How often must oil record books be maintained onboard tankers?

The maintenance of oil record books on board tankers must be done continuously, meaning entries should be made in real-time as operations occur. This practice is essential for ensuring accurate and up-to-date records of all oil-related operations, including the transfer, discharge, and disposal of oil and oily mixtures.

Real-time documentation helps in maintaining an accurate account of activities and facilitates immediate audits and inspections, thus enhancing compliance with regulatory requirements. It also plays a crucial role in preventing marine pollution by providing a detailed and chronological account that can be reviewed by authorities to ensure that the tanker is operating within acceptable environmental standards.

While the other options suggest various frequencies for maintaining records, they do not align with the strict requirements set by international regulations such as MARPOL Annex I, which clearly stipulate the need for immediate and continuous recording.