Consequence modelling will be used to simulate the Major Accident Events (MAE) raised from the scenario identification. The consequence modelling will be carried out using SHELL, FRED version (4.0). SHELL, FRED Flyer, including technical capabilities and benefits is presented in Appendix 2. FRED consequence modeling software stands for Fire, Release, Explosion and Dispersion
developed by SHELL global solutions. This software calculates in graphical displays and detailed reports the previously mentioned hazardous consequences and presents their extent and degree of danger.
FRED determines the heat radiation contours from different fire scenarios depending on the amount of fuel burning, type of fuel and wind direction. It calculates a fluid release flow rate depending on the fluid pressure, the size and location of the hole. Also, It calculates the explosion overpressure contours resulting from the ignition of released gas inside confined space depending on the type of the fuel exploding and the degree of confinement. Finally it performs gas dispersion calculation and calculates the gas concentration contours in fraction of the lower explosive limits depending on the type of gas released, release rate, wind stability, wind speed and surface roughness.
SHELL FRED has the following simulation modules (deta iled list): • Tank Top Fire
• Pool Fire • Trench Fire
• Gas Jet Flame (known rese rvoir pressure) • Gas Jet Flame (Known mass flow rate) • Shell BLEVE
• BLEVE (TNO) • Temperature Rise
• Pressurised release (known reservoir pressure) • Pressurised release (known mass flow rate) • Pressure relief valve
• Blowdown • Two-Phase Blowdown • LPG two -phase • Explosion CAM • Explosion TNO • Explosion TNT • Dense gas dispersion
• Gaussian dispersion (in stantaneous) • Gaussian dispersion (Continuous)
• Gaussian dispersion (Non boiling liquid pool) • Heat Up
• Vessel Burst • Bubble Plume
This hazardous consequence simulation is normally carried out in order to optimize the design, while on the other hand it will be used in this study to estimate the degree of danger raised from the hazardous events on the facilities under study in order to assess the associated risks.
Process conditions: Component Weight Fraction norm Mole Fraction norm Critical Temp °C Critical Pressure bara Molecular Weight kg/kmol Atmos BP °C Freeze Pt °C Heat of Comb kJ/kg n-Butane 0.0321 0.0100 152.1 37.41 58.12 -0.5001 -138.4 45742.7 Propane 0.0487 0.0200 96.7 41.91 44.1 -42.1 -187.7 46383.8 Ethane 0.0829 0.0500 32.18 48.08 30.07 -88.6 -182.8 47514.8 Methane 0.7966 0.9000 -82.6 45.35 16.04 -161.5 -182.5 50043.9 Nitrogen 0.0155 0.0100 -146.9 33.56 28.01 -195.8 -210 0 Carbon dioxide 0.0243 0.0100 31.06 72.86 44.01 -86.9 -56.6 0 • Temperature = 30 °C • Pressure = 70 bara
• Pressure downstream of release = 1.013 bara • Use standard atmospheric pressure = yes • Release source = Vapor space
Hole & release geometry: Hole geometry:
• Failure type = Custom
• Hole diameter = 0.1 / 0.025 / 0.005 m • Discharge coefficient = 0.8
Pipe :
• Pipe length = 100 m • Pipe diameter = 0.2 m
• Pipe surface roughness = 4.6e -005 m • Sum loss coefficient = 0
Release:
• Release height = 1 m
• Release angle from vertical = 90 deg
• Release angle, clockwise from North = 90 deg Weather:
• Temperature = 40 °C • Relative humidity = 50 % • Wind speed = 1 & 10 m/s
• Direction wind is going to = 180 deg (measured clockwise from North) • Atmospheric stability conditions define by = Pasquill class
Thermal radiation:
• Radiation contours = 1.5, 2.5, 6.3, 12.5, 32 kW/m² • Height at which plan view contours to be plotted = 0 m
• Cross flame distance at which side view contours to be plotted = 0 m Dispersion:
• Surface roughness = 0.01 m • Contours to plot:
• Plot type = LFL/UFL
• Sampling time = Instantaneous Technical Notes:
• Shell Fred includes two methods of inputs to the discharge modelling, one is “known reservoir pressure” and the second is “known release mass flow rate”. The scenario was selected as “known reservoir pressure” in order to represent the maximum desired flow rate through the hole. • The following table provides values of absolute roughness, e, in metres for materials used in the
construction of open channels. (Note that where pipes have become corroded, surface roughness can increase 10-fold):
Table 15.1: Values of Absolute Roughness
• Pipe surface roughness was selected as 4.6e-005, which represents the steel material.
• Different wind speeds were selected for the gas dispersion and heat radiation modelling, basically 1 m/s and 10 m/s.
• Dispersion sampling time was selected to be “Instantaneous”, which represents the worst-case scenario (stricter than 10 minutes sampling).
Table 15.2: Values of incident radiation intensity
• Different Pasquill stability classes were selected for the gas dispersion and heat radiation modelling, basically B (Unstable) and E (Stable).
Table 15.3: Pasquill Stability Classes
Numbe r Class Description
1. A Very Unstable 2. B Unstable 3. C Slightly Unstable 4. D Neutral 5. E Stable 6. F Very Stable
• The following table contains the criteria for the Pasquill stability classes, taking time of day, wind speed and cloud cover influences into account.
Table 15.4: Pasquill Stability Classes
• For the flammable gas dispersion modelling, the upper flammability limits (UFL) and lower
flammability limits (LFL), have been selected as dispersion contours for representing different gas composition cloud contours.
• For the toxic gas dispersion modelling, modelling values in the terms of parts per million (ppm) shall be selected as concentrations of interest for toxic gases dispersion. (As highly toxic gases, shall represent a critical safety factor in designing such types of facilities).
• Obstacles on the level over which the plume is dispersing will have a tendency to break up the plume. This effect is quantified in the gas dispersion models by a surface roughness. Typical
surface roughness lengths, as used in many models, are given in the following table. (The roughness values are NOT the actual size of the obstacles on the ground ).
Table 15.5: Values of Surface Roughness
• Failure Types supported by Shell FRED is listed in the following table, indicating the typical hole diameter resulting from the failure.
Table 15.6: Typical Hole D iameters
• Fire characteristics can be summarized in the following table.