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Proceedings of the IAHR International Symposium on Ice, II, pp. 516-525, 1981
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The salinity of artificial built-up ice made by successive floodings of
sea water
ANALYZED
- -
Ser
THl
IN21d
National Research
Conseil national
no.
1042
,
I
*
Council Canada
de recherches Canada
c. 2
B r n
- - J
THE SALINITY O F ARTIFICIAL BUILT-UP ICE
W
E
BY SUCCESSIVE FLOODINGSO F SEA WATER
by N. Nakawo and R. F r e d e r k i n g
Reprinted f r o m
P r o c e e d i n g s , IAHR International Symposium on Ice, Quebec 1981 Vol.
II
p. 516-
525 DBR P a p e rNo.
1042 Division of Building R e s e a r c h62-
ar-
23
g l B ~ ~ ~ ~ ~ P Q U E
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C N,--
l ~ @ * T P r i c e $1.00 OTTAWA N R C C 20379SOMMAIRE
Une f a s o n d'augmenter 1 1 6 p a i s s e u r d'une couche d e g l a c e c o n s i s t e
il
l a r e c o u v r i r d'eau. La g l a c e e s t constitu'ee d e couches s u c c e s s i v e s d ' e a u d e mer gel'ee. La s a l i n i t d d e l a g l a c e a i n s i f o d e joue un r a l etres
important dans l e s c a r a c t g r i s t i q u e s mgcaniques d e c e t t e g l a c e .On
a proc6dE2
d e s o b s e r v a t i o n s pr'ecises l o r s d e l a c o n s t r u c t i o n d'une plate-forme en g l a c e a r t i f i c i e l l e . La s a l i n i t ' e observge d t a i t d e l ' o r d r e d e20
% o , donc moindre que c e l l e de l ' e a u d e mer q u i d t a i tB
l ' o r i g i n e d e l a formation d e l a g l a c e , s o i t-30 X o .
Presque l a moiti'e d e l a p e r t e en s e l a v a i t eu I.ieu pendant l a p s r i o d e d e g e l d e l a couche ; l ' a u t r e moiti'e au c o u r s d e l a formation d e s a u t r e s couches s u c c e s s i v e s . Les a u t e u r s concluent q u ' i ly
a une b a i s s e d e l a s a l i n i t ' e dans l e s e n s h o r i z o n t a l e t d a n s l e s e n s v e r t i c a l pendant l a c o n s t r u c t i o n d e l a plate-fonne a p r e s a v o i r mesurd avec pr'ecision l a s a l i n i t c , d e s couches minces e t l e ddplacement d e s t e i n t u r e s .R . F r e d c r k i n g Q c s e n r c h n f f i c e r I l i v i s i o n of Bui l d i n g R c s c a r c h Canada N a t i o n a l R e s c a r r h C n u n c i l o f Canada O t t a w a , O n t a r - i o , Canada One method of t h i c k e n i n g a n i c e s h e e t i s " f r e e f l o o d i n g " : i c e i s b u i l t up b y s u c c e s s i v e f l o o d i n g and f r e e z i n p of s e a w a t e r l a y e r s . The s a l i n i t y of t h e b u i l t - u p i c e i s of g r e a t i n t e r e s t b e c a u s e i t n l a y s a n i m p o r t a n t r o l e i n e s t a b l i s h i n g m e c h a n i c a l p r o p e r t i e s . D e t a i l e d o b s e r v a t i o n s on s a l i n i t y of f l o o d e d E a t e r and b u i l t - u p i c e were c a r r i e d o u t d u r i n g c o n s t r u c t i o n of a n i c e p l a t f o r m . I c e s a l i n i t y was g e n e r a l l y a b o u t 2020 which i s s i g n i f i c a n t l y l o v e r t h a n t h e s a l i n i t y o f t h e o r i g i n a l s e a w a t e r (-30x0). Almost h a l f of t h e " l o s t s a l t s " d i s a p p e a r e d d u r i n g t h e f r e e z i n g p e r i o d o f a l a y e r ; t h e r e m a i n d e r were l o s t d u r i n g s u b s e q u e n t f l o o d i n g s ,
D e t a i l e d s a l i n i t y , t h i n s e c t i o n and d y e m i g r a t i o n measurements were used t o p o s t u l a t e p r o c e s s e s of l i o r i z o n t a l a s w e l l a s v e r t i c a l d e s a l i n a t i o n d u r i n g c o n s t r u c t i o n of t h e i c e p l a t f o r m .
'
P r e s e n t a d d r e s s : A p p l i e d P h y s i c s S e c t i o n , F a c u l t y o f E n g i n e e r i n g , Hokkaido U n i v e r s i t y , S a p p o r o , J a p a n 060Floating ice platforms have provided a very successful means for carrying out exploratory drilling in the Canadian arctic islands [I]. Since the first ice platform was constructed and used for drilling in 1974 the trend has been toward increasing rig
loads and lengthened drilling periods. Consequently, efforts have been devoted to developing an improved basis for ice platform design [2] as well as platform construction 131.
The technique used in constructinc) an ice platform is "free flooding", i.e., ice
built up in layers by successive flooding and freezing of sea water pumped from beneath the ice cover. Grain structure and salinity of built-up ice plays an important role in establishing its mechanical properties. Observations have shown that the salinity of the built-up ice is significantly lower than that of the original
sea water. An explanation of this desalination process is of considerable interest.
This paper presents field data on spatial and temporal distribution of salinity in built-up ice and discusses processes that control these distributions.
Description of Observation Site -
The field observations were made 14 to 20 January 1979, at Panarctic's Desbarates
6-73 well site which was located 76' 42' and 105' 57' W, northeast of Melville Island.
Two platforms were constructed at the site: one for the drilling operation and the other for the drilling of a relief well if necessary. The actual measurement program was carried out on the relief platform.
Two pumps located near the centre of the platform lift sea water from beneath the ice and discharge it on the surface. Average sea water salinity was 30.8 +0.6%0. Distribution of the water was controlled by periodic adjustment of the discharge direction of the pump nozzle. The resulting platform was elliptical in shape with maximum and minimum diameters of approximately 200 m and 100 m respectively. There was no confinement to the water flow and "free flooding" resulted in a platform with
maximum thickness at the centre, tapering to the n a t u ~ a l ice thickness at the edge,
i.e., the platform surface sloped slightly downwards from the centre (inclination about 0.4").
At the time of the field program the total ice thickness of the platform was
about 3 m at the centre and 1.8 m at the edge (natural ice thickness). The duration
of a single flood was 0.5 to 1.0 h, with a resulting layer thickness of 10 to 20 mm.
Flooding frequency was once every 3 to 5 h. Altogether 31 floods were made during the
observation period for a total ice build-up of almost 0.5 m.
Detailed observations were made along a radial line running outward about 60 m
from one of the pumps. This line corresponded to one of the short axes of the plat- form so the observation area covered a representative section extending from the centre to the edge. The field observations included measurements of salinity of the
sea water and Suilt-up ice and also temperature in the built-up ice.
A core was
r e c o v e r e d and r e t u r n e d t o t h e l a b o r a t o r y f o r f u r t h c r s a l i n i t y rneasuremcnts and a n a l y s i s o f g r a i n s t r u c t u r e .
R e s u l t s
S a l i n i t y o f t h e s u r f a c e l a y e r was measured a t a number o f p o s i t i o n s i n t h e o b s e r - v a t i o n a r e a s e v e r a l t i m e s d u r i n g t h e o b s e r v a t i o n p e r i o d . The s u r f a c e l a y e r samples were r e c o v e r e d s h o r t l y b e f o r e t h e n e x t f l o o d . l i o r i z o n t a l d i s t r i b u t i o n o f t h e s u r f a c e
l a y e r s a l i n i t y i s shown i n p a r t ( I ) ) o f 1:igurc I . N o t c t h : ~ t 111) t o 3 0 m , sllrf:lcc l n y c r
50 I I I I I !
A, DISCHARGED WATER AT PUMP 0 ( b )
0, RUNNING WATER O N SURFACE
40
1
C, RUNNING WATER O N SURFACE
1
F i g u r e 1 V a r i a t i o n s o f l a y e r t h i c k n e s s ( a ) and s a l i n i t y (b) with i n c r e a s i n g d i s t a n c c from pump D I S T A N C E FROM P U M P ,m
s a l i n i t y i s a b o u t 25%, b u t beyond 30 rn i t i n c r e a s e d t o about 35%,. T h i s f i g u r e a l s oshows t h e s a l i n i t i e s o f pump d i s c h a r g e w a t e r and w a t e r r u n n i n g on t h e s u r f a c e d u r i n g
f l o o d i n g . These d a t a show an i n c r e a s e i n s a l i n i t y w i t h i n c r e a s i n g d i s t a n c e from t h e
pump. P a r t ( a ) o f F i g u r e 1 p r e s e n t s a v e r a g e l a y e r t h i c k n e s s a s d e t e r m i n e d by rneasur-
i n g t o t a l i c e b u i l d - u p o v e r t h e o b s e r v a t i o n p e r i o d and d i v i d i n g by t h e number o f
f l o o d s i n t h a t p e r i o d ( 3 1 ) . Each f l o o d d i d n o t c o m p l e t e l y c o v e r t h e a r e a beyond 30 m,
which a c c o u n t s f o r t h e r e d u c t i o n i n a v e r a g e l a y e r t h i c k n e s s beyond 30 m .
From a c o r e t a k e n 18.5 m from t h e pump, v e r t i c a l t h i n s e c t i o n s were made and a
v e r t i c a l s a l i n i t y p r o f i l e e s t a b l i s h e d ( F i g u r c 2 ) . t a c h s a l i n i t y v a l u e r e p r e s e n t s t h e a v e r a g e o f t h a t p a r t i c u l a r l a y e r . The s u r f a c e s a l i n i t y o f t h e v e r t i c a l c o r e (25%,) c o r r e s p o n d s w e l l w i t h t h e a v e r a g e s u r f a c e s a l i n i t i e s shown i n F i g u r e 1 . The v e r t i c a l
Figure 2
Vertical salinity profile obtained for a core sample taken 1979-01-20 at a distance of 18.5 m from pump
profile shows that to a depth of 0.4 m salinity was in the range of 20 to 22%,. Below 0.4 m , there was a further decrease in salinity. The boundaries of the individual flood layers can be quite clearly distinguished as dark lines in the thin sections.
An enlargement of the profile at a depth of 0.28 m (reference level) is shown in Figure 3. Tl~e boundaries as shown in the thin section ,re indicated by the dashed lines. l?qe salinity profiles within a layer show a characteristic "double S"
distributi-on3 i.e., salinity is high on the layer boundary, decrea,;es to a minimum at the quarter point in the layer, increases to ,L peak at the mid-point in the layer, decreases to another minimum 2t the three-quarter point, and finally to another maximum on the boundary. Grain structure also shows a characteristic pattern: fine- grained granular ice on the boundaries, bands of elongated grains at the quarter and chrec-quarter points in the layer, and a very irregular band is in the middle of the layer.
Figure 3 Detailed salinity profile for four flooded layers in the core sample
Measurements were also made of temperature and salinity changes with tine in the
upper layers of the built-up ice. Part (a) of Figure 4 indicates time variations of
temperature at different depths in the built-up ice d u r i n ~ several flooding 2nd freez-
ing cycles. The temperature cilrves labelled Layers A.
B
and C were measured withinthe aost recently flooded layer. At the bcqinning of a flood these temperatures were
high as the relatively warm (-2'C) sea water was discharged onto the ice surface.
A rapid decrease in temperature followed the completion of the flooding. At greater depths in the ice, temperature fluctuations were attenuated and a phase shift was evident. Below a depth of about 200 mm ice temperature did not respond to individual floodings.
Part (b) of Figure 4 presents results of periodic salinity measurements of
particular layers during several flooding and freezing cycles. In the case of each flood the first point was the salinity of the sea water. Subsequent samplings showed a gradual decrease in salinity to about 25%, at the end of the freezing period.
During successive floodings (Layers
B
and C) there was a further decrease in thesalinity of the underlying ice (Layer A) to 20 to 22%, (the same salinity as found at intermediate depth in the vertical profile (Figure 2)).
In addition to the salinity, temperature and thin section observations, a dye
experiment (using Rhodamine B) was carried out to visually trace the directions of
TIME,
hIbl
3-0 0 - - ,-
o...t
\O'...
-*
%"..
..
LAYER C-
\ LAYER B1.
O b \"\ b-...-
VR
A \ \"
1
'
.,
-
.
-
: : . - o-
LOODING A FLOODING B FLOODING C
Figure 4 Time v a r i a t i o n o f t e m p e r a t u r e ( a ) a n d s a l i n i t y ( b ) f o r t h r e e flooding c y c 1 c s measured 1 9 7 9 - 0 1 - 1 8 a t a d i s t a n c e of 18.5 m from pump
b r i n e movements. The r e s u l t s o f t h e experiment a r e i l l u s t r a t e d i n Figure 5 . A f t e r
t h e c u r r e n t s u r f a c e l a y e r , a , had f r o z e n , a t r e n c h , 0 , was c u t normal t o t h e water
flow d i r e c t i o n . The t r e n c h was t h e n f i l l e d with dyed water and allowed t o f r e e z e . The
next f l o o d i n g produced l a y e r y . A f t e r l a y e r y had f r o z e n , a v e r t i c a l s e c t i o n was c u t
a c r o s s t h e t r e n c h . Figure 5 shows t h a t t h e dyed a r e a extended h o r i z o n t a l l y , i n t h e
water flow d i r e c t i o n , and downward, i n d i c a t i n g t h a t t h e r e was b r i n e movements i n t h e s e d i r e c t i o n s .
Another dye experiment was c a r r i e d o u t i n t h e a r e a where t h e v e r t i c a l c o r e was
F i g u r e 5 V e r t i c a l s e c t i o n s c h e m a t i c a n d p h o t o g r a p h o f d y e m i g r a t i c 1: e x p e r i m e n t
d a y s l a t e r , when t h e c o r e was r e c o v e r e d , t h e dyed zone e x t e n d e d downward from l a y e r 2 2 t o l a y e r 18. When a n o t h e r c o r e was r e c o v e r e d two months l a t e r , t h e dyed zone extended f u r t h e r downward t o l a y e r 1 3 . By t h i s t i m e , however, t h e c o l o u r i n t e n s i t y of t h e dye
had g r e a t l y r e d u c e d . Also n o t e w o r t h y was t h e a b s e n c e o f any a p p a r e n t upward m i g r a t i o n
o f d y e . D i s c u s s i o n
The dye e x p e r i m e n t s d e m o n s t r a t e d , i f o n l y i n a q u a l i t a t i v e f a s h i o n , t h a t b r i n e
moves v e r t i c a l l y downward and h o r i z o n t a l l y i n t h e b u i l t - u p i c e . I t i s p o s s i b l e ,
however, t o make some f i r s t o r d e r q u a n t i t a t i v e e s t i m a t e s of t h e s e b r i n e movements. The r e s u l t s p r e s e n t e d i n F i g u r e 1 showed an i n c r e a s e i n s a l i n i t y w i t h d i s t a n c e from t h e
pump. Combining t h e s a l i n i t y d a t a w i t h t h e a v e r a g e l a y e r t h i c k n e s s , a mass b a l a n c e
was c a r r i e d o u t on t h e s a l t i n a s e c t o r of r a d i u s 60 m from t h e pump and a r c 1 r a d i a n
( s e e T a b l e I f o r r e s u l t s ) . Area 1 c o m p r i s e s t h e zone from t h e pump o u t t o 30 m;
a r e a ? t h e zone from 30 m t o t h e p o s i t i o n where t h e a v e r a g e s u r f a c e l a y e r s a l i n i t y i s
e q u a l t o t h e s e a w a t e r ( 4 3 . 5 m); and a r e a 3 t h e zone from 4 3 . 5 m t o 60 m . Row 1 of
t h e t a b l e g i v e s t h e mass o f s a l t i n e a c h a r e a f o r an e q u i v a l e n t l a y e r o f s e a w a t e r ; row 2 t h e a c t u a l mass of s a l t i n t h e s u r f a c e l a y e r o f i c e ( c a l c u l a t e d from measured
T a b l e I . S a l t c o n t e n t d i s t r i b u t i o n i n an a v e r a g e l a y e r
Area 1 Area 2 Area 3 T o t a l
- - - -
Row 1 S a l t i n Water, kg 398 374 412 1184
Row 2 S a l t i n I c e , kg 323 340 457 1120
Row 3 D i f f e r e n c e , kg -75 -34 +4 5 -64
s ; ~ l i r l i t y and l a y e r t h i c k n e s s ) ; and row 3 t h e d i f f e r e n c e s . Area 1 e x p e r i e n c e d about a 20% Loss o f s a l t ( d e s a l i n a t i o n ) , a r e a 2 a b o u t a 10% d e c r e a s e and a r e a 3 a b o u t a 10% g a i n i n s a l t . Over t h e t h r e e a r e a s t h e r e was a n e t l o s s o f 5 % . From t h e s e numbers t h e r e i s e v i d e n c e o f a h o r i z o n t a l r e d i s t r i b u t i o n o f s a l t , d e c r e a s e i n a r e a s 1
and 2 and i n c r e a s e i n a r e a 3 , a s w e l l a s an o v e r - a l l l o s s o f 5 % from t h e s u r f a c e l a y e r due t o v e r t i c a l d r a i n a g e . In a r e a 1 d u r i n g t h e i n i t i a l f r e e z i n g p e r i o d , d e s a l i n a t i o n was e q u a l l y t h e r e s u l t o f v e r t i c a l and h o r i z o n t a l b r i n e movement. To q u a n t i f y t h i s breakdown more a c c u r a t e l y , e x p e r i m e n t a l measurements o f p e r m e a b i l i t y of b u i l t - u p i c e would be needed. V i s u a l e v i d e n c e o f h o r i z o n t a l b r i n e movement was i n d i c a t e d by t h e o b s e r v a t i o n o f h i g h s a l i n i t y b r i n e s e e p i n g from t h e o u t e r edge o f t h e t o p f l o o d e d l a y e r a t t h e end o f t h e f r e e z i n g p e r i o d . S i m i l a r o b s e r v e d i n d i c a t i o n s o f h o r i z o n t a l b r i n e movements were n o t e d d u r i n g f l o o d i n g e x p e r i m e n t s a t P o i n t Barrow [ 4 ] . The d r i v b g f o r c e f o r t h i s movement c o u l d be i n t e r n a l p r e s s u r e g e n e r a t e d w i t h i n a l a y e r d u r i n g f r e e z i n g . The r e s u l t s i n T a b l e I were f o r t h e i n i t i a l f r e e z i n g o f t h e s u r f a c e l a y e r . I f an a v e r a g e s a l i n i t y f o r a r e a 1 c o r r e s p o n d i n g t o s u c c e s s i v e f l o o d s (20%,) were u s e d , t h e p r o p o r t i o n o f d e s a l i n a t i o n due t o v e r t i c a l b r i n e d r a i n a g e would i n c r e a s e t o 10%. A s shown i n F i g u r e 2 t h e s a l i n i t y o f t h c b u i l t - u p i c e below t h e s u r f a c e l a y e r s i s a b o u t 20%, which i s s u p p o r t e d by t h e o b s e r v a t i o n s p r e s e n t e d i n F i g u r e 4 . I t a p p e a r s t h a t a f t e r t h e i n i t i a l d e s a l i n a t i o n (down t o 20%.) f u r t h e r v e r t i c a l b r i n e movements a r e by a d i s p l a c e m e n t p r o c e s s , i . e . , s a l i n i t y o v e r t h e d e p t h o f 0 . 0 5 t o 0 . 4 m d o e s n o t change. For d e p t h s below 0 . 4 m t h e d e c r e a s e i n s a l i n i t y can be a t t r i b u t e d t o t h e e f f e c t s c ~ f te m p e r a t u r e and t e m p e r a t u r e g r a d i e n t i n promoting b r i n e movements [S].
The d e t a i l e d s a l i n i t y p r o f i l e and t h i n s e c t i o n i l l u s t r a t i n g g r a i n s t r u c t u r e ( F i g u r e 3) can be e x p l a i n e d a s f o l l o w s . The h i g h s a l i n i t y and f i n e g r a i n e d s t r u c t u r e o f t h c i c e a t t h e t o p and bottom o f each l a y e r would r e s u l t from r a p i d f r e e z i n g when t h e s e a w a t e r comes i n c o n t a c t w i t h t h e c o l d i c e s u r f a c e and a i r . Subsequent i c e growth would o c c u r from b o t h t h e t o p and bottom o f t h e l a y e r , b u t a t a s l o w e r r a t e which would a l l o w more o f t h e b r i n e t o be e j e c t e d from t h i s p a r t o f t h e l a y e r . Here one would e x p e c t a lower s a l i n i t y and a l a r g e r g r a i n e d columnar s t r u c t u r e . The c e n t r e of t h e l a y e r , t h e l a s t p a r t t o f r e e z e , would have a h i g h e r s a l i n i t y and a more
Conclusions
An analysis of the results suggests that there are three stages of desalination. In the first stage, freezing period of the top layer, salinity is reduced from 31%, to 25%. as a result of vertical and horizontal brine movements. The second stage sees a further reduction in salinity to 20%. during the next two floods. The salinity remains approximately constant at this value until such dcpth i s reached where the ice
temperature is higher than -lS°C. During construction the tcmpcrature rc:~ches this level at a depth of about 0.5 m [3]. This leads to the third stage of desalination which is caused by warming of the ice. An extension of the third stage of desalination occurs in the spring with seasonal warming of the ice cover.
In addition to providing information on the desalination processes, the results given in this paper show the difficulties that would be experienced in trying to simulate in the laboratory the open system process for building up the ice that occurs in the field.
Acknowledgements
The authors would like to acknowledge the support provided by Panarctic Oils Ltd. and FENCO Consultants Ltd., and the assistance of D. Wright, Technical Officer, Division of Building Research, National Research Council of Canada, in carrying out the field measurements.
This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Division.
References
[l] Baudais, D.J., Masterson, D.M., and Watts, J.S. A system for offshore drilling in the arctic islands. 25th Annual Technical Meeting of the Petroleum Society of CIM (Canadian Institute of Mining and Metallurgy), Calgary, Alta., May 1974, Paper No. 374029.
[2] Masterson, D.M., Anderson, K.G., and Strandberg, A.G. Strain measurements in floating ice platforms and their application to platform design. Can. J. Civ. Eng., Vol. 6 , No. 3, p. 394-405, 1979.
[3] Nakawo, M. Heat exchange at surface of built-up ice platform during
construction. Cold Regions Science and Technology, Val. 3, p. 323-333, 1980. [4] Dykins, J.E., and Funai, A.I. Point Barrow Trials - P.Y. 1959; Investigations of
thickened sea ice. U.S. Naval Civil Engineering Lab., Port Hueneme, Calif., Tech. Report R185, 1962.
[5] Adams, Jr., C.M., French, D.N., and Kingery, W.D. Solidification of sea ice.
T9E
S A L I N I T Y O?AAFITI?'ICIAL BUILT-TJ'P I C E
MADE
BY S U C C E S S I V E
FLOODINGB O F SEA WATE2
by
?4.Nakawo and
R.
rrederking
Discussion by Andrew Assur,
VSA C W E L
The detailed study of salinity distribution after flooding is
certainly a welcome contribution. The desalination is quite modest,
i.e. the resulting salinity is still quite high. At low temperatures
(approx.
- 2 5 O ~ )this would not matter, but under warmer conditions a
gradual deterioration could take place. What are the observations?
dhat are the salinity changes after several months?
Zeply to discussion
Some observations carried out in early Yay, when ice temperatures
were approaching
- l O ° C ,
still did not show signs of deterioration or
further significant desalination.
B ylate June, however, there were
obvious signs of deterioration and ice salinities were of the order
of
5%..
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