Theoretical ice growth and melt

Figure 10. Dates of significant ice formation at the Phillips Platform with respect to freezing degree-days (assuming a base temperature of 3.9C).

Cook Inlet marine ice data

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The twice-per-day tides bringing in warm, saline wa-

Table 5. Ice design criteria for Cook Inlet pe-

ter from the Gulf of Alaska,

troleum industry platforms. (After Visser

The tides moving ice from freshwater areas where it

1992, p. 148.)

grows more rapidly to more saline water where it

tends to melt, and

Design Criteria

North- and northeast-trending winds during the win-

ter, which move ice down-Inlet to areas of melting.

Original

API (1988)

Design parameter

design

and amounts of solar radiation and snow cover on the

Ice thickness, level ice (m)

1.1

0.60.9

ice. Also, southern air masses with above-freezing air

Ice thickness, rafted ice (m)

1.21.5

temperatures can sometimes blanket the region for

Compressive strength,

3.8

3.44.1

unconfined (MPa)

days at a time, causing rapid decay of a well-

Compressive strength,

300

275330

established ice cover. Occasionally volcanic eruption

confined (MPa)

has been known to melt the ice cover. This was in-

40 106

42 106

Maximum load (MN)

deed the case in January 1976, when an eruption of

Mount St. Augustine during a 5-day period raised the

water temperature at the Dolly Varden platform

(approximately 24 km NNE of West Foreland) from

2.2 to +1.0C. At the same time the ice concentration

in the Inlet reportedly went from 95% to less than 50%

(Schulz 1978).

3.3 Theoretical ice growth and melt

Sea ice growth and decay is a highly complex sub-

Compared to other arctic regions where multi-year

ject that relates to a wide array of environmental fac-

ice and icebergs occur, Cook Inlet ice conditions might

tors, such as air temperature, solar input, salinity of

be considered mild. However, they are significant be-

the water and the ice, and exposure to wind, waves,

cause the first offshore developments in ice-infested

and tides. In Cook Inlet it is especially difficult to pre-

waters for the petroleum industry were built in Cook

dict ice conditions because of the extreme magnitudes

Inlet. Four oil fields and one natural gas field were

and constant fluctuation of the variables discussed in

developed in the early 1960s in Upper Cook Inlet. The

Section 3.2. However, since Poole and Hufford (1982)

first fourteen offshore drilling and production plat-

determined a reasonable correlation between accumu-

forms were built during 19641968 (Visser 1992), and

lated FDDs and Upper Inlet freeze-up, temperature

environmental data necessary for their design were

appears to be the best single indicator of the severity

largely nonexistent. Some of the earliest ice environ-

of the ice season. In this section we calculate theoreti-

ment studies in Cook Inlet were financed by the pe-

cal ice sheet thickness values for Cook Inlet using

troleum industry to ensure that the structures were con-

simple models based on the average air temperatures

structed to withstand the forces imposed by the moving

in Anchorage. The models assume that when the mean

ice pack. The fact that there are now hundreds of plat-

daily air temperature is less than a specified value, ice

form-years of performance experience without a ma-

will form, and when the temperature is higher than a

jor failure testifies to the competence of the structural

specified value, the ice will melt.

designs. Several published sources describe the data,

LaBelle et al. (1983) stated that Brewster's calcu-

procedures, and assumptions that engineers used to

lations of ice growth in Cook Inlet were made using a

design the offshore and along-shore structures in Cook

reformulation of Zubov's (1945) model:

Inlet (e.g., Blenkarn 1970, Sanderson 1988, Visser

1992, Utt and Turner 1992, Bhat and Cox 1995). Visser

I 2 + 50 I = 8R

(1)

(1992) listed parameters that governed design of the

original platforms and compared them with recom-

where I is the ice thickness (cm) and R is the number

mended guidelines that were later adopted by the

of freezing degree-days that had accumulated in An-

American Petroleum Institute (API 1988). These are

chorage, assuming a base temperature of 0C. Solv-

shown in Table 5. The original design assumed a level,

ing the quadratic equation in the manner of Bowditch

undeformed ice thickness of 1.1 m, which the API later

(1979) yields a form of the equation from which the

amended to 0.60.9 m in its guidelines. The API also

ice growth, Ig, can more easily be determined as a func-

recommends a design thickness of 1.21.5 m for rafted

tion of the sum of FDDs. That is,

ice.

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