presumed to occur outside the waters of the United States or any other nation.

  1. MARPOL has been adopted by nations with authority over 95 percent of the world’s merchant vessels. In the United States, MARPOL has been adopted through the Act to Prevent Pollution from Ships (33 U.S. Code 1901 et seq.)

  2. Discharges of bilge water and certain cargo tank washings from tankers are the only permitted discharges under MARPOL.

  3. Discharge of fuel oil sludge is prohibited.

  4. In order to assist vessels in complying with these prohibitions against discharge of excess quantities of oil and oil sludge into the water, waterfront facilities (where commercial vessels are received) are required to establish and maintain oil waste reception facilities. These oil waste reception facilities are suppose to have sufficient capacity to receive and process all oily wastes generated by all vessels calling at the facility.

Failure to have reception facility capability is supposed to result in vessels being denied entry to the facility. For example, International Maritime Organization’s (IMO) 1998 list of waste reception facilities includes 1047 such facilities in various ports in the United States.

Operational Discharges from Cargo Tanks of Oil Tankers
Regulatory Background

During normal operations, certain tankers may discharge into the sea an amount of oil contained in the ballast and tank washings.

Under regulation 13 of MARPOL 73/78, tankers of 20,000 tonnes deadweight and above are required to have segregated ballast tanks (SBT), dedicated clean tanks (CBT), and/or crude oil washing systems (COW), depending on the vessel type, when they were built and their size. Regulation 13F adopted in 1992 restricts routing of ballast piping through cargo tanks and vice versa. These measures are designed to reduce operational and accidental pollution from tankers due to ballasting and tank washing.

Generally, crude oil carriers of 20,000 deadweight and above and product tankers of 30,000 tonnes deadweight and above delivered since 1983 must have SBT. Segregated ballast tanks are ballast tanks that are completely separated from the cargo oil and fuel oil systems, and which are permanently allocated to the carriage of water ballast. SBT greatly reduces the likelihood of oily ballast discharge, as there are sufficient segregated ballast tanks for normal operation in ballast. For these vessels, ballast may be allocated to cargo tanks only when needed to insure the safety of the vessel in particularly severe weather.

Unlike SBT, the piping systems for clean ballast tanks (CBT) may be common or connected with the cargo oil pump and piping systems. There are only a few CBT tankers operating today.

Crude oil washing is a system of cleaning cargo tanks using the dissolving action of crude oil to reduce clingage and sludge. Crude oil washing eliminates or reduces water washing, and thereby reduces operational oil pollution.

Regulation 9 of MARPOL limits the amount of oil that may be discharged into the sea to 1/15,000 of the total cargo oil volume for tankers built prior to the implementation of MARPOL73 (commonly referred to as Pre-MARPOL tankers), and 1/30,000 of the total cargo oil volume for MARPOL tankers. The requirement that the oil content of discharged effluent cannot exceed 15 ppm has the practical effect of limiting operational discharge to amounts much less than these maximum values.

Table E-2 gives the number and average deadweight for tankers in year 1999, and Table E-3 presents the age profile. More than two-thirds of the current fleet has SBT or double hull (DH) arrangements. MARPOL Regulation 13G requires mandatory retirement for single hull tankers at 30 years of age. A revision to regulation 13G currently under review will phase out all Pre-MARPOL tankers by 1 January 2007, at which time all MARPOL compliant tankers will have either SBT or DH arrangements.

SBT and Double Hull Crude Oil Carriers

According to industry sources (INTERTANKO, unpublished), SBT and double hull crude oil carriers will water wash 3-4 cargo tanks twice a year for inspection purposes. INTERTANKO estimates an average of 6,000 m3 of wash water per tanker per year is discharged. Assuming an oil content of 15 ppm of oil, operational oil discharge is approximately 90 liters (0.08 tonnes) per year per tanker. Assuming 2/3 of the 1,782 crude oil carriers have SBT or DH arrangements, the total estimated discharge per year is as follows:

SBT and DH Crude Oil Carriers: 1,782•(2/3)•0.08 = 95 tonnes

For non-compliance vessels, where the ODMS is not working properly or intentionally bypassed, the estimated average discharge is 38 tonnes per year per tanker.

TABLE E-2 Summary of Number and Deadweight of Tankers for Year 1999 (Lloyd’s Register, 1999)


No. of Tankers

Deadweight (millions of tonnes)

Average Deadweight (tonnes)

Crude Oil Tanker




Product Tanker




Bulk dry/Oil Tanker








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