Geotechnical site investigations
In January this year Fugro started the geotechnical site investigations. These covered the area where the 2.5km by 2.5km island will be created as well as the adjacent 1,059km² offshore wind farm zone.
Fugro geotechnical project manager Kasper Speth says that the geotechnical investigations sought to answer questions that the surveys were unable to. These include: “How stiff [is the soil]?” and: “Will we run into any soft clays?” Across the wind farm zone Fugro’s investigations were more spread out while it carried out a much denser grid of CPTs across the smaller island site, notes Speth.
He adds: “The boreholes also had different requirements than the standard boreholes that you would do for the wind farm investigation, because at this point, it’s still not known what type of island they will build.”
The island could be a jacked structure, or it could be reclaimed from the sea using sand.
“For each of these structures, you need different types of information about the subsurface,” explains Speth. Because of this Fugro performed a very broad investigation on the energy island site.
Fugro’s geotechnical investigations consisted of a seabed phase of CPT tests followed by a phase of soil sampling. After this the team continued with another round of CPTs in case they had not gone as deep as they needed to initially.
Fugro’s geotechnical vessel the Normand Mermaid mobilised on the 17 January and covered the wind farm site.
This vessel was equipped with the Seacalf Mk V Deepdrive – an advanced system for seabed CPTs. This large unit can push a cone up to 60m into the seabed with 20t of thrust.
Fugro then mobilised three downhole drilling vessels – the Fugro Scout, the Gargano, and the Highland Eagle – across the wind farm site. The Fugro Scout and the Gargano also worked on the artificial island site.
Each of these vessels was equipped with a specialised wireline CPT tool called Wison Mk V Ecodrive.
“This [tool] is a nice innovation because usually for these kinds of systems we use hydraulic oil to get the pressure in the downhole tool,” Speth notes. “But with this one, we’ve used an eco-fluid which means that if you have leakage, there’s no environmental damage.”
Crucially, the geotechnical team used geophysical data from the previous survey campaign to “make sure that there was no debris or UXOs across the site” while they were carrying out investigations. “[The data] was also used to assess the risk of shallow gas [trapped natural gas] which was low on the whole site,” adds Speth.
Puncturing shallow gas pockets during geotechincal operations could put Fugro vessels and crew at risk.
The last two vessels finished work in June. Now all the samples have been gathered, they will be analysed in Fugro’s various geotechnical laboratories.
Cable route surveys
In March this year Fugro sent vessels to site to begin the surveying work for the cable route. This runs for 100km north of the site and 140km south of the site.
The geophysical survey work is a similar investigation to what Fugro did for the energy island site. The offshore component of the survey is performed by the Fugro Frontier.
“Again, that will perform a very detailed bathymetry survey, and we are performing ground truthing by doing grab sampling,” says Padwalkar.
As part of this Fugro will capture about 3kg of soil samples at 1km intervals along the pipeline route.“We also are carrying out a magnetometer survey to again ensure that there are no magnetic anomalies along the cable route while we are doing the cable lay,” adds Padwalkar.
The second part of the campaign will include a shallow geotechnical investigation of seabed sections. The smaller Fugro Seeker vessel will perform this work at water depths of 10m to 2m.
Following this, Fugro will undertake rapid airborne lidar surveys at the landfall points of the cable route.
Then Fugro’s Energy Scout will carry out shallow geotechnical investigations using its Blue Snake geotechnical system – an innovative new technology. Blue Snake integrates CPT and sampling technology so that data can be captured in a single pass with testing completed at the same time and at the same location along the cable route. The technology integrates a vibrocorer and 10t CPT into a single frame with a customised launch and recovery system.
“This unit helps not only speed up the acquisition of the samples, but gives us a good correlation between the samples that we acquire,” says Padwalkar.
On completion of the geotechnical scope, Fugro will carry out a utilities inspection involving a desktop study of all the possible locations of telecom company communication cables on the seabed.
In the next package Fugro will map these out using a remotely operated vehicle inspection from the Atlantic Dweller vessel.