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Sea ice properties on the USCGC Healy ice trials
Jones, S. J.; Kirby, C.; Meadus, C.; Tucker, W.; Gagnon, J.; Elder, B.
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SEA ICE PROPERTIES ON THE USCGC HEALY ICE TRIALS
Stephen J. Jones, Craig Kirby and Chris Meadus
National Research Council of Canada, Institute for Marine Dynamics, P.O. Box 12093, Stn. A, St. John’s, Newfoundland, Canada A1B 3T5
Walter Tucker, John Gagnon and Bruce Elder
US Army Cold Regions Research and Engineering Laboratory, Hanover, NH 03755, USA
ABSTRACT
During the USCGC Healy ice trials in Baffin Bay in April/May 2000, the sea ice thickness and strength, as well as the snow thickness, were measured. The thickness of the ice was measured by augering, by an electromagnetic (EM) method, from cores, and with an Over-The-Side-Video (OTSV). Good agreement was found from these methods in the first year ice, but less good for the two cores of multi-year ice. The snow thickness was measured by point survey measurements and from the OTSV. The OTSV method gave snow thicknesses consistently greater than the point measurements. These different methods are described and the results obtained are discussed. Sea ice strength was determined by drilling cores either by hand or with the RapidCore equipment, measuring temperature, salinity and density, and then calculating flexural strength. The mean strength of the first year ice was 315 kPa (46 psi) and the mean strength of two multi-year cores was 714 kPa (104 psi).
INTRODUCTION
Ice trials on the new USCGC Healy were conducted in Baffin Bay in April/May 2000. This paper describes the ice property measurements carried out by NRC’s Institute for Marine Dynamics (IMD), and the US Army’s Cold Regions Research and Engineering Laboratory
POAC ‘01
Ottawa, Canada
Proceedings of the 16 International Conference onth Port and Ocean Engineering under Arctic Conditions POAC’01 August 12-17, 2001 Ottawa, Ontario, Canada
(CRREL). These measurements were essential to determine whether the new ship met its icebreaking requirements of “continuous icebreaking at 3 knots through 4.5 ft (1.37 m) of ice of 100 psi (690 kPa) strength”.
APPARATUS AND METHOD
The Over-The-Side-Video (OTSV) was a downward looking video camera attached to the side of the ship that was used to observe pieces of ice upturned on edge by the ship. From the video screen, which had been calibrated, the thickness of the ice was measured. Time,
latitude and longitude were included on each frame to determine the location of each
measurement.For many of the individual ship tests, ice thicknesses were documented in detail by drilling and by using an electromagnetic (EM) induction technique. Two teams would measure thickness proceeding backwards along the ship track after the test(s) were completed. One team used a power-driven 2 inch auger to drill and measure thickness at 50 to 100 m intervals. Snow depth was recorded at each drill hole using a ski pole with attached measuring tape. A second team followed the first, towing a kayak containing a Geonics Ltd. EM-31 electromagnetic induction instrument. This team made EM-31 measurements and snow depth readings at 5 m intervals. The EM-31 instrument measures the strength of a secondary
magnetic field beneath the ice bottom induced by a primary field generated by the instrument (Haas et al., 1997; Kovacs and Morey, 1991). The apparent conductivity measured by the instrument is a function of the distance from the antenna of the instrument to the conductive seawater, which is the ice and snow thickness corrected for antenna height. The relationship between apparent conductivity and distance was developed by fitting coincident drill hole and apparent conductivity measurements:
872 . 0 / ) 7 . 54 ln( 11 . 8 z ) 1 ( i = − σi −
where zi is the distance from the antenna to the seawater, and σi is the measured apparent
conductivity. The relationship is very similar to that presented by Haas et al. (1997). The RapidCore was an apparatus that allowed an ice core to be retrieved without anybody going onto the sea ice. It was lifted by crane from the deck of the ship onto the ice, and it then drilled a core up to 1.5 m thick, controlled from the ship. After drilling, the RapidCore was crane lifted back onto the deck, the core was retrieved and measurements of core temperature, density, and salinity were made. From these measurements, the ice flexural strength was determined for correlation with the full-scale tests. On some occasions, personnel went onto the ice and retrieved cores by manual drilling.
RESULTS
Ice and Snow Thickness Results
The OTSV provided 161 ice thickness measurements from 26 ship performance tests. These measurements have been compared with values obtained from the ice cores. Fig. 1 shows a comparison for ice thickness, and Fig. 2 for snow depth. The ice thickness values from cores and OTSV measurements are in good agreement for first year ice, as shown in Fig. 1. The
test L_000509_1715, a backing and ramming test in multi-year ice, is the only case in which the ice thickness is significantly different. This is probably because there was only four OTSV measurements, and just one core thickness, in a multi-year sheet that could have had a significant variation in thickness. It was also found that the effectiveness of the OTSV was limited to level ice runs when broken ice pieces consistently turned up on edge. During backing and ramming operations, and in areas of deep snow cover, the OTSV did not give sufficient measurements opportunities. Multi-year ice does not turn on its side as readily as first-year ice, mainly because of its greater thickness, and this may be another reason the OTSV is less useful in multi-year ice.
The snow depth data are not in such good agreement, as shown in Fig. 2. Typically the snow depth measured by the OTSV was greater than the probe depth. Of 16 comparable tests, 13 of the OTSV measurements are greater than the probe depth, of which six are greater by at least 5 cm. The reason for this disagreement is not clear. Perhaps the probe is stopped by a hard layer of snow that is not seen on the video records. Perhaps the video scale calibration was not correct, but if so the ice depths would not be in such good agreement with the cores as seen in Fig. 1.
EM measurements
Fig. 3 shows the EM thickness measurements and the drilled hole ice thickness for Test 0419. In the figure shown, the ice is of uniform thickness of approximately 80 cm between 300 and 1100 m from the end of the test, and then changes abruptly to 50 cm from 1300-1700 m from the end of test. The drilled holes are in good agreement with the EM measurements. The snow depths are also in good agreement with the snow depth measured at the drill holes. The data for two ship performance tests were tabulated as shown for example in Table 1.
Ice Core Measurements
The chief object of the ice core measurements was to obtain ice flexural strength values. Whether the RapidCore equipment, shown in Fig 4, was used or cores were obtained by hand drilling, the measurement procedure was the same.
As soon as possible after retrieving the core, holes were drilled into its centre every 10 cm to measure temperature. The eventual strength obtained from the calculations is very dependent on this temperature, so it is essential to do the measurement as soon as possible and to use a reliable, calibrated, thermometer. To this end, all the thermistors used on the USCGC Healy for the temperature measurements were checked for zero error by immersing them in a well-stirred, freshwater ice, bath. The zero error of each was then marked on the thermistor, and one instrument was discarded because of an excessive zero error of +0.9oC. After the temperature measurement, the cores were taken into one of the Healy’s cold rooms
maintained at –10oC, where they were cut into 10 cm lengths and each piece was measured for length, diameter, and then weighed on an electronic balance. This allowed the density of each 10 cm length to be determined. Density was also measured by immersion of a sample in a beaker of freshwater and measuring the force needed to just submerge the sample. The pieces were then placed in clean plastic containers and allowed to melt. The following day,
usually, the salinity of the meltwater was measured at a temperature of about 20oC, using a YSI Model 30/50 FT, Salinity, Conductivity, and Temperature meter. This instrument had been previously calibrated at CRREL. From these measurements of salinity, temperature and density, the brine volume was calculated from the equations of Cox and Weeks (1983). Figure 5 shows the results obtained on core 39 for salinity, density and temperature. These are typical of the results obtained on the first year sea ice cores. The only exception were the two multi-year ice cores obtained, and Fig. 6 shows the results for one of those, core 37. Note the much reduced salinity in the multi-year ice and the more uniform density measurements. The flexural strength was then calculated using the equation (Timco and O’Brien, 1994):-
) 88 . 5 exp( 76 . 1 ) 2 ( σf = − νb
where σ is the flexural strength in MPa, and f ν is the brine volume expressed as a fraction. b Table 2 shows the flexural strength results obtained for all cores. With the exception of the two cores obtained on the 9th May, all were first year sea ice and their mean strength was 315 kPa or 46 psi. The two multi-year cores, 36 and 37, had a mean strength of 715 kPa or 104 psi.
Density Measurements
As mentioned above two methods of density measurements were tried. The first, and the one used for the above calculation of strength, was simply weight divided by volume calculated from length and diameter measurements. The greatest error in this method arises from the diameter measurement as the core sample is rarely a perfect cylinder, and no lathe was available to make it uniform. The second was to weigh the sample in air and then immerse it in a beaker of cold fresh water and measure the force needed to just immerse the sample. This method avoids measuring the volume directly because the density is calculated from the equation: i i w i M F M + × =ρ ρ ) 3 (
where ρ is the density of the ice, i ρ is the density of the cold fresh water in the beaker w assumed to be 1.0 Mg/m3, M is the mass of the ice in air, and i F is the additional force needed to just submerge the ice sample. F was measured by floating the ice in a beaker of cold fresh water placed on an electronic balance, taring the balance reading to zero, pressing down on the ice to just submerge it and noting the force required to do so, in kg.
The chief disadvantages of the first method are that it allows brine to drain out of a sample before the weight is known, and requires an accurate diameter measurement. The second method allows water to replace any drained brine but also to fill any air pockets that might have existed. Thus one might expect the second method to give higher densities than the first, and this is what was found as shown in Table 3, which shows data for two cores studied. A similar result was found for all 20 cores measured. The mean values for all the cores were
0.914 Mg/m3 for the second, submergence, method and 0.910 Mg/m3 for the first, or volume, method.
Fortunately the equations for determining brine volume, and hence strength, are not very sensitive to changes in density. For example, if the density changes from 0.90 to 0.94, a large change, the strength changes by only 12 kPa (1.7 psi). However, if the temperature of the ice core changes by 1oC, the strength changes by 48 kPa (7 psi). This would not be true,
however, if one was using total porosity rather than brine volume. Ideally, total porosity should be used, but in deriving their equation 2 above, Timco and O’Brien (1994) found that ice density information was not available in the literature and hence they used brine volume instead. Therefore, the temperature measurement is far more important than density when using brine volume rather than total porosity, to calculate ice strength. This re-emphasizes the need for an accurate, calibrated thermometer, and a measurement made as soon as possible after retrieving the core.
SUMMARY AND CONCLUSIONS
The sea ice properties on the USCGC Healy trials were successfully measured with various pieces of equipment. Ice thicknesses measured from the OTSV, core lengths, drilled holes and an EM technique were in good agreement with each other for first year ice. Snow thickness measured from the OTSV was nearly always greater than point measurements, and the reason was not directly determined. Ice core strengths were determined from salinity, temperature, and density measurements and gave mean values of 315 kPa (46 psi) for first-year ice and 714 kPa (104 psi) for multi-first-year ice. The calculated strength measurements, based on brine volume rather than total porosity, were shown to be very dependent on the temperature measurement, and not significantly dependent on the density.
REFERENCES
Cox, G.F.N., and Weeks, W.F., 1983. Equations for determining the gas and brine volumes in sea-ice samples. J. Glaciol., 29 (102), p.306-316.
Haas, C., Gerland, S., Eicken, H., and Miller, H., 1997. Comparison of sea-ice thickness measurements under summer and winter conditions using a small electromagnetic induction device. Geophysics, 62(3), p. 749-757.
Kovacs, A., and Morey, R.M., 1991. Sounding sea-ice thickness using a portable electromagnetic induction instrument. Geophysics, 56, 1992-1998.
Timco, G.W. and O’Brien, S., 1994. Flexural strength equation for sea ice. Cold Regions
Fig. 1. Comparison of the OTSV measured ice thickness with cores obtained. Note the general good agreement except for test L_000509_1715 as explained in the text.
Fig. 2. Comparison of snow depth measured by the OTSV with probe method. With the exception of one test, the snow depth measured by the OTSV was always greater then the
depth measured by the probe.
0 50 100 150 200 250 300 350 400
Trial Num ber
OTSV Average Thickness Core Thickness 1 Core Thickness 2 0 5 10 15 20 25 30 35 40 45
Trial Num ber
O TSV A verage D epth Core Depth 1 Core Depth 2
Fig. 3. The results of the EM thickness probe compared to drilled holes for ship performance test 0419
Fig. 5. Salinity, temperature and density of core 39, typical of the first year sea ice cores.
Fig. 6. Salinity, temperature and depth of core 37, a multi-year sea ice core -8 -6 -4 -2 0 2 4 6 5 35 65 95 125 155 1 85 215 2 45 2 75 30 5 33 5 D e pth cm 0 .6 0 0 .6 5 0 .7 0 0 .7 5 0 .8 0 0 .8 5 0 .9 0 0 .9 5 S alinity T em p D ensity -8 -6 -4 -2 0 2 4 6 8 10 5 15 25 35 45 55 65 75 85 95 105 115 125 135 Depth cm 0.88 0.89 0.90 0.91 0.92 0.93 0.94
Table 1 An example of the tabulated thicknesses from the EM technique and the drilled holes.
Table 3. Density of sea ice measured by the two methods described in the text
Mass Submer. Density Density Dia. L in Air FB Mass M/(M+F) M/Vol.
M F * Rho W mm mm g g Mg/m3 Mg/m3 Core #1B Top 71.7 56.7 207 23 0.900 0.904 Mid 71.7 53.8 194 18 0.915 0.893 Bottom 70.7 42.2 144 16 0.900 0.869 avg.= 0.905 0.889 Core #2B Top 71.5 55.6 193 18 0.915 0.865 Mid 71.4 49.7 185 18 0.911 0.930 Bottom 70.7 44.2 155 15 0.912 0.893 avg.= 0.913 0.896 Level 0411
Level 0411, measurements made 4/10/00 Position 63 19N, 62 10W
EM Ice Snow Drill snow Drill ice Freeboard
Avg 0-1250 m 60.55 no snow meas 15.14 60.43 0.61
St Dev 7.44 at EM sites. 4.57 6.55 1.42
Level 0414
Level 0414, measurements made 4/14/00 Position 63 59N, 62 17W
EM Ice Snow Drill snow Drill ice Freeboard
Avg 0-965 m 111.6 18.6 18.7 98.9 4.3 stdev all 33.63 9.38 9.59 27.96 5.00 Avg 0-250 m 150.0 21.7 20.8 134.3 7.8 stdev 15.04 6.72 9.83 4.55 4.02 Avg 350-800 m 88.0 14.8 15.1 83.9 4.2 stdev 12.28 6.07 6.87 20.01 2.66 Avg 850-965 m 122.0 27.8 20.7 84.0 0.7 stdev 42.26 15.36 13.61 13.08 8.02
Table 2. List of all ice cores taken and strengths obtained
CORE # DATE UTC LAT. LONG. THICK. STRENGTH STRENGTH
TIME cm kPa psi Phase I 1A 10 Apr 63 19 62 10 60 350 51 1B 10 Apr 1500 63 21 62 07 78 325.2 47.2 2A 10 Apr 63 19 62 10 55 351 51 2B 10 Apr 1715 63 17 62 08 54 350.4 50.8 3 11 Apr 1325 63 19 62 20 59 292.8 42.5 4A 11 Apr 63 19 62 10 66 335 49 4B 11 Apr 2230 63 19 62 10 66 341.5 49.5 5A 11 Apr 63 17 62 10 59 370 54 5B 11 Apr 2345 63 17 62 10 61 374.5 54.3 6A 14 Apr 62 59 62 12 127+ 488 71 6B 14 Apr 1305 63 59 62 12 131 442 64.1 7A 14 Apr 63 58 62 15 68+ 407 59 7B 14 Apr 1615 63 58 62 15 82 360.7 52.3 8 14 Apr 1740 63 56 62 17 82 375.2 54.4 9 14 Apr 63 56 62 18 77 320.3 46.5 10 14 Apr 1825 63 55 62 19 97 404.3 58.6 11A 19 Apr 1322 64 04 61 30 41 226 33 11B 19 Apr 1330 64 05 61 29 39 216.2 31.4 12A 19 Apr 1530 64 05 61 27 90 273 40 12B 19 Apr 1530 64 05 61 24 100 302.3 43.8 13A 19 Apr 2116 64 04 61 32 80 295 43 13B 19 Apr 2130 64 04 61 32 79 297.6 43.2 14 19 Apr 2315 64 03 61 34 45 215.4 31.2 15 19 Apr 2355 64 04 61 34 58 271.5 39.4 16 20 Apr 0030 64 04 61 34 44 261.2 37.9 17 20 Apr 1321 64 03 61 10 91 213.9 31 18 20 Apr 1625 64 04 61 11 102 285.4 41.4 19 20 Apr 1710 64 03 61 11 88 254.1 36.9 20 20 Apr 1900 64 04 61 11 87 244.5 35.5 21 21 Apr 1230 64 06 61 17 88 245.3 35.6 22 21 Apr 1630 64 07 61 14 93 259.4 37.6 23 21 Apr 1900 64 08 61 17 93 265.8 38.6 24 21 Apr 2107 64 08 61 20 96 258.9 37.6 25 21 Apr 2400 64 09 61 23 92 293.5 42.6 26 22 Apr 0030 64 09 61 21 86 303.1 44 27A 22 Apr 1755 64 08 61 18 86 243 35 27B 22 Apr 1810 64 08 61 19 88 228.3 33.1 28A 23 Apr 1140 64 08 61 21 88 237 34 28B 23 Apr 1140 64 08 61 21 90 267.3 38.8 304 44 Phase II 29 3 May 2211 68 56 63 26 138 370 54 30 4 May 1100 approx 68 57 63 26 131 429 62 31 4 May 1945 68 55 63 25 117 404 59 32 5 May 1335 68 51 63 18 90+ 394 57 33 5 May 1843 68 49 63 16 135 318 46 34 6 May 1726 68 30 63 10 172 356 52 35 7 May 1405 68 26 63 08 164 361 52.4 36 9 May 1113 68 35 61 04 213 734 107 37 9 May 1424 68 35 61 03 363 695 101 38 10 May 1532 68 30 61 05 63 192 28 39 11 May 2243 68 02 61 03 140 343 50 40 15 May 1750 64 12 61 34 122 306 44.4 347 50
Mean Phase 2 excl. 36/37 Mean Phase 1
