Section 29(2)(c) in The Merchant Shipping (Prevention of Pollution by Oil from Ships) Rules, 2010
(c)the arrangement of slop tank or combination of slop tanks shall have a capacity necessary to retain the slop generated by tank washings, oil residues and dirty ballast residues and the total capacity of the slop tank or tanks shall not be less than three percent of the oil-carrying capacity of the ship, except that the Central Government may accept,-(i)2% for such oil tankers where the tank washing arrangements are such that once the slop tank or tanks are charged with washing water, this water is sufficient for tank washing and, where applicable, for providing the driving fluid for educators, without the introduction of additional water into the system;(ii)2% where segregated ballast tanks or dedicated clean ballast tanks are provided in accordance with rule 18, or where a cargo tank cleaning system using crude oil washing is fitted in accordance with rule 33: