The draft of ice plates and unevenness of their bottoms af- fect the resistance coefficient used to calculate the shearing resistance generated by flow. Although the maximum draft of ice plates during the observation period was approximately 2m, drafts as great as 3m were locally observed. It was also confirmed that the ice plate bottoms were not consistently but rather, irregularly uneven.
5. Verification and consideration of Ice Force
5-1 Comparison between design and actual values Using collected data, the comparison between measured tension and design values determined in the above- men- tioned design calculation formula was conducted over 2 peri- ods, for which aerial photographs of the inflow of ice floes are provided. The results shown in Table 7 indicate that the design values fell below the actual values in both periods. Because the flow velocity and the length of influenced ice field range were small during these periods, the tension was extremely small so that there were no structural or function- al problems. However, this small value of design calculation may lead to underestimation of the environmental forces of ice plates.
Table 7 Design and actual tension values
5-2 Examination of resistance coefficient
Regarding the thickness of ice plates and unevenness of their bottoms, no two- dimensional measurements were con- ducted for the resistance coefficient of ice plates (Cw=0.0011) used in design. According to research conducted by Kawai et al.2）, the fluid resistance of ice plates depends on the shear- ing force caused by the friction between the bottom surfaces of ice plates and water, and the shearing coefficient is greatly influenced by the unevenness of the bottom surfaces of ice plates.
During the observation periods, component of wind force was small and wind forces were negligible. Thus, the resis- tance coefficient Cw, which was treated as a constant in de- sign, is re- examined as an item relating to calculation of the load caused by flow.
The load strength applied by flow, τ wt, which consists of the shearing force applied by flow, Fsw, and the form resis- tance associated with flow, FDw, can be expressed as follows:
where
ρ w: sea water density
wo: unit weight of sea water
g : gravitational acceleration
Csw: friction coefficient between sea water and floating structures(=0.007)
COW: form resistance coefficient between sea water and floating structures(=0.6)
r : length of ice plates
H' : draft of ice plates (m)
(Values Csw and Cow were estimated using results of various model experiments on fluid force conducted by Ueda et al.3)
Meanwhile, in the design calculations, the force applied by flow was calculated as a single unit with Eq. (1), without splitting it into the shearing force and the form resistance. However, Eq. (6) was derived by substituting the length of influenced ice field range R with r. Then, by comparing the sum of Eqs. (4) and (5), and Eq. (6), Eq. (7), which expresses Cw with Csw and Cow, was derived. By substituting Csw and Cow, Cw can be obtained with Eq. (8) from the relation between H' and R.
Because H' was approximately 2m according to field obser- vation results, R was changed in cases where H'=lm, 2m, 3m and 4m, and Cw obtained with Eq. (8) is shown in Fig.7. When R was small, Cw was large because it was greatly influenced by the form resistance, but when R was large, Cw tends to be small due to less significant influence. When R=1,250m, which is a design condition of the facility, it was found that Cw for every H was 0.011 or less, the value used in design calculation.
