In this article, we will delve into the design and construction aspects of ships capable to ply in ice. This is also related to the concept of ‘Ice Class’ which deals with classification guidelines and norms that dictate the construction design of these vessels.
Recapitulating, ships sailing in icy waters essentially must have the following capabilities:
• Ploughing through ice ridges and sheets for ice-going vessels and icebreakers without compromising structural integrity
• Making channels, clearing through ice floes or loose ice
• Low Ice Resistance
• Unhindered propulsion through ice cusps and fragments
• Avoidance of freezing of machinery and equipment
• Protection of propeller, rudder and appendages, if any from contact damage through the ice
• Waning the feasibility of structural damage due to grazing, rubbing, grinding, banging of ice during clearance
• Higher propulsive efficiency and EEDI, though lesser in varying degrees for ice-capable ships
Depending on the design of the hull form, the broken ice can either:
• Be displaced underneath the vessel by submergence
• Flow aft and get lodged in the propeller’s wash
• Drift sideways
As described in the previous article, the ice responsive abilities are different based on whether the vessel is an ice-strengthened, ice-going or an ice breaker.
As per the classification, the design of the hull form plays a pivotal role in defining the same.
Some key factors in the hull form are stem angle, buttocks, beam, and flare, waterlines. As the main capability of an ice-class ship lies in its ice-negotiating and breaking capability without a compromise of its operational utility, certain crucial parameters are taken into consideration such as:
• Determination of the likely loads (based on ice characteristics, expanse, areas of the voyage, temperatures, ice covers and most importantly, the operational nature of the ship)
• Framing system
• Allowable and limit-strength design values, the overall detailed design (make, dimensions, hull form, propulsion system, power output etc.)
• applicable to the designated vessel
• Hull Material
• Paints and coatings
• Design of propellers based on the feasible zones of navigation
Ostensibly these are invariably applicable to more or less all forms of ships. However, when plying in ice, the importance of these becomes manifold.
For instance, in any other vessel, loads encompass principally hydrostatic and hydrodynamic loads along with secondary ones like wind or weather loads.
Obviously, in icy waters, ice becomes the biggest factor. The strength analysis of ice coupled with its properties is already discussed in the previous articles. The modes of ice negotiation and breakage were also briefed.
From a design point of view, a vessel plying in icy regions is subjected to ice in the lower portions of the hull as obvious. So, needless to say, that only those areas of the hull are given special consideration in terms of ice-strengthening.
Before carrying out our discussion further into the design and classification aspects of ice-worthy vessels, let’s us briefly touch upon the crucial aspect of ice resistance.
As we know, ships already are encountered with frictional, wave resistance and form drag in open water along with other smaller addendums like an appendage or air resistance.
Of course, it goes without saying that in icy waters the ice poses a value of resistance which irreversibly affects the propulsion and powering problems of the vessel.
To a layman, the resistance imposed by ice may appear to be akin to the ones posed by the road to a moving vehicle, which in elementary physics is simply coined as ‘friction’ force. But here it is more complex. The resistance encountered by vessels in icy waters is an interplay of various phenomenon exhibited by the intact or broken (brash) ice during its interaction with the hull.
As we have discussed in our previous versions, when a vessel encounters a sheet of ice in its course, it rams into it breaking it off to floes, fragments, ridges etc. The aggregate ice resistance is comprised of the resistance due to:
- The crushing or breaking of ice, which can be visualised as more of a mechanical resistance
- Turning/drifting and moving of the ice floes sideways
- Resistance due to shear failure of ice
- Submergence of the broken ice.
- Level Ice resistance
Let’s begin with the last one. Level ice resistance is the simplest form of ice resistance. Suppose, the icebreaker or an ice-worthy vessel is stuck abaft or affront an expanse of level ice or simply an ice sheet afloat in water.
During this process, the stem of the vessel will tend to interact with the ice cover for the purpose of breaking it. The stem of the hull imposes a pushing force tending to barge through the ice which in turn triggers high moduli of reaction force from Newton’s 3rd law.
This will provide a considerable degree of resistance to the ship tending to move ahead or astern (if the ice sheet is behind) to the effect citing the interplay of the hull contact and the reaction forces. The situation can be best compared to a tug towing a larger vessel.
The maximum towing force occurs at zero speed (when the towing line is fully taut and the larger vessel just begins to move), also known as the situation of maximum bollard pull. So, in this situation as well, the pushing force (analogous to towing force) value oscillates from zero (at the zeroth time) to a maximum constant value (after some time, when the ice sheet just begins to crush).
So the net ice resistance, in this case, is measured as the time-average value of the ‘pushing force’ (like the towing force of tugs). Maximum ice resistance (static) occurs at zero speed and is higher than at low speeds (when the vessel just begins to move through the crushed ice).
As the ice gradually starts breaking, the velocity of the vessel increases as expected along with higher applied thrust and the ‘ice resistance due to velocity’/ dynamic resistance increases linearly with velocity. An obvious physical property dictating the level of ice resistance is the thickness of the ice.
Resistance during crushing can be reckoned interchangeably with the Resistance in Level Ice. Just an obvious difference is that resistance due to crushing encompasses all forms of ice including uneven ridges as well. So, the resistance force in level ice can be considered as a subset of the resistance categories due to crushing.
Here, resistance due to ice is principally due to the time-variant horizontal component of the reaction forces during the ship-ice interaction. This is well depicted in the following figure.
The resistance component due to shear failure follows local crushing resistance in most cases. The ice-capable design is aimed at inducing higher moduli of vertical components of contact force.
This breakage of ice at a distance away, as we know can be attained in two ways: One by constant contact force, wherein the ice may fail due to ‘buckling’ or ‘bending’. Another by partially ‘crawling’ or climbing the ice cover and breaking it by ‘bending’. The disparity in terms of bending and buckling lies in the angle of interaction and friction (between ice and the structure).
The second probability of the first case and the second case is more common. As we had already discussed, the application of compressive loads, in this case, increases flexural stresses due to the reaction force of the ice which is somewhat ‘hydraulically elastic in nature’.
Now an obvious question pops up: How can the ice be brittle exhibit elastic properties? The answer lies in the expanse of ice cover. Long sheets of ice over a region have a certain extent of elastic behaviour analogous to a strip of metal!
This aspect makes its fail by buckling and predominantly, bending. However, we omit to delve deep into the complex mechanics of ice elasticity. That’s why in larger ice covers, crushing is not that important.
In both cases, there cracks propagate radially until the point of failure due to flexural stresses. Flexural stress values increase and after sometime approaches the limit stress of bending. This is dependent on the contact area as a larger contact area subjugated to continual loading over time leads to shear forces. This is visualized by the formation of radial cracks at varying distances from the point of application.
Spontaneously, the local stresses increase reflected in the form of what we had contemplated the crushing resistance. Due to excessive lateral loads, after a certain tenure of crushing, the ice slab may break/fail at a certain distance due to shear failure, remember? (If not, read the previous articles of this series!)
The breakage of the ice sheet appears at a certain concentric radial crack a definite distance away from the point of contact. This creates the shear resistance, which is ten times lower than the resistance posed due to crushing!
Thus, once ice fails due to shear failure, the resistance is decreased manifold. To put it simply, the ice resistance during the ship-ice interaction increases linearly (predominantly crushing till the point of shear fracture and then gradually decreases till reaching a constant value when the resistance is predominantly resistance due to shear failure and the crushing failure diminishes).
This is illustrated below:
Now after the breakage of ice, the remnant components of ice resistance are posed by the submergence of ice fragments, turning/drifting of ice sideways and sometimes just the movement of ice floes in opposing directions.
Resistances due to turning or lateral movement of ice and submergence are mainly triggered by the simple grazing of the ice against the hull plating creating mechanical friction.
The effect is more pronounced in these two phenomena as the under hull region and the side plating, being the most prone zones of ice contact during submergence and turning, respectively. As understood, the under hull part is affected by the reverse grazing of the ice bits moving aft after submergence, whereas for the side shell it is the constant grazing, impact and collision of the ice floes.
The fate of the broken off ice largely depends on the geometry of the hull, which we shall discuss in the next section. Then the ice fragments disintegrate after the interaction with the stem pressure of the hull they tend to come in contact with the side. This is also aided by the buoyant forces and the hydrodynamic forces where these ice floes tend to cling to the hull plating.
Here the surface of contact is mostly flat or even as opposed to the stem where the sharp minimum area tends to wield high pressure required for crushing of the ice. So, the ice follows the contours of the hull amounting to the resistance due to grazing or rubbing or sliding. This ice may now recede further sideways or move underneath the hull depending on the hull form and ice behaviour.
The broken ice can also be set into a rotational motion causing resistance beyond the scope of this article. However, it can be worth saying that the hull form of the vessel and the resultant surface area is of paramount importance as the frictional resistance components of ice are all dependant on this.
Another way of breaking up of ice resistance can be as RI = RB + RV, where RB and RV are breaking and speed-dependent parts of ice resistance respectively. The speed-dependant part is again divided into submergence and grazing, as both of them involve a relative motion between the vessel and the ice surface.
Estimation Of Ice Resistance
Reiterating, the ships in ice waters are encountered with the net ice resistance which is comprised as follows: ii) Open-Water Resistance iii) Pure Ice Resistance
Riw =Rop +Ri
Now the main challenge is the determination of ‘Pure Ice Resistance’ as it gives an idea about the ice capability of the hull in addition to its open-water capability.
Methods for estimation of ice-resistance involve:
- Model Tests
- Full-scale trials
- Empirical and numerical approaches based on past collective data.
To begin with, in the case of full-scale trials, the estimation of ‘pure ice resistance’ is based on the fact that the net ice resistance, as shown above, is composed of the open water resistance and ‘pure ice resistance’ itself. Now, Wave resistance is computed empirically from given equations.
Moving on to frictional resistance, it is calculated from model experiment data and extrapolated from existing data. So, now the last variable that needs to be calculated is the Net Resistance itself. Two methods exist
1) Shaft Thrust Method
2) Shaft Torque Method.
More common is the first one.
The estimation process includes an interplay of full-scale trials and model test. The entire aspect works on a dramatically proven hypothesis that the thrust deduction factor, t is the same in open water and in icy conditions. Though no mathematical reasoning has been provided for the same, it can be argued that the resistance offered by ice shards is similar to waves in open seas, which are minimal in icy waters and in level ice conditions when the vessel is treading forward to break the ice. The absence of one and omnipresence of another in either of the cases strike a balance! Now from open water model tests, the thrust deduction factor tp (= ti) is deduced using plot data of tp v/s Jm (advance coefficient = Ship advance Speed (V)/ Propeller Shaft rotating speed (n)* Diameter of Propeller (D)). For computing Jm, all the parameters in terms of ship speed, shaft rotation speed and propeller diameter are available from full-scale trials. Moreover, the shaft thrust in icy water conditions (Tbi) is also obtained from engine readings in trial. Hence, from the definition of thrust deduction factor itself, the net resistance in icy waters (Riw) is calculated. Now as the equation goes,
Riw = (1-tp)*Tbi
Also, the data of resistance in open waters is computed from either model data or extrapolation from full-scale data. Subtracting the latter from the net resistance in icy waters, as per the above equation, the resistance of pure ice is obtained.
The calculation of the various resistances can be calculated from purely model tests as well as per standard procedures like one prescribed by ITTC. The main challenges in model testing lies in the laying of ice, maintenance of ice in the ambient temperatures and accurate measures of resistance posed by ice.
Various methods of model ice production in model tank are used like ‘spraying’ and ‘seeding’. Spraying refers to the process of simply spraying several layers of fine-grained ice and allowing them to settle successively. Seeding is a bit complicated process as it involves spraying a single layer of ice and allowing it to grow in the ambient controlled conditions.
Ship-model correlation scale is similar to the basis and practises of other model tank tests. In Dynamic similarities, Froude similarity (read through our previous articles on resistance) is definitely maintained along with Cauchy similarity which involves a ratio of inertial and elastic forces (related to ice). A swathe of expansive ice exhibits elastic behaviour, remember?
To enhance relation to full-scale conditions, model testing tanks also simulate ice conditions in various feasible forms like:
- Level Ice
- Predawn ice
- Ice floes
- First-year Ridges
- Rubble ice
- Brash ice
Preparation of model for ice testing is similar to that of open-water model tests. However, special care is taken to account for the unavoidable friction between the model and ice. Model data about hull form and propulsion should be presented beforehand. Since the model ice can impose a significant amount of forces on the model ship, it’s built to suitable ice forces in the tanks without structural damage.
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