A jack-up is a self-elevating platform: it arrives floating at the site, lowers three or four legs to the seabed and jacks itself up out of the water. Diving comes in before it's positioned, to check the seabed, and afterwards, to inspect the legs, the spudcans and the footprints left in the sediment. It's an environment with risks of its own, starting with the most obvious one: the structure can move.
Of all the plataformas offshore, the jack-up is the one that looks least like the popular idea of an oil platform, and at the same time the most numerous in shallow water. It doesn't float while it works: it lifts itself up on its own legs and sits still, like a giant table planted on the seabed.
That particular feature completely changes the kind of diving it generates. And above all it changes the list of things that can go wrong.
What it actually is
A jack-up, or self-elevating platform, is a unit with a floating hull and three or four lattice legs that pass through that hull from top to bottom. In transit, the legs are raised and the unit behaves like a barge: it's towed or sails under its own power to the site.
Once there, the jacking system lowers the legs until they touch bottom. Then, instead of raising the legs, the mechanism works in reverse: the unit jacks itself up, climbing its own legs until the hull sits several metres above sea level. From that point it's a fixed structure, with no roll, no pitch and no need for anchoring or dynamic positioning.
Its uses vary: drilling wells in shallow water, personnel accommodation, installing and maintaining offshore wind turbines, installation work and decommissioning jobs. The limitation is depth: the length of the legs sets how far it can go, which confines it to the continental shelf.
The legs and the spudcans
Each leg ends in a wide foot called a spudcan, usually conical in shape, whose job is to spread the load over the sediment. As the unit jacks up, that foot digs in until the ground balances the weight.
Before the position is signed off, a preload is carried out: the unit is deliberately ballasted, usually by flooding tanks, to put each leg under a load higher than it will see in service. It's a proper load test. If the ground is going to give way, it's better that it does so there, in a controlled way, while the unit is still close to the surface.
Diving before positioning
The most delicate moment in a jack-up's life runs from when it arrives at the site until it's elevated and stable. And that's where diving has a specific role: checking the seabed.
What's being checked is that there's nothing that shouldn't be there directly beneath each leg. Nothing means pipelines, umbilicals, submarine cables, abandoned wellheads, construction debris, scrap or any buried structure. A spudcan landing on a pipeline is a major failure and a risk to the unit itself.
The other issue is footprints. When a jack-up moves off, it leaves a crater behind with the ground disturbed. If another unit is positioned nearby afterwards, a leg can slide into it and cause a sudden tilt. Locating and characterising old footprints is part of the prior survey, and where depth allows it the diver provides what the sonar can't: close-range detail.
Most of this survey work is done with geophysical methods and ROVs. Diving comes in as a complement in shallow water and whenever something needs to be touched, measured or removed.
Diving with the unit jacked up
With the platform already in position the recurring work begins, which is the bulk of the diving hours associated with a jack-up.
Leg inspection
The legs are lattice steel structures, with welded joints, exposed to current, waves and corrosion throughout the campaign. They're inspected like any offshore structure: a general survey, cleaning the areas of interest and a detailed examination of the joints and welds, using whichever visual inspection method applies under the plan.
The splash zone deserves a section of its own. It's the band that alternates between air and water with every wave, where corrosion is most aggressive and where working is uncomfortable and dangerous in equal measure: the diver moves in and out of the water with every movement of the sea, without the stability of being fully submerged.
Spudcans and the seabed
The penetration of each foot is checked, along with the condition of the ground around it and whether there's scour: the erosion the current produces in the sediment next to the spudcan. Progressive scour reduces the effective support and, if it continues, requires intervention with seabed protection.
Cathodic protection and anodes
As with any submerged steel structure, it's checked that the cathodic protection system is still working: anodes with enough material left, sound electrical connections and potentials within the protection range.
The risks a jack-up adds
Diving next to a jack-up has hazards that don't occur on a genuinely fixed structure.
The first is that the unit can move. It's resting on the seabed, not founded into it. Differential settlement, sliding into an old footprint, or a punch-through — the sudden penetration of a leg as it breaks through a hard layer and finds soft ground underneath — can tilt the platform without warning. That's why diving never takes place during jacking up, jacking down or preload operations: during those phases the water around the legs is a no-go zone.
The second is falling objects. With the hull raised above the water, anything dropped up top falls exactly where the diver is. Simultaneous operations management and exclusion zones aren't bureaucracy: they're the only real barrier against that.
The third is the current between the legs. The structure accelerates and deflects the flow, creating zones where the current is noticeably stronger than in open water. A umbilical diver swept by the current into a latticed member is a serious problem.
The fourth is the unit's own water intakes and discharges. Anything that draws in or expels water has to be isolated and locked out before anyone goes in, and that isolation has to be documented. It's at the heart of the permit to work.
What working there is like
For the diver, a jack-up is about as comfortable a posting as it gets. The unit doesn't move, so there's no seasickness and no weather windows as narrow as on a vessel; the dive point is stable and working conditions on deck are good.
In exchange, it's a dense industrial environment, with lots of people, a lot going on at once, and a demanding permit regime. Anyone coming from harbour work usually notices the jump: nothing here starts without signed paperwork.
Diving is almost always surface-supplied, at moderate depths, without needing saturation. That makes it one of the reasonable entry points into offshore work for whoever has the right certification and is looking for their first serious campaign away from the harbour. The subsea services contractors that work for shallow-water operators are the natural place to look.
Frequently asked questions
What's the difference between a jack-up and a semi-submersible?
A jack-up rests on the seabed on its legs and works out of the water, without movement. A semi-submersible floats and holds position with mooring or dynamic positioning. The first only works in limited depths; the second can work where the seabed is far below.
What is a spudcan?
The foot that caps each of a jack-up's legs: a wide piece, usually conical or truncated-conical, that spreads the load over the sediment. It's the part that digs in and that leaves a footprint on the seabed when the unit moves off.
Why does diving take place before positioning a jack-up?
To verify that the seabed is clean and suitable: that there are no pipelines, cables, debris or footprints from a previous location right where the legs are going to land. Landing on an old footprint or on buried infrastructure is one of the scenarios operators most want to avoid.
What is a punch-through?
The sudden penetration of a leg as it breaks through a resistant layer of ground and finds much softer ground underneath. The unit tilts abruptly. It's a known geotechnical risk, which is why the seabed survey before positioning is so detailed.
Does diving happen once the platform is jacked up?
Yes, and it's the most common situation: inspecting the legs, the spudcans, the splash zone and any associated subsea structure. The condition is that the unit is in a stable, declared state, with the work authorised by permit.
Do you need specific experience to work on jack-ups?
Not as a separate qualification, but yes as experience. What's valued is having worked in an offshore environment with strict procedure: permits to work, isolations, coordination with the unit and discipline in communication with the surface.
Published 1 September 2026 · By Arthur More, professional saturation diver
Photo: L.C.Nøttaasen · CC BY via Openverse
This article is part of our in-depth guide to how dangerous commercial diving really is — the complete picture, with sources.
This article is part of our in-depth guide to how dangerous commercial diving really is — the complete picture, with sources.
This article is part of our in-depth guide to how dangerous commercial diving really is — the complete picture, with sources.
This article is part of our in-depth guide to how dangerous commercial diving really is — the complete picture, with sources.