Design of Ice Breaking Ships 

By Kaj Riska

Shorten version. Full article is available as PDF attachement at the end of this page

The design of ice capable ships includes reaching an adequate performance, adequate hull and machinery strength and proper functioning of the ship in ice and in cold weather. Good ice performance requires hull shape that has a low ice resistance as well as allows different manoeuvres required. Good ice performance includes also a good propulsion thrust which can be achieved with propeller design and also designing the hull lines so that propeller-ice interaction is minimized. The adequate strength is achieved commonly by selecting a proper ice class and following the class rules. The designer must have some insight about ice loads in order to select the structural arrangement. This chapter describes the requirements for materials, equipment and general arrangement.

1. Designing an Ice Capable Ship

Understanding how ice is acting on a ship forms the basis of design of ships for ice. In this chapter some aspects of ship design for ice are covered, and mostly in a qualitative way. The reason for the qualitative approach is that no single and exact method for any aspect of ship design for ice exists. The designer is mostly forced to search for literature and then applies in various depths a multitude of methods found – and the final design is then a synthesis of results from different sources that the designer deems most appropriate. This judgement is at best if it is based on earlier experience; this makes ice design a difficult area as most valid experience is feedback from designs that have been realized and are operating in ice. The following outline about design aspects of ice capable ships should be considered as a general overview about design; the more exact numbers must be supplied by the detailed methods selected.
The design starting point is usually a functional specification (an example given in the box below) outlining the ice performance required. This specification is made often with interaction between the owner and a designer so that the different requirements are in balance. The balance of the different requirements ensures that no single requirement drives the design. Below is shown an extract of a detailed functional specification. Often the designer is given a free hand how the requirements are met but sometimes the owner has a clear idea of how the ship shall look like.
The General Ice Performance Requirements
• Average escort speed: The average speed in all normal ice conditions in the operational area must be at least 8 – 12 knots;
• Level ice ahead: The ship speed must be at least 13 knots in 50 cm thick level ice with a flexural strength of 500 kPa and thin snow cover. The ship must be able to proceed with a 3 knots speed in 1.5 m thick level ice;
• Level ice astern: The ship must be able to go astern with 7 knots speed in 70 cm thick level ice (flexural strength 500 kPa, thin snow cover);
• Manoeuvring capability: The ship must be able to turn on spot (180o) in 70 cm thick level ice (flexural strength 500 kPa, thin snow cover) in max. 2.5 minutes. The ship must be able to turn out immediately from an old channel with 5 m thick side ridges;
• Old channels: The ship must be able to maintain a high speed in old channels. Especially in a channel corresponding to the requirement of IA Super ships, she has to maintain at least 14 knots speed;
• Ridge penetration: The ship has to be able to penetrate with one ram (initial speed 13 knots) a ridge of 16 m thickness;
• Channel widening: The ship has to be able to make a 40 m wide channel in 50 cm thick ice (500 kPa, thin snow cover) at speed 4 knots;
• Performance in compressive ice: The ship must be able to maintain a 9 knots speed in compressive ice of thickness 50 cm.
• Temperatures: Air temperature -35 + 30 and sea water temperature -1 +32. 

2. Historical Development of Ice Capable Ships

A short historical note on the development of icebreakers and ice going ships is presented by noting the major steps in the evolution. The first ice breaking ships appeared in mid 1840’s in Hudson River in the US and in the Elbe River in Germany. First dedicated icebreakers appeared in 1860’s and 1870’s in the St. Petersburg and Hamburg harbours. Before the turn of the century several dedicated sea-going icebreakers were in service. The development of merchant ships for ice started towards the end of 19th century. The year-round navigation in the Baltic started in 1877 with the introduction of the ship Express II sailing between the ports of Turku and Stockholm. The hull design of this ship and many similar ones followed that of the icebreakers; only the machinery power was larger in icebreakers.
Ships that were intended to sail independently in ice evolved in 1950’s in the Soviet Union with the emergence of the Lena- and Amguema-series of ships (the latter is also called Kapitan Gotskij series). These ships had an icebreaking bow shape and a high strength for Arctic trade. Several series of Arctic ships has been built to Soviet and Russian owners (e.g. Norilsk and Norilsk Nikel-series) and to Finnish owners (Lunni-series) – the Canadian ships MV Arctic and MV Umiak 1 should be mentioned also. Since the early times the icebreakers and ice breaking ships have developed much based on several technological innovations, some of which are mentioned below.

The general arrangement of icebreakers and also ice going ships has changed little during the years. The largest change in the arrangement took place in 1970’s when the superstructure was changed into deck house i.e. no accommodation was placed in the hull, Fig. 2. The reason for the change was partly to increase the height of the bridge to improve the visibility and partly to avoid the noise and vibration caused by ice in the crew accommodation.

 Machinery of icebreakers has experienced many changes since the early icebreakers with steam engines and fixed pitch propellers. The economy and torque capability of steam engines was improved much with the introduction of diesel-electric machinery (diesel main engine with generators and electrical propulsion motors). The first diesel-electric icebreaker was the Swedish IB Ymer in 1933. The diesel-electric machinery is more expensive than a direct diesel drive but the torque performance of a fixed pitch propeller with a direct drive is not good; the solution for this is the use of controllable pitch (CP) propellers. These became common in early 1980’s in merchant ships. The bow propellers were introduced in icebreakers in the end of 19th century (first European bow propeller icebreaker is the Finnish Sampo). The bow propeller improves the ice breaking capability by reducing the forces required to break ice and by reducing the friction. Only lately the bow propellers have been made superfluous by the introduction of so called Z-drives (azimuthing propulsion units); the first icebreaker with azimuthing propulsion was the Finnish multi-purpose icebreaker Fennica in 1993.

Strength of ship hull and machinery is still mostly designed based on experiences from earlier ships. When damages caused by ice have occurred, strengthening of the structures is indicated. These experiences have been collected into the rules of the classification societies and thus most of the strength design is even nowadays done following the classification society rules. The Baltic is the most active sea area for ice navigation and it is natural that the experiences from Baltic are followed worldwide. The experience from ship damages is reflected in the strength level used in the Finnish-Swedish Ice Class rules. Already these short notes from the historical development of ice design show how closely the design of ice capable ships is linked with the experience from earlier designs. As the collection of feedback is not a straightforward task by any means, those designers that can follow the performance of their design in ice operation have an advantage.

3. Performance in Ice

Ship performance in ice consists of ability to break ice and to manoeuvre in ice – these capabilities have been defined in the functional specification. The capability of breaking ice is measured in uniform ice conditions (level ice, brash ice) by the speed at which certain ice thickness can be broken. Ice ridges and multi-year ice floes are distinct ice features and the capability in these is measured by the ability to penetrate these. The speed that the ship makes in ice is determined by the ice resistance determined by ice properties, and the hull shape and main dimensions as well as the thrust provided by the propulsion. The manoeuvring performance is similarly determined by the transverse forces provided by the rudder(s)/azimuthing thrusters and the resisting forces mainly due to ice. It is thus clear that the performance in ice is influenced by the resisting forces and the propulsive forces and these can be improved (resisting forces minimized and propulsive forces maximized) by hull shape and propulsion design, respectively.

Ice Resistance

Ice resistance refers to the time average of all longitudinal forces due to ice acting on the ship. These ice forces are divided into categories of different origin;
  •  Breaking forces;
  •  Submergence forces; and
  •  Sliding forces.
In different ice conditions the relative importance of these components varies; in level ice the breaking component is usually the largest but in brash ice or in smaller ice floes the other two components become more important. The breaking force is related to the breaking of the ice i.e. to crushing, bending and turning the ice. Submergence is related to pushing ice down along the ship hull whereas the sliding forces include frictional forces. Usually the velocity dependency of the ice resistance is attributed to the last component. A sketch of ice resistance experienced by a ship is shown in Fig. 3.

4. Machinery Layout

Main task of ship machinery is to produce the required thrust for ship propulsion. Main components of the ship propulsion machinery are the main engine, power transmission and the propeller. Each of these is described with the point of view of design for ice.

Machinery Alternatives

There are many alternatives for how the machinery layout can be realized. Most common machinery layout in ice classed tonnage a diesel engine or engines with a direct shaftline transmission (with or without gears) and a fixed pitch propeller (FPP) or controllable pitch propeller (CPP). An alternative is to use diesel electric propulsion where the power transmission is electric and separate electric propulsion motors supply the torque to FP propellers. Also gas turbines have been used as main engines or combined diesel/gas turbine solutions, where the gas turbine is used as a booster when high power is required.
The main difference in the machinery layout of an ice classed (ice going) ship as compared with open water ships is the operating regime of the propeller – in ice going ships the propeller load varies a lot depending on the ice conditions. The continuous load could be anything between the open water load to torque in bollard condition. If much ice is acting on the propeller, the ice torque can exceed the torque given by the engine; then the propeller slows down. Diesel engines have a relatively small RPM range where they can deliver full power, see Fig. 16, and thus direct drive diesel solutions may stall which often leads to stopping of engines in situations with heavy ice load. An improvement is to use a CP propeller that can adjust to the increased torque by decreasing the propeller pitch and in this way maintain the RPM. Even better solution is to use electric propulsion motors with diesel main engines and generators. Electric motors can maintain the torque in a large RPM range, thus the diesel-electric machinery is very efficient in conditions where occasional high torque loads are encountered. Diesel electric propulsion is used in most ice-going ships.

Propulsion Design

Classic propulsion system layout in icebreakers is single screw with a single rudder. The increased power led to twin screw solutions and then to introduction of bow propellers. The advantages of bow propellers were noticed in road ferries in the USA. The decrease of ice resistance due to bow propellers is attributed partly to a lubrication effect and partly to decrease in breaking resistance. 
The development of propulsion systems led finally to Urho class icebreakers with two bow and two stern propellers, two rudders and diesel electric propulsion. These ships that were constructed in mid 1970’s can be considered the last conventional icebreakers. The development of the propulsion arrangement of icebreakers is shown in Fig. 19 where the first icebreakers are from late 19th century.
The design of propulsion systems for ice going ships has experienced two large steps towards more advanced systems since 1970’s. The strength of CP propellers became adequate for ice conditions in 70’s and now CPP’s are widely used in ice going merchant ships. CPP’s are applied with a direct shaft drive in merchant vessels while in diesel electric machinery applications there is no need for a CPP. Another major step forward occurred with the construction of the Finnish multipurpose icebreakers, MSV Fennica and Nordica, in 1990’s, see Fig. 20. In these icebreakers azimuthing thrusters were used first time in icebreakers. Azimuthing thrusters offer a superb manoeuvring capability and they also replace the bow propellers because the propeller wash of the azimuthing thrusters can be used to flush ice along the ship hull. Since the construction of Fennica and Nordica, azimuthing thrusters have become most common propulsion system in icebreakers and ice breaking supply ships.
The propulsion system general design is based on balancing economic matters with the required performance. Diesel electric machinery with azimuthing thrusters (either podded or direct drive) seems to offer clearly the most advantages – this solution is also most expensive. Thus the most common application in merchant ships is a diesel direct drive with a CPP. As the requirements for icebreaking ships are more stringent, the diesel-electric drive is common in these.

5. Hull and Machinery Strength

Design of ship hull structures requires knowledge of the ice loads acting on different regions of the ship hull. Even if the structural design usually follows class rules, it is important in conceptual design phase to have an idea about the magnitude of ice loads and quantities describing the loads.

Definition of Local Ice Load

As ice loads arise from contact with an ice edge, it is commonly assumed that the load acts mostly on a load patch (area of non-zero ice pressure) that is narrow in vertical direction and long in horizontal direction. In case of an impact with multi-year ice of rounded shape, the load patch can be of more irregular shape. Load patch is idealized as a rectangular patch for structural response calculation of local shell structures like plating, main frames, stringers and web frames. This idealization is sketched in Fig. 24.

Ice pressure

How the ice pressure is conceived has varied much and there still is quite large controversy how to treat it. Often ice pressure is described by the average pressure on the area considered. Usually this area is the gauge area but also some geometric considerations may determine the area – if for example the load is observed on a pile of straight face towards level ice, then this area can be assumed to be D·hi where D is pile diameter and hi ice thickness. Observations of the ice pressure on smaller areas have suggested that considerable variation in local ice pressure magnitude exists inside the nominal contact area. The nominal contact area is defined by the geometry of the cross section between the ice feature and the structure – like the area D·hi mentioned above. Several different theories about ice pressure have been suggested.
The earliest model for ice pressure is to treat it uniform and proportional to compressive strength of ice (Korzhavin 1971). The proportionality factors depend on the shape of the contact surface and on ‘quality of contact’ whereas the dependence on ice temperature and strain rate was included in the definition of ice compressive strength. In the 1970’s much research was done to clarify these proportionality factors (see for example Cammaert and Muggeridge 1988) but when it was realized that the measured compressive strength of ice depends much on the testing methods and specimen preparation quality (see e.g. Kendall 1978 and Tuhkuri 1996), the popularity of the use of the Korzhavin Equation has diminished.
The highest values of ice pressure are coupled with ice failure by crushing. ‘Crushing’ is a general description of ice failure into small particles. As ice must be broken along the whole contact surface, it is clear that some flow of crushed ice from the centre of the contact must take place. Russian scientists have analyzed the flow of crushed ice assuming that the crushed ice is viscous fluid. The situation of the flow is depicted in Fig. 26. Based on this assumption and Reynolds thin film fluid flow equations the following form for the pressure have been derived (Kurdjumov & Kheisin 1976, Popov & al. 1968)

Ice Class Rules

The determination of scantlings of ship structures and also more generally the design of ship structures follows some rules that classification societies have given. The classification societies and also some maritime authorities (Finnish and Swedish Maritime Administrations and Transport Canada) have developed rules for designing ice capable ships. These ice class rules define several different ice classes depending on the severity of ice conditions. Ice class rules define the scantlings of the hull and shaftline structures and give some requirements for ship performance in ice and structural arrangement. At present there are three main sets of ice class rules: the Finnish-Swedish Ice Class Rules (FSICR), the Russian Maritime Register of Shipping (RMRS) ice rules and the unified Polar Class (PC) rules of the International Association of Classification Societies (IASC).
The FSICR (2008) contain requirements for ship hull, ship machinery and also for ship performance in ice. Four different ice classes are defined and also the open water ships have their own ice class notations (II and III). This is because the fairway dues are dependent on the ice class – higher ice class ships pay less fairway due as these ships use less icebreaker support.

The Finnish-Swedish ice classes are

1. ice class IA Super; ships with such structure, engine output and other properties that they are normally capable of navigating in difficult ice conditions without the assistance of icebreakers, maximum level ice thickness 1.0 m;
2. ice class IA; ships with such structure, engine output and other properties that they are capable of navigating in difficult ice conditions, with the assistance of icebreakers when necessary, maximum level ice thickness 0.8 m;
3. ice class IB; the same as above for ice class IA except that maximum level ice thickness 0.6 m;
4. ice class IC; the same as above for ice class IA except that maximum level ice thickness 0.4 m;
5. ice class II; ships that have a steel hull and that are structurally fit for navigation in the open sea and that, despite not being strengthened for navigation in ice, are capable of navigating in very light ice conditions with their own propulsion machinery;
6. ice class III; ships that do not belong to the ice classes referred to in paragraphs 1-5.
FSICR are intended for ships navigating in the Baltic following the operational practice used there i.e. ships are escorted by icebreakers in the worst ice conditions. The design point in the FSICR is the elastic limit; and the scantling equations have been modified through the years so that the damage frequency has reached an acceptable level. Measurements of the structural response of the hull structures have shown that the yield point is reached about once a week (Muhonen 1991); also that the yield point in plating is reached more often than in the frames – this suggests a correct structural hierarchy in FSICR. The highest machinery and hull loads and the performance requirement do not have a common design ship-ice interaction scenario as the largest ship response occurs in different kinds of scenario. The design scenarios for hull, machinery and performance are stated in Table 2.
The Finnish ice class rules have evolved since the first rules published in year 1890. The first rules gave just requirements for the general arrangement. The first rules for scantlings were published in 1920. These were so called ‘percentage rules’ as the scantlings were increased a certain percentage from the open water values. These rules were slightly modified in 1932 and 1962. When the year-round navigation to all Finnish ports started in 1960’s, the ship damages due to ice started to increase sharply. This experience from ice damages led to new ice rules in year 1971 – these were the first joint Finnish-Swedish rules and also the first modern ice rules in the sense that the ice load was stated explicitly. These rules have been revised several times (1985, 2002 and 2005) and the present rules stem from 2008.
The requirements for scantlings are based on ensuring an adequate safety of ships. The performance requirement (stated also as a powering requirement) is based, on the other hand, on ensuring an efficient winter navigation system. All ships fulfilling the requirement for an ice class set by Finnish or Swedish maritime authorities and bound to/from Finnish or Swedish ports get icebreaker escort. If the ice capability of ships would be low, many icebreakers would be needed to escort all ships (or the waiting times would be intolerably long), and the winter navigation system would be very expensive to maintain. Thus the merchant ships are required to have some ice capability so that the escort distances in ice will be shorter and escort speed higher.
The Finnish-Swedish ice class rules have been adopted by most of the classification societies (all except RMRS) – the FSICR have been described as an ‘industry standard’ for first year ice conditions even if they are intended only to Baltic. The classification societies follow their own notations, but the basic rules are the same as FSICR. The equivalent notations are stated in the table 3.
The short survey of ice classes show that it is difficult to select an ice class based solely on the ice class descriptions. The ice class that a ship should have is in principle set by the ice conditions and the required safety level – but in practice the required ice class is decided by the requirements of the maritime authorities. In Finland and Sweden the maritime authorities set the required ice class for each port in the Traffic Restrictions. These requirements develop when winter proceeds. Russian and Estonian authorities follow roughly a similar procedure; only the requirements are slightly lower than to Finland and Sweden. The Canadian system is called the Arctic Ice Regime Shipping System (AIRSS 1996) – in this system an Ice Numeral is calculated based on the prevailing ice conditions and ship ice class, and if the numeral is negative, the ship cannot enter the area. The selection of a suitable ice class must take into account what the authorities require in different ice conditions.

6. Winterisation Aspects

Winterization refers to those design aspects that are influenced by cold weather or ice cover, but are not covered in the structural design of hull or machinery covered by the ice rules. Thus for example the ballast water heating, sea chests’ operation without clogging by ice, deck equipment operation and avoiding or mitigating the effects of ice accretion are areas where the cold weather should be taken into account. The term ‘winterisation’ sometimes alludes to a situation where a ship is designed according to the open water practice and then the winterization aspects are added on top of this open water design. This does not result in good solutions as the cold weather and ice should be taken into account from the beginning.
In the conceptual design phase most of the winterization aspects are not very prominent as these can be dealt with during the basic design phase – this does not mean, however, that these aspects are somehow second in importance. If winterisation aspects are not taken into account, often the ship operability and functioning is impaired. Some winterization aspects should be, however, taken into account from the early design phases. One of these is the protection for icing. Often heating i.e. ice melting is offered as a solution. Heating can be a solution for icing only in small areas like control boxes, control equipment etc. Mostly a better solution for icing is protection; the forecastle can be covered so that winches are protected, outside gangways could be indented into the deckhouse etc. 
Another matter that influences the early design is the required visibility from the wheel house. Icebreakers and other ships that must navigate actively in ice must do many manoeuvres that bring them close to other ships and also other obstacles like offshore structures. In order to successfully operate in close proximity of other ships, visibility from the wheel house must be good in all directions. This has led to a cockpit concept in wheel house design, see Fig. 39. In this concept all the ship operation is concentrated at one location, usually on the starboard side. No helmsman is required as the officer of watch operates the ship from one position. The visibility in all directions, especially forward, aft and sideways to starboard must be good.

7. Conclusion

The design for performance and safety in ice has been covered in this chapter. The approach has been to give an overview of design aspects that must be taken into account rather than giving exact calculation methods. The design for ice is still more an art than a science and thus the designer must combine knowledge, sometimes conflicting, from different sources. Here the aim is to give some background for the designer and for a general interested reader of what the design for ice entails.
Several aspects in design for ice are still somewhat controversial; nozzles in ice, bulbous bows in ice, the pressure-area relationship to mention a few. The designer must use his/her own experience in making the design decisions. The design methods for ice do not have a single methodological background like the hydrodynamics where Navier-Stokes equations prevail. The approach used is mostly a collection of different methods from beam/plate theory, hydrodynamics, fracture mechanics etc. The methods in this approach are of an ad hoc type, all parts in the methods used should be roughly right. A first hand insight from ships operating in ice is invaluable in developing and using design methods incorporating results from several disciplines. The correct balance of different factors seems to be best achieved if the designer has some insight on ice action on ships. Thus designers who have gained insight from the feedback from the operation of earlier designs are in a good position.


The author in grateful for the invitation from Prof. Hayley Shen to write this chapter. Further, the author acknowledges the input from practical ship designers at ILS Oy who have forced me to apply the theoretical knowledge about ship-ice interaction in concrete ship design aspects.


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    RISKA, K. & LEIVISKÄ, T. & NYMAN, T. & FRANSSON, L. & LEHTONEN, J. & ERONEN, H. & BACKMAN, A. 2001. Ice Performance of the Swedish Multi-purpose Icebreaker Tor Viking II. The 16th International Conference on Port and Ocean Engineering under Arctic Conditions (POAC), 12-17 August 2001, Ottawa, Canada, pp. 849-866. Results from full scale trials of the first Swedish multi-purpose icebreaker
    RISKA, K. & UTO, S. & TUHKURI, J. 2002: Pressure Distribution and Response of Multiplate Panels under Ice Loading. Cold Regions Science and Technology, 34(2002), pp. 209-225. Theory for the influence of structural flexibility on the ice pressure distribution
    RISKA, K. & BREIVIK, K. & EIDE, S. & GUDMESTAD, O. & HILDÉN, T: 2006: Factors Influencing the Development of Routes for Regular Oil Transport from Dikson. Proc. of ICETECH’06, Banff, Canada, 7 p. Ship behavior in compressive ice and also in penetrating ice ridges.
    RUNEBERG, R. 1888/89. On steamers for winter navigation and ice-breaking. Proc. of Institution of Civil Engineers, Vol. 97, Pt. III, p. 277-301. First presentation known to the author about ice resistance. The work is based on the early year-round navigation between Finland and Sweden in 1877
    SANDERSON, T. 1988: Ice mechanics, Riska to offshore structures. Graham & Trotman, London, 253 p. The second text book about ice action on offshore structures
    SODHI, D. & TAKEUCHI, T. & NAKAZAWA, N. & AKAGAWA, S. & SAEKI, H. 1998: Medium-scale indentation tests on sea ice at various speeds. Cold Regions Science and Technology, Vol. 28, pp. 161 – 182. Test results from a large Japanese research project on structure-ice contact as well as an analysis of results including ductile-to-brittle transition speed
    SU, B. & RISKA, K. & MOAN, T. 2010: A numerical method for the prediction of ship performance in level ice. Cold Regions Science and Technology vol. 60, pp. 177–188. A model simulating the breaking pattern at the ship bow and prediction of the ship progress in ice
    TUHKURI, J. 1996: Experimental investigations and computational fracture mechanics modeling of brittle ice fragmentation. D.Sc. thesis, Acta Polytechnica Scandinavica, Mechanical Engineering Series No. 120, 105 p. A rigorous analysis of the crack propagation and formation of the active contact zone in structure-ice interaction
    Biographical Sketch
    Kaj A. Riska graduated from the Helsinki University of Technology in Naval Architecture as MSc. in 1978 and DSc. in 1988. He worked at the Technical Research Centre of Finland 1977 – 1988 as the group leader for Arctic Marine Technology. 1989-1991 he was a senior researcher for the Academy of Finland. 1992-1995 he was the director of Arctic Offshore Research Centre and 1995-2005 professor of Arctic Marine Technology at the Helsinki University of Technology. Since 2005 he has been the partner of the company ILS Oy and since 2006 Professor at the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway. He and his PhD students are investigating the models to describe the ice action on ships and their application in various ship design aspects.

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