The Chad Risk Assessment

Abo Homos-Nubaria Pipeline QRA

Flammability Assessment

14.1 General

An assessment of all flammable and combustible materials present on the facilities is required to determine those materials that can be excluded from further assessment in the QRA due to low flammability and hence the low probability of ignition.

In order for a fire to start there must be an ignition source of sufficient heat intensity to cause ignition. However, after a fire has started, the heat necessary to sustain combustion is typically supplied by the combustion process.

A flammable gas or vapour burns in air only over a limited concentration range. Below a certain concentration in air, the Lower Flammability Limit (LFL), the mixture is too ‘lean’, and above a certain concentration in air, the Upper Flammability limit (UFL), the mixture is too ‘rich’ to sustain combustion.

The concentrations between these limits constitute the flammable range.

Flammability limits vary between hydrocarbon gases. For example, propane is flammable between 2.1 and 9.5% v/v. Process streams consist of a mixture of hydrocarbons and on loss of containment the flammability limits depend on the composition of the gas or vapour that is released to air.

The flash point of a flammable liquid is the temperature at which the vapour pressure is sufficient to result in a concentration of vapour in air above the liquid corresponding to the lower flammable limit.

On loss of containment or where open to the atmosphere, a hydrocarbon liquid that has a flash point below ambient temperature is readily ignitable. A liquid with a high flash point, could also ignite if raised in temperature above its flash point by an external heat source, if released as a high pressure spray that promotes vaporisation, or if soaked into lagging (insulating materials). Flash point is the main parameter in the hazard classification of flammable liquids.

The flammability of materials has been assessed using the Flammability Hazard Ranking from NFPA 325M under the categories summarised in Table 13.1. Flammable liquid classes referred to in Table 13.1 are explained in IP15.

In general, materials with a flammability rating of 3 and 4 are readily ignited and present a greater fire hazard than materials with flammability rating of 1 or 2 that require pre-heating (e.g. by an existing fire) before ignition can occur.

30

Abo Homos-Nubaria Pipeline QRA

Table 13.1: NFPA 325M Flammability Rating

Flammability Description

Rating

This degree includes flammable gases, liquids and class IA flammable

4liquids. The preferred method of fire attack is to stop the flow of material or to protect exposures while allowing the fire to burn itself out.

This degree includes class IB and IIC flammable liquids and materials

3

that can be easily ignited under almost all normal temperature conditions. Water may be ineffective in controlling or extinguishing fires in such materials.

This degree includes materials that must be moderately heated before ignition will occur and includes class II and IIIA combustible liquids and

2solids and semi-solids that readily give off ignitable vapours. Water spray may be used to extinguish fires in these materials because the materials can be cooled below their flash points.

This degree includes materials that must be pre-heated before ignition

will occur, such as class IIIB combustible liquids and solids and semisolids whose flash point exceeds 93.4ºC, as well as most ordinary

1combustible materials. Water may cause frothing if it sinks below the surface of the burning liquid and turns to steam. However, a water fog that is gently applied to the surface of the liquid will cause frothing that will extinguish the fire.

The properties of the various flammable materials present on the facilities are summarised in Table 13.2. The flammability of the various inventories is discussed in further detail in the following sections.

Table 13.2 physical properties of Selected Flammable / Combustible materials

14.2Process Hydrocarbons

NFPA Flammability Index = 4

As a result the process hydrocarbon inventories are classified as highly flammable, corresponding to an NFPA Flammability Index rating of 4. Based on this rating the fire and explosion hazards associated with the inventories are further assessed in the QRA.

31

Abo Homos-Nubaria Pipeline QRA

Consequence Modelling Input Data

Consequence modelling will be used to simulate the Major Accident Events (MAE) raised from the scenario identification.

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.

The following simulation modules are included (detailed list):

•Tank Top Fire

•Pool Fire

•Trench Fire

•Gas Jet Flame (known reservoir 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 (instantaneous)

•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.

32

Process conditions:

Process conditions:

•Temperature = 50 °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 = 0.8 / 0.4 / 0.025 m

•Discharge coefficient = 0.8

Pipe:

•Pipe length = 10000 m

33

Abo Homos-Nubaria Pipeline QRA

•Pipe diameter = 0.813 m

•Pipe surface roughness = 4.6e-005 m

•Sum loss coefficient = 0

Release:

•Release height = 0 m

•Release angle from vertical = 0 / 90 deg

•Release angle, clockwise from North = 0 deg

Weather:

•Temperature = 40 °C

•Relative humidity = 75 %

•Wind speed = 2 & 10 m/s

•Direction wind is going to = 180 deg (measured clockwise from North)

•Atmospheric stability conditions define by = Pasquill class

•Pasquill class = "F" stable & "D" Unstable

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:

•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.

•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 2 m/s and 10 m/s.

•Dispersion sampling time was selected to be “Instantaneous”, which represents the worstcase scenario (stricter than 10 minutes sampling).

34

Abo Homos-Nubaria Pipeline QRA

Sensitivity Analysis

The sensitivity analysis shall be performed in order to determine the worst case parameters (or the combination of the worst case parameters), which shall be utilized in the consequence modelling.

The sensitivity analysis shall include the following parameters:

1.0Release flow rate:

The release flow rate depends on the size of the hole assumed to leak, which can be summarized as follows:

•Catastrophic leak or full bore rupture (presents maximum release rate).

•Major leak or half bore rupture (presents minimum release rate).

2.0Release pressure:

The release pressure depends on the process design and operating pressure of the released materials, which can be summarized as follows:

•The proposed maximum pipeline design pressure.

•The proposed minimum pipeline design pressure.

3.0Release temperature:

The release temperature depends on the process design and operating temperature of the released materials, which can be summarized as follows:

•The proposed maximum pipeline design temperature.

•The proposed minimum pipeline design temperature.

4.0Ambient temperature:

Ambient temperature varies from high ambient temperatures in the summer to low ambient temperatures in the winter, which can be summarized as follows:

•The proposed maximum ambient temperature in the summer is 40 (oC)

•The proposed minimum ambient temperature in the winter is 5 (oC)

5.0Relative humidity:

35

Abo Homos-Nubaria Pipeline QRA

Relative humidity varies from high relative humidity in the summer to low relative humidity in the winter, which can be summarized as follows:

•The proposed maximum relative humidity is 90 %.

•The proposed minimum relative humidity is 50 %.

6.0Wind speed:

Wind speed varies from high wind speed to low wind speed depending on the weather conditions, which can be summarized as follows:

•The proposed maximum wind speed is 10 m/s (presents very unstable weather conditions).

•The proposed minimum wind speed is 2 m/s (presents very stable weather conditions).

7.0Wind stability:

Wind stability presented as Pasquill stability classes varies from very unstable weather to very stable weather depending on the weather conditions, which can be summarized as follows:

•The proposed very unstable weather is (A).

•The proposed very stable weather is (F).

Different Pasquill stability classes are represented in the following table:

Each of the previously mentioned parameters shall be checked with all other parameters are constant. (I.e. these parameters shall be checked one by one, and for each case all other parameters shall remain unchanged in order to determine the worst case scenario for each parameter).

From the sensitivity analysis for the gas dispersion, it can be concluded that:

1.The gas dispersion distances shall be increased by higher release flow rate.

2.The gas dispersion distances shall be increased by higher release pressures.

3.The gas dispersion distances shall be increased by lower release temperatures.

4.The gas dispersion distances shall be increased by higher ambient temperatures.

36

Abo Homos-Nubaria Pipeline QRA

5.The gas dispersion distances shall be increased by lower relative humidity.

6.The gas dispersion distances shall be increased by lower wind speeds.

7.The gas dispersion distances shall be increased by higher weather stability.

From the sensitivity analysis for the heat radiation, it can be concluded that:

1.The jet flame heat radiation distances shall be increased by higher release flow rate.

2.The jet flame heat radiation distances shall be increased by higher release pressures.

3.The jet flame heat radiation distances shall be increased by higher release temperatures.

4.The jet flame heat radiation distances shall not be affected by ambient temperatures.

5.The jet flame heat radiation distances shall not be affected by relative humidity.

6.The jet flame heat radiation distances shall be increased by higher wind speeds.

7.The jet flame heat radiation distances shall not be affected by weather stability.

From the sensitivity analysis performed, it has been concluded that there is a combination of set of parameters that gives the worst case scenarios for the gas dispersion and heat radiation, while on the opposite side; there is a combination of set of parameters that gives the mild case scenarios for the gas dispersion and heat radiation.

Both cases (the worst cases and mild cases) can be simulated using consequence modelling, however only the worst case scenarios for gas dispersion and heat radiation shall be governing in this report in order to present a conservative approach leading to conservative QRA results.

From the sensitivity analysis, the following parameters have been selected to represent the worst case scenario parameters and shall be utilized in the consequence modelling analysis:

•The proposed maximum ambient temperature in the summer is 40 (oC),

•The proposed minimum relative humidity in the winter is 50 %,

•The proposed minimum wind speed is 2 m/s (presents very stable weather conditions),

•The proposed very stable weather stability class is (F).

37

Abo Homos-Nubaria Pipeline QRA

Ignited Release Scenario

14.1 Generic Causes of Release

Historically, incidents involving the loss of containment of flammable/combustible material in the oil and gas facilities have occurred due to causes including:

•Human error associated with operations and maintenance activities;

•Corrosion

•Erosion

•Fatigue/vibration/vortex shedding;

•Brittle fracture (e.g. due to low temperatures embrittlement);

•Impact (e.g. due to dropped object, projectile, impacts…etc.);

•Creep;

•Natural causes (e.g. storm, earthquake…etc);

•Operation beyond design envelope;

•Inappropriate choice of materials; and

•Inadequate design.

In relation to the new facilities, thorough design and the implementation by Company of an appropriate Safety Management System will ensure many of the causes listed above are either avoided or significantly reduced in potential.

14.2Generic Causes of Ignition

Historically, the causes of ignition of released flammable/combustible material in the oil and gas facilities have included:

•Flames/direct heat;

•Hot surfaces;

•Hot work (e.g. welding, flame cutting, grinding);

•Mechanical sparks;

•Electrical equipment not classified for hazardous areas;

•Faulty electrical equipment;

•Lightning;

•Engines;

•Distressed equipment (e.g. overheated bearings);

•Impact energy (e.g. tools, dropped objects, projectiles);

•Chemical energy;

•Static electricity;

•Illicit smoking; and

•Hot soot particles.

38

Abo Homos-Nubaria Pipeline QRA

Similar to causes of release, the above listed causes of ignition on the new facilities will be either avoided or significantly reduced in potential through thorough design and the implementation by Company of an appropriate Safety Management System.

39