Forty three miles out from the English coast the North Sea floor is not one thing. It is sand in places, stiff clay in others, and the depth over it shifts across the site.
So the 95 steel piles hammered into it were never going to be identical.
The last of them went down this August. The campaign had been running since the spring of the year before.
Between the longest and the shortest sits a 55 foot gap.
Why no two of these could be cut to one drawing
A monopile is a hollow steel cylinder driven into the seabed until the ground grips it. Nothing else holds it upright.
That grip depends entirely on what the ground happens to be. Dense sand takes hold over a shorter buried length than soft clay does, and deeper water puts more sideways load on the same tube, so the two pull against each other everywhere.
This one covers about 118 square miles.
Which means the design could not be one drawing repeated ninety five times. Every position got its own survey, its own soil profile, and its own length of steel cut to match before anything left the yard.
What came out of that is a graded set rather than a batch. The shortest runs about 220 feet and the longest reaches 275 feet, and the weights climb along the same curve.
The biggest one, measured against things on land
The record piece is the one everybody photographed. It stands 275 feet and weighs close to 1,984 US tons.
The developer offered its own yardsticks for that number. Roughly the height of fifteen giraffes, a width matching the length of a London bus, and the mass of about 250 African elephants.
Across the full set the weights start near 1,323 US tons and climb to that figure. The widest of them measures almost 35 feet across.
Added together the foundations swallowed more than 220,000 US tons of steel, spread across an area the developer puts at close to forty three thousand football pitches.
The collar that goes on top of every one of them
None of these piles carries a turbine directly. A second component goes on first.
The transition piece is a steel collar about 65 feet tall and 26 feet across, weighing more than 440 US tons by itself. It carries the boat landing, the access platform, the ladders and the internal deck a technician needs to reach anything at all.
Bolted onto the pile head, it also takes out whatever lean the driving left behind. A tube hammered into varied ground almost never finishes perfectly plumb, and the collar is where that error gets corrected rather than passed up the tower.
Ninety five of those went on as well. Stack one on its pile and the pair lands near three hundred elephants on the same scale.
How one of these actually gets into the seabed
The ship doing this work is a jack up. It arrives floating, lowers four legs to the seabed, and then lifts its own hull clear of the water on them.
That is the whole trick. A floating crane rolls with the swell and a jack up does not.
Each pile arrives lying flat and has to be stood on end before anything else can happen. The handling gear for that was built specially for this job, designed, fabricated and fitted to the ship in under two years, and the marine contractor said that preparation is what made the lift possible at all.
Upright, the pile is gripped, lowered and then struck repeatedly by a hydraulic hammer until it reaches design depth. The first one set the European mark for the largest monopile placed from a jack up vessel.
Rock goes down around the base afterward as scour protection. A current running past a new obstacle pulls sand out from under it, and without the armour the pile would slowly lose the top of its own grip.
What is still to come before any of it turns
All ninety five foundations and all ninety five collars are in the water, and trade coverage logged the final one in the middle of August.
The turbines are the part still going up. The first went on in the spring.
Each carries blades 377 feet long, the longest ever built and put to work in Britain, and all two hundred and eighty five come out of a single factory on the Humber.
One rotation of a single rotor is put at enough electricity to run a British household for more than four days. At full output the site is rated at 1.4 gigawatts, enough for the equivalent of more than a million homes and change.
Whether the array beats its schedule is another question, and American projects running the same sequence have learned that the hard way, including a Virginia farm that reached the grid late. Here the target is the end of the year, and the developer has not moved it.
