All crude oil washing must be completed before the ship
leaves its final port of discharge. Where any tank is crude oil washed
while the ship is at sea between multiple discharge ports, the tank
shall be left empty and available for inspection at the next discharge
port before commencing the next ballast voyage. This inspection may
consist of multiple sounding/dipping of the bottom of the tank when
the tank is empty. Departure ballast tanks shall be ballasted prior
to departure from port so that examination of the surface of the ballast
water can be made. In the latter case the guidance given in 4.2.10(b) is relevant to the inspection.