VINTERSJÖFARTSFORSKNING. AN EXPERIMENTAL STUDY ON THE EFFECT OF SPEED ON THE ICE RESISTANCE OF A SHIP Phase I for Winter Navigation Board

Transcription

1 STYRELSEN FÖR VINTERSJÖFARTSFORSKNING WINTER NAVIGATION RESEARCH BOARD Research Report No 73 Teemu Heinonen AN EXPERIMENTAL STUDY ON THE EFFECT OF SPEED ON THE ICE RESISTANCE OF A SHIP Phase I for Winter Navigation Board Finnish Transport Safety Agency Finnish Transport Agency Finland Swedish Maritime Administration Swedish Transport Agency Sweden

2 Talvimerenkulun tutkimusraportit Winter Navigation Research Reports ISSN ISBN

3 FOREWORD In its report no 73, The Winter Navigation Research Board presents the outcome of the phase I of the project on an experimental study on the effect of speed on the ice resistance of a ship. The performance of an ice-going vessel is usually measured as the maximum ice thickness where the vessel can operate. Therefore previous studies on the ice resistance of a ship have focused on speeds below 1 knots. On the other hand, icebreakers operating at the Baltic Sea seldom navigate in ice conditions which correspond to the maximum thickness. In addition, maritime administrations have started to emphasize the need for higher escort speeds, typically recent requirement definitions have been 12 to 13 knots. The purpose of this work was to experimentally study the speed dependency of the breaking component and the restraining effect of the ice sheet on the wave making of the ship, and also gather information about the ice breaking process at high speeds for future research. The Winter Navigation Research Board warmly thanks Mr. Teemu Heinonen for this report. Helsinki and Norrköping June 214 Jorma Kämäräinen Finnish Transport Safety Agency Peter Fyrby Swedish Maritime Administration Tiina Tuurnala Finnish Transport Agency Stefan Eriksson Swedish Transport Agency

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5 The Ice Technology Partner AARC Report A-443 An Experimental Study on the Effect of Speed on the Ice Resistance of a Ship Phase I for Winter Navigation Board September 21 Teemu Heinonen Research Engineer Aker Arctic Technology Inc. Merenkulkijankatu 6, FI-98 Helsinki, Finland Tel Fax Business ID:

6 An Experimental Study on the Effect of Speed on the Ice Resistance of a Ship The Phase I report consists of two parts. The first part is a report of Teemu Heinonen s Master s Thesis and the second is a report of verifying model tests made at Aker Arctic Technology Inc. Suggestions for phase 2 are made on the ground of these tests. 1 Background The performance of an ice-going vessel is usually measured as the maximum ice thickness where the vessel can operate. Therefore previous studies on the ice resistance of a ship have focused on speeds below 1 knots. On the other hand, icebreakers operating at the Baltic Sea seldom navigate in ice conditions which correspond to the maximum thickness. In addition, maritime administrations have started to emphasize the need for higher escort speeds, typically recent requirement definitions have been 12 to 13 knots. This would provide more fluent traffic. The effect of speed on the ice resistance is poorly known and one of the main reasons for this is that the combined effect of the open water resistance and the ice sheet is unknown. The ice resistance is often divided into different components and speed dependencies of these components are unclear. Especially the speed dependency of the breaking component is debated. Also the knowledge of doing model tests at higher speeds is limited. The purpose of this work was to experimentally study the speed dependency of the breaking component and the restraining effect of the ice sheet on the wave making of the ship, and also gather information about the ice breaking process at high speeds for future research. The study was done as a Master s Thesis at Aalto University, School of Science and Technology. The effect of speed was studied experimentally: both full-scale and model tests were done. 2 Theory A brief look at the theory of ice breaking helps to understand the purpose of the work and methods used. A superposition principle is often used when determining the ice resistance. The superposition principle means that the total resistance in ice is a sum of the open-water and the ice resistance (Equation (1)). This assumption is not true as the open water resistance and ice resistance are linked together but the principle is used because of simplicity and the relatively small open water resistance at low speeds. But when the speed increases, the ship starts to form waves. The ice sheet will affect on the wave making and ice pieces which are pushed below the hull will affect the flow around the hull. Therefore the determination of the ice resistance on high speeds is difficult. R IT R R, (1) OW I 2-2

7 where R IT is the total resistance in ice, R OW is open-water resistance and R I is ice resistance. The ice resistance is usually divided into components: a breaking, a submersion and a velocity components are often used (Equation (2)). The velocity component is often used in semi-empirical calculations to take the speed effect into account. The speed dependencies and relative magnitudes of different components are debated. Especially speed dependency of the breaking component is debated in literature: some references consider it speed dependent while others do not. R I R R R, (2) B S V where R B is the breaking component, R S is the submersion component and R V is the speed component. 2-3

8 3 Full-scale tests Full -scale tests were performed in with icebreaker Kontio (Table 1). Table 1: Main particulars of IB Kontio (Arctia 21). Built Length, OA Breadth, max Draft, max Displacement m 24,2 m 8, m 913 ton Power Propellers Bollard pull Speed, open water Speed, 8 cm ice 15 MW 2 16 ton 18,5 kn 1 kn During these tests the bow of the ship was filmed from a pilot lift as the ship moved in level ice (Figure 1). Total four test runs were done along the normal operation of the icebreaker in the Bay of Bothnia. Test sites are presented in Figure 2. During tests, the icebreaker advanced with a few different power levels which corresponded following speeds: 2-3 knots, 12 knots, 14 knots and full power which gave knots speed. The ice thickness was around 3 cm in all tests. Recorded videos were later analyzed in order to visually study the breaking process and the wave-making in ice. Velocity and position were measured by using a gps-device. Power levels and propeller rpm were also recorded from instruments on the bridge. Figure 1: The bow was filmed from the pilot lift. 3-4

9 Figure 2: Test sites: Test 1 - red, Test 2 - blue, Test 3 - yellow, Test 4 - green 3.1 Full-scale observations and results The most important data from the full-scale tests were visual observations of the icebreaking process. Following observations were made: - The width of the broken channel increases as the speed increases (Figure 3 & Figure 4) - The block size reduces as the speed increases (Figure 5 & Figure 6) - The icebreaking process is different at high speeds (above 12 knots) than at lower speeds: ice raises up with the bow wave changing the contact point and angle between the hull and ice - Ice hits the hull in more perpendicular angle and more crushing occurs (Figure 7 & Figure 8) - Bow wave seemed to bend the ice sheet broken before any contact with the hull The classical ice resistance was determined by calculating the thrust of the ship from the propeller rpm and thrust curves and then reducing the corresponding open water resistance. The results are presented in Figure

10 The Ice Technology Partner Figure 3: Channel at 12 knots. Figure 4: Channel at 16 knots. Figure 5: Block size at 2-3 knots speed. Figure 6: Block size at 12 knots speed. Figure 7: Ice pieces hitting the hull in nearly perpendicular angle. Figure 8: Lot of crushing happens at high speeds. Aker Arctic Technology Inc. Merenkulkijankatu 6, FI-98 Helsinki, Finland Tel Fax Business ID:

11 Figure 9: Results from full-scale tests. 3-7

12 4 Model tests Model tests were performed in the ice model basin of Ship Laboratory (Aalto University) during Total four ice sheets were done: the same tests were driven in two different ice thicknesses (3 cm and 5 cm in full-scale, the bending strength was 55 kpa in full-scale in all tests). Firstly, tests were performed in level ice and breaking patterns and block sizes were analyzed from photos. Secondly, breaking patterns were cut in a new ice sheet and tests were performed in presawn ice. Four different speeds were used: 2 kn, 6 kn, 12 kn and 16 kn in full-scale. In addition, open-water tests were performed. All tests were done without propulsion as the model was towed. 4.1 The model The model of icebreaker Otso (sistership of IB Kontio) was used in tests. In order to study the effect of the ice sheet to the wave-making, waterlines and frames were drawn into the bow of the model and cameras were filming the bow. The model was towed and no propulsion was used. The resistance was measured by using a force sensor. The model is shown in Figure 1 and Figure 11. Data of the model and the ship is presented in Table 2. A turbulence stimulator wire was used in open-water tests. A 5 kg counterweight was used in all tests. Figure 1: The model of IB Otso. The waterlines are drawn with 5 cm intervals (1 meter in full-scale). Figure 11: Stern of the model. Propeller shafts were plugged. 4-8

13 Table 2: Model and ship data. Ship Model Scale 2 Lenght at designwatrline L DWL [m] 9 4,5 Length overall L OA [m] 99 4,95 Length of the bow L BOW [m] 18,9 Length of the parallel midship L PAR [m] 62,5 3,1 Breadth at design waterline B DWL [m] 23,35 1,17 Draft T [m] 7,3, Test program Four ice sheets were produced. First was the level ice and then presawn ice which was done according to the observed breaking patterns in level ice. The breaking pattern was done by doing longitudinal cuts and cuts parallel to the waterline angle. The distance between longitudinal cuts defined the length of the ice pieces while the distance between oblique cuts defined the width of the cusps. The speed of the model was determined according to the Froude s model law: VS VM, (3) where is scale factor ( = 2) and subscripts M and S refer to the model and to the ship respectively. Four different speeds were used:,23 m/s;,69 m/s; 1,38 m/s and 1,84 m/s, which correspond to the following full-scale speeds: 2 kn, 6 kn, 12 kn and 16 kn. Test runs were done in two separate lanes (Figure 12). Figure 12: Principle of how tests were performed. Speeds are in full-scale. Tests are named as Test X.X where the first number refers to the test day and the second one refers to the speed (Table 3). 4-9

14 Table 3: Test program. Testday 1 Level 15 mm ice Test 1.1 Test 1.2 Test 1.3 Test 1.4,23 m/s,69 m/s 1,38 m/s 1,84 m/s Testday 2 Presawn 15 mm ice Test 2.1 Test 2.2 Test 2.3 Test 2.4,23 m/s,69 m/s 1,38 m/s 1,84 m/s Testday 3 Level 25 mm ice Test 3.1 Test 3.2 Test 3.3 Test 3.4,23 m/s,69 m/s 1,38 m/s 1,84 m/s Testday 4 Presawn 25 mm ice Test 4.1 Test 4.2 Test 4.3 Test 4.4,23 m/s,69 m/s 1,38 m/s 1,84 m/s Testday 5 Open-water Test 5.1 Test 5.2 Test 5.3 Test 5.4,23 m/s,69 m/s 1,38 m/s 1,84 m/s The thickness and flexural strength of the model ice is determined according to the scale: h i, M h f, M i, S f, S (4) (5) Tests were performed in two different ice thicknesses: 15 mm and 25 mm (3 cm and 5 cm in full scale). The flexural strength of the model ice was 27,5 kpa which corresponds 55 kpa in full-scale. Open-water tests were done in the same tank as the ice tests. Approximately 1 minutes was waited between the tests in order to have a calm water surface. 4-1

15 5 Model test results Results were analyzed with Microsoft Excel. The mean value of the resistance was defined from an interval where the signal was as constant as possible. Measured resistances are presented in Table 4 and Figure 13. The time history of the resistance for every test is presented in Appendix 1. Measured ice properties are presented in Table 5. The Young s Modulus E could not be measured in tests with 15 mm ice as the ice was too thin to support the measuring device. The E/ f ratio represents the stiffness of the ice sheet and according to a common rule of thumb it should be over 1. Otherwise the ice sheet will be too elastic. Table 4: Measured resistances. Test number Resistance [N] 23,7 8,4 158,1 189,1 18,1 41,4 76,9 16,2 Test number Resistance [N] 36,8 17,3 227,6 314,4 24,5 6,2 117,3 155,9 Test number Resistance [N],6 8,8 34,1 74,7 Mitattu kokonaisvastus Vastus [N] ,2,4,6,8 1 1,2 1,4 1,6 1,8 2 Nopeus [m/s] Tasainen 15 mm Tasainen 25 mm Esisahattu 15mm Esisahattu 25mm Figure 13: Measured resistances. Blue = 15 mm level ice, pink = 25 mm level, yellow = presawn 15 mm, green = presawn 25 mm. 5-11

16 Table 5: Measured ice properties in different tests. Test No. h i [mm] f [kpa] c [kpa] E [MPa] E/ f ,9 42, ,25 52, ,5 25,45 4, ,5 25,5 52, ,5 29, , ,5 15, ,2 26, ,9 32,3 26, ,35 34,95 26, ,95 35,25 26, , , ,9 27,2 42, ,2 19,7 42, ,8 42, , , Analyzing results Model tests gave considerably high results compared to full-scale results. High resistance values occurred only in ice tests while open water tests correlated well with previous tests with the same model. It is possible that the high speed generates some unknown phenomena which increases the resistance but results were unrealistically high already at 6 knots speed which is a rather typical model test speed. Therefore it is difficult to estimate if the phenomena are due to high speed. Model test results were verified in another tests by the Aalto University s Ship Laboratory staff. Results were the same, so there were no errors in original measurements. Even though it is not possible to study absolute resistance values, it is possible to study speed dependencies of different resistance components. The ice resistance was also calculated with semi-empirical calculation methods and results were compared to full- and model-scale results. The method of Lindqvist (1989) and the method of Riska et al. (1997) were used The breaking component It was assumed that no breaking happens in presawn ice which means that the breaking component can be calculated directly by subtracting the measured total resistance in presawn ice from the level ice resistance. Therefore the breaking component R B can be calculated as following: R B RIT, level RIT, presawn, (6) where R IT,level is the total resistance in level ice and R IT,presawn is the total resistance in presawn ice. 5-12

17 No thickness correction was made as it was assumed that the open-water resistance is related to the ice resistance. The thickness correction would have affected the open-water resistance part and therefore was not used. The strength correction was made and the corrected breaking resistance for the model R B,M is following: R B R, M B f f,, t arg et measured, (7) where subscript target refers to the target value and measured to the measured real value. The force is extrapolated to the full-scale by the scale to the power of three. The breaking component for the ship R B,S is calculated as following: R. (8) 3 B, S RB, M The calculation method is presented in Table 6 and Table 7. Results are drawn in Figure 14. Table 6: The calculation of the breaking component in 15mm ice. Fr R IT,level R IT,presawn R B RIT, level RIT, presawn R B R f, t arg et, M B f, measured,3 23,6 N 17,1 N 6,5 N 6, N 48,1 kn,1 8,1 N 42, N 38,2 N 37,2 N 297,2 kn,21 157,5 N 79,4 N 78,2 N 84,5 N 675,7 kn,28 188,3 N 14,3 N 84,1 N 9,6 N 725,1 kn R 3 B, S RB, M Table 7: The calculation of the breaking component in 25 mm level ice. Fr R IT,level R IT,presawn R B RIT, level RIT, presawn R f, t arg et B, S RB, R B, M RB f, measured,3 33,2 N 23,6 N 9,5 N 9,1 N 72,6 kn,1 114, N 61,3 N 52,7 N 51,2 N 49,3 kn,21 235,3 N 117,7 N 117,6 N 111,7 N 893,4 kn,28 316,1 N 16,8 N 16,8 N 159,9 N 1279,3 kn M 5-13

18 Murtokomponentti Voima [kn] Nopeus [kn] 3 cm jää 5 cm jää Figure 14: The breaking component. Pink colour = 5 cm thick ice, Blue = 3 cm thick ice. X-axis is speed in knots and Y-axis is force in [kn]. According to model tests the breaking component is speed dependent as it increases linearly with speed (Figure 14). In thinner ice the breaking component decreased at the highest speed. This could be due to the reason that the bow wave breaks the ice as it seemed to do in fullscale tests but the more probable cause is a strange behaviour of the model ice in the test. The ice seemed to become slush which was thrown on the ice sheet (Figure 15). Apparently the increased crushing did not model correctly. The results are way too high as the breaking component alone in 3 cm ice is higher than the measured ice resistance in full-scale. In addition to the unrealistic values there are also other uncertainties in the experimental breaking component. As the block size reduces while speed increases, it is difficult to determine the corresponding breaking pattern. It is possible that the block size was too small in some tests which would result to too high breaking component and too low submersion component. Also the wave making is different in presawn ice than in level ice as the bow wave is smaller in level ice. The percentage of the breaking component from the total resistance is presented in Table 8. Figure 15: Model ice did not behave realistically at high speed tests in thin ice. 5-14

19 Table 8: The percentage of the breaking component from the total resistance. The breaking component s percentage from the total resistance Ice Thickness,23 m/s,69 m/s 1,38 m/s 1,84 m/s 15 mm 25 % 46 % 53 % 48 % 25 mm 27 % 45 % 47 % 51 % Total resistance and Ice resistance According to the equation (1) the ice resistance R I is calculated simply by subtracting the openwater resistance from the total resistance in ice. Now the thickness correction is taken into account as the open-water resistance is not present. The thickness correction is done separately for level ice and presawn ice as tests were done on different days and the ice had different properties. The breaking component R B is calculated as following: R B X X hi, t arg et hi, t arg et R I level R, I presawn h,, (9) i measured h, i, measured where the X has value of 1,75. The strength correction is made to the breaking component and finally the ice resistance is following: R I, M R B f, t arg et f, measured R I, presawn h h i, t arg et i, measured X. (1) The total resistance, ice resistance and submersion and friction resistance of the model are presented in Figure 16Figure 17. The last-mentioned represents the resistance due to submerging ice pieces and ice sliding against the hull. It is determined by subtracting the openwater resistance from the total resistance in presawn ice. The breaking resistance is acquired by subtracting the submersion resistance from the ice resistance. The open water-resistance is acquired by subtracting the ice resistance from the total resistance. The percentage of the breaking component from the ice resistance is presented in Table 9 and the percentage of the ice resistance from the total resistance is presented in Table 1. Table 9: The percentage of the breaking component from the ice resistance. The breaking component s percentage from the ice resistance Ice Thickness,23 m/s,69 m/s 1,38 m/s 1,84 m/s 15 mm 26 % 52 % 66 % 78 % 25 mm 28 % 49 % 56 % 66 % Table 1: The percentage of the ice resistance from the total resistance The ice resistance s percentage from the total resistance Ice Thickness,23 m/s,69 m/s 1,38 m/s 1,84 m/s 15 mm 97% 89 % 78 % 6 % 25 mm 98 % 92 % 84 % 75 % 5-15

20 Mallin vastus 15 mm jäässä Vastus [N] ,,5 1, 1,5 2, Nopeus [m/s] Jäävastus Kokonaisvastus Upotus- ja likukomponentti Figure 16: The model's resistances in 15 mm (3cm) ice. Pink = ice resistance, blue = total resistance, yellow = submersion resistance. Y-axis = resistance in [N], X-axis = speed in [m/s]. Mallin vastus 25 mm jäässä Vastus [N] ,,5 1, 1,5 2, Nopeus [m/s] Jäävastus Kokonaisvastus Upotus- ja liukukomponentti Figure 17: The model's resistances in 25 mm (5cm) ice. Pink = ice resistance, blue = total resistance, yellow = submersion resistance. Y-axis = resistance in [N], X-axis = speed in [m/s]. 5-16

21 The ice resistance of the ship is calculated as following: R. (11) 3 I, S RI, M The ice resistance determined by model tests is presented in Figure 18 and Figure 19. Also the calculated ice resistances (Lindqvist and Riska) (friction coefficient,1 and = 1 kg/m used) are presented in the figures along with the full-scale measurements. It can be seen that the model tests gave considerably higher results. The calculation of the ice resistance is presented in Appendix 2. Laivan jäävastus 3 cm jäässä 12 1 Jäävastus [kn] , 5, 1, 15, 2, Nopeus [kn] Täysmittakaavakokeet Kokeellisesti määritetty vastus Lindqvist Riska Figure 18: Ship's ice resistance in 3 cm ice. Blue = full-scale measurements, Pink = model tests, Yellow = Lindqvist, Green = Riska Laivan jäävastus 5 cm jäässä Jäävastus [kn] , 5, 1, 15, 2, Nopeus [kn] Kokeellisesti määritetty vastus Lindqvist Riska Figure 19: Ship's ice resistance in 5 cm ice. Pink = model tests, Yellow = Lindqvist, Green = Riska 5-17

22 Table 11: The percentage of the breaking component from the ice resistance according to the Lindqvist method. The breaking component s percentage from the ice resistance Ice Thickness 2 kn 6 kn 12 kn 16 kn 3 cm 34 % 4 % 44 % 45 % 5 cm 41 % 45 % 48 % 49 % According to model tests the total resistance increases linearly with speed (Figure 16 & Figure 17). The classical ice resistance is determined by reducing the open water resistance from the total resistance. It should be noted that the ice resistance is uncertain at high speeds because of big assumptions related to the superposition principle. The ice resistance also increases linearly with speed but starts to reduce when the speed is over 12 knots. The biggest contributor for the reduction is the submersion component which starts to decrease at high speeds. If the reduction of the submersion component is real, it might mean that less ice is going under the hull. The breaking component is relatively larger of the two and the increase of the ice resistance happens mainly because of the breaking resistance Wave making and other visual observations Recorded videos revealed that the ice sheet reduces wave making. A clear bow wave starts to form when the speed is over 12 knots. The bow wave is bigger in presawn ice than in level ice. The height of the bow wave is presented in Figure 2. It is determined from the videos at the frame No. 9. Results should be evaluated with caution as it was difficult to see exactly where the bow wave was located at higher speeds. Keula-aallon korkeus kaarella 9 12, Aallon korkeus [mm] 1, 8, 6, 4, 2,,,,5 1, 1,5 2, Nopeus [m/s] Tasainen 15 mm jää Esisahattu 15 mm jää Tasainen 25 mm jää Esisahattu 25 mm jää Figure 2: The height of the bow wave for the model measured on frame No. 9. X-axis the speed in m/s and Y-axis the height in mm. Blue = 15 mm level ice, pink = 15 mm presawn ice, yellow = 25 mm level ice, light blue cross = 25 mm presawn ice Waves did not break the ice sheet and widen the channel at higher speeds as they did in fullscale. Even though the ice sheet did not break into pieces, cracks were formed into the ice sheet (Figure 21, Figure 22 and Figure 23). Cracks were longer and there were also more cracks in level ice. This suggests that there is a higher pressure under the ice when the wave making is prevented which could mean that there is also a higher pressure against the hull. This might increase the viscous resistance. 5-18

23 The crack lengths were measured only in 25 mm ice as they were difficult to spot and a proper method was not found at the first week. The distance between cracks in presawn ice was 58 cm, while in level ice it was 56 cm. Ice pieces under the hull affect the viscous resistance but their effect was not studied. Figure 21: Cracks which have formed into the 25 mm level ice sheet. Figure 22: Cracks which have formed into 25mm presawn ice sheet. Figure 23: Distances between the cracks in level ice. Lengths are measured to the middle of the channel. 5-19

24 5.1.4 Feasibility of model tests and semi-empirical calculation methods for high speeds It is difficult to say about the reliability of model tests as results were too high. It can not be said that model tests are unreliable at higher speeds as the results were high already at regular model test speeds. The icebreaking seemed to be quite different in model scale than in full-scale. The ice sheet did not break into pieces around the ship and the icebreaking process seemed to be much smoother. The usage of presawn ice is difficult at higher speeds as the determination and making of the breaking pattern is difficult. Also the different wave making will affect the results from presawn ice. A bigger scale should be used when doing tests in thin ice to reduce errors and uncertainties in model ice. The modelling of crushing is more important as the speed increases. Semi-empirical calculation methods gave relatively good results although a bit high (Figure 18). On the other hand it is questionable to use calculation methods, which are designed for breaking process at waterline, at speeds above 1 knots as the breaking process is different. 6 Conclusions The effect of speed on the ice resistance of a ship was studied experimentally by doing fullscale and model-scale tests. Visual observations made in full-scale and also in model-scale showed that the ice breaking-process changes when the speed is over 1 knots. The bow wave raises ice up changing the contact point and angle between the hull and ice. More crushing occurs. The bow wave seemed to break the ice before any contact with the hull but this was not confirmed in model tests. The model test results were too high for unknown reasons. This makes it hard to discuss about the reliability of model tests and current scaling methods. Verifying model tests should be arranged and after them tasks for the future research should be decided. Even though model test results were peculiar, it was possible to study speed dependencies. The breaking component is linearly dependent of speed along with the total resistance. The classical ice resistance also increases linearly with speed but starts to decrease as the speed is over 12 knots. The ice sheet reduces wave making and possibly increases the pressure against the hull. Semi-empirical calculation methods gave a little too high resistance values when compared to full-scale results and their use should be carefully considered at speeds over 1 knots as the breaking process is different. 6-2

25 7 Model tests performed in august 21 As the results obtained in the Master s Thesis were peculiar, new tests were performed at Aker Arctic Technology in order to verify previous results in a different type of model ice. Both model basins use fine grain model ice, but with different additives: Aalto uses alcohol while AARC uses salt. AARC also has fresh water layers in the ice. The first test was a towing test at 15 mm level ice with speed of 1,84 m/s (16 kn at full scale) and it was performed exactly in a similar way to the one at the Aalto University. The test was filmed from several angles including underwater footage and a high-speed camera footage of the bow. Also an open water test was performed, but this time without the turbulence stimulator. It was thought that the assumption of ice pieces waking the turbulence could be wrong. These tests were done The model was the same as in previous tests but propeller shafts were not plugged. It was considered that the towing of the model could influence the resistance as the movement of the model is forced. Therefore a test with propulsion was performed in order to have a natural movement for the model. The model was equipped with two propellers and rudders. First, the correct rpm to get the desired (1,84 m/s) speed in ice was determined. This was done in open water tests by towing the model at desired speed and adjusting the rpm so that the model produced extra thrust equal to the previously obtained ice resistance. When the correct rpm was found, a test in 15mm level ice was performed. The rpm of the propellers was adjusted correctly before moving the model. When the model was in ice and accelerating, propellers were switched on. These tests were performed and A summary of tests is presented in Table 12. Table 12: A summary of tests performed in august 21. Date Type Speed Ice thickness Measured quantities Towing test, 1,84 m/s 15 mm Resistance, speed level ice Towing test,,23 m/s;,69 m/s; - Resistance, speed open water 1,38 m/s; 1,84 m/s Open water test with propulsion 1,84 m/s - Resistance, speed, rpm Propulsion test, level ice 1,84 m/s 15 mm Speed, rpm 7.1 Results This time the ice resistance was corrected a bit differently than previously: R I f, t arg et.33 R I, measured. 67 R f, measured I, measured h h i, t arg et i, measured X, (12) where constant X was 1,6 according to previous tests with same model (before autumn 29). Also the percentage of breaking component was taken according to previous test data. 7-21

26 R I,measured is the measured ice resistance without any corrections. It is acquired by subtracting open-water resistance from the measured total resistance. Time histories of the measured quantities and the defined extra thrust are presented in Figure 24,Figure 25,Figure 26 and Figure 27. Resistance Resistance [N] Time [s] Figure 24: The measured resistance in 15 mm level ice, towing test Extra thrust produced at 1,84 m/s 12 1 y = x Extra thrust [N] Propeller rotation [1/s^2] Figure 25: Extra thrust versus propeller rpm according the open water tests made at

27 Propeller rpm RPM Time [s] Figure 26: The measured propeller rpm at Velocity Velocity [m/s] Time [s] Figure 27: The measured velocity at The ice resistance in propulsion tests was determined from Figure 25 according to the measured propeller rpm and speed. This is the measured ice resistance R I,measured. Then the corrected ice resistance is calculated with equation (12). The calculation of ice resistances in both cases are presented in Table 13 and Table

28 Table 13: The calculation of the ice resistance in 15 mm level ice, towing test. R IT R OW R I,measured h i,measured f,measured E/ R I 187,61 N 77,64 N 19,97 N 16,5 mm 26,29 kpa ,85 N Table 14: The calculation of the ice resistance in 15 mm level ice with propulsion. Mean speed Mean RPM R I,measured h i,measured f,measured E/ R I 1,84 m/s 85,2 95,99 N 15,5 mm 23,93 kpa ,58 N The obtained resistance values are little lower than previous ones but they are still too high when compared to the full-scale. 7.2 Visual Observations Again, visual observations turned out to be extremely useful when analyzing the breaking process. The propulsion did not change the breaking process nor the cracks formed into the ice sheet as the phenomena seemed to be identical in both tests. Similar cracks as in tests performed at Aalto University were formed onto the ice sheet (Figure 28). The mean value between the cracks was 36 cm in the towing test and 39 cm in the test with propulsion. There was a junction between cracks and it was situated on average 145 cm from the middle of the channel. In propulsion test the junction was impossible to determine due to slushy layer above the near edge. The cracks reached to the sides of the towing tank which are 4 meters from the middle of the channel. Figure 28: Cracks which are formed into the ice. Small holes have been pushed into the ice in order to make cracks more visible. The high-speed camera revealed the breaking process in more detail (Figure 29): first, a crack is formed in front of the bow. Then the ice starts to rise with the wave. The ice is not bend down and submerged, instead it is pushed aside and it sinks near the shoulders. Finally the model 7-24

29 hits the edge of the intact ice where the crack had initially formed and the cycle begins again. The bow hits the ice edge at a constant time interval and the distance travelled between the cracks was approximately 36,8 cm which corresponds well with measured crack intervals. The distance is approximately 7,4 meters when converted to full-scale. Underwater footage revealed that there is a slowly moving, almost still, layer of ice onto the bow and then there are waves of ice moving (from bow to stern) above the layer (Figure 3). The waves appear at constant time intervals which is the same as bow hitting the ice edge. Therefore it is likely that these waves are due to crushing formed in the contact between the bow and intact ice sheet. Figure 29: A crack is formed in front of the bow (1), ice is not pushed down (2), the bow hits the edge of intact ice sheet and pushes the old ice behind. Figure 3: There is a slowly moving layer of ice onto the bow and there are waves of crushed ice (marked with arrows) going over them. Comparison of the model test footage with the full-scale footage revealed some differences. Similar cracks are formed also in full-scale (Figure 31) but their interval is approximately 4 meters (7,4 m according to model tests). Also the bow wave seems to be higher in full-scale (2 m-2,5 m) than in model scale (1,3 meters). The ice sheet breaks/crushes into pieces when in 7-25

30 contact with the hull of the ship but in model scale big ice pieces hit the bow and are pushed to sides without breaking them. Figure 31: Cracks in front of the bow. 7.3 Conclusions and Suggestions for Phase 2 Verifying model tests revealed possible reasons for the high resistance values. The model ice is not brittle enough. Too big ice pieces are broken and the pieces are pushed aside above the ice sheet: the bow is unable to sink them. Also the model ice sheet seems to be preventing the wave making more than in full-scale. This probably changes the pressure field and flow around the hull. In addition, the sheet does not break into pieces around the channel. The stuck layer of ice onto the bow surely raises the resistance. Reasons for the slow movement should be resolved. Possible reasons could be due to high counter pressure in front the bow which pushes the ice pieces firmly against the hull. The prevented wave-making could amplify this effect. The model ice seems to be the biggest contributor to the high ice resistance. Therefore the future research should focus on producing more brittle model ice. This could also affect to the movement of ice pieces around the hull. Also some sort of turbulence stimulators could help to avoid ice pieces to get stuck onto the bow. The test in 6 knots speed should be repeated in order to verify its results and to have underwater footage at smaller speed. Then new, more brittle, model ice should be tried to produce. One way could be to reduce the amount of salt in ice. After this, results and videos are analyzed and future testing decided according to them. Tests are kind of trial and error as there is no previous knowledge of such tests. Results show which phenomena can be ruled out and which need more investigation. Good underwater footage and high-speed video footage are crucial when investigating the high speed ice breaking. 7-26

31 8 Refrences: Arctia Oy. 21 Technical information of IB Kontio (webpage). [ Read Lindqvist, Gustav A Straightforward Method for Calculation of Ice Resistance of Ships POAC 89 Vol. 2, s Riska, Kaj. & Wilhelmsson, Max. & Englund, Kim. & Leiviskä, Topi Performance of Merchant Vessel in the Baltic Winter Navigation Research Board, Research Report No

32 Appendix 1 (1/1) Voima [N] Aika [s] Test 1.1 The mean value of resistance is determined from a time interval of 8,1 31,6 seconds Voima [N] Aika [s] Test 1.2 The mean value of resistance is determined from a time interval of 11,6 17,9 seconds. 8-28

33 Appendix 1 (2/1) Hinausvoima [N] Aika [s] Test 1.3 The mean value of resistance is determined from a time interval of 11,2 18,4 seconds Voima [N] Aika [s] Test 1.4 The mean value of resistance is determined from a time interval of 17,1 23,1 seconds. 8-29

34 Appendix 1 (3/1) Hinausvoima [N] Aika [s] Test 2.1 The mean value of resistance is determined from a time interval of 13,8 43,7 seconds Hinausvoima [N] Aika [s] Test 2.2 The mean value of resistance is determined from a time interval of 9,7 19,9 seconds. 8-3

35 Appendix 1 (4/1) Hinausvoima [N] Aika [s] Test 2.3 The mean value of resistance is determined from a time interval of 15, 2,5 seconds Hinausvoima [N] Aika [s] Test 2.4 The mean value of resistance is determined from a time interval of 11,8 2,6 seconds. 8-31

36 Appendix 1 (5/1) Voima [N] Aika [s] Test 3.1 The mean value of resistance is determined from a time interval of 16,8 52,9 seconds Voima [N] Aika [s] Test 3.2 The mean value of resistance is determined from a time interval of 13,8 24,9 seconds. 8-32

37 Appendix 1 (6/1) Voima [N] Aika [s] Test 3.3 The mean value of resistance is determined from a time interval of 11,6 19,7 seconds Voima [N] Aika [s] Test 3.4 The mean value of resistance is determined from a time interval of 11,4 16,6 seconds. 8-33

38 Appendix 1 (7/1) Voima [N] Aika [s] Test 4.1 The mean value of resistance is determined from a time interval of 22,3 61,3 seconds Voima [N] Aika [s] Test 4.2 The mean value of resistance is determined from a time interval of 14,2 25,6 seconds. 8-34

39 Appendix 1 (8/1) Voima [N] Aika[s] Test 4.3 The mean value of resistance is determined from a time interval of 21,4 26,6 seconds Voima [N] Aika [s] Test 4.4 The mean value of resistance is determined from a time interval of 28,6 32,9 seconds. 8-35

40 Appendix 1 (9/1) Voima [N] Aika [s] Test 5.1 The mean value of resistance is determined from a time interval of 43,6 17, seconds Voima [N] Aika [s] Test 5.2 The mean value of resistance is determined from a time interval of 27, 59, seconds. 8-36

41 Appendix 1 (1/1) Voima [N] Aika [s] Test 5.3 The mean value of resistance is determined from a time interval of 21, 33,7 seconds Voima [N] Aika [s] Test 5.4 The mean value of resistance is determined from a time interval of 13,2 2,7 seconds. 8-37

42 Appendix