Complying to the Convention on the International Regulations for Preventing Collisions at Sea (COLREGs) is one of the key challenges for Maritime Autonomous Surface Ships (MASS) to perform autonomous navigation. We propose a rule-based approach utilizing operationalized situation statements to assess vessel encounters with regards to COLREGs. The statements contain basic informational pieces to describe the relation between two vessels. For processing with a rule-based inference system, the statements are grouped into rule sets to determine applicable COLREGs rules for an encounter situation. Implementation results are shown in the context of the B ZERO project, outlining how the approach can be utilized onboard a seagoing ship.