Section 25(3)(c) in The Merchant Shipping (Prevention of Pollution by Oil from Ships) Rules, 2010
(c)Suction wells may be neglected in the determination of the value hi provided such wells are not excessive in area and extend below the tank for a minimum distance and in no case more than half the height of the double bottom and if the depth of such a well exceeds half the height of the double bottom, hi shall be taken equal to the double bottom height minus the well height.