Underwater cutting and welding

Oxygen arc cutting and arc welding underwater require greater skill and stamina than the same work in a dry end stable environment. The underwater environment imposes several limitations and restrictions on both the equipment and the operator, and the restriction of short bottom times at greater depths for surface oriented divers makes efficient working important to getting the job done in a reasonable time.[1]^{: Ch. 1}

The diving equipment encumbers the diver, water movement, unstable footing, and restricted visibility make it worse. The rapid cooling of water makes good weld quality difficult to achieve, so when practicable solutions using cutting are preferred to solutions requiring welding in salvage operations.[1]^{: Ch. 1}

Underwater construction is industrial construction in an underwater environment. There is often, but not necessarily, a significant component of commercial diving involved.[2][3] It is a part of the marine construction industry.[4] Underwater welding may be used, and for repair work cutting of steel or concrete may be necessary.

Marine salvage is the process of recovering a ship and its cargo after a shipwreck or other maritime casualty. Salvage may encompass towing, re-floating a vessel, or effecting repairs to a ship. Protecting the coastal environment from spillage of oil or other contaminants is also a high priority. Before the invention of radio, salvage services would be given to a stricken vessel by any ship that happened to be passing by. Most salvage is carried out by specialist salvage firms with dedicated crew and equipment.[5]

The techniques applied in marine salvage are largely a matter of adapting available materials and equipment to the situation, which is often constrained by urgency, weather and sea conditions, accessibility of the site, and financial considerations. Diving is slow, labour-intensive, dangerous, expensive, constrained by conditions, and often inefficient, but may be the only, or most efficient, way to do some tasks.[6]^{: Ch. 4}

Ships husbandry is all aspects of maintenance, cleaning,[7] and general upkeep of the hull, rigging, and equipment of a ship. It may also be used to refer to aspects of maintenance which are not specifically covered by the technical departments.[8] The term is used in both naval and merchant shipping,[9] but naval vessel husbandry may also be used for specific reference to naval vessels.[10]

Maintenance and repair of marine structures like submarine pipelines

Diver cutting through a steel beam during welding operations

The most commonly used of the available technologies are oxygen-arc cutting and shielded metal arc cutting. For arc cutting and welding, the workpiece must be cleaned and grounded before an arc can be struck, and the quality of the cut will depend on the surface condition of the workpiece. The ground clamp must be firmly attached to bare clean metal.[1]^{: Ch. 2}

Clean metal cuts better than corroded or growth-encrusted metal. Steel covered with ordinary mill scale and one or two thin coats of paint may be cut easily. Thick scale, thick paint, barnacles and similar marine growth make cutting difficult and should be removed first. An ordinary paint scrapper is useful in removing light scale and paint, but for heavier growths, a high-pressure water jet cleaning tool may perform the cleaning process satisfactorily. If possible, both sides of the metal should be cleaned before cutting. Fouling and heavy corrosion on the back of the metal can clog the cut and prevent the cutting jet from blowing through. If inaccessible, striking the area to be cut with a heavy sledge hammer may sufficiently loosen scale on the opposite side.[1]^{: Ch. 2}

Attaching a grounding cable with a clamp

Wet welding is done with both the diver and the work completely submerged, and shielded metal arc welding is the most widely used process.[1]^{: Ch. 1} Most wet welding is done using direct current with the electrode negative, also known as straight polarity, as the electrode holder lasts longer with this polarity, but sometimes better results can be achieved with DC electrode positive, also called reverse polarity.[1]^{: Ch. 3} Alternating current is not used for safety reasons.[1]^{: Ch. 2}

Diver uses a grinder to trim down a repair patch

Underwater oxygen arc cutting, shielded metal arc cutting, and wet welding are performed with basically the same high current DC welding equipment, to which special electrode holders and an oxygen supply are added for Oxygen-arc cutting. A knife type safety switch is recommended for making and breaking the welding circuit as it is obvious at a glance whether it is closed or open. Additional accessories include scrapers, grinders, and wire brushes high pressure waterjetting and abrasive waterjetting equipment which are used for surface preparation for both underwater cutting and welding operations.[1]^{: Ch. 4}

The diving equipment used is basic surface-supplied diving equipment with voice communications, fitted with welding shields that can be hinged down over the faceplate to protect the diver's vision. Suitable heavy duty rubber gloves and overalls may be used in addition to a diving suit appropriate to the general underwater conditions.[1]^{: Ch. 4}

Rigging and access equipment may be needed appropriate to the specific task.

Serious injury or death may result if adequate precautions are not followed during underwater cutting or welding operations.[1]^{: Ch. 3}

The life-threatening hazards of underwater cutting and welding are associated with the electrical current and gases used or generated during the process:

• If the diver passes between the position of the ground connection and the electrode, they will becom part of the electrical circuit, which can cause electrical shock injury. Electrical shock produced by alternating current can prevent voluntary relaxation of the muscles controlling the hand. Consequently, the diver may be unable to let go if his body or equipment accidentally becomes part of an AC electrical circuit, so AC power is not used for underwater cutting or welding. DC shock is less dangerous, but still unpleasant at the least. Keeping out of the circuit requires continual attention and awareness by the diver.[1]^{: Ch. 3}
• Extremely hot electrodes and workpieces can burn the diver through protective clothing and do severe damage to diving equipment, such as umbilical hoses.[1]^{: Ch. 3}
• The welding current generates explosive gas mixtures, which may be trapped in adjacent structure and accumulate, and if detonated could harm the diver and other equipment and structure.[1]^{: Ch. 3}
• Welding currents may cause accelerated galvanic corrosion.[1]^{: Ch. 3}
• Oxygen partial pressure increases with water depth, thereby creating a fire hazard in habitat environments. At depths greater than 60 FSW, gas filled welding habitats should be filled with an inert gas.[1]^{: Ch. 3}

• Marine salvage – Recovering a ship or cargo after a maritime casualty
• Underwater work – Work done underwater during diving operations
• Clearance diver – Navy diver specialist with explosives
• Commercial diving – Professional diving on industrial projects
• Offshore diving – Diving outside the territorial waters of a country
• Underwater construction – Industrial construction in an underwater environment