In this way the hours under way in each segment can be obtained. Through the repeated calculation of hours under way in each segment, the distribution of the running days at sea can be derived for the entire route. Ship speed under icebreaker escort was assumed to be the same as that of the icebreaker, and the ship speed was calculated using the ice index derived for the Arktika. Appendix 5-2 shows the correlation between ice index and the ship speeds of the icebreaker and three types of ship used in the simulation.
Simulation program
The simulation calculations of shipping costs were conducted for all combinations of four routes and three ship types. The Russian data were used to specify ice conditions in the NSR, defined by segment, year and month. Each ship left the port of Yokohama in a specified month and year and passed in succession through approximately 130 segments in the NSR. The time to pass through each segment was calculated and costs arising in each segment were derived in a series of repeated calculations. Although the algorithms of the calculation are simple, the colossal data that must be processed in this simulation required a special suite of application software. Three modes of navigation were treated, which depended on the presence or absence of an icebreaker escort.
Navigation modes and the decision to use an icebreaker escort
If the ice index is sufficiently high, ice conditions are favorable enough that ships may navigate the NSR unescorted, even if some ice is present. This mode is called independent mode. As ice conditions deteriorate the ice index drops, obliging icebreaking cargo ships to travel under icebreaker escort. This second scenario is called escort mode. The point on the ice index at which icebreaker support becomes necessary, called INc, is derived from the relation between average ship speed and ice index. Icebreaker support is deemed necessary when average ship speed falls to 3 knots, and this point therefore becomes INc. INc is -4 for 25BC and 40BC and -1 for 50BC. It is assumed that an icebreaker can escort a cargo ship without any standby time at the starting point of the route segment where the escort is requested. Modern satellites have enabled remarkable progress in technology for sea ice monitoring, so that icebreaker escorts can be arranged well in advance of voyages. Icebreaker escort is assumed to be provided in units of one day, and average speed is assumed to be 10 knots. Once escort begins, the computer code searches escort mode segments for the next 12 segments (240NM, including the segment in which escort began), and if the computer finds any segment less than INc. the escort is continued (this process is called the "watching mode"). When no escort mode segment is found in the next 12 segments, escort mode can be switched into independent mode at the next segment.
