Underwater Ship Welding: Repairs Below the Waterline

A harbor ferry comes in for its special survey and the ultrasonic gauge finds pitting on the shell plating that no one can see from the deck. The yard’s quote for a drydocking runs into six figures and costs three weeks of lost sailings, so the owner asks the question every marine superintendent eventually asks: can this be welded with the ship still in the water? The answer is yes, but only if you understand which underwater ship welding method you are buying, and what it can and cannot deliver.

Underwater Ship Welding: What It Is and When It Makes Sense

The survey found pitting, the drydock quote ran into six figures, and the sailing schedule has no slack. So the question on the table is whether a welder can fix that plating while the ship floats. Yes, ships can be welded afloat, and it happens constantly in commercial ports. The catch is that “underwater welding” is not one technique with one quality level. It is a family of methods, and the one you specify determines the weld quality you get, the inspection you can prove, and the classification society’s answer when the surveyor climbs aboard.

Two Families: Wet Welding and Dry Hyperbaric Welding

Underwater welding splits into two broad categories. Wet welding is performed directly in open water: a diver-welder strikes an arc on the submerged steel with a waterproofed, flux-covered electrode, essentially the same shielded metal arc welding process used on deck, adapted for immersion. Dry hyperbaric welding takes place inside an enclosure, a habitat or a cofferdam, that is sealed against the sea and filled with a breathable gas mixture at the surrounding pressure, so the weld zone stays dry and the arc behaves much as it does in air. That distinction is the whole ballgame: wet welding is fast and cheap but hard to inspect, while dry welding is slower and far more expensive but can approach code-quality work.

Two boundaries around the term are worth drawing. Underwater welding is not underwater cutting, which uses exothermic or arc processes to remove steel rather than join it, even though the same dive team often performs both. And it is not internal tank welding, where the work happens inside a drained, ventilated cargo or ballast space with the hull dry. Underwater welding means joining metal that is genuinely submerged or exposed in the splash zone.

Why Owners Choose It

The commercial case is straightforward. Underwater welding is frequently the least expensive repair option because it avoids pulling the structure out of the water and the downtime that goes with it. Owners generally reach for it for three reasons:

• Avoiding drydocking cost and lost sailings. A patch welded afloat can cost a fraction of a slot in a repair basin and keeps the vessel earning.
• Emergency or interim repairs. A damaged frame or holed plate can be made watertight enough to let the vessel steam to a yard on its own bottom instead of under tow.
• Structures that cannot leave the water. Harbor piles, sea chests, offshore jackets, jetties and berth structures have no drydock to go to.

The Trade-Offs Nobody Should Skip

Honesty matters here. Wet welds cool extremely fast, so they tend to be harder, more porous and more prone to hydrogen cracking than shop welds, and visibility is often poor enough that inspection quality suffers. Work is weather, tide and current dependent. And a classification society will not automatically accept a repair simply because it was welded: procedures, welder qualification and surveyor attendance usually have to be arranged in advance, following guides such as AWS published codes and standards. A practical rule of thumb is that most wet welds on ships are temporary or non-critical repairs, not permanent renewals of primary structure.

A Short History and Where the Work Actually Happens

The dry side of the discipline began in 1932, when Soviet metallurgist Konstantin Khrenov demonstrated that an electric arc could be sustained and used for welding underwater, opening the field to systematic development (Hyperbaric welding). Operational dry hyperbaric welding has since been carried out below 400 m, with laboratory hyperbaric chambers simulating conditions down to roughly 2,500 m. Pressure climbs by about one atmosphere for every 10 metres of seawater, which is why depth limits are driven more by diving physiology and logistics than by the arc itself.

For ships, none of that extreme depth is relevant. The overwhelming majority of vessel work takes place in shallow harbours and the splash zone, a few metres below the surface, alongside piers and in berth pockets, conditions that also make coordination, ventilation and sound marine welding safety practice easier to manage. Repairing hull plating afloat is a routine harbor job, not a deep-sea expedition. And understanding hull corrosion in the first place helps you judge which defects deserve an afloat weld and which should wait for the yard.

Wet Welding vs Dry Hyperbaric Welding: The Two Families Explained

Ask ten people what “underwater welding” means and you will get ten versions of the same vague idea: a diver with a torch, sparks in the water, done. In practice the industry splits into two genuinely different operations, and the difference decides everything that follows — cost, quality, how much inspection you can do, and whether the repair counts as permanent. One keeps the weld puddle in direct contact with the sea. The other takes the water out of the equation by rebuilding a small dry room around the work.

Knowing which family you are buying is the single most useful thing a superintendent can understand before signing a work order.

Side-by-Side: What Actually Differs

Factor	Wet welding	Dry hyperbaric welding
Working environment	Diver-welder works directly in open water; the arc is exposed to the sea	Work area sealed inside a habitat or cofferdam, purged and filled with a breathable gas mix at ambient pressure
Typical processes	Shielded metal arc welding with a waterproofed electrode, direct current; some flux-cored and friction work	Gas tungsten arc welding preferred; SMAW and GMAW used in larger habitats
Achievable weld quality	Sound and serviceable, but affected by extreme cooling, porosity and hydrogen pickup	Approaches topside quality because heat input, preheat and cooling rate can be controlled
Inspection confidence	Limited by visibility, water turbidity and difficult NDT access	Comparable to surface work: visual inspection, video records and most NDT are practical
Depth practicality	Technically simple, so it scales to depth, but diver exposure governs	Physiologically limited by the diving regime long before the welding runs out of capability
Relative cost	Lower mobilization, small spread, fast to start	High: habitat, gas monitoring, fume extraction, larger surface support
Typical shipboard uses	Temporary repairs, securing inserts, patch work, getting a vessel to a yard	Permanent plating renewal, sea chest repairs, class-approved structural work

Wet Welding: A Stick Electrode in Open Water

Wet welding is essentially shielded metal arc welding performed with a waterproofed electrode while the diver is submerged. The flux coating is formulated to burn in a wet environment and generate a shielding bubble of gas around the arc; the diver watches that bubble, not the arc itself. Current is direct current, almost always straight polarity in practice, supplied from an insulated surface set through a heavy-duty isolation switch operated by a surface attendant.

Electrode size drives the amperage. In the field you will commonly see welders working in the 300–400 amp range depending on electrode diameter, coating formulation and depth, with the operator trimming current as the arc length and position change. Flux-cored consumables exist as an alternative that deposits metal faster, and friction welding has been demonstrated for deeper repairs where an arc becomes unstable, but stick welding with a purpose-made underwater electrode remains the workhorse. It is cheap to mobilize, needs almost no support equipment, and can be done in a few metres of harbor water on short notice.

The trade-offs are equally plain. The puddle is quenched by seawater, which raises hardness, encourages porosity and drives hydrogen into the deposit. That is why wet welding is generally restricted to lower carbon equivalent steels — a point that matters a great deal when someone asks a diver to weld a high-strength hull frame. Electrode handling is critical too; underwater electrodes need careful storage and selection because a damp or damaged coating behaves unpredictably once the arc is struck.

Dry Hyperbaric Welding: Rebuilding a Dry Room at Depth

Dry hyperbaric welding seals a habitat around the work area and fills it with a gas mixture held at the surrounding water pressure, so the diver-welder works in gas rather than water. Because the joint is dry, ordinary metallurgy applies: gas tungsten arc welding is the usual choice because it gives the tightest control of heat input and puddle shape, and the operator can preheat, apply post-heat and let the joint cool at a controlled rate instead of slamming into cold seawater. Visual inspection and NDT become realistic rather than optimistic.

The cost is a much larger surface spread — gas plant, monitoring, communications, fume extraction, tender diving support — and depth limits set by decompression and gas physiology, not by the welding equipment.

One Term, Not One Quality Level

This is where specifications go wrong. AWS D3.7 is a classification framework for underwater welds rather than a single acceptance standard: it sorts results into classes that reflect serviceability, and the purchaser has to state which class the job must meet. Welder qualification sits alongside it — ISO 15618 covers qualification testing for hyperbaric wet and dry welding, so a certificate is process-specific, not a blanket licence to weld anything below the waterline.

The Physics of Welding Underwater: Pressure, Bubbles and Hydrogen

Underwater ship welding only makes sense once you understand what the water is doing to the arc, the metal and the diver simultaneously. On land an arc sits inside a calm, predictable shield of gas. Submerged, it fights hydrostatic pressure, a boiling shielding bubble, brutally fast cooling and a hydrogen problem that ends in cracks. Understand those forces and the clauses in a welding specification stop looking bureaucratic and start looking like survival instructions.

Hydrostatic Pressure and the Arc

Water pressure climbs by roughly one atmosphere every ten metres of depth. A diver welding a hull in ten metres of water works at about 2 bar absolute; one at a hundred metres works at roughly 11 bar absolute. That pressure acts straight onto the arc column. As ambient pressure rises, so does arc voltage, the arc root contracts into a narrower, harsher spot, and the arc grows less stable and more prone to extinguishing.

The practical fallout is immediate: the same electrode and amperage that produce a smooth bead in a harbour behave differently at depth, with lower process efficiency, heavier arc blow and slower travel speed. The relationship is well documented in hyperbaric welding research, and it is precisely why a dry hyperbaric procedure must be qualified at the depth it will actually run. A procedure qualified at 3 bar carries no automatic validity at 11 bar.

Worked pressure example: at 30 m a diver sits at about 4 bar absolute, roughly four times surface pressure; at 100 m it is about 11 bar absolute. Even a modest harbour depth of 15 to 30 metres places real load on the arc and on the diver’s equipment.

The Shielding Bubble: Steam Inside the Shield

In wet welding there is no gas bottle and no nozzle. The shield is a bubble, generated mainly by decomposition of the electrode’s flux coating and reinforced by steam boiling off the weld pool itself. For a few milliseconds at a time it performs a shielding gas bottle’s job.

The problem is that it is not clean. Water contacting molten metal flashes to steam and is drawn into the envelope, so the shield ends up contaminated and the arc wanders as bubble pressure fluctuates. Expect more spatter, more porosity and less consistent fusion than an equivalent dry weld. That is also why shielded metal arc welding remains the dominant wet process: the flux creates and feeds its own shield, something no gas line reproduces as reliably underwater. Flux-cored wires and friction processes earn their niches, but stick electrodes stay the diver-welder’s default tool.

Quench Rates, Hardness and Hydrogen Cracking

Water extracts heat from a weld far faster than air ever could. A bead that would cool in air into soft, ductile microstructure can quench underwater into hard, brittle phases that crack under their own shrinkage stress. The same rapid cooling traps gas before it escapes, which is where much of the weld’s porosity originates. Fast cooling, a brittle structure and hydrogen is the classic cracking recipe, and underwater welding supplies hydrogen from several directions at once:

• Dissociated water: the arc splits water vapour in and around the bubble into atomic hydrogen, small enough to dissolve directly into the molten pool.
• Moisture: damp or badly stored electrodes, wet coatings and the surrounding water itself.
• Coatings and paint: burning through primer or anti-fouling releases hydrogen-bearing compounds alongside other contaminants.
• Rust and marine growth: corrosion products trap moisture and add inclusions as well as hydrogen.

Once dissolved, hydrogen migrates into the heat-affected zone and waits. Given tensile stress and a hard microstructure it can open cracks hours after the diver has surfaced, which is why inspection timing matters as much as inspection method. This is the reason wet welding is generally restricted to low carbon equivalent steels, with the restriction tightening as depth increases and cooling accelerates. Higher-strength, higher-carbon-equivalent steels are simply too crack-sensitive to weld wet with any confidence.

The Human Side of the Physics

The same water that quenches the weld works on the person making it. Turbidity, silt and bubble plumes can cut working distance to a few centimetres, and much wet welding is done by feel with a finger in the joint. Water drags on the diver, so every movement of stinger and cable fights that loading and fine positional control becomes exhausting. Even rod that is comfortably all-position on land behaves differently in a diver’s cold, tired hands.

Heat leaves the diver nearly as fast as it leaves the weld, making hypothermia a genuine planning item even in temperate harbours, and chilled, stiff hands degrade bead quality directly. Sound behaves differently too, travelling roughly four times faster in water than in air, yet divers normally hear the surface through umbilical speakers, so communication protocols between diver and surface operator must be explicit and disciplined.

All of this is why underwater welding is regulated as commercial diving work first and welding work second, under frameworks such as the HSE’s UK diving regulations and guidance like the AJ Marine welding guide for marine practice. Grasp the physics, and the ambitious idea of “just weld it in the water” turns into a procedure that survives contact with reality.

How Wet Welding Is Actually Done: A Step-by-Step Walkthrough

Wet welding looks crude from the quay: a diver disappears over the side, a few bubbles rise, and an hour later there is a fresh bead on the hull. Behind that bead is a tightly controlled sequence of paperwork, permits and procedure. Skip a step and you do not get a bad weld, you get an incident. Here is how a typical wet hull repair actually runs, from drawing review to post-weld check.

1. Job Scoping and Drawing Review

Before anyone gets wet, the superintendent pulls the ship’s structural drawings and identifies the plating thickness, the steel grade, and what sits behind the repair area: tanks, sea chests, echo sounder transducers, anodes, cable runs. The scope decides everything downstream. A seal weld over a small pit is a different job from an insert plate in a highly stressed area, even if both are “stick welding underwater.”

2. Class Approval and the Weld Specification

Most ships are classed, and class societies treat underwater welds seriously. The owner submits the proposed repair method, and the society responds with conditions: what weld class is acceptable, whether the repair is classed as temporary or permanent, and what NDT is required afterward. As the AWS codes and standards library makes clear, the governing specification, not the welder’s preference, defines acceptance.

3. Dive Plan, Hot Work Permit and Isolation

The dive supervisor produces a dive plan covering depths, bottom times, gas supply, umbilical management and contingency. Alongside it sits the hot work permit: the port or terminal’s written authorization to strike an arc, with fire watches, gas testing of adjacent tanks, and notification to the port authority. Cathodic protection in the area may need to be temporarily switched off, because stray current makes arcs wander and accelerates corrosion elsewhere.

4. The Surface Team and the Isolation Switch

The welder-diver is not alone. A surface operator runs the power source and, critically, controls a heavy-duty isolation switch that makes and breaks the welding current. The diver signals “arc on” and “arc off,” and the current is cut whenever the diver is repositioning, changing electrodes, or simply moving across the hull. That switch is the single most important safety device on the job, because a live electrode holder in a conductive environment is unforgiving.

5. Cleaning to Bare Metal

The repair area is chipped, brushed and gouged back to bright steel. Marine growth, paint, rust and old coating all contaminate the arc and feed hydrogen into the puddle. In practice this cleaning step takes longer than the welding itself, and it is the step most often rushed. If the diver cannot see bare metal, the weld should not start.

6. Fit-Up of the Patch or Insert

For a repair plate, the diver fits a pre-fabricated patch or insert and tacks it in place. Fit-up tolerance underwater is poor compared with a shipyard bench: a diver in gloves, in current, in limited visibility cannot chase tight gaps. Gaps are bridged with generous root openings and multiple stringers rather than a single heavy pass.

7. Electrodes, Current and Technique

Wet welding is essentially stick welding with waterproofed, specially fluxed electrodes stored in sealed packaging until the moment of use. The flux creates a shielding bubble around the arc, and its decomposition products help protect the puddle from the water. Current is set on the surface per the qualified procedure, usually somewhat higher than the same electrode would run in air, because the arc is stiffer and the surroundings cooler.

The diver strikes the arc, keeps a short arc length, and drags rather than weaves. Beads run in a planned sequence with proper overlap, typically about half the bead width, to temper the previous pass. Between passes, slag is chipped away as thoroughly as visibility allows. One genuine advantage: the slag blanket left on a completed bead slows cooling slightly, buying the weld a few precious seconds against the water’s relentless heat sink. It is not nearly enough to eliminate the rapid cooling that raises hardness and porosity, but it helps.

8. Seal Passes and Post-Weld Checks

Final seal passes close the perimeter, and the diver then records the finished weld on video, notes the extent, and flags any visible porosity or undercut for the surveyor. Back on deck, the documentation goes into the dive report and weld map for the next survey.

What Wet Welding Cannot Do

Honesty matters here. Wet welds cannot be reliably preheated, positional welding is awkward, fit-up must be coarse, and inspection confidence is limited compared with a dry weld. Because of that, most wet welds on ships are temporary or non-critical repairs intended to keep the vessel trading until a proper yard repair. Treating one as a permanent fix in a highly stressed structural area is how repairs fail.

Dry Hyperbaric Welding: Habitats, Cofferdams and Controlled Quality

Some ship repairs can’t rely on a temporary patch. When a surveyor needs a weld that will hold for the long haul, the priority is simple: remove the sea. This is the purpose of dry hyperbaric welding. By isolating the work area and pressurizing it to match the surrounding water, a diver-welder can lay a joint in a dry, controlled space. The method is costly, but for critical structural repairs, it’s often the only way to achieve a provable, code-level weld.

Habitats: A Sealed Workshop Under Pressure

A habitat is a positive-pressure enclosure that is clamped, sealed, and grouted around the damaged area. Once secured, water is displaced and replaced with a breathable gas mixture at ambient pressure. At shallow depths, compressed air is often used, but as depth increases, helium-based heliox mixes are necessary to avoid nitrogen narcosis. A slight positive pressure is maintained throughout the job, forcing any potential leak outward rather than allowing seawater in.

Access is made through an entry lock or trunk, acting much like an airlock to keep the interior dry. Once inside, divers can work without a bulky full diving helmet and breathe the habitat gas directly. For this to remain safe, three elements are non-negotiable:

• Gas monitoring: Oxygen, carbon dioxide, and humidity must be tracked continuously.
• Communication: A constant link between the diver, supervisor, and surface crew is required.
• Fume control: Welding plumes must be extracted from the breathing zone, then vented or scrubbed.

With these systems in place, the habitat becomes less of a diving bell and more of a pressurized workshop, one where precision work becomes realistic again.

Cofferdams: A Practical Compromise

From Dry Arc to Proved Quality

The largest advantage of either option is control. With water removed, nearly all of the defects inherent to wet welding are avoided. Gas tungsten arc welding is the process of choice for most dry hyperbaric jobs due to its stable arc, precise filler addition, and controllable heat input. More importantly, shielding gas remains effective, and a dry environment allows for pre- and post-weld heat treatment where required. This makes it viable to weld tougher steels and tackle tricky joints, such as those found on aluminium vessels. For these alloys, wet welding is rarely suitable for structural work, and meeting the joint preparation requirements outlined in Aluminum Boat Welding: The Complete Guide for Strong Hull Joints is far easier to achieve when dry.

Quality control is also a major factor. Inside a habitat or dry cofferdam, visual inspection, magnetic particle testing, and ultrasonic examination can all be conducted on site before demobilization. Any defects found can be corrected while the welder is still there. This traceability is what allows repairs to be qualified against documented procedures, drawing from published references such as the AWS codes and standards library.

Depth, Cost and the Push to Mechanize

Despite the process itself being sound, the true ceiling for dry hyperbaric welding is not the arc, but the diver. Operationally, most jobs are limited to depths of less than about 400 m. Decompression requirements, diver physiology, and the sheer scale of chamber and gas support all contribute to this limit. As noted in the overview of hyperbaric welding, laboratory simulations have reached depths of roughly 2,500 m, proving the weldability is not the issue.

These constraints come at a price. A dry hyperbaric job demands extensive surface spread: dive support, gas blending, positioning gear, decompression chambers, hot-work control, and a large crew. With procedure qualification, surveyor attendance, and post-repair NDT added on top, costs rise drastically. For this reason, dry welding is reserved strictly for critical structural work.

Mechanization offers a way forward. On offshore pipeline girth welds, automated systems are already used inside habitats to lay consistent beads, with divers handling consumables and repositioning. Hull geometry is far more awkward for these machines, so manual welding still dominates on ships. Still, as monitoring improves and automation becomes more compact, dry conditions will remain the gold standard wherever a repair simply has to be proved.

Ship-Specific Repairs: Where Underwater Welding Is Used on Vessels

Ask a marine superintendent where underwater welding actually gets used on a ship and the answer is narrower than most people expect. Afloat welding is not a substitute for a planned steel renewal program. It targets defects that are accessible from the water, small enough to manage in one or two dives, and important enough that docking the vessel would cost more than the repair itself. Knowing which repairs fall into that category, and which do not, separates a realistic scope from a hopeful one.

The Repairs That Commonly Happen Afloat

The typical list is short and repeats across vessel types:

• Shell plating patches and insert plates. Pitting, gouges, or small corrosion wastage on bottom or side plating, cut out and replaced with a fitted insert welded in place.
• Sea chests and gratings. Corrosion or damage inside the sea chest recess, including renewal pieces and restraints for the grating.
• Bilge keels. Leading edges dented or torn by contact, straightened, built up, or re-welded to the hull connection.
• Rudder plating and pintle areas. Localized damage to the rudder blade skin, with pintle and heel bearing work treated far more cautiously because it involves fit and alignment, not just fusion.
• Stern frames and thruster tunnels. Cavitation wear, impact scoring, and local build-up, usually in shallow, sheltered water with good access.
• Splice pieces at damaged frames. Internal frames damaged by grounding, tied back to the shell with a splice welded from outside where the tank cannot be entered.
• Emergency repairs. Temporary patches, crack arrest holes, and doubler plates that let a damaged vessel sail safely to a yard under its own power.

That last category is the one the industry leans on most. Underwater welding earns its reputation in the gap between “damaged” and “safe to move.” As the hyperbaric welding overview notes, wet welding was developed largely for repair work where taking the structure out of service was not an option, and that logic carries straight into ship repair. For a broader view of how marine welding work is structured in practice, see our AJ Marine Welding guide.

Temporary or Permanent: What Classification Requires

Classification societies do not treat “an underwater weld” as a single quality level. For a permanent repair that must count toward the ship’s class condition, the society generally expects an approved welding procedure, qualified welders working to that procedure, and a surveyor present or consulted before, during, or after the work. Thickness gauging usually precedes the job to establish how much sound metal remains, and follows the repair to confirm the insert or patch has been fused and dimensioned correctly. This mirrors the qualification logic in the AWS codes and standards library, where the specification states the required class rather than assuming it.

Temporary repairs are a different animal. They exist to make the vessel safe for a voyage, are recorded as such, and expire at the next survey or upon arrival at a repair yard. Confusing the two is expensive: an owner who assumes a diver-welded doubler will satisfy a special survey learns otherwise when the surveyor asks for the procedure qualification and the NDT report.

Repairs Deliberately Left Ashore

Propellers are the clearest example. Even a small crack on a blade is normally handled by removal and ashore repair or replacement, because propeller bronze is metallurgically sensitive, the assembly must be balanced, and geometry is verified against the original pitch profile. Welding a blade afloat makes that verification effectively impossible. Cast stern gear, rudder stocks, and highly stressed castings get similar caution, as do high-carbon-equivalent and high-strength hull steels, where rapid underwater cooling risks hard, crack-prone zones. Consumables for these materials demand careful selection, and that selection logic is covered in our alloy steel welding electrodes resource.

Before the Arc Is Struck

Work on hull plating adjacent to fuel oil or ballast tanks cannot begin until the space has been gas freed, tested, and isolated, and a hot work permit has been issued. In-port work additionally requires coordination with the port authority or harbourmaster, notification of adjacent berths, and agreement on dive windows so that traffic, ferry movements, and swinging room are accounted for. The OSHA commercial diving requirements set the baseline for the diving side of that permit; the ship’s own safety management system usually adds layers on top.

Repair type	Likely method	Typical setting
Shell plating insert	Wet SMAW, or cofferdam dry weld for permanent work	Sheltered berth, surveyor attending
Sea chest renewal	Wet weld temporary; cofferdam or habitat permanent	Alongside, tanks gas freed
Bilge keel repair	Wet SMAW build-up and fillet repair	Shallow water, good visibility
Rudder skin patch	Wet weld, dimensional check ashore afterward	Alongside or at anchor
Frame splice	Wet weld from outside, splice fitted to shell	Adjacent ballast tank gas freed
Emergency doubler or crack arrest	Wet SMAW, explicitly temporary	Any accessible damaged area
Propeller blade crack	Not welded afloat; removed and repaired ashore	Repair yard

The shipyard-versus-afloat decision comes down to three questions: is the defect accessible from the water, does it require code-quality fusion and full NDT, and is the vessel’s schedule better served by staying alongside? When the answers are yes, yes, and no, drydocking wins. When they are yes, no, and yes, an afloat repair done to a clearly stated specification is the professional choice.

Safety on an Underwater Welding Job: The Risks Nobody Sees From the Quay

Standing on the quay, an underwater weld looks deceptively calm: a diver in the water, a barge overhead, a cable running to a generator. What the observer cannot see is the stack of hazards stacked between the electrode and the diver’s body. Underwater welding sits at the intersection of two regulated, inherently risky trades, commercial diving and hot work, and the people who survive long careers in it are the ones who treat every job as a dive first and a welding job second.

Electric Shock: The First and Constant Risk

Wet welding puts an energized electrode within arm’s reach of a human being suspended in conductive seawater. Seawater conducts current far more readily than fresh water, so a leakage path that might be tolerable in a shop tank becomes serious in a harbor. The controls are well established:

• Direct current only. Alternating current is not used for underwater wet welding because the risk of shock at the moment of arc initiation is unacceptable.
• Open-circuit voltage limits. Welding sets are specified with low open-circuit voltages suitable for underwater work, and the equipment must be designed for the duty rather than adapted from a land-based machine.
• Insulation and isolation switching. The diver never strikes an arc on their own authority. A surface operator controls a heavy-duty isolation switch that makes and breaks the circuit, and electrode changes happen only with the circuit open.
• Dedicated grounding discipline. The return path is arranged deliberately so current does not wander through the ship’s structure, piping, or the diver.

The regulatory baseline in the United States comes from OSHA’s commercial diving standards, which set requirements for equipment, supervision, and safe work practices. Note that these figures and rules are revised over time, so the current published standard always governs.

Diving Physiology: Risks Independent of Welding

Even with perfect electrical discipline, the dive itself carries hazards that a welder working in a shop never faces. Hyperbaric environments impose real physiological limits:

• Decompression sickness. The primary occupational risk, and one that exists whether or not the diver strikes an arc. Nitrogen dissolved in tissue under pressure must be released gradually, and shortcuts on the dive table can be disabling or fatal.
• Nitrogen narcosis. Judgement degrades at depth in a way the diver often cannot self-assess, which is one reason depth limits and dive supervision matter more than production targets.
• Oxygen toxicity. Breathing gas mixtures at depth requires monitoring, because partial pressure, not just percentage, determines risk.
• Hypothermia. Water pulls heat away from the body far faster than air. A long weld sequence in cold harbor water degrades dexterity long before the diver notices.
• Entrapment, differential pressure, and umbilical loss. Suction from pumps, closing valves that create differential pressure across an opening, or a snagged and severed umbilical are the classic killers. Permit systems, valve logs, and standby divers exist because memory is not a control.

Fume, Gas and Fire in Habitats

Dry hyperbaric work adds a different hazard set. Welding and cutting inside a sealed habitat generate fume that accumulates in a small gas volume, so extraction, gas analysis, and ventilation are mandatory rather than optional. Oxygen enrichment from leaking cylinders, ignition of non-metallic components, and the presence of hydrocarbon vapors all turn a habitat into a fire risk. This is where hot work permit discipline either works or fails: permit issuance, atmospheric testing, fire watch, and rescue arrangements must be in place before the electrode is energized. Understanding shielding and reactive gases generally is worth reviewing in our guide to welding gas supply and safety.

Reported Long-Term Effects

The chronic picture is less dramatic but honestly documented. Divers report a metallic taste during and after underwater welding and cutting, attributed to galvanic breakdown of dental amalgam in the presence of stray current. More substantively, a UK Health and Safety Executive research report investigated possible cognitive and musculoskeletal effects in divers who weld underwater, and HSE diving guidance continues to frame health surveillance as part of good practice. These findings do not disqualify the trade, but they argue for exposure tracking, medical surveillance, and honest career planning.

Who Regulates the Work

The regulatory map differs by jurisdiction and by where the vessel sits. OSHA covers occupational diving under 29 CFR 1910 Subpart T and construction diving under 29 CFR 1926 Subpart Y. Offshore work typically follows IMCA guidance and industry codes, while flag-state and port-state authorities add their own requirements, including hot work notification and salvage or diving contractor approval. Qualifications themselves are framed by standards such as AWS D3.7 and ISO 15618, which cover welder competence rather than dive competence, and electrode selection discipline is familiar territory for anyone who has worked through alloy steel electrode selection and use.

The practical takeaway is simple. Before any underwater weld is scheduled, confirm the dive contractor’s certifications, the permit regime, the standby and rescue arrangements, and the medical surveillance status of the divers. A welder is a commercial diver first, and the dive plan, not the weld procedure, decides whether everyone goes home.

Inspection, Codes and Proving the Weld Is Good

Here is the uncomfortable truth about underwater ship welding: the arc is rarely the hard part. Proving the weld is sound is. Water scatters light, hides the root of a joint and defeats the inspection tools a workshop floor takes for granted, which is why assuring the integrity of wet welds is genuinely difficult. Inspection is treated as a discipline of its own, and the specification matters more than the arc.

Start With the Specification, Not the Dive

The repair document must define three things before anyone suits up: weld class, acceptance criteria and qualifications. AWS D3.7, the Guide for Underwater Welding, classifies underwater welds so acceptance criteria match the intended service. Class A is comparable with surface welds and is realistically only achievable by dry hyperbaric work. Class B is visual-quality work for non-critical service. Class C covers temporary and emergency repairs meant to be replaced at the next yard visit. Selling a wet weld on ferry shell plating as Class A is how owners get surprised at survey.

Around the class sits the paperwork that decides whether a repair is accepted: a qualified welding procedure specification with procedure qualification records, welder qualification to ISO 15618 for wet and dry hyperbaric work, and classification society approval with a surveyor in attendance. Welders entering this field usually start with a formal aluminium welding course or an equivalent steel programme before diving, because the codes do not change once the welder is wet.

Diver Visual Inspection and Its Limits

The first line of assurance is still a diver with eyes and a camera: cleaning the weld to bare metal, removing slag and recording the finished bead on video with a scale in frame. In clear water this catches gross defects like crater cracks, undercut and missed fusion at the toes. The weaknesses are plain: turbidity, silt stirred by the diver’s fins, awkward lighting and the fact that nobody holds position like a machine. Video is evidence, not proof of internal soundness.

Underwater NDT: What Actually Works

Because nobody can see inside a weld underwater, the burden falls on non-destructive methods that survive immersion:

• Ultrasonic thickness gauging is the everyday workhorse: it measures remaining plate thickness around a repair, which is how pitting is tracked and parent material judged before and after welding.
• Magnetic particle inspection uses diver-held probes and fluorescent or visible particles to reveal surface-breaking cracks in ferrous plating.
• Alternating current field measurement (ACFM) is widely used for underwater crack detection. It reads field disturbances caused by an induced current, sizing cracks without stripping coatings or perfect cleaning, a real advantage in a harbor.
• Flooded member detection uses pressure or acoustic methods to find members that have cracked through and filled with water, a routine check on offshore jackets and framed ship structures.
• Cathodic protection potential surveys are commonly combined with weld and coating inspections in a single dive, since CP condition directly affects corrosion rates around any repair.

Documentation: The Paper That Proves It

An underwater repair is only as good as its records: weld maps locating every bead and insert plate on the shell, signed dive reports listing depth, visibility, conditions and electrodes used, NDT reports with A-scan traces or ACFM data, and as-built records for the next special survey. Air arc welding and carbon arc gouging must be logged too, since gouging out a defective bead changes the plate thickness a surveyor will later gauge.

Troubleshooting Common Underwater Weld Defects

Defect	Likely causes underwater	Practical fix
Porosity	Steam and hydrogen in the shielding bubble, fast quench, dirty or painted surface	Clean to bare metal, use low-hydrogen waterproofed electrodes, hold current in range, run smaller beads
Slag inclusion	Poor interpass cleaning, awkward diver position, insufficient bead overlap	Chip and brush between passes, increase overlap, sequence beads so each washes the toes
Lack of fusion	Low current, fast travel, poor fit-up, arc wandering in the bubble	Raise current within procedure, slow the drag, fix fit-up gaps instead of filling them
Undercut	Excessive current, wrong electrode angle, arc too long as the diver drifts	Reduce current, correct angle, cover pass along the toes
Cracking	High carbon equivalent steel, hydrogen pickup, rapid cooling, restrained joint	Do not weld wet: preheat is unreliable, use dry hyperbaric or cofferdam with controlled cooling

Welding over coatings or marine growth is the most common root cause behind the first three rows, and proper surface preparation and cleaning discipline prevents it before the arc is struck.

None of this changes the central lesson: specify the weld class up front, qualify the procedure and the diver, inspect with the best method the water allows, and write down what you did. An underwater weld that cannot be inspected to its acceptance criteria should never be signed off as a permanent repair.

Cost, Planning and the Mistakes That Ruin a Repair

The arc time on a hull patch is rarely the expensive part. What owners actually pay for is everything that surrounds those beads: the dive spread, the conditions, the paperwork and the proof that the weld is sound. Understanding that cost structure is what separates a smart afloat repair from a bill that ends up exceeding the drydocking you were trying to avoid.

What Drives the Cost of an Afloat Repair

• Dive spread size. A support vessel or work barge, dive team, compressor or gas panel, hot-water suit supply, umbilicals, lighting, welding sets and a tender all bill by the day, whether the diver is burning electrode or waiting out a tide.
• Depth. Greater depth means longer bottom times, more gas consumption, stricter decompression schedules and fewer available divers per shift.
• Water temperature and visibility. Cold water shortens productive bottom time and pushes heating costs up. Poor visibility turns a four-hour job into a full day of cleaning and摸索 by touch.
• Weather and tidal windows. Current and sea state decide when a diver can work at all. Lost days are usually absorbed by the client, not the contractor.
• Port charges, permits and hot work administration. Port authority notifications, line handling and permits add fixed cost before anyone gets in the water.
• Inspection and class attendance. NDT, surveyor day rates and procedure or welder qualification are significant line items. Labour is not cheap either: AWS reports entry-level underwater welder pay around $62,000 and a national median near $68,000 based on 2025 Lightcast data, with experienced saturation divers earning well above that.
• The cost of failure. A rejected weld means re-diving, gouging out, re-welding and re-inspecting, usually at the contractor’s cost or in a dispute. In the worst case it forces an emergency drydocking.

When Drydocking Wins

Afloat repair is not the cheaper option by default. It wins for a single, well-defined area of sound steel in reachable depth with tolerable visibility. Drydocking wins when any of the following apply:

Factor	Afloat repair	Drydocking
Cost shape	Labour, spread and inspection heavy	Yard, scaffolding and lost sailings
Weld quality ceiling	Wet welds limited; class may accept only temporary repairs	Dry, positional, code-quality welds with full NDT
Inspection	Visual and limited NDT underwater	Any method, including radiography
Scope	Best for one or two small areas	Wins when steel renewal is extensive or spread over several locations
Other work	None possible	Coating, anode renewal, propeller and running gear work at the same time

If the class surveyor cannot accept the weld class required in the wet condition, the decision has already been made for you. Codes and acceptance criteria sit in documents such as AWS’s codes and standards publications, and the specification must state what is being bought.

Mistakes That Ruin a Repair

1. Welding over paint, rust or marine growth. Contamination produces porosity and lack of fusion. Clean to bare, bright metal, as covered in our AJ Marine welding guide.
2. Wrong polarity or current. Too little current gives lack of fusion; too much burns through thin plating and worsens the hydrogen problem.
3. Insufficient overlap between beads. Cold-lapping adjacent runs leaves channels that leak and crack. Positional work is unforgiving, so follow a proven all position welding rod guide when the joint will not sit flat.
4. No qualified procedure. A job welded without an approved WPS and qualified welder has no defensible acceptance basis at the next survey.
5. Bad grounding. Stray welding current can damage nearby equipment and accelerate corrosion if the grounding path is wrong.
6. Ignoring cathodic protection. Anodes that protect the hull can make arc initiation unstable, and welding near them can disrupt the system’s potential.
7. Wetting high-carbon-equivalent or high-strength steel. Rapid quench plus hydrogen is a cracking recipe; see our notes on alloy steel welding electrodes for why filler selection matters.
8. Skipping the hot work permit and adjacent tank gas freeing.
9. No post-repair inspection. An undocumented weld is, to a surveyor, an unwelded one.
10. Unrealistic schedules. Compressing a dive plan to meet a sailing date is how shortcuts happen.

The Superintendent’s Pre-Job Checklist

• Define the weld class required, and whether it is temporary or permanent.
• Verify procedure and welder qualification for underwater work.
• Agree acceptance criteria with the class in writing before the dive.
• Plan the NDT method and who pays for it.
• Document everything: weld map, dive report, NDT results and as-built records for the next survey.

Do that, and the question of whether the ship can be welded afloat stops being a gamble and becomes a straightforward engineering decision.

Careers, Technology and the Future of Afloat Repair

If you have read this far, you already know that underwater ship welding sits at an unusual crossroads. It asks one person to be a competent welder and a competent commercial diver at the same time, and then to perform in water where visibility, temperature and time on the bottom are all working against them. That combination is exactly why experienced personnel stay in short supply, and why the career remains one of the better-paid technical trades in the marine sector.

The Career Path, Honestly Described

The route is rarely short. Most people build a welding foundation first, in a shop or a trade program, learning arc basics, metallurgy and positional work on dry land where mistakes are cheap. An Aluminium Welding Course: What You Learn, Costs & Careers is a reasonable example of the sort of structured grounding that helps before anything aquatic begins. Only then does commercial diving school start: physics and physiology, dive tables, mixed gases, underwater cutting, chamber operations and the rescue drills that make up the real bulk of the training.

After certification, the first years are typically harbor and nearshore work, the kind of shallow, muddy, low-visibility job described throughout this article. Offshore contracts and saturation work come later, and with them longer decompression schedules, larger teams and considerably more money. The trade-offs are physical. Diving is demanding on joints and backs, schedules are irregular, and long-term health monitoring matters. The AWS guidance on the underwater welder career path is a sensible starting point for anyone weighing the commitment, alongside the UK HSE diving resources on competence and safety expectations.

What Is Changing Underwater

Technology is moving, though slower than marketing suggests. Mechanized and automated hyperbaric welding systems have been developed for pipeline and deepwater work, where consistency beats a diver’s hand in cold, dark conditions. Friction stitch welding has emerged as a deepwater repair technique that avoids conventional arc welding in water altogether, joining material below the melting point and sidestepping the quench and hydrogen problems that limit wet welds. As summarised in the overview of hyperbaric welding, dry habitat work remains the route to code-quality results, while wet welding stays the pragmatic option for shallow, non-critical repairs.

On the support side, ROV-deployed cleaning and cutting tooling, better underwater magnetic particle and ACFM inspection, and digital weld documentation are gradually replacing paper dive reports and photo folders. Records captured on the diver’s tablet now flow straight into the vessel’s maintenance file, which matters enormously at the next survey. Related advances in Alpha Laser Welding: Systems, Uses, and Safety Guide show how precision heat sources are reshaping repair work on land; underwater, the equivalent gains come from automation and sensing rather than handheld devices.

Where the Industry Is Heading

Expect more condition monitoring and predictive repair, with ultrasonic and corrosion data triggering interventions before survey deadlines do. Tighter emissions and coating regulations are changing hull maintenance cycles too, since a cleaner, better-coated hull needs less emergency steelwork. The skills shortage is unlikely to resolve itself, which keeps leverage with people who can prove both dive and weld qualifications. Regulatory expectations are firm and getting clearer, from OSHA’s commercial diving rules to flag-state and port-state requirements.

Closing Checklist: Afloat or Ashore?

• Weld afloat for shallow, accessible, non-critical work where downtime cost dominates and class accepts a wet or temporary repair.
• Dock the vessel when the joint is structural and critical, the steel is high-strength or high carbon equivalent, inspection access is poor, or permanent code-quality results are required.
• Fix the specification first: weld class, acceptance criteria, qualified procedure, qualified welder, approved NDT plan.
• Document everything: weld maps, dive reports, NDT results and as-built records for the next survey.
• Never skip the permit, the isolation check or the post-repair inspection.

No new robot, coating or sensor removes the need for qualified people and honest inspection. The future of underwater ship welding is better tools in the hands of divers who still know when the right answer is to stop, surface, and tow the job to a yard.

Frequently Asked Questions

Can you really weld a ship while it is still in the water?

Yes. Divers can weld hull plating, sea chests, bilge keels and rudder areas afloat using either wet welding or a dry habitat or cofferdam. Whether it is allowed depends on the repair’s criticality, the classification society’s requirements, and the weld quality specified.

Is underwater welding as strong as welding in a shipyard?

Not usually with wet welding, which tends to produce harder, more porous welds with less consistent fusion. Dry hyperbaric welding in a habitat gets much closer to yard quality because heat input and cooling can be controlled, but it costs considerably more.

How deep can underwater ship welding be done?

Most ship repair work happens in shallow harbors and the splash zone, often under 50 m. Wet welding becomes progressively harder and less reliable with depth, while dry hyperbaric welding has been performed operationally to around 400 m, limited mainly by diver physiology.

Why can’t underwater welders use alternating current?

Alternating current increases the shock hazard in a conductive medium and offers no practical advantage underwater. Wet welding practice uses direct current with a surface-controlled isolation switch so the circuit is only live during the actual weld.

Do underwater welds get inspected?

Yes, but with limits. Divers perform visual inspection with video records, and magnetic particle, alternating current field measurement and ultrasonic thickness methods are used underwater. Radiography is generally impractical, which is why wet welds carry more inspection uncertainty.

Is underwater welding legal for permanent ship repairs?

It can be, if the classification society approves the procedure and the weld meets the specified class. Many afloat welds are accepted only as temporary repairs to allow the vessel to reach a yard, so the approval status must be confirmed before work starts.

How much does an underwater ship welding repair cost?

There is no standard price. Cost depends on the dive spread, water depth and conditions, whether a habitat is needed, NDT and class attendance, and mobilization. Even a modest afloat repair can be expensive, but it is often far cheaper than an unscheduled drydocking plus lost operating days.

Can propellers or castings be welded underwater?

Generally no. Propellers and many cast components involve alloys and stress conditions that make in-water welding unreliable, and they also need balancing and detailed inspection. They are normally repaired or replaced ashore.

What qualifications should I ask for before hiring an underwater welding contractor?

Ask for commercial diver certification from an accredited school, welding certification and procedure-specific qualification to ISO 15618 or an equivalent standard, approved welding procedures, insurance, dive team composition, and a documented NDT plan.