The two main types of welding used underwater are wet welding and dry (hyperbaric) welding. Wet welding happens directly in the water with no barrier between the welder and the surrounding environment, while dry welding takes place inside a sealed chamber that pushes water away from the work area using pressurized gas. Each method serves different purposes depending on depth, weld quality requirements, and budget.
Wet Welding: The Most Common Method
Wet welding is the go-to technique for most underwater repair work. The welder-diver works directly in the water, holding an electrode to the metal just as a welder on land would, except everything around them is submerged. The two most common processes are shielded metal arc welding (stick welding) and flux-cored arc welding, both of which use electrodes with special coatings that create a gas bubble around the arc as they burn. That bubble shields the molten metal from the water just long enough for the weld to form.
Wet welding has been used reliably at depths of about 100 meters (330 feet), and welds on carbon steel structures have been made as deep as 200 meters (660 feet). The appeal is simplicity: there’s no need to build or transport a chamber, which makes wet welding faster to set up and significantly cheaper. For emergency repairs on ship hulls, offshore platform legs, or underwater pipelines, this speed matters.
The trade-off is weld quality. Water cools the molten metal rapidly, which creates a harder, more brittle zone around the weld. The electric arc also splits water molecules into hydrogen and oxygen, and that hydrogen can diffuse into the weld metal. The gas bubble surrounding the arc can contain up to 92% hydrogen. When hydrogen gets trapped in rapidly cooled steel, it creates internal pressure that leads to cracking, a problem known as hydrogen embrittlement. Higher-strength steels are especially vulnerable because their microstructure is less forgiving of these stresses. This is why wet welds are typically reserved for less critical structural repairs rather than load-bearing joints on primary infrastructure.
Dry Hyperbaric Welding: Higher Quality at Higher Cost
Dry hyperbaric welding solves the quality problem by removing water from the equation entirely. A sealed chamber, called a habitat, is built or positioned around the joint to be welded. The water is then displaced with a gas mixture, giving the welder a dry environment to work in. The main welding processes used inside these habitats are gas tungsten arc welding (TIG) and shielded metal arc welding (stick), though flux-cored and gas metal arc welding are sometimes used as well.
The catch is that the pressure inside the chamber matches the water pressure outside. At 100 meters deep, that’s roughly 11 times atmospheric pressure. This hyperbaric environment changes the physics of welding in measurable ways: arc voltage increases with ambient pressure, the rate of short-circuit metal transfer speeds up, and the dense gas pulls heat away from the weld faster. Every welding parameter, from current to travel speed, has to be specifically optimized for the pressure at that particular depth. Getting this right is the central challenge of hyperbaric welding research.
Despite those complications, the results are far superior to wet welding. Dry hyperbaric welds can meet the same standards as welds made on the surface, which is why this method is used for critical repairs on subsea oil and gas pipelines, nuclear infrastructure, and other structures where failure isn’t an option.
Dry Spot Welding: A Compact Alternative
A smaller-scale version of dry welding uses a portable dry spot habitat, essentially a clear plastic box fitted with sponge or flexible rubber seals that covers only the welding head and a small area around the joint. It moves along with the welder rather than enclosing the entire work zone. This approach offers some of the quality benefits of a full habitat without the cost and logistics of building a large chamber underwater. However, dry spot welding hasn’t seen as much development or widespread adoption as either full habitat welding or wet welding, so it remains a niche option.
Weld Quality Classifications
The American Welding Society’s D3.6M code defines three classes of underwater welds. Class A welds must be comparable to above-water welds and undergo the most rigorous testing, including radiographic, ultrasonic, and visual inspection. These are the standard for critical structural work and are almost always produced using dry hyperbaric methods. Class B welds are for less critical applications and have somewhat relaxed acceptance criteria. Most wet welds fall into this category. Class O welds are a flexible designation that must meet the requirements of another specified code, allowing engineers to tailor standards to the specific project.
Electrical Safety Underwater
Welding in water creates an obvious electrocution risk, so the entire electrical setup is designed around preventing stray current from reaching the diver. All underwater welding uses direct current (DC) rather than alternating current (AC), because DC is less likely to cause fatal cardiac disruption if accidental contact occurs. A critical piece of safety equipment is the knife switch, a heavy-duty 400-amp isolating switch positioned between the welding machine and the diver. A surface tender controls this switch, energizing the circuit only when the diver is in position and ready to strike an arc. When the diver needs to change electrodes or reposition, the tender cuts the power. This simple on/off control is the single most important safeguard against electrocution.
Hazards Beyond the Weld
The welding itself is only part of the danger. Decompression sickness, where nitrogen bubbles form in the bloodstream during ascent, accounts for an estimated 20 to 25 percent of underwater welding fatalities. The welding arc also generates toxic gases including hydrogen sulfide and carbon monoxide. In wet welding, these gases mix with the diver’s breathing environment and can cause respiratory damage and neurological problems with repeated exposure. Long-term, many underwater welders experience chronic joint damage and neurological issues from repeated decompression cycles, which contributes to a reduced life expectancy compared to the general population.
Robotic Systems for Deep Water
Human divers can only work so deep, which is driving interest in robotic alternatives. Germany’s DFKI Robotics Innovation Center has developed a submersible welding robot that uses a stereo camera system and artificial intelligence to detect weld seams and plan the welding process autonomously. The robot’s modular arm has six degrees of freedom, a two-meter reach, and can operate at depths up to 6,000 meters, far beyond what any human diver could survive. It uses a flux-cored arc welding process adapted for underwater conditions. The system can be mounted on a crawler vehicle or lowered on a guide system, making it adaptable to different infrastructure types. While no fully autonomous robotic welding system is commercially available yet, this technology points toward a future where the deepest, most dangerous repair work no longer requires a person in the water.