World Wide Icebreakers

Among modern European sea-going icebreakers with non-traditional solutions as to the hull shape and rudder-propeller system one should distinguish Swedish icebreaker Oden built in 1988. The icebreaker has a simplified box-like form of hull with a flat stem and a wider, in comparison with main hull, forward end changing in the bottom wedge and making a cleaner wider channel as well as improving the maneuverability of the icebreaker. Besides, to raise the icebreaking capability, icebreaker Oden is equipped with powerful water washing and fastacting heeling systems. To reduce the circulation radius of the icebreaker special side attachments were used, which abut against ice when icebreaker is heeling creating additional turning moment. On the icebreaker a motor-reduction unit with two nozzle controllable pitch propellers is used.

Among the largest foreign icebreakers built during last 30 years there were American icebreakers Polar Star and Polar Sea of the same type constructed in 1976-1977 with a combined propulsion unit consisting of a diesel-electric and afterburner gas turbine installation and Japanese icebreaker Shirase of 1982 construction with a diesel-electric unit of 26 500 kW. On icebreakers of the Polar Star type for the first time in the practice of icebreaker building controllable pitch propellers were used, power being directly delivered form gas turbines via reduction gear to propeller shafts. These icebreakers are used mainly for the research in the Arctic and also, as icebreaker Shirase, for the supply of Antarctic stations.

Specially for the Antarctic, Finland delivered to Argentine in 1978 a twin- shaft icebreaker Almirante Irizar with a diesel electric plant of 14 MW. One should mention as well German research icebreaking ship Polarstern built in 1982 and used for Arctic and Antarctic expeditions. This ship has icebreaking hull shape close to a traditional one with forward end in the form of a concave wedge, motor reduction unit with a power of about 15 MW and two nozzle CPP. Particular feature of the hull shape of Polarstern, as of a ship designed for long transits in open water, is presence of a box-like keel 1 m high protruding beyond the basic line and playing the role of a passive stabilizer.

For carrying out conventional icebreaking works on the escorting of ships in the Arctic and freezing seas Canada for the period in question completed its fleet by a series of three icebreakers of the Pierre Radisson type with a diesel-electric plant of about 13 MW and one icebreaker Henry Larsen, also with a traditional diesel-electric unit of about 18 MW, but with an AC propeller drives. In the mid-seventies Sweden and Finland have set in operation icebreakers of the Finnish construction of the Atle type having four-shaft diesel-electric propulsion units with two stern and two bow propellers.

Particular feature of the last decade of the XX-th century in the field of icebreaker building was further improvement of conventional hull lines and rudder-propeller systems as well as the construction of new icebreakers of multipurpose type. The latters during winter period provide for the escorting of ships in freezing seas and in summer serve as supply vessels of offshore drilling rigs. As an example of that may be the construction in Finland of icebreaker Fennica which was put into operation in 1993. The second icebreaker of this type Nordica was delivered in 1994. It is envisaged to use these ships not only for icebreaking operations in winter in the Baltic Sea, but also as offshore supply vessels in the North Sea during summer period. Such purpose predetermined rigid requirements to ensure, along with high ice performance, good seaworthiness and maneuverability in open water. Accordingly, a combined icebreaking and seaworthy hull shape of the new ship was elaborated and two azimuthing propellers with a power of 7 500 kW each specially manufactured by Aquamaster-Rauma were used as a main propulsion rudder unit. Besides, three bow thrusters with a ship’s dynamic positioning were installed. Thanks to the possibility of directing propeller jet sideways the icebreaker was capable of making the channel with a width much larger than that of the icebreaker proper.

In 1998 the shipyard Aker Finnyrds Oy delivered to the Finnish Maritime Administration a new multipurpose icebreaker Botnica having smaller dimensions and power in comparison with icebreakers of the Fennica type (see table 2.2), but is equipped with Azipod propulsion system. The icebreaker is designed to escort ships in winter principally in the Gulf of Finland. In summer, she, as Fennica, serves as an offshore supply vessel in the North Sea. Besides, drilling equipment may be installed aboard the ship. Her functions include operations on rescue, patrolling and escorting of ships, oil spills combating.

In 1999 the polar diesel-electric icebreaker WAGB-20 Healy built in New Orleans at shipyard Avondale Industries to order of the US Coast Guard was put into operation. Along with its direct purpose, icebreaker Healy is constructed for the research works in high latitudes. Icebreaker Healy has a traditional improved hull shape permitting to work practically under any ice conditions keeping acceptable seaworthiness. Being a research ship the icebreaker is equipped with laboratories and living spaces for the accommodation of 50 scientists. On the icebreaker there is also a helipad with a hangar for a board helicopter. This icebreaker is the last one of the sea-going icebreakers built in the XX-th century.

In the Soviet Union, a whole series of nuclear ice breakers was constructed. The first- of the series, the ice breaker "Lenin," which displaced 16,000 tons was built in 1957. The nuclear power plant produced 44,000 hp., which permitted the ship overcome ice a few meters thick at a speed of 2 knots. The speed in open water was 18 knots. Even more high-powered icebreakers, such as "Sibir' ", "Arktika" and "Rossiya" are now in operation.

In 1988, the USSR launched the first nuclear ice-breaking cargo vessel, "Sevmorput' ", with a nuclear power plant capable of delivering 40,000 hp and a freight-carrying capacity of 33,000 tons. The vessel is about 300 m long, with a beam of 32.2 m. She is intended for transportation in Arctic regions. The "Sevmorput" nuclear powered lighter-aboard container carrier is the only icebreaking nuclear powered freight ship in Russia.

Introduction to Icebreakers

Icebreakers traditionally break ice in two alternative ways, namely: either by plowing continuously through the ice sheet relying on the downward force applied by a specially configured, highly raked bow structured to break the ice; or by a technique known as "boxing" or "ramming."

In plowing, a specially configured highly raked bow structure acts like a plow blade that runs under the ice sheet. The displacement of the vessel is that the bow runs under the ice sheet and the vessel is thus displaced downwardly. A moment is presented to the underside of the ice sheet. When the moment becomes sufficient to cause rupture of the ice, complete failure of the ice sheet occurs. This action causes the ice to plow over. This provided a very effective icebreaker as long as the thrust that was supplied by the power plant was sufficient to cause the bow to displace itself under the water, and thus to exert this moment. However, when the bow hits a pressure ridge it can no longer penetrate the ice because it is completely dependent upon the thrust produced by the power plant on the screws.

In boxing, an icebreaker runs its bow onto an ice sheet too thick to be broken by continuous plowing until the ship breaks through the ice at about which time the ship is either at rest in the ice or nearly so; after the ice is at least partially broken, the icebreaker is backed off the ice into the track of broken ice until it is clear of the ice sheet, and again driven to ram into and to ride up onto the ice.

Conventional ice breakers rely upon the mass of the vessel to accomplish breakage of the ice during both continuous plowing and boxing modes of operation. The forward end of an icebreaker may be ballasted to increase the effective portion of the overall mass of the vessel applied to the ice sheet, especially where the vessel becomes stuck on the ice during boxing of very thick ice sheets.

The effectiveness of an icebreaker, measured in terms of the thickness of ice capable of being broken during boxing mode operation, has been determined primarily by the displacement (total weight) of the vessel and by the efficiency with which the specially configured bows of these vessels transferred forward momentum and weight of the vessel downwardly to the ice. The basic objective has been to apply sufficient force downwardly to the ice or by the use of an upwardly acting icebreaker bow structure to cause the ice to break into pieces and to separate from the ice sheet.

The ratio of propulsive horsepower to displacement in icebreakers traditionally has been rather limited considering the task expected of such vessels. Propulsive horsepower has been limited to prevent the vessel from being driven so far up onto a thick ice sheet during boxing mode operation that the vessel cannot be backed off the ice. Bows for icebreakers also are designed to limit hull advance onto an ice sheet to the point where the vessel can be backed off if beached.

The development of ships designed for passing through thick ice has led to more and more powerful engine outputs, and these in turn render the economy of operation of such ships questionable, due to the rather high costs of the machinery. In the course of this development, the already long ago introduced shape of a pointed bow or forecastle in icebreaker vessels has generally been retained but for minor modifications, although this configuration entails various drawbacks, especially for the passage through continuous ice sheets. Although the inclined stem of an approximately wedge-shaped cross-section will be pushed onto the ice sheet by means of the propeller thrust and subsequently the stem fractures the ice sheet by its weight, this fracturing is essentially confined to a central region only. The channel thus formed has subsequently to be widened to conform to the width of the ship, and this widening is accompanied by the so-called "shoulder effect" which is very wasteful in terms of the energy required for overcoming this effect. Another drawback is that floes may become jammed in the narrow channel between the ship's side wall and the rigid ice sheet, and produce high frictional resistance forces.

When an icebreaker clears a channel in the ice for the passage of merchant ships, the ice floes sliding along the bottom and the side walls of the icebreaker vessel entail the further drawback that these floes will again emerge and then float in the opened channel behind the icebreaker vessel, in thereby impeding the passage of merchant ships. For these reasons the designers of icebreaker vessels heretofore have always attempted to arrive at an icebreaker design which allows to clear ice-free channels.

In the case of ships having a favourable icebreaking bow shape, break lines are produced in the ice in front of the ship and run at right angles to the longitudinal axis of the ship. Thus, ice floes are obtained, which initially have a width corresponding to that of the hull. These ice floes are forced under the hull, at a certain depth are broken in planned manner into two halves and are then led away to the side. In this process, the ice floe with its original surface is guided by the outer plating of the prow. At its leading edge the floe is supported against the unbroken ice. The supporting force acts on the unbroken ice in bottom to front sloping manner and its action direction is opposed with one component to the force intended to crush the ice in front of the ship into the aforementioned ship-wide floes and consequently reduces the breaking force. The ship must simultaneously apply the longitudinal component of this force through increased propeller or screw thrust. As the longitudinal component of the breaking force must also be consumed by the propeller thrust, the supporting of the broken floe and all floes behind it in the forwards direction has a double effect on the propeller thrust, in which all floes "stick" under the ship's bottom jointly have a double action, firstly due to the frictional force component opposing the thrust and secondly through the frictional force component opposing the breaking force. The first frictional force component consequently requires a higher propeller thrust, even if the friction is absorbed by surroundings other than the frontally positioned ice. The second component cancels out part of the breaking force, so that through a further increase in the thrust and consequently the ship must be moved even higher onto the ice than if no frictional force were present.

In all icebreakers, the support of the broken ice floe and the ice floes behind it in the forward direction has a multiple effect in the propeller thrust. This forward support would not be necessary if there were no frictional forces between the hull plating and the ice surface of the submerged ice floes. These frictional forces can never be completely prevented, but the magnitude thereof is decisively dependent on the contact between the ice surface and the outer plating. Vital importance is attached to the presence of a lubricating film between the outer plating and the ice. Even if no water can penetrate the space between the outer plating and the ice, in the case of a sufficiently large relative movement between ship and ice, the frictional heat in itself forms a lubricating film from the melted ice. In addition, the thermal state of this region is such that the melting heat is greater than the heat removed. However, this no longer applies in the case of slow ships movements, i.e. in the limit range of icebreaking by the ship and with very low outside temperatures. The small frictional heat produced at such a low speed is rapidly removed again by the very cold, surrounding ice and by the very cold steel of the hull plating, so that at the most there is dry friction leading to a very high frictional force.