sentences:  Other classes are subject to limitations on time of year, required escort (always with a vessel of higher ice class) and ice conditions.The ABS database includes hundreds of Arctic Ice Classed ships, including many Arctic Research Vessels, and the entire productions of certain shipyards.The Russian Maritime Register of Shipping (RMRS), established in 1913, has a long history of classing icebreakers and ice-strengthened vessels, and today maintains its own set of ice class rules for vessels navigating in freezing non-Arctic and Arctic seas.