Saturation diving

Saturation diving is an ambient pressure diving technique which allows a diver to remain at working depth for extended periods during which the body tissues become saturated with metabolically inert gas from the breathing gas mixture. Once saturated, the time required for decompression to surface pressure will not increase with longer exposure. The diver undergoes a single decompression to surface pressure at the end of the exposure of several days to weeks duration. The ratio of productive working time at depth to unproductive decompression time is thereby increased, and the health risk to the diver incurred by decompression is minimised. Unlike other ambient pressure diving, the saturation diver is only exposed to external ambient pressure while at diving depth.
The extreme exposures common in saturation diving make the physiological effects of ambient pressure diving more pronounced, and they tend to have more significant effects on the divers' safety, health, and general well-being. Several short and long term physiological effects of ambient pressure diving must be managed, including decompression stress, high pressure nervous syndrome, compression arthralgia, dysbaric osteonecrosis, oxygen toxicity, inert gas narcosis, high work of breathing, and disruption of thermal balance.
Most saturation diving procedures are common to all surface-supplied diving, but there are some which are specific to the use of a closed bell, the restrictions of excursion limits, and the use of saturation decompression.
Surface saturation systems transport the divers to the worksite in a closed bell, use surface-supplied diving equipment, and are usually installed on an offshore platform or dynamically positioned diving support vessel.
Divers operating from underwater habitats may use surface-supplied equipment from the habitat or scuba equipment, and access the water through a wet porch, but will usually have to surface in a closed bell, unless the habitat includes a decompression chamber. The life support systems provide breathing gas, climate control, and sanitation for the personnel under pressure, in the accommodation and in the bell and the water. There are also communications, fire suppression and other emergency services. Bell services are provided via the bell umbilical and distributed to divers through excursion umbilicals. Life support systems for emergency evacuation are independent of the accommodation system as they must travel with the evacuation module.
Saturation diving is a specialized mode of diving; of the 3,300 commercial divers employed in the United States in 2015, 336 were saturation divers. Special training and certification is required, as the activity is inherently hazardous, and a set of standard operating procedures, emergency procedures, and a range of specialised equipment is used to control the risk, that require consistently correct performance by all the members of an extended diving team. The combination of relatively large skilled personnel requirements, complex engineering, and bulky, heavy equipment required to support a saturation diving project make it an expensive diving mode, but it allows direct human intervention at places that would not otherwise be practical, and where it is applied, it is generally more economically viable than other options, if such exist.

History

On December 22, 1938, Edgar End and Max Nohl made the first intentional saturation dive by spending 27 hours breathing air at 101 feet sea water in the County Emergency Hospital recompression facility in Milwaukee, Wisconsin. Their decompression lasted five hours leaving Nohl with a mild case of decompression sickness that resolved with recompression.
Albert R. Behnke proposed the idea of exposing humans to increased ambient pressures long enough for the blood and tissues to become saturated with inert gases in 1942. In 1957, George F. Bond began the Genesis project at the Naval Submarine Medical Research Laboratory proving that humans could in fact withstand prolonged exposure to different breathing gases and increased environmental pressures. Once saturation is achieved, the amount of time needed for decompression depends on the depth and gases breathed, and does not increase with further exposure. This was the beginning of saturation diving and the US Navy's Man-in-the-Sea Program.
The first commercial saturation dives were performed in 1965 by Westinghouse to replace faulty trash racks at on the Smith Mountain Dam. In the same year, the Conshelf III experiment was carried out by Jacques Cousteau and a group of divers for three weeks at the depth of 100 m.
Peter B. Bennett is credited with the invention of trimix breathing gas as a method to eliminate high pressure nervous syndrome. In 1981, at the Duke University Medical Center, Bennett conducted an experiment called Atlantis III, which involved subjecting volunteers to a pressure of 2250 fsw, and slowly decompressing them to atmospheric pressure over a period of over 31 days, setting an early world record for depth-equivalent pressure in the process. A later experiment, Atlantis IV, encountered problems as one of the volunteers experienced euphoric hallucinations and hypomania.
Early experiments in saturation diving were often done in underwater habitats, usually serviced by surface vessels, but in relatively exposed waters the advantage of being able to abandon the site in bad conditions encouraged the use of saturation facilities on board the support vessel, where logistics are simplified.
The history of commercial saturation diving is closely linked to offshore oil and gas extraction. In the early 1960s exploration of the North Sea started on the premise that the Dutch gas fields might extend under the sea. This was borne out when the Gulf Tide drill rig hit the Ekofisk reservoir in 1969, and in 1971 Shell oil found the Brent oilfield between Norway and Shetland. From this time to the 1990s the industry developed the procedures and equipment for saturation diving from pioneering and experimental, with a somewhat dubious safety record, to a mature industry with greatly improved occupational health and safety.
The first US Navy saturation procedures were published in the US Navy Diving Manual revision 2 of 1979. They allowed an upward excursion to start decompression, used a constant decompression rate deeper than 60 msw and varied rates from 60 msw to surface. Decompression was interrupted for a night stop and an afternoon stop, and partial pressure of oxygen was constant deeper than the fire risk zone, after which oxygen fraction was limited.
When the North Sea drilling started, there was little diving support infrastructure in Europe, and the high wages attracted divers from the Gulf of Mexico oilfields, who introduced the fibre reinforced resin lightweight demand helmets from Kirby Morgan, hot water suits from Diving Unlimited International, and the U.S. Navy Diving Manual, at the time the leading set of offshore diving procedures. Research and development money was available, and new technical developments were supported by the European Economic Community. A major challenge was developing saturation diving practices suitable to the common North Sea depth range of 100 to 180 m.
During the early drilling stages most of the diving work was for relatively short periods and was generally suitable for bell bounce diving, but the development of oilfield seabed infrastructure required much longer diver interventions, and saturation diving procedures were developed to suit. By 1982, a large amount of shallow maintenance work was becoming necessary, which brought in more air diving to service the rigs. By 2017 about 80% of North Sea diving was heliox saturation diving and the other 20% shallow air diving.
Excursion dives without decompression stops can be done both upward and downward from saturation storage pressure within limits, allowing the divers a range of working depths, and if work is required beyond excursion range, the divers can be compressed or decompressed in storage to suit the changed depth range. Further work was done by the United States Navy Experimental Diving Unit on excursion dives from February 1974 to June 1976, and the results published in the 1984 U.S. Navy Diving Manual. These tables used a partial pressure of oxygen of 0.35 to 0.4 bar during decompression, with quite slow decompression rates, which varied with the depth, getting slower as the depth decreased, with a 6-hour stop from midnight and a two-hour stop from 14:00 and a gas fraction limit for oxygen of 22% for the last part of the ascent to reduce fire risk. The tables allowed decompression to start directly after return from a dive provided there had not been an upward excursion, as this was found to increase the risk of bubble development.
At the same time, the commercial diving contractor Compagnie maritime d'expertises had been developing slightly different decompression procedures, in which the oxygen partial pressures were higher, between 0.6 and 0.8 bar, and the ascent rates were faster to take advantage of the high P_O₂. Continuous decompression without night stops was used, and excursions were allowed. Over time these were revised to use lower P_O₂ and slower ascent rates, particularly at the shallower depths. Competing tables were thought to be used to gain competitive advantage, so in 1988 the Norwegian Petroleum Directorate organised a conference on saturation decompression safety under Val Hempleman, and in 1990 a conference to harmonise the saturation tables to be used in the Norwegial sector of the North Sea used input from five contractors. In 1999 the NORSOK U100 standard was published, which was a compromise using aspects of several of the tables, and has proven in use to be sufficiently conservative with a good safety record.
In the 1980s the Royal Navy were using an oxygen partial pressure of 0.42 bar for decompression from saturation, which is slightly higher than the 0.40 bar of the US Navy table. This reduced the time for decompression by a small percentage.
The 1981 salvage of the gold on HMS Edinburgh at 256 m was the first commercial use of helium reclaim systems,and set a new depth record for sustained commercial saturation diving.
Saturation decompression in the Brazil oilfields took a slightly different route, and was originally based on company tables, until Brazil produced their own legislation in 1988, similar to that of the UK's Health and Safety Executive. In 2004 revised legislation was closer to the COMEX procedures.
There have been no major research projects since the Norwegian deep diving contracts, and since then commercial procedures have evolved through cumulative empirical adjustments based on the experience of freelance personnel moving between companies, changes due to mergers and takeovers between contractors, and guidance by regulations, industry standards and client requirements. By 2017 the system had settled into a chamber P_O₂ of 0.5 bar while deeper than 15 msw, and limited to 22 to 23% at the end of decompression to limit fire risk.