If a tanker can avoid putting water into her cargo tanks the possibilities of cargo contamination and of corrosion of those tanks is substantially reduced but when a tanker discharges its cargo it must take on ballast water to maintain a seaworthy condition. In the past tankers took this ballast into unwashed cargo tanks and on the ballast passage discharged this oily water into the sea and replaced it with clean sea water. Load on top procedures whereby the tank washings were retained on board after decanting and discharging overboard as much water as possible, greatly reduced the oil going into the sea but did not eliminate it. The discharge of tank washings or dirty ballast into the sea is now banned under the International Convention for the Prevention of Pollution from Ships (Marpol/73/78). All ships registered in states which have ratified this convention and any ships trading from or to states which have ratified, must comply with the requirements of the convention, and this means that virtually all tankers in international trade must comply. The precise details of the regulations as they apply to the various sizes of tankers, whether crude or product, and whether existing or newbuildings, within the convention’s definitions of newbuldings, are complex. Broadly speaking, all newbuildings where the orders were placed after June 1979 or were delivered after June 1982 must have Segregated Ballast Tanks (SBT) if over 20,000 tons deadweight for crude carriers, or over 30,000 tons if product carriers. For existing ships, there are more alternatives which are not detailed here. US Coastguard Regulations require vessels over 20,000 tons to have SBT or CBT. SBT as distinct from Dedicated Clean Ballast (CBT), is completely separated from the cargo system having its own lines, pumps and tanks.