AU680741B2 - Wreck cargo recovery method - Google Patents

Abstract

Deep-water wreck cargo recovery method using a heavy-duty pincer assembly (2) deployed from the drill string (3) of a dynamically positioned drill ship (1) and controlled via an underwater module (6) fitted with propulsion means, as well as a shock absorbing device (10) in the lower portion of the drill string. During the demolition stage, the pincer assembly rips open the wreck to provide access to the cargo. During the recovery stage, the pincer assembly is fitted with side walls and is used as a bucket. The method is particularly useful for recovering ores from modern ships and for recovering hazardous materials (e.g. radioactive materials).

Description

WRECK CARGO RECOVERY METHOD.

The present invention relates to a method of recovering a cargo on board a wreck that has sunk in deep water.

Numerous wrecks exist at the bottom of the oceans containing cargoes of value which cannot be utilized at present by conventional methods because of depth.

Present methods generally make use of divers and so they are restricted to shallow waters.

Recovery of a cargo enclosed inside a wreck is a very difficult task. Access needs to be made through the wreck. This access may be provided through the hatches if the configuration so allows (wreck lying horizontally on the bottom). It may be made through the hull if the wreck is on its side or upside-down on the bottom.

It is common practice for the wreck to be cut up by means of explosives or oxygen cutting torches used by divers. Once access has been provided, the cargo is recovered by conventional hoisting means buckets operated from anchored barges).

Cargoes of value have thus been utilized from depths of less than 500 meters For several years, in rescue applications, use has been made of very strong dredging buckets to demolish wrecks that constitute a danger to navigation. Such buckets are actuated by cables under the control of divers. Their closure force and their traction force can reach 50 metric tons and thus allow large pieces of wreck to be extracted by tearing off plates.

When cargoes are in deep water, utilization has been possible only from wrecks that present few difficulties of access, either because the cargo is lying on the bottom due to the ship breaking up during the shipwreck, or because the ship was made of wood that has deteriorated greatly. Manned or remote controlled submarines have been used for this purpose with success.

Various attempts have been made with cargoes in deep water enclosed on board modern wrecks made of steel, but no method has been found to be technically and economically satisfactory. Dismantling by means of explosives has been tried in association with the use of lightweight buckets handled by means of cables. The effectiveness of explosives has been disappointing at great depth.

The method of the invention enables both the problem of dismantling the hull, and the problems of recovering the cargo to be solved, even at great depth.

During the '70s, the first dynamically positioned drilling vessel was developed for oil prospecting in deep water (300 m to 3,000 In the world, there exist at present about 15 vessels that are capable of drilling in deep waters. They are fitted with heavy derricks capable of exerting vertical forces of 500 metric tons. Pounding compensators enable the motion of the vessel to be compensated while the tool is in contact with the ground.

The size of the well beneath the derrick is such (5 m x m) that it is possible to handle large packages.

The object of the invention consists in using a large-capacity pincer from such a vessel by means of an underwater module for moving the pincer and for actuating it under the control of cameras.

The invention relates to a method of using a dynamically positioned drilling vessel to recover a cargo enclosed on board a wreck that has sunk in deep water.

The method is characterized by the fact that the wreck is dismantled by means of a pincer and of a shock absorber device located near the bottom of the drill string.

The invention also relates to a device for operation from a dynamically positioned drilling vessel to dismantle a wreck sunk in deep water. The device is characterized in that it is constituted by a pincer and a shock absorber device disposed near the bottom of the drill string.

The closure force of the jaws of the pincer as actuated by means of hydraulic actuators is such (several hundreds of metric tons) that it can manage to tear the sides off the wreck so as to gain access to the cargo.

During the recovery stage, the pincer is fitted with cheeks and it is used like a bucket having a capacity of several cubic meters.

During the demolition stage, the pincer is clamped powerfully onto the structure that is to be dismantled.

The pincer is then hoisted vertically so as to tear off the piece held between the jaws. The sizes of the pieces removed vary as a function of the tear lines obtained.

Some pieces resist more strongly than others, and it is sometimes impossible to tear them off without running the risk of breaking the drill string. The pincer is then moved a little, and in general after a few attempts, the piece is torn off. This work of destruction by clamping onto pieces and tearing them off is quite different from the work of taking samples by conventional grapplers.

The shock absorber device is essential in implementing the method. At great depths, under maximum traction force, the total elongation of the drill string is about m. The accumulated energy in the "mass-spring" system is such that without the shock absorber device, when tearing takes place, the assembly comprising the module and the pincers would rise with great acceleration, to a height well above the equilibrium position. This would give rise to uncontrolled buckling of the drill string and would run the risk of breaking it.

Operating times are relatively lengthy (one rising and lowering cycle per day to a depth of 3,000 m), however such operations can be economically profitable given the value of certain cargoes of ore, for example.

The device enabling the method to be implemented is described below with reference to the accompanying drawings, in which: Figure 1 is a section through the apparatus of the invention; Figure 2 is a section through the underwater module; and Figure 3 is a section through the pincer.

Figure 1 shows the apparatus in section installed on board a drilling vessel The apparatus comprises a pincer installed at the end of the drill string (3) and handled vertically by means of the derrick and the turntable A control module causes the pincer to move horizontally by means of thrusters and serves to operate it hydraulically. An umbilical cord paid out from a winch transmits power and control to the underwater module. It is attached along the drill string by means of fast-acting fixing collars On the surface, the vessel is held by dynamic positioning to within a circle of radius 5 m. At the bottom, the pincer is displaced vertically by means of the drill string and horizontally by means of the thrusters. In a preferred embodiment of the invention, the thrusters are steered by rotating the drill string directly from the turntable A shock absorber device (10) made up of a series of circular plates is disposed near the bottom of the drill string so that the drill string cannot be put into compression when contact is suddenly lost with the wreck (11) during a tearing-off action. An uncontrolled compression force would run the risk of damaging the drill string by buckling its section. The energy that is stored when the drill string is put under tension is very large because the string lengthens by several meters when subject to a force of several hundreds of metric tons.

Figure 2 is a section through the underwater module A hydraulic pump (12) driven by an electric motor (13) powers the motors of the thrusters (14) and the hydraulic actuators of the pincer via a set of electrically controlled valves (15) and a compensation tank (16).

At the base of the module, a wide-angle camera (17) and a floodlight (18) enable the situation of the pincer relative to the wreck (11) to be observed.

At its top, thrusters (14) are disposed diametrically opposite each other about the drill string The assembly is housed in a cylinder (19) made of thick metal sheet, so as to protect the equipment while working inside the wreck The drill string (3) passes through the module and also through the shock absorber device The diameter of the cylinder and the diameter of the plates constituting the shock absorber device (10) are such as to allow them to pass through the turntable during on-board handling.

Figure is a section through the pincer The jaws (20) are fitted with lips (21) and with teeth (22) of large dimensions. In conventional manner, they are actuated by actuators (23) and they are synchronized by means of gearing Removable cheeks (25) are bolted onto the sides of the jaws (20) so as to transform them into scoops while recovering the cargo.

The inside volume of the framework is trapeziumshaped in section being open at its bottom end.

This volume is organized to protect a camera two floodlights and a depth sounder A device for injecting clear water makes it possible to form a cone of clear water in the field of the camera (27) and is intended to increase visibility when the pincer is at the bottom of a hole in a volume of water that is completely cloudy because of the sediment in suspension and the lack of current inside the wreck.

The apparatus makes it possible to identify the target while the pincer is resting wide open on its teeth.

The clear water is pumped from the surface by means of drilling pumps and it is conveyed to the pincer through the drill string It could equally well be pumped from the underwater module by means of a pump situated inside the module, for example.

The method and the apparatus of the invention are described with respect to an embodiment that is preferred but not limiting. Thus, the drilling vessel could be replaced by a floating support fitted with a derrick and a special drill string. The thrusters fitted to the underwater module could be disposed in a different known manner so as to displace the pincer without it being necessary to steer it by means of the drill string.

The method and the apparatus of the invention apply to recovering any kind of cargo, including radioactive substances and nuclear reactors enclosed inside deep wrecks.

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