Where jack-ups run out

By Jack Berryman, engineer, MintMech

Most of the fixed-bottom offshore wind built so far has been installed from something standing on the seabed. A jack-up puts its legs down, lifts its hull clear of the water and becomes, for the duration, a fixed platform. Everything above it – the crane, the hammer, the deck spread – is designed on the assumption that the thing underneath is not moving.

That assumption is worth a great deal. It is also the first thing to go.

What a jack-up actually needs

Three things, and depth is only the obvious one.

Enough leg. A jack-up cannot work in water deeper than its legs, less the airgap it needs above the wave crest and the penetration the seabed demands. The usable depth is always meaningfully less than the leg length.

Ground that will hold. The spudcans have to bear the platform’s weight and its environmental loading without settling unacceptably. Layered ground – a firm crust over something softer – carries the risk of punch-through, where the leg breaks through the strong layer and drops. That is a serious event, and it is a function of the ground, not the vessel. We have written about it separately.

A weather window long enough to get up and down. Jacking is not instantaneous, and a platform part-way through the operation is at its most exposed.

None of these fail gracefully. They constrain the work, and then at some point they stop it.

What deeper water changes

Push a fixed-bottom project further offshore and several things move at once.

The water gets deeper, which is the constraint everyone names. But the ground is often less forgiving too – glacial deposits, boulder beds, chalk, rock at or near seabed. Monopiles have grown to diameters that make the driving energy substantial. And the further from shore, the shorter and less predictable the workable weather.

The result is a set of sites where a jack-up either cannot stand at all, or can stand but not economically – and where driving a pile is a poor answer even if you can get a vessel there.

Why driving is not always the answer

Driven piling is fast, well understood and the right method in a great deal of ground. It has three recurring problems in the ground that deep-water fixed-bottom sites tend to present.

Hard ground. A pile that meets rock refuses. Drive-drill-drive works, but it means bringing drilling capability to the job anyway – at which point the question is which method is primary.

Fatigue. Driving damages the pile. In hard driving, the accumulated fatigue over thousands of blows becomes a design case in its own right.

Noise. Underwater noise from impact piling is a consent constraint in most European waters, and mitigation adds cost and complexity to every operation.

None of this makes driving wrong. It makes it one option among several, and the ground decides.

Drilling the foundation in, from a floating vessel

The alternative is to stop trying to stand on the seabed, and instead work from a vessel that stays afloat and references the bottom rather than itself.

That inversion is the whole idea. A dynamically-positioned vessel holds station on the surface. The work – the drilling, the pile, the loads – is referenced to a seabed frame that lands on the bottom, reacts the drilling and handling loads into the ground and guides and clamps the string. The vessel moves. The hole does not.

From there the method follows the ground:

In hard ground, cased open hole. A sacrificial casing is advanced to the seabed directly behind the cutters to hold the overburden open, and a rock socket is drilled through it. The pile lands into the completed socket, is clamped by the seabed frame and grouted in place. Because the drill string can be recovered and the casing left on the seabed with the hole open, the vessel can leave for weather and come back to it – which matters more than it sounds when the weather window is the binding constraint.

In soft or unstable ground, the pile goes in with the hole. The pile is pinned to the bottom-hole assembly and installed as the bore advances, under-reaming so the bit passes through. The ground never gets the chance to close on an open hole.

Both routes end the same way: a pile at a known depth, in competent ground, grouted, with the position and verticality set by the frame rather than by how well the vessel was holding station.

The heave problem sits underneath all of it

None of this works if the vessel’s motion reaches the bit.

A drill string suspended from a moving vessel puts that motion straight into the hole – variable weight on bit, poor advance, damaged cutting structure. Holding the load steady while the vessel moves is the enabling capability for every method described above, and it is one of the reasons this work has historically stayed on jack-ups. It is also a capability in its own right, and one we build.

The vessel is often already there

The equipment that drills a foundation pile is closely related to the equipment that takes the site investigation samples in the first place.

A geotechnical marine drilling vessel already has the derrick, the hoisting, the heave compensation and the deck handling. What it lacks is the pile-installation package – the seabed frame, the tooling, the grout spread and the conversions that tie them to the vessel. We have a documented method for that conversion, engineered since 2019 and independently validated in 2023 under an Offshore Wind Growth Partnership framework.

The commercial consequence is worth stating plainly: the vessel that takes the soil samples can install the foundations those samples inform, without a purpose-built piling vessel entering the picture. On a market with a constrained installation fleet, that is not a small thing.

What this means for a project

If a site is being written off as too deep, or the foundation method is being chosen around the limits of an available jack-up, it is worth testing that assumption early – while the engineering is still cheap to change.

The questions that decide it are the ordinary ones: what the ground actually is, what holding capacity the foundation needs, what the weather window realistically allows and what is already in the fleet. The answers point at a method. They usually point at it well before anyone has committed to a vessel.

Tell us the duty – the depth, the ground and the foundation – and we will tell you which method fits, and what it would take.