Lifting bag
A lifting bag is an item of diving equipment consisting of a robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy. The heavy object can either be moved horizontally underwater by the diver or sent unaccompanied to the surface.
Lift bag appropriate capacity should match the task at hand. If the lift bag is grossly oversized a runaway or otherwise out of control ascent may result. Commercially available lifting bags may incorporate dump valves to allow the operator to control the buoyancy during ascent, but this is a hazardous operation with high risk of entanglement in an uncontrolled lift or sinking. If a single bag is insufficient, multiple bags may be used, and should be distributed to suit the load.
There are also lifting bags used on land as short lift jacks for lifting cars or heavy loads or lifting bags which are used in machines as a type of pneumatic actuator which provides load over a large area. These lifting bags of the AS/CR type are for example used in the brake mechanism of rollercoasters.
Physics of buoyant lifting
The volume of the bag determines its lifting capacity: each litre of air inside the bag will lift a weight of 1 kilogram, or each cubic foot will lift about 62 pounds. For example, a bag can lift a underwater object.A partially filled bag will accelerate as it ascends because the air in the bag expands as the pressure reduces on the ascent, following Boyle's law, increasing the bag's buoyancy, whereas a full bag will overflow or blow off excess volume and maintain the same volume and buoyancy providing it does not descend. A bag which leaks sufficiently to start sinking will lose volume to compression and become less buoyant in a positive feedback loop until stopped by the bottom.
Breakout
The force required to lift a submerged object from the bottom can be split into two main components:- Apparent weight, which is the weight of the object less the buoyancy of its displacement.
- Breakout forces due to embedment in the bottom, which can be negligible, or in some cases the major part of the load.
- Use of mechanical or hydraulic excavation to loosen the sediments holding the load.
- Use of a "Dead Man Anchor" - a large heavy weight - and restraining cable to prevent the bag from moving away too far, so that the buoyancy can be corrected to more closely match the load.
- Use of shallow bags with long cables to the load to provide breakout, which will only lift a short distance before surfacing, after which the load can be lifted further by staged lifts or direct lift by close-coupled bags.
Stability of the load
Types and construction
Underwater lifting bags are lifting equipment and as such may be required to comply with safety standards.Open lift bags (parachute lift bags)
Parachute lift bags are open at the bottom. When full any extra or expanding air will spill out. The shape of an open lifting bag should distribute the volume in a vertical rather than a horizontal direction so that the open end of the bag always remains underwater. If the open end reaches the surface, air will escape from the bag and it may sink.The simplest version are two-sided bags, either joined round the edges or folded and joined along two sides. Webbing straps may be stitched to doubler patches which are then glued or welded to the bag on light duty bags, but on large and heavy duty bags there are usually strips of bag material bonded to the bags which form flat retaining tubes for the webbing which is threaded through the tubes and may be withdrawn for maintenance and inspection. heavy duty open bags are generally conical with a domed top or a reversed truncated cone top, and may have several straps from the lifting point at the bottom, through the guide tubes on the sides, to a crown ring of webbing or steel at the top, to spread the load evenly over the fabric of the bag.
Parachute lift bags cannot be overfilled and are suitable for lifts where there is a large pressure change, and where it may be necessary to capsize the bag to stop a runaway lift.
Some lift bags can be converted from open to closed by screwing a cover onto the bottom opening.
Propeller lifting bags
Installation and removal of propellers is a specialised application where there is usually very little clearance above the load, which is usually at least partly underneath a vessel. Lift bags for this application partly enclose the propeller when in use, and are required to hold the propeller in the correct alignment for fitting to the shaft.It may be necessary to ballast the lower blade to keep the propeller upright. and details of the rigging will depend on the precise geometry of the propeller, such as blade aspect ratio, skew, and number of blades. The propeller lift bag will cover the upper part of the propeller, but cannot project below the line of the top of the shaft. It will support the propeller by at least two blades, for stability, and the propeller will generally by slung with a gap at the top, for maximum tip clearance. The bag may be shaped with a relatively horizontal top or a curve to follow the blade disc. Attachment to the blades must be on both sides for stability, and the slings must not harm the blade surfaces or damage the leading or trailing edge.
Closed lift bags (camels)
Closed lift bags have an overpressure valve to prevent internal pressure from exceeding ambient pressure by more than a set amount Closed lift bags are intended for use at or near the surface, as they retain the air even in rough seas. They are available in several configurations, including horizontal cylinder, vertical cylinder, teardrop and pillow.Rapid deployment
Rapid deployment lift bags have a scuba cylinder mounted on the outside which contains sufficient air to inflate the bag at a specified depth. The bag can be attached to the load and when ready, the valve is opened and the diver swims clear while the gas is released into the lift bag.Dump valves
Dump valves are used to release air from the bag when in the water. They can be operated manually at the valve by a diver or may be remotely operated by a pull-cord, which may be operated by a diver or attached to a weight which will automatically open the valve if the weight is lifted off the bottom. Some dump valves can be operated in both these ways. One system operates by pressing on the top or pulling a line attached to the bottom to actuate the spring-loaded valve. The dump valve may be a screw-in quick change system, and the spring tension may be adjustable.Use
Dynamic lifts
When the empty lift bag is attached to the load and the lift is made by controlling the volume of inflation air it is referred to as a dynamic lift.Direct lift
The bag or set of bags is used to lift the load directly to the surface. This is simple, but there is a risk if the lift bag is too large and cannot be vented fast enough the lift may get out of control and ascend so fast that the bag breaks the surface, capsizes and collapses, losing so much air that it then cannot support the weight of the load, and will then sink back to the bottom. If there is a marker buoy attached it will at least not be lost. A lift bag which is only slightly larger than needed to support the load will ascend more slowly, and is less likely to capsize at the surface, as excess air will be spilled continuously during the ascent.Staged lift
Lift bags are used to bring the load up in stages: a long chain or sling is used to connect the load to a lift bag just below the surface, which is filled to break out the load and lift it until the bag reaches the surface, then a second bag is used to bring the load up further. This procedure continues until the load has been raised sufficiently. Advantages of this method are a more controlled lift, the facility to use a larger capacity for initial breakout without risk of a runaway. Disadvantages include the requirement for divers to work on or near the lifting gear when under load.Buoyancy assisted lift
The lift is controlled by a line from the surface vessel, and the load is reduced by a lift bag with a volume too small to support the weight of the load when full. This allows a faster lift by the winch. The lifting gear must be capable of supporting the load if the bag fails, or must be arranged to fail safely.A buoyancy assisted lift is a common procedure for recreational divers to assist the recovery of the shotline or anchor, which would otherwise be pulled up manually. A small lift bag attached to the shot is partially filled by the last diver to leave the bottom, and after surfacing the crew pull up the line and the air in the bag expands as it ascends, providing more assistance to the crew. In this application a runaway lift is not usually a problem, and the bag size is not critical.