MEDIUM SCALE MODELLING OF ICE RIDGE SCOURING OF THE SEABED, PART II: CONSOLIDATION AND PHYSICAL PROPERTIES

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 2002 International Association of Hydraulic Engineering and Research MEDIUM SCALE MODELLING OF ICE RIDGE SCOURING OF THE SEABED, PART II: CONSOLIDATION AND PHYSICAL PROPERTIES Knut V. Høyland 1, Pavel Liferov 1,2,3, Per Olav Moslet 1, Sveinung Løset 1,3 and Basile Bonnemaire 3 ABSTRACT The ridge described in Part I had a keel depth of 1.1 m and was monitored with thermistor strings for 4 weeks. The thickness of the consolidated layer was 0.2 m 19 hours after the formation and it finally became (h c, f ) m. The oceanic heat flux during this initial phase was estimated to be roughly 2.2 kw/m 2. Different tests were done on ice from the consolidated layer, and the uniaxial compressive strength was MPa, the salinity was between 2 and 5.5 ppt (similar to level ice in the area) and the density was between 886 and 956 kg/m 3 (level ice spanned from 783 to 906 kg/m 3 ). INTRODUCTION Sea ice ridges are formed by compression or shear in the ice cover. In many Arctic and sub-arctic areas ice ridges give the design forces for marine structures. However, the forces and deformations mechanisms involved in an ice ridge-structure interaction are not clear. When estimating the forces from first-year ridges on structures one needs the thickness of the consolidated layer (h c ) and some mechanical properties as input (Blanchet, 1998). One may divide the lifetime of first-year ice ridges into two parts: the consolidation part and the deterioration part. We are in the following only concerned with the growth (the consolidation) of the ridge. The consolidation can be separated into two phases; an initial phase and a main phase. At the end of the initial phase a consolidated layer (h c ) can be defined and the temperature in the ice blocks below this layer is at the freezing point of the seawater. In other words the negative energy of the cold ice blocks has been transported into the surrounding water and is spent on freezing new ice (consolidation of the ridge). The duration of the initial phase and the amount of new ice that is created during this phase (the initial thickness of the consolidated layer, h c, 0 ) is not clear. The basic energy 1 The University Courses on Svalbard, Norway 2 Barlindhaug Consult a/s, Tromsø, Norway 3 The Norwegian University of Science and Technology, Trondheim, Norway

2 balance (the first law of thermodynamics) in a first-year ice can be expressed as follows: E sur E ocean = E lat E init (1) where E sur is the energy that is transported upwards into the surrounding air, E ocean is the energy that comes from the surrounding water, E init is the potential energy stored in the ice blocks and E lat is the energy released to freeze new ice. The left-hand side is the net energy transfer into/out of the ridge and this has to be balanced by a change in the internal energy of the ridge (right-hand side). We assume that the change in internal energy is expressed either as a temperature change (E init ) or a phase change (E lat ). The energy loss from the surface can be calculated with the help of meteorological data as done in simulations of level ice growth (see e.g. Maykut and Understeiner, 1971), and during the main phase this flux can be balanced with the latent flux (the growth of the consolidated layer) in a numerical conductive simulation (Høyland, 2002a). However, in the initial phase this becomes more complicated, convective heat transfer dominates as the heat transfer between the ocean (oceanic flux, q ocean ) and the ridge is vital. The oceanic flux is a function of the hydrodynamic conditions, the shape and the permeability of the ridge, the temperature, the size and the porosity of the ice blocks from which the ridge is made. These factors determine how much of E init that can be used to freeze new ice. There is also heat exchange between the ridge keel and the ocean during the main phase, though it does not influence the growth of the consolidated layer significantly. However, it seems to be of importance for the consistency (i.e. the mechanical properties) of the unconsolidated rubble. Comparison of ridges in the Baltic and in the Van Mijen fjord showed considerable difference in rubble consistency (Høyland, 2002b). SITE AND EXPERIMENTAL SET-UP The ridge was produced 13 March and monitored until 9 April 2002 as described in Part I (Liferov et al., 2002). The salinity of the seawater was about 34 ppt. The geometry and the consolidation was examined by mechanical drilling (2" drillbit) and the temperatures were monitored by installing thermistor strings. Two strings were installed in the ridge after the production 13 March and logged the temperatures in the ridge until 9 April. Dataloggers were attached and the logging frequency was every five minute from 13 March until 14 March 20.00, after that every hour. The strings are produced by EBA Engineering ( and the dataloggers by Lakewood Systems Ltd. ( The strings had 16 sensors with a vertical spacing of 20 cm. Samples were taken for compression and salinity tests 17 April and for salinity and density examinations 4 April. The compression tests were done on a closed loop testing machine, the strain rate was ɛ = 10 3 s 1 and the temperature was T = 16 C. The samples were cylindrical, oriented vertically in the ridge and the length varied between 165 and 190 mm, and the diameter was 70 mm. RESULTS AND ANALYSIS Geometry and consolidation The surface of the ridge was 3.7 m times 3.7 m and the keel depth was 1.1 m of which about 0.4 m was unconsolidated rubble at the time of testing. The sail was 0.2 m, and this gives a keel depth to sail height ratio of 5.5 which is a bit above the average for

3 first-year ridges found by Timco and Burden (1997). The ice blocks were initially about 0.15 m thick. The consistency of the rubble was soft, probably eroded by the currents in the fjord, and it was not possible to determine the porosity by mechanical drilling. The current velocity some hundred meters further out the fjord was about 2-4 cm/sek. The Freezing Degree Days (FDD) is a simple way of expressing the energy that the cold air pulls out of the ice (Eq. 2). The FDD was 20 C days for the initial phase and 290 C days during the main phase. FDD = (T f T air ) t (2) Fig. 1 gives the temporal development of the thickness of the consolidated layer (h c ). The temperature data shows that the ridge did not consolidate further after 3 April (the second time we did mechanical drillings) hcons (m) String 1 String 2 Drilling Time (days from ) Figure 1: The thickness of the consolidated layer given by temperature measurements and drillings The figure shows that the initial phase lasted roughly one day, and that h c, 0 reached at least 0.2 m. A rough estimate for the oceanic flux during the initial consolidation can be done by applying Eq. 1 and calculate the different terms as given below. The energy loss through the surface is estimated by the use of the measured temperature gradients in the consolidated layer (averaged every hour) and Fourier s law: E sur = k T h t (3) where k is the thermal conductivity of the sea ice, T the temperature, h the vertical distance and t the time. The initial energy content of the ice blocks is found from: E init = ρ c (T init T f ) h s+k (4)

4 where ρ is the density, c the specific heat capacity calculated after Schwerdtfeger (1963), T init is the initial temperature of the ice blocks, T f is the freezing point and h s+k the total (sail + keel) thickness of the ridge. The latent heat released during the initial phase: E lat = ρ l pi η t h c (5) where l pi is the latent heat of pure ice, η t the total porosity of the ridge and h c the growth of the consolidated layer. The macro porosity is the ratio of the volume of non-sea ice material to the total volume. However, a total porosity may be defined by including the porosity of the sea ice (brine and air). The total porosity in the following is defined as the volume of liquid divided by the total volume. See Høyland (2002a,b) for a discussion of different ridge porosities. We estimated the total porosity by finding E sur for the main phase as showed above and assuming that all this energy originated from released latent heat (Eq. 5). The total porosity then became 0.40 in three out of four estimates. This corresponds to a macro porosity of 0.34, which is a reasonable value, see e.g. Leppäranta et al., (1995) and Veitch et al., (1991). In the last case the porosity was higher (0.64/0.54) so this part of the ridge probably contained a pore. With the values given in Table 1, Eq. 6 gives an oceanic flux of 2.2 kw/m 2 in average during the first 19 hours of consolidation. Simulations on a Baltic ridge (Høyland, 2002a) indicates that the oceanic flux was 0.5 kw/m 2 if the duration of the initial phase was one week. Both of the values are rough estimates but they clearly indicate that the oceanic heat loss in connection with ridging is substantially more than up through level ice (often estimated in the range 1 10 W/m 2 ). Table 1: The values used to estimate q ocean k T/ h t ρ c T init h s+k l η t (W/m 2 ) (s) (kg/m 3 ) (kj/ C kg) ( C) (m) (kj/kg) (-) q ocean = (E init + E sur E lat ) t (6) Salinity, density and compressive strength The salinity and density results are shown in Fig. 2. The salinity is similar to earlier data on salinity of level ice and ridges in the Van Mijen fjord, the brine seems to be drained efficiently from the consolidated layer so the salinity becomes similar to that of level ice (Høyland, 2002b). The compression tests are shown in Figs. 3 and 4. The results are comparable to what was found by Høyland et al. (2000): MPa, Veitch et al. (1991): MPa, and

5 Figure 2: The salinity and density of the consolidated layer, note that the unit of the density is 10 times kg/l Figure 3: The compression tests on the consolidated layer, the samples were taken from the following depths (cm): 1(0-25), 2 (25-45), 3 (45-70) and 4(70-88). Frederking and Wright (1982): MPa. The tests performed by Frederking and Wright were not done with a closed loop testing machine and the results vary a lot more. However, the amount of available data on mechanical and physical parameters of first-year ice ridges is limited, and it is not clear what numbers could be used for e.g. compressive strength in a ridge-structure interaction model. A key point is to examine spacial distribution of the parameters, Høyland et al. (2000) indicates that the consolidated is a less homogenous material than level ice. That is, it may contain more weak points and thus

6 7 Compressive strength (MPa) Strength-depth Strength-salinity Salinity/depth (ppt/dm) Figure 4: The strength vs depth and salinity fail easier. CONCLUSIONS An artificial ridge was produced in the Van Mijen fjord on Svalbard the spring 2002, the ridge was examined with respect to consolidation and physical/mechanical properties, the main conclusions are: The ridge had a sail of 0.2 m and a keel depth of about 1.1 m The consolidated layer reached a thickness of m after 4 weeks of consolidation. The initial phase was estimated to last for about one day and the initial thickness of the consolidated layer h c, 0 became 0.2 m. The oceanic flux during the first 19 hours was estimated to 2.2 kw/m 2. The compressive strength of the consolidated layer was between 2.9 and 5.9 MPa. The salinity of the consolidated layer ranged from 2 to 5.5 ppt The density of the consolidated layer and level ice was kg/m 3 and 783 to 906 kg/m 3, respectively. ACKNOWLEDGEMENT We would like to thank the Arctic Technology students at UNIS the spring of 2002, especially Rüdiger Biedorf whose assistance during the experiments was vital, Jean Sebastian L Heureux who did the compression tests and Elena Rudakova who did the density measurements and helped us with the temperature data. We would also like to thank the

7 mining company on Svalbard; Store Norske Spitsbergen Kulkompani (SNSK) who let us eat and sleep in Svea and helped us with logistics. We could not have performed these experiments without their assistance. REFERENCES Blanchet, D. Ice loads from first-year ice ridges and rubble fields. Can. Journal of Civil Engineering 25: (1998). Frederking, R.M.W. and Wright, B. Characteristics and Stability of an Ice-Rubble Field Issungnak, February-March NRC Technical Memorandum, No. 134, NRC, Ottawa, Canada (1982) Høyland, K.V. Simulations of the consolidation process in first-year ice ridges. Journal of Cold Regions Science and Technology 34(3): (2002a). Høyland, K.V. The consolidation of first-year ice ridges. Journal of Geophysical Research (in press 2002b). Høyland, K.V., Kjestveit G., Heinonen, J and Määttänen, M. LOLEIF ridge experiments at Marjaniemi; The size and strength of the consolidated layer. In: Proc. of the 15th Int. Symp. on Ice (IAHR), Vol. 1, Poland (2000) Leppäranta, M., Lensu, M., Koslof, P. and Veitch, B. The life story of a first-year sea ice ridge. Cold Regions Science and Technology 23: (1995). Liferov, P., Løset, S., Moslet, P.O., Bonnemaire, B. and Høyland, K.V. Medium scale modelling of ice ridge scouring of the seabed, Part I: Experimental set-up and basic results. In Ice in the Environment: Proc. of the 16 Int. Symp. on Ice (IAHR), New Zealand (2002) submitted. Maykut, G.A. and Understerner, N. Some results from a time-dependent thermodynamical model of sea ice. Journal of Geophysical Research 76(6): (1971). Schwerdtfeger, P. The thermal properties of sea ice. Journal of Geophysical Research 4: (1963). Timco, G.W. and Burden, R.P. An analysis of the shapes of sea ice ridges. Cold Regions Science and Technology 25: (1997). Veitch, B., Lensu, M., Riska, K., Kosloff, P., Keiley P. and Kujala, P., Field observations of ridges in the northern Baltic Sea. In: Proc. of the 11th Int. Conf. on Port and Ocean Eng. Arc. Cond. (POAC), Canada (1991)