Underwater cutting and welding

Last updated December 20, 2023

Underwater cutting and welding are metalworking techniques used by underwater divers in underwater construction, marine salvage and clearance diving applications. Most underwater welding is direct current wet stick welding, and most underwater metal cutting is immersed oxygen-arc and shielded metal-arc cutting, though other technologies are available and sometimes used. These processes are mostly applied to steel structures as that is the most common arc-weldable material used in the underwater environment.^[1]

Scope

Oxygen arc cutting and arc welding underwater requires greater skill and stamina than working in a dry and 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}

Applications

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 maritime accident. 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, labor-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]

Underwater cutting

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}

It is easier to cut clean metal 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 be additionally used. 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 sledgehammer may sufficiently loosen scale on the opposite side.^[1]^{: Ch. 2}

Oxygen-arc cutting

Oxygen-arc cutting, also referred to as burning, is a process in which the metal is cut by oxidizing the heated metal, which is then blown away by the gas, it can use steel tubular electrodes or exothermic electrodes, which continue to burn independently of the arc once ignited and the oxygen is flowing.^[1]^{: Ch. 1} Oxygen-arc is preferred where practicable because it cuts plain and low carbon steel well. It is usually used with a constant current DC welding generator set on straight polarity, and the arc should ignite as the electrode touches the work. The tip of the electrode is consumed and must be replaced frequently as it burns down to a stub.

The tubular steel electrode consists of a steel tube with a waterproofed flux coating which is applied during manufacture. A typical electrode is 14 inches long with a 5/16-inch outer diameter and a bore diameter of slightly less than 1/8 inch. The waterproof flux coating is similar to the coating on welding electrodes, and both promotes easy striking and maintenance of the arc and the gases it produces form a bubble around the arc which keeps the water off for long enough to burn the heated metal under the arc. The flux is also an electrical insulator, which protects the diver in case of accidentally touching the electrode, and it prevents side arcing in tight spaces. ^[1]^{: Ch. 2}

Exothermic electrodes will continue to burn once ignited as long as the oxygen flows, so the diver may have the current switched off after starting the cut, although the system will cut metals faster if the power remains on as the arc produces additional heat. The exothermic electrodes can melt or burn through almost any materials after they have been ignited.^[1]^{: Ch. 2}

Exothermic electrode cutting is simple and easy to learn, and can be used to cut thin metal when the power is off. Cutting speed is fast and ferrous and non-ferrous metals of any thickness, and a wide range of non-conductive materials like concrete and rock can be cut. They do not require a high operating current, and cam be ignited from a 12 volt automotive starter battery.^[1]^{: Ch. 2}

Shielded metal arc cutting

Shielded metal arc cutting cuts the metal by melting it with the intense heat of the arc, without burning it with oxygen, which works better for carbon steel plate less than 6mm thick and for non-ferrous and corrosion resistant metals of any thickness.^[1]^{: Ch. 1}

Underwater shielded metal arc cutting is a simple process. Almost any waterproofed mild steel stick-type electrode can be used and can cut corrosion resistant steels and non-ferrous metals of any thickness. No oxygen is required, and the heat of the arc simply melts the metal which either drains away by gravity pushed out of the kerf by the diver using the electrode tip.^[1]^{: Ch. 1} Relying on gravity is not preferred because the water cools down the molten metal too fast.

Underwater welding

Wet welding is done with both the diver and the workpiece completely submerged, where shielded metal arc welding is the most widely used process.^[1]^{: Ch. 1} Most wet welding is done using direct current (DC) 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}

Shielded metal arc welding

Shielded metal arc welding is produced by heating with an electric arc created between a flux-covered metal electrode and the work. The arc creates intense heat, generally between 7,000 to 11,000 °F (3,900 to 6,100 °C), concentrated in a very small area. It results in melting of the parent metal parts, the core wire and some of the flux covering. Other elements of the flux covering decompose to form a gaseous shield around the arc. This shield protects the molten metals from contamination by the surrounding atmosphere, in this case mainly superheated steam. As the electrode melts, small drops or globules of molten metal are formed, forced across the arc and deposited on the work into a molten pool which solidifies, forming a bead of weld metal. The drops or globules do not simply fall into the pool by gravity but are forced by the electric current flow. Otherwise, overhead welding would not be possible.^[1]^{: Ch. 3}

The wet welding process used for salvage operations is usually a simple underwater joining technique. The equipment used is commercially manufactured shielded metal arc equipment and waterproofed electrodes. Minimal ancillary devices are needed. These include lighting, staging and hand tools. The advantages to wet welding are that the underwater welder can work freely on any portion of complex structures or on sections with restricted access, whereas other underwater welding techniques may encounter access difficulties. Patching can be done faster and at less cost because no time is lost in construction and installation of enclosures. Standard welding power sources and equipment are used, so a wet welding job can be easily initiated at remote job sites. Wet welding also allows more freedom of patch design and size of patch sections.^[1]^{: Ch. 3}

Equipment

Underwater oxygen arc cutting, shielded metal arc cutting, and wet welding are performed with essentially 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 immediately evident 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.

Safety

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 become 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 of their 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}

Related Research Articles

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as brazing and soldering, which do not melt the base metal.

Shielded metal arc welding (SMAW), also known as manual metal arc welding, flux shielded arc welding or informally as stick welding, is a manual arc welding process that uses a consumable electrode covered with a flux to lay the weld.

Submerged arc welding (SAW) is a common arc welding process. The first SAW patent was taken out in 1935. The process requires a continuously fed consumable solid or tubular electrode. The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.

<span class="mw-page-title-main">Arc welding</span> Process used to fuse metal by using heat from an electrical arc

Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals, when cool, result in a binding of the metals. It is a type of welding that uses a welding power supply to create an electric arc between a metal stick ("electrode") and the base material to melt the metals at the point of contact. Arc welding power supplies can deliver either direct (DC) or alternating (AC) current to the work, while consumable or non-consumable electrodes are used.

MAPP gas was a trademarked name, belonging to The Linde Group, a division of the former global chemical giant Union Carbide, for a fuel gas based on a stabilized mixture of methylacetylene (propyne), propadiene and propane. The name comes from the original chemical composition, methylacetylene-propadiene propane. "MAPP gas" is also widely used as a generic name for UN 1060 stabilised methylacetylene-propadiene.

Plasma cutting is a process that cuts through electrically conductive materials by means of an accelerated jet of hot plasma. Typical materials cut with a plasma torch include steel, stainless steel, aluminum, brass and copper, although other conductive metals may be cut as well. Plasma cutting is often used in fabrication shops, automotive repair and restoration, industrial construction, and salvage and scrapping operations. Due to the high speed and precision cuts combined with low cost, plasma cutting sees widespread use from large-scale industrial computer numerical control (CNC) applications down to small hobbyist shops.

A cofferdam is an enclosure built within a body of water to allow the enclosed area to be pumped out or drained. This pumping creates a dry working environment so that the work can be carried out safely. Cofferdams are commonly used for construction or repair of permanent dams, oil platforms, bridge piers, etc., built within water.

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas. A filler metal is normally used, though some welds, known as 'autogenous welds', or 'fusion welds' do not require it. When helium is used, this is known as heliarc welding. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. TIG welding is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing stronger, higher-quality welds. However, TIG welding is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.

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Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, most notably gas metal arc welding and gas tungsten arc welding. Their purpose is to protect the weld area from oxygen, and water vapour. Depending on the materials being welded, these atmospheric gases can reduce the quality of the weld or make the welding more difficult. Other arc welding processes use alternative methods of protecting the weld from the atmosphere as well – shielded metal arc welding, for example, uses an electrode covered in a flux that produces carbon dioxide when consumed, a semi-inert gas that is an acceptable shielding gas for welding steel.

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References

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 U.S. Navy Underwater Cutting and Welding Manual (PDF) (Change B ed.). Washington, D.C.: Department of the Navy, Naval Sea Systems Command. 1 June 2002.{{cite book}}: |work= ignored (help) This article incorporates text from this source, which is in the public domain .
↑ Brown, J. Mariah (27 January 2011). "Underwater Construction". buildipedia.com. Retrieved 1 August 2020.
↑ "Civil underwater construction". www.ducmarinegroup.com. Retrieved 1 August 2020.
↑ "Our industry". imca-int.com. International Marine Contractors Association . Retrieved 15 September 2020.
↑ Urbina, Ian (December 28, 2015). "Maritime 'Repo Men': A Last Resort for Stolen Ships (Published 2015)." The New York Times". The New York Times. Archived from the original on 8 November 2020. Retrieved 12 October 2020.
↑ U.S. Navy Salvage Manual (PDF). Vol. 1: Strandings, Harbor Clearance and Afloat Salvage S0300-A6-MAN-010. United States. Navy Department. Supervisor of Salvage and Diving. 31 May 2013. Archived (PDF) from the original on 5 April 2023. Retrieved 28 March 2023.
↑ Nelson, Mark. "Ship's Husbandry". Jackspeak of the Royal Canadian Navy. ReadyAyeReady.com. Retrieved 19 March 2017.
↑ Staff. "1: Hull Maintenance". Ship Husbandry (PDF). Royal Australian Navy Apprentice Training Establishment.
↑ Jensen, Milinda D. (3 March 2004). "Underwater Ship Husbandry Combines Navy, Civilian Divers into Successful Team". Story Number: NNS040303-02. Naval Sea Systems Command Public Affairs. Retrieved 19 March 2017.
↑ Staff (2014). "Naval Vessel Husbandry". Strategy Logistics. Retrieved 19 March 2017.

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[USN_Cutting_and_welding_manual-1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 U.S. Navy Underwater Cutting and Welding Manual (PDF) (Change B ed.). Washington, D.C.: Department of the Navy, Naval Sea Systems Command. 1 June 2002.{{cite book}}: |work= ignored (help) This article incorporates text from this source, which is in the public domain .

[Brown_2011-2] Brown, J. Mariah (27 January 2011). "Underwater Construction". buildipedia.com. Retrieved 1 August 2020.

[DUC-3] "Civil underwater construction". www.ducmarinegroup.com. Retrieved 1 August 2020.

[IMCA-4] "Our industry". imca-int.com. International Marine Contractors Association . Retrieved 15 September 2020.

[NYT_2015-5] Urbina, Ian (December 28, 2015). "Maritime 'Repo Men': A Last Resort for Stolen Ships (Published 2015)." The New York Times". The New York Times. Archived from the original on 8 November 2020. Retrieved 12 October 2020.

[Salvage_manual_010-6] U.S. Navy Salvage Manual (PDF). Vol. 1: Strandings, Harbor Clearance and Afloat Salvage S0300-A6-MAN-010. United States. Navy Department. Supervisor of Salvage and Diving. 31 May 2013. Archived (PDF) from the original on 5 April 2023. Retrieved 28 March 2023.

[Nelson-7] Nelson, Mark. "Ship's Husbandry". Jackspeak of the Royal Canadian Navy. ReadyAyeReady.com. Retrieved 19 March 2017.

[boatregister-8] Staff. "1: Hull Maintenance". Ship Husbandry (PDF). Royal Australian Navy Apprentice Training Establishment.

[Jensen_2004-9] Jensen, Milinda D. (3 March 2004). "Underwater Ship Husbandry Combines Navy, Civilian Divers into Successful Team". Story Number: NNS040303-02. Naval Sea Systems Command Public Affairs. Retrieved 19 March 2017.

[Stratlog-10] Staff (2014). "Naval Vessel Husbandry". Strategy Logistics. Retrieved 19 March 2017.

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