31.—(1) In every tanker of Class VII(T) of 500 tons or over arrangements for purging and gas freeing shall be such as to minimise the hazards due to the dispersal of flammable vapours in the atmosphere and to flammable mixtures in a cargo tank.
(2) When the ship is provided with an inert gas system the cargo tanks shall first be purged—
(a)in accordance with the provisions of the Guidelines for Inert Gas Systems; or
(b)if the ship does not need to be provided with an inert gas system complying with the requirements of regulation 30(1) but is a ship mentioned in regulation 30(2)(a), (b) or (c) in accordance with the provisions of paragraph 12 of Part IV of the Guidelines on Inert Gas Systems or,
until the concentration of hydrocarbon vapours in the cargo tanks has been reduced to less than 2 per cent by volume. Thereafter, gas-freeing may take place at the cargo tank deck level.
(3) When the ship is not provided with an inert gas system, the operation shall be such that the flammable vapour is discharged—
(a)through the vent outlets as specified in regulation 23 of the Merchant Shipping (Cargo Ship Construction) Regulations 1997(1); or
(b)if the ship is one constructed on or after 1st February 1992, through outlets at least 2 metres above the cargo tank deck level with a vertical efflux velocity of at least 30 metres per second during the gas-freeing operation; or
(c)through outlets at least 2 metres above the cargo tank deck level with a vertical efflux velocity of at least 20 metres per second and through devices (other than flame screens) complying with Schedule 1 in the Merchant Shipping Notice MSN 1671 so as to prevent the passage of flame into the cargo tanks,
until the flammable vapour concentration in the outlet has been reduced to 30 per cent of the lower flammable limit. Thereafter, gas-freeing may be continued at the cargo tank deck level.