21st IAHR International Symposium on Ice Ice Model Tests in

21st IAHR International Symposium on Ice
"Ice Research for a Sustainable Environment", Li and Lu (ed.)
Dalian, China, June 11 to 15, 2012
© 2012 Dalian University of Technology Press, Dalian, ISBN 978-7-89437-020-4
Ice Model Tests in Compressive Ice
Mikko Suominen*, and Pentti Kujala
Aalto University, Department of Applied Mechanics, Aalto, Finland
*
[email protected]
Dynamic and compressive ice fields cause difficulties for ships operating in ice. Ships might get
stuck or even get damaged in a compressive ice field due to the compressive forces. Especially
ships with a long parallel midship are vulnerable to compressive ice forces. In the EU FP7
project called SAFEWIN the risks and impacts of compressive ice fields on ships are
investigated. Ice model tests are conducted in the ice basin of Aalto University within the project
with a tanker model in compressive ice fields. The aim of the tests is to determine the increase of
ice resistance resulting from compression in the ice field. In addition, ice loads and ice pressures
are measured on the bow shoulder and the parallel midship areas simultaneously with load
sensors and tactile pressure sensor sheets. In order to determine the increase of ice resistance and
ice loads resulting from different compressive situations, the tests are conducted in compressive
level ice fields and in closing channels with different ice thicknesses and different compression
levels. The resistance and ice loads are measured with and without compression in the same ice
sheet and the measurements are compared between. The compressive force in the ice field and
closing channel are created with pushing plates mounted on the testing rig and the compressive
force is measured from the plates with load sensors. Test procedures and set-up are described in
this paper and the measurements of the rig-imposed ice forces are given. The preliminary
assessments of the added resistance and ice loads are presented.
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1. Introduction
Dynamic and compressive ice fields cause difficulties for ships operating in ice. Ships might get
stuck or even get damaged in a compressive ice field due to the compressive forces. Especially
ships with a long parallel midship, such as tankers, are vulnerable to compressive ice forces. As
the oil tanker traffic is increasing in the Baltic Sea, the risks related to oil spill due to dynamic
and compressive ice are increasing.
Studies on ship in compressive ice field are rare. Compressive situations are difficult to catch and
full scale tests and instrumentation of a ship is expensive. Ice model tests offer a possibility to
study how ships feel compressive ice field. The first ice model tests in compressive ice field
were conducted in a joint research project between Helsinki University of Technology,
Laboratory of Naval Architecture and Marine engineering, and Academy of Sciences in USSR,
Institute for Problems in Mechanics (Kujala et al., 1991).
In the tests towing force, speed and maximum compressive ice forces at midship were recorded
and studied in level ice, channel, compressive level ice and compressive channel. The test
showed that resistance increases significantly in compressive ice as the resistance in compressive
channel was twice the resistance in level ice (Kujala et al., 1991). The research conducted in the
joint project was summarized by Riska et al. (1995). The summary presented an early method to
estimate the additional resistance due to compression. In addition, an initial method on predicting
ice loads at midship was presented.
A program to study operation of large tankers in ice infested waters and under Arctic conditions
was started in 2005 (Riska et al., 2006). The ice model test program included tests in
compressive ice field. In the tests in compressive ice field, the focus was on the added resistance
due to compression. As a result, a model for calculating the resistance in compressive ice was
developed.
New ice model tests were conducted in compressive ice fields in the EU FP7 project called
SAFEWIN. The tests were conducted in level ice, channel, compressive level ice and in closing
channels with different ice thicknesses and different compression levels. In the tests, the added
resistance and added ice loads and ice pressure resulting from different compressive situations
were studied. This paper describes the test procedures and set-up and calibrations of the used
measurement devices. The preliminary assessments of the added resistance and ice loads are also
presented.
2. Tests Procedure
The tests were conducted in the ice basin of Aalto University. The sides of the basin are 40
meters and the depth is 2.8 meters. The basin has a bridge which extends from side to side and
can be moved over the basin. A smaller carriage moves beneath the bridge and can travel
throughout its length. Used model ice is GE-ice (granular ice of ethanol solution) (Jalonen, 1990).
Model ice is produced by spraying. The bridge moves back and forth over the basin and sprays
small droplets of ethanol solution into cold air. The droplets cool in the air and drop to the
surface forming slush ice. After spraying the target ice properties are achieved with consolidation
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treatment where the temperature is decreased below -15°C. Before the actual test runs, ice
thickness and ice properties are measured.
The tests were conducted in level ice, compressive level ice, closing channel and in open channel
to study the added resistance and added loads resulting from compression in the ice field. The
test programme consisted of six test series where the ice thickness and compression level were
varied, see Table 1. During the tests, ice resistance, ice loads and ice pressure were measured on
the bow shoulder and the parallel midship areas simultaneously with load sensors and tactile
sensor sheets. In addition, compressive force added to the ice sheet was measured from pushing
plates with load sensors. Instrumentation is described in more details in its own chapter below.
Figure 1 presents the layout of the tests arrangement. The tests were performed by towing the
model with a winch across the basin with the speed of 0.5 m/s. The first test of each test series
was level ice tests with no compression. The second test was compressive level ice tests. For the
second test the model was moved to the other side of the basin. The ice sheet was sawed straight
under the bridge and the pushing plates of the bridge were lowered to the water level, see Figure
2. The bridge was adjusted to push the ice sheet with the speed of 0.003 m/s and the model was
towed across the basin. The speed of 0.003 m/s was chosen to represent a static compression in
the ice field as it was the lowest possible speed the bridge could move.
Figure 1. Layout of the test arrangement.
Figure 2. The pushing plates lowered to the
water level.
After the level ice tests, the tests were performed in closing channel and open channel. The
procedure in closing channel tests was similar with the compressive level ice test. After the
model had been towed one model length, the bridge started to close the channel by pushing the
ice sheet between the model and the bridge, see Figure 3. The towing continued until the model
reached the other side of the basin. Closing channel test was repeated until the ice sheet between
the channel and the bridge was broken apart. Open channel tests were conducted after the ice
sheet was unusable for closing channel tests.
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Figure 3. A picture from a closing channel test.
In closing channel tests, different closing speeds were used to simulate different compression
levels. The closing speeds of 0.01, 0.02 and 0.03 m/s were used to simulate low, moderate and
severe compression respectively. The closing speeds were chosen based on Russian system of
observing ice compression. The criterion for defining the different levels of compression in this
system is based on the distance the channel closes after the ship. In low compression, the channel
closes 0.75 nm after the ship and in moderate and severe compression the channel closes 0.5 and
0.25 nm after the ship respectively. The ice thickness and closing speeds of the channel used in
different test series are presented in Table 1.
Table 1. Ice thickness and closing speed of the channel and the ship model in test series.
Test series number
Ice thickness
[mm]
1
2
3
4
5
6
41
29
23
29
29
24
The used closing speeds
of the channel [m/s]
0.003, 0.03
0.003, 0.03
0.003, 0.03
0.003, 0.01, 0.02
0.003, 0.01, 0.02, 0.03
0.003, 0.01, 0.02, 0.03
The speed of the
ship model [m/s]
0.5
0.5
0.5
0.5
0.5
0.5
The model used in the tests is a model of 21 300 DWT general cargo carrier Credo designed by
FKAB. The ship was chosen to represent typical tanker / cargo vessel operating in the Baltic Sea
as it has Swedish-Finnish ice class 1A Super and it has long parallel midship section. The used
model scale was 1:25.
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3. Instrumentation and Calibration
The model was instrumented with load sensors and tactile pressure sensor sheets, hereafter
referred as pressure foils. Towing force was measured with a load sensor in order to determine
the ice resistance. For this purpose, the sensor was attached from one end to the bow of the
model and to the towing rope from the other end. The load sensor was calibrated with dead
weights before the tests.
Ice loads on the bow shoulder and the midship area were measured with load sensors and
pressure foils. In order to measure ice loads with load sensors, two panels were cut out from the
model and instrumented with load sensors, hereafter referred as load panels. The dimensions of
the load panels were 0.3 m × 0.2 m (width × height) at midship and 0.12 m × 0.2 m at bow
shoulder. The centerline of the load panels in vertical direction was at the water line. The
pressure foils were then fixed with tape on top of the panels. Figure 4 presents the locations of
the load panels and pressure foils.
Figure 4. Locations of load panels and pressure foils. Inner rectangles represent load panels and
outer rectangles pressure foils.
The load panel at the bow shoulder was instrumented with one three-axial load sensor which
measures loads to x-, y- and z-directions, see Figure 5. Three-axial load sensor was chosen for
ice load measurements at bow shoulder as the hull of the model curves at this area meaning loads
to different directions might occur. The load panel at midship area was instrumented with three
one-axial load sensors which were oriented to measure loads acting normal to the hull surface,
see Figure 5. As can be seen from Figure 5, all load sensors were mounted between the panels
and supporting structure inside the model.
Figure 5. The load panel at the bow shoulder instrumented with three-axial load sensor (on the
left) and the load panel at the midship instrumented with three one-axial load sensors (on the
right).
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All load sensors were calibrated with dead weights before mounting to the ship model. After the
mounting of the sensor, additional calibrations were conducted with the load panels. In these
calibrations, the model was turned to stand on its side and 600 g dead weight was placed at
different locations on the panels and loads were recorded, see Figure 6 and Figure 7. As can be
seen from Figure 6 and Figure 7, the measurements correspond the dead weight loading.
Figure 6. Locations of dead weight loading (on the left) and the measured load with load panels
(on the right) at midship.
Figure 7. Locations of dead weight loading (on the left) and the measured load with load panels
(on the right) at bow shoulder.
The pressure foils used in the tests are part of tactile sensor system called I-SCAN. The system
consists of a sensor sheet, handle and measuring PC. The sensor sheet used in the tests is model
5350N, see Figure 8. The width of the measuring area is 439.9 mm and the height is 480.1 mm
having 44 pressure cells to horizontal direction and 48 to vertical direction. The elements of the
pressure foils have voids between each other. Due to the voids, the actual measuring area of the
pressure foil is 64% of the total element area. Before fixing the sheets to the model, the sheets
were calibrated with dead weights at the temperature of 0°C according to the system manual of
Tekscan.
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Figure 8. Sensor sheet taped on the model side.
The bridge above the basin has pushing plates which extend from side to side. In order to
measure the force inserted to the ice sheet, 10 m of this span was instrumented with load sensors,
see Figure 2. The centerline of the instrumented part was located 25 meters away from the
starting side of the test runs. Load sensors were mounted between the pushing plates of the
bridge and plates placed in front of these plates which were connected with hinges above, see
Figure 9. Altogether four plates were placed in front of the pushing plates, each 2.4 m wide, and
two load sensors were mounted on each plate. In the tests, the line load measured from
instrumented pushing plates is assumed to be constant over the total length of the pushing plates.
The load sensors were calibrated with dead weights before mounting to the pushing plates.
Load sensors
Figure 9. An instrumented pushing plate.
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4. Results
The first run of a test series was conducted in level ice and the second one in compressive level
ice and the resistance and loads on model hull and pushing plates were measured. The
compression in compressive level ice tests is considered to be static compression as compression
existed in the ice sheet, but the speed of the sheet was close to zero. As the bridge was adjusted
to move 0.003 m/s, the channel behind the ship model did not closed and the intact ice sheet
carried the load inserted by the pushing plates.
The measurements in level ice and compressive level ice showed that the static compression in
the ice field does not increase the resistance of a ship. Figure 10 presents an example of the
resistance measured in level ice and compressive level ice tests and the line load measured from
pushing plates in the compressive level ice measurement. The first peaks in the resistance
measurements results from the acceleration of the model and are therefore neglected. The
measurements presented in Figure 10 were conducted in the same ice sheet, which thickness was
29 mm. As can be seen from Figure 10, the resistances in level ice and in compressive level ice
are close to each other and no added resistance due to compression can be observed. The same
observation was made from the other measurements in level ice.
Acceleration phase
Figure 10. Comparison of the resistance between level ice and compressive level ice. Line load
inserted to the ice sheet with pushing plates presented with grey line.
Figure 11 presents typical time histories of measured line load on midship in level ice and in
compressive level ice. Figure 11 shows that line loads are similar in level ice and compressive
level ice and no added loads loads resulting from the compression can be observed. In addition,
the frequency of the ice loads at midship is low. The ice load measured with the pressure foil is
the sum of the measured ice load on the elements at the same area as the load panel divided by
0.64, because the measuring area is 64% of the total area as explained above. Figure 11 shows
that measurements with the load panel and the pressure foil correlates well. Similar correlation
was observed in other tests also.
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Figure 11. The time histories of measured line loads with the load panel at midship in level ice
and with the load panel and pressure foil in compressive level ice in the ice thickness of 29 mm.
Figure 12 presents typical time histories of measured line loads on bow in level ice and in
compressive level ice. The direction of the measured ice load is to y-direction in ship coordinate
system. The time histories of measured ice loads to y-direction at bow area are similar in level
ice and in compressive level ice. When the average ice loads at bow to y-direction were studied,
no clear added load due to compression was observed. In some cases, the measured ice loads on
bow shoulder were smaller in compressive level ice than in level ice. The maximum ice loads to
x- and z-direction were less than 10% of the measured maximum loads to y-direction. Therefore
measurments to x- and z-direction are not presented and discussed in this paper.
Figure 12. Measured ice load at bow shoulder load panel in y-direction in level ice and
compressive level ice.
As mentioned above, tests were conducted also in closing channel with different closing speed of
the channel. These cases are considered to be dynamic compression as the ice sheet is moving.
Figure 13 shows a measured resistance during closing channel test with line load measured from
pushing plates as a function of channel width in front of the model. The ice thickness in the tests
was 29 mm and the closing speed of the channel was 0.02 m/s, which represents moderate
compression. The negative values in channel width indicates open channel in front of the ship,
zero value a closed channel and positive values rafting ice sheet.
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End
Figure 13. Resistance in a closing channel test and the measured line load with pushing plates.
Ice thickness in the test was 29 mm and the closing speed of the channel was 0.02 m/s.
Figure 13 shows that the resistance is increasing as a function of decreasing channel width. After
the channel has closed, the resistance is not increasing although the ice in front of the model is
rafting, but it stabilizes around a certain value. Comparison between Figure 10 and Figure 13
shows that the resistance is significantly higher in closing channel test when the compression is
dynamic, although the line load measured from pushing plates are close to each other and the ice
thickness is the same.
Figure 14 and Figure 16 present the measured line loads on load panels at midship and bow
shoulder at the same test as the resistance presented in Figure 13. Figure 15 presents the line load
measured with the pressure foil at midship at the same area as load panel at the same test run.
Other closing channel tests gave similar measuring results as presented in these Figures. As can
be seen from Figure 16, line loads at bow shoulder area in closing channel test are similar to
level ice and compressive level ice, see Figure 12. This indicates that dynamic compression in
the ice field does not increase ice loads at bow area. Vice versa, study of Figure 14 and Figure 15
show that dynamic compression has a significant impact on the line loads occurring at the
midship, compare to Figure 11. The maximum line load and frequency of ice impacts increase in
great extent. Comparison of Figure 14 and Figure 15 with Figure 16 suggests that line loads in
dynamic compression are similar at midship and bow shoulder area.
Figure 14. Line load measured on the load panel at midship.
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Figure 15. Line load measured on the pressure foil at midship at the same area as the load panel.
Figure 16. Line load measured on the load panel at bow shoulder to y-direction.
5. Conclusions
The measurements showed that a static compression in the ice field does not increase the
resistance nor the ice loads occurring on model hull both at the bow shoulder and midship. In
some cases, the measured resistance and ice loads on bow shoulder were smaller in compressive
level ice than in level ice. The measurements showed that ice loads on ship hull mainly occur at
bow and bow shoulder area. The frequency and magnitude of the ice loads are significantly
lower at midship when compared to the bow shoulder in level ice and static compressive cases.
One possible reason for lower resistance and loads in level ice with static compression is that
small cracks, not visible, have form in the compressed ice sheet. The cracks weaken the ice sheet
and make it easier for the ship to break the ice. As the speed of the ice sheet is slow compared to
the model speed, the midship does not have contact with the edge of the channel and no friction
related added resistance occurs at midship.
Closing channel tests showed that dynamic compression has an impact on the resistance and on
the frequency and magnitude of the ice loads occurring at midship. The resistance is increasing
as a function of the width of the channel in front of the model. After the channel had closed, the
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resistance stabilized. The situation could be considered as dynamic compression in level ice. The
resistance was significantly higher in dynamic compression than in static compression or in level
ice.
Furthermore the closing channel tests showed that the line loads at midship are similar to the line
loads at bow shoulder. Comparison of the line loads in static and in dynamic compression
showed great increase in frequency and in magnitude in dynamic situation. The measurement
results indicate that the added resistance due to dynamic compression results from added line
loads at midship area. The resistance increases when the magnitude and frequency of the line
loads at midship increase. Furthermore, the magnitude and frequency of line loads at midship
area depends on the speed of the ice sheet towards the model side.
Acknowledgments
The model tests presented in this paper were conducted within EU FP7 project called SAFEWIN.
The funding came from European Commission, which is here gratefully acknowledged. The line
drawings of the ship model were received from FKAB, which enabled to conduct the model tests
with a ship model of an existing ship. Their contribution is also gratefully acknowledged. The
support from VTT personnel Kari Kolari and Pieti Marjavaara is also recognized. Furthermore
the writers want to thank all the project partners AS2CON, Arctic and Antarctic Research
Institute, Finnish Meteorological Institute, Finnish Transport Agency, ILS Oy, Stena Rederi AB,
Swedish Maritime Administration, Swedish Meteorological and Hydrological Institute, Tallinn
University of Technology and AS Tallink Group. Special thanks to Jonathan Howell and Andrew
Manuel for processing the collected data and the staff of Aalto University.
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