GB2252082A - A method of raising objects from the sea bed - Google Patents

Abstract

A flexible envelope, of fabric, plastic sheet or similar material, is made buoyant by being filled with water of lower salinity than the surrounding medium, and used to lift loads underwater. The lift can be generated in situ by pumping water into the envelope after it has been attached to the load through a hose from the surface, and reduced by either releasing the water from within the envelope or pumping it back to the surface again. By combining these two operations, the lift can be exactly controlled. Several envelopes can be used to lift a single load, and the load itself can be made up of small objects placed in a suitable container by other means. The envelopes may be towed into position, or may be provided with their own means of propulsion.

Description

A METHOD OF RAISING OBJECTS FROM THE SEA BED.

Introduction Two'methods are commonly used to raise large objects, such as sunken ships, from the sea bed. Firstly, the object may be lifted directly with cables and a crane mounted on a suitable vessel. This is open to many objections. For very heavy objects, a large and very costly lifting vessel must be employed. The cables may tangle if more than one is needed to sustain the weight. At very great depths, the weight of the steel cables is a significant part of the total load. Steel cables have little compliance, and so will transmit wave movements direct to the load, considerably increasing peak stress. These last two problems can be overcome by using cables made of synthetic fibre with approximately the same density as s water. However, these are costly, and, if they break, the large amount of stored energy can cause serious accidents.

The second lifting method employs air-bags. A balloon is attached to the load, and air is pumped into it, generating lift equivalent to the water displaced. A variation of this is to close all the apertures on a wreck, and fill it with air ; the wreck itself then acts as its own balloon. Although it is simple and relatively cheap, this method suffers from being virtually uncontrollable. Normally, extra lift, over and above the weight in water, is required to break the object free from the bottom. Once the load starts to move upward, the air in the balloon expands, further increasing the lift. The rate of ascent therefore increases, until the load virtually leaps out at the surface. Since the air-bag usually has an open bottom, the air is often spilled at the surface, so the load descends to the bottom again.

There are other problems at very great depths. The air must be pumped down from the surface at a pressure at least equal to that at the sea bed; hence, powerful pumps and very heavy pressure hose must be used. Furthermore, the solubility of a gas is proportional to its partial pressure (Henry's Law), so a considerable proportion of the air actually supplie will be lost by dissolving in the sea-water.

The system proposed seeks to combine the simplicity and cheapness of the air-bag system with the excellent control of the direct lift method.

The Proposed Technique Basic Considerations The technique proposed is equivalent to the air-bag method, with the crucial modification that the air is replaced by fresh water. Depending upon salinity, sea water is roughly 2% more dense than fresh water; hence, a bag containing 1 cu. m of fresh water immersed in sea-water will experience a lift of approximately 20kg. Since the compressibility of the two fluids will be identical, this lift will be independent of depth, and will not increase as the load rises. This principe has been applied, in a slightly different form, in the"Bathyscaphe", which is basically a self-contained manned underwater balloon, with buoyancy provided by a large volume of oil. However, it would clearly be impractical, on both pollution and economic grounds, to pump large amounts of oil into the sea when there is, inevitably, a risk of leakage. A leak of fresh water, on the other hand, is unlikely to have any serious consequences.

The surface pressure required to pump fresh water down to the"balloon"will be 2% of the pressure at the latter. For example, the pressure at 2000 m depth in sea water is roughly 200 Bar, say 3000 p. s. i., but the static pressure required to pump fresh water down to this depth will be only 4 Bar, say y 60 p. s. i., which is well within the range of ordinary water pumps. This low pressure will allow wide, thin-walled hoses, such as standard fire-hoses, to be used. Since the stresses will be relatively low, a wide range of materials can be used to construct these hoses; it would clearly be useful to arrange that the net specific gravity of the hose full of fresh water was roughly unity, so that the hose was supporte by the water, and therefore not subjected to tensile stress.

A balloon filled with fresh water will need to be roughly fifty times the volume of an air-bag to provide the same lifting force. Quite large volumes of water will be requiredfor example, to generate 5000 tonnes lift, approximately 250000 tonnes of fresh water will be required, equivalent to a 78 m diameter sphere, although in practice the water would probably be distributed between several smaller bags. However, this is not a serious difficulty. Fresh water is cheap, and can be carried to the salvage site either in tankers or in"Dracones" ; indeed, many ships distil several tonnes of fresh water per day, which may well be enough for modest lifts. Using the latter, virtually all operations could be carried out using quite small, conventional vessels, as against the costly lifting barges used for conventional salvage with cables.

Hence, this technique should have considerable economic advantages.

Construction The stresses in the water-filled balloon will be low, so very light material, such as thin plastic sheet, can be used.

The actual balloon itself would resemble a hot-air balloon and would be designed by the same general methods. For light loads, the fabric itself could sustain the stress, but for heavy loads, the best method would be to renforce the seams between the gores with suitable rope or tape. This method, which is well known in hot air balloons gives the possibility-of minimising the stresses in the fabric itself by allowing it to bulge out between the seams-it is generally accepted that the local stresses in such a structure fall with decreasing local radius of curvature. Another method of transmitting the load to the fabric envelope is to use a net over the top of the balloon; however, this might increase the danger of tangling underwater. The supply hose would be connected to the top of the balloon ; the bottom could be either open, as in a hot-air balloon, or closed, although an over-pressure valve would he required.

Dynamic Behaviour and Control The large volume of the balloon has a significant avantage, in that it will act as a very effective damper, and will slow down the ascent. The mass of the fresh water will al. so contribute to the control. The"extra"lift, over and above the weight of the object, required to detach the latter from the sea bed is often considerable, and with air-bags, or cables, which have little mass in themselves, this excess lift will cause the object to accelerate once it has broken free.

With the method proposed, however, the excess lift must accelerate not only the object itself, but also the mass of the fresh water. These two features will ensure that the object ascends steadily.

The steady, highly damped movement of the balloon and its load offers ideal conditions for buoyancy control by pumping water into and out of the envelope. Pumping water in from the surface presents few problems, but removing water by the same method would inevitably be slow. Although the static pressure required to drive water down is low, the resistance of, say, 2km of hose would be considerable, so the hose should be as wide as practicable. This presents no difficulties if the tube is made of thin, flexible material, but such a tube will collapse if the pressure inside falls at all below the pressure e outside, so the only pressure available to drive fresh water to the surface will be that due to the difference in hydrostatic heads. This problem can be overcome in several ways. The fresh water in the bag can be diluted and displaced by sea water pumped from the surface, either through the primary hose, or a second one. The fresh water can be released by a valve at the top of the balloon, controlled from the surface :- the pressure differential between the top and bottom of the latter will drive it out. These methods waste the fresh water. This could be avoided by a pump attached to the balloon controlled from the surface to assist in returning the water to the surface.

All the control methods relying on pumping water to or from the surface will be relatively slow, since they will be limited by the inertia of the water in the hose. For fine control, it will probably be necessary to provide remotely controlled dump valves at the bottom end of the pipes, so the flow can be diverted away from the balloon quickly even If it cannot be stoppe. With these precautions, it should be possible to "hover"the balloon and its load at any desired depth if required, for example to avoid wave action on the surface.

Similarly, the balloon system will allow objects to be lowered, as well as raised, under complete control. It is often required to place pumps, etc. on underwater platform, no mean task with h cables from the surface; the balloon system would completely isolate the load from wave action, etc and allow it to be lowered under complete control.

Unlike normal lifting with cables, the technique proposed does not require the surface vessel to be exactly positioned with respect to the load. This is a considerable avantage, since keeping a conventional salvage vessel exactly on station normally requires either multiple anchors or precise navigation by satellite. Indeed, if buoyancy control was not required, all connection between the salvage vessel and the load could be severed once the load had started to lift, providing the water connection to the balloon had a non-return valve: the ascent could be followed by a transponder on the balloon. However, this loose connection makes it difficult to get the balloon to an exact point on the sea bottom. It would therefore be necessary to tow the deflated balloon (which could be packed in a suitable container if necessary to reduce drag) to the load on the bottom with a suitably modifie ROV (Remotely Operated Vehicle) during the descent. In the limit, for work in strong currents, the balloon could be streamlined, like a conventional airship, and provided with its own propulsion motors.

In very great depths of water, the time taken for the balloon to ascend and descend could be a significant disadvantage, particularly if a large number of small objects are to be recovered. In this case, the balloon could be attached to a suitable carrier, which was loaded by one or more ROV'se Final Recovery Like the conventional air-bag system, this technique will not allow the recovered object to be lifted on board another vessel; cranes, or similar devices will be necessary. However, it would be possible to tow the object, suspended below the surface to avoid wave turbulence, to shallow water or to a shore-based lifting facility. Other final recovery techniques, such as"Camels"or semi-submersible barges, could also be used. Once the load was at the surface, it would also be possible to replace some or all of the fresh water with compresse air, which would lift the load much closer to the surface.

Claims (10)

1. A METHOD OF RAISING OBJECTS FROM THE SEA BED.

   CLAIMS 1. A method of lifting loads underwater using one or more flexible envelopes, of fabric, plastic sheet or similar material, which can be filled with water of lower salinity than the surrounding medium, and are thereby rendered buoyant and capable of lifting the load.
2. 2. A method, as in Claim 1, in which the envelopes are reinforced with ropes or tapes to spread the weight of the load.
3. 3. A method, as in Claim 1, in which the envelopes have nets spread over them to distribute the load.
4. A A method, as in Claim 1, in which the envelopes are open at the bottom.
5. 5. A method, as in Claim 1, in which the envelopes are equipped with a valve to release the internal pressure in the envelopes.
6. 6. A method, as in Claim 1, in which the water is pumped into the envelopes through a hose from the surface.
7. 7. A method, as in Claim 1, in which the water within the envelopes can be displaced by the surrounding medium pumped into it.
8. 8. A method, as in Claim 1, in which the water within the envelope can be released by remotely controlled valves to control the buoyancy.
9. 9. A method, as in Claim 1, in which the envelopes are provided with remotely controlled pumps to assist in the addition or removal of water.
10. 10.-A method, as in Claim 6, wherein the hose connection is provided with a non-return valve.

    1-1. A method, as in Claim 1, in which the envelopes are provided with remotely controlled means of propulsion. 12 A method, as in Claim 1, in which the envelopes are used to lift a suitable container which is itself loaded by other means.

    13 A method, as in Claim 1, in which the medium within the envelopes is replaced wholly or partly with air when the load reaches the surface.