A Pilot experiment to measure arctic pack-ice driving forces

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A Pilot experiment to measure arctic pack-ice driving forces

Croasdale, K. R.; Comfort, G.; Frederking, R. M. W.; Graham, B. W.; Lewis,

E. L.

https://publications-cnrc.canada.ca/fra/droits

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A Pilot Experiment to Measure

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Reprinted from

Proceedings of the Ninth Conference (POAC87) University of Alaska Fairbanks, Fairbanks. Alaska August 17-21,1987

Port and Ocean Engineering Under Arctic Condition3

Vol. 111, 1988 p. 381-395 (IRC Paper No. 1623)

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A PILOT EXPERIMENT TO MEASURE ARCTIC PACK-ICE DRIVING FORCES

K . R. Croasdale

Esso Resources Canada Limited. Calgary. Alberta, C A N A D A

G. Comfort

Arcrec Canada Limited. Kanata. Ontario. C A N A D A R . Frederking

National Research Council of Canada, Ottawa, Ontario, C A N A D A

B. W . Graham

1. D. Engineering Canada Inc., Winnipeg, Manitoba, C A N A D A E . L. Lewis

Institute of Ocean Sciences, Sidney, British Columbia, C A N A D A

A b s t r a c t Pack-ice d r i v i n g f o r c e s a r e synonymous w i t h t h e l i m i t i n g f o r c e s t h a t c a n b e t r a n s n i t t e d t h r o u g h pack i c e . Knowledge of t h i s t o p i c is i m p o r t a n t i n r e l a t i o n t o :

-

i c e l o a d s on s t r u c t u r e s ( t h e l i m i t i n g d r i v i n g f o r c e a p p r o a c h )

-

i c e motion m o d e l l i n g and f o r e c a s t i n g

-

a r c t i c s h i p r o u t i n g U n t i l r e c e n t l y , l i m i t i n g pack-ice

f o r c e s had been s p e c u l a t e d upon but

n e v e r measured. T h i s paper d e s c r i b e s a p i l o t experiment t o measure t y p i c a l v a l u e s f o r pack-ice d r i v i n g - f o r c e s a c r o s s a w i d t h of s e v e r a l k i l o m e t r e s i n t h e s o u t h e r n Beaufort Sea. Zn-situ i c e p r e s s u r e s e n s o r s were i n s t a l l e d n e a r t h e middle of a m l t i - y e a r f l o e . The f l o e was s u b j e c t t o c o n v e r g i n g i c e c o n d i t i o n s and p r e s s u r e r i d g i n g o c c u r r e d around it. C o r r e sponding compressive i c e s t r e s s e s i n t h e

f l o e were measured from which t h e

a v e r a g e pack-ice f o r c e s a s s o c i a t e d w i t h t h e o b s e r v e d r i d g i n g c o u l d b e i n f e r r e d .

This is a reviewed and edited version of a paper presented at the Ninth International Conference on Port and Ocean Engineering Under Arctic Conditions, Fairbanks, Alaska. USA. August 17-22. 1987. Q The Geophysical Institute.

University o/ Alaska, 1987.

The paper d e s c r i b e s t h e background t o t h e p r o j e c t , t h e g e n e r a l approach, t h e e q u i p e n t used, and t h e r e s u l t s o b t a i n e d . I n t r o d u c t i o n The maximum s u s t a i n a b l e i n t e r n a l s t r e s s e s w i t h i n pack i c e a r e synonymous w i t h t h e d r i v i n g f o r c e s which pack i c e can e x e r t on l a r g e i c e f e a t u r e s embedded

w i t h i n i t ; hence t h e term pack-ice

d r i v i n g f o r c e s . Pack-ice d r i v i n g f o r c e s a r e of i n t e r e s t w i t h r e s p e c t t o a r c t i c o p e r a t i o n s f o r s e v e r a l r e a s o n s . . F i r s t ,

under c e r t a i n c i r c m s t a n c e s

,

i c e l o a d s on s t r u c t u r e s a r e i n f l u e n c e d by pack-ice

d r i v i n g f orcee. Second. i c e motion

f o r e c a s t models r e q u i r e r e a l i s t i c i n p u t o f i n t e r n a l pack-ice s t r e n g t h . T h i r d , t e c h n i q u e s f o r p r e d i c t i n g a r e a s o f p r e s s u r e d pack i c e a r e i m p o r t a n t i n a r c t i c s h i p o p e r a t i o n s . I n t e r m of i c e f o r c e models, t h e l i m i t - f o r c e c o n d i t i o n ( C r o a s d a l e 1980, 1984) i s governed by pack-ice d r i v i n g f o r c e s . This i s i l l u s t r a t e d conceptu- a ' l l y i n F i g u r e 1. A t h i c k i c e f e a t u r e (e.g. m u l t i - y e a r f l o e o r i c e i s l a n d ) is b e i n g f o r c e d a g a i n s t a s t r u c t u r e by moving pack i c e . Once t h e i c e f e a t u r e

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i s a t r e s t , t h e i n t e g r a t e d d r i v i n g f o r c e i s made up of t h e pack-ice d r i v i n g f o r c e , t h e wind d r a g and, i f a p p l i c a b l e , t h e c u r r e n t d r a g . I f t h i s t o t a l f o r c e i s l e s s t h a n t h a t r e q u i r e d t o l o c a l l y f a i l t h e t h i c k i c e a t t h e s t r u c t u r e , t h e n i t w i l l be t h e l i m i t - f o r c e l o a d which w i l l govern.

LIMIT FORCE

FIGURE 1 : LIMIT-FORCE ICE LOAD GOVERNED BY DRIVING FORCES (F = wL

+

WIND DRAG) I t i s s u g g e s t e d t h a t t h e pack-ice d r i v i n g f o r c e a c r o s s t h e f l o e w i d t h i s governed by t h e f a i l u r e of t h e pack i c e a c r o s s t h e w i d t h of t h e lodged f l o e . The p r o c e s s of l a r g e s c a l e i c e f a i l u r e w i t h i n pack i c e h a s been d i s c u s s e d by s e v e r a l i n v e s t i g a t o r s (e.g. H i b l e r 1980; Rothrock 1975; P a r m e r t e r 6 Coon 1973). It h a s been p o i n t e d o u t t h a t w i t h i n pack

i c e , t h e most common form of i c e

d e f o r m a t i o n and i c e f a i l u r e is p r e s s u r e r i d g i n g . F u r t h e r m o r e , o b s e r v a t i o n s of p r e s s u r e r i d g e s i n d i c a t e t h a t o v e r much of t h e i r l e n g t h . t h e f a i l u r e p r o c e s s a p p e a r s t o be one o f f l e x u r a l f a i l u r e r a t h e r t h a n i c e c r u s h i n g . P r e v i o u s i n v e s t i g a t o r s have s p e c u l a t e d on t h e f o r c e s n e c e s s a r y t o c r e a t e p r e s s u r e r i d g e s . Reviews of p r e v i o u s work on e s t i m a t i n g p r e s s u r e r i d g e b u i l d i n g f o r c e s have been COP

d u c t e d by V i v i t r a t and K r e i d e r ( 1 9 8 1 ) . C r o a s d a l e (1980, 1984) and o t h e r s (e.g.

Michel 1983). Also, model t e s t i n g h a s

been used a s a n a p p r o a c h t o

b e t t e r q u a n t i f y i n g r i d g e - b u i l d i n g f o r c e s

(e.g. Abdelnour 6 Croasdale 1986).

T y p i c a l v a l u e s f o r e s t i m a t e s of ridge- b u i l d i n g f o r c e s and t h e i r s o u r c e s a r e g i v e n i n T a b l e 1. The range of v a l u e s i s a t l e a s t one o r d e r of magnitude. The l o w e s t v a l u e s a r e t h o s e a s s o c i a t e d w i t h g e o p h y s i c a l s c a l e i c e motion p r e d i c - t i o n s , and averaged f o r c e s o b t a i n e d from e n e r g y approaches. The h i g h e s t v a l u e s a r e t h o s e o b t a i n e d c o n s i d e r i n g i c e

f a 1 lu r e a c r o s s a narrow width. Given

t h e f a c t t h a t r i d g e b u i l d i n g a c r o s s a

wide f r o n t w i l l probably not be

s i m u l t a n e o u s , t h e n both t h e high v a l u e s

( a c r o s s a narrow width) and t h e l o v

v a l u e s ( a v e r a g e d over s e v e r a l km) a r e n o t n e c e s s a r i l y i n c o m p a t i b l e . The o t h e r p o i n t of s i g n i f i c a n c e i s t h a t i n T a b l e 1 no d a t a e x i s t s which i s based on f u l l - s c a l e measurements. I t was t h i s l a c k of r e a l d a t a which p r o v i d e d t h e r a t i o n a l e and i n c e n t i v e f o r t h i s p r o j e c t . The o b j e c t i v e s of t h e p r o j e c t were t o a s s e s s how measurements

of a v e r a g e pack-ice f o r c e s could be

accomplished, and t o conduct a f i e l d

p i l o t e x p e r i m e n t . (See Croasdale and

o t h e r s 1986, f o r t h e f u l l p r o j e c t d e s c r i p t i o n and d e t a i l e d r e s u l t s ) . O v e r a l l Approach Recognizing t h a t i t i s t h e s p a t i a l l y a v e r a g e d r i d g e - b u i l d i n g o r p a c k - i c e f o r c e s a c r o s s a

wide

f r o n t t h a t a r e of i n t e r e s t , t h e g e n e r a l concept f o r m e a s u r i n g them was t o measure i n t e r n a l i c e s t r e s s a t t h e c e n t r e of a m u l t i - y e a r f l o e i n a n a r e a of converging pack i c e ; t h i s i s shown c o n c e p t u a l l y a s F i g u r e 2. T h i s approach u s e s t h e i n s t r u m e n t e d m u l t i - y e a r f l o e a s a l a r g e t r a n s d u c e r which s e n s e s t h e a v e r a g e pack-ice f o r c e s a p p l i e d t o i t . It was r e c o g n i z e d a t t h e o u t s e t , t h a t t h e s u c c e s s of t h i s approach depended on: o An a b i l i t y t o r e l i a b l y measure low i c e s t r e s s e s . o An a b i l i t y t o i n t e r p r e t t h e i n t e r n a l s t r e s s e s n e a r t h e c e n t r e o f t h e f l o e i n t e n a s of pack-ice f o r c e s a t t h e p e r i m e t e r .

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T a b l e 1 E s t i m a t e s of r i d g e b u i l d i n g f o r c e s ( v a r i o u s s o u r c e s ) C r o a s d a l e t e s t s C r o a s d a l e 2-D t h e o r y f o r f r a c t u r e 0.35 t o 1.1 x l o 5 6 r i d e u p (1 t o 2m i c e ) o Confidence o r knowledge t h a t t h e measurements e i t h e r r e p r e s e n t e d a l i m i t t o t h e p a c k - i c e f o r c e s , o r c o u l d be c o r r e l a t e d w i t h p r e s s u r e r i d g i n g around t h e f l o e . During t h i s p r o j e c t , e a c h of t h e above i s s u e s was a d d r e s s e d . The f i r s t i s s u e was a d d r e s s e d by d e v e l o p i n g and t e s t i n g , i n a c o l d room, newly d e s i g n e d i c e s t r e s s s e n s o r s s u i t a b l e f o r measuring r e l a t i v e l y low i c e s t r e s s e s . The second i s s u e was a d d r e s s e d p a r t l y by m a t h e m a t i c a l m o d e l l i n g and p a r t l y by examining t h e r e s u l t s from t h e f i e l d . The t h i r d i s s u e was a d d r e s s e d i n t h i s experiment by c o r r e l a t i n g i c e s t r e s s e v e n t s w i t h f l o e m o t i o n , and a c t u a l o b s e r v a t i o n s of p r e s s u r e r i d g i n g around t h e f l o e . T h i s p a p e r c o n c e n t r a t e s o n t h e f i e l d experiment and d i s c u s s i o n of t h e r e s u l t s . F i r s t , however, a b r i e f overview of t h e d e s i g n of t h e s e n s o r s and a l s o t h e c o l d room t e s t s i s provided.

The I c e S t r e s s S e n s o r s 6 Cold Room T e s t s Measuring s t r e s s e s w i t h i n a n i c e f e a t u r e i s n o t s i m p l e . mainly because t h e d e f o r m a t i o n b e h a v i o u r of i c e under s t r e s s i s not s i m p l e . I c e i s a m a t e r i a l c l o s e t o i t s m e l t i n g p o i n t . I t s deformation c h a r a c t e r i s t i c s a r e t h e r e f o r e s t r a i n - r a t e dependent and t h e y a l s o vary w i t h t e m p e r a t u r e . T h i s is t h e main r e a s o n why one cannot simply measure t h e s t r a i n i n t h e i c e and c o n v e r t i t t o s t r e s s u s i n g a n e l a s t i c modulus, a s one would w i t h l i n e a r e l a r t i c m a t e r i a l s such a s m e t a l s . N e v e r t h e l e s s , d e s p i t e t h e s e d i f f i c u l t i e s , t h e t h e o r y of i n c l u s i o n s h a s been s u c c e s s f u l l y used t o d e v e l o p an i n t e r p r e t a t i o n p r o c e d u r e f o r s e n s o r s i n s e r t e d i n a n i c e s h e e t (Metge and o t h e r s 1975). Based on p r e v i o u s work by t h e a u t h o r s and o t h e r s , t h e technology of i n - s i t u i c e s t r e s s s e n s o r s i s f a i r l y well understood. A t t h e same t i m a ,

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FLOE SUBJECT TO OVERALL

ICE PRESSURE SENSORS NEAR FLOECENTRE MEASURE AVERAGE COMPRESSIVE STRESS THROUGH THICKNESS ACTIVE RIDGING UNDER OVERALL PACKCONVERGENCE

FIGURE 2: OVERALL SCHEME FOR MEASURING PACK-ICE FORCES

however, i t was r e c o g n i z e d t h a t p r e v i o u s T a b l e 2 u s e of i c e p r e s s u r e s e n s o r s had been

d i f f e r e n t from i n t h i s p r o j e c t . They R e l a t i o n s h i p between a v e r a g e f l o e had been used m o s t l y i n t h e r a n g e of i c e compressive s t r e s s ( o ) and r t d a e - p r e s s u r e e x p e c t e d l o c a l l y i n f r o n t of b u i l d i n g f o r c e (w) f o r a Cfloe t h i c k n e s s

f i x e d s t r u c t u r e s , e.g.. 0 t o 3500 kPa. of 5 m Whereas, f o r t h i s p r o j e c t , s e n s o r s w i t h

a working range of a b o u t 0 t o 100 kPa were r e q u i r e d . ( T a b l e 2 i n d i c a t e s t h e r e l a t i o n s h i p between t h e a v e r a g e r i d g e - b u i l d i n g f o r c e (w) and t h e a v e r a g e S t r e s s a t t h e c e n t r e of a 5 m t h i c k f l o e , a s s m i n g u n i f o r m c o m p r e s s i v e s t r e s s . )

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The i s s u e o f m e a s u r i n g i n a r e l i a b l e , unambiguous way t h e s e low s t r e s s l e v e l s c o u l d n o t be t a k e n l i g h t l y ( d e s p i t e t h e f a v o u r a b l e p r e v i o u s e x p e r i e n c e w i t h s t r e s s s e n s o r s ) . P o t e n t i a l problem a r e a s v e r e c o n s i d e r e d t o be: o C o n f i g u r i n g t h e s e n s o r t o have high s e n s i t i v i t y y e t m a i n t a i n i n g a c o n s t a n t and k n w n i n c l u s i o n f a c t o r r e g a r d l e s s of t h e e f f e c t i v e i c e modulus. o , T h e e f f e c t s of long-term z e r o d r i f t . o The e f f e c t s o f t e m p e r a t u r e changes. o The e f f e c t s o f f r e e z e - i n s t r e s s e s and t h e i r d i s s i p a t i o n . o I n s t a l l a t i o n and o p e r a t i o n a l p r o c e d u r e s i n r e l a t i o n t o t h e above i s s u e s .

These problems were a d d r e s s e d i n t h e f i r s t p a r t of t h i s p r o j e c t which i n v o l v e d t h e d e s i g n of t h e i c e s t r e s s s e n s o r s and t h e i r t e s t i n g i n a c o l d room. I n t h e s e n s o r d e s i g n phase i t was concluded t h a t e x i s t i n g s e n s o r technology c o u l d be a d a p t e d t o measure low i c e s t r e s s e s . Based on p r e v i o u s work by t h e a u t h o r s ( C r o a s d a l e and o t h e r s 1986) i t was f e l t t h a t i n t e r n a l s t r a i n - g a u g e d e l e m e n t s . a s used i n t y p i c a l p a n e l s e n s o r s , c o u l d be used s i n g l y ( o r i n a n a r r a y of t h r e e ) i n a s e n s o r which w u l d be a b o u t 1.0 t o 1.5 cm t h i c k and up t o a b o u t 40 cm i n d i a m e t e r . It was c a l c u l a t e d t h a t a r e s o l u t i o n o f a b o u t 1 kPa was a c h i e v a b l e w i t h a n i n c l u s i o n f a c t o r c l o s e t o 0.9. S e n s o r s based o n t h i s c o n f i g u r a t i o n were b u i l t f o r c o l d room t e s t i n g . The o t h e r t y p e o f s e n s o r b u i l t f o r t h i s p r o j e c t was based o n a small h y d r a u l i c f l a t j a c k ( o r t o t a l p r e s s u r e c e l l ) connected t o a p r e s s u r e t r a n s d u c e r w i t h a s u i t a b l e range. The major c o n c e r n s w i t h t h i s t y p e of s e n s o r u e r e complete e l i m i n a t i o n o f a i r from t h e h y d r a u l i c f l u i d and t h e e f f e c t s of t h e r m a l expansion. S u c c e s s f u l use of t h e s e t y p e s o f s e n s o r s by o t h e r s s u g g e s t e d t h e s e problems c o u l d b e overcome. I n a n y c a s e . i t was i n t e n d e d t o monitor t e m p e r a t u r e o f t h e s e n s o r s i n t h e i c e , a n t i c i p a t i n g t h a t any obvious changes of c a l i b r a t i o n w i t h t e m p e r a t u r e c o u l d be c o r r e c t e d . T e s t i n g o f t h e s e n s o r s involved c a l i b r a t i o n i n a p r e s s and a l s o w i t h i n loaded i c e blocks. Both s e n s o r s

-

e x h i b i t e d v e r y c o n s i s t e n t and Linear r e s p o n s e when t e s t e d i n a p r e s s .

R e s u l t s o b t a i n e d from t e s t i n g t h e s e n s o r s i n l o a d e d i c e b l o c k s were not a s good, t h e r e b e i n g some n o n - l i n e a r i t y and a p p a r e n t h y s t e r e s i s . It v a s not c l e a t whether t h e s e e f f e c t s were r e a l s e n s o r c h a r a c t e r i s t i c s , o r due t o o t h e r c a u s e s , e.g. s t r e s s - r e d i s t r i b u t i o n i n t h e loaded i c e b l o c k s w i t h time. I n any c a s e , i t

was judged t h a t t h e s e n s o r s had a good enough performance f o r t h i s p i l o t e x p e r i m e n t , a l t h o u g h f u r t h e r s e n s o r t e s t i n g and d e v e l o p u e n t w u l d have been d e s i r a b l e (and s h o u l d be done b e f o r e more comprehensive f i e l d measurements a r e implemented).

The F i e l d Deployment

The f i e l d program commenced on A p r i l 11, 1986 and was completed by May 5, 1986. The g e n e r a l l o c a t i o n was t h e Canadian B e a u f o r t Sea about 40 km

o f f s h o r e i n a b o u t 30 m of water. T h i s was c l o s e t o t h e Gulf Canada Resources c a i s s o n d r i l l i n g s t r u c t u r e ( t h e Molikpaq). P r i o r t o going t o t h e f i e l d , s y n t h e t i c a p e r t u r e r a d a r imagery of t h e i c e i n t h e v i c i n i t y of t h e Holikpaq was i n s p e c t e d . S e v e r a l m l t i - y e a r f l o e s were i d e n t i f i e d a s p o t e n t i a l c a n d i d a t e s f o r t h e experiment. On one of t h e s e f l o e s , which was a b o u t 16 bn e a s t of t h e Uolikpaq, Gulf had placed a s a t e l l i t e - r e p o r t i n g ARGOS buoy.

-

.

This f l o e was i n s p e c t e d d u r i n g a f i e l d v i s i t and was s e l e c t e d a s t h e f l o e f o r t h e experiment. I t appeared t o be o f r e l a t i v e l y u n i f o r m t h i c k n e s s , was a good s i z e ( a b o u t 4.5 x 2.5 h), and had t h e advantage of t h e ARGOS buoy, by which d a i l y p o s i t i o n s c o u l d be obtained. On t h e f i r s t v i s i t t o t h e f l o e , a r a d i o beacon was i n s t a l l e d i n o r d e r t o a s s i s t t n f i n d i n g t h e f l o e on subsequent v i s i t s . For redundancy, a second r a d i o beacon was i n s t a l l e d a day l a t e r . With t h e ARGOS buoy, and t h e r a d i o beacons, no d i f f i c u l t y was e x p e r i e n c e d f i n d i n g t h e f l o e d u r i n g t h e c o u r s e of t h e experiment, d e s p i t e t h e f a c t t h a t t h e f l o e d r i f t e d a t o t a l of about 100 Ian.

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Approximate f l o e s h a p e and dimensions were o b t a i n e d by s c a l i n g o f f t h e SAR imagery, t h e s e a r e shown i n F i g u r e 3. NEW

+

i

0 0 5 1 0 POSITION OF SENSORS

FIGURE 3: FLOE SIZE AND SHAPE

The s i t e l o c a t i o n on t h e f l o e was s e l e c t e d t o be a p p r o x i m a t e l y i n t h e middle of t h e f l o e w i d t h . The s i t e was s e l e c t e d o n t h e b a s i s o f being r e l a t i v e l y f l a t a n d , a s b e s t a s c o u l d be judged. f r e e from m a j o r c r a c k s ( a l t h o u g h a snow c o v e r of between 0 and about 10

cm i n h i b i t e d o u r a b i l i t y t o judge t h e q u a l i t y of t h e i c e ) . The i c e t h i c k n e s s i n t h e v i c i n i t y was measured t o be i n t h e range of 1.7 m t o 3 m. Two g r o u p s of f o u r s e n s o r s were i n s t a l l e d w i t h i n 25 m of e a c h o t h e r . T a b l e 3 d e s c r i b e s e a c h s e n s o r group. The i c e s t r e s s - m e t e r s were deployed i n a " s t a r " r o s e t t e p a t t e r n a t e a c h l o c a t i o n a s shown i n F i g u r e 4. These r o s e t t e s were a r r a n g e d s u c h t h a t t h e arras of t h e r o s e t t e were a l l comprised of t h e same t y p e of i c e s t r e s s - m e t e r . A redundant s t r e s s - m e t e r of t h e a l t e r n a t e t y p e was p l a c e d i n p a r a l l e l w i t h one of t h e r o e e t t e arms t o a l l o w d i r e c t comparison between t h e two s e n s o r t y p e s . S e n s o r s

i n each r o s e t t e were p l a c e d a minimum 1 m a p a r t . T h e r m i s t o r s were used t o m o n i t o r t h e t e m p e r a t u r e of t h e s e n s o r s i n t h e i c e ( a t t h e s e n s o r d e p t h s ) and t h e a i r t e m p e r a t u r e .

GROUP f SENSORS GROUP 2 SENSORS

~ 1 2 0 819 FIGURE 4: SITE LAYOUT

P r i o r t o i n s t a l l i n g t h e s e n s o r s i n t h e i c e , t h e y were a g a i n checked t o be f u n c t i o n i n g (by s t a n d i n g on them) and z e r o ' s were s e t . Data r e c o r d i n g commenced a s s o o n a s t h e y were flooded.

Photographs of s e n s o r i n s t a l l a t i o n and t h e g e n e r a l s i t e l a y o u t a r e shown i n F i g u r e 5.

The s e n s o r s d e s i g n a t e d Croup 2 were i n s t a l l e d f i r s t ( o n A p r i l 13, 1986). T h i s took a b o u t 8 hours. The s e n s o r s were i n s t a l l e d i n d r y s l o t s , p r e p a r e d u s i n g a n a u g e r , c h a i n saw and i c e c h i s e l . They were f r o z e n i n t o t h e i c e by i n i t i a l l y c o v e r i n g t h e s e n s o r s w i t h f r e s h w a t e r which was t r a n s p o r t e d from Tuktoyaktuk. One day l a t e r , when t h i s w a t e r had f r o z e n , h o l e s were d r i l l e d i n t h e s e n s o r s l o t s which allowed s e a w a t e r t o f l o o d t h e remainder of t h e s l o t s . S e n s o r s were p l a c e d i n t h e d e p t h range of one h a l f t o one t h i r d t h e i c e t h i c k n e s s from t h e s u r f a c e . T h i s p u t them a t a b o u t t h e 1 m d e p t h . On A p r i l 14, 1986, t h e s e n s o r s d e s i g n a t e d Croup 1 were i n s t a l l e d u s i n g s i m i l a r Eechniques, e x c e p t t h a t s e a w a t e r was used t o f l o o d t h e s l o t s r a t h e r t h a n f r e s h w a t e r ( b e c a u s e o f t h e l o g i s t i c a l d i f f i c u l t i e s of b r i n g i n g enough f r e s h w a t e r t o t h e s i t e ) . It was judged t h a t t h e i c e t e m p e r a t u r e was

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s u f f i c i e n t l y low t h a t t h e d i f f e r e n c e i n i c e modulus between f r e s h a n d s a l i n e i c e would n o t a f f e c t t h e s e n s o r r e a d i n g s .

Removal o f t h e s e n s o r s was a c c o m p l i s h e d u s i n g a h o t w a t e r g e n e r a t o r o n May 1 (Group 2) and May 3 (Group 1 ) . Zero r e a d i n g s w e r e t a k e n f o r a l l s e n s o r s .

F i e l d d a t a was l o g g e d o n - s i t e and r e t r i e v e d by p h y s i c a l l y a c c e s s i n g t h e i c e f l o e . Raw time s e r i e s d a t a was r e c o r d e d c o n t i n u o u s l y d u r i n g t h e f i e l d m o n i t o r i n g p e r i o d . A i r a n d s e n s o r t e m p e r a t u r e s w e r e r e c o r d e d a t h o u r l y i n t e r v a l s a l o n g w i t h t h e d a t e a n d time ( i . e . , h o u r and m i n u t e ) a t w h i c h t h e s e m e a s u r e m e n t s w e r e made. I c e s t r e e a - m e t e r d a t a was i n i t i a l l y r e c o r d e d a t a f i f t e e n s e c o n d p e r c h a n n e l s c a n r a t e . S u b s e q u e n t l y , t h e s c a n r a t e was r e d u c e d t o 6 0 s e c o n d s p e r c h a n n e l t o a l l o w a g r e a t e r t i m e p e r i o d b e t w e e n s i t e v i s i t s . S t r e s s Measurement R e s u l t s D u r i n g t h e d a y s f o l l o w i n g i n s t a l l a t i o n , t h e s e n s o r s a p p e a r e d t o r e s p o n d t o f r e e z e - i n s t r e s s e s a n d t e m p e r a t u r e t r a n s i e n t s . Most s e n s o r s e x h i b i t e d c y c l i c o u t p u t s which c o u l d be c o r r e l a t e d w i t h t h e d a i l y v a r i a t i o n s i n a i r t e m p e r a t u r e and n o " r e a l " s t r e s s e v e n t s a p p e a r e d t o o c c u r . During t h i s time, t h e f l o e was i n a d i v e r g i n g i c e f i e l d , a s o f f s h o r e winds opened l e a d s between t h e pack i c e and t h e f a s t i c e ; t h e f l o e b e i n g c a r r i e d a l o n g by t h e g e n e r a l m o t i o n o f t h e pack i c e , s e e F i g u r e 6. Af'ter a b o u t A p r i l 1 8 , none o f t h e s e n s o r s e x h i b i t e d a n y c y c l i c o u t p u t t h a t c o u l d b e l i n k e d w i t h v a r y i n g a i r t e m p e r a t u r e . T h i s c o u l d have been d u e t o t h e d r i f t i n g snow r e e s t a b l i s h i n g t h e s n w c o v e r , which was i n t h e r a n g e o f 20-30 cm i n t h e s e n s o r a r e a .

Smor Rnrillaeter Smor !hsor FBrufacturer

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FIGURE 6: FLOE MOVEMENT DURING STUDY

Commencing a b o u t A p r i l 20, t h e predominant wind d i r e c t i o n changed from o f f s h o r e t o onshore. The f l o e became s u b j e c t t o a c o n v e r g i n g i c e c o n d i t i o n a s t h e l e a d s c l o s e d up and t h e pack i c e was d r i v e n a g a i n s t t h e l a n d f a s t i c e . During t h i s p e r i o d , a s e r i e s of a p p a r e n t s t r e s s e v e n t s were r e c o r d e d . S e v e r a l of t h e s e e v e n t s c o u l d be c o r r e l a t e d w i t h t h e o c c u r r e n c e of new r i d g i n g i n t h e f i r s t - y e a r i c e a t t h e edge of t h e f l o e ( s e e F i g u r e 3).

The e v e n t which gave t h e h i g h e s t s t r e s s e s is shown i n F i g u r e 7. The

maximum stress change v a r a b o u t

2 2 kPa. A l l t h e s e n s o r s i n t h e Group 1 a r r a y responded t o t h i s e v e n t . Two of t h e s e n s o r s i n t h e Group 2 a r r a y responded, b u t a t l o v e r l e v e l s . Another f o u r s t r e s s e v e n t s t o o k p l a c e d u r i n g t h e p e r i o d A p r i l 21 t o May 3. The s i g n a - t u r e s of t h e s t r e s s o u t p u t s v e r e s i m i l a r t o t h o s e shown i n F i g u r e 7 , but t h e i r magnitudes v e r e lower.

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I n t e r p r e t a t i o n of R e s u l t s D i s c u s s i o n o f R e s u l t s I n t h i s p a p e r , a f u l l p r e s e n t a t i o n and d i s c u s s i o n of r e s u l t s i s n o t p o s e i b l e ; However, t h e f o l l o w i n g c a n be concluded from e x a m i n a t i o n of t h e r e s u l t s . o Although d u r i n g t h e s e t t l i n g - i n p e r i o d , t h e s e n s o r s appeared t o respond t o t h e r m a l changes a s w e l l a s s t r e s s e v e n t s , t h e s i g n a t u r e s of t h e r e s p o n s e s were q u i t e d i f f e r e n t . The t h e r m a l c h a n g e s c r e a t e d o u t p u t s which c o r r e l a t e d w i t h v a r i a t i o n s i n a i r t e m p e r a t u r e , whereas t h e s t r e s s o u t p u t s were s p i k y and s i m i l a r t o i c e f a i l u r e s t r e s s s i g n a t u r e s o b t a i n e d i n o t h e r s i t u a t i o n s . Also t h e o u t p u t s d u e t o t h e r m a l e f f e c t s became v e r y s m a l l a f t e r t h e t e m p e r a t u r e t r a n s i e n t s had d i s s i p a t e d and a s n w c o v e r had r e e s t a b l i s h e d . o B e t m e n s t r e s s e v e n t s , t h e o u t p u t s from t h e s e n s o r s were v e r y c o n s t a n t , e x h i b i t i n g v i r t u - a l l y no d r i f t . On removal. a 1 1 b u t one s e n s o r came back t o w i t h i n 2 kPa of t h e i n i t i a l zero. o The s t r e s s e v e n t s c o u l d be l i n k e d i n a l m o s t a l l c a s e s t o new r i d g i n g around t h e f l o e . The h i g h e s t s t r e s s e v e n t was l i n k e d t o t h e c r e a t i o n of r i d g e s w i t h s a i l s up t o 4 m high. o Changes i n f l o e motion, p r i o r t o i t b e i n g s u b j e c t t o c o n v e r g i n g i c e , d i d n o t g e n e r a t e measurable i n t e r n a l s t r e s s e s i n t h e f l o e . o T h e r e were problems of c o n s i s t e n c y of s t r e s s r e a d i n g s between t h e two a r r a y s . I t i r n o t p o s s i b l e t o be s u r e why t h e s e d i f f e r e n c e s o c c u r r e d . P o s s i b l e e x p l a n a t i o n s a r e ;

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D i f f e r e n t s e n s o r s used i n t h e two a r r a y s ,

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D i f f e r e n t i n s t a l l a t i o n methods,

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The p r e s e n c e of c r a c k s i n t h e f l o e . The e v e n t s h w n i n F i g u r e 7 y i e l d e d t h e h i g h e s t and most c o n s i s t e n t s t r e s s o u t p u t s . It i s of i n t e r e s t t o expand t h e t i m e s c a l e f o r t h e gauge-type s e n s o r s d u r i n g t h i s e v e n t ; t h i s has been done i n F i g u r e 8. It will be noted t h a t s t r e s s peaks from t h e t h r e e s e n s o r s i n t h e r o s e t t e a r e not synchronous. T h i s i m p l i e s t h a t t h e d i r e c t i o n of p r i n c i p a l s t r e s s i s changing q u i t e s i g n i f i c a n t l y w i t h time. T h i s is not u n r e a s o n a b l e g i v e n t h a t r i d g i n g e v e n t s around t h e f l o e a r e p r o b a b l y n o t s i m u l t a n e o u s a c r o s s t h e 5 Ian of f l o e l e n g t h . A p o i n t t o r e c o g n i z e when examining t h e expanded time o u t p u t s i s t h a t i c e s t r e s s was b e i n g measured o n c e e v e r y minute. It i s p o s s i b l e t h e r e f o r e , t h a t some of t h e h i g h e r p e a k s have been l o s t . Looking a t t h e n a t u r e of t h e t r a c e s , however, i t i s t h e a u t h o r s ' o p i n i o n t h a t e x t e n s i o n s t o t h e r e c o r d e d peaks due t o t h i s e f f e c t would be l e s s t h a n about 10 t o 20%. A s i m p l e i n t e r p r e t a t i o n of t h e o u t p u t s g i v e s t h e h i g h e s t r e c o r d e d s t r e s s change a s 35

-

13 = 22 kPa. T h i s o c c u r r e d on gauge 2 a t about 1520 hours on A p r i l 22, s e e F i g u r e 7. Hw can we i n t e r p r e t t h i s i n t e n a s of a n a v e r a g e c o m p r e s s i v e s t r e s s through t h e f u l l f l o e t h i c k n e s s of 1.8 m? There a r e two c o r r e c t i o n s which must be made; one i s i n r e l a t i o n t o t h e gauge i n c l u s i o n f a c t o r , t h e o t h e r i s i n r e l a t i o n t o t h e d i s t r i b u t i o n o f s t r e s s through t h e i c e t h i c k n e s s . Aa-regards t o t h e l a t t e r , w e d o not have enough i n f o r m a t i o n t o q u a n t i f y t h e s t r e s s d i s t r i b u t i o n . I n a more compre- h e n s i v e e x p e r i m e n t , a t l e a s t t v o s e n s o r s , one above t h e o t h e r through t h e i c e s h e e t t h i c k n e s s would g i v e a n i n d i - c a t i o n o f s t r e s s d i s t r i b u t i o n . I n t h i s t r i a l deployment t h i s was not done. Another a p p r o a c h would be t o c a l c u l a t e an i c e modulus d i s t r i b u t i o n through t h e i c e t h i c k n e s s , based on t e m p e r a t u r e and s a l i n i t y d i s t r i b u t i o n . For s t r a i n c o m p a t i b i l i t y , t h e d i s t r i b u t i o n of i c e modulus would be an i n d i c a t i o n of t h e s t r e s s d i s t r i b u t i o n (assuming a uniform c o m p r e s s i v e l o a d a p p l i e d a t t h e f l o e e d g e ) . I n t h i s experiment we d i d n o t o b t a i n t h e t e m p e r a t u r e and s a l i n i t y p r o f i l e s , s o t h i s cannot be done.

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ELAPSED TIME

-

hr

ELAPSED TlME

-

hr

We can s p e c u l a t e , hawever, t h a t i n view of t h e f a c t t h a t t h e s e n s o r was above t h e mid-point of t h e i c e t h i c k n e s s , t h a t i t is probably r e a d i n g a s t r e s s which i s h i g h e r t h a n t h e a v e r a g e s t r e s s t h r o u g h t h e f l o e t h i c k n e s s ; how much h i g h e r i s a m a t t e r of s p e c u l a t i o n . The a u t h o r s ' s u g g e s t t h a t t h e s e n s o r c o u l d be r e a d i n g a s t r e s s which i s no more t h a n 1.0 t o 1.3 h i g h e r t h a n t h e a v e r a g e s t r e s s through t h e f l o e t h i c k n e s s . The i n c l u s i o n f a c t o r e f f e c t t e n d s t o push t h e measured s t r e s s i n t h e o t h e r d i r e c t i o n , i.e.. lower. From t h e c o l d room t e s t s i t was concluded t h a t t h e i n c l u s i o n f a c t o r f o r t h e gauge s e n s o r s was i n t h e r a n g e of 0.35 t o 0.9. It was n o t e d , however, t h a t t h e i n c l u s i o n f a c t o r tended t o t h e h i g h e r v a l u e when t h e s e n s o r had been p r e s t r e s s e d f o r a n l n b e r o f h o u r s . ( A l s o , t h a t t h e method of t e s t i n g t h e s e n s o r s i n f i n i t e i c e b l o c k s c o u l d have been a major c a u s e of v a r i a t i o n i n t h e i n c l u s i o n f a c t o r ) . For t h e e v e n t we a r e examining, t h e s e n s o r had been under a compressive s t r e s s from t h e i n i t i a l f r e e z e - i n . f o r s e v e r a l d a y s , and from t h e b e g i n n i n g of t h e s t r e s s e v e n t f o r s e v e r a l hours. T h e r e f o r e . i t i s s u g g e s t e d t h a t t h e i n c l u s i o n f a c t o r was probably c l o s e r t o 0.9 t h a n t o 0.35. We w i l l assume a l i k e l y range of i n c l u s i o n f a c t o r

i s

between 0.75 and 0.9. W e c a n now combine t h e t m e r r o r bands. I f a i s t h e nominal i c e s t r e s s o u t p u t , t h e n t h e r a n g e of i n t e r p r e t e d a v e r a g e compressive s t r e s s a c i s g i v e n by V) w 15 I n o t h e r words, t h e most l i k e l y 0 v a l u e of h i g h e s t a v e r a g e c o m p r e s s i v e to _{s t r e s s} _{t h r o u g h} _{t h e} _{f l o e} _{t h i c k n e s s} 5 measured d u r i n g t h i s p r o j e c t was a b o u t 1.33 x 22 = 29 kPa, t h e l o w e s t v a l u e of 23 23 4 23.8 24 2 24 6 25 t h e peak s t r e s s e v e n t b e i n g a b o u t 0.85 x ELAPSED TlME - hr 22 = 19 kPa.

FIGURE 8: MAXIMUM STRESS EVENT

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EXPANDED TlME SCALE

Based on t h e s e r a n g e s of v a l u e s f o r t h e peak compressive s t r e s s measured I n

t h e f l o e , what can we s a y about t h e a v e r a g e pack i c e o r r i d g e - b u i l d i n g force; a c t i n g a t t h e f l o e edge? The most s i m p l i s t i c i n t e r p r e t a t i o n i s t o a s s m e a uniform f l o e w i t h a uniform

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r i d g i n g o r pack-ice f o r c e around it. I n t h i s c a s e , t h e a v e r a g e r i d g i n g load (w) c a n be c a l c u l a t e d a s w - a t

-

C where a i s t h e a v e r a g e c o a p r e s s i v e s t r e s s throug% t h e i c e t h i c k n e s s ( t ) , where t h e e t r e s s is measured. Using t h i s a p p r o a c h , w i t h t = 1.8 m, y i e l d s Such a v a l u e

is

q u i t e c o m p a t i b l e w i t h t h e r a n g e of p o s s i b l e r i d g e b u i l d i n g f o r c e s shown i n Table 1. Note t h a t f o r t h i s p r o j e c t ( b u t not p r e s e n t e d i n t h i s p a p e r ) , a s e r i e s of d i f f e r e n t edge l o a d d i s t r i b u t i o n s and f l o e t h i c k n e s s d i s t r i b u t i o n s were examined i n r e l a t i o n t o t h e i r i n f l u e n c e on t h e c o m p r e s s i v e s t r e s s e s a t t h e f l o e c e n t r e . The work i n d i c a t e d t h a t t h e p o t e n t i a l e r r o r a s s o c i a t e d w i t h t h e a s s m p t i o n o f a s i m p l e uniform edge l o a d i n g would g e n e r a l l y be l e s s t h a n a b o u t f 25%. O t h e r p o t e n t i a l e r r o r s a s s o c i a t e d w i t h t h e p r e v i o u s s i m p l e i n t e r p r e t a t i o n i n c l u d e t h e e f f e c t s of a c t i v e c r a c k s i n t h e f l o e . With t h e l i m i t e d n a t u r e o f t h i s t r i a l e x p e r i m e n t , i t i s n o t p o s s i b l e t o q u a n t i f y t h e p o s s i b l e magnitudes of t h e above p o t e n t i a l e r r o r s . T h e i r c u m u l c t i v e e f f e c t c o u l d be a d d i t i v e o r b a l a n c i n g . At t h i s s t a g e , one might s p e c u l a t e t h a t t h e i r e f f e c t s , p l u s o t h e r p o t e n t i a l e r r o r s a l r e a d y d i s c u s s e d , c o u l d g i v e a t o t a l e r r o r band o f a b o u t f 100%. T h i s would t h e n l e a d u s t o t h e r e s u l t t h a t t h e maximum pack-ice f o r c e s o r r i d g i n g f o r c e s measured d u r i n g t h i s e x p e r i m e n t were i n t h e range of about

0.25 t o 1.0 x l o 5 N/m. C o n c l u d i n g Remarks T h i s was t h e f i r s t p r o j e c t ( a t l e a s t i n t h e p u b l i c domain) t o f o c u s o n t h e measurement of pack i c e i n t e r n a l s t r e s s e s , w i t h t h e s p e c i f i c aim of o b t a i n i n g a v e r a g e r i d g i n g f o r c e s a c r o s s a wide f r o n t . It was a p i l o t p r o j e c t

and i t was recognized t h a t l i m i t e d r e s o u r c e s f o r t h e f i e l d deployment would r e s u l t i n l e s s t h a n a n i d e a l set of m e a e u r e w n t s , which would l e a d t o u n c e r t a i n t i e s i n i n t e r p r e t a t i o n . T h i s proved t o be t h e c a s e . and y e t , i n t h e a u t h o r s ' o p i n i o n t h e r e s u l t s a r e p l a u s i b l e and f o r t h e f i r s t time t y p i c a l i n t e r n a l s t r e s s e s i n a f l o e s u b j e c t t o l i m i t i n g pack-ice f o r c e s (1.e.. r i d g i n g f o r c e s ) have been obtained.

Some d i f f i c u l t i e s were a s s o c i a t e d w i t h c o n s t r u c t i o n of t h e s e n s o r s and t h e c o l d room t e s t s . P r o b l e m a s s o c i a t e d w i t h t e s t i n g t h e s e n s o r s i n l a r g e i c e b l o c k s i n c l u d e d : o F r e e z i n g - i n t h e s e n s o r s w i t h o u t c r a c k i n g t h e i c e b l o c k s and a l s o e n s u r i n g t h a t t h e i c e was i n i n t i m a t e c o n t a c t w i t h t h e s e n s o r s . o Applying a uniform a t r e s s t o t h e i c e b l o c k s . o Non-uniform s t r e s s d i s t r i b u t i o n i n t h e i c e b l o c k s due t o v a r i a - t i o n i n i c e modulus c a u s e d by t e m p e r a t u r e v a r i a t i o n s i n t h e i c e b l o c k s . o The c r e a t . i o n of i n t e r n a l s t r e s s i n t h e i c e b l o c k s due t o temper- a t u r e v a r i a t i o n s i n t h e i c e b l o c k s , t h e s e i n t e r n a l s t r e s s e s b e i n g a s g r e a t a s t h e a p p l i e d low s t r e s s e s . I n h i n d s i g h t , t h e t e m p e r a t u r e c o n t r o l i n t h e c o l d room was p r o b a b l y much more c r i t i c a l t h a n o r i g i n a l l y t h o u g h t . F u t u r e c o l d room t e s t i n g o f low s t r e s s s e n s o r s s h o u l d be done w i t h t h e i c e b l o c k s under i s o t h e r m a l c o n d i t i o n s . I n t h e f i e l d , t h e s e n s o r s a p p e a r e d t o respond b e t t e r t h a n i n t h e c o l d room, i n t h a t d r i f t was minimal and t h e o u t p u t s from t h e s e n s o r s was remarkably s t a b l e between t h e o b v i o u s s t r e s s e v e n t e . T h i s t e n d s t o s u p p o r t t h e s u g g e s t i o n t h a t some of t h e p e r c e i v e d s e n s o r problems i n t h e c o l d room t e s t s were d u e t o t h e f a c t o r s mentioned above.

On t h e o t h e r hand, t h e o u t p u t s from some of t h e s e n s o r s i n t h e f i e l d were n o t c o n s i s t e n t . In g e n e r a l , t h e o u t p u t s from t h e Croup 2 s e n s o r s were l o v e r t h a n

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Group I and some of t h e Group 2 s e n s o r s showed no response t o s t r e s s e v e n t s . I t

i s s p e c u l a t e d t h a t t h i s was p r i m a r i l y due t o t h e i n s t a l l a t i o n method which was d i f f e r e n t f o r t h e Group 2 s e n s o r s t h a n t h e Group 1 s e n s o r s , and c o u l d h a v e r e s u l t e d i n i n c o m p l e t e c o n t a c t between t h e s e n s o r s and t h e i c e . However, t h e p r e s e n c e of c r a c k s i n t h e f l o e c o u l d a l s o be a p o s s i b l e e x p l a n a t i o n f o r t h e l a c k of c o n s i s t e n t agreement between t h e two groups of s e n s o r s . The l o g i s t i c s of t h e f i e l d program proceeded very mr~oothly. Also, t h e d a t a r e c o r d i n g system f u n c t i o n e d v e r y v e l l . w i t h o n l y some minor m a l f u n c t i o n s ( a b o u t 98% o f t h e d a t a was c a p t u r e d ) . The measured v a l u e s of t h e i n t e r n a l s t r e s s e s i n t h e f l o e d u r i n g t h e i c e s t r e s s e v e n t s could be l i n k e d t o t h e o b s e r v e d r i d g i n g e v e n t s . No i n t e r n a l s t r e s s e s were r e c o r d e d when t h e f l o e changed speed o r d i r e c t i o n . The r e s u l t s from t h i s p r o j e c t a r e s u f f i c i e n t l y e n c o u r a g i n g t o recommend t h a t a more comprehensive f i e l d p r o j e c t b e conducted. I d e a l l y , t h e i n g r e d i e n t 8 of s u c h a p r o j e c t s h o u l d be a s f o l l o w s : o It should be c o n d u c t e d i n a n a r e a w i t h a m i x t u r e of m u l t i - y e a r and f i r s t - y e a r i c e and which i s f a i r l y dynamic. o Two m u l t i - y e a r f l o e s o f d i f f e r e n t s i z e s s h o u l d be i n s t r u m e n t e d . r a t h e r t h a n one. o Each f l o e s h o u l d be i n s t r u m e n t e d w i t h a t l e a s t two r o s e t t e s o f s e n s o r s n e a r t o t h e c e n t r e o f t h e f l o e , and w i t h a d d i t i o n a l r o s e t t e s c l o s e r t o t h e e d g e s of t h e f l o e . o A t l e a s t two l e v e l s i n t h e t h i c k n e s s of t h e f l o e s h o u l d b e i n s t r u e n t e d a t e a c h r o s e t t e . o C o n s i d e r a t i o n s h o u l d a l s o be g i v e n t o d e p l o y i n g s u r f a c e s t r a i n meter r o s e t t e s a s w e l l a s s t r e s s s e n s o r s .

o The sampling r a t e s h o u l d be more f r e q u e n t t h a n e v e r y minute ( a t l e a s t f o r some of t h e t i m e , f o r some of t h e s e n s o r s ) . 0 Temperature and s a l i n i t y p r o f 1 l e s of t h e i c e c l o s e t o t h e s e n s o r s s h o u l d be g a t h e r e d . o The f l o e s s h o u l d be s u r v e y e d t o g i v e b e t t e r t h i c k n e s s v a r i a t i o n d a t a t h a n was p o s s i b l e i n t h i s p r o j e c t .

o Ridges forming around t h e f l o e s should be recorded i n t e r m s of i c e t h i c k n e s s , p r o f i l e s and e x t e n t . o The s e n s o r s used c o u l d be s i m i l a r t o t h o s e used i n t h i s p r o j e c t b u t t h e r e a r e some u n c e r t a i n t i e s a s s o c i a t e d w i t h t h e s e n s o r s which s t i l l need t o be a d d r e s s e d . A modest t e s t program ( b u i l d i n g on t h e e x p e r i e n c e of c o l d room , t e s t i n g i n t h i s p r o j e c t ) s h o u l d e n a b l e t h e u n c e r t a i n t i e s t o be r e s o l v e d . T h i s work s h o u l d be done w e l l i n advance of t h e f i e l d work. I n summary, t h e p r o j e c t demon- s t r a t e d t h a t t y p i c a l pack i c e f o r c e s c a n b e measured u s i n g a n i n s t r u m e n t e d m u l t i - y e a r f l o e a 8 a t r a n s d u c e r . There remain some o u t s t a n d i n g i s s u e s of i n t e r p r e t a - t i o n which can probably o n l y be r e s o l v e d by c o n d u c t i n g a n o t h e r f i e l d program a s recommended above. However, t h e measurements o b t a i n e d d u r i n g t h i s p r o j e c t w i l l

be of some h e l p t o s c i e n t i s t s and

e n g i n e e r s i n v o l v e d i n pack i c e m o d e l l i n g and i c e f o r c e p r e d i c t i o n s .

Acknowledgements

T h i s p r o j e c t was conducted under c o n t r a c t t o t h e Government of Canada. We v i s h t o t h a n k t h e s u p p o r t l n g

government e n t i t i e s which i n c l u d e :

Supply and S e r v i c e s Canada, Unsolicited

P r o p o s a l s Program; Department o f

F i s h e r i e s and Oceans; Canada P u b l l c

Works.

Gulf Canada R e s o u r c e s d e s e r v e s

s p e c i a l mention f o r p r o v i d i n g accom-

modation a t t h e i r b a s e camp l n

Tuktoyaktuk. The s t a f f a t Tuk Base were

e x t r e m e l y h e l p f u l , and c o o p e r a t e d w i t h e n t h u s i a s m and i n t e r e s t . Mr. Brian Wright i s thanked f o r a r r a n g i n g t h i s , s u p p o r t , a s a r e o t h e r Gulf s t a f f who provided a d v i c e and i n f o r m a t i o n on o f f s h o r e c o n d i t i o n s , t h e s e i n c l u d e d

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D r . R. P i l k i n g t o n and Dr. V. Neth. D r . M c h e l Metge. who was s t a b i l i t y e n g i n e e r o n t h e Molikpaq a t t h e time,

i s

t h a n k e d f o r h i s encouragement and i n t e r e s t i n t h e p r o j e c t , and t h e very v a l u a b l e i n f o r m a t i o n he provided o n o f f s h o r e i c e c o n d i t i o n s . R e f e r e n c e s Abdelnour, R. and K.R. C r o a s d a l e , 1986. I c e Forces A s s o c i a t e d w i t h Ridge B u i l d i n g ; Model T e s t s and R e s u l t s Compared w i t h T h e o r e t i c a l Models. P r o c e e d i n g s o f IAHR I c e s y m p o s i m , U n i v e r s i t y o f Iowa, 1986. C r o a s d a l e , K.R., 1980. Some Implica- t i o n s of I c e N d g e s and Rubble F i e l d e o n t h e d e s i g n of A r c t i c O f f s h o r e S t r u c t u r e s . P r o c e e d i n g s of NRC Workshop on Sea I c e Ridging, Calgary. T e c h n i c a l Memo 134. N a t i o n a l Research C o u n c i l o f Canada. Ottawa.

C r o a s d a l e , K.R., 1984. The L i m i t i n g D r i v i n g F o r c e Approach t o I c e Loads.

_{- -}

~ r o c e e d i n g m of o f f

shore

Technology Conference. Houston, OTC Paper, OTC

6 7 16.

C r o a s d a l e , K.R., B.W. Graham, G. Comfort and R.W. M a r c e l l u s , 1986. E v a l u a t i o n o f I c e P r e s s u r e S e n s o r s and S t r a i n Meters i n a Large T e s t Basin. P r o e c e d i n g a o f fAAR Ice S y m p o e i u , U n i v e r s i t y of Iova, 1986.

C r o a s d a l e . K.R., G. Comfort and B.W.

Graham, 1986. A r c t i c Pack-ice Forces R e s e a r c h P r o j e c t . 1986. Report t o

Government of Canada, Dcpt. of Fisheries and Oceans. I n s t i t u t e f o r Ocean S c i e n c e s . S i d n e y , BC, Canada. H i b l e r , W.D. III,, 1980. Modelling a

V a r i a b l e Thicknkss Sea I c e Cover. Monthly Weather Review Vol. 108 No.

1 2 , December 1980.

M e l l o r , M., 1983. Mechanical Behaviour of Sea I c e . U.S. Army CRREL -&nograph 83-1, 1983.

-

Michel, B . , 1983. Elements of I c e Dynamics i n t h e A r c t i c Pack I c e . ~ i o c e e d i ngs of 8 t h I n t e r n a t i o n a l POAC Conference, N a r s s a r s s u a q . G r e e n l a n d , 1987.

Nevel. D.E., 1983. P r e s s u r e Ridge Forces. P r o c e e d i n g s of Seventh I n t e r n a t i o n a l PO AC C o n f e r e n c e , T e c h n i c a l Research C e n t r e of F i n l a n d , H e l s i n k i . 1983. P a r m e r c e r , R.R. and M.D. Coon. 1973. On t h e Mechanics o f P r e s s u r e Ridge Formation i n Sea I c e . P r o c e e d i n g s of O f f s h o r e Technology Conference

,

Houston, OTC Paper 1810, 1973.

P r i t c h a r d , R.S., 1977. The E f f e c t of S t r e n g t h on S i m u l a t i o n e of Sea I c e ~ ~ n a m i c s . P r o c e e d i n g s of 4 t h I n t e r n a t i o n a l POAC Conference, Memorial U n i v e r s i t y . S t . J o h n ' s , Newfoundland. 1977.

Rothrock. D.A., 1975. The E n e r g e t i c s of P l a s t i c Deformation of - - Pack I c e by Ridging. J. Geophys. Res. 8 0 , 4514- 4519, 1975.

V i v i t r a t . V. and J.R. K r e i d e r , 1981. " I c e Force P r e d i c t i o n Using a Limited D r i v i n g Force Approach. P r o c e e d i n g s o f Of f s h o r e Technology Conference

,

Houston. Paper 0TC4 11 5 , 198 1 . Metge,M.,A.Strilchuk and P . T r o f i w n k o f f . 1975. On Recording S t r e s s e s i n Ice. P r o c e e d i n g s of IAHR I c e 1975 Symposium, Hanover, N.H. pp.459-468.