Underwater lift bags are a means of lifting an object underwater independent of surface support. The lift bag utilizes buoyancy created by displacing sea water with air. Lift bags come in various shapes and size. Once in place the bag is filled with air creating a buoyant sphere that is greater than the weight of the object being moved.
There are two types of lift bags, Parachute, which is open on the bottom and Sphere which is closed. The most commonly used lift bag in the offshore oil industry is the Parachute bag. This type of bag can be rigged to empty its contents in the case of an uncontrolled ascent.
[I]n any lifting activity a good lift plan should be developed and followed. The first step is to evaluate what is being lifted:
Is the lift Dynamic or Static? Dynamic lift is when the object is being lifted off bottom and relocated. Static lift is when the object being lifted straight off bottom and is restrained to the bottom. Even though during a dynamic lift the object is being relocated if at all possible there should always be some type of hold back rigging to prevent uncontrolled ascent of the load and the bag.
Confirm the weight of what is being lifted. Construction drawings should be used if possible. If buried take into consideration mud suction.
Once it is determined what is being lifted then the next step is to develop the lift plan:
Determine the safety factor to be used for rigging, (See Appendix #3 for recommendations)
Calculate the size of the lift bags and rigging. A lift bag that has a buoyancy capacity much larger than the planned lift should be avoided.
Determine center of gravity on the object being lifted
Calculate the number of bags to be used and their spacing. Spacing must be enough to allow the lift bags to completely fill and not interfere with each other.
Determine rigging points
Determine hold back rigging points
Determine where inverter line should be secured.
Determine a lift bag deflation plan.
Once the plan is in place then the divers that are executing the plan are selected:
Confirm the divers are trained to safely carry out the plan
Confirm the divers understand the plan and have input into the plan.
If possible a trial run can be performed in a diving tank to confirm the diver has proper working knowledge and operating experience.
Improper use of lift bags has caused injuries and fatalities in the past! Read the rest of this document to learn more!