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.

Discharge of oily water within 50 nautical miles from shore is prohibited. Therefore, operational discharges from tankers in North American waters were presumed to be zero. Intentional discharges within U.S. and Canadian waters are believed to be small due to rigorous enforcement programs, and such intentional discharges are considered as part of the input from spills.

By 1999, approximately two-thirds of the tankers operating worldwide had either SBT or double-hull arrangements. SBT and double-hull crude oil carriers will water wash 3 to 4 cargo tanks twice a year for inspection purposes. Assuming full compliance with MARPOL and an oil content of 15 ppm for the discharged effluent, the total discharge per year from SBT and double-hull tankers is estimated as 95 tonnes per year.

The remaining one-third of the crude oil carriers are Pre-MARPOL vessels, and they carry ballast in their cargo tanks. For MARPOL compliant operations, the committee estimated that 1/200,000 of the cargo oil moved on these Pre-MARPOL vessels is discharged with the ballast. This assumes that 30 percent of the deadweight is discharged as ballast with an oil content of 15 ppm. If all Pre-MARPOL tankers operate in compliance with MARPOL, the total estimated discharge is 5,000 tonnes per year.

The outflow factor (oil discharge expressed as a fraction of the oil moved) is estimated as 1/2,000,000 for SBT and double-hull product tankers, and 1/200,000 for Pre-MARPOL product tankers. Assuming all vessels are MARPOL compliant, the estimated operational discharge is 380 tonnes per year for SBT and double hull product tankers, and 1,900 tonnes per year for non-SBT product tankers.

Thus, if all tankers operate in strict compliance with MARPOL, the total operational discharge from cargo washings and ballast discharge is estimated to be 7,400 tonnes per year. It is recognized that not all vessels comply with the regulatory requirements, and the following levels of compliance were assumed when calculating worldwide operational discharges:

Greater than 125,000 DWT

99 percent compliance

20,000 to 125,000 DWT

95 percent compliance

Less than 20,000 DWT

90 percent compliance

The rationale for this degree of non-compliance is that: 1) not all tankers fly the flag of a State that is party to MARPOL 73/78; 2) not all tankers that fly the flag of a State party to MARPOL 73/78 operate in compliance with the discharge criteria; 3) there will be equipment failures on board ships; and 4) there are not adequate reception facilities worldwide. The higher degree of compliance allocated to the larger tankers was justified on the basis that the majority of the larger tankers fly the flag of a State party to MARPOL 73/78, and the majority of larger tankers are on longer voyages that facilitates compliance with MARPOL 73/78.

When these levels of compliance were factored into the analysis, the best estimate for total operational cargo oil discharge from tankers was calculated to be 36,000 tonnes per year. Due to the uncertainty in estimating the frequency of non-compliance and the expected discharge in the event of non-compliance, the minimum estimate is 18,000 tonnes (one-third of the best estimate), and the maximum estimate is 72,000 tonnes (twice the best estimate).7

In the 1975 NRC, 1985 NRC, and 1990 IMO reports, the best estimates for operational discharges from tankers were 1,080,000 tonnes/year, 710,000 tonnes/year, and 158,600 tonnes per year, respectively. The new estimate of 36,000 tonnes/year follows this trend of reduced operational discharges from tankers. Further reductions are anticipated in future years when the tanker fleet is fully double-hulled.

Atmospheric Deposition (from Tankers)

Tankers emit volatile organic compounds (VOC) during loading, during crude oil washing operations, and during the course of the voyage. The amount of VOC emissions depends on many factors, including the properties of the cargo oil, the degree of mixing and temperature variations experienced during the voyage, and whether a vapor recovery system is employed during loading operations. Precise measurement of VOC loss from tankers is difficult. Cargo insurance companies will typically exclude coverage for loss of 0.5 percent of a crude oil cargo as normal variation between loading and unloading ports. This is the upper range of potential uncovered loss.

INTERTANKO conducted an extensive research program to investigate the behavior of crude oil during transport (INTERTANKO, 2000). About 2,024 crude oil and VOC samples were taken during a total of 361 voyages. On the basis of CRUCOGSA study and further theoretical analysis, INTERTANKO estimates that total VOC emissions is approximately 0.20 percent of the crude oil movements, roughly half of these emissions occurring during transport and half during the loading process. In an information paper submitted to IMO MEPC 47 by INTERTANKO, they note that for the more volatile types of crude oil, the loss rate is between 0.4 percent and 0.6 percent. A loss rate of 0.2 percent is applied in this study.

Approximately 3.3 million tonnes of petroleum products were moved by tanker vessels in 1999, including about 2.4


The 33 percent factor used to develop a minimum estimate and the 100 percent factor applied to develop maximum estimate are somewhat subjective and reflects the committee’s confidence in the data available and the methods and assumptions used to complete the calculation.

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