The Chad Risk Assessment
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Abo Homos-Nubaria Pipeline QRA
Likelihood Data
1.1Process Release
A summary of historical pipeline failure data from some of the best sources of data for landbased pipelines are summarized in the following table (based on E&P Forum, CONCAWE, PARLOC and EGIG):
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Table 21.1 Comparison of Pipeline Failure Rate Data |
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No. |
Source |
Failure rate / year / Km |
1 |
US Gas Pipelines (1985 – 1994) |
1.66 X 10-4 |
2 |
US hazardous liquid pipelines (1986 – 1998) |
8.05 X 10-4 |
3 |
CONCAWE European Oil Pipelines (1990 – 1998) |
3.25 X 10-4 |
4 |
European Gas Pipeline Incident Data group (1970 – 1997) |
4.77 X 10-4 |
The pipeline leak frequency shall be extracted to be (2.2 X 10-4).
1.2Ignition Probability
The probability of ignition depends on the availability of a flammable mixture, the flammable mixture reaching an ignition source and the type of ignition source (energy etc.).
The ignition sources may include:
•Hot work
•Faults in electrical equipment
•Faults in rotating equipment
•Ignition caused by combustion engines or hot surfaces
•Automatic ignition in the event of a fracture or rupture
•Static electricity
•Open flame
Generic ignition probabilities have been taken from Lees.
Ignition probability data are provided for gas release based on mass release rate.
Typical ignition probability data are given in Table 21.2.
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Table 21.2 Ignition Probability Data |
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Mass Release Category |
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Mass Release Rate (Kg / Sec) |
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Ignition Probability |
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Gas |
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Oil |
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Minor |
< 1 |
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0.01 |
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0.01 |
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Major |
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1 – 50 |
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0.07 |
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0.03 |
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Massive |
> 50 |
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0.3 |
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0.08 |
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Abo Homos-Nubaria Pipeline QRA
Risk Assessment
1.1Risk Assessment Basis
Risk shall be determined for both workers and public using international risk management guidelines as a reference. The risk will be compared with international risk acceptance criteria.
Risk assessment will comprise the following items:
•Failure rate,
•Ignition probability,
•Occupancy,
•Vulnerability.
Where:
Failure rate: is the failure frequency.
Ignition Probability: is the likelihood of a release to become a fire or explosion.
Occupancy: is the personnel presence in the area. [10 workers assumed to be present outdoors].
Vulnerability: is the likelihood that the specific person will be fatally injured by the effect of the event (determined from the consequence modelling software).
1.2Risk Assessment for Buried Underground Pipeline
Individual risk for buried underground pipeline
IR = 6.6 X 10-8 per year
Societal risk for buried underground pipeline
SR = 6.6 X 10-7 per year
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Abo Homos-Nubaria Pipeline QRA
Risk Evaluation
Risk assessment shall be evaluated based on the international risk acceptance criteria (Figure 22.1).
The ALARP principle has been adopted for risk evaluation. The ALARP region is that point at which the time, effort difficulty and cost of further risk reduction become out of proportion compared with the amount of risk reduction achieved.
The international risk acceptance criteria are presented in the following figure.
UN ACC EPTA BLE REG IO N
W orkers
M ax imum tolerable limit
1 in 1000 per year
P ublic
AL A R P Benchm ark existing installations
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1 in 5,000 per year |
M aximum tolerable limit |
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1 in 10,000 per year |
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AL ARP OR TO LERA BILI TY REG IO N |
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ALA RP Be n ch m a rk n e w ins ta lla tion s |
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AL ARP OR TO LERA BILITY |
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R EG IO N |
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1 in 50,000 per year |
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(R isk m ust be dem onstrated to have |
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M inim um to lerable lim it |
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be en reduced to a level w hich is |
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1 in 100,000 per year |
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practicable with a view to cost/benefit) |
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M inim um tolerable lim it |
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AC CEPTABLE REG IO N |
1 in 1 m illion per year |
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AC CEP TABLE REG IO N |
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IN DIV IDU AL RISK TO W OR KER S |
IN DIV IDU AL RISK TO TH E PUBLIC |
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(including c ontractor em plo y ees) |
(all those no t d irectly inv olved w ith co mpa ny |
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a ctiv ities) |
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Figure 22.1 International Risk Acceptance Criteria
From the risk assessment and the international risk acceptance criteria the risk evaluation for individual and societal risks for both pipeline orientations are presented in the following tables.
Table 22.1 Buried Underground Pipeline Orientation Risk Evaluation Summary Table
No |
Risk Type |
Calculated Risk |
ALARP Limits |
Risk Acceptance |
1.0 |
Individual Risk |
6.60E-08 |
1.0E-03 to 1.0E-05 |
Acceptable ( ) |
2.0 |
Societal Risk |
6.60E-07 |
1.0E-04 to 1.0E-06 |
Acceptable ( ) |
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Abo Homos-Nubaria Pipeline QRA
Risk Reduction Measures (Recommendations)
Risk reduction measures (Recommendations) may include reducing the risk by several technically feasible methods, generally are as follows:
•Measures to eliminate the risk.
•Measures to reduce the exposure of personnel to the hazards.
•Measures to reduce the frequency of occurrence.
•Measures to mitigate the consequences if the event does occur.
•Measures to improve evacuation in case of emergency (event occurs).
It has been concluded that the risk falls within the Acceptable limits for the individual risk to workers and public for the pipeline. However, the following measures (recommendations) should be adhered:
•Ensure pipeline design, commissioning, start-up, construction and operation is complying with code requirements (ASME B31.8 Gas Transmission and Distribution Piping Systems).
•Ensure Signs or markers shall be installed where it is considered necessary to indicate the presence of a pipeline at road, highway, railroad, and stream crossings. Additional signs and markers shall be installed along the remainder of the pipeline at locations where there is a probability of damage or interference (ASME B31.8 requirement).
•Signs or markers and the surrounding right-of way shall be maintained so markers can be easily read and are not obscured (ASME B31.8 requirement).
•The signs or markers shall include the words “Gas" (or name of gas transported) Pipeline,” the name of the operating company, and the telephone number (including area code) where the operating company can be contacted (ASME B31.8 requirement).
•Ensure Overpressure protection is provided by a device or equipment installed in a gas piping system that prevents the pressure in the system or part of the system from exceeding a predetermined value (ASME B31.8 requirement).
•Emergency Response plan (ERP) to include means for detection pipeline leak or rupture also, means for safe and quick isolation of the damaged section of the pipeline.
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Abo Homos-Nubaria Pipeline QRA
Uncertainty Analysis
Uncertainty analysis is performed to define the uncertainty data in the input model, underestimation of consequences, neglected items, assumed points, exaggeration points or overestimation.
In this QRA study all input data have been selected based on the worst case scenarios. The selection of the worst case scenarios shall result in conservative design leading to conservative results.
A list of worst case scenarios and conservative assumptions include the following:
•Selection of the minimum wind speed of 2 m/s with stability class "F" in order to represent a "Very Stable" weather conditions in the consequence modelling calculations.
•Selection of the maximum ambient temperature of 40 C in the consequence modelling calculations.
•Selection of the minimum relative humidity of 50% in the consequence modelling calculations.
•The maximum operating pressure has been selected as the simulation pressure.
•Simulation sampling time has been selected as "Instantaneous" in stead of 10-minutes sampling to investigate the maximum plume length in order to achieve conservative results.
•Selection of the maximum hole size to be the controlling case. The selected 2-hole sizes are 32inch, representing catastrophic failure as a full bore rupture and 16-inche representing major leak as a half bore rupture.
•The release direction (release orientation) has been selected in the direction towards the populated area under study representing the worst case directional orientation.
•The prevailing wind direction has been selected in the direction towards the populated area under study representing the worst case wind direction.
•The highest failure rate has been selected as the basis for failure data.
•The highest ignition probability has been selected for all ignition probability data.
•The vulnerability of all hazardous events has been selected on the worst case scenario.
Hence, there is no uncertainty in the QRA calculations and all the calculated risks are certain.
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Abo Homos-Nubaria Pipeline QRA
References
1.NFPA 325M,
2.FRED Version (4.0) documentation,
3.Frank P. Lees, Loss Prevention in the Process Industries, 2001,
4.API-581, Risk Based Inspection recommended practice,
5.E&P Forum,
6.Project Documents.
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Abo Homos-Nubaria Pipeline QRA
Appendix-1 FRED Simulation Cases for PRS
Author |
EcoConServ |
Company |
EcoConServ |
Department |
HSE |
Revision |
0 |
Notes |
QRA - Consequence Modelling |
Revision date |
24 July 2011 |
1.3Table of Contents
70Bar - Full Bore Rupture [32 Inch] - Vertical Release 70Bar - Half Bore Rupture [16 Inch] - Vertical Release 70Bar - Minor Leak [1 Inch] - Vertical Release
70Bar - Depressurization [10 Inch] - Vertical Release Explosion [Confined space]
1.470Bar - Full Bore Rupture [32 Inch] - Vertical Release
1.4.1Scenario Summary
1.4.1.1Scenario
Scenario = 70Bar - Full Bore Rupture [32 Inch] - Vertical Release Fluid = Natural gas
1.4.1.2 Process conditions
Calculate at = User input pressure
Temperature = 40 °C
Pressure = 70 bara
1.4.1.2.1Pressure downstream of release
Pressure = 1.013 bara
Use standard atmospheric pressure = yes
1.4.1.2.2Release from
Release source = Vapour space
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Abo Homos-Nubaria Pipeline QRA
1.4.1.3 Hole & release geometry
1.4.1.3.1Hole geometry
Failure type = Custom
Hole diameter = 0.799 m
Discharge coefficient = 0.8
1.4.1.3.2Pipe
Pipe length = 0 m
1.4.1.3.3Release
Release height = 0 m
Release angle from vertical = 0 deg
Release angle, clockwise from North = 0 deg
1.4.1.4 Weather
1.4.1.4.1Ambient conditions
Temperature = 40 °C Relative humidity = 75 % Wind speed = 10 m/s
Direction wind is going to = 0 deg (measured clockwise from North)
1.4.1.4.2Atmospheric stability conditions
Define by = Pasquill class
Pasquill class = D Neutral
1.4.1.5 Thermal radiation
Radiation contours = 1.58, 4.73, 6.31, 9.46, 37.5 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
1.4.1.6 Dispersion
Surface roughness = 0.1 m Contours to plot:
47226.4 ppm 148597.8 ppm Plot type = LFL/UFL Sampling time = Instantaneous
1.4.1.7 Release summary
Mass flow rate = 5571.0 kg/s
Flux = 11110.9 kg/m²/s
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Abo Homos-Nubaria Pipeline QRA
Static exit pressure = 35.59 bara
Exit temperature = -55.14 °C
Exit density = 35.6 kg/m³
Exit velocity = 312.1 m/s
Residence time = 0 s
Vapour fraction at exit = 1 mol/mol
Expanded exit velocity = 623.4 m/s
Air equivalent source diameter = 3.079 m
1.4.1.8 Release Composition
Molecular Weight of Release = 18.13 kg/kmol
Component |
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Weight |
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Mole |
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Critical |
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Critical |
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Molecular |
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Atmos |
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Freeze |
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Heat of |
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Fraction |
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Fraction |
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Temp |
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Pressure |
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Weight |
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BP °C |
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Pt °C |
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Comb |
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norm |
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norm |
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°C |
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bara |
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kg/kmol |
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kJ/kg |
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n-Butane |
0.0321 |
0.0100 |
152.1 |
37.41 |
58.12 |
-0.5001 |
-138.4 |
45742.7 |
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Propane |
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0.0487 |
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0.0200 |
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96.7 |
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41.91 |
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44.1 |
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-42.1 |
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-187.7 |
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46383.8 |
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Ethane |
0.0829 |
0.0500 |
32.18 |
48.08 |
30.07 |
-88.6 |
-182.8 |
47514.8 |
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Methane |
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0.7966 |
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0.9000 |
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-82.6 |
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45.35 |
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16.04 |
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-161.5 |
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-182.5 |
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50043.9 |
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Nitrogen |
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0.0155 |
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0.0100 |
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-146.9 |
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33.56 |
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28.01 |
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-195.8 |
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-210 |
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0 |
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Carbon |
0.0243 |
0.0100 |
31.06 |
72.86 |
44.01 |
-86.9 |
-56.6 |
0 |
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dioxide |
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1.4.1.9 Reservoir summary (at reservoir pressure)
Bubble point temperature = n/a
Dew point temperature = n/a
Vapour fraction = 1
1.4.1.9.1Properties of phases
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Vapour |
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Liquid |
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Molecular weight (kg/kmol) |
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18.13 |
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0 |
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Density (kg/m³) |
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55.5 |
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0 |
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Enthalpy (kJ/kmol) |
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193.3 |
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0 |
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Entropy (kJ/kmol*K) |
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-29.46 |
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0 |
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Cv (kJ/kg*K) |
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1.725 |
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Cp (kJ/kg*K) |
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2.627 |
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Sound velocity (m/s) |
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414.8 |
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Viscosity (e-3 kg/m*s) |
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0.01354 |
0 |
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Surface tension (e-3 N/m) |
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0 |
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0 |
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Abo Homos-Nubaria Pipeline QRA
1.4.2Jet Fire
1.4.2.1Jet Fire Summary
Flame length (of frustum) = 233.6 m
Cone width of flame base = 34.82 m
Cone width of flame end = 103.3 m
Flame lift-off = 121.9 m
Flame angle from vertical = 24.55 deg Flame angle, clockwise from North = 0 deg
Surface emissive power = 258.1 kW/m² Fraction of heat radiated = 0.05906
Total combustion power = 264788.1 MW Heat of combustion = 47529.6 kJ/kg
1.4.2.2 Side view
Raw plot data
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