Virginia Trunk Limitations DCS Risk Calculations

Abstract

Abstract : The incremental increase from prior decompression sickness (DCS) risk estimates due to waiting an additional 5 seconds for the escape hatch to fully open is estimated at only 0.1 to 0.3% DCS risk. This estimate applies to 600-foot escapes both from an unpressurized ship with a variety of pressurization profiles, and for the 11 foot (about 1 1/3-atmosphere) saturation case. The increase is clearly minor and well within the uncertainty of the base predictions provided previously. Estimation of six hundred foot escape after saturation from 23 feet is harder to answer. The main mathematical model (USN93) predicts a DCS rate of 13.5%, and after adding 2 standard deviations for model uncertainty and a 30% bias correction, this yields an upper bound of 21%. For the same escape from 11 feet pressurization, the same upper bound gives an escape DCS probability of 9 %. However, USN93 is known to be less accurate with saturation exposures in that range. A recent UK model with the same limitations provides similar results. After examining the raw data of human saturation exposures, and a recent model of them by Lillo and colleagues, it appears that 600 foot escape from 23 feet saturation carries about an 8% risk of DCS. The risk estimates refer to the full range of DCS symptoms. An unpublished description of expected severity by Thalmann and Weathersby suggested that a DCS risk at less than 10% would result in a probability of very few permanent problems and that DCS risk of 10-20% would include some cases requiring recompression within a few hours . Better estimates could be made from a purpose-built submarine escape DCS model, which we have proposed to develop. Much better estimates should be available in one year, at the successful completion of the project DISSUB escape from depths greater than 600 fsw.