2.4 Service ships
Three service ships were used for the simulation. They are all hypothetical ships, although their performance and principle dimensions were confirmed employing ice tank tests and past design data. WP4 conducted the preliminary design for two icebreaking bulk/container ships with 40,000 and 25,000 dead weight tons (Juurmma et al., 1998). A type of ships should have been selected following cargo flow analysis in current and future, although the whole phase II project are running in parallel and not allowed to wait for the result of cargo flow analysis done in WP3. Currently there is little demand for specified cargoes in the NSR, and it also makes difficult to select an optimum type of ships for the simulation. Thus, a sort of open bulk carriers is selected as a type of ships to comply with multi cargo demand in future as a compromised solution. These two ships are able to carry dry bulk including ore, timber and containers.
The Ship and Ocean Foundation provided 50,000DWT bulk carrier design that was developed by the NSR steering committee in Japan (SOE 1998). This ship enhances open water performance and cargo volume rather than icebreaking capability. It would be suitable to examine the effects of less powered ship and larger cargo volume.
Table 2.4.1 summarizes the principa1 dimensions for three ships and Arctic class selected as an escort icebreaker.
40,000 DWT Icebreaking Bulk/Container Carrier
The design was aimed to enhance both icebreaking capability and dead weight up to 8 month independent navigation and 40,000DWT respectively. This ship is assumed to deploy in Northerly route and has a draft of 12.5m. Figure 2.4.1 shows the general arrangement and rough body plan. This ship features the DAS (Double-Acting Azipod Ship) with Azipod as well as adopted by MT Lunni. The Azipod rotates 360｡?nd realize the full DAS concept, therefore the Azipod works as pulling mode (the propeller comes first). The propulsion is provided with two full rotating Azipod-units podded syncrhronous AC motors rating 14MW each. Four medium-speed engines rating 7.92MW each generate the AC electricity. The motors are controlled by cycloconverters. The schematics of diesel electric systems are depicted in Figure 2.4.2. The going astern mode is usual employed in moderate ice conditions without multi-year ice to preclude multi-year fragments from hitting against Azipods that are unable to bear high ice load. Therefore, the conventional icebreaking bow form is adopted in order to go ahead in heavy ice condition including multi-year fragments. The cargo ship has 8 cargo holds and 6 of them are divided in upper and lower holds. The cargo holds are protected with double bottom and double sides along the whole cargo length. The upper and lower holds are suitable for timber and ore respectively. When containers are laden, pivot type hatches in the middle of cargo holds will be kept open. The ship carries 825 TEU in cargo holds, 846 TEU on deck, and 1671TEU in total when used as a container ship.
The icebreaking capability was estimated by the ice tank test using 1/30 scale model. From the ice tank test results, it is estimated that the vessel breaks 1.2m thick level ice at 3 m/sec. with ahead mode, and also 1.8m and 1.85m thick ice at 1.1m/sec. and 1.0m/sec. with astern mode respectively. As for ridge, tests astern were made in two ridges with thickness corresponding to 15 to 20m and length to 180m in full scale. According to the tests, the vessel is capable to penetrate the ridge continuously by turning the Azipod to and fro. However, when going ahead, the vessel was not able to move continuously, and three rammings were necessary to break through a ridge of 15m thickness in full scale. In open water service speed is 14.5 knots with 15% sea margin and required power will be 15.8MW that are supplied by two engines.