FAQs for Underwater Lifting Bags

What is the Archimedes Principle?

This explains the nature of buoyancy and why certain objects become heavier when removed from underwater. It basically states that any object, whether wholly or partially submerged in a liquid, experiences an upward thrust equal to the weight of the liquid displaced.

What is positive, neutral, negative and buoyancy?

If the weight of the a submerged object is less than the weight of the displaced liquid, then it has positive buoyancy and will float. Should the two weights turn out to be equal then the submerged object will have neutral buoyancy, leaving negative buoyancy (or sinking) where the weight of the object is greater than the liquid it displaces.

Do I need more buoyancy in a lake than in the ocean?

The buoyant force of a liquid is dependent on its density, which is its mass per unit volume. Fresh water with a density of  62.354 lbs per cubic foot offers less upward thrust than salt water, which has a density of  63.936 lbs per cubic foot. Therefore, less buoyancy is required to lift an object in the ocean, than the same object if it's in a lake or river.

Why do you need pressure relief valves on the bag?

One of the major challenges associated with buoyancy recover systems is controlling the ascent velocity once actual lift has commenced. As the bag ascends to the surface, the gas inside it will expand, increasing its net buoyancy, which in turn increases the ascent rate. The valves are used to slow down this rate and in some cases on totally enclosed systems prevent the bags from bursting. It is recommended the ascent rate never exceed 2-3 ft per second.

What might be the result of too fast an ascent rate?

Should a lift bag rise at a rate faster than 10 feet per second, a phenomena known as velocity head may develop. This results when the force due to the upward motion reacts against the top face forming a pressure head, deforming the lift bag and causing it to become unstable. In an exreme situation this may cause the bag to dump air, lose buoyancy and return the load to the bottom.

Can the object be towed after the lift with the bags still attached for buoyancy?

This may be achieved using totally enclosed bags. However, towing speed should be kept to minimum (no more than 2-3 knots) depending on sea state. Excessive speed could cause pressure to build up in front of the bags and reduce volume and lift.

Additional Notes on the use of underwater lift bags:

When using totally enclosed lift bags, it is essential that they are secured in a such a way that they remain horizontal during the lifting operation.  If they deviate, the air will migrate to the higher end of the bag causing deformation and loss of efficiency.

When bags are securely attached, partially inflate the bags prior to commencing the lift. This ensures that all straps are correctly positioned and the bags assume their proper shape.

Under no circumstances should underwater lift bags be used as proof load bags for testing lifting equipment such as cranes. They are not designed to handle dynamic forces subjected during crane lifting parameters.

Extreme care should be taken when using lift bags to overcome seabed suction or freeing mechanically locked or snagged equipment. A hold back Strap and anchor should be available which is heavier than the up thrust created by the lift bag.