MARPOL Annex VI entered into force on 19 May 2005 and Regulations 14 and 18 define the method of controlling Sulphur Oxide (SOx) emissions on a global basis and in defined protected areas called Sulphur Emission Control Areas (SECAs).

The aim of the legislation is to reduce SOx emissions from ships to reduce the acidification of the atmosphere and the resulting acid rain. This is to be achieved by setting a limit on the sulphur content in marine fuels.

Marpol Annex VI, Regulation 14 requires the following:

• A limit on the sulphur content on any fuel used onboard ship, this must not exceed 4.5% m/m.
• The sulphur content on any fuel used onboard a ship operating in a SECA must not exceed 1.5% m/m.
• Alternatively, an exhaust gas cleaning system or other approved technological method of reducing total SOx emissions from main and auxiliary engines and boilers to a maximum of 6.0g SOx /kWh when operating in a SECA. Controls are also set on effluent discharges from such cleaning systems.
• Details of the change over operation from high sulphur fuel to low sulphur fuel when entering a SECA are to be recorded in a log book and also when changing over to high sulphur fuel when leaving a SECA for an uncontrolled area.. The procedure is to ensure that all fuels exceeding the 1.5% sulphur limit are flushed out of the fuel system prior to entering a SECA.

Marpol Annex VI, Regulation 18 establishes requirements for the quality, sampling and delivery of fuel oil and the keeping of bunker delivery note records.