Laying Underwater Pipelines and Cables

Laying underwater pipelines and cables is fraught with difficulty and would be almost impossible without the use of buoyancy bags. Buoyancy aids make the process of moving and laying underwater objects a much simpler process through the use of a relatively simple technology based on a simple concept.

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IBUs used for pipeline buoyancy

The ‘Overpressure’ Principle

Inflatable buoyancy aids are made from flexible materials and become squashed under pressure but expand as pressure decreases. They work on the principle of overpressure. As the bag is lowered deeper into the water, it decreases in volume because the air inside gets compressed and occupies less volume.

On the other hand, as it is raised the air expands and the volume of the bag increases. Buoyancy is equal to the mass of the water displaced by the bag, which is proportional to the volume of the bag. It follows, therefore, that buoyancy decreases as a bag is lowered deeper into the water and increases as it rises up. Using an air hose attached to the bag, more air can be pumped into it whilst in a compressed state. Then as the bag is raised it expands in volume quickly and a huge amount of buoyancy is created. Even a small buoyancy aid can easily lift a weight equivalent to five family cars.

ALBs and ILBs

The two basic types of buoyancy units are ALBs (Air Lifting Bags) and ILBs (Inflatable Buoyancy Units). ALBs are used in operations such as cable floatation where an object needs lifting from the sea bed. They have just one point of attachment, which means that they will remain upright at all times and can be used at any depth. ILBs are used to increase, for example, pipeline buoyancy close to the surface or the buoyancy of any static object just under the water. They cannot be used at great depths and have multiple points of attachment.

Generally, ALBs are the only option when salvaging a vessel or lifting a cable or pipeline deep under water. If an object such as a pontoon or a section of pipeline that is waiting to be laid needs to be floated on the surface of the water, then ILBs are the correct choice. However, there are many complications when working underwater and the choice is not always so simple so it is well worth consulting the experts before starting any work.

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