Identification of differential pressures subsea during diving operations

A member has reported an incident in which, during diver intervention work on a subsea pipeline, considerable suction was encountered when the diver attempted to separate a pair of flanges. The diver noted the suction before the flange was completely separated and called a stop to operations before any injury occurred or damage resulted.

An investigation revealed that there was confusion and lack of understanding regarding the difference between the internal pipeline pressure and the subsea ambient pressure caused by the water depth.

Although negative pressure (suction) is something that divers are especially concerned about and aware of, it is not always a generally understood hazard that is readily comprehended or identified by persons not normally involved in diving operations, i.e. the concept of standing in and working in a space where the ambient pressure is greater than that contained in a void which is being opened, for example, when a diver opens a bolted flange on a pipeline.

The following example was given:

• If a pipeline from a platform extends to the seabed at 150msw, the absolute pressures and differential pressure effects are considerably different for a surface worker on an installation and the diver on the seabed. If the installation pressurises a pipeline to 10 Bar gauge (11 Bar absolute), this shows itself as a net positive pressure of 10 Bar to the person on the installation and a net negative pressure of 5 Bar to the diver at 150msw (16 Bar absolute).

Because of the opportunity for confusion and the potential for two completely different hazards (retained pressure and suction) it is important that any pressures noted in project procedures, permits, risk assessments, dive plans etc. are done so in a clear, concise and objective way ensuring that there is no room for misinterpretation or misunderstanding.

When recording or stating pressures in documents where there is scope for misinterpretation due to differential pressures during diver intervention works, all pressures should be noted as the actual pressure value, such as Pressurise pipeline to 12 bar gauge rather than (as is often the case) relative or referenced pressure values such as Pressurise pipeline to 2 bar greater than seabed ambient. If not completely unambiguous, instructions should also state where the pressure is measured e.g. whether on the installation, or subsea.

Further information is available in the following IMCA guidance:

• IMCA international code of practice for offshore diving
• Guidelines for isolation and intervention: Diver access to subsea systems