3-2.1 Cofferdams and Caissons. Cofferdams and caissons are used at the waters edge or in the splash zone and below the water surface. While the structure excludes the surrounding water from the work area, its upper section is open to the atmosphere. This technique is depth-limited by the physical size of the cofferdam. Typical applications include ship's hulls, dock and harbor facilities.
3-2.2 Mini-Habitats. Mini-habitat welding makes use of a small, easily portable, gas-filled enclosure. Often mini-habitats are transparent plexiglas boxes, which are placed over the joint by a diver. Water is displaced by an inert gas or air supplied from the surface. In this instance welding is performed at elevated ambient pressures. Depending on the size of the enclosure, the diver is partially immersed in water (only the diver's hands and the welding torch are inside the habitat). This method requires adequate visibility and is limited to areas with clear access.
In general, dry welds are better than wet welds because cooling rates are slower. Both of the devices described above are methods of excluding water from the weld area, thereby creating a dry environment where dry welds and welds requiring more advanced techniques may be produced. In salvage, most underwater welding consists of joining fairly simple steel patches to hull plating. Therefore, wet welding may be acceptable for most patching applications.
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