event trees

Scenario: "Gas leak in plant area"

Start with the initiating event gas leak occurs. Then branch by gas detection works / fails, then shutdown or equipment isolation works / fails, then ignition / no ignition, then if stated operator present / absent or injury / no injury. Final outcomes should be things like safe shutdown, no harm, fire, explosion, or operator injured.

Scenario: leak detected and all equipment shuts down

After the initiating leak, write the first branch as detection succeeds / fails. If it succeeds, branch to shutdown succeeds, then usually end with no ignition / no injury or controlled outcome. If detection fails, continue to possible ignition and escalation.

Scenario: nearby boiler/pump almost certainly ignites gas if leak not detected

After detection fails, write ignition occurs as the next branch. Then, if the scenario mentions an operator passing through, branch to operator present / absent and then injury / no injury.

Scenario: hydrogen/gas leak with operator injury probability

Use this order: leak → detection works/fails → electrical equipment shuts down/does not shut down → ignition/no ignition → operator in area/not in area → injury/no injury. This mirrors the exam style very closely.

Scenario: flammable liquid release

Start with release of flammable liquid, then branch immediate ignition / no immediate ignition. If no immediate ignition, branch into delayed ignition / no delayed ignition. Final outcomes should be pool fire, flash fire, vapour cloud explosion, toxic/environmental exposure, or no harmful consequence depending on the question wording.

Scenario: gas release from vessel or pipeline

Start with gas release, then branch into immediate ignition / no immediate ignition. If immediate ignition, final outcome is often jet fire. If no immediate ignition, branch into delayed ignition / no delayed ignition. Delayed ignition can give flash fire or explosion; no ignition gives dispersion / exposure.

Scenario: toxic gas release

Start with toxic release, then branch detected / not detected, isolation successful / failed, public or operator exposure / no exposure, then injury/fatality / no injury. If ignition is not relevant, do not force it into the tree.

Scenario: overfill or LOC that may ignite

Start with loss of containment / overflow, then branch immediate ignition / no immediate ignition. If no immediate ignition, then delayed ignition / no delayed ignition. Final outcomes can be pool fire, flash fire, explosion, environmental contamination, or safe dispersion/containment.