21.—(1) In every ship constructed on or after 1st July 1986 the pumping and piping arrangements serving any tank designated for the carriage of a Category B substance or a Category C substance shall be such that, if the tank were filled with water and pumped out under favourable pumping conditions, the quantity of residue retained in the tank’s associated piping and in the immediate vicinity of the tank’s suction point would not exceed—
(a)in the case of a tank designated for the carriage of a Category B substance, 0.1 cubic metres, or
(b)in the case of a tank designated for the carriage of a Category C substance, 0.3 cubic metres.
(2) In every ship constructed before 1st July 1986 the pumping and piping arrangements serving any tank designated for the carriage of a Category B substance or a Category C substance shall be such that if the tank were filled with water and pumped out under favourable pumping conditions—
(a)the quantity of residue retained in the tank’s associated piping and in the immediate vicinity of the tank’s suction point would not exceed—
(i)in the case of a tank designated for the carriage of a Category B substance, 0.3 cubic metres, or
(ii)in the case of a tank designated for the carriage of a Category C substance, 0.9 cubic metres; or
(b)until 2nd October 1994 (on which date this sub-paragraph shall cease to have effect) the quantity of residue retained in the tank and its associated pumping system, when added to the surface residue assessment of the tank calculated in accordance with Appendix A to the IMO Standards would not exceed—
(i)in the case of a tank designated for the carriage of a Category B substance, 1 cubic metre or 1/3,000th of the capacity of the tank, whichever is the greater, or
(ii)in the case of a tank designated for the carriage of a Category C substance, 3 cubic metres or 1/1,000th for the capacity of the tank, whichever is the greater.
(3) The Secretary of State may exempt from any of the requirements of this regulation any ship which is so constructed and operated that—
(a)ballasting of cargo tanks is not required, and
(b)cargo tank washing is required only for the purposes of repair or dry docking,
subject to such conditions as he thinks fit.
(4) An exemption granted in accordance with Regulation 5A(6) or (7) of MARPOL 1973/78 by or on behalf of a state party to MARPOL 1973/78 to a ship registered in that state shall be treated as equivalent to an exemption granted by the Secretary of State.