Imagine an offshore wind farm, with scores of turbines lined up in uniform across a stretch of horizon. Erecting just one of those turbines in the hostile offshore environment is undoubtedly a great feat of engineering. But establishing an entire wind farm isn’t just a case of repeating that process multiple times – it can pose a challenge on a whole new scale. 
One reason for this can be traced back millennia. At the Codling Wind Farm site, located in the Irish Sea, the sea bed was ravaged during the last Ice Age. Great trenches were cut through the bedrock, which then filled with softer materials. Those variations in the sub-sea landscape necessitate very careful planning and analysis before any turbines can be installed.
“The Codling site is a fairly complicated site in that we have different ground conditions,” says Ed Sly, engineering manager at Codling. “We have a number of channels that ran through the site historically, which were formed in the last Ice Age. These have now been in-filled with softer materials. So we have a mixture of sands and gray clays and silt throughout the site. And what will happen is we will end up with a need for different designs at different locations within the site.”
And wind farms need to be fully prepared before construction starts, as any delays can cause major problems.
The wind energy sector is growing at gale force, with the 117 gigawatts of capacity installed in 2023 more than double that of the previous year. That total is forecast to grow to more than 180 gigawatts by 2028, with offshore accounting for around one-fifth.
As a result, speed is key, and the necessary equipment is constantly in demand. Wind farms also represent big investments, so need to be running profitably as soon as possible. And of course, there’s always a level of danger working offshore – so completing the project more quickly reduces the risk faced by the team.
“Ultimately, with any offshore campaign, we want to do it as efficiently as possible, and obviously we’re having to encounter things like weather, so whether downtime is a serious consideration,” says Sly. “So anywhere we can make the process more efficient, that’s beneficial.”
But that’s easier said than done – particularly in the Irish Sea, notorious for strong currents churning below the surface. Taking on the challenge was geo-data specialist Fugro, which deployed a range of cutting-edge technologies.

Mapping the sea bed

Codling is a 50:50 venture between Fred Olsen Seawind and EDF Renewables. It’s set to be Ireland’s biggest offshore wind farm, projected to produce 1,300 megawatts of power to homes in Dublin and beyond. Despite the challenges of the variations of the sea bed, the site was chosen due to its proximity to the Irish capital.
To map out those variations, Codling contracted Fugro to do a site investigation campaign, comprising bore hole sampling and cone penetration tests (CPTs).
“This is just one part of our site investigation approach,” says Sly. “Initially, we were informed by geophysical survey, which identifies things like the bathymetry and the subsurface make-up of the ground conditions, and then we further test and proof this using these intrusive methods such as CPTs and boreholes.”
Using this data combined with around two decades of site investigation, a map was created not just of the seabed, but tens of metres of the land below. It’s not possible to analyse the entire area, of course, so data modelling is used to fill in the gaps.

Battling the elements

Fugro has a range of technologies to ensure the testing is completed as quickly and comprehensively as possible, even in the face of extremely challenging conditions.
“Engineering for the offshore environment is one of the trickier places to try and produce a piece of kit, because you’ve got the salinity, and you’ve got the corrosion,” says Ross Frazer, principal analysis engineer at Fugro. “Anything that moves will generally start moving at the beginning of the contract, but you want it to be moving at the end of the contract.
“Lots of corrosion-resistant materials are used. For anything that moves, we will generally use nylon-based, or oil-impregnated nylon, as a material to run items on. That keeps them sliding without any corrosion-causing issues. For the [hydraulic] rams, we’ll use stainless steel rods so we don’t get corrosion and get them binding up all of our pins. In anything that moves we’ll go in with a non-metallic bush between the pin and the steel material. Again, that lets us get it in at the beginning of the job, and, more importantly, take it apart at the end of the job.
“The paint specification is quite important to keep things going. The moving aspect is the more difficult piece to keep working. So, we do select our components very carefully for our anything that goes subsea. We tend to use open section for our structures so that we don’t have trapped air so nothing will implode. It’s something we give a lot of thought to.”

Innovation at sea

For companies like Fugro, there’s a balance to be struck between optimising data gathering while taking the conditions into consideration. One neat solution that tackles both requirements is Fugro’s sliding deck, which was attached to its Excalibur jack-up platform for the Codling project.
It allows the whole drilling set-up to be moved by up to eight metres, to give access to new sampling sites without having to perform a ‘bump over’ – the labour-intensive process of moving the whole platform.
Melanie Zacheis, geotechnical team leader at Fugro, explains the benefits: “Having the cantilever deck, which allows us to slide approximately eight metres, means that if we for some reason are unable to move locations due to poor weather, we can slide the deck across, and that will allow us to do another borehole or to gain more information when we would have lost that time period otherwise. If we meet an obstruction in the borehole, usually a cobble or boulder, that we can’t get past, or it damages some testing instruments to the point where it then blocks the hole, It also means we don’t have to move the jack-up to do a retest. Again, we can just slide that deck across to carry on testing as a bump over location.”
The cantilever deck was developed by Fugro for an earlier contract at the Codling site and was originally used on a different jack-up. When it was moved to Excalibur, the technology was finessed further, creating the opportunity for the sliding mechanism to be developed.
“We wanted to raise it above the deck level so that whenever we towed the jack-up, we didn’t get waves hitting the cantilever,” says Frazer. “That was one of our concerns, because if the wave hits the cantilever, then you’ve got to reduce your towing conditions. If you reduce your towing conditions, that means you’ve got smaller weather windows to get to site, which is a delay to the contract. So we raised it up above the deck to help us with that, keep our operability up. And then when we put it on Excalibur, we had the benefit that it that we could put it on a moving system that gave us eight metres of sliding to the stern.”

Underwater protection

Equipment needs protection below the water as well as above, and to that end, Fugro developed its riser casing, which covers its drilling and testing kit. This was particularly important during the Codling site project, due to the strong currents in the Irish Sea.
“We needed to design a very robust riser casing system that basically runs from the jack-up platform down to the seabed, within which you run all our clever geotechnical investigation casings, drill strings and tooling,” says Matthew Chappell, regional service line director nearshore at Fugro.
And it’s not just a case of making the casing robust – as with all the equipment used in subsea analysis, a high level of sophistication is required.
“The 20-inch casing is designed to resist significant wave heights and the high currents there, including the effects of vortex-induced vibrations,” says Frazer. “So if you have a long, flexible pipe and high current area, what can happen is the pipe will shed vortices on one side of it, then it will shed vortices on the other side of it, and the whole casing vibrates. As it vibrates, it can get up to about three times the current load that it was expecting. The current can also cause massive fatigue damage, so the conductor would fail with cracks propagating throughout it.
“We fitted vortex-shedding strakes on the on the casing, that prevent the vortex-induced vibration at the expense of requiring a little bit more strength on the actual casing, but it means it can sit there in all the currents that we were expecting to experience. It can sit there in quite large wave conditions, which then keeps the uptime that protects the inner pipes for the duration of the hole, and we can just carry on drilling regardless of the conditions outside of the outside of the casing.”

The next step

Once the data has been collected, it has to be processed. For the Codling project, Fugro is overseeing the analysis.
“For this particular project, Codling has got an external party that is doing the engineering and design, so we’re doing everything up to the laboratory report,” says Zacheis. “So we’re reporting to the lab. We’ll do our assessment to make sure that we’re confident the laboratory results that we are giving them are representative of the samples tested so that we can give them a sense of confidence in the results that will then go on to their designers, in this case, to then make that decision.”
The reports don’t necessarily give a ‘yes/no’ conclusion – more helpfully, they give the information required to work out how to tackle conditions at a given point.
“Sometimes it could conclude that a site is just not suitable,” says Zacheis. “But with other sites, and this is more likely with Codling, it’s: ‘OK, so how do we get these turbines in?’ And each turbine might need a slightly different design. So for instance, the turbines that are going through the gravel may need a different design to the turbines that are going through the slightly softer material.”
One takeaway from Fugro’s analysis at the Codling site was the amount of gravel, which was greater than had been anticipated. 
“Our main concern before going out for the site investigation at the Codling wind farm was actually gravel beds,” says Zacheis. “So gravel can be very difficult to collect, and also a risk to our tools going down.
“And it proved there was a lot more gravel there than the clients were expecting. So while it didn’t really cause too many issues from a site investigation perspective, from an engineering perspective for the actual wind turbines, it is creating some concern for them because the gravel beds were a lot thicker than they expected.
“It has changed Codling’s ground model of the area and one of their clay units has been split into two. Because they have found that softer area which behaves differently to the slightly harder area. So again, it’s changed the ground model, and therefore their planning and engineering going forward. That meant they had to schedule extra laboratory testing to really be able to characterise that softer area that isn’t throughout the whole site. It’s only at two or three locations that they’ve found this softer unit.
“For me as a geotechnical engineer and geologist, that’s the bit I found the most interesting. It’s really nice to see in areas that you think are very well understood, that we’re still learning and we’re still updating ground models based on our site investigations. Which just goes to reinforce how important site investigations are.”
There’s more work to be done at Codling: such is the complexity of construction at an offshore wind farm, further analysis will be needed.
“Certainly we need more SI at the moment, we’ve covered a reasonably small percentage of the total number of final WTG locations, so we will be going back offshore and conducting a further site investigation campaign,” says Sly. “We anticipate that we will have another campaign that will involve CPTs this time only. We’re happy that we’ve acquired a lot of borehole data, and that’s given us the samples that we need to use.”
The project is also currently applying for planning permission. Should that be successful then the Codling wind farm, armed with huge amounts of data on the landscape beneath the waves, will be ready to provide around a million Irish homes with renewable energy – making the Emerald Isle even greener.