POAC 09 Luleå, Sweden Proceedings of the 20th International Conference on Port and Ocean Engineering under Arctic Conditions June 9-12, 2009 Luleå, Sweden POAC09-68 MODEL TESTING OF RIDGE KEEL LOADS ON STRUCTURES PART IV: PRELIMINARY RESULTS OF FREEZE BOND SHEAR STRENGTH EXPERIMENTS Ada H. V. Repetto-Llamazares 1, Peter Jochmann 2, Karl-Ulrich Evers 2 and Knut V. Høyland 1 Norwegian University of Science and Technology (NTNU), Trondheim, NORWAY 2 Hamburg Ship Model Basin (HSVA), Hamburg, GERMANY ABSTRACT A series of experiments intended to study freeze-bond strength in model ice were performed in the Hamburg Ship Model Basin (HSVA) during November 2008. The ice used in the experiments was the same model ice used to build the model ridges described in Part II of the publications. Freeze-bond strength in direct shear was measured as a function of initial ice temperature, available time to develop the freeze-bond and normal stress applied to the freeze bond samples. Freeze-bond strength was also measured in rafted layers from the model ice ridges. A total of 58 experiments divided into four series were performed. The results from the controlled experiments and the freeze-bonds from the model ridges indicate that the former are representative of the actual phenomena happening in the model ridges. A decreasing tendency of the freeze-bond strength versus increasing submersion time was found, which agrees with previous results from full scale tests. A slightly decreasing tendency of freeze-bond strength with decreasing initial ice temperature was found. This does not agree with previous observations. The variation of freezebond strength versus normal stress showed an increasing tendency with increasing normal stress until a normal load of 1200 Pa where the freeze-bond strength decreases. 1. INTRODUCTION Ice ridges may represent the design load for ships, coastal, and offshore structures in many arctic and subarctic marine waters. Different approaches to calculate the ridge loads on structures are being used. The methods can be classified into three groups (Timco et al., 2000): model tests, analytical models and numerical models. No matter which approach is chosen, the confidence on the obtained results is affected by the fact that the ridge-structure interaction process is not well understood. And a great part of the uncertainties have their root in the lack of knowledge about the physical properties of the ridges and their evolution with the thermal and mechanical history of the ridge. Choosing model tests to estimate ridge loads on structures adds the uncertainty of the model ridges physical properties and their evolution with time. The scaling method, which is the mathematical way in which the model scale ridges properties are related with the full scale ridges properties is not well understood either (Høyland, 2007). Several authors argue that the initial failure of ice rubble is reached when freeze bonds between ice blocks fail (Ettema and Urroz, 1989, Surkov and Truskov, 1993, Surkov et al., 2001, Shafrova et al., 2004 and Liferov, 2005). Moreover Liferov and Bonnemaire (2005) discuss that this initial
failure of the rubble may correspond to the peak load. Thereafter, the study of freeze bonds is of vital importance when dealing with ice ridge loads on structures. This paper presents preliminary results from experiments where the strength of artificially created freeze-bonds made from model ice was tested on direct shear. The objective of the experiment was to study the dependence of the freeze bond shear strength on three parameters: the initial temperature of the ice, the normal stress applied to the freeze-bonds while they are being formed and tested, and the submersion time available for the freeze-bond to form. The experiments were performed during November 2008 in the Hamburg Ship Model Basin (HSVA) as part of a set of experiments intended to study the characteristics of the model ridges used for ridge-subsea structure interaction testing (Serré et al., 2009a, Serré et al., 2009b and Repetto-Llamazares et al. 2009). The paper presents preliminary results of the tests. 2. EXPERIMENTAL SET UP The experiments described in this paper were done using the same ice used to build the ridges presented in Repetto-Llamazares et al. (2009), Serré et al. (2009a) and Serré et al. (2009b). The bonded samples were tested in direct shear and the force at which failure occur (F c ) divided by the initial surface area (A) of the ice block was defined as the shear capacity (τ c ) of the freeze bond or the freeze-bond strength. 2.1. Direct Shear Testing Device A direct shear testing device was designed based on the experimental set-up proposed by Ettema and Schaefer (1986). Figure 1 depicts the basic design. It consists of two wooden frames that hold the freeze bond on its top and bottom. The bottom frame is fixed to a wooden table where a hydraulic piston is mounted. The piston pulls at constant velocity the upper frame until the ice specimen fails. Connected to the piston there is a load cell (sensitivity 1 kn/mv/v, accuracy of 0,12 N and a range of 2kN) and a displacement sensor. Both of them are connected to an amplifier, analog filter and AD converter. Data is recorded using DIAdem software as the computer interface. The hydraulic system was built by Friesecke and Hoepfner and allows for a maximum load of 10 kn, maximum displacement of 100 mm and maximum velocity of 100 mm/s. The velocity used during all the experiments was set to 0,7 mm/s, which lies in between the velocities interval used in Ettema and Schaefer (1986) (they found no variation of shear capacity of the ice between velocities in the range of 0.44 mm/s and 0.84 mm/s). (a) Pulling Piston σ Weight (Confinment) (b) Bottom Frame Detail 170 mm Bonded Ice Blocks 150 mm Movable end Fixed Frame Freeze Bond Area (15 ~ 20) mm 100 mm Screw Small rail Figure 1. (a) Basic design of the direct shear testing device. (b) Detail on the movable end of the bottom (fixed) frame.
2.2. Experimental Procedure The experimental procedure is described in Figure 2. Samples with dimensions around 160 mm by 160 mm in plane and with a thickness equal to the ice-sheet ice thickness (h i ) were cut from the ice-sheet using a hand saw, placed in plastic bags and put inside freezers until the ice samples reached the desired initial temperature (T i ). The ice thickness was between 31 mm to 34 mm with few samples of 40 mm. The air temperature of the cold room (T air ) was set to be the similar to T i. When the samples reached T i, they were taken out of the freezers and cut with a band saw to the desired final size of 140 mm by 140 mm (the thickness was left untouched). The freeze bonds were made by putting two ice pieces in contact (in the air), placing a wooden plate on top of them and adding metal weights until reaching the desired confinement (normal stress, σ) (See Figure 3). The wooden plate was intended to redistribute the weight along the totality of the surface area of the upper ice block. The complete set of ice, plate and weight were immediately placed in a secondary basin containing water with similar salinity and temperature (T w ) as in the large ice tank. The water salinity of the secondary basin (S sb ) increased from 7 ppt to 13 ppt during the experiments (Table 1). The dimensions of the tank were 1430 mm long, 940 mm wide and 800 mm deep. It was provided with wooden shelves so that it was easy to submerge the samples and to take them out for testing (Figure 3). After a certain submersion time (Δt) the samples were taken out of the water and tested in the direct shear testing device using the same weight that was used during the submersion. 1 14 cm HSVA model ice Experimental Procedure Freeze-Bond Strength Experiments 5 14 cm σ h i 2 σ 4 T = T i T water Δt 3 Figure 2. Basic description of the experimental procedure used to perform the freeze bond strength experiments. A total of 58 experiments separated in four series were performed. The parameters varied during the experiments were T i, σ and Δt. Table 1 presents their values in each experiment. Series 10100 were performed while leaving T i and Δt constant and varying σ. Three to four samples were prepared with each σ. Series 10200 were done at constant T i and σ while varying Δt, and again, three to four samples were prepared for each Δt in order to be able to obtain average values of shear capacities. While testing, some samples were broken or tested in a failure mode that was not expected, so the number of samples (n) actually used with each σ and each Δt is given in
Table 1. Samples corresponding to experiments 10410 and 10510 were taken from rafted layers of ice found in model ice ridges 3000 and 4000 described in Repetto-Llamazares et al. (2009). Table 1. Parameters used in each experiment: initial temperature of the ice (T i ), air temperature when making and testing the freeze bonds (T air ), normal stress applied while making the freeze-bond and while testing them (σ), number of samples used (n), time of submersion of the samples (Δt), salinity (S sb ) and temperature (T w ) of the water where the samples were submerged Test # T i ( o C ) T air ( o C ) σ (Pa) n Δt (hr) S sb (ppt) T w ( o C) 125 4 10110-7.5-7.5 637 3 10.7-0.8 20 1205 3 2040 3 12-0.83 125 1 10120-14 -12 637 2 10.3-0.79 20 1205 2 2040 2 12.2-0.83 147 3 10130-1.2-1.6 295 3 637 3 20 13.0-0.85 2040 3 3 1 10210-7.5-7.5 637 3 4.5 12-0.83 2 10 2 1 10220-12 -14 637 2 4.5 12.2-0.83 3 10 185 1 10410 365 1 Ridge -1.3-2 3000 305 1 1380 1 20 7.0-0.5 10510 Ridge 4000-1.7-1 205 1 205 1 245 1 187 1 9 7.0-0.5 2.3. Selection of parameters 2.3.1. Normal Stress - Confinement The normal stresses used during the experiments were selected according to the procedure presented in Ettema and Schaefer (1986) and Mellor (1980) to calculate the vertical component of the internal stresses exerted by floating ice rubble at a certain depth inside a rubble field. Ettema and Schaefer (1986) calculated a maximum normal stress of 4000 Pa for a 10 m depth full scale rubble field, which is a good estimation of a full scale ridge keel. For a model scale ridge of
keel depth around 50 cm, the vertical component of the internal stresses would be around 600 Pa. So a range of normal stresses was selected to resemble both full scale and model scale situations. The maximum normal stress was selected to be 2000 Pa instead of 4000 Pa due to practical reasons related with the experimental set-up. The minimum confining stress was selected assuming a rubble field of just two rafted ice blocks. Two intermediate stresses then chosen. The force balance performed to calculate the needed weights to achieve the desired normal stresses is depicted in Figure 3 and expressed in Equation 1. Water Level Wooden Shelf Sample F load F b Weight (Load) F wp Wooden plate F wi N Shelf of basin Figure 3. Scheme of how the ice blocks were loaded during submersion in the secondary basin and the forces involved. Where F load + F wi + F wp F b = N (1) F load = Force exerted by the weight on top of the ice. F wi = Weight of the two pieces of ice. F wp = Weight of the wooden plate. F b = Buoyancy force generated when submerging completely the ice blocks. N = Normal force (reaction of the shelf). The set up was such that the sample was almost completely submerged when using only the wooden plate on top of the freeze-bond. That is why the following approximations presented in Equation 2 and 3 are valid. F wi + F wp F b ~ 0 (2) F load ~ N = σ A (3) In the case of the minimum loads selected, the ice blocks were just placed in the water with the wooden plate on top and no extra weight. Thus in the force balance N = 0, leaving F b as responsible for the normal stress (σ = F b /A). Consequently, the calculation is inverted and by knowing the density of the water and the volume of the ice-blocks the buoyancy force can be calculated and hence the normal stress applied to the ice blocks (it could have been done by other methods, such as weighting each sample and wooden plate before submerging it or using ice density and ice volume). For those samples where only the wooden plate on top the calculation of the buoyancy includes the uncertainty corresponding to the calculation of the sample volume. The uncertainty arises from the method used to estimate ρ i. The ice density was calculated as the
average between the values measured before submerging the samples to create the freeze-bond and after taking it from the freezers. This was done just for three samples, assuming a similar value for all the experiments. Nevertheless, the interest of these experiments is to have a first approach on how the freeze-bond strength varies with normal stress, so the uncertainty is not relevant. The normal stress and the surface area used for series 10400 and 10500 during submersion was not controlled since the blocks were extracted from the ridges after the model tests had been performed. After the blocks were taken out of the water, they were cut squared-shaped with side dimension depending on each sample original size, varying between 95 mm to 140 mm. The weight applied to the freeze-bond during testing was recorded and afterwards the normal stress applied during testing was calculated. This is the load reported in Table 1. 2.3.2. Temperatures Two extreme temperatures were selected to have as much separation between the measured points as possible. The warmer temperature was selected to be close to the ice temperature used to build the model ridges (T i = -1.6 o C) and the coldest one was set at -14 o C which was easily reached and maintained by the refrigeration system of the cold room. An intermediate temperature was then selected to be able to evaluate better any possible trend in the data. The initial temperature of the ice used in the experiments #10410 and #10510 is approximately known and is taken as the ice-sheet temperature measurement at 20 mm depth before the ridge was built (See Repetto-Llamazares et al. (2009) for more details). It is reported in Table 1. 2.3.3. Submersion Time The scaling of time in a small scale freeze bond experiment is not completely understood (Shafrova and Høyland, 2008). Thus the approach was to perform preliminary tests to asses the shortest submersion time at which the freeze-bond was strong enough to take it out of the water and handle it to test in the direct shear device. It was found that for ice at T i = -14 o C, freezebonding was present after 1 min of submersion. Unfortunately it was not possible to measure its strength with our experimental set-up. The movable end of the bottom frame was not pressing hard enough so the two bonded ice blocks were tilted by the piston moment applied in the upper frame instead of failing the freeze bond (See Figure 4). A submersion time of 5 minutes was also tested, but the ice remained to be too strong. After that, it was decided to set the smallest Δt using warmer ice. When using ice of T i = -3 o C, freeze bonding was not observed until Δt = 1 hr. Consequently this was the minimum submersion time selected for the experiments (Table 1). The longest time was selected to be similar to the life-time of the model ridges (time between ridge building and ridge-structure interaction test, Repetto-Llamazares et al., 2009). The submersion time of experiments #10410 and #10510 is calculated as the interval of time between ridge building (See Repetto-Llamazares et al. (2009) for more detail) and freeze-bond testing. It is reported in Table 1.
Figure 4.Two bonded ice blocks tilted by the piston moment applied in the upper frame instead of failing in the freeze-bond. 3. RESULTS Figure 5 shows two representative plots of the curves obtained when testing the freeze-bonds in the direct shear device. Two distinct failures mode have been observed, referenced as Failure mode A (Figure 5 (a)) and B (Figure 5 (b)). Approximately 25 % of the samples failed in mode A Figure 5 (a) while the remaining 75 % failed in mode B (Figure 5 (b)). (a) 120 Test 10120 - = 1205 Pa - Sample #2 F C 100 (b) 28 Test 10130 - = 637 Pa - Sample #1 F C 25 20 Force (N) 100 80 60 Force (N) 50 0 Force (N) 24 20 16 12 Force (N) 15 10 5 224 225 226 227 228 229 230 231 tim e (s) 40 446 448 450 time (s) 8 20 4 0 450 475 500 525 time (s) 0 220 240 260 280 300 320 time (s) Figure 5. Representative plots of the curves obtained when testing the freeze-bonds in the direct shear device. (a) Failure mode A. (b) Failure mode B. The freeze-bond shear capacity (τ c ) was obtained from the curves by selecting the maximum value of the force before failure (F c as shown in Figure 5) and dividing it by the surface area of the ice-blocks. A total of 58 curves similar to the ones presented in Figure 5 have been analyzed to obtain the shear capacity of the freeze bonds for each experiment. Figure 6 summarizes the results of series #10100 and #10200 by a plot of τ c vs. Δt (Figure 6 (a)) and τ c vs. σ (Figure 6 (b)). Each point was calculated as an average of the n measurements available and the error bar corresponds to two standard deviations.
(a) (b) 4500 4000 Submersion time variation Avg 10210 T ~ -7 C Avg 10220 T ~ -14 C 12000 10000 Normal Stress Variation Avg 10130 T ~ -1,2 C Avg 10 110 T ~ -7 C 3500 Avg 10120 T ~ -14 C 3000 8000 c (Pa) 2500 2000 1500 c (Pa) 6000 4000 1000 2000 500 0 2 4 6 8 10 12 14 16 18 20 22 t (hr) 0 500 1000 1500 2000 (Pa) Figure 6. Summary of the obtained results: shear capacity of the freeze-bonds as function of the (a) submersion time (b) normal stress. The results obtained for series #10400 and #10500 are showed in Figure 7 (Figure 7 (a) is also presented in Part II of the publications (Repetto Llamazares et al., 2009) (a) 2600 2400 2200 (b) 2600 2400 2200 19 Pa 141 Pa c (Pa) 2000 1800 1600 1400 1200 Ridge 3000 c avg = (192 +/- 52) Pa Ridge 4000 c avg = (148 +/- 56) Pa 1000 0 500 1000 1500 2000 25000 (Pa) c (Pa) 2000 1800 1600 1400 1200 25 Pa 21 Pa 21 Pa Ridge 3000 Ridge 4000 31 Pa 19 Pa 37 Pa 1000 8 9 10 11 12 13 14 15 16 17 18 19 20 21 t (hr) Figure 7. Shear capacity from experiments #10410 and #10510 as function of (a) normal stress (b) submersion time (the number on the side of each point correspond to the normal stress applied while the freeze bond was being tested).
4. DISCUSSION 4.1 Data Trends Failure Mode A (Figure 5 (a)) was associated with higher shear capacities than the ones registered for ductile-like behavior. At the moment, the character of the curve at failure does not seem to be related with any particular parameter of the experiments. The failure mode of the samples appeared to be random. Figure 6 (a) shows a slightly decreasing tendency of the shear capacity of the freeze-bonds when Δt increases. Shafrova and Høyland (2008) suggest that the freeze-bond strength increases with the submersion time until a certain Δt critical where it starts to decrease. It seems that the Δt used in the experiments were long enough to be in the last part of the curve, where the variation of the strength of the freeze-bonds is small and of decreasing tendency. Both Figures 6 (a) and (b) show a tendency to have weaker freeze-bonds when the initial temperature of the ice decreases. This is opposite of what was expected but it can be due to the several experimental uncertainties discussed below (section 4.1). Colder samples will develop a shear capacity faster and can be more sensitive to the submersion time. Figure 6 (b) shows a slightly increase of τ c for the two smaller normal stresses used (147 Pa and 637 Pa) while it increases faster for 1205 Pa, decreasing again for 2040 Pa. This change in behavior with normal stress (from increasing to decreasing) was not expected. It is thought to be related with the small number of samples tested. It appears to be certain randomness in weather the sample fails in mode A or B (the former having associated higher freeze-bond shear capacities). When testing the freeze-bonds made using normal stresses of 1205 Pa, most of the specimens failed in mode A. Thus the average value for their shear capacity was higher. Testing more samples under each normal stress would help to understand this aspect better. A detailed discussion of Figure 7 (a) is presented in Repetto-Llamazares et al. (2009). The analysis in this paper is restricted to show that the values obtained from the controlled experiments and the values obtained from rafted layers in model ice ridges are consistent. This gives certain insight into the validity of the controlled experiments to represent the freeze-bonds present in model ridges. Figure 7 (b) shows an increasing tendency of the shear capacity of the freeze-bonds with increasing Δt. This is opposite to what was found in the controlled experiments. Most likely, the discrepancy is due to the sampling process from the model ridges: only rafted layers were sampled since they were the only pieces with freeze-bonds of sizes big enough to be tested in the direct shear device. Thus, there was not randomness in the sampling process. 4.1 Experimental Uncertainties There are several parameters that are expected to influence the shear capacity of a freeze-bond. Besides the ones used as control parameters during these experiments (initial temperature of the ice, normal stress and submersion time), it can be mentioned for example, the real contact area between ice blocks (which will be affected by the surface roughness) and ice properties (ice density, salinity, porosity, thickness, etc). The selection of the control parameters of our experiments was done by identifying the parameters we expected to be the ones dominating the freeze-bonds strength, based in previous publications (Ettema and Schaefer (1986) and Shafrova and Høyland, 2008) and available experimental conditions. From the large scatter in the data, we
realize that some of the other parameters must have a considerable effect on the freeze-bond strength. Related with these parameters, some observations were made while testing: The ice surface was uneven, varying greatly between samples. There were samples with higher air content than the other ones. Top and bottom of the ice sheet was used randomly when samples were put together to form the freeze bond. Measurements of density and salinity were only done for very few representative samples. So the density, salinity and air content of each sample is not known. The ice thickness of the block used to prepare the freeze-bonds varied between 30 and 40 mm We think that the dominating effect generating the spread in the data is probably the one coming from the quality of the initial contact surface of the ice blocks. On the other hand the experimental set-up was not optimal. The system used to fix the movable end of the bottom frame was not the optimum one, so there were occasions in which the complete freeze-bond sample was tilted due to the moment applied by the piston and the lack of good fixing of the bottom frame (Figure 4). In some situations we were able to stop the experiment, press the bottom frame harder into the ice and test it again. But in other cases, the freeze-bond failed before the piston could be stopped. In the later case, the ice failed in a different mode than shear. If the tilting was small, the values of shear capacity were close to the properly measured ones, and in those cases the measured values were averaged with the others. When the tilting was large, the difference of the shear capacity was up to 300% different from the properly measured values. Thus those measurements were not used to calculate the average shear capacity. For future experiments, improvements on the experimental method should include either a better control over the initial contact area of the ice-blocks or a higher repetition of the experiments under the same set of control parameters to get a better estimation of the probability distribution of the freeze-bond shear capacity. Regarding the experimental set up, it is important to have a better fixing system for the bottom frame of the direct shear device to be sure that high shear capacities are not being left out due to an experimental limitation in their measurements. It is also of interest to measure the shear capacity of the intact ice to be able to compare with the freezebond shear capacity. 5. CONCLUSIONS Freeze-bond strength tests have been performed in the Hamburg Ship Model Basin during November 2008 using a direct shear device design especially for this purpose. A total of 4 series of experiments were done while varying initial temperature of the ice, normal stress and submersion time of the bonded ice blocks. Two of the series were done by measuring freezebonds from rafted layers of two model ice ridges. The results from the controlled experiments and the actual freeze-bonds present in the model ice ridges indicate that the controlled experiments are representative of the phenomena occurring in the ridges. A decreasing tendency of the freeze-bond strength versus increasing submersion time was found, which agrees with previous results from full scale tests. A slightly decreasing tendency of the freeze-bond strength with decreasing initial ice temperature was found. The
variation of freeze-bond strength versus normal stress showed an increasing tendency with increasing normal stress until a confinement of 1205 Pa where the freeze-bond strength decreases. The reason for this behavior is not clear and has to be investigated in more detail. The results were a good starting point in the study of the freeze bond shear capacity of ice as function of key parameters. Some drawbacks of the experimental set up were identified and possible solutions have been proposed. ACKNOWLEDGMENTS The work described in this publication was supported by the European Community's Sixth Framework Programme through the grant to the budget of the Integrated Infrastructure Initiative HYDRALAB III, Contract no. 022441(RII3). The author(s) would like to thank the Hamburg Ship Model Basin (HSVA), especially the ice tank crew, for the hospitality, technical and scientific support and the professional execution of the test programme in the Research Infrastructure ARCTECLAB The authors furthermore gratefully acknowledge the PETROMAKS programme, the Research Council of Norway, PetroArctic and StatoilHydro for funding this study and thanks to Hanne Hagen, Andrea Haase, Christian Lønøy and Vegard Asknes for their help during the model tests. REFERENCES Ettema and Schaefer (1986) Experiments on freeze-bonding between ice blocks in floating ice rubble. Journal of Glaciology, Vol. 32, No. 112, pp. 397-403. Ettema, R. and Urroz, G.E. (1989). On internal friction and cohesion in unconsolidated ice rubble. Cold Regions Science and Technology, No. 16: 237 247. Liferov, P. (2005). Review of ice rubble behaviour and strength, Part II: Modelling. Cold Regions Science and Technology, (41): 153 163. Liferov, P. and Bonnemaire, B., (2005). Review of ice rubble behaviour and strength, Part I: Testing and interpretation of the results. Cold Regions Science and Technology, No. 41: 135 151. Mellor M. (1980). Ship resistance in thick brash ice. Cold Regions Science and Technology, Vol. 3, No. 4, pp. 305-321. Repetto-Llamazares A. H. V., Høyland K. V., Serré N., Evers K-U and Jochman P, 2009. Model testing of ice ridge loads on structures part ii: ridge building and physical properties. Submitted to Port and Ocean Engineering under Arctic Conditions (POAC) 2009, Luleå, Sweden. Serré N., Pavel L. and Evers K-U. 2009a. Model testing of ridge keel loads on structures part I: Test set up and main results, submitted to Port and Ocean Engineering under Arctic Conditions (POAC) 2009, Luleå, Sweden. Serré N., Pavel L. and Jochmann P. 2009b. Model testing of ridge keel loads on structures part III: Investigation of model ice rubble mechanical properties., submitted to Port and Ocean Engineering under Arctic Conditions (POAC) 2009, Luleå, Sweden. Shafrova, S., Liferov, P., and Shkhinek, K. N. (2004). Modelling ice rubble with a pseudodiscrete continuum model. In Proc. of the 17th Int. Symp. on Ice (IAHR), Saint-Petersburg, Russia, pp. 265-273.
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