Global ice impact forces on the CCGS Amundsen, Beaufort Sea

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Global ice impact forces on the CCGS Amundsen, Beaufort Sea Johnston, Michelle

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Global Ice Impact Forces on the CCGS

Amundsen

Beaufort Sea, August 2011

M. Johnston

1 2 3 4 H1 H2

Technical Report, CHC-TR-085

April 2012

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Global Ice Impact Forces on the CCGS

Amundsen

Beaufort Sea, August 2011

M. Johnston

Canadian Hydraulics Centre National Research Council of Canada

Montreal Road Ottawa, Ontario K1A 0R6

FINAL REPORT prepared for: Canadian Coast Guard

Ottawa, ON

Imperial Oil Resources Ventures Ltd. Calgary, AB ArcticNet Laval, QC University of Manitoba Winnipeg, MB Technical Report, CHC-TR-085 April 2012

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multi-year ice floes in the Beaufort Sea. Global impact forces were derived from an inertial measurement system called MOTAN. In general, global forces of up to 9.0 MN were measured for maximum impact speed from 3.1 to 10.2 kt. Most of the impacts were conducted with two floes of known thickness and strength. Only two floes were sampled during the two-week program, but they characterized a wide spectrum of multi-year ice. Floe B1S1 was the thinner of the two floes (7.2 m), quite warm, saline and relatively weak (the depth-averaged borehole strength of the two test holes was 7.3 MPa and 4.5 MPa). Floe B1S2 was thicker (8.0 m), colder, less saline and stronger (the depth-averaged borehole strength of the uppermost 7 m of ice was 22.0 MPa). As anticipated, global impact forces and ship accelerations were higher during collisions with the thicker, stronger floe than the weaker floe, for comparable ship speeds. The highest surge acceleration (1.03 m/s²) occurred during the final ram with Floe B1S2 which was conducted at a maximum impact speed of 6.7 kt. That collision generated the highest global impact force of the program: 9.0 MN. The highest heave acceleration (0.84 m/s²) and sway acceleration (0.46 m/s²) occurred during rams with Floe B1S1, while transiting to a natural harbour within the floe. Global forces from 18 impacts with isolated multi-year floes floating in open pack ice were comparable to many of the rams with Floe B1S1, for a given impact speed. Comparison of the Commanding Officer’s expected ice severity for the 18 impacts indicated that even experienced personnel can have difficulty ascertaining the competency of multi-year ice. Impact forces and accelerations measured by MOTAN on the CCGS Amundsen were comparable to MOTAN-derived measurements on the CCGS Terry Fox from oblique impacts with bergy bits. The maximum heave, surge and sway accelerations on the CCGS Terry Fox were respectively 0.57 m/s², 0.56 m/s² and 1.24 m/s² during the Bergy Bit Trials, with measured global impact forces of up to 10.6 MN. Global impact forces and accelerations on the CCGS Amundsen during this study were not nearly as high as the impact forces measured by other instrumentation systems on ships operating more aggressively in Beaufort Sea multi-year ice.

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10.0 References... 52 Appendix A: Methodology for On-ice Sampling ... A-1 Appendix B: August 14 Rams Accessing Natural Harbour ... B-3 Appendix C: August 17 Rams with Sampling Area, Floe B1S1 ... C-2 Appendix D: August 19 Rams with Sampling Area, Floe B1S2... D-2 Appendix E: August 24 Transient Impacts with Isolated Floes ... E-1

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List of Figures

Figure 1 Schematic of MOTAN: sensor and software... 3

Figure 2 Location of MOTAN sensor with respect to VCG, LCG and centreline ... 4

Figure 3 Approaches used to calculate resultant forces... 6

Figure 4 Location of sampled multi-year ice floes ... 8

Figure 5 Floe B1S1 as identified from satellite imagery ... 9

Figure 6 Aerial perspective of ship secured in natural harbour of Floe B1S1 ... 10

Figure 7 Ship track of rams needed to reach natural harbour of Floe B1S1, 14 August ... 11

Figure 8 Global force and ship speed for rams reaching natural harbour in Floe B1S1 ... 15

Figure 9 Bow prints from rams with Floe B1S1, natural harbour (rams 1 to 6)... 16

Figure 10 Bow prints from rams with Floe B1S1, natural harbour (rams 7 to 11)... 17

Figure 11 Topography of Floe B1S1 from the ice surface ... 18

Figure 12 Temperature, salinity and strength profiles of Floe B1S1... 20

Figure 13 Ship track showing rams with sampling area of Floe B1S1, 16 August ... 22

Figure 14 Global force and ship speed for rams with sampling area of Floe B1S1 ... 24

Figure 15 Bow prints from rams 1 to 6 with sampling area of Floe B1S1 ... 25

Figure 16 Bow prints from rams 7 to 11 with sampling area of Floe B1S1 ... 26

Figure 17 Floe B1S2 as identified from satellite imagery ... 27

Figure 18 Aerial perspective of ship secured against Floe B1S2 ... 28

Figure 19 Temperature, salinity and strength profiles of Floe B1S2... 30

Figure 20 Ship track showing four rams conducted with sampling area of Floe B1S2 ... 31

Figure 21 Global force and ship speed for 4 impacts with sampling area of Floe B1S2 ... 33

Figure 22 Bow prints left by rams with sampling area of Floe B1S2 ... 33

Figure 23 Global force, ship speed and shaft output for impacts with isolated floes... 38

Figure 24 Isolated Floes #1 to #5a... 39

Figure 25 Isolated Floes #6 to #11... 40

Figure 26 Isolated Floes #11a to 16... 41

Figure 27 Ice severity compared to (a) global impact force and (b) surge acceleration ... 42

Figure 28 Comparison of five years of drill-hole measurements on multi-year floes ... 43

Figure 29 Properties of two different floes sampled during the voyage:... 44

Figure 30 CCGS Amundsen accelerations at the CG from impacts with multi-year ice ... 46

Figure 31 Global impact forces versus ship speed ... 48

Figure 32 Forces on the CCGS Amundsen compared to other ships ... 50 Figure NRC dual acting borehole indentor... A-3

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List of Tables

Table 1 Particulars of CCGS Amundsen... 4

Table 2 Rams Approaching Natural Harbour, Floe B1S1: Global Forces and Ship Motions.... 12

Table 3 Summary of thicknesses along drill-hole transects on Floe B1S1 ... 19

Table 4 Rams with Sampling Area of Floe B1S1: Global Forces and Ship Motions... 21

Table 5 Summary of ice thicknesses measured on Floe B1S2 ... 28

Table 6 Rams with Sampling Area of Floe B1S2: Global Forces and Ship Motions... 31

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Global Ice Impact Forces on the CCGS

Amundsen

Beaufort Sea, August 2011

1.0 Introduction

This report quantifies the whole-ship motions and global ice impact forces on the CCGS Amundsen from 44 collisions with multi-year ice floes in the Canadian Beaufort Sea, August 2011. In addition to the ship-based MOTAN measurements, NRC-CHC also undertook on-ice measurements of the thickness and strength of two floes impacted by the ship. The on-ice measurements were made as part of a jointly-funded study to characterize Extreme Ice Features. The complete results of that study are included in the NRC-CHC report “Field Measurements on Multi-year Ice in the Beaufort Sea, August 2011” (Johnston, 2011). Excerpts of that report are included here, in terms of the thickness and strength of the two sampled floes, since it has a direct bearing upon the how the ship responded to impacts with the two floes.

The ship motions and global impact forces for 22 impacts with Floe B1S2 (the first floe of known thickness and strength) are presented, along with 4 ship impacts with Floe B1S2 (the second floe of known thickness and strength). Results from 18 impacts with isolated multi-year floes of unknown thickness and strength are also discussed in terms of the ship response that was expected by the Commanding Officer prior to impacting the floe, compared to his assessment of the impact after the fact. Supplementary information about the total power from the ship’s shaft output is provided for impacts with isolated floes1, but that information was not available for the collisions with Floes B1S1 and B1S2.

Having MOTAN measurements, in combination with the documented properties of two of the impacted multi-year floes, provides a very unique set of data. It is only the second study during which global ice forces were measured from ship impacts with floes of known thickness and strength2. This valuable data set can be used to validate numerical ice models, physical ice models and to assist the designer in performing calculations for direct design of hull scantlings of the future Polar Icebreaker. Results from this study will also be used to help determine which multi-year floes are capable of inflicting damage on ships, making it relevant to the ongoing NRC-CHC project ‘Establishing Damage Criteria for Multi-year Ice’ (for Transport Canada). The Canadian Coast Guard provided the motivation for this project. They are currently designing a new Polar Icebreaker to replace the CCGS Louis S. St-Laurent. The new icebreaker is intended to operate farther north, for a longer period each year, than currently possible with the CCGS Louis S. St-Laurent. Additional funding participants for this work include the University of Manitoba and Imperial Oil Resources Ventures Ltd (IORVL) through ArcticNet. The on-ice portion of the study was funded solely by IORVL.

1

Measurements made by Avron Ritch Consulting, Ltd. Data provided to NRC-CHC by ArcticNet.

2

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2.0 MOTAN

Traditionally, ice impact forces on ships are estimated from the deflections measured by strategically placed strain gauges in the ship’s bow (for local forces) and/or along the ship’s length (for global forces). Two of the earliest trials that used strain gauges to measure local ice impact forces on a ship involved the CCGS Louis S. St-Laurent (Noble et al., 1978; Blount et al., 1981). The strain gauge technique was used to estimate global ice impact forces on ships such as the M.V. Robert LeMeur and Canmar Kigoriak (Ghoneim et al., 1984), M.V. Arctic (German & Milne and VTT, 1985), USCGC Polar Sea (Minnick et al., 1990) and the USCGC Polar Star (Minnick and St. John, 1990). One of the drawbacks of this technique is that installing strain gauges can be invasive, labor intensive and costly.

The National Research Council of Canada’s Canadian Hydraulics Centre (NRC-CHC) developed an inertial measurement system called MOTAN to provide an alternate approach to measuring global impact forces on ships. MOTAN, which stands for MOTion ANalysis, was initially developed to measure the motions of ships and floating structures in a wave basin or towing tank (Miles, 1986). The system has since undergone extensive modifications to permit its use for estimating the force of ship-ice impacts.

The first full-scale deployment of MOTAN was undertaken in the year 2000 in order to determine the viability of using full-scale ship motions to back-calculate ice-induced global impact forces. Continued improvements have been made to the MOTAN system over the past 10 years in order to provide better estimates of global ice impact forces on ships. Johnston (2006) provides an overview of the ice-strengthened ships on which MOTAN has been installed and how experience from each of those installations lead to an improved system. Johnston et al. (2008-a, 2008-b) provide the most in-depth discussion of MOTAN to date, along with a comparison of how the MOTAN-derived forces on the CCGS Terry Fox compare to two other, independently operated force measurement systems. That full-scale study of the CCGS Terry Fox represents the most comprehensive validation study performed on MOTAN to date.

2.1 MOTAN physical sensor

The MOTAN system consists of two parts (1) a physical sensor to measure ship motions in six degrees of freedom and (2) specially developed software to calculate first, the whole ship motions and second, the global exciting forces and moments, as shown in Figure 1. The physical MOTAN sensor uses three accelerometers and three rotational rate sensors to measure respectively the ship’s total acceleration, including earth’s gravity component, and its three-dimensional angular rotational rates, each of which is resolved along the instantaneous positions of the X, Y and Z body-axes of the ship. The six analog voltage signals from MOTAN are recorded by a standard data acquisition system. MOTAN is equipped with a low-pass hardware filter to remove vibrations higher than 5 Hz, since those frequencies are not characteristic of the ship’s global response to impacts.

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Specially developed software called MOTAN103 is used to solve the nonlinear equations of motion relating the 18 motion components (see Figure 1) to the body-axes accelerations and rotational rates from the six MOTAN inertial motion sensors. In order to use the whole ship motions to calculate an impact force for the rigid body, the 18 motion components must be referenced to the ship’s centre of gravity. That, in turn, requires knowing the distance from MOTAN to the vertical centre of gravity (VCG), longitudinal centre of gravity (LCG) and the ship’s centerline. The distance of the MOTAN sensor from a known reference point was obtained while installing MOTAN on the CCGS Amundsen in June 2011 (D. Pelletier, personal communication) and then used to determine MOTAN’s position from the VCG, LCG and centreline (Figure 2). The VCG and LCG were obtained from the ship’s stability report, which was provided by the Commanding Officer of the CCGS Amundsen, Captain Thibault, for “Condition 6: Navigation en glace hivernale” since it was considered to be most representative of the ship’s operating condition in August 2011 (Captain Thibault, personal communication). The ship’s particulars for Condition 6 are listed in Table 1.

MOTAN

Rate Acceleration

Physical Sensor:

3 accelerometers & 3 angular rate sensors dimensions: 260 mm x 160 mm x 100 mm weight: 1.88 kg Computer Software Displacement Surge x x x Sway y y y Heave z z z Pitch θ θ θ Roll φ φ φ Yaw ψ ψ ψ x ’ y’ z’ Heave Pitch Roll Yaw Sway Surge

arrows used to show coordinate system and (positive) sign convention

MOTAN10 Whole-ship Motions (displacements,rates and accelerations)

EFM Global Impact Forces (determined using output

from MOTAN10)

a b

Figure 1 Schematic of MOTAN: sensor and software

3_{MOTAN10 was developed in 2008 in preparation for the Transport Canada study “Long-term Monitoring of}

Global Loads on the CCGS Louis S. St-Laurent”. MOTAN10 is an improved version of the previous software for calculating the whole-ship motions of full-scale ships because it removes the spurious oscillations in surge (prior to an impact event) introduced by data processing.

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Table 1 Particulars of CCGS Amundsen

Design Particularsa

Length overall 98.33 m

Beam 19.51 m

Total Power 10 MW

Particulars for Case 6b:

Displacement, Condition 6 7105 t

Draft at amidships 6.43 m

LCG, longitudinal centre of gravity, aft of amidships 0.16 m

VCG, vertical center of gravity, above keel 7.24 m

a

design particulars obtained from Canadian Coast Guard website

b

from stability booklet provided by Captain Thibault, Commanding Officer CCGS Amundsen

6.43 m, draft at midship ice knife approximately 8 m aft of FP centreline MOTAN sensor + + centre of gravity 87.93 m 6.0 m 4.9 m 7.24 m 1.1 m aft perpendicular (AP) forward perpendicular (FP) AP FP 4.9 m fwd of LCG 6.0 m above VCG 1.1 m stbd. of centreline 4.9 m

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The MOTAN software EFM, for Exciting Forces and Moments (Figure 1), is key to using the inertial measurement system to calculate ice impact forces on ships. In effect, the software uses the entire suite of whole ship motions and information about the ship’s particulars, to back-calculate the force that would have been required to produce the measured ship’s response. EFM uses the six linear coupled differential equations included in Salvesen et al. (1970) and McTaggert (1997) to calculate three global exciting forces and three global exciting moments at the ship’s origin. Calculating the exciting forces and moments with EFM requires information about the ship’s characteristics, as well as its hydrodynamic coefficients and hydrostatic coefficients (McTaggert, 1997). These coefficients were developed for the CCGS Amundsen for “Condition 6: Navigation en glace hivernale” for a wide range of frequencies at operational ship speeds from 1 to 12 kt (0.5 to 6.2 m/s).

EFM provides two options for calculating the resultant global impact force on the ship (see Figure 3). The two approaches are (1) the SO Approach and (2) the POI Approach. The POI Approach can be used to calculate the resultant force at the point of impact (POI) provided the impact location is known with reasonable accuracy and that the contact area covers a relatively small area. If the impact location is not known with sufficient accuracy or if the contact area is spans a larger area, global impact forces should be calculated at the ship’s origin (SO) using the SO Approach. The two approaches are described below.

2.3.1 The SO Approach to Calculating Global Impact Forces

The SO Approach calculates the resultant global impact force at the ship’s origin (SO) from the three global exciting forces (F1, F2 and F3) output by EFM. Equation (2) shows the force

components that are taken into account when using the SO Approach to calculate the resultant global impact force.

2 3 2 2 2 1 F F F + + = SO at force Resultant (2) where;

F1 = longitudinal force component, surge force

F2 = lateral force component, sway force

F3 = vertical force component, heave force

2.3.2 The POI Approach to Calculating Global Impact Forces

EFM provides a second approach for calculating a global resultant force when the force can be assumed to act at a single point – the point of impact (POI) - rather than over a large contact area. The so-called POI Approach calculates a resultant global impact force from the global exciting force in surge (F1), the global exciting moment in pitch (F5) and the global exciting

moment in yaw (F6). The vertical (Fz_pitch) and lateral (Fy_yaw) force components are obtained by dividing respectively the global exciting moments in pitch (F5) and yaw (F6) by the longitudinal

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see Figure 3). Equation 3 shows the three force components that are needed to calculate the resultant global impact force using the POI Approach.

2 _ 2 _ 2 1 (Fz pitch) (Fy yaw) F POI at force Resultant = + + (3) where;

F1 = longitudinal force component, surge force

Fz_pitch = vertical force component, F5 /Xab

Fy_yaw = lateral force component, F6 /Xab

The longitudinal moment arm, Xab, can be determined accurately only if the impact force is assumed to act at a single point and if the impact location can be determined with reasonable accuracy. The most-straightforward case for determining the POI occurs for symmetrical (head-on) ship impacts that have a relatively small contact area (i.e. produce a small bow print in the ice). In that case, the impact location can be approximated as a single point that is assumed to act at the intersection of the ship’s waterline and the forward perpendicular (Xab of 43 m – one half the distance between the FP and AP for the CCGS Amundsen, see Figure 3).

ship’s origin (SO) ship’s origin (SO) x-axis z-axis y-axis Yaw moment F₆ Pitch moment F5 F₁ F_{y_yaw} F_{z_pitch} x-axis point of impact (POI) point of impact (POI) x-axis ship’s origin (SO) y-axis z-axis Surge Force F₁ Sway _Force F₂ Heave Force F₃ Pitch moment, F₅ Roll moment, F₄ Yaw moment, F₆ Xab Xab 87.93 m

Figure 3 Approaches used to calculate resultant forces (1) SO Approach and (2) POI Approach (AP aft perpendicular, FP forward perpendicular)

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CCGS Terry Fox Bergy Bit Trials showed that the SO Approach was conservative (produced higher forces) than the POI Approach (Johnston et al., 2008-b). Resultant global forces for the 51 examined oblique impacts resulted in forces from 0.9 to 10.6 MN for SO Approach and from 0.5 to 6.7 MN for the POI Approach. For the Bergy Bit Study, the POI Approach was preferred method of calculating impact forces because (1) the ship-ice contact area was relatively small, (2) global forces were calculated from first-hand knowledge of the impact location, which was well-documented and (3) results from the POI Approach were in better agreement with independent force measurements from the strain gauge technique, which was fully expected to have provided a reliable estimate of the impact force.

In this report, multi-year ice impacts with the CCGS Amundsen produced substantially larger contact areas than generated by the CCGS Terry Fox during the Bergy Bit Study. Therefore, global impact forces were calculated here using the SO Approach because the bow prints left after each ram clearly showed that the ship-ice contact area cannot be approximated by a single point, as it was during the Bergy Bit Trials. As expected, approximating substantial contact areas as a single point that was not well known in position caused the POI Approach to yield unacceptably low global forces for the CCGS Amundsen rams in comparison to the SO Approach. A full discussion of the comparison of results from the SO Approach and POI Approach is beyond the scope of this report, but it is fully expected that this subject will be discussed in a future publication based upon the knowledge gained during this study.

The reader is advised to keep in mind that the global impact forces cited in this report are best estimates of the forces generated by collisions with multi-year ice. The uncertainty in the forces calculated by MOTAN will remain unresolved until additional validation studies are conducted; such a study would require using MOTAN and at least one other independent instrumentation system to measure global impact forces from impacts with, preferably, thick multi-year ice. The time-series traces of the raw and processed accelerations for each of the impacts discussed in this report are presented in the Appendices. It should be noted that the raw accelerations are different than the processed accelerations for the following reasons: (1) the raw accelerations were measured where MOTAN was located whereas the processed accelerations characterize ship motions at the centre of gravity, (2) the raw accelerations include earth’s gravity component and (3) the raw accelerations do not reflect the cross-coupling that naturally occurs in six degrees of freedom (see Johnston et al., 2004).

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3.0 Voyage of CCGS Amundsen during Test Program

The objective of this Joint Industry Participant (JIP) field program was to measure the forces on the ship from impacts with floes of known thickness and strength. The investigation took place during Leg 2A (11 to 25 August 2011) during operations at the eastern edge of the Beaufort Sea pack ice. The concentration of old ice in this area ranged from 4 to 6/10ths, with lesser amounts of thick first-year ice and/or open water. The total ice concentration in this area ranged from 7 to 9/10ths. Initially, it was fully expected to find multi-year ice floes of suitable thickness and integrity in the area designated as “Box 1” (Figure 4), which had been previously selected from satellite imagery by the JIPs onboard the vessel.

The ship itself was used to assess the integrity of the ice in Box 1: a floe was considered unsatisfactory if it split upon impact, if the ice cusps overturned by the ship were too thin and/or the ice presented minimal resistance to the ship. Since none of the multi-year floes in Box 1 were considered satisfactory, it was decided to proceed 200 km north, where satellite imagery suggested more substantial floes existed. Indeed, the two multi-year floes that were sampled during the field program were located about 100 to 150 km from the northwest coast of Banks Island (Figure 4). Floes B1S1 and B1S2 were sampled from 15 August to 18 August.

Floe B1S1 (15, 16 Aug) Floe B1S2 (17, 18 Aug) Banks Island Viscount Melville Sound Beaufort Sea Prince Patrick Isl Sachs Harbour 10/10ths Old Ice Concentration 9 to 9/10ths 7 to 8/10ths 4 to 6/10ths 1 to 3/10ths <1/10th “Box 1”

Figure 4 Location of sampled multi-year ice floes

“Box 1” shows the area that was initially selected for measurements. Only the old ice concentration for 15 August is shown.

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equipment had returned from Floe B1S2. The incident was deemed to have placed the health and safety aspect of the on-ice program in jeopardy, which effectively terminated on-ice activities. On 20 August the ship departed the sampling area and sailed approximately 150 km east to Banks Island; it was thought prudent for the ship to take shelter from the storm that was taking shape in the Beaufort Sea. The ship remained off the coast of Banks Island until the storm passed, then departed for the region where the two floes had been sampled, to observe the effect of the storm on the ice conditions. The ship departed this area on 24 August for the journey to Sachs Harbour, where scientific personnel departed the ship.

4.0 Floe B1S1

On 14 August, the ship arrived at Floe B1S1 (74°51.3'N, 128°17.7’W). This floe was a massive fragment of a landfast hummock field (10 x 30 km, Figure 5) that had dislodged from coast of Prince Patrick Island in early July 2011. Satellite imagery had been used to track Floe B1S1 as it drifted from Prince Patrick Island to where it was encountered by the CCGS Amundsen, 250 km south. A helicopter reconnaissance was conducted to determine the region of Floe B1S1 best suited for on-ice measurements, to estimate the ice thickness with a helicopter-based EM (HEM) system and to suggest where the ship could penetrate the floe without too much difficulty.

B1S1

Figure 5 Floe B1S1 as identified from satellite imagery (courtesy of Canadian Ice Service)

While the helicopter was out, the ship approached Floe B1S1 from what was mostly open water to the east (Figure 5). Skirting along the edge of the floe provided an excellent opportunity to observe that the floe had an extensive amount of recently rubbled ice and that some regions of the floe’s keel were quite porous. When the helicopter returned, the Captain was informed that the aggregate floe of multi-year ice and deteriorated first-year/second-year ice was about 5 m thick, based upon HEM measurements. The ship then proceeded along the southern edge of Floe B1S1 until reaching an area where a relatively straight transit could be made to an open water

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lead inside the floe, roughly 1 km from the floe’s edge. The intention was to use that lead as a natural harbour to secure the ship so that the on-ice sampling area could be accessed directly from the ship.

On 14 August, a total of 11 rams were needed to cover the 1 km distance from the floe edge to the natural harbour. On-ice measurements were conducted on 15, 16 August after which 10 rams were conducted with the ice between drill-hole Transects 2 and 4 (see Figure 6), where the ice thickness and strength had been well documented.

1 2

3

4 H1

H2

Figure 6 Aerial perspective of ship secured in natural harbour of Floe B1S1

The approximate location of the four drill-hole transects and the location of the two boreholes (H1, H2) where property measurements were made is shown. Photo courtesy of K. Hochheim.

4.1 Floe B1S1: Ramming Needed to Arrive at Natural Harbour

A considerable amount of icebreaking was needed to reach the natural harbour of Floe B1S1. The CCGS Amundsen proceeded to force its way into Floe B1S1 for about one hour, during which time it conducted 11 ramming cycles. Figure 7 shows the ship track that was made reaching the natural harbour. Often, the ship would be directed into the same bow print that was left by the previous ram. The offset in the ship track for successive rams resulted because the floe was drifting, generally to the southeast, which explains why the individual rams do not appear to overlap.

Each ramming cycle lasting several minutes, on average (see Figure 8). The forward portion of the ramming cycle lasted about one minute, as the ship approached the floe from about a ship length or so, to impact the ice at a controlled speed. The reverse portion of the ramming cycle took about two minutes, as the ship backed off the floe (or slid off the floe, in some cases) and

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peak impact force about 30 seconds before the ship reached its maximum speed (Figure 8). A similar effect has been noted in records from MOTAN installations on other ships. A phase lag is to be expected because the ship will take time to slow due to its forward momentum, but the problem was exacerbated by the ship’s GPS system and the MOTAN system having different timestamps. Data logged by two independent systems are extremely difficult to synchronize in time – this problem has plagued many ship-based research projects. For the CCGS Amundsen trials, the MOTAN system had been designed so that the timestamp from the ship’s GPS could be written to the actual MOTAN file however, upon boarding the ship in August, it was found that the GPS feed for the MOTAN system had been removed. At that point, the only recourse was to obtain the ship’s speed from GPS files that were logged independently of MOTAN. Since the GPS recorded data at a sampling rate of only 1Hz (1 s) and was subsequently averaged over 5 seconds to remove data spikes, the ship speed can related to the MOTAN data in general terms only; MOTAN data are recorded at a sampling rate of 20Hz (0.05 s).

Table 2 lists the processed translational ship accelerations (surge, sway and heave accelerations in m/s²) and rotational displacements (roll, pitch and yaw in degrees) for the ship’s centre of gravity. Following is a short description of the rams that were needed to reach the natural harbour. Appendix B provides detailed records of the accelerations, forces and ship speed on a ram-by-ram basis. The bow prints left by each ram are shown in Figure 9 (Rams 1 to 6) and Figure 10 (Rams 7 to 11) because they clearly illustrate the damage zone and the penetration distance that was gained for each ram. The ice failed in crushing in most cases, with some evidence of flexural failure and floe splitting.

Floe B1S1:

rams required to reach natural harbour

Ram #1 10 11 8 9 7 6 5 3 4 2 general direction of floe drift

Figure 7 Ship track of rams needed to reach natural harbour of Floe B1S1, 14 August (floe was drifting to southeast)

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Table 2 Rams Approaching Natural Harbour of Floe B1S1: Global Forces and Ship Motions Impact Time of impact (s) Global impact force (MN) Max. speed during ram (kt) Surge accel (m/s²) Heave accel (m/s²) Sway accel (m/s²) Roll (deg) Pitch (deg) Yaw (deg) 1 13097 6.3 7.5 0.32 0.17 0.20 3.2 0.7 0.7 2 13308 7.8 8.4 0.36 0.47 0.38 1.8 1.4 0.9 3 13522 5.0 7.4 0.49 0.21 0.19 1.9 0.8 1.3 4* 13747 5.7 7.5 0.64 0.29 0.20 1.3 1.0 1.0 5 13950 7.4 8.7 0.84 0.29 0.23 2.9 1.9 0.9 6 14100 4.8 7.8 0.45 0.24 0.18 3.4 0.7 0.3 7 14313 7.4 9.3 0.32 0.19 0.46 3.1 0.7 1.3 8 14482 7.5 8.2 0.30 0.32 0.33 2.1 0.5 1.4 9 14765 3.4 6.3 0.27 0.17 0.21 3.4 0.9 0.6 10 14947 4.8 6.0 0.20 0.18 0.21 0.8 0.4 0.4 11 15177 3.8 5.8 0.12 0.16 0.24 2.1 0.6 0.7

* surge acceleration record in Appendix B indicates ice knife contacted floe

Ram 1: Excellent contact was made between the ship and the ice during Ram 1 as the ship impacted the floe in the bow print made by the previous ram4. The 7.5 kt impact speed produced a peak impact force of 6.3 MN (Figure 8). The “V” like notch in the bow print that Ram 1 produced likely occurred because the ship’s ice knife5 struck the floe edge (Figure 9). The surge acceleration record in Appendix B does not reveal the rapid ship deceleration that often occurs when the ice knife contacts multi-year ice, which suggests that the ice penetrated by the ship during this ram was not hard enough to cause the ship to decelerate abruptly (as happened during Ram 4 of this series, see below). MOTAN registered considerable deceleration during the impact (0.32 m/s² surge acceleration), 0.20 m/s² of sway acceleration and 3.2 degrees of roll. Ram 2: This ram generated the highest force of the 11 rams needed to reach the natural harbour. The 8.4 kt ram produced a global impact force of 7.8 MN. The bow print clearly indicates that the ice knife contacted the floe (Figure 9). The blue colour of the crushed debris floating in the bow print confirmed that Floe B1S1 was indeed multi-year ice, which can be characterized by a wide range of surface conditions (Johnston and Timco, 2008-c; Johnston et al., 2009). Bridge observations noted that Ram 2 produced considerable ride-up and roll, which the substantial accelerations along each of the ship’s three axes confirmed: surge deceleration (0.36 m/s²), heave acceleration (0.47 m/s²) and sway acceleration (0.38 m/s²).

4_{Although referred to as Ram #1 in this report, the first ram with Floe B1S1 actually occurred immediately prior to}

this impact. It was not described here because the author was not on the bridge at the time. 5

the ice knife is used to avoid excessive beaching, submersion of the deck aft, and to promote failure of the ice. The ice knife on the CCGS Amundsen is about 8 m aft of the forward perpendicular (Figure 2), so is almost certain to have contacted the ice during some of the collisions (Captain Julien, personal communication).

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significant (0.49 m/s²), with lesser accelerations in heave and sway (0.21 m/s² and 0.19 m/s² respectively). Bridge observations noted that the impact caused substantial ride-up (pitch) but MOTAN registered higher rotations in roll (1.9 degrees) and yaw (1.3 degrees) than pitch (0.8 degrees). Ram 3 was significant enough for the author to use the hand-held device on the bridge to mark the event with a “double-trigger”, as shown by the two vertical lines in Figure 8. The ship penetrated the floe by about 60 m by the end of the third ram, achieving a distance of roughly 20 m with each impact.

Ram 4: The ship did not progress as far during Rams 4 to 7 as it had during the previous three rams, likely because of the ridged feature on the ship’s starboard side (Figure 9, ram 4). This 7.5 kt ram caused a global impact force of 5.7 MN. The surge acceleration record in Appendix B shows that the ship decelerated (in surge) from 0.64 m/s² to near zero in just 0.3 seconds. This extremely rapid deceleration is believed to have been caused by the ice knife contacting the floe. It was the first (and only) ram of the series to show such a rapid deceleration in surge.

Ram 5: The 8.7 kt speed of this ram produced an impact force of 7.4 MN, the highest surge deceleration of this series (0.84 m/s²) and significant rotations in roll (2.9 degrees) and pitch (1.9 degrees). The hand-held device on the bridge was used to mark the event as a “triple trigger” as shown by the three vertical lines in Figure 8. The dramatic ship response was caused when the ship was deflected by the ridged feature on its starboard side (Figure 9). The impact generated a crack on the ship’s port side. Ram 5 caused substantially higher pitch (1.9 degrees) than any of the other rams in this series (see Table 2).

Ram 6: This 7.8 kt ram generated an impact force of 4.8 MN and 3.4 degrees of roll. The ship impacted the floe, rode-up onto the ridge on the ship’s starboard side and then was deflected to port, inducing a major roll response in the ship. The bow print left by Ram 6 (Figure 9) contained a layer of crushed, compacted ice on the ship’s starboard side – similar layers of pulverized ice were observed in the bow prints during the CCGS Terry Fox Bergy Bit Trials (Johnston et al., 2008-b). The ship response during Ram 6 was significant enough to warrant the hand-held trigger be depressed four times (vertical lines in Figure 8). The acceleration record in Appendix B shows that MOTAN measured a maximum (raw) transverse acceleration of 2.1 m/s² where it was installed 6 m above the ship’s CG (Figure 2), which is comparable to the raw transverse accelerations that were measured on ships operating aggressively in the Beaufort Sea in the 1980s6. Note that the transverse sway acceleration on the CCGS Amundsen took place over a 5 second interval (Appendix B), but it is not known how quickly the transverse accelerations changed on the other ships.

Ram 7: This 9.3 kt ram provided the ship with enough momentum to push beyond the ridged ice, towards a more benign area of the floe (Figure 9). Ram 7 produced a global impact force of 7.4 MN. Several smaller forces occurred during the run-up to the peak force of Ram 7. Each of the impacts was marked by a single trigger (vertical lines in Appendix B). The bow print generated by Ram 7 indicates that the ice on the port side of the ship failed in flexure (Figure 10) and that the impact produced a large crack in the ice on the port side of the bow.

6_{The most significant transverse accelerations of comparable displacement ships operating in the Beaufort Sea was}

0.19 g’s on the Kigoriak, 0.14 g’s on the MV Robert Lemeur and 0.15 g’s on the MV Kalvik, at amidships (Browne, 2006). The author reported raw accelerations, as opposed to the processed accelerations calculated by MOTAN which are more indicative of the ship motions along the six degrees of freedom.

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Ram 8: Since Ram 7 had cleared the way for the ship to enter an easier area ice, it is not surprising that Ram 8 resulted in greater penetration than any of the preceding runs. Roughly 160 m of ice was transited during Ram 8, as opposed to the 20 to 60 m of penetration gained by each of the previous rams (see ship track in Figure 7). The 8.2 kt ram caused two closely-spaced impact forces: a 7.5 MN force, followed by a lesser force three seconds later. After the secondary impact, the ship entered a fissure in the ice until it again contacted ice that caused a series of small impacts. The ship then reversed in preparation for the final few rams that were needed to reach the natural harbour.

Ram 9: The 6.3 kt speed of this ram caused an impact force of 3.4 MN and 3.4 degrees of roll – a roll response rivaled only by Ram 6. Observations from the bridge noted that Ram 9 produced significant ship motions.

Rams 10 and 11: The portion of Floe B1S1 that was impacted during the final two rams was more weathered-looking than most of the relatively level ice transited during the previous nine rams (see Figure 10). The surface topography and freeboard of the ice in this area were comparable to the area of Floe B1S1 that would be sampled during the subsequent two days of on-ice measurements (as discussed below). The ice in this area of the floe may have been more competent, but the global impact forces from the final few rams were lower than the previous rams (4.8 MN and 3.8 MN) likely because the close proximity of open water (the natural harbour) permitted fissures to propagate through the ice (ice fragments could move away from the ship) which would have reduced the impact force.

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0 2 4 6 8 10 14200 14400 14600 14800 15000 15200 15400

Elapsed time (sec)

G lobal F or c e ( M N ) an d S hi p s pee d ( k t)

global force ship speed trigger

7 8 9 10 11 0 2 4 6 8 10 13000 13200 13400 13600 13800 14000 14200

Elapsed time (sec)

G lob al For c e ( M N ) and S hi p s p eed (k t)

1 2 3 4 5 6

Figure 8 Global force and ship speed for rams needed to reach natural harbour in Floe B1S1 (a) rams 1 to 6 and (b) rams 7 to 11. Note: ship speed averaged over a 5 second interval, vertical lines

show event markers from trigger.

(a)

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4.2 Property Measurements of Floe B1S1

Floe B1S1 was sampled for two days (15, 16 August). On Day 1, the NRC-CHC field party of four used the drill-hole technique7 to measure the full thickness of ice at 10 holes along Transect 1 and then focused upon measuring the temperature, salinity and strength of the ice in boreholes H1 and H2 (see Figure 6). Ice cores from the first borehole (H1) were used to measure the ice temperature and salinity, after which a series of borehole strength tests were conducted. Borehole H2 was merely augured (no cores) to provide a hole in which to conduct borehole strength tests. Although it was initially intended to measure the properties in a level area of ice and a deformed area of ice, that was not possible because it would have required man-hauling two very heavy sleds of equipment across rugged terrain8. As the photograph in Figure 11 shows, Floe B1S1 was considerably more imposing from the ice than from the air, as is usually the case. On Day 2, drill-hole thicknesses were conducted towards the stern of the ship, where the Commanding Officer had decided to impact the floe. Transects 2 and 4 were made perpendicular to the floe edge and Transect 3 was made parallel to the floe edge, as shown in Figure 6.

4.2.1 Thickness of Floe B1S1 from drill-hole transects

The ice thickness of the 49 drill holes on Floe B1S1 ranged from 3.6 to 9.3 m. The average thicknesses of the four transects was comparable: 6.3 to 7.7 m, with standard deviations of ±0.6 to 1.5 m (see Table 3). The ice along Transect 1 was the most uniform in thickness (±0.6 m) and Transect 3 had the greatest variability in thickness (±1.5 m).

Figure 11 Topography of Floe B1S1 from the ice surface

7

Appendix A describes how the thickness, temperature, salinity and strength of the ice was measured.

8

A snow machine had been rented to haul NRC-CHC equipment over hummocked multi-year ice but it was not removed from its shipping location (on top of one of the cargo modules) until after Floe B1S1 had been sampled.

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Table 3 Summary of thicknesses along drill-hole transects on Floe B1S1 No. of drill holes Length of transect (m) Average thickness (m) Maximum thickness (m) Minimum thickness (m) Transect 1 10 90 7.5 ± 0.6 8.5 6.9 Transect 2 9 80 7.7 ± 1.2 9.3 5.8 Transect 3 9 80 6.3 ± 1.5 7.6 3.6 Transect 4 19 190 7.1 ± 1.3 8.9 4.8 Floe B1S1 49 7.2 ± 1.4 9.3 3.6

4.2.2 Temperature, Salinity and Strength of Floe B1S1

Coring the 7.2 m thick ice at H1 was an interesting experience. The uppermost three metres of ice produced a solid core that, although warm and saturated, had integrity. However, a very slushy mixture, intermixed with a few larger fragments of ice, was all that could be captured with the core barrel below the 3 m depth. This was not entirely unexpected because many of the drill-holes made earlier that morning along Transect 1 revealed soft ice that presented little resistance to drilling. Although the 3 m long core from H1 did not contain voids – nor were voids felt at that location when drilling for thickness – voids had been encountered in many of the drill holes along Transect 1, beginning at an ice depth of 2.2 m and occurring intermittently until the bottom of the floe had been reached.

Temperature: Since ice temperature measurements can only be made on a solid, continuous ice core, the temperature profile of H1 was limited to the uppermost 3 m of ice. Below that depth, it was not possible to obtain a solid core. The temperature of the top ice surface was 0°C and gradually decreased to a minimum of -1.7°C at the 3 m depth (Figure 12-a). The average temperature of the uppermost 3 m of ice was -0.9°C.

Salinity: Ice salinity measurements also require retrieving ice cores, but even slushy, unconsolidated samples can be used, since the samples are melted. However, it is difficult (sometimes impossible) to determine the depth to which the loosely consolidated samples correspond. The salinity profile in Figure 12-b extends to the 3.8 m depth, rather than terminating at the 3 m depth like the temperature profile. The salinity of the floe in the uppermost 3.8 m ranged from 0 to 3.1‰. The average salinity of that layer was 1.8‰. The low-salinity layer extended only to a depth of 0.20 m, which was just part way through the 0.6 m ice freeboard at H1. Note also that the ice salinity was quite high for a floe that originated from a landfast hummock field off Prince Patrick Island - traditionally, landfast multi-year hummock fields are expected to be quite old, and therefore would be expected to have a low salinity layer several metres thick, at least. Floe B1S1 may have originated from the coast of Prince Patrick Island, but property measurements suggest that the ice was not very old (or consolidated).

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Strength: Results from the borehole strength tests speak volumes about the integrity of Floe B1S1: the ice at boreholes H1 and H2 was 7 m thick but only the uppermost several metres had ‘respectable’ strength (Figure 12-c). Here, ‘respectable’ strength is loosely defined as a peak borehole ice pressure greater than 5 MPa. Strengths in the uppermost three metres of ice from Floe B1S1 ranged 7.3 to 15.6 MPa whereas the bottommost 4 m of ice consistently returned peak ice pressures of 5.5 MPa, or less. To provide a context for these measurements, Johnston and Timco (2008-b) showed that 1.3 m thick first-year ice in early July (at 74°N) had a depth-averaged strength of approximately 9 MPa. In comparison, the 7 m thick ice on Floe B1S1 had a depth-averaged strength of 7.3 MPa and 4.5 MPa.

Max ice pressure (MPa)

-8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 0 5 10 15 20 Hole 2 Hole 1 Hole 1 = 7.2 m Hole 2 = 7.3 m -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 0 1 2 3 4 Ice salinity (‰) H1 = 7.2 m thick Hole 1 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 -2 -1 0 1 2 Ice temperature (C) Ice dep th ( m ) H1 = 7.2 m thick Hole 1

Figure 12 Temperature, salinity and strength profiles of Floe B1S1

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4.3 Floe B1S1: Controlled Impacts with Sampling Area

The second series of rams with Floe B1S1 involved the region of ice between drill-hole transects 2 and 4 (Figure 6). The rams were conducted on 16 August, upon concluding the second day of on-ice measurements. To assist the Commanding Officer when aiming for the sampling area, each of the drill-holes had been marked with biodegradable, orange fluorescent paint before departing the floe. A rough, generalized map of the thicknesses along the transects was shown to the Commanding Officer so that he was aware of the thickness of ice that he would penetrate. Figure 13 shows the ship track that was made during this series of rams. The rams were conducted at approach speeds of 5.2 to 10.2 kt – the widest possible range of speeds still within the Commanding Officer’s comfort level, given the ice thickness. The rams were conducted in a controlled manner, whereby the approach speed was gradually increased based upon the ship’s response to the previous ram. Sometimes, the Commanding Officer would overshoot his intended approach speed, which is entirely understandable given the difficulty of directing a 7100 t ship towards a targeted area of ice, at a specific speed. In general, the ship gained about 20 to 40 m penetration during each ram. Table 4 summarizes the global impact forces, translational accelerations and rotational displacements associated for the rams. The plot in Figure 14 shows the global forces that were measured during this ramming series. The reader is referred to Appendix C for more detailed records of the accelerations and forces for each ram.

Table 4 Rams with Sampling Area of Floe B1S1: Global Forces and Ship Motions

Impact Time of impact (s) Global impact force (MN) Max speed during impact (kt) Surge accel (m/s²) Heave accel (m/s²) Sway accel (m/s²) Roll (deg) Pitch (deg) Yaw (deg) 1 5795 6.5 5.6 0.20 0.28 0.25 1.5 0.9 0.6 2 5928 5.8 5.2 0.34 0.38 0.27 1.3 1.3 0.5 3 6110 4.1 6.8 0.36 0.17 0.23 0.8 0.9 0.6 4* 6265 5.8 6.0 0.65 0.22 0.17 0.8 1.0 1.6 5* 6469 4.6 7.1 0.51 0.14 0.13 0.9 0.5 0.3 6 6636 2.7 7.2 0.26 0.09 0.09 0.4 0.3 0.3 7 6822 4.6 7.8 0.24 0.23 0.16 1.2 0.6 0.8 8 7044 5.0 7.7 0.13 0.18 0.24 1.7 0.3 1.5 9** 7857 1.9 5.4 0.16 0.08 0.04 0.3 0.4 0.1 10 8150 6.3 8.8 0.50 0.36 0.31 1.9 1.6 1.5 11* 8430 6.1 10.2 0.49 0.37 0.28 1.4 1.1 0.7

* the surge acceleration record in Appendix C indicates that ice knife contacted the floe ** impact 9 occurred while ship was maneuvering into position, not from a ram

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Floe B1S1:

impacts with sampling area Ram #1

11 8 10 7 6 5 3 4 2 general direction of floe drift

Figure 13 Ship track showing rams with sampling area of Floe B1S1, 16 August

(since the ship speed was reported at a 1 second interval, the data markers are more widely spaced during the advance portion of the ram - which took place at a faster speed)

Ram 1: The Commanding Officer did a remarkable job aligning the ship with the sampling area, even though the orange drill-hole markers were barely visible from the bridge. The aiming process was complicated further by the ship’s approach distance being severely restricted for the first few rams – the ship could not back up too far because the natural harbour was, at most, one and a half ship lengths wide at that location (see Figure 6). Ram 1 drove the ship into an area that was only about 40 m away from Transect 4, where the ice was 5 to 8 m thick. The collision made a very distinct notch in the ice. The 5.6 kt ram produced three distinct peaks in the force record (see Appendix C). The first impact generated the highest force (6.5 MN), followed by two smaller peaks. The impact produced maximum accelerations of 0.20 m/s² in surge, 0.28 m/s² in heave and 0.25 m/s² in sway.

Ram 2: This 5.2 kt ram produced a maximum force of 5.8 MN (Figure 14). The impact resulted in a heave acceleration of 0.38 m/s², surge acceleration of 0.34 m/s² and sway acceleration of 0.27 m/s². The impact was noteworthy because it also produced 1.3 degrees of roll and pitch, with a lesser amount of yaw (0.4 degrees).

Ram 3: The Commanding Officer thought that the ice encountered during Ram 3 was more solid than the previous two rams. That may have been because the impact resulted in a surge deceleration (0.36 m/s²) that was higher than the previous two rams. The maximum impact force was, however, less than the first two rams (4.1 MN), despite the collision being conducted at a higher impact speed (6.8 kt). By the end of the third ram the ship had penetrated the floe by about 50 m. The orange paint marks along Transect 3 could be seen about 50 m in front of the ship.

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Ram 4: The fourth ram, which was conducted at 6.0 kt, generated an impact force of 5.8 MN and caused the greatest surge deceleration of this series of rams (0.65 m/s²). The detailed records in Appendix C show that the surge acceleration rapidly decreased from 0.65 m/s² to near zero as the ice knife contacted the floe. Ram 4 produced the highest ship response in yaw (1.6 degrees) of this series of rams. The bow print showed a well-defined notch and a layer of pulverized ice extending about 10 m along the port side of the imprint (Figure 15). The collision caused a fissure to form in the ice off the ship’s port side.

Ram 5: This 7.1 kt ram generated a peak force of 4.6 MN. As the ice knife came up hard against the floe, the surge deceleration rapidly decreased from a maximum of 0.51 m/s² to near zero. The notch in the bow print in Figure 15 confirmed that the knife did indeed contact the floe, as did the Commanding Officer’s descriptive comment “oof” as the ship came up solid against the floe.

Ram 6: This ram was unremarkable because the 7.2 kt impact speed produced a force of just 2.7 MN (Figure 16). The ship-ice interaction resulted in 0.26 m/s² of surge deceleration, but minimal accelerations in heave and sway. Less than 0.4 degrees of rotation was experienced in roll, pitch and yaw. Ram 6 drove the ship up to the 7 to 8m thick ice along Transect 3, which was roughly 100 m from the edge of the floe.

Ram 7: Since the ship had severed a huge ice wedge from the floe during the previous ram, the next few rams took a slightly different approach angle. Ram 7 was directed against a fresh area of ice towards the end of Transect 2, where the ice was 8 m thick. The 7.8 kt ram caused a double peak in the force record, with a force of 4.6 MN being the higher to the two peaks (see Appendix C). The impact produced 1.2 degrees roll, likely because there was open water on the ship’s port side.

Ram 8: This 7.7 kt impact generated a global force of 5.0 MN. The floe was impacted in the same notch that was created during the previous ram. The collisions produced a hard hit on the ship’s starboard side, and then the ship veered to port. Ram 8 caused a considerable amount of roll and yaw (1.7 and 1.5 degrees respectively) again because open water existed on the ship’s port side.

Nudge 9: This event was a “nudge” (1.9 MN) that occurred when the ship contacted the edge of the floe with its port side aligning for the next run. The nudge came from the pie-shaped area of ice on the ship’s port side (Figure 16). The photograph also shows several of the orange paint marks from drill-hole Transect 4. The glancing impact did not register much of a response with MOTAN, nor was it expected to, since the ship was merely maneuvering for the next impact. Ram 10: This ram was significant enough to warrant four triggers from the hand-held device on the bridge. The ship impacted an area of Transect 2 where the average ice thickness was 7.1 m (±1.3 m) and the ice freeboard ranged from 0.5 to 0.9 m (see the two orange paint markers in Figure 16). The same notch in the ice that was attacked during Rams 7 and 8 was impacted at a speed of 8.8 kt, producing a maximum force of 6.3 MN. Significant accelerations occurred along all three of the ship’s axes during the ram: 0.50 m/s² in surge, 0.36 m/s² in heave and 0.31 m/s² in sway. The ram also caused the highest ship response in pitch (1.6 degrees) and roll (1.9 degrees) of this series of rams.

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Ram 11: The final ram, by which time the ship had penetrated the floe about two to three ship lengths, was significant enough that it caused the Commanding Officer to later explain that he thought the ship might have been (temporarily) beached on the floe – thankfully, it was not because the ice was slippery enough to allow the ship to slide back off the floe. The 10.2 kt ram caused a maximum force of 6.1 MN. The ram produced accelerations along the ship’s three axes comparable to those for Ram 10.

0 2 4 6 8 10 5600 5800 6000 6200 6400 6600

Elapsed time (sec)

G lo ba l For c e ( M N ) an d S hi p s p ee d (k t)

1 2 3 4 5 0 2 4 6 8 10 6600 6800 7000 7200 7400 7600 7800 8000 8200 8400 8600

Elapsed time (sec)

G loba l F or c e ( M N ) an d S h ip s p ee d ( k t)

6 7 8 9 10 11

Figure 14 Global force and ship speed for rams with sampling area of Floe B1S1 (a) rams 1 to 5 and (b) rams 6 to 11

(a)

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Figure 15 Bow prints for rams 1 to 6 with sampling area of Floe B1S1 Circles show drill-hole locations along transects

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Figure 16 Bow prints for rams 7 to 11 with sampling area of Floe B1S1 Circles show drill-hole locations along transects.

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5.0 Floe B1S2

After concluding operations on Floe B1S1 on 16 August, the ship traveled 33 km to the northwest to investigate another deformed multi-year ice floe on which to make measurements (Figure 17). Several targets were explored before finally selecting a hummocked area of ice (about 200 m by 400 m across) within a 6.5 by 6.5 km multi-year floe. The sampling area was designated as Floe B1S2. This floe drifted in a region of 6/10ths old ice (see Figure 4) and 1/10ths thick first-year ice for a total ice concentration of 7/10ths.

B1S2

Figure 17 Floe B1S2 as identified from satellite imagery (courtesy of Canadian Ice Service)

Since the ragged edge of Floe B1S2 made it quite difficult to secure the ship to the floe, the Commanding Officer proceeded to use the ship to mechanically ‘clean’ the floe edge in hopes of providing a cleaner edge for attaching the ship. This was of limited use, because it caused the floe to become more ragged. Eventually, the ship was secured to the floe in the area shown in Figure 18. The figure also shows the three drill-hole transects along which a total of 40 thickness measurements were made over the two-day period (17, 18 August). Table 5 provides a summary of the drill-hole thicknesses. Two boreholes were also made along the most prominent hummock in the area (H1 and H2, Figure 18) in order to measure the ice temperature, salinity and strength to a depth of 7 m.

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1

2

3

H1 _H2

Figure 18 Aerial perspective of ship secured to Floe B1S2

The approximate location of the four drill-hole transects and two boreholes in which ice property measurements were made are also shown (photo courtesy of S. Prinsenberg)

Table 5 Summary of ice thicknesses measured on Floe B1S2 No. of drill holes Length of transect (m) Average thickness (m) Maximum thickness (m) Minimum thickness (m) Transect 1 20 190 8.3 ± 2.4 14.8 5.1 Transect 2 10 90 8.0 ± 3.0 15.7 5.3 Transect 3 10 90 7.3 ± 1.5 9.9 5.7 Floe B1S1 40 8.0 ± 2.3 15.7 5.1

5.1 Thickness of Floe B1S2 from Drill-hole Transects

The thickness of the ice along the three transects ranged from 5.1 to 15.7 m. The average thickness of the three transects was quite similar (8.3, 8.0 and 7.3 m) and standard deviations ranged from ±1.5 to 3.0 m. Floe B1S2 had an average ice freeboard of 0.9 m, although the freeboard at the individual drill holes ranged from 0.2 m to 2.3 m. All three drill-hole transects avoided the 2.7 m high hummock on which boreholes H1 and H2 were made. Since an inordinate amount of time and effort would have been spent measuring the thickness of this multi-year hummock, it was decided to characterize the hummock during the strength portion of the work instead.

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The drilling experience on this floe was considerably different than on Floe B1S1. The ice on this floe was thicker and more consolidated than the previous floe. In some holes, the bottom ice was soft; however voids were not encountered in most of the drill holes. Frequently, there was considerable difference in the integrity of the ice in adjacent holes, and even within the same hole. Past experience has shown that to be typical of multi-year ice in late summer.

5.2 Temperature, salinity and strength of Floe B1S2

Two boreholes were made in the most prominent hummock of Floe B1S2. The first hole (H1) was merely augured to produce a hole for strength measurements (14.3 m thick, 2.7 m freeboard). The second borehole (H2) was made about 10 m away. Due to time constraints, it was not possible to measure the ice thickness and freeboard at H2. Having conducted those measurements at H1, it was thought prudent to spend the time obtaining ice cores instead. Cores extracted from H2 to a depth of 7 m revealed competent, consolidated ice – which certainly had not been the case on the previous floe. That stands to reason, because this floe (Floe B1S2) had a less deteriorated surface and older-looking hummocks than the previous floe (Floe B1S1).

Temperature: The uppermost metre of ice at H2 was near isothermal at 0°C. The temperature of the ice decreased to minimum of -4.8°C at a depth of 5.2 m (Figure 19-a). The ice temperature was relatively uniform from depths 5.2 to 6.4 m, then gradually warmed to -3.7°C at the 7 m depth. The average temperature of the uppermost 7 m of ice was -2.6°C. The ice temperature profile is somewhat erratic because problems were encountered retrieving cores from the hummock. A considerable amount of time (and patience) was needed to retrieve remnant core pieces from the borehole with a ‘fishing’ tool. Still, it was impossible to retrieve all of the pieces, and many of the fragments that were retrieved were too small to measure the ice temperature. As a result, temperature measurements are not available for two depth ranges (3.9 to 4.2 m; 5.4 to 6.2 m). It should also be noted that the time spent recovering core fragments sometimes resulted in the ice temperature being altered, particularly if the fragments floated in a water-filled borehole. By late-summer, water typically creeps into the borehole even if the full thickness of ice has not been penetrated.

Salinity: The salinity the uppermost 7 m of H2 ranged from 0.1 to 2.1‰, for an average of 1.0‰. The salinity profile in Figure 19-b shows three relatively distinct regions: (a) the uppermost 2.0 m of ice where salinities were less than 0.2‰, (b) the region between 2.0 to 3.6 m, where the salinity ranged from 0.1 to 1.8 ‰ and (c) depths 3.6 to 7.0 m where salinities typically were near 1.8‰.

Strength: A full set of borehole strength tests were conducted in H1 but strength tests were conducted only at depths 4.2 to 6.6 m in H2. Since there was limited time available for conducting tests in H2, it was decided to start at the bottom of the hole to capture the coldest, strongest ice first, and then move up towards the top ice surface. The strength of the hummocked ice was respectable at all depths. The average strength of the uppermost 7 m of ice was 22.0 MPa. The ice strength at the various depths ranged from 11.1 to 30.8 MPa (see Figure

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19-c9). It is very important to note that even though it was late summer and the ice was relatively warm (on average -2.6°C), strength tests showed the ice at all depths to have appreciable strength.

Max ice pressure (MPa)

-8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 0 5 10 15 20 25 30 35 Hole 2 Hole 1 Hole 1 = 14.2 m Hole 2 = ? m -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 0 1 2 3 Ice salinity (‰) Hole 2 H2 = ? m thick -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 -6 -4 -2 0 2 Ice temperature (C) Ic e dep th ( m ) H2 = ? m thick Hole 2

Figure 19 Temperature, salinity and strength profiles of Floe B1S2

5.3 Floe B1S2: Controlled Impacts with Sampling Area

After the on-ice measurements had been completed on 18 August, the ship prepared to impact the sampling area. Having communicated to the Commanding Officer that Floe B1S2 was considerably thicker and stronger than the previous floe, he prudently decided to impact the floe very cautiously and to limit the number of rams with this formidable floe. Four rams were conducted with the sampling area at speeds from 3.5 to 6.7 kt. Not surprisingly, each ram made only the slightest indentation in the ice, as illustrated by the ship trajectory of the four rams in Figure 20. Table 6 lists the particulars for each ram.

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These strengths represent the maximum pressure attained at each test depth. The peak pressure for all of the strength tests in each borehole occurred before the maximum indentor travel had been reached.

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Table 6 Rams with Sampling Area of Floe B1S2: Global Forces and Ship Motions Impact time of impact (s) Global impact force (MN) Max speed during ram (kt) Surge accel (m/s²) Heave accel (m/s²) Sway accel (m/s²) Roll (deg) Pitch (deg) Yaw (deg) 1 8712 4.6 3.1 0.47 0.25 0.18 0.7 0.9 0.3 2* 8982 7.2 3.6 0.60 0.33 0.26 0.9 1.0 0.4 3* 9207 7.1 4.8 0.82 0.34 0.34 0.5 1.1 0.7 4* 9473 9.0 6.7 1.03 0.59 0.30 3.3 1.9 0.3

* surge acceleration record in Appendix D indicates that ice knife contacted the floe

Floe B1S2:

rams with sampling area

Ram #1 #2 #3 #4 general direction of floe drift

Figure 20 Ship track showing four rams conducted with sampling area of Floe B1S2

Ram 1: The first ram with Floe B1S2 was conducted at a speed of 3.1 kt. The ship struck the floe about 30 m to the left of Transect 1 – exactly where the Commanding Officer intended the strike to occur. The average thickness of ice along Transect 1 was 8.3 m (± 2.4 m). In general, the ice along Transect 1 was slightly thinner at the floe edge and increased as the transect extended into the floe. Ram 1 produced a deep, hard hit right on the bow. The 3.1 kt ram produced a force of 4.6 MN (Figure 21). Maximum accelerations of 0.47 m/s² in surge, 0.25 m/²

Global ice impact forces on the CCGS Amundsen, Beaufort Sea

Archives des publications du CNRC

Global Ice Impact Forces on the CCGS

Amundsen

Beaufort Sea, August 2011

M. Johnston

Technical Report, CHC-TR-085

April 2012

Global Ice Impact Forces on the CCGS

Amundsen

Beaufort Sea, August 2011

Table of Contents

List of Figures

List of Tables

Global Ice Impact Forces on the CCGS

Amundsen

Beaufort Sea, August 2011

1.0 Introduction

2.0 MOTAN

3.0 Voyage of CCGS Amundsen during Test Program

4.0 Floe B1S1

5.0 Floe B1S2

B1S2