Prior to the adoption of "Unified Requirements for Polar Class Ships", Det Norske Veritas (today DNV GL) maintained its own set of requirements for ships operating independently in freezing sub-Arctic, Arctic and Antarctic waters. Ships operating in first-year winter ice with pressure ridges could be assigned class notation ICE-05, -10, or -15 where the number indicated nominal ice thickness used for structural design; for example, for ICE-05. Vessels expected to encounter multi-year sea ice and glacial ice inclusions could be assigned more stringent requirements class notation with POLAR-10, -20, or -30. Finally, vessels intended for icebreaking as their main purpose could be assigned an additional class notation "Icebreaker" after the ice class, e.g. POLAR-10 Icebreaker.