Salvage diving is the diving work associated with the recovery of all or part of ships, their cargoes, aircraft, and other vehicles and structures which have sunk or fallen into water. 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. 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.
Most salvage diving is commercial work, or military work, depending on the diving contractor and the purpose for the salvage operation, Similar underwater work may be done by divers as part of forensic investigations into accidents, in which case the procedures may be more closely allied with underwater archaeology than the more basic procedures of maximum cost/benefit expected in commercial and military operations.
Clearance diving, the removal of obstructions and hazards to navigation, is closely related to salvage diving, but has a different purpose, in that the objects to be removed are not intended to be recovered, just removed or reduced to a condition where they no longer constitute a hazard. Many of the techniques and procedures used in clearance diving are also used in salvage work.
The US Navy considers the recovery of sunken or wrecked naval craft, submarines, human remains, critical items of equipment needed to determine the cause of a mishap, including classified and sensitive materials to be within the scope of their salvage and recovery operations.
For stranded and floating vessels, a detailed hull survey includes the parts of the ship that are underwater. These will be external areas below sea level, and any internal areas that are flooded. If sea conditions or access are unsuitable for external survey, internal survey will have to be more comprehensive as the information about one side must be extrapolated to provide needed information about the inaccessible side. The diving survey includes::Ch. 2
The dive team should be briefed on all structural damage found inside the hull so that they can check for underwater damage in the same areas. When practicable the work of the divers should be minimised as diving is slow, labor intensive, dangerous and expensive work. Tidal flow can change the conditions and limit diving operations, and also affect the condition of the vessel and seabed in the vicinity. Where scouring is likely it should be monitored by regular underwater inspections. Video records allow comparison to estimate the rate of scour or deposition.:Ch. 2
Underwater patching is almost always done by divers. As much patch fabrication and rigging as possible should be done out of the water to minimise diving time. Small leaks are generally sealed off and made watertight by wooden plugs and wedges, small wooden patches and concrete boxes, small steel plate patches or combinations combinations of these, caulked and sometimes additionally sealed with epoxy resin or fire-reinforced resins. Small steel patches for minor leaks are usually fitted with gasket material to seal against the damaged hull. Major patching is characterized by extensive diving work and include detailed underwater surveys, measurements, and major underwater cutting and welding operations to prepare and fit the patch.:Ch.10
If a ship is pumped out while the deck is submerged the top of the deck is loaded by hydrostatic pressure and may require shoring to support the load. This is generally done by divers and is time-consuming and expensive. The load may also be compensated by compressed air in the space if practicable.:Ch. 6
Detailed information of the layout and structure of the vessel to be salvages and the type and location of cargo are useful for planning and essential for the actual salvage operation. Information acquired during the planning stage can greatly facilitate the actual operation.
If the vessel is to be raised, details of the cause of sinking and the extent of damage is required.
Useful information can be gathered from ships plans, cargo manifests, loading plans, interviews with witnesses and survivors, photographs and official reports of similar accidents.
The choice of salvage systems depends on the specific conditions of the job. 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. Manned submersibles and atmospheric diving suits can go deeper than ambient pressure diving without decompression obligations, and have advantages of human vision and judgement, and when working without tethers have good maneuverability, but dexterity is compromised, and cost is high. Tethered unmanned ROUVs eliminate the risk to human life of manned systems and are available with a wide range of capabilities which can be matched to the operational requirements, and are not limited by operator fatigue.:Ch.1
Underwater search and recovery operations are used to locate, identify, observe and recover specific objects from the seafloor. Typically, search and recovery operations are conducted as two distinct phases - the search includes detection and identification of the target and, in some cases, direct inspection. Operations are generally planned to suit expected conditions, but plans should be sufficiently flexible to allow for changes to suit actual conditions.:Ch. 2
The equipment available for underwater searches ranges from simple equipment like grapples and draglines to complex acoustic technologies and magnetic field sensors.
Remotely operated vehicles are platforms that can carry sensors underwater and maneuver them in proximity to the object. They are limited to operating in relatively small areas because the support ship must loiter almost directly over the ROV and the umbilical limits the system’s maneuverability. An ROV can be very effective at locating small isolated targets in a known debris field, and visually identifying a target in good visibility.:Ch. 2
The effectiveness of a search can be expressed by how thoroughly and efficiently the search area is examined. Systematic examination of the search area is achieved by following a predetermined pattern that suits the conditions of the search.:Ch. 2
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.
The choice between surface oriented and saturation diving is based largely on depth and the amount of decompression anticipated.