Methods of approximation and determination of human ...

1 Methods of approximation and determination of human vulnerability for offshore major accident hazard assessment Contents Introduction Estimation of harm - general issues: toxic agents Probits (probit functions) Specified level of toxicity (SLOT) dangerous toxic load (DTL) or significant likelihood of death (SLOD)/approaches used by HSE Immediately dangerous to life or health (IDLH) concept Blast overpressure Direct effects of blast Indirect effects Fragments Whole body displacement Thermal radiation Physiological effects Pathological effects Severity and consequences of exposure The thermal dose unit Probit functions for thermal dose estimation Effects of clothing Suggested thermal dose fatality criteria Hydrocarbon combustion products - 1 - Suggested harm criteria for exposure to hydrocarbon combustion products Carbon monoxide Carbon dioxide Oxygen depletion Combined effects of carbon monoxide, carbon dioxide and oxygen depletion Toxic agents (gases, liquids or solids)

2 Toxic gas effects (mainly NOx, NH3, SO2 and HF) Combination of hazardous exposures and estimation of impairment Effects of other gases Hydrogen sulphide Exposure to hydrocarbon vapours Smoke / obscuration of vision Hypothermia Introduction 1 This appendix provides a summary of information relating to the effects of the hazards offshore personnel may be exposed to in the event of an incident and is intended for use in the preparation and evaluation of risk assessments. 2 In the assessment of survivability or fatality probability, during a major accident on an offshore or onshore installation, it is important to take into account the following factors: Information prior to fire (alarms) Development of incidents Reaction times of personnel Emergency procedures and preparedness Escape time and distance to safety Type of hazard (toxic gas, thermal radiation, blast etc.)

3 Protection and attenuation effects ( shielding or reflection) Harm levels as a function of time (dose) Total exposure time (accumulated dose) Other critical aspects like visibility, toxic gases, explosion loads etc - 2 - Estimation of harm 3 In order to estimate the level of harm from a hazardous agent it is necessary to provide a means to quantify the exposure in terms of the intensity, duration of exposure and consequences of effect. This is usually achieved by an estimation of the received dose and a comparison of this against, statistically manipulated, experimental data to determine the probability of harm to an exposed population or individual. Vulnerability criteria can be established to determine dose levels that result in specific consequences.

4 In this guidance the indicative criteria provides: The threshold of harm above which, protection is required to prevent impairment of the functions an individual requires for escape or to avoid becoming a fatality ( survivability) and, A means for the estimation of fatality probability should dose levels exceed the harm threshold and adequate protection is not present. 4 There are two main approaches for the determination of the effects of received dose: the use of Probit Functions and the determination of Harmful Dose (typically applied to toxic or thermal hazards) Probits (probit functions) 5 Probits account for the variation in tolerance to harm for an exposed population.

5 The fatality rate of personnel exposed to harmful agents over a given period of time can be calculated by use of probit functions that typically take the form: Y = k1 + k2(ln V) (Equation 1) Where: Y = probit, (value range representing 1 fatality) a measure of the percentage of the vulnerable resource that might sustain damage. Fatality probability can then be determined by evaluation of Y on a probit transformation chart such as that provided by Finney 1971 (see Annex 2). k1 + k2 = Constants V = the product of intensity or concentration of received hazardous agent to an exponent n and the duration of exposure in seconds or minutes For thermal radiation, V= I4/3t and is called the thermal dose, with units (kW/m2)4/3 seconds.

6 6 A modified thermal dose unit, V , may be provided where account is taken of additional protection or other mitigation. - 3 - When applied to thermal radiation hazards V (= . V) and is called the effective dose. equals a factor that may be included to account for such issues as the variation in skin area exposed to thermal radiation. ( for normally clothed population and if clothing has been ignited (Lees 1994). 7 For toxic gas V equals the product of gas concentration to an exponent n and time in minutes. Concentration can be reported in parts per million (ppm) or milligrams per cubic metre (mg/m3), thermal radiation probits may use radiation units of watts or kilowatts so care is needed to ensure the correct probit is applied with the correct units.)

7 Probits may be obtained for almost any hazardous agent but are typically available for thermal radiation, toxic gas and blast overpressure effects. For example, several probit functions have been developed for NH3, SO2, and HF. The following are examples commonly used for each gas where the probit equation described earlier takes the form: Y = k1 + k2ln(Cnxt) (Equation 2) Where; C = hazard concentration (ppm) and t = time in minutes Table 1: Probits for hazardous gases taken from NORSOK Z013 (DNV / Scanpower) Substance K1K2n LC50 5 min exposure Ammonia 2 15240 Sulphur Dioxide 1 3765 Hydrogen Fluoride HF 1 11845 8 There are many published probits for estimating fatality levels from exposure to harmful agents for example the values listed in Table 2 are taken from Lees (2005) using the Perry & Articola (1980) values for Ammonia, Chlorine and Hydrogen Fluoride.

8 Many probit sets have been produced by various sources including Louvar and Louvar (1998) and the Green Book (1992) and there can be significant variation in the dose effect estimates for each probit equation for the same hazard. Table 2: Sample probits taken from Lees (2005) and estimated dose effects. LC1 (ppm) LC50 (ppm) Material K1K2n 5 min 30 min 5 min 30 min - 4 - Acrolein 93 16 291 48 Ammonia 15057 6147 28264 11539 Benzene 18096 7388 22545 9204 Carbon monoxide 11810 1968 22169 3695 Chlorine 173 71 613 250 Hydrogen chloride 3464 577 11106 1851 Hydrogen sulphide 897 256 1543 441 Nitrogen dioxide

9 2 160 65 367 150 Phosgene 77 13 145 24 Sulphur dioxide 1241 207 3764 627 Toluene 5352 2614 51965 25377 Hydrogen Fluoride 1 19652 3260 39184 6531 Hydrogen Cyanide 564 161 969 277 9 Probit Analysis is an approximate methodology but it does allow quantification of consequence resulting from exposure. However, care must be taken in probit equation selection as the estimate fatality probability can vary significantly for the same hazardous agent depending on the probit selected.

10 This is demonstrated later [see para 51]. Care must also be taken to ensure the units of concentration are appropriate for the probit equation used. Specified level of toxicity (SLOT) or significant likelihood of death (SLOD) 10 The probit approach is one means of estimating the level of fatality for exposure to a hazardous agent and an alternative is the application of the Specified Level of Toxicity (SLOT) or Significant Likelihood Of Death (SLOD) approaches proposed by HSE. 11 The SLOT approach is described by Turner and Fairhurst (1993) and involves the use of the most relevant toxicity data available that is then extrapolated for use on man. In its usual application the estimated dose is termed SLOT Dangerous Toxic Load (or SLOT DTL).