Preliminary tests of a novel method of manipulating heavy loads underwater are described. The method exploits the difference in density between salt and fresh water, the latter being contained in an envelope of light fabric. Buoyancy can be controlled within very fine limits, and the method is safe, can be used at any depth and is suitable for both lifting and emplacement. Design features, costs and possibilities for this technique in general use are briefly discussed.


An observer on the shores of Loch Linnhe in late September, 1993, would have seen a small tug towing a large pontoon carrying two "Portakabins" out from Fort William. The pontoon was anchored fore and aft in 25m of water and the tug returned to Fort William, re-appearing with a motley assortment of scrap-iron, roughly welded together to give a total weight (in air) of four tons, suspended from an "A" frame in the stern; this was dropped alongside the pontoon. Next day, a heavily laden barge, with an engine-driven building-site pump on its bows, was towed out. This was moored alongside the pontoon and various gear was taken on board, including a large red canvas valise easily carried by two men. After some preparations - connecting up a 6″ hose, a small-bore hose and various electrical cables to a thick Nylon disc - divers entered the water. A heavy steel fitting on the end of a long bundle of red and yellow light Nylon cloth was passed down to them, and they submerged. The cloth bundle was paid out from the valise as the divers descended until the top end, carrying the Nylon disc, was thrown overboard. After a short interval, the divers surfaced, and the pump on the barge was started. After about forty minutes, during which the barge rose perceptibly, there was a slight stir in the water, and a red and yellow object, looking like a gigantic striped jellyfish, rose gently to the surface. The pump was stopped, and an operator turned a handle on a cylinder attached to the smaller hose. The "jellyfish" gently submerged again. The operator returned the handle to its former position, and the pump was re-started; after a few minutes, the "jellyfish" re-appeared.

The observer would have witnessed the first trial of an entirely novel method of manipulating heavy loads underwater. The valise contained a modified hot-air balloon, with a load-cell and lifting eye at the bottom, and a hose connection and remotely controlled dump-valve at the top. The divers connected the lifting eye to the four-ton weight, and the balloon was filled with fresh water pumped down from the barge. Since the density of fresh water is roughly 2% less than that of sea-water, the lift of the balloon increased steadily as it filled - had the observer been on board, he would have been able to follow the increasing lift on a computer screen - until the load rose to the surface. Opening the dump-valve released some fresh water, allowing the load to sink again, simulating an emplacement. The lift could be controlled to better than 10kg.

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