Q$3.$ A potentially hazardous installation $($PHI$)$ site situated close to a housing estate

contains a vessel for the storage of liquefied petroleum gas. It has been estimated

that a major release of the contents of the vessel could occur once every one

hundred years $($frequency $=\frac{0\mathrm{.}01}{?}$ year$).$ Such a release, together with the presence of

an ignition source $($probability $p=0\mathrm{.}05),$ could lead to a flash fire or a vapour cloud

explosion on site. Alternatively, if the wind is in a certain direction $(p=0\mathrm{.}6)$ and

there is stable wind speed of less than $8\frac{m}{s}(p=0\mathrm{.}5),$ a vapour cloud may drift to

the housing estate where it could be ignited $(p=0\mathrm{.}85).$

$($a$)$ Using the data provided, construct an event tree to calculate the expected frequency

of fire$/$explosion both on site and on the nearby housing estate.

$(11$ marks$)$

$($b$)$ Assuming that fire$/$explosion will result in fatality, with reference to the Hong

Kong Risk Guidelines adopted by the Coordinating Committee on Land$-$use

Planning and Control relating to Potentially Hazardous Installations $($CCPHI$)$ to

assess the off$-$site risk levels of PHIs, comment on the significance of the results

obtained in the above part Q$3.($a$).$

$(8$ marks$)$

$($c$)$ Outline, with examples, control options to minimize the risks to the housing estate.

$(14$ marks$)$