Underwater thermal cutting burns steel with an exothermic electrode fed with oxygen. It works at any diveable depth, but it generates hydrogen and oxygen that can build up in enclosed spaces and explode. That's why the procedure — purging, venting and electrical control from the surface — matters as much as the technique itself.
Cutting steel underwater is one of the most routine tasks in commercial diving: removing a damaged section of structure, cutting a window into a hull, freeing a jammed pipe, or decommissioning a platform at the end of its life. Water doesn't stop steel from burning; it just changes the conditions. What really changes compared with cutting at the surface is the risk, and that risk is what dictates the procedure.
What underwater thermal cutting is
Two families of technique are used underwater. Thermal cutting melts or burns the metal through a chemical reaction and an electric arc. Mechanical or "cold" cutting separates it by abrasion or by force: hydraulic saws, discs, diamond wire, shears or abrasive water jet. Each has its own ground, and the choice is almost never a matter of the diver's preference, but of what the risk assessment allows.
Within thermal cutting, the dominant method in professional work is oxy-arc cutting with an exothermic electrode, also known as oxy-arc cutting. Other variants — the classic hollow steel electrode or plasma systems — have been relegated to specific uses.
How the exothermic electrode works
The electrode is a hollow tube packed with metal rods, usually alloys that burn readily in the presence of pure oxygen. Pressurised oxygen from the surface runs through the inside of the tube. The sequence is simple to describe and takes practice to execute well:
- The diver rests the tip of the electrode against the workpiece or against a striker plate.
- He calls for current from the panel. The tender closes the switch and an arc is struck.
- The arc ignites the electrode's inner charge, which from that point burns on its own, fed by the oxygen.
- The jet of molten metal and slag is blown clear by the oxygen pressure itself, opening up the cut.
The practical consequence of the electrode burning by chemical reaction is that once lit, it keeps burning even if the current is cut. That's why it can cut materials that don't conduct electricity — concrete, cast iron, non-ferrous alloys, even reinforced concrete — and it's also why it's dangerous: it doesn't go out when you let go of the trigger, it goes out when the oxygen is shut off.
The equipment and the split of responsibilities
An underwater cutting torch is an insulated clamp with a trigger or oxygen valve and a jaw that holds the electrode. Behind it are three things the diver doesn't handle: the DC power source, a positive cut-off switch on the surface, and the oxygen line with its regulator. The switch is a safety item, not a convenience. The circuit is only live when the diver explicitly asks for it over comms and the tender confirms it. Once the cut is finished, it's opened again.
The standard polarity is DC with the electrode negative. Reverse polarity drastically speeds up electrolytic degradation of the electrode holder and of any metal in the diver's equipment, and it's banned in underwater work. This detail, along with correctly earthing the workpiece, is part of the pre-checks documented before anyone goes into the water.
The risk that shapes the procedure: the gas pocket
The cutting reaction produces gas in quantity. Some is hydrogen from the dissociation of water in the arc; some is oxygen that hasn't reacted. That mixture is explosive across an extremely wide range of proportions, and the water doesn't neutralise it: it pushes it upward.
While the diver is working on open structure, the bubbles escape and nothing happens. The problem appears when there's a ceiling above the cutting point: the inside of a hull, a tank, a caisson, a dead-ended pipe, the underside of a plate, the gap between two bulkheads. There the gas builds up until it forms a pocket, and a single spark, or the cutting front itself breaking through the plate, is enough to set it off. The great majority of serious underwater cutting accidents documented over the last few decades trace back to this mechanism, not to burns or electrocution.
Hence the rules every supervisor applies without argument:
- Never cut upward inside an enclosed space without venting it first.
- Cut vent holes at the highest point of the compartment so the gas can escape continuously.
- Purgar Purge tanks and pipework that have held hydrocarbons, and certify in writing that they are gas-free before heat is applied.
- Break off cutting periodically on long jobs inside structures, to let the gas disperse.
- Keep the body clear of the line of ejection of slag and of the likely path of a deflagration.
Ventilation isn't a formality. It's the measure that separates a normal day's work from an accident that, in a confined space, leaves no room to react, as any review of the real risks of commercial diving.
Thermal cutting versus mechanical cutting
Thermal cutting is fast, tolerates awkward geometries and doesn't need a machine clamped onto the piece. In exchange, it brings heat and generates gas. Mechanical cutting — hydraulic saws and shears, diamond wire, abrasive jet — is slower and needs rigging up, but introduces no source of ignition.
That's why, in practice, there are situations where cold cutting stops being an alternative and becomes mandatory: pipework that has carried hydrocarbons, hulls with adjacent tanks, environments with a risk of flammable atmosphere, or jobs on installations where the client expressly bans hot work. In structure decommissioning the two are combined: cold cutting for the compromised sections and oxy-arc cutting for open structural steel. A good part of the underwater hydraulic tools in a campaign's container exist precisely to cover those cases.
There's a third, less obvious factor: what's left of the material afterwards. Thermal cutting leaves a heat-affected zone and an irregular edge. If the piece is going to be welded again to quality requirements, the edge will need preparing; on hyperbaric welding jobs with code requirements, that clean-up is already built into the procedure.
What training is required
Cutting isn't a separate ticket you pick up online: it's part of the working competencies taught on recognised commercial diver courses and then consolidated on the job. In the programmes of commercial diving schools it appears as a practical module within the underwater-tasks block, alongside wet welding, the use of hydraulic tooling and rigging.
What separates a diver who cuts well from one who cuts fast is judgement: reading the structure before striking up, identifying where gas could get trapped, telling the supervisor what he sees, and stopping when something doesn't add up. That authority to stop is spelled out in the permit to work and doesn't depend on anyone's seniority. If you want to know how you get here, the route is set out in how to train as a commercial diver, and the map of companies that hire for this kind of work, in the company directory.
Frequently asked questions
Can steel be cut underwater with an ordinary torch?
No. Conventional surface oxy-fuel cutting relies on a preheat flame, which water extinguishes. Underwater, the exothermic electrode is used instead, which burns by an internal chemical reaction fed with oxygen and needs no external flame.
Why is the current switch operated from the surface and not by the diver?
Because it's a safety measure. With a positive cut-off switch at the surface, the circuit is only live for the strictly necessary time and at the diver's explicit request over comms, which cuts the risk of an accidental arc and of electrolytic degradation of the equipment.
What's the main hazard of underwater thermal cutting?
The build-up of hydrogen and oxygen in enclosed spaces above the cutting point. That gas pocket is explosive and has been the cause of most of the serious accidents on record. It's controlled by venting, cutting vent holes and purging before heat is applied.
Does the exothermic electrode cut concrete?
Yes. Since the reaction doesn't depend on the material conducting electricity, the exothermic electrode cuts concrete, cast iron and non-ferrous alloys as well as steel. The arc is only used for the initial ignition.
When is thermal cutting banned and cold cutting made mandatory?
When there's a risk of a flammable atmosphere: pipework and tanks that have held hydrocarbons, enclosed compartments that can't be vented, or installations where the client bans hot work. In those cases diamond wire, hydraulic saw, shears or abrasive jet are used instead.
Is a specific certification needed to cut underwater?
No, it isn't a stand-alone qualification. Thermal cutting is part of the practical syllabus of recognised commercial diver courses and is certified within the general qualification, backed up by experience logged in the dive log.
Published 24 August 2026 · By Arthur More, professional saturation diver
Photo: PiktourUK · CC BY via Openverse
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.