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.

Icebreakers

An icebreaker is a ship that is intended to break ice in order to escort merchant vessels, do ice management or carry out some other special task in ice. Usually these kinds of vessels are pure icebreakers, ice breaking supply vessels or cruise ships (modified usually from icebreakers). The borderline between different vessels is not exact. The supply vessels are intended to operate in various tasks at offshore oil and gas fields and thus they are not intended to be used in escort duties - this is reflected for example in that these supply vessels do not have a towing notch.

Breaking ice with ships was not possible before the advent of steam power. By the end of the nineteenth century, only fixedpitch, screw-type propellers driven with steam power were installed on new icebreakers. Early icebreakers were not powerful, and the hull form was basically adapted from open water hull shapes by sloping the bow angles more to create a vertical force to break the ice in bending.

Advances in shipbuilding technology resulted in the creation of the icebreaker, a vessel strong enough to not only withstand the crushing power of the ice, but to break through it. This technology finally opened most of the ice covered Arctic Ocean to military, scientific and commercial interests. In the last decades of the twentieth century, significant developments in icebreaking technology took place through the application of modern marine technology to the design and the operation of polar ships. As a result, ships can travel to remote polar regions that were deemed impenetrable only a few years ago. Basically, polar ships have become larger, stronger, and more powerful. In addition, innovative ideas have been implemented to improve propulsion systems and reduce the resistance encountered during icebreaking.

The operation of icebreakers to support maritime trade is usually included in the infrastructure given by the port state. The icebreaker support is given in Finland by the Merenkulkulaitos (Finnish Maritime Administration), in Sweden by Sjöfartsverket (Swedish Maritime Administration), in Estonia by Veteede Amet (Estonian Maritime Administration), in Latvia by by the Port of Riga, in Denmark by the Danish navy, in the USA by the US Coast Guard, in Canada by the Canadian Coast Guard and in Japan by the Japanese Navy i.e. the Self Defence Force in Japan.

In Russia the state offers the ice breaking services through the state company RosMorPort but there is also some private ice breaking services. An example of private ice breaking services is given by Gazprom icebreaker that were intended to assist the oil export from the Prirazlomnaja oil field. There does not exist economic competition anywhere in the world in ice breaking services as there does not exist any free icebreakers to participate in the competition. Only in the Okhotsk Sea there exists some competition in offering offshore supply services.

The ice breaking performance reflects the capability of the ship in ice. This breaking performance does not necessarily make a good icebreaker. The requirements for a good icebreaker include a sufficient speed in the ice conditions of the operational area (in the Gulf of Bothnia for example 10-12 knots in 80 cm thick ice) and good manoeuvrability in ice for escort operations. Even a technically good icebreaker is not performing well in escort services if the crew cannot use all the capability invested into their ship. The experienced icebreaker crews indicate short escort times and larger escort speeds. The average escort speed is one of the main measures of a good, smooth winter navigation system. The ice breaking capability and the bollard pull help in assessing the capability of a ship in escort operations.

By 1900, it was well understood that, while ships with blunt bows are efficient in breaking level ice in sheltered areas, such as rivers, lakes and other protected areas, their performance in rubble ice is poor because they have a tendency to push broken ice ahead of themselves. On the other hand, ships with wedge-shaped bows and sharp stems did not have any tendency to push rubble ice. This experience led to all sea-going ships built between 1901 and 1979 having a wedged-shaped bow and a sharp stem. Over the years, the wedge-shaped bows became known as "conventional" bows, and the other shapes as "unconventional" bows.

In a different area pertaining to icebreakers, attempts have been made reliably to predict the areas in which energy is expended by an icebreaker operating during both continuous and boxing modes. It has been estimated that of the total energy expended by an icebreaker in breaking ice, 5% of the energy is consumed in actually breaking the ice, 80% is consumed in moving the ice out of the way of the vessel and in overcoming the buoyancy of the ice, and 15% is consumed in overcoming conventional hull resistance.

Ice floes which have become broken and move laterally by the ship's prow under the unbroken ice cover laterally limit the fairway channel formed and surround the stern of the ice breaker in such a way that the ice is drawn into the propellers as a result of the increased water speed produced by the propeller thrust deduction and the ice is chopped by said propellers, so that an increased propeller power is required. With increased propeller thrust deduction further disadvantageous effect occur, namely the ocean bed is washed out by the backwash and is moved to the rear and side if the ocean bed is made from soft material, so that protuberances and depressions form on said bed, which lead to navigational problems, particularly for ships following the ice breaker or in the case of sternway travel of the actual ice breaker. In addition, the propellers can be damaged by the ice flows broken by the prow during forward propulsion and moved back by the thrust deduction into the fairway channel.

The recent development in ships for traversing thick ice sheets has tended in the direction of higher and higher propulsive outputs which in turn render questionable the economy of such vessels, due to the increased operational costs of the machine units. In all of these developments, the conventional pointed shape of the forecastle of the icebreaker vessel has been retained, with the exception of minor variations, although this configuration is disadvantageous, particularly in continuous ice sheets: The inclined stem with a substantially wedge-shaped configuration will be pushed onto the ice sheet by propeller thrust and fractures by its weight the ice sheet but merely in a substantially central region. This broken ice region must then be widened into a channel having the width of the ship. This widening brings about the so-called "shoulder effect" which demands much energy. Another drawback is the fact that ice floes tend to become jammed in the narrow channel between the ship's side walls and the lateral edges of the rigid ice sheet, thereby resulting in additional friction forces.

Other types of icebreakers have been described in which weight was transferred cyclically in the vessel to induce pitching and other movements of the hull purportedly in resonance with the corresponding natural periods of the vessel in such movements. The patent describes the use of counterrotating eccentric weights in the vessel, and the shifting of water ballast fore and aft in the vessel; comment is also made therein that water can be pumped into and out of the vessel, all to pitch the vessel, purportedly in resonance with the natural period of the hull.

It is known that icebreakers which are equipped with rotating weight systems may operate to produce cyclic induced motion of the vessel at a frequency of 30 cycles per minute and preferably much greater. Induced pitch experienced at these frequencies by such mechanisms increased the icebreaking efficiency of such vessels. It has been found, for example, that when the induced motion system was operated, the bow of the vessel experienced vertical excursions of 10cm. (total amplitude) at the rate of 30 times a minute.

Thus, it has been discovered that the induced motion frequencies far exceeds the natural motion frequencies of usual icebreaking vessels. Such frequencies had no relation whatever to the natural frequencies of the hulls in question. It has been suggested that much higher frequencies (on the order of 120 cycles per minute) of induced motion would be even more effective. It is highly significant the such induced pitching motions are of small amplitude. It is also known that ballast systems, while effective to produce pitch in a ship under static conditions, cannot be used effectively to produce forces with sufficient rapidity to attain something of a ship's natural rythm of pitch. Thus, prior practical experience with induced hull movements for icebreakers involved high frequency, low amplitude movements resulting from effects internal to the hull.

With the introduction of low-friction coatings and auxiliary systems, the capabilities of present icebreakers are greatly enhanced so that they can make steady progress in all types of ice conditions. With sufficient displacement, power and auxiliary systems, future icebreakers that can operate independently year-round in the Arctic are well within the known technology and operational experience.

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Military

Introduction to Icebreakers

Icebreakers