Joining Metals Down Below
Engineers have known of the welding arc’s underwater operation capacity for over a century. Van der Willingen developed waterproof electrodes in 1946 and British Admiralty executed the first underwater weld. Steel is the most commonly welded material underwater.
Used earlier for emergency repairs and salvage operations at less than 9m depths, underwater welding is now used to build super-sized ships, ship maintenance-repair, and installation-maintenance-repair of offshore structures viz. pipelines, oil rigs, and platforms that have recently come up in large numbers.
Marine or Underwater Welding eliminates the need to pull the structure out of water saving tons of time, money, and complications. But it is tougher than in-air welding because greater pressure, higher cooling rates, and weld-joint hydrogen content make the arc unstable and the joint porous and less tough.
Types, Pros, & Cons
Fusion welding processes for deep-sea construction employ free-burning arc operated in the localized dry region created around the weld zone at elevated pressures. Free-Burning Arcs mainly operate in the surrounding gas and, secondarily, in the vapor they generate. Its plasma is formed freely in space.
Underwater Welding types:
- Wet Welding: executed without any physical barrier between water and weld joint with water-proof welding-rods i.e. electrodes. The arc burns in the cavity formed inside the flux covering designed to burn slower than the electrode’s metal barrel
Practiced up to 2,500m depth, it uses Shielded Metal Arc Welding (SMAW) / Stick Welding and Flux Cored Arc Welding (FCAW)
- Dry Welding: enacted in dry, sealed chambers created around the weld-zone. Gas Tungsten Arc Welding (GTAW) is most popularly employed. Such welds are currently executed up to 300m depths
Chambers are normally made of steel and customized to accommodate structural members. The dry, warm chamber with its environmental control system (ECS) enables controlled conditions for welding and superior post-weld heat treatments, greater diver safety, and non-destructive weld-joint testing
- Local Cavity Welding: implemented in a local dry chamber. Gas Metal Arc Welding (GMAW) is largely employed although Laser Welding is also used
Weld properties are better than that of wet welding. It meets classification society requirements for up to 200m depth. Weld-joint hydrogen-content is 5-21ml/100g-Fe. The method does not permit making welding process observations
Dry Welding Types:
- Hyperbaric Welding: executed in sealed chambers with a breathe-able helium-oxygen mixture at or slightly above ambient pressure. GTAW is mostly used for quality welds complying with code and X-Ray requirements
- Cofferdam Welding: enacted in closed-bottom-open-top enclosures at one atmosphere pressure
- Dry Welding at One Atmosphere: in chambers maintained at one atmospheric pressure
- Dry Habitat Welding: performed at ambient water pressure in water-free chambers large enough to accommodate diver-welders completely
- Dry Spot Welding: implemented in gas-filled chambers large enough only for diver-welder’s arm
- Dry Chamber Welding: practiced at ambient pressure in chambers with space only for the diver-welder’s head and shoulders
Diver-Welders have greater freedom of movement during wet welding. This allows them to access spots not reached under dry welding making wet welding more efficient, effective, and economical. It is used for emergency welding
because it does not require the time-consuming setup of dry chambers.
Rapid Quenching of wet-welded weld-joints by surrounding water lowers its impact-strength and ductility while increasing undesired porosity. Quenching is rapid cooling of heated materials that also increases tensile strength. In the operating 5000C-8000C temperature range, cooling rate is 560C-4150C/s.
Another challenge is Hydrogen Embrittlement. Hydrogen dissolves in weld-joint and the heat affected zone (HAZ) making the joint brittle and fissure-crack prone. Molecular hydrogen, water-vapor, and carbon-monoxide cause porosity. Electrode covering, water depth, and arc stability determine intensity of porosity.
Dry welding chambers provide safety for diver-welders, controlled welding environment, better surface monitoring viz. pipe alignment and joint preparation, and non-destructive testing (NDT) with immediate defect correction. This provides high-quality welds free from quenching and embrittlement.
Cost is dry welding’s major limitation. The arc loses intensity at greater depths thereby increasing voltage requirements and expenses. A single weld job costs around $80,000 and the custom-made chambers are normally discarded after operations.
Surprisingly, drowning risk is not considered. Perhaps because drowning is the first barrier and is factored in standard practices. The depth, exhausting nature, number of dives, time spent underwater, and dive-repetitiveness aggravate the hazards of:
- Electric Shocks: minimized through safe electrical procedures
- Explosion: neutralized by preventing hydrogen and oxygen gas-pocket build up in wet welding
- Nitrogen Narcosis: the drowsiness caused by inhaling air under-pressure at below 100feet depth when nitrogen enters bloodstream. Decompression chambers, emergency air supply, and stand-by divers reduce this hazard
- Undetected Defects: inspection of wet-welded joints is tough
An ideal underwater welding process must provide reliable welds through flexible, cost-effective operations with good visibility and allow the operator to support himself while welding. Equipment must be inexpensive, electric-shock proof, provide 20cm/min welding speed, and automation-compatible.