Forced refreezing method for the formation of high strength ice structures

A method for accelerating construction of a load bearing ice island, formed by either sea water spraying or flooding techniques, of higher quality or in a shorter time or both than would otherwise be possible. The method involves forced refreezing of spray ice by application of a vertical stream of cold ambient air, as produced by a fan or other devices described, directly downward on the ice surface or by application of the downwardly directed air stream to an impounded mass of sea water. The specific application for the process is construction of improved load bearing structures as used in Arctic regions in support of offshore hydrocarbon exploration and production activities.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved method for accelerating the 
freezing of ice, initially formed by the freezing of a sea water spray or 
impounded sea water, and more particularly to an improved method to form 
an engineered load-bearing ice structure of high quality and in a shorter 
time than normally could be obtained. 
Rapid freezing of sea water is important in certain applications such as 
the construction of load-bearing ice structures in offshore Arctic regions 
where such structures are employed in conjunction with hydrocarbon 
exploration and production and in the construction of airfields, roads, 
camps and the like. In these applications, sea water is used exclusively 
as the aqueous medium and construction is usually started as soon as the 
ambient air temperature is sufficiently low to cause freezing of the sea 
water. It is economically advantageous to be able to cause the freezing of 
sea water to proceed as rapidly as possible so that load-bearing 
structures may be constructed in a relatively short period of time so as 
to extend to the maximum degree possible the utility of the manufactured 
structure. 
A method commonly employed to form ice structures involves the propelling 
of sea water through the air as essentially a stream of sea water and over 
significant horizontal distances. The volume of the continuous stream may 
range up to 30,000 gallons per minute from a single nozzle used to propel 
the salt water over the needed distance. The air, by virtue of its low 
temperature with respect to the nominal freezing temperature of sea water 
(-1.6 to -2.0 degrees C depending on salinity), acts as a coolant. The 
formation of droplets and the interaction of the sea water stream/droplet 
spray with cooler air results in freezing of the projected droplet spray. 
The efficiency of freezing depends on efficient heat exchange between the 
sprayed droplets and air. Formation of water droplets and the size of the 
droplets ultimately governs freezing efficiency at any ambient air 
temperature less than the nominal freezing temperature of the sea water. 
At the spray nozzle, the bulk of the sea water is in the form of a solid 
stream of water having high momentum in order to cover the desired 
relatively large horizontal distance. In the vicinity of the nozzle, shear 
and turbulent forces along the periphery of the water stream initiate 
droplet breakup and segregation. Along the trajectory of the 
stream/droplet spray, wind forces and gravitational forces promote 
increasing droplet breakup and segregation. Maximum droplet breakup, in 
the absence of significant wind forces, occurs at the apogee of the stream 
trajectory. The surface tension of the sea water is the fundamental 
property which governs how soon discrete water droplets will form and 
their size distribution for any imposed set of ambient conditions. 
Load-bearing ice structures are also commonly built by forming a berm or 
dike and then flooding the impounded area with sea water, the process 
being repeated, after freezing of the sea water, as necessary until a 
desired thickness of ice has formed. Ice structures which are used as the 
support unit for large drill rigs are themselves large. Construction may 
require one or more months. It is necessary, therefore, to accelerate the 
ice construction phase so as to allow maximum time for drilling activities 
prior to the onset of the Spring thaw. The more or less routine 
application of flooding-spraying technology in conjunction with offshore 
Arctic application is described in the prior art, U.S. Pat. No. 4,048,808 
being a typical example. 
In accordance with this invention, it has been discovered that the 
governing property of a high volume sea water stream is formation of water 
droplets varying in a size from 1 to about 3 mm in diameter. These 
droplets freeze in the form of hailstones, which are rounded or spherical 
masses of ice. The interior of the frozen droplets commonly contain liquid 
water of high salinity consistent with finite freezing rates and 
thermodynamic constraints that govern the freezing of saline solutions 
which have a true eutectic. Successful ice construction requires that the 
projected sprayed material which falls to the surface have a liquid 
content. Some droplets crush on impact releasing additional brine. The 
fallen material undergoes partial melting and then refreezing. Excess 
brine drains either away from the structure by virtue of its reduced 
freezing temperature, caused by partial evaporation during flight and by 
salt rejection that occurs simultaneously with freezing or remains 
entrained in the porosity of the spray ice. On impact with the ground, the 
brine is released and there is some partial melting of the frozen 
material. The newly formed slush then refreezes upon exposure to ambient 
temperature air. The refreezing which occurs after impact is the phenomena 
that is responsible for strength development in sprayed ice. 
In ice construction, where the aim is to build a substantial load-bearing 
structure of a relatively large dimension, dry snow is undesirable and 
detrimental because snow contributes to a general weakening of the 
manufactured structure and snow does not possess the substantial strength 
of ice. 
Sea water spray construction of ice islands is a complex process that 
includes several important phenomena which collectively control the 
properties of the manufactured structure. Sea water is usually applied as 
a spray. The freezing of the spray is controlled by ambient climactic 
conditions, the volume of spray and the size distribution of water 
droplets within the spray. Spray ice, which consists of a mixture of ice 
and brine and/or precipitated salt may, depending upon ambient temperature 
and wind conditions, partially remelt upon impact and then slowly 
refreeze. Typically, spray ice construction is a cyclic process where sea 
water is sprayed for a period of time and then spraying is terminated to 
allow refreezing of the sprayed surface. The cycle is then repeated as 
necessary to produce the desired structure. Internal structure of spray 
ice reflects the cyclic nature of its formation. 
Manufactured ice consists of alternating layers of relatively hard ice 
immediately underlain by a much thicker layer of much softer material. The 
internal structure of an ice island is a direct reflection of the 
techniques used for its construction. 
The basic methodology for construction of an ice island using sea water 
spraying techniques, consists of freezing a sea water spray by the cooling 
action of ambient temperature air on the spray. Since sea water must be 
sprayed in large volumes over considerable horizontal distances, nozzles 
are selected primarily for their throwing or spraying distance. This 
requirement places rather stringent controls of the size of water droplets 
which form in the spray. It is the discrete water droplets which 
ultimately freeze and fall to the ground. 
As droplets form in the spray, they freeze in the form of spherical 
hailstones consisting of ice. The cores of many of the larger hailstones 
contain brine significantly more saline than the source sea water due to 
partial evaporation of sprayed sea water and salt rejection during the 
freezing process. Upon impact, some hailstones shatter releasing brine. 
Depending upon ambient temperatures, some free, unfrozen brine may also 
reach the ground unfrozen but concentrated by partial evaporation. The 
spray may reach heights above ground surface of two hundred (200) feet or 
more. Air temperature differences between the maximum height attained by 
the spray and ground level can also encourage partial remelting of spray 
ice. 
The saline brine contacts previously sprayed and frozen material and causes 
partial melting of this material. The residue brine as a consequence of 
the partial remelting decreases in salinity. The newly formed slush is 
then slowly refrozen by the action of the ambient air. The slush refreezes 
from its surface downward. As the initial upper surface refreezes, lower 
levels of the slush are insulated from direct air contact and they freeze 
at a lower rate. As a result of this process, the sprayed ice consists of 
cyclic deposits of hard ice immediately underlain by softer material that 
was prevented from fully freezing. If spraying is stopped and then resumed 
at a later time, the newly fallen material will cause partial remelting of 
the previously frozen surface. Thus, the thickness of the hard ice surface 
is probably never as great as it was when originally formed just before 
resumption of spraying. 
A thermal gradient exists from the sea water-ice interface to the ice-air 
interface. Thermistor arrays are usually buried in an ice island during 
construction, and temperature data derived from these devices graphically 
demonstrate the heat transfer phenomena. Thus, partial remelting of newly 
formed spray ice is also a reflection of heat transfer from the warmer sea 
water to the colder free ice surface. 
The primary factors that govern spray ice construction can be summarized as 
follows: (1) the freezing dynamics of a sea water spray, and (2) the 
refreezing of spray ice. 
In the past, researches have concentrated on understanding spray freezing 
phenomena. Essentially, no attention has been devoted to the problem of 
spray ice refreezing. The dominating importance of spray ice refreezing 
can be readily understood when it is noted that during a typical twenty 
four (24) hour period, sea water may be sprayed for ten (10) hours or less 
whereas the remainder of the twenty four (24) hour period is spent waiting 
for spray ice to refreeze. Any improvement resulting in a diminution of 
the time required to refreeze spray ice may have dramatic and significant 
impact on overall construction time and cost. 
The time required to refreeze spray ice after a spraying period is the 
major factor that influences the time required to build an ice structure. 
It would be desirable, therefore, to provide improved and relatively 
simple methods for accelerating spray ice refreezing. 
SUMMARY OF THE PRESENT INVENTION 
In brief, the present invention focuses on acceleration of the formation of 
load bearing ice structures and more particularly to the acceleration of 
the refreezing of ice structures during their construction. In one form, 
the method of this invention involves use of a conveyance to move a 
ventilation fan across the newly deposited ice surface. Normally, 
refreezing of spray ice occurs by ambient air cooling. Wind blows cool air 
horizontally across the ice surface. However, the efficiency of the 
process is limited by thermal effects which retard heat heat transfer when 
the ice surface initially refreezes thereby insulating lower lying 
material from the direct cooling effects of ambient temperature air. 
Furthermore, wind velocity in the boundary layer adjacent to the ice 
surface may be a small fraction of wind forces at higher levels above the 
ice surface. 
The method of the present invention involves forced refreezing by directing 
a vertical column of air downward on the ice surface with sufficient force 
to disrupt the surface material and, thereby, to cause cooling to a 
greater depth than would be otherwise possible. The roughened air-blown 
surface may then be resmoothed by a rake attached to the ventilation fan 
conveyance. Another approach involves mounting the fan directly on 
self-contained power units. Other methods for direction of air columns 
downward in a spray ice surface include use of helicopters of hydrofoils 
operated over the desired area or tracked vehicles or use of winches and 
cranes to support or transport any one of a number of different well known 
devices to move a vertical air column across the spray ice surface. 
Ice construction using flooding techniques is effective and routinely 
practiced in Arctic regions because it is possible to freeze a shallow 
impounded mass of sea water. Cooling occurs at the water-air interface. An 
intrinsic property of water is the attainment of maximum density at a 
temperature slightly above its freezing temperature. This property allows 
for more uniform cooling of a large impounded water mass. 
The forced refreezing method can, therefore, equally be applied to the 
accelerated freezing of impounded sea water. 
Application of the forced refreezing method, whether applied to the 
refreezing of spray ice or to the accelerated freezing of impounded sea 
water, will significantly improve the mechanical properties of the ice 
structure, where improvemnt in load-bearing strength and shear resistance 
is desirable. This improvement is obtained because refreezing of spray ice 
or accelerated freezing of impounded sea water, occurs over a greater 
depth range, by virtue of the forced refreezing of the downward directed 
air column which contacts the spray ice or impounded sea water over a 
greater vertical depth than could be obtained normally by the action of 
wind blowing more or less horizontal with respect to the local ground 
surface. 
In accordance with the present invention, enhanced cooling or forced 
refreezing of spray ice or forced freezing of impounded sea water can be 
accomplished by use of a large downward-facing fan that is moved over the 
freshly sprayed or flooded surface to decrease the heat transfer 
resistance between the ambient temperature and surface temperature. There 
are two important factors that work together to increase the freezing 
speed considerably. These two factors are that the heat transfer 
coefficient is much greater in stagnation flow, compared to parallel flow; 
and, in a related aspect, the blowing arrangement ensures that the cold 
far-field temperature is brought in closer proximity of the surface. 
Virtually any technique for moving fan, or other source of downwardly 
directed frigid air, across a surface may be employed. By the present 
invention, it is the movement of large volumes of cold ambient temperature 
air downward against a layer of freshly prepared spray ice or impounded 
sea water which is important and for the purpose of more quickly and 
completely freezing or refreezing the surface material. The air stream 
produced by the fan can be controlled so that spray ice or impounded sea 
water may be cooled over a greater depth than is possible by natural 
cooling due to wind movement horizontally across the spray ice or 
impounded sea water surface. This more efficient cooling will lead to more 
complete freezing and refreezing and, thereby, production of a stronger 
structure in a shorter time. 
In Arctic regions, it is common practice to employ wheeled and tracked 
vehicles in conjunction with ice island and other types of construction 
activities. Modification of these devices by addition of the ventilation 
fan is practical, feasible, and by means disclosed herein, beneficial in 
providing for more rapid and complete freezing and refreezing of spray ice 
and impounded sea water. Application of the methods disclosed herein will, 
therefore, significantly shorten the time normally required to fabricate 
an ice structure and, therefore, reduce construction costs. Furthermore, 
application of the disclosed methods will result in ice structures having 
greater inherent load-bearing capacity and resistance to shear, by virtue 
of more complete freezing, than could otherwise be reasonably expected by 
application of what is generally recognized to be standard and accepted 
ice structure construction practice. 
An obvious implication of the forced refreezing method is its extension to 
ice construction involving primarily the preparation of offshore ice 
roads, camps, air fields, parking ramps and the like. 
It is apparent from the foregoing brief description that the present 
invention offers many advantages over the prior art methodology. These and 
other advantages and other objects are made more clearly apparent from a 
consideration of the several forms in which the present invention may be 
practiced. Such forms are described and forms of the various apparatus 
which may be used in the practice of this invention are illustrated in the 
present specification. The forms described in detail are for the purpose 
of illustrating the general principles of the present invention; but it is 
to be understood that such detailed description is not to be taken in a 
limiting sense.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, load-bearing ice structures may 
be fabricated from frozen sea water and in those geographic areas and at 
those times of the year in which the ambient air temperature is below 
about minus one degree C. The fabrication of ice structures, in accordance 
with the present invention, also contemplates the continued maintenance of 
a site in those regions amenable to construction of ice structures. Thus, 
for example, roads or aircraft runways and the like may be partially 
completed by conventional construction and completed or processed in 
accordance with the present invention. 
There are two basic modes of practicing the improved ice construction 
methodology of the present invention. In one mode, a spraying technique, 
as described, may be used. In the other a berm is formed to impound sea 
water and thereafter the construction proceeds in accordance with this 
invention. 
Ice construction applications involving the freezing of sea water sprays 
benefit from a reduction in the time required to refreeze partially melted 
spray ice. In similar fashion, more rapid freezing of impounded sea water 
would be desirable and beneficial. Accelerated rates of freezing of spray 
ice and impounded sea water can be obtained by directing a controlled 
column of frigid ambient air vertically downward against the surface to be 
frozen. The air temperature should be at least below about minus one 
degree C. in order to effect freezing of sea water. 
As mentioned, in the use of spraying techniques, the spraying operation, in 
addition to providing for the formation of ice particles, by the freezing 
of water drops, results in the formation of a slush ice which is of a 
salinity greater than the normal salinity of sea water. The slush ice is, 
in effect, a residue having a salinity somewhat higher than that of the 
sea water initially frozen from the droplet spray. As noted, the 
refreezing of this slush ice is responsible for the development of 
strength in the formation spray formed ice structures. In the case of spay 
ice construction, it is this refreezing which adds to the time of 
construction and which is needed in order to develop the desired strength 
of the load-bearing ice structure. 
By the present invention, an initial ice structure is formed. For the 
purposes of this invention, the initial ice structure is that initially 
formed at the start of the construction and which, in effect, forms the 
base upon which the final ice structure is constructed. Overall, the 
process is cyclical, involving spraying, freezing and refreezing, and 
spraying etc., a cycle that is repeated until the structure is completed. 
By the present invention, the freezing and refreezing portion of the cycle 
is shortened and the nature of the frozen product, in terms of its load 
carrying qualities, is improved over prior practices. To effect this 
improvement, it is necessary to effect reasonably rapid freezing of the 
slush ice or impounded ice, in order to achieve a depth of frozen ice 
which enhances the loadcarrying ability of the finished ice structure. 
By the present invention, this is accomplished by the formation of an 
initial ice structure, either by spraying or impounding procedures, 
followed by directing downwardly towards the surface of the initial ice 
structure a controlled column of frigid ambient air. Since the surface of 
the initial ice structure possesses sufficient integrity to support 
weight, vehicles may be used to transport equipment intended to generate a 
downwardly vertically directed column of air. Thus, the methodology 
involves traversing the initial ice structure while directing the column 
of air against the surface of the ice structure. in general the entire 
surface of the initial ice structure is traversed, although this may not 
be necessary for those portions intended not to be significant 
load-bearing regions of the completed ice structure. 
After the first pass, additional sea water is sprayed or added to the 
impounded area and the process is repeated. In those instances in which 
the surface of the initial ice structure is such that it is undesirable to 
use ground vehicles, a helicopter may be used in which case the main rotor 
down wash forms the controlled column of air which is directed against the 
ice surface. 
As an example of the type of vehicles which may be used, reference to the 
drawings, FIGS. 1 through 3, which illustrate typical land vehicles of the 
type used in the Arctic region. As illustrated in FIGS. 1 and 1a, a 
ventilation fan 10 and its associated speed control and electric power 
generator 12 are mounted on a wheeled platform 15 that is towed behind a 
wheeled primary power unit 20. The power unit 20 may, for example be a 
unit known commercially as a ROLL-E-GONE power unit. 
The air rate is adjusted so as to disturb the spray ice surface with air 
penetration into the spray ice or, alternatively, into a layer of 
impounded sea water. Disruption and dispersion of spray ice is minimized 
by placement of a shroud 25 about the fan which also serves to channel the 
column of frigid air downwardly. Disrupted and refrozen spray ice may be 
converted to a smooth surface by passage of the rake 30 located at the end 
of the fan platform 15. In use, the vehicle traverses the initial ice 
structure while the fan blows a column of frigid air downwardly towards 
the surface. One pass is usually sufficient, depending upon the capacity 
of the fan and the rate of travel. If necessary a partial or added pass 
may be made, as needed. Thereafter, spraying is continued or additional 
sea water is added to the impounded area formed by the berm. 
Alternatively, the fan conveyance of FIGS. 2 and 2a may be employed, in 
which cases, the various components, such as the fan 50 and the generator 
52 are mounted on the bed 55 which is combined into a single power unit. 
The shroud 65 is located as illustrated, with the rake 66 mounted on the 
end of the bed. The unit illustrated in FIGS. 3 and 3a is similar to that 
of FIGS. 2 and 2a except that the vehicle is a tracked vehicle 75, as 
shown. 
In use, a layer of spray ice of six (6) to twelve (12) inches thickness is 
formed. Sea water spraying would then cease for the period required to 
freeze the deposited material by passage of the fan. Sea water spraying or 
flooding would then resume and the cycle of spraying or flooding followed 
by forced refreezing would continue as necessary until an ice structure of 
desired size were built. 
It will be apparent from the above detailed disclosure that various 
modifications may be made, based on the above detailed disclosure, and it 
is understood that such modifications as will be apparent to those skilled 
in the art are to be considered within the scope of the present invention 
as set forth in the appended claims. So, for example, the passage of a 
helicopter over an impounded body of sea water would be but another 
instance of the application of the present invention. Similarly, the 
passage of a hydrofoil or hovercraft, which is a vehicle that moves on a 
cushion of air, over a spray ice surface or a body of impounded sea water, 
can be seen to be but another embodiment of the forced refreezing method.