Choosing the right buoyancy bag for the Job

Basic Concepts

Because flexible buoyancy relies solely on internal overpressure to work, a flexible buoyancy unit will decrease in volume as it goes deeper because the air inside becomes compressed and occupies a smaller volume. Conversely, as it rises, the air inside will experience a lesser surrounding pressure and so expand to provide a greater volume.

As the amount of buoyancy (lift) is equal to weight of the volume of water displaced, it follows that if no air is gained or lost, the amount of lift varies in proportion to the depth.

In other words, a lift bag going down is compressed and provides decreasing buoyancy while a bag coming up expands and provides increasing buoyancy.

Types of Bags

There are two basic types of flexible buoyancy units with fundamental differences:

Air Lifting Bags. (ALBs)

These are designed to dynamically lift objects from depth and rise up through the water column dumping excess air from the open bottom as they ascend. Air can be fed in through the ¾” valve on the top or from an airline in the open bottom while the dump valve operated by a lanyard at the bottom of the bag allows good control of the buoyancy. The rig only requires a single point fixing which allows them to remain upright and stable regardless of the angle of the load.

Seaflex models provide the maximum buoyancy for the least overall height and their robust construction makes them easy to use and hard to damage. The larger bags can be towed at speeds up to 1.5kts on the surface in moderate sea states.

Inflatable Buoyancy Units (IBUs)

Sometimes referred to a fully enclosed bags or pontoons, these units have the primary purpose of decreasing the weight of a static underwater object or providing buoyancy to objects on, or very close to the surface. They are not designed or suitable for dynamic lifting from any depth. Their rigging requires multiple fixing points at set spacing and they must be kept close to horizontal at all times. Their capacity to dump excess air through Pressure Release Valves (PRVs) is limited and any outside pressure variation must be carefully controlled.

Choosing a suitable lifting bag.

Air Lifting Bags (ALBs) are really the only sensible choice for vessel salvage and the recovery of underwater objects although they also used by offshore contractors for static buoyancy at any depth as they only need a single attachment point and are so easy to handle underwater.

When used for vessel salvage, ALBs need the minimum number of fixings, always remain stable and are easy to fill and control. During the lift they accept the high angles of trim that often occur when raising a vessel and can survive extreme situations with little or no damage. Their strength, ease of use underwater and predictable behaviour make them the most effective and safest option for any ‘lifting operation’.

If the load simply needs lightening or supporting on or close to the surface, then Inflatable Buoyancy Units (IBUs) are the best choice. Typical applications are vessel draft reduction where pre-set fixings can be used or for pipe pulls where there is no possibility excessive trim angles.

They are totally unsuitable for salvage work because of the many and accurate fixing points that are required and their unpredictable behaviour when filling. There is no control of the buoyancy and they are extremely susceptible to expensive damage if the vessel lifts at one end before the other, as is almost always the case. The extra time and complexity required underwater coupled with their unpredictable and uncontrollable behaviour make these units unsafe and uneconomic for salvage work.

Conclusion

While these notes hope to describe the basic characteristics of bags there are just about as many different solutions as problems, so please don’t hesitate to contact us with any specific requirements not covered by these basic comments above.

If we can provide any useful information - we’ll be very happy to do so.