Figure C-12 Monthly changes in average sea speed in the regional west route in each sea region (25BC, averages for 1957-1990)
Supplementary discussion of the ships used in the simulation
40,000DWT icebreaking bulk/container ship
The design was aimed to enhance icebreaking capability for unescorted navigation for eight months in the NSR as well as dead weight up to 40,000DWT. Figure C-13 shows the general arrangement and rough body plan of such a vessel. This ship, as an open bulker, is capable of carrying either bulk or container cargo. To carry a container cargo, hatches are opened to store 825TEU of containers in the hold and 846TEU on the deck for a total container cargo capacity of 1671TEU. The propulsion system consists of two Azipods. The Azipod was successfully adopted in the icebreaking tanker Lunni. Propulsion is provided by two full rotating Azipod-units podded synchronous AC motors rating 14 MW, 28 MW in total. Four medium-speed engines rating 7.92MW each generate the AC electricity. The motors are controlled by cycloconverters. In the relatively gentle first-year ice, the bow and stern are reversed, the ship moves stern-first and the propeller pulls the ship forward, performing icebreaking in a mode of operation called stern mode. Like the bow, the stern is also designed to be effective at icebreaking. Because the Azipods can be rotated 360°, a rudder is not needed, and the vessel boasts outstanding turning ability. The astern mode is usually employed up to moderate ice conditions without multi-year ice to preclude hard multi-year fragments from hitting against the Azipods, which are unable to bear high ice loads. When the ship encounters harsh multi-year ice, the bow turns forward to propel the ship in the conventional bow mode. When a ship is expected to encounter only moderate first-year ice, the bulbous bow design can be adopted, providing excellent performance in calm water. Ships designed in this fashion are called Double-acting Azipod Ships (DAS). However, this could not be done for NSR navigation, since multi-year ice is abundant in the eastern part of the NSR. The icebreaking capability of the ship was verified by model tests in ice tank. In stern mode the ship can move at 1m/s through level ice 1.8m thick, and in bow mode it can move at the same speed through level ice 1.2m thick. The vessel adopts stern mode to pass through ridges, by operation of Azipod, turning left and right to wash out broken fragments of the ridge keel, away from the surrounding of the ship. This action also reduces pressure at the bottom of the ridge, causing the ridge to sink downward and keeping the ship moving steadily forward. The ship was verified by model tests to be able to pass through a first-year ridge, equivalent to a ridge with 15-20m keel in full scale. In normal bow mode, the ship had to ram three times to pass through the ridge with keel 15m deep. In calm water the ship operates with only two generators (four in ice). Given output of 15.8MW and a sea margin of 15%, the ship's speed under these conditions is 14.5knots.
