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Indicative human vulnerability to the hazardous agents present offshore for application in risk assessment of major accidents

SPC/Tech/OSD/30

Purpose

This document provides guidance on the application of indicative human vulnerability criteria for risk assessment when applied by the offshore industry.

It applies to the assessment of major accident hazards and the consequences of acute exposure in terms of impairment and survivability of persons exposed. SPC is not to be used in the assessment of chronic or long-term exposure to any of the hazardous agents described.

Background

This SPC provides guidance to inspectors in support of OSD’s offshore intervention guide (APOSC). It provides indicative criteria on human vulnerability to the hazards encountered offshore during an emergency for use in risk assessment.

There is a supporting document to this SPC entitled “Methods of Approximation and Determination of Human Vulnerability for Offshore Major Accident Hazard Assessment"

The main objective of an adequate and appropriate vulnerability model for exposure to a harmful agent is to predict the degree of harm that a given level of exposure may cause. These harm criteria enable an estimation of consequence that, when combined with an appropriate estimation of the likelihood of such exposure, allows a robust estimate of risk. The establishment of a consistent set of criteria for use in fatality assessments is of paramount importance in offshore risk analyses as it provides uniformity in the risk assessment output. Vulnerability needs to be assessed not only for fatal outcomes, but also where it could seriously affect the mental or physical performance of personnel, reducing their ability to survive an incident because of injury or reduced decision-making capability.

Safety Case assessors should apply the criteria in this document to confirm that an adequate range of suitable impact criteria has been used to demonstrate risks are ALARP.

Offshore risk assessment, such as that provided in a “compliance demonstration”, should be based upon the impact of exposure to the following acute physiological effects:

Risk assessment principles

For a risk assessment to be deemed adequate it must include a suitable and sufficient demonstration that all hazards that may be present, or produced as the result of an incident, have been identified and appropriately assessed. Evaluation of a particular hazardous agent should reflect the likelihood and consequences of its release. In the event of low consequence exposures and low exposure probability qualitative risk assessment by competent persons may be all that is required. As the consequence of exposure becomes more severe and / or the probability of exposure increases, then the complexity of risk assessment required for the achievement of a compliance demonstration may also increase. In certain circumstances the achievement of compliance will require the application of fully quantified risk assessment (QRA), with some qualitative arguments, to justify the adequacy of assumptions and understanding of uncertainty in the calculations.

Irrespective of its complexity, a risk assessment should:

It is proposed that the consequences of exposure to a hazardous agent (or agents) should be based on the effects of the dose received. Dose is estimated on the basis of the product of intensity of exposure (possibly to some exponent) and duration of exposure (typically in minutes or seconds).

QRA provides a numerical estimation of the acute effects of hazard exposure. The value of this estimate can then be compared to a numeric criterion to assess the acceptability of the risk or used to assist with risk ranking and the determination of appropriate risk reduction measures (i.e. by input in to CBA). However the risk assessment process cannot be considered suitable or sufficient without inclusion of the effects of lower exposures. Some doses may not be sufficient to produce death or severe injury but could be sufficient to impede escape or evacuation possibly resulting in an increased risk of death or serious injury.

The precision, complexity and depth of risk assessment, whether qualitative or quantified, should be commensurate with the level of risk posed by the installation. To prevent the under estimation of risk the use of optimistic values should be avoided. While a pessimistic approach will produce an over estimate of risk, with its associated costs to establish ALARP, this is favoured by many because it has the benefit of dealing with the uncertainty of the inputs in to the analysis. To prevent the, potentially unreasonable, cost burden associated with pessimistic inputs while avoiding an underestimate of risk it is recommended that risk assessment input parameters are determined on a “Cautious best estimate” basis.

Possible areas requiring specialist assessment

A compliance demonstration may utilise vulnerability predictions that are markedly different from those given in this document or the “Methods of Approximation and Determination of Human Vulnerability for Offshore Major Accident Hazard Assessment”. Under such circumstances clear and adequate justification for the use of the alternative criteria should be included as part of the demonstration.

Establishment of “survivability criteria”

The most typical hazard exposure levels that an offshore worker would be able to survive, without being significantly impaired, are summarised in Table 1. These hazard exposure levels are provided for use in the assessment of accidents where emergency action will be required to recover or mitigate an incident. It should be noted that these values are not for use as occupational exposure limits. Estimates of the lethal dose required to produce a 50% fatality level in the exposed population has also been included as this can be typically used to estimate fatality levels in large or dispersed groups of exposed individuals. The use of 50% fatality dose is best applied where the exposed population experiences the reduction of hazard effect as the distance from the source increases. In such cases it is reasonable to assume that the numbers of individuals inside the 50% fatality zone that survive are equivalent to those beyond this region who receive a lesser but still fatal dose. The 1% lethality dose (LD1) is also included to indicate the value representing 1- 5% fatality, 50% hospitalisation and severe distress for the remainder of an exposed population.

To account for the potential for short duration exposure (i.e. during escape to or evacuation from the TR) of up to a few minutes and exposure of longer duration (i.e. occupancy of the TR or control room for up to 30 minutes) two sets of criteria are provided in Table 1.

Note: Throughout this SPC the following units will be typically used:

“N/A” indicates no value available at the time of preparation of Table 1.

Where other units have been used to define the criteria in this SPC the reader should consult “Methods of Approximation and Determination of Human Vulnerability for Offshore Major Accident Hazard Assessment” where clear explanation is provided.

It is also cautioned that when preparing or assessing a compliance demonstration care is taken with units such as concentration as many errors have been noted in the available literature and relevant texts.

Suggested survivability and fatality criteria

Tables 1 and 2 provides criteria to estimate:

Table 1: Suggested short and long term survivability for the principle major accident hazards offshore

Hazardous agent Survivability criteria
Short term exposure Long term exposure

Overpressure (barg) (out of doors)

0.21

N/A

Thermal radiation (kWm-2)

4.0
(< 3 minutes)
6.0
(Maximum)
(1.5 minutes)

1.6
(10 minutes)
(Without heavy clothing)

Air temperature (ºC)

140
(5 mins)

75-80
(60 min)

Thermal stress

Skin wettedness (%)

N/A

< 85

Sweat rate (g/hr)

N/A

< 390

    Heat storage (w-h/m2)

< 50

< 60

    Water loss (g)

N/A

3250

Cold water immersion (1)

30 minutes
@ 5ºC

60 minutes
@10ºC

Carbon dioxide (ppm) (2)

30000
(3%)
(15 minutes)

20000
(2.0%)
(30 minutes)

Carbon monoxide (ppm)

1000
(5 min)

500
(30 min)

%Carboxyhaemoglobin
in Blood (COHb) (3)

< 15
Dim vision, Impaired dexterity

< 10
Headache, impaired vision

Oxygen depletion (% 02)

15
(5-10min)

17-18
(60 min)

Hydrogen sulphide (ppm)

300
(5 min)

200
(30 min)

Noise (dBA)

115
(30 s)

100
(15min)

Total body impact (m/sec)

3

N/A

Smoke Obscuration Visibility (metres)

3
(primary)
(compartment)

10
(escape route) (4)

“C5” Hydrocarbons

3000ppm
(10 min) (5)

N/A

Details and explanatory text are to be found in “Methods of Approximation and Determination of Human Vulnerability for Offshore Major Accident Hazard Assessment”.

  1. Maximum exposure time doubles (in minutes), from 0ºC, for each 5ºC increase, at 0ºC maximum exposure time is 15 minutes.  
  2. Preliminary values only and are currently subject to review.
  3. %COHb is an accumulative effect due to continuing respiration of carbon monoxide and as smokers may have up to 5% COHB they would be expected to cross exposure thresholds more quickly.
  4. This may remain at 3m where floor guide lights are fitted and are operational.
  5. The higher molecular weight (“heavier”) alkanes have a proportionally higher potency as anaesthetics (e.g. n-heptane is around 10 times more potent than n-pentane).

Table 2: Suggested fatality criteria (based on HSE toxicology)

Hazardous Agent Fatality Criteria
LC1 (ppm)
(5 min exposure)
LC50 (ppm)
(30 min exposure)

Ammonia

8695

5859

Hydrogen bromide

2440

1627

Hydrogen chloride

4740

2550

Hydrogen cyanide

196

120

Hydrogen fluoride

2400

700

Hydrogen Sulphide

795

841

Carbon Monoxide

8025

1900

Nitrogen oxides (as NO2)

139

144

Sulphur dioxide

965

1576

Methanol

160400

89000

Carbon dioxide

86028

91700

Other Hazards
  Onset of fatality Estimated 50% fatality
Miscellaneous

%Carboxyheamaglobin
in Blood (COHb) **

40-50
Confusion, Collapse

50-70
Convulsions & Death

Oxygen Depletion (% 02)

8-10
(< 5 minutes)

6-8
(<10 minutes)

Air Temperature (ºC)

160

N/A

Instantaneous events

Total Body Impact (m/sec)

6.4

16.5

Overpressure (barg)
(out of doors)

0.25-0.35

0.5

Short Duration Exposure Hazards

Thermal radiation (kWm-2)

kWm-2 (time)
12.5 (32s)
6.0 (90s)
4.0 (160s)

kWm-2 (time)
12.5 (1 min)
6.0 (3 min)
4.0 (5 min)

** %COHb is an accumulative effect due to continuing respiration of carbon monoxide and as smokers may have up to 5% COHB they would be expected to cross exposure thresholds more quickly.

Further information

For further information please contact OSD3.2 0151 951 3144

Updated 2012-03-14