very advantageous for this project. The width of the of the vessel is about close to 100m and the length is for two and a half times that, 300 metres maybe. It’s even with such a large vessel where we’re finding the deck space tight, but most of the vessels have nothing like that deck space. So we’re lucky to have it.”

Being able to drop the template and drill down onto the seabed and then use the vessel for other activities is more efficient than traditional top drive drilling where the vessel would host the drill. “One of the one of the advantages of the drill is it’s effectively apart from the umbilical it’s completely detached from the vessel itself. So it will be deployed from the vessel. But once it is subsea the only connection back to the vessel is an umbilical, which allows us to carry on with other work on deck at the same time.”

Lifting the 1000t template down onto the seabed won’t challenge the 7000t capacity cranes. What is more challenging is the complexity and variety of the seabed which means that a range of foundation depths are required from 25m down to 47m.

Scaling up the drill

The team is so confident in the effectiveness of the new drilling technology, they are already planning a large-scale version of this drill that will be able to bore the sockets for piles that are over 9m diameter anywhere in the world from Taiwan to the US and France. 

“Certainly we’re seeing huge growth in the Americas and if you look at the Asia Pacific region, we’re seeing developments in Taiwan and Japan. There’s also Korea and Vietnam looking to progress developments,” says Julia Roope, Fugro’s global business development manager for offshore wind. It is her job to keep track of the growing industry. 

Each of these new markets are busily creating roadmaps to assess where their resources are greatest and how to get this power to the demand centres. Back in the UK we have a different kind of challenge around scaling up to meet newly generation targets.

“We’ve really seen the offshore wind market develop hugely over the past 10 to 20 years.  it’s grown hugely. It’s gone through a commercialization phase. It’s now fully industrialised and it’s become a core energy generating technology.” 

And then last year the government announced the UK Offshore Wind Sector deal which committed the UK to 30GW of wind energy by 2030. This was then increased to 40GW a few months later. To achieve 40GW by 2030 the UK needs to more than double the number of turbines offshore, even though they are producing 6 times more capacity than the original 2MW turbines that kickstarted the industry in 2000. And to ensure that the government maintains a supportive policy environment, the industry has to keep innovating to drive down costs. 

The first step for developers then is to choose the right site and then learn as much as possible about it. Data is everything such as geotechnical data about the seabed where the turbines are to be constructed. This is done by taking borehole samples from the sites and sending them to the lab for analysis.

That lab is run by Alana Horton. Sited at Wallingford in Oxfordshire the laboratory has a throughput of approx. 13,000 geotechnical lab tests per year, 4,000 of which are advanced soil tests.  Alana says that this enables them to carry out more extensive testing than anywhere else in the world. “We do testing right from the most basic classification testing, so kind of your particle size distribution, particle density, that kind of thing. And right through your whole suite of triaxials, odometers, CRS tests.”

CRS stands for constant rate of strain testing and  measures the compression response of soil. “So we do cyclic testing, cyclic triaxials, cyclic simple shear. And we also do resonant columns, which is probably one of the most advanced tests that we do. We’re also involved in quite a lot of research collaboration projects with our clients, where we’re developing new ways of testing things.”

So what do all these tests do? Put simply they are designed to explore some aspects of the soil response which may affect the turbine.

The most simple characterisation tests are to measure particle size distribution and particle densities but it quickly becomes more complex with things like automated odometer testing which repeatedly compresses the soil sample to assess its deformation response.  “Basically we emulate the stresses that that soil has been through before. And that would help to know, help our clients to know what stress that soil can be exerted to in the future,” says Alana.

And then there is another form of shear testing called triaxial testing, with overall capacity increasing significantly to cope with increasing demand from offshore wind. “Something that is really important with offshore wind is the cyclic testing that we do. So basically, we do cyclic triaxial testing and we also do cyclic simple shear tests. And so with both of those, we take the samples to large strain, catastrophic failure.

One of the more sophisticated tests are the resonant column machines which vibrates the sample which then….“allows us to create a degradation curve, which is critical for components of design, mainly within pile design, which is really important for the monopiles for wind turbines.”

Where the degradation curve refers to the stiffness of the soil. 

As the offshore wind industry grows Alana is definitely feeling the impact,“Just because the sites are so large, and they require so much testing to be able to get them through. So we are trying to implement as much as we can to try and capture that upturn in the markets and keep up with that increase. So we have, even just in the last year we’ve actually been investing in more equipment. So we have more simple shear machines, more triaxials, more resonant columns, more ring shears, shear boxes, we invested in the automatic odometers. And there’s going to be a lot more investment over the next years. Definitely.”

Better modelling

Getting extensive and detailed data about wind farm sites is not just important for the placement and performance of the physical infrastructure. It is also helping geotechnical engineers to create better predictive models for designing foundations. Offshore wind foundations can account for as much as 30 per cent of the capital cost of a wind farm, so getting the design right is critical. Designing a foundation for an offshore structure requires an interaction between the geotechnical engineers who are concerned with the soil andthe structural engineers who of course design primarily the structure and the foundation. “As geotechnical engineers, we have to give them enough information to go ahead with the design. That’s typically done in that we give a soil a spring, so forced displacement spring along the pile or the foundation,” says Scott Whyte, geotechnical engineer at Fugro.

And the analysis performed to calculate these soil reaction springs, that are used by the structural engineers for the design, use the soil testing that Alana described earlier.

“So essentially, we’re trying to give them some reaction curves or springs that capture the response of the soil,” says Scott.

But the response of the soil and hence the soil reaction curves isn’t always the same throughout the life of the turbine because wind turbines experience cyclic loading, so they’re exposed to repeat loading.  These springs can stiffen, or they can degrade in stiffness. So they can get softer, they can get stronger, and this is why the models are developed – to accurately capture the soil behaviour and the better they are, the more accurately they can represent the true behaviour of the soil when performing foundation design analysis.

“So the finite element analysis technique and has been used for decades by engineers to predict basically the forced displacement of structures,” says Scott. “And when you do that analysis, which is what we are doing for offshore wind turbine foundations, the thing that is very, very important is something called the constituent model, which is just the bit of the code that predicts the stress/strain in response to the deformation of materials and in this case, its soil but it but it could be any other material, but in this case, its soils…….And what we’ve developed is a suite of models based on lots and lots of laboratory data, data from lots of soils across the North Sea, to better capture the response of certain features of soil behaviour that will drive the response of these of this analysis essentially.”

In essence the models have been developed to better predict the site-specific soil response with the aim to make design analysis more accurate, and hence the overall foundation design more cost effective by relying less on empirical design methods. The results are already making savings. First of all the team used traditional techniques to design a monopile for an offshore turbine. This led to a 7.5m monopile that was 40m long. They then redesigned it with the new modelling approach with better performance analysis and found these could actually be reduced to 30m.“If you scale that across a wind farm with 100, turbines it is clearly a huge saving,” says Scott.

But Scott says that the most important thing is that the models are getting closer to reality and reflect a trend in the industry towards continuous improvement through collaboration.

And this continuous improvement in the industry from its ever increasing turbines to the improved data capture and modelling, is what has brought UK offshore wind prices down to an all time low, seen The Netherlands embark upon the first subsidy free farm and led to offshore wind markets emerging all over the world. 

The good news for both emerging and mature markets is that 20 years of experience in the UK and Europe is being used to continually improve site design and construction methods  – from new modelling tools to state of the art drills. And as Julia said, if countries like the UK are serious about achieving carbon neutrality, then rapid expansion of offshore wind is vital.