Lift Bags

In conducting underwater work, it is sometimes necessary to move heavy and bulky objects, such as instrument arrays, support'towers, anchors, pallets of construction materials, construction tools and equipment, and similar objects. It may also be necessary to safely float equipment, materials, and assembled construction products on the surface of the water (e.g., armored cable assemblies). This can be achieved underwater by the diver using lift bags and float balloons, and from the surface using cranes and winches.

There are several lift bag systems that can be used in underwater operations. They are:

• Open- and closed-bottom commercial lift bags

• 8.4-ton salvage pontoons

• Variable-buoyancy zipper lift bags

• Fixed-displacement Kevlar lift bags 2.5.1 Open-Bottom Lift Bags

The buoyancy of open-bottom lift bags is increased by admitting air into the bottom opening of the bag and decreased by exhausting air through a top-mounted, manually-operated, air exhaust valve or by venting air through a vertical zipper as dis cussed in Section 2.5.3. The buoyancy of these lift bags is difficult to accurately control because of the on-off nature of the exhaust valves and the change in gas volume (buoyancy change) with depth. A slight ascent of the bag allows the air to expand and buoyancy to increase, which can cause the lift bag and its attached load to accelerate toward the surface. Conversely, a descent compresses the air, causing a loss of buoyancy.

Open-bottom lift bags have been used the most by the UCTs because of their availability and simplicity. These bags are typically used for moving relatively light loads around underwater. However, controlling a 10-ton load would pose considerable difficulty due to the size and mass of the load. There is a high potential to lose air out the bottom of the bag as it breaches the surface. The bag may then lose buoyancy and drop the load back to the bottom. These lift bags, as illustrated in Figure 2-26, are available in sizes from 50 to 12,000 pounds. Reliability of open-bottom lift bags is generally good. The PMS requirements are simple, requiring only cleaning and exhaust

Bag Work For Underwater Construction

WARNING The ascent and descent rates of all lift bags can rapidly exceed allowable levels for divers. Operators must ensure that all divers are clear of the path before deploying the bag.

valve lubrication. Leaky exhaust valves are easily replaced. They are also easily transported and stored because of their compactness.

2.5.2 Closed-Bottom Lift Bags

Closed-bottom lift bags are generally used for lifting objects from the seafloor to the surface and not the reverse.

Commercial closed-bottom lift bags come in a variety of sizes and shapes as illustrated in Figure 2-27. They are used for many different applications including salvage, cable laying, surface floats, and general underwater construction support. Some closed-bottom lift bags, such as the heavy duty pillow-shaped bags, can be used for assisting in land construction: for example, an appropriately sized deflated boulder bag placed under an object makes it suitable for lifting vehicles, pontoons, etc.

Ocean Salvage Tubes

Boulder Bags

Figure 2-27. Closed-bottom lift bags.

Boulder Bags

Figure 2-27. Closed-bottom lift bags.

Closed-bottom lift bags are commercially available in sizes from 200 to 20,000 pounds (possibly larger ones are available). The totally enclosed design helps prevent the loss of air when the assembly is bobbing at the surface.

Because the bags are totally enclosed, they are equipped with overpressure vent valves that dump the expanding air as the bag rises to the surface. The rate of ascent of the bag will be determined by the dump valve capacity and the excess buoyancy of the bag during ascent.

Bag manufacturers design the dump valves to provide adequate venting from the bag as it rises through the water column provided that the load being lifted is as least 80 percent of the bag's rated lift capacity. Avoid excessive buoyancy. Too light a load can result in:

• The lift rising too rapidly.

• Inability of the dump valve to exhaust the expanding gas.

• Excessive internal pressure.

• Potential rupture of the bag whereby the load will fall back to the seafloor.

WARNING For safety reasons, select closed-bottom lift bags so that the underwater weight of the lifted object is at least 80 percent of the bag's lift capacity.

The totally enclosed, tear-shaped salvage lift bags are ideally suited for lifting a load from depth and holding it on the surface. The tear-drop shape and single lift point provide a stable configuration through the water column. The CANFLEX Model DLE10 shown in Figure 2-28 is available

Pontoon Lift

Figure 2-29. Pontoon lift bag.


Figure 2-30. Pumpkin floats.

WARNING Pontoon lift bags are not recommended for use in an excess buoyancy mode.

Bottom from the ESSM Pool. It has a 22,000-pound lift capacity, weighs 210 pounds, and has an ascent rate of 66 ft/min when the underwater load is at least 80 percent (at least 17,000 pounds) of the bag capacity. The bag measures 8.S feet in diameter by 8.5 feet long when inflated.

Figure 2-28 CAN FLEX salvage lift bag.

Pontoon (horizontal cylindrical-shaped) lift bags such as those shown in Figure 2-29 are best suited for shallow water lift. There is a general concern for the stability of the bags as they are being filled. If they fill unevenly, excessive side loading can be put on the straps causing uneven lifting and possibly tearing the lift bag loose from the straps.

Pontoon lift bags can be effectively used for REDUCING the weight of underwater objects to supplement controlled lifting from other means such as winches.

Figure 2-29. Pontoon lift bag.

Pumpkin Floats, Figure 2-30, are available in the TOA. Their primary UCT application is for floating shore cables during their installation. The floats, which have a 250-pound buoyancy, are tied to the cable as the cable is payed out from the cable vessel or shore. Once the cable is landed, the float lines are cut by swimmers or from a small boat to deploy the cable. It is very important that the float be properly tied to the cable to prevent slipping or loosening during deployment. The recommended procedure is demonstrated in Figure 2-31.

Figure 2-30. Pumpkin floats.

2.5.3 Variable-Buoyancy Lift Bags

The Diver Lift System provides three different sizes of open-bottom, variable-buoyancy lift bags with the following lift capacities: 220 to 500 pounds, 500 to 1,250 pounds, and 1,000 to 3,000 pounds. Because the bags are not rigid and open bottom, air must be continually added during descent to the seafloor.

Each lift bag (Figure 2-32) has features permitting buoyancy adjustment and fine trimming to accommodate maneuvering loads. The lift


Finish by tying to cable using a clove hitch plus two half-hitches for backout.


Finish by tying to cable using a clove hitch plus two half-hitches for backout.

Figure 2-31. Method for tying pumpkin float to cables.

bridle includes internal rigging, which allows the load to be lifted in or out of the water from the top of the bag. This is a feature that is not available in most underwater lift bags.

The in-water weight capacity of each bag is adjusted by setting the position of the leakproof zipper. Excess air from filling or expansion during ascent is vented through the zipper opening. The maximum lift is attained with the zipper fully closed.

Ascent/descent rate and hovering of the bag are controlled by the diver. A lanyard operated iris-type diaphragm valve is mounted on the top of each bag to accommodate regulated release of air. The lift system contains a fill valve assembly that is attached via a hose to an air supply (compressor or scuba) tanks.

Detailed procedures for operating the diver lift system are included in the Diver Lift System O&M manual. Training in use of the lift bag is highly recommended. The training should include in-water experience with ascent, descent, hovering, and load transfer in and out of the water.

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  • Olga
    What is diver in bagwork for underwater construction?
    4 years ago

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