398039 Escape Routes Selection for Offshore Units Based on Quantitative Risk Assessment Results
Escape, evacuation and rescue is the process of getting installation personnel to a place of safety given an emergency condition, where such place may be on or off the installation, depending on whether evacuation is required in the course of a particular emergency situation.
Ideally, personnel that survive from direct effects of an accidental scenario will try to reach a temporary refuge (TR) area. According to SOLAS and other requirements, as DNV Classification Rules:
All escape routes shall have sufficient clearances to allow personnel to move quickly and safely around the installation in an emergency
Signs indicating preferred direction of escape shall be strategically positioned along escape routes.
Escape routes shall be as straight and direct as possible leading persons away from potential hazards towards the TR by the shortest route possible.
The objective of this paper is to evaluate the different escape routes in an offshore installation and propose the best alternatives of escape for personnel in the process area (top side) up to temporary refuge.
Several factors impact on the ability of a person to escape during an emergency, being one of the main factors the possible effects of accident events towards the escape routes, reducing the chance of people to arrive at the refuge.
According to NORSOK Z-013, the following accidental and environmental load categories shall be used when distinguishing between different types of hazards and loads that shall be assessed and compared separately against the defined risk acceptance criteria for loss of main safety functions:
Heat loads (e.g. due to HC processing leaks, riser/pipeline leaks, blowouts or fires in combustible material).
Smoke and toxic loads
Explosion loads. This includes static pressure loads, dynamic pressure loads etc
Impact loads (e.g. collision loads from vessels, helicopters drifting icebergs etc, dropped object loads from lifting operations, falling ice etc.)
Extreme environmental loads such as: from wind, wave, current earthquake
In order to assess the impairment and evaluate the escape routes, the analysis shall followed the steps below:
Step 1: Review of all alternative escapes, from each manned area to the TR: considering the defined routes in the unit, both main and secondary;
Step 2: Calculate for each escape route the impairment cause by the different accidental scenarios, identified and evaluated through the Quantitative Risk Assessment, considering elements 1 to 3 above;
Step 3: Calculate the time for personnel to get from the area affected by the accidental scenario up to the refuge area;
Step 4: Assess from the impairment values, which escape route is the least impacted and that leads the lower time to escape;
The critical features of the mustering process are the availability of the escape routes and the time to escape. The escape routes may become impaired (i.e. not accessible for personnel in normal clothing) due to high thermal radiation, high smoke concentrations or strong explosion (causing blockage of escape routes due to damaged equipment, explosion panels, etc.).
This impairment frequency is calculated as the sum of the frequency of occurrence of each accidental scenario evaluated by the Quantitative Risk Assessment which effects extrapolate the limit criteria defined for fire, explosion, toxic and smoke.
An example of impairment criteria is:
Fire: duration of pool fire above 10 minutes; duration of jet fire above 5 min;
Radiation: above 5kW/m2 for more than 30 minutes; above 37,5 kW/m2 will cause fatalities
Visibility: The escape limit is given by a visibility of 10 m
Explosion overpressure above 0.35 bar
The time to escape is calculated considering extension of each alternative escape, defined on Step 1 above and the typical escape speeds for level walkways/corridors, stairs and ladders.
The outputs obtained from the assessment are:
Impairment frequency for the alternative escape routes and the manned areas;
Graphic map of impairment frequency of each alternative on the layout of the offshore unit;
Comparison of alternative escape routes impairment and time to escape.
The results can be applied by the offshore unit to enhance its Emergency Plan, reviewing the escape ways for each accidental scenario, including the drill of the alternative escapes with personnel on board and reinforcing the best escape from each manned area (as the one with the lower frequency of impairment and that can lead people to temporary refuge in the shortest time).