4. Technological Aspects of NSR Navigation
4.1 Ice-transiting Ships
The most salient characteristic of the NSR is the presence of sea ice. Not only is this sea ice a significant barrier to the feasibility of ship operations, reducing their speed, it is a hazard as well, which can at times cause damage to their hulls and propulsion systems. Navigation of these unique waters requires the design and construction of special vessels, generically referred to as ice-transiting ships. These ships are classified as ice-worthy/ice-strengthened ships, icebreaking ships and/or icebreakers, depending on their tasks and icebreaking capabilities. More often ice-transiting ships are simply called icebreaking ships. To ensure that icebreaking ships can ply these polar seas safely and efficiently, these ships are equipped with a range of unique structural features and safety requirements not seen in vessels designed for service in open water. In this section, we deal first with the general features of icebreaking ships and the regulations that govern them. Next we provide an overview of the icebreaking ships, in particular icebreakers and icebreaking freighters, drawing on specific examples of this type of vessel that are used in Russia. Russia possesses the world's largest fleet of icebreakers and ice-transiting ships and, through its NSR shipping activities, boasts more experience in this field of navigation than any other country. Finally, this section will survey recent research and development in the field of icebreaking ships, including some unique hull forms recently developed for operations in ice, with particular reference to an R&D project conducted in Japan on NSR commercial vessels.
4.1.1 Requirements for Icebreaking Ships
(1) General Characteristics
When ships navigate ice-covered waters, ice loads act on their hulls through contact with sea ice. This ice load can damage the ship's hull or its appendages, and even sink the ship in the worst cases. To ensure the safety of crew and cargo, ice-transiting ships must have solid hulls and mechanisms for reducing the ice load. In the 16th and 17th centuries, the Russians developed ships called koch and lod'ya for navigation near the coast of the icy White Sea. Some 18m long and built for a capacity of 30 people, these vessels had a uniquely round shape. Thanks to this design, when these ships were beset in ice and unable to move, the action of the ice pushed the ship up and out of harm's way, rather than crushing it (WP-28). This round hull form was born out of efforts to compensate for the technical shortcomings of the Russians, who could not build sufficiently sturdy ships at that time, rather than as an idea for protecting ships from ice loads. Much later, when Fridtjof Nansen commissioned Colin Archer to design the Fram, Archer adopted this idea (Figure 4-1-1). A cross-section of the Fram appears as a semicircle, in which both its stem and stern were rounded.
This concept of using a round hull form to lighten the ice loads was carried into the age of steel ships. One example from Japan is the Fuji, an Antarctic research vessel constructed in 1965; like the Fram, the Fuji featured a semicircular cross-section. In contrast, the Shirase, a successor to the Fuji built in 1982, abandoned the round hull form in favor of a hull form slightly slanted on both sides (Figure 4.1-2). With this design, if the ship were beset in ice the ice pressing on the hull would not be crushed by compression but fractured by bending, lightening the ice loads on the hull. Although great strides have been made in the knowledge of ice load and the technology of ship structure design, ice load remains a fearsome threat to the ships in ice. Compared to other hydrodynamic loads, ice load concentrates a great amount of load in a relatively small area. To withstand this ice load, even today, the hulls of icebreaking ships are reinforced with vastly more steel than conventional vessels require.