Salvage diving
In the case of ships it may also refer to repair work done to make an abandoned or distressed but still floating vessel more suitable for towing or propulsion under its own power.
[1][2][3] The recreational/technical activity known as wreck diving is generally not considered salvage work, though some recovery of artifacts may be done by recreational divers.
[2] Commercial salvors will generally undertake an operation if it is likely to be sufficiently profitable, taking into account the known hazards and risks.
Small steel patches for minor leaks are usually fitted with gasket material to seal against the damaged hull.
[1]: Ch.10 When all or part of the main deck of a sunken ship is submerged, flooded spaces cannot be dewatered until all openings are sealed or the effective freeboard is extended above the high water level.
In salvage, one method of doing this is to build a temporary watertight extension of the entire hull of the ship, or the space to be dewatered, to the surface.
[1]: Ch.10 Complete cofferdams cover most or all of the sunken vessel and are equivalent to extensions of the ship's sides to above the water surface.
[1]: Ch.10 Diving work on cofferdams often involves clearing obstructions, fitting, and fastening, including underwater welding, and where necessary, caulking, bracing and shoring the adjacent structure.
Divers can work efficiently in shallow water, but the practicality decreases rapidly with depth and has an absolute limit determined by current technology.
They bring the advantages of human vision, judgement and high dexterity manipulative skills, but these are offset by depth limitations, dive duration, risk, support requirements and cost.
[3]: Ch.1 Underwater search and recovery operations are used to locate, identify, observe and recover specific objects from the seafloor.
[5] Scuba is not authorised for most salvage work by commercial or naval operators due to relatively high risk in comparison to surface supplied techniques, though naval operations may use scuba for non-penetrative work in good visibility and relatively shallow depths.
[2] The choice between surface oriented and saturation diving is based largely on depth and the amount of decompression anticipated.
