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.
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.
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.
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.
Acknowledgements
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.