AWS WHC-4.10:2010

Underwater Welding and Cutting
Scope : When the exploration and production of energy resources moved offshore, the petroleum industry provided a significant but challenging incentive for the further development of underwater welding technology. Prior to this, underwater â??wet weldingâ? was used infrequently and with highly unpredictable results. Hyperbaric dry welding was not yet developed. Underwater cutting was generally limited to salvage projects and the removal of piling and inland waterway obstructions. The need for reliable underwater welding repair methods increased as the number of offshore structures increased, incurring accidental damage during and after installation and metal fatigue and corrosion as the structures aged. As the materials used in offshore structures and subsea pipelines escalated from readily weldable low-carbon steel to high-strength steels that were susceptible to hydrogen-induced cracking, the development of new and better welding materials and procedures kept pace. Some of the notable events in the progress of underwater welding and cutting are recorded in Figure 10.1. From a metallurgical perspective, producing a sound weld underwater is comparatively complex. However, wet welding processes, such as shielded metal arc welding (SMAW) and flux cored arc welding (FCAW), closely resemble welding in air in that the welding arc and molten metal are shielded from the environment (water or air) by gas and slag produced by decomposition of the flux-coated electrodes or flux-cored wire. The welding of a ring stiffener to a tubular member is an example of wet welding, as shown on the cover page of this chapter. Underwater dry welding, defined as any welding in which water is excluded from the immediate vicinity of the arc by a mechanical barrier, is done at ambient pressure in a chamber from which water has been displaced.1, 2, 3 Depending on the size and configuration of the chamber, the welder/diver may be completely in the chamber or only partially in the chamber, and may work in conventional welding attire, diving gear, or a combination of both. Underwater welding and cutting processes are used during the installation of new offshore structures, subsea pipelines and hot taps, docks and harbor facilities, and also for additions or modifications to underwater structures. However, these processes are most often required for repairs to existing structures, such as the following: 1. Repair of damage to offshore structures caused by corrosion and fatigue; 2. Repair or replacement of structural members damaged during installation, by objects falling overboard from boats, boat collisions, or other accidental damage; 3. Repair or replacement of damaged subsea pipeline sections and pipeline manifolds; 4. Repair of corrosion and collision damage to harbor facilities such as sheet and H-piling, and to cover openings resulting when sheet pile has been driven out of interlock during construction; 5. Repair and replacement of tubular braces and supports of docks and mooring dolphins for tankers; 6. Permanent or temporary repairs to holes in ship and barge hulls and to hulls and pontoons of semisubmersible oil drilling ships; and 7. Repair of damage to steam dryers, feedwater spargers, internal reactor components, and leaks in pool liners in nuclear power plants. This chapter covers wet welding and dry welding and the processes and methods that are most commonly used. Properties of weldments and specifications for qualifying welders and welding procedures are referenced, and inspection methods and procedures are discussed. Although the arc welding processes are the most commonly used, friction welding (FRW) is in use in some underwater applications. Friction-related processes such as friction stir welding (FSW) and friction stitch welding, considered experimental by most during the first decade of the 2000s, will probably become accepted practice in underwater applications. However, the arc welding processes remain popular because of their simplicity and ease of deployment. Underwater cutting processes are discussed, including oxygen arc cutting (OAC) and water arc cutting,flux cutting (OC-F), plasma arc cutting (PAC) and shielded metal arc cutting (SMAC), oxyfuel gas cutting and several variations; also thermal cable cutting, oxygen lance cutting (OLC) and oxygen gouging (OG). The Safe Practices section near the end of the chapter illustrates how the recommended precautions and safety procedures of the welder/diver and the support team, the equipment, and welding conditions must be coordinated to counter the hazards associated with underwater welding and cutting. A chapter on underwater welding was published in the Welding Handbook for the first time in the 8th edition. The contributors to that chapter and their works are listed in the Bibliography, along with the contributors to this updated 9th edition chapter.