Line pipes can be connected by mechanical connectors or welding. Threaded and coupling (T&C) or pin and box connectors are used for drilling riser and top tensioned riser connections. However, welding is more commonly used for offshore pipelines due to its proven technology and lower cost than mechanical connectors.

The range of welding techniques used for pipeline construction includes Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Submerged Arc Welding (SAW), Flux Cored Arc Welding (FCAW) and Gas Tungsten Arc Welding (GTAW). SAW is used on third generation lay barges for double jointing. SAW is a high heat input, high dilution process. Therefore, the chemistry of the linepipe being welded has a large influence on the properties of the final weld.

Another type of welding that can be done underwater is Hyperbaric welding. Hyperbaric welding is the process of welding at elevated pressures, normally underwater. Hyperbaric welding can either take place wet in the water itself or dry inside a specially constructed positive pressure enclosure and hence a dry environment. It is predominantly referred to as "hyperbaric welding" when used in a dry environment, and "underwater welding" when in a wet environment. The applications of hyperbaric welding are diverse—it is often used to repair ships, offshore oil platforms, and pipelines. Steel is the most common material welded.

Wet underwater welding directly exposes the diver and electrode to the water and surrounding elements. Divers usually use around 300–400 amps of direct current to power their electrode, and they weld using varied forms of arc welding. This practice commonly uses a variation of shielded metal arc welding, employing a waterproof electrode. Other processes that are used include flux-cored arc welding and friction welding. In each of these cases, the welding power supply is connected to the welding equipment through cables and hoses. The process is generally limited to low carbon equivalent steels, especially at greater depths, because of hydrogen-caused cracking.

The hazards of underwater welding include the risk of electric shock to the welder. To prevent this, the welding equipment must be adaptable to a marine environment, properly insulated and the welding current must be controlled. Commercial divers must also consider the occupational safety issues that divers face; most notably, the risk of decompression sickness due to the increased pressure of breathing gases. Many divers have reported a metallic taste that is related to the galvanic breakdown of dental amalgam. There may also be long term cognitive and possibly musculoskeletal effects associated with underwater welding. Therefore, underwater welder should be highly certified.

Video of how underwater welding is done can be seen in the video below:

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Source:

Bai, Yong and Bai, Qiang. Subsea Pipelines And Risers. USA: Elsevier Inc. 2005.

https://www.youtube.com/watch?v=aE4Dtl2IBJo

https://en.wikipedia.org/wiki/Hyperbaric_welding