At the request of the UK Department of Energy, a survey was made of all commercial air dives carried out in the UK sector of the North Sea during 1982 and 1983, to determine whether or not this form of diving resulted in an unacceptable incidence of Type I1 decompression sickness (DCS) The report of the survey (Shields and Lee, 1986) was submitted to the Department of Energy in mid-1986, and resulted in a Diving Safety Memorandum (DSM 7/86) which placed restrictions on the permissible bottom time at given depths.
The survey was based on a retrospective analysis of dive logs submitted by the divine contractors In all. 25 740 man-dive records were examined and analysed undeF15 parameters, such as dive depth and time, table depth and time, type of thermal protection, etc Surface decompression diving accounted for approximately 60% of the dives, no-stop diving for 30%, and in-water decompression for only 10% The dives using in-water decompression had, in general, a much less severe hyperbaric exposure than those using surfacing decompression
During the survey period, 79 cases of DCS were recorded, 44 with Type I manifestations, and 35 with Type I1 (neurological) manifestations Although the nature of the data made conventional statistical manipulation impossible, an attempt was made to determine the significance of four contributing factors the severity of the hyperbaric stress, the decompression procedure used, the extent of safety-factoring, and the type of thermal protection used An index of dive severity (the Decompression Penalty Index) was proposed, based on a notional decompression time required for any given exposure, a dive with a DP Index of less than 30 was considered to be modest, one with a DP Index of between 30 and 60 to be of moderate severity and one exceeding-a DP Index of 60 to be severe.
Of these contributing factors, the most powerful influence was the severity of the hyperbaric stress (that is, increasing depth and/or time of the dive) (Table 1) As dive severity increased, there was a greater incidence of DCS When this was(Table 1 and Table 2 are available in full paper) controlled for, there was no obvious difference between the use of in-water stops and surface decompression, although this comparison was only possible on dives of modest severity Thermal protection also had an effect on DCS, with a higher incidence resulting from the use of actively heated (hot-water) suits This influence was only apparent, however, on dives of severe hyperbaric stress Of particular concern was the increasing proportion of the Type II DCS on such dives, where hot-water suits were used (Table 2)
The conclusion from the report was that, although the overall incidence of DCS was commendably low, the total number of dives was such that an unacceptable number of individual cases of neurological DCS were accumulating A ‘no-Type-II’ line was drawn on the depth/time plot of all the dives, to the left of which there would be an ‘acceptable’ incidence of Type II DCS (Fig 1) and this line was used to determine a recommended bottom time at any depth These times are the basis of DSM 7/86 (Table 3)