Underwater cutting is used for salvage work and for cutting below the water surface on piers, dry docks, and ships. The two methods most widely used are oxyfuel gas cutting (OFC) and oxygen arc cutting (AOC).
Technique- The technique for underwater cutting with OFC is not materially different from that used in cutting steel in open air. An underwater OFC torch embodies the same features as a standard OFC torch with the additional feature of supplying its own ambient atmosphere. In the underwater cutting torch, fuel and oxygen are mixed together and burned to produce the preheat flame. Cutting oxygen is provided through the tip to sever the steel. In addition, the torch provides an air bubble around the cutting tip. The air bubble is maintained by a flow of compressed air around the tip. The air shield stabilizes the preheat flame and at the same time displaces the water from the cutting area.
Special Equipment- The underwater cutting torch has connections for three hoses to supply compressed air, fuel gas, and oxygen. A combination shield and spacer device is attached at the cutting end of the torch. The adjustable shield controls the formation of the air bubble. The shield is adjusted so that the pre-heat flame is positioned at the correct distance from the work. This feature is essential for underwater work because of poor visibility and reduced operator mobility caused by cumbersome diving suits. Slots in the shield allow the burned gases to escape. A short torch is used to reduce the reaction force produced by the compressed air and cutting oxygen pushing against the surrounding water.
Gases- As the depth of water at which the cutting is being done increases, the gas pressures must be increased to overcome both the added water pressure and the frictional losses in the longer hoses. Approximately 3.5 kPa (1/2 psi) for each 300 mm (12 in.) of depth must be added to the basic gas pressure requirements used in air for the thickness being cut.
Methylacetylene-propadiene(MPS), propylene, and hydrogen are the best all-purpose preheat gases, because they can be used at any depths to which divers can descend and perform satisfactorily. Acetylene must not be used at depths greater than approximately 6 m (20 ft), because its maximum safe operating pressure is 100kPa (15 psi).
No great difficulty is experienced in underwater severing of steel plate in thicknesses from 13 mm (1/2 in.) to approximately 101 mm (4 in.) with the oxyfuel gas cutting torch. Under 13mm (1/2 in.) thickness, the constant quenching effect of the surrounding water lowers the efficiency of preheating. This requires much larger preheating flames and preheat gas flows. Cutting oxygen orifice size is considerably larger for underwater cutting than for cutting in air. A special apparatus for lighting the preheat flames under water is also needed.
Oxygen Lance Cutting (LOC)
The LOC process can also be used underwater. The lance must be lighted before it is placed underwater; then piercing proceeds essentially the same as in air. The process produces a violent bubbling action which can restrict visibility.
This is another underwater cutting process used to cut ferrous and nonferrous metals in any position. Underwater electrodes for AOC are steel tubes with a waterproof coating. A fully insulated electrode holder equipped with a suitable flash-back arrester is required. See OXYFUEL GAS CUTTING, OXYGEN LANCE CUTTING, and OXYGEN ARC CUTTING.