Despite all efforts to reduce maritime accidents through improved safety measures and technological advances in navigational aids, there will be occasions oil or other harmful substances are spilled from a vessel as a result of:
- Striking a wreck or other obstacle
- Fire and/or explosion
- Failure or breakdown of machinery or equipment which results in impairment of the safety of navigation
- Structural failure
- Storm damage and ice damage
In most accidents, the master, takes immediate action to ensure the safety of his crew, the preservation of the ship and to stop or limit the loss of cargo. This may also involve the operation for salvage of the ship. Salvage is a super-specialty maritime operation which is concerned with saving a vessel from being damaged further, saving her cargo, saving or limit environmental damage from a ship which has experienced an accident.
The organization which does this operation of salvage is called Salvor. As per the International convention on salvage, 1989”A Salvage operation means any act or activity undertaken to assist a vessel or any other property in danger in navigable waters or in any other waters.”
Salvage is a highly technical and complex process which, whenever possible is best placed in the hands of professional salvors. This blog is a brief description of the more usual salvage methods which will be interesting to be read by all mariners, engineers and navigators alike.
- Lightering: In many cases of stranding and grounding of a ship, a theoretically simple solution is to make ship light so that it again floats in lower depth. This can be achieved by transferring the cargo into suitable vessels(or ashore),by bringing the receiving vessel as close to the casualty as is safe and transferring cargo from the casualty to the receiving vessel, as in case of tankers by means of transfer hoses and where necessary, utilizing fenders, portable pumps and inert gas equipment. When the necessary reduction in the draft has been obtained, towing is usually employed to refloat the vessel under controlled conditions.
- Air lift: the displacement of water by air pressure, particularly where tanks have been opened to the sea during the casualty, is another means of obtaining lift to free a damaged vessel. Whenever possible, cargo or bunkers should be removed from damaged tanks prior to pressurization in order to minimize the risk of pollution or otherwise a water interface between the cargo and damaged areas open to the sea should be maintained in order to prevent the expulsion of pollutants by the pressurized air above the liquid.
- Tidal lift and heaving: This method require the employment of heavy rigging called “beach gear”. These are heavy pulling equipment (fitted usually on floating platforms or on salvage tugs) used to pull out the stranded vessel. In the tidal lift method, beach gear is drawn up tight at low tide and uses the lift on salvage vessel from the rising tide to pull on the stranded vessel. Great forces are involved, but the energy expended by the salvage ship is not large as most of the lift is provided by the tidal sea water and the beach gear mostly pulls out the ship in the right direction towards deeper waters. As shown in the figure the ground “leg” of the beach gear, consists of an anchor and wire rope to carry the load.
During the heaving method, the force is provided by heaving on the towing lines with a winch or hydraulic ram.In” heaving gear”, at the end of the ground leg, a series of blocks is used to increase the pull. Several legs of beach gear may be needed to free a large vessel. Heavy weather conditions may provide the energy needed to move the vessel in the direction of the tow.
- Towing: Unlike the previous method which relies on seawater tidal energy, Towing relies on the power supplied from the salvage vessel to generate the moving force. Modern Tugs with powerful and efficient propellers and rudders with bow thrusters provides an effective pulling force for the salvage operation. Practically, combinations of towing with other methods are used for the salvage operation.
For a towing attempt, a wire rope is used as the towline, sometimes in conjunction with a nylon spring and/or a chafing chain. Modern salvage tugs are fitted with powerful towing winches and often carry tow wires up to 2000 meters in length and 76 mm in diameter.
- Refloating or Breaking out:
There are several methods for refloating the stranded vessel and cargo lightering, is often the quickest and most effective method.
- If the sea-bed is exposed during the low tide, the ground beneath the ship can be excavated by several techniques.
- Another method of reducing the refloating force is to use pontoons, or even barges, rigged alongside, to support part of the weight of the vessel.
It should be clearly understood that salvaging a ship is a complex process and invariably comprises of one or more of the methods mentioned above and is a trial and error process. Even with these conventional methods many new novel methods are tried and tested for complex salvage operation throughout the world.
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