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Proceedings of the 20th International Conference on Port and Ocean Engineering under Arctic Conditions, 2009-01-01

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Preliminary testing of a new ice impact panel Gagnon, R.; Bugden, A.; Ritch, R.

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National Research Council Canada Institute for Ocean Technology Conseil national de recherches Canada Institut des technologies oc ´eaniques

IR-2009-10

Institute Report

Preliminary testing of a new ice impact panel.

Gagnon, R.; Bugden, A.; Ritch, R.

Gagnon, R.; Bugden, A.; Ritch, R., 2009. Preliminary testing of a new ice impact panel. 20th International Conference on Port and Ocean Engineering under Arctic Conditions, 9-12 June 2009, Luleå, Sweden POAC09-33 : 10 p.

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POAC09-33

PRELIMINARY TESTING OF A NEW ICE IMPACT PANEL

R. Gagnon 1, A. Bugden 1 and R. Ritch 2

1

Institute for Ocean Technology, National Research Council of Canada St. John's, NL, Canada

2

Avron Ritch Consulting Ltd., Vancouver, B.C., Canada

ABSTRACT

A modular ice impact panel, intended for future ship / bergy bit collision tests, has been designed that incorporates new pressure-sensing technology. A single module of the panel, with sensing area of 1 m2, has been fabricated and tested in the lab by dropping heavy masses of freshwater ice on it from heights up to 1.8 m. The central component of the Impact Module is a large cast acrylic block, 1 m x 1 m x 0.5 m, housed in a strong steel frame. The Impact Module design is made possible by the strength and transparency characteristics of the acrylic. The Module utilizes two types of pressure sensors, one consisting of strain-gauged acrylic cylinders (2.5 cm diameter) that are imbedded in the face of the acrylic block at 9 locations. The other technology is an opto-mechanical system consisting of many thin narrow strips (13 mm x 0.9 m x 4 mm) of acrylic that cover the impacting face of the acrylic block. Elastic flattening of the slightly convex bottom surface of the strips occurs when pressed against the face of the block during impacts. A high-speed video camera, positioned behind the transparent block, records the deformation of the strips from which the corresponding pressure is determined from calibrations. The effective square-shaped unit sensing area for this technology is 13 mm x 13 mm, implying that the Impact Module has the equivalent of about 4500 symmetric-shaped pressure sensors on its surface. The acrylic block is supported on four large flat-jack load cells that measure the impact loads. The tests demonstrated that the Impact Module is very robust and the pressure-sensing technologies yield consistent results. The large capacity load sensors were also shown to be functional.

INTRODUCTION

In June 2001 the first Bergy Bit Impact Field Study was conducted using the CCGS Terry Fox. This successful program demonstrated that a strong vessel could be instrumented to obtain detailed load and pressure distribution data during intentional impact experiments with bergy bits (Gagnon et al., 2008). Bergy bit impacts are a major hazard for tankers and other vessels operating off the East Coast of Canada, and similarly in the Arctic regions where resource development, tourism, mineral transport, etc are on the increase. The recent sinking of the Explorer cruise ship in the Antarctic following a collision with what was likely a bergy bit illustrates this point. Large chunks of multiyear sea ice pose similar hazards in the Arctic regions.

POAC 09

Luleå, Sweden

Proceedings of the 20th International Conference on

Port and Ocean Engineering under Arctic Conditions June 9-12, 2009 Luleå, Sweden

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The first Bergy Bit Impact Field Study was a success, however, maximum loads attained were limited due to the side-mounted orientation of the impact panels used. A new panel has now been designed to be mounted at the centre of the ship’s bow to facilitate more aggressive impacts. With this configuration much higher bergy bit impact loads can be achieved at moderate ship speeds while remaining within acceptable levels of deceleration.

The new impact panel configuration, with its more sophisticated pressure-sensing technology, will provide detailed information about the temporal and spatial evolution of pressure during ice impacts throughout a significantly higher load range (0-20 MN) than the first field study.

Prior to fabricating the full-scale impact panel for the Terry Fox vessel it was decided to fabricate one of the six impact modules intended for the panel and to test the device extensively to confirm its robustness and sensing capabilities.

Impact Panel Details

The impact panel intended for field use is modular in design and is 3 m wide and 2 m in height at the front impacting surface (Gagnon, 2008). Figure 1 shows its location on the Terry Fox bow. The top of the panel is at the waterline. The panel consists of six 1 m x 1 m modular sensor inserts, each with its own independent array of sensors for measuring load and pressure. The sensor inserts fit into the slots of the box-like structure of the panel, referred to as the backing structure. The backing structure is fabricated using 1” steel plate, welded at all joins. To enable divers to weld the panel to the hull of the Terry Fox it is necessary to do the attachment as a multi-stage process. First a mounting frame must be welded to the hull (Figure 1). The back end of the frame conforms

to the vessel and its shape enables full access for welding all of the conforming metal surfaces to the hull. The front of the mounting frame has several seating pads that mate and bolt to matching seating pads on the back end of the backing structure. Once the backing structure is bolted on then the sensor inserts are individually installed in the six slots. Alternatively, they may all be installed prior to bolting the backing structure on. This strategy is similar to that used during the first Terry Fox field study. Removal of the panel from the vessel is accomplished in the reverse order. All electrical cables from the inserts would feed up through a protective conduit welded to the upper hull running from the top of the panel up the side of the ship’s bulwarks from where the cables continue on to the data acquisition hut situated on the vessel’s fore-deck.

Figure 1. Impact panel on the CCGS Terry Fox bow. Note the spaces for the modular sensor inserts.

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Impact Module Description

A staged strategy has been adopted for the design and fabrication the impact panel since some of the technology is novel and it is prudent to conduct tests in the lab prior to committing to the final details. A 1/ 6 portion (one sensor insert) of the panel (Figure 2) has been fabricated for extensive testing at IOT and in the Structures Lab of Memorial University. This instrument is referred to as the Impact Module. Final details of the full impact panel design are contingent on the results of the lab tests using the Impact Module. Following the proofing of the impact panel design, the Impact Module will be used as a research tool for on-going laboratory studies of ice impacts. The details of the Impact Module construction and functioning are given below. These are essentially the same details that apply to the full impact panel.

The central component of the Impact Module is a thick transparent block of acrylic measuring 1 m x 1m x 0.46 m (Figure 2). The block rests on four flat-jack type load cells. These cells are metallic envelopes filled with hydraulic fluid. Load applied to the surface of the cell translates into pressure in the fluid, which is monitored by a pressure gauge. The thickness of the acrylic block gives it the flexural strength to withstand high loads while supported at its four corners. The block is held firmly against the load cells by two bolts that pass part-way through opposing corners of the block and that are secured with nuts and strong springs. Also, around the top edge of the block there is a securing plate that bolts to the steel side plating of the Impact Module with nuts and springs. This protects the edges of the block where a thin stainless steel sheet, that covers the new pressure-sensing technology, is screwed to the block while enabling the load from an ice impact to transfer to the flat load cells.

The top surface of the block is covered by new pressure-sensing technology. This new technology consists of many strips of acrylic, 13 mm wide and 4 mm in thickness and ~ 1 m in length, mounted side-by-side on the block’s surface. Each strip has a gentle curvature (0.23 m radius) across its width on the face that touches the large acrylic block (Figure 3). Pressure applied to the opposite side of the strips causes them to flatten against the block’s surface. The degree of flat contact, i.e. the width of the contact, is a direct measure of the pressure applied and

Figure 2. Sectional view of the Impact Module. (From Gagnon (2008)).

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can be calibrated. This type of sensor is very robust and has been used successfully in a number of ice crushing studies (Gagnon and Bugden, 2007; Gagnon and Bugden, 2008; Gagnon and Daley, 2005), in single and multiple strip configurations. Its range of sensitivity is about 0-60 MPa. The pressure sensor strips are covered by a thin sheet of stainless steel that is the contacting surface with the ice during impacts. The data acquisition system for the pressure-sensing strips is a fast high-resolution video camera (AOS X-PRITM ), operating at 250 images/s with resolution of 1280 x 1024 pixels, located at the bottom of the block as shown in Figure 2. When not pressed against the acrylic block the strips make very little contact, and when pressure is applied more contact occurs due to elastic flattening of the strips’ curved surfaces (Figure 3). To enhance the visibility of the flattened areas of the strips a thin sheet of opaque white plastic film (MonokoteTM) is loosely situated between the acrylic strips and the large acrylic block. This thin sheet provides a bright white representation of the flattened areas of the acrylic strips where they are pressed against the acrylic block during loading. The only light source inside the acrylic block is from a large number of horizontally oriented LED’s located around the block perimeter near the top. Light from these LED’s normally internally reflects off the top inner block surface. When an acrylic strip is flattened against the film-covered block the internal reflection is frustrated, and the light passes through the block’s surface to illuminate the white plastic film where the strip makes contact. Hence the portion of strip contact, appearing as white, becomes visible to the camera. In principle a pressure strip could be cut into many arbitrarily short lengths, each serving as a separate sensor that does not influence adjacent sensors. For example, cutting the strips into symmetric pieces 13 mm x 13 mm would amount to covering the top surface of the block with

Figure 3. Schematic showing how the new pressure-sensing technology functions. Two of the light rays from the source at the left internally reflect off the block’s internal surface where there is no contact with the strips. The center ray illumines the ‘white’ acrylic strip since the internal reflection is frustrated where the strip is elastically flattened against the block due to the pressure. The curvature of the strips is exaggerated for illustrative purposes. (From Gagnon (2008)). Note: the strips are clear acrylic, however the thin opaque white plastic film (not shown) between the block and the strips essentially behaves as a thin coat of resilient white paint on the strips’ bottom surfaces.

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approximately 4500 pressure-sensing units. Cutting the strips into segments and handling so many small pieces, however, isn’t necessary in practice. By using a continuous rather than segmented strip one allows for a small amount of influence between adjacent portions of the strip due to its flexural strength. The net result is a slight averaging effect in the pressure at each point along the strip associated with the small influence of material up to a 6 mm distance on either side of that point, as observed during calibration tests. This, along with a similar slight effect from the stainless steel sheet may be ignored. The automated data analysis technique (below) obtains pressure measurements at 0.79 mm increments along each pressure strip, that is, at each pixel in the image, amounting to many thousands of measurements for the whole sensing surface. In addition to the pressure-sensing strips the top surface of the block has an array of 9 strain-gauged pressure-sensing plugs recessed in it. These are small cylinders of acrylic (2.5 cm diameter) that are located and secured in flat-bottomed holes (with slightly larger diameter) machined in the surface to the exact depth corresponding to the height of the plugs. The end of each cylinder is glued to the acrylic at the bottom of the hole with Loctite 5900TM glue. Each plug has two strain gauges attached at opposite sides with sensitivity along the axis of the plug. Load applied to the top of a plug causes it to contract in length and this registers on the strain gauges. The plugs are essentially pressure transducers that have the same mechanical properties as the block itself. The sensors are useful both for corroborating the output from the pressure-sensing strips and as backup sensors in the event there is a problem with the strip sensors.

Results from Preliminary Tests

The preliminary tests conducted on the Impact Module were impact experiments involving dropping substantial masses of freshwater ice onto the Module’s surface. The Module was situated at the back end of IOT’s towing tank facility for the tests, where the reinforced concrete floor was strong and furthermore

the Module was located directly over a supporting column under the floor (Figure 4). This was more than adequate to support the weight of the Module (~ 2000 kg) in addition to the impulse loads from the ice impacts. Rectangular column-shaped blocks of ice with masses varying from 92 to 130 kg were dropped from heights ranging from 0.9 to 1.8 m above the Module. The ice blocks were grown in a commercial facility where no particular emphasis was placed on controlling texture or fabric of the grain structure. The blocks were clear and nearly free of air bubbles and had generally large grains. These factors were not important since these

Figure 4. Impact test setup. The Impact Module is located below the suspended ice mass. The netting is to protect staff and equipment from flying ice pieces.

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preliminary tests were not intended as a study of ice, but rather a test of the Impact Module. However it was desirable that the ice blocks stay intact well enough during impacts to impart good impulsive loads, as was the case.

The impacting end of an ice block was roughly shaped like a truncated pyramid with rounded edges to concentrate the initial impact loading. At the opposite end of the block a hole was augured through the ice so that a lifting strap could be passed through. In addition to the high-speed camera located inside the Impact Module that was used to record the pressure strip data, another high-speed camera (Photron Ultima APX-RSTM) recorded the impacts from outside the Module. The recording rates used for this camera varied from 500 to 1500 images/s. These records were very useful for observing behaviour of the ice, such as spalling events, that could be corroborated with the observed pressure patterns recorded by the Impact Module’s internal camera. Figure 5 shows the full view from the high-speed camera located inside the Impact Module in the case where no load is applied, that is, when no flattened portions of the pressure-sensing strips are visible since there is no pressure.

During a typical test the ice sample was raised with a crane, that also incorporated a weight scale, and positioned above the desired impact spot on the Module. All impacts were centered on one of the strain-gauged pressure plug sensors so that the output of the plugs and the pressure strip sensors could be compared. Most tests were conducted over the center strain gauge plug and a single test was conducted over each of two side located strain gauge plugs. The precise value for the drop height was not controlled during the setup, however the external high-speed camera provided data for accurate determination of impact velocity and associated drop heights. Ice blocks were shaped in a cold room at –10oC, however by the time the blocks were actually tested, approximately ½ hr after leaving the cold room, the surface was near or at 0oC, as evidenced by melting in places. Hence there was a temperature gradient in the ice samples. The release mechanism used

for the ice blocks during tests was a WichardTM Quick Release Shackle, generally used for yachting purposes, that provides easy release of heavy loads. This prevented any strong side forces at the lifting strap during release that could induce unwanted rotation of an ice block during its fall.

Impact tests on this scale can be hazardous so various procedures were instituted regarding handling and lifting the ice. Also, a polypropylene net (Figure 4) was used to prevent large flying ice pieces from hitting staff and sensitive equipment during the impacts.

Figure 5. Full view from the high-speed camera located inside the Impact Module. Five of the pressure-plug sensors are visible at the four sides and center. Air bubbles left over from the casting of the acrylic block are also visible.

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While these drop tests are seemingly simple to configure and perform, they are nevertheless extraordinary due to the information that can be gleaned from the combination of sensors inside the Impact Module and the secondary high-speed camera viewing the impacts from outside the Module. Figure 6 shows three images, separated in time by 0.004 s from left to right, of the ice block from Test#5 at impact. The first image at the left occurs near the first instant of contact where little damage or fracture has occurred in the ice. The second image shows the block 0.004 s later after a crack has occurred and a portion of ice has broken off from the back right side. The third image at the right shows more clearly the ice that has broken from the back side of the block (arrow) resulting from a spalling event. Debris moving rapidly out of the zone of contact is also visible in the second and third images.

Figure 7 shows the view of the pressure-sensing strips from inside the Impact Module for the first three images that show ice contact and loading. Only the central portion of the camera’s view that contains the pressure information is shown. These images were captured at approximately the same times as the images shown in Figure 6. The first image at the left shows the region of ice contact in the pattern of pressure depicted by the portions of the pressure strips that are flattened against the underlying large acrylic block. The second image was taken following the spall event that was seen initiating and progressing in the second and third images of Figure 6, as captured by the external viewing camera. In Figure 7 the spalling event is clearly evident as the abrupt change in pressure on the left side of the ice contact region, that is, the change from thick line contact (high pressure) in the left image to thin line contact (lower pressure) in the center image. Note that these events occur rapidly over just a few consecutive camera images captured at 250

Figure 6. Images from a drop test on the center of the Impact Module captured by the external high-speed camera. The left image shows the first contact of the ice block with the Module and is located directly over the center strain gauge plug. The middle image was captured 0.004 s later after some fractures associated with a spalling event at the far side of the ice block had occurred. The right image was captured 0.004 s after the middle one and shows a chunk of ice breaking from the right back side of the block and contact zone (arrow) during the spalling event that initiated shortly before the second image. The ice block had a mass of ~122 kg and impact velocity of 6.0 m/s. The camera was operating at 1500 images/s and every 6th image is shown consecutively so that the three images correspond to the images in Figure 7 where that camera was running at a slower rate (250 images/s).

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images/s. At any given location, the width of an individual pressure strip contact zone is a measure of the pressure at that point. The pressure indicated by the pressure strip contact lines near the strain gauge plug in the left image in Figure 7 corresponds to roughly 12 MPa. Similar analysis of ice crushing behaviour using two camera views, and the same multiple pressure strip technology, has been reported before (Gagnon and Bugden, 2008), however the present tests are the first ice impact experiments ever conducted with this level of richness in pressure information and corroborative data.

Figure 8 shows the time series record for the strain gauge plug sensor for the same test shown in Figures 6 and 7. Markers have been inset to indicate the times at which the images from the internal camera shown in Figure 7 were captured. The record shows a very sharply rising initial pressure, corresponding to ice contacting and loading the Impact Module before any fracturing has occurred, followed by more complex pressure changes associated with failure of the ice immediately contacting the sensor. The peak pressure attained was about 14 MPa, in reasonable agreement with the output from the pressure strip sensors. Note that the pressure strips have not yet been accurately calibrated and therefore only estimates are quoted here based on calibrations from former experiments using similar technology.

The outputs from the four flat-jack load cells supporting the acrylic block in the Impact Module (Figure 2) were barely visible above the ambient noise of the sensors, including impact-induced resonance in the system, since these relatively small-scale ice impacts generate a small load (< 100 kN) compared to the calibrated load range intended for these load cells during the full-scale

Figure 7. Three consecutive images from the camera inside the Impact Module for the same drop test shown in Figure 6. The camera capture rate was 250 images/s and only the central portion of the images is shown. The images correspond to the three images shown in Figure 6. The left image shows the first contact of the ice block with the Module before ice damage has occurred and where the pressure plug sensor is encompassed by the ice contact zone. The middle image shows the sudden reduction of pressure on one side of the original contact zone due to a spalling event. The right image shows more clearly the reduction in size of the contact zone due to the spalling event when the ice that has broken away no longer exerts any pressure on the Impact Module. The right edge of an image in this figure is parallel to and towards the left foreground edge of the Impact Module in Figure 6.

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field tests. The crude estimates of load are nevertheless in the expected range for these small-scale tests. A better test of these devices will occur in the loading frame fabricated at MUN that has been specifically designed to apply large loads to the Impact Module.

Pressure strip data from the tests could be obtained by direct manual analysis of the images, that is, through measurements of the pressure strip contact width, as in Figure 7, at many locations in the field of view. Although laborious, the uniqueness and value of the data would justify this approach if it was necessary. However, a method for automated analysis has been developed that yields the same accuracy with much less human effort. Briefly, the method involves dramatically increasing the signal-to-noise of an image by subtracting out all information except the pressure strip signal. This is done by simply subtracting a null image, that is, one where no pressure is showing anywhere, from the images that are showing pressure strip signals. This leaves only the pressure strip signal on a black background. Enhancements other than the image subtraction, such as brightness and contrast filtering, are not required and would degrade the image information. The image resulting from the subtraction routine is then edge detected using a Sobel operator that outputs the edges of the pressure strip contact areas. Contrast between the pressure strip images and the black background is high so that the edge detection works very well even though the pressure strip images appear somewhat dim (Figure 7), due to limitations of the lighting. A computer program then detects the separation between the edges at each pixel along the length of the strip in the image. Each measurement is converted to pressure according to a calibration equation.

Figure 8. Time series of the output from the center strain gauge plug during the same test as shown in Figures 6 and 7. Markers (dark circles) indicate the times when images were captured by the internal camera of the Impact Module, where the first three at the left correspond to the images in Figure 7. The exposure time for each image is from one marker to the next (i.e. 1/250 s).

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Conclusions

Successful preliminary impact tests of the Impact Module were conducted using masses of freshwater ice. The tests showed consistent results for pressure measurements from the Impact Module’s two types of sensors. The robustness of the sensors was also demonstrated for multiple impacts at test pressures up to the highest recorded in this test series (~18 MPa). The pressure strip and strain gauge plug sensor technologies are both capable of measuring rapidly changing pressures. The pressure strip technology yields an accurate mapping of the pressure in the rapidly changing contact zone during impacts with ice. The large flat-jack load cells were shown to be operative and capable of registering these relatively small impact loads.

In fabricating the Impact Module and conducting these tests some important and easily remedied issues have been identified to insure the best outcome from the future field tests. One is the issue of vibration of the camera within its housing during impacts. The flexible component of the camera housing has been identified and strategies will be implemented to eliminate the problem in the Impact Module and in the design of the large impact panel to be used in the field tests. Secondly, the undesirable air bubbles in the acrylic block were the result of a manufacturing problem with the casting process. Such bubbles will be avoided in casting the new acrylic blocks for the large impact panel. In any case the system, and analysis technique, works even with the bubbles. Finally, with simple modifications the view of the internal camera in the Impact Module can be easily increased to encompass 95% of the impact surface whereas presently it senses about 85%.

Acknowledgements

The authors would like to thank the Program of Energy Research and Development (PERD), Transport Canada and IOT for their financial support of this research.

REFERENCES

Gagnon, R.E. (2008). A New Impact Panel to Study Bergy Bit / Ship Collisions. Proceedings of 19th IAHR International Symposium on Ice 2008, Vancouver, British Columbia, Canada, Vol. 2, 783-790.

Gagnon, R.E. and Bugden, A., 2007. Ice Crushing Tests Using a Modified Novel Apparatus. Proceedings of POAC-07, 235-244.

Gagnon, R.E. and Bugden, A., 2008. 2-Dimensional Edge Crushing Tests on Thick Sections of Ice Confined at the Section Faces. Proceedings of 19th IAHR International Symposium on Ice 2008, Vancouver, British Columbia, Canada, Vol. 2, 763-771.

Gagnon, R.E. and Daley, C., 2005, “Dual-axis video observations of ice crushing utilizing high-speed video for one perspective”, Proceedings of POAC 2005, Potsdam, New York, Vol. 1, 271-282.

Gagnon, R., Cumming, D., Ritch, A., Browne, R., Johnston, M. Frederking, R., McKenna, R. and Ralph, F., 2008. Overview accompaniment for papers on bergy bit impact trials. Cold Regions Science and Technology, 52, 1–6.

Figure

Figure  1.  Impact  panel  on  the  CCGS  Terry  Fox  bow. Note the spaces for the modular sensor inserts
Figure 3.  Schematic showing how the new pressure-sensing technology functions. Two  of  the  light  rays  from  the  source  at  the  left  internally  reflect  off  the  block’s  internal  surface  where  there  is  no  contact  with  the  strips
Figure  4.  Impact  test  setup.  The  Impact  Module  is  located below the suspended ice mass
Figure  5.  Full  view  from  the  high-speed  camera  located  inside  the  Impact  Module
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