that shows the ice conditions including ice categories, their ice concentrations and average navigation velocity. The data like Figure 2.5.5 are useful to verify the relation between ice index and velocities. CASPPR adopted ice numerals that are calculated from ice class and ice conditions. An icebreaking vessel does mot allow to enter into area that ice numeral is negative.
Ship Speed in escorted navigation
An icebreaking cargo ship has to maintain a sufficient speed in a broken channel made by a leading icebreaker. When selecting the power level of the SA‐15 class, they were designed to attain the most efficient icebreaking capability when escorted by the Arktika class icebreakers that are the major ships in the NSR icebreaker fleet. Figure 2.5.6 indicates the dependence of speed moving through the broken channel made by the Arktika class icebreakers. The Arktika class icebreakers can navigate at approximately 2 knots in the 2m thick compact ice. While the SA-15 class can navigate slightly over 2 knots and thus the compatibility to the icebreaker can be well balanced. The data stored for 15-year operation enables to derive possible navigation days as a function of ice breaking capability. Figure2.5.7 represents the relation between independent navigation days and icebreaking capability escorting the SA-15 while ensuring safety operation against a relative heavier ice condition with four year return periods. The SA-15 class can continuously break ice approximately one meter thick. Thus, the SA-15s can independently navigate for：one month in transit navigation and in the eastern navigation;2.5 months in the western Arctic area;3 months in the western Kara Sea and 6 months in the Pechora Sea.
