Underwater cutting services on ships are mainly applied in the case of deformed blades, to ship wrecks going for scrap, to plates or pieces of plates needing replacing.
Underwater cutting is a particularly useful object, in a wide range of applications, since it is used for rescue operations, preparation of plates prior to welding, damage repair from accidents. There are two types of electrodes used for oxygen-arc (oxy-arc) cutting tubular-steel and the exothermic types (Ultra-thermic) which provide excellent cutting results. Oxygen-arc cutting (OAC) uses tubular –steel rods which are composed of a hollow, solid steel tube that is waterproofed with a flux coating. The electrode holder is equipped with an oxygen passage. Preheating is done by striking an arc between the hollow electrode and the metal to be cut. Once the arc is struck, the cutting oxygen lever is depressed, oxygen flows through the hollow electrode, and the cutting begins. The metal oxidizes and is blown away. They perform efficient cutting and work very well on clean steel, because of the need for electrical continuity they cannot burn through heavy corrosion or marine growth efficiently. Exothermic Cutting (Ultra thermic cutting) uses an electrode filled with small fuel wires and high volume oxygen to burn continually without an arc. Once the rod is ignited the electrical current can be shut off. The heat is maintained by thermo-chemical reaction sustained by the exothermic materials. Cast iron, stainless steel and non-ferrous metals do not oxidize; therefore, underwater cutting becomes a melting process. They perform efficient cutting and will burn through steel, concrete, rock, coral, marine growth and other non-conductive materials when the power is off.
Advantages of the Steel-Tubular Electrodes
- Metals up to 2 inches in thickness can be cut.
- Cutting is performed rapidly.
- Neat, trim, narrow cuts are produced.
- The power required is within the capacity of a 400-ampere welding power supply.
- There is less electrode waste because the electrode must be in constant contact with the metal being cut to sustain an arc.
Disadvantages of the Steel-Tubular Electrodes
- The burning time of the electrode is short (approximately one minute).
- Requires good visibility because it produces a narrow gap.
- A welding machine is required.
- Higher amperage requirement
Advantages of Exothermic Electrodes
- They will cut thin metal when the power is off.
- Cutting is performed rapidly.
- They will cut all ferrous and non-ferrous metals.
- They are applicable to all metal thicknesses.
- The power required is within the capability of a 200-ampere welding power source.
- A 12-volt battery can be used as an ignition source.
Disadvantages of Exothermic Electrodes
- Larger volume of oxygen is required than with steel-tubular electrodes.
- Burning time of the electrode is short; 45 to 55 seconds.
- Contact with the work is not required to sustain ignition, thus electrode waste can occur.
Underwater welding, as well as underwater cutting ensures speediness of the work rate at very low cost, with equally good results, which makes them irreplaceable in the field of marine structures. Underwater cutting has many common points as with those carried out on the surface. The difference is in the special equipment required and qualified divers engaged in it, who strictly abide by the safety rules.
Underwater cutting services on ships are mainly applied in the case of deformed blades, to ship wrecks going for scrap, to plates or pieces of plates needing replacing such as a detached bilge keel.
Along with the engine, the propeller plays a primary role in the ship’s performance. Essential is its significance in the handling, speed and acceleration of the ship, engine life, fuel consumption and even safety. Therefore, any deformations of the propeller blades can be particularly detrimental to the proper functioning of the ship.
There may be various types of damage. Sometimes the damage is minor, so recovery is simple. But others are larger as deformations which require cutting portions of one and of the opposite blade, and smoothing out so that propeller can regain its hydrodynamic shape.
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