Responding to the challenges posed by the offshore petroleum industry’s move into ever-deeper waters, commercial diving contractors have kept pace with the demands placed on them by introducing technologies more appropriate to working at such depths.
While traditional surface-based diving methods were logistically easier to deploy, (and adequate for many of the tasks conducted on air), any cost benefit savings diminished with depth. Mixed gasses were expensive to use and the diver was still limited in his effective working bottom time by the need to carry out lengthy and non-productive decompression schedules: Factors that helped to prolong the successful completion of a job while eroding profits.
Based on the assumption that once the tissues in a diver’s body become completely saturated with an inert gas then their decompression obligations will be the same – regardless of whether they remain at depth for hours, days, or even weeks – the US Navy began, in the late ‘fifties, to experiment with the concept now known as saturation diving.
First successfully used by a commercial diving contractor in 1965, saturation divers remain in a pressurised environment equivalent to the depth at which they will be working; either until the job is completed or until they are rotated back to surface pressure and replaced by another “sat” team.
Living in a deck-mounted hyperbaric chamber where they continuously breath a gas mix appropriate to the depth at which they will be working, “sat” divers will often remain under pressure for thirty days or longer, only needing to carry out one lengthy decompression procedure at the end of their tour.
With none of those necessary interruptions that accompany surface diving operations saturation diving systems are now regularly employed in water depths ranging between 50- and 400-metres plus.
Establishing a regular work and sleep pattern that can allow for around-the-clock operations, divers ‘commute’ to the job site from their surface habitat in a similarly pressurised diving bell.
Usually carrying two- or three-man teams, the bell locks onto the main chamber. Once the divers are inside and the pressure locks are sealed the bell is lowered down to the work site. Connected to the surface by a lifting wire, stabilising wires and an umbilical hose that provides the divers with gas; hot water to heat their suits; and diver-to-diver and diver-to-surface communications, the internal pressure of the bell remains above that of the surrounding water; a practical application of Boyle’s Law that allows divers to safely exit and re-enter the bell through the opened lower hatch.
Bell run times, (the time taken for the bell to lock-off from the hyperbaric living chamber until locking back on at the end of a working shift.), are usually in the region of eight hours. While one person – the bellman – always remains in the bell to act as tender; stand-by diver; and to tend monitor the life support systems, the working diver can remain ‘locked-out’ from the bell for four hours or more before switching roles with the bell man.
Unlike surface-based diving operations, heading for the surface in an emergency is not even an option for saturation divers. Beyond any immediate assistance the bell becomes the divers only refuge. Designed as a self-contained life support system with complete instrumentation and scrubbers to eliminate dangerous build-ups of carbon dioxide, a diving bell carries its own on-board gas supply should the surface to bell umbilical pressure fail.
And in the event that the lifting cable snaps and the divers have to endure a lengthy wait for a rescue attempt, most bells now carry survival packs that include water, energy foods and thermal protection suits.
Although deploying a saturation diving system is costly the advantages are many: Personnel and equipment requirements change very little regardless of the water depth: Working together for longer periods of time, a saturation diving team tends to be more productive than their surface-based counterparts: Effective working time is increased dramatically: Adverse weather conditions are less likely to interrupt diving operations; and with larger, more technically competent crews, sophisticated equipment and a heavy emphasis on back-up redundancy, saturation diving has proven itself as a safer alternative to surface-based diving.