Because R during the observation periods compared in 5-1 ranged from 20 to 120m, Cw was in the range greater than 0.011 in the design value, as shown in Fig.6. Therefore, Cw, which corresponded to R, was obtained and used to calculate tension by using Cw, to compare actual and design values. The results are shown in Fig.8. It shows that the actual and calculated values on Feb22 closely corresponded, and on Feb.21, it could be confirmed that, compared with de- sign values, actual and calculated values tended to closelycorrespond, although a certain degree of dispersion was seen.
Fig. 8 Comparison between actual and calculated tension values
5-4 Comparison by changing resistance coefficient
In Fig.7, the actual value measured at 13:00 on Feb21 was almost twice the calculated value. When confirming this by aerial photographs, it was found that this time corresponded to the time of large ice- plate collisions, leading us to believe that an impulsive force acted.
Because load is considered in design as static external force, impulsive forces caused by ice plate collisions were not taken into account. If, however, large ice plates collide when the length of influenced ice field range is within the design condition (R=1,250m), the impulsive force is not likely to ex- ert an acute influence on tension. Therefore, it can be thought that there is no problem in design.
However, in cases of short length of influenced ice field range, design conditions for facilities other than those studied here, the impulsive force generated by large ice plate colli- sions or other factors can pose major design problems.
6. Summary
Based on the results of field observations and subsequent examinations, the items confirmed regarding the tension of the main wires are summarized below:
(1) Although tension responds to flow velocity during inflow, no correlation was found between them. It was confirmed that external force is not solely generated by fluid force.
(2) Tension is greatly affected by the length of influenced ice field range and resistance associated with the form and thick- ness of ice plates. When the length of influenced ice field range is relatively short, in particular, it was confirmed that the resistance coefficient increases.
(3) Impulsive force generated by large ice plate collisions has little effect when the length of influenced ice field range is relatively long, as seen the facilities observed here, but may pose major design problems when considering installation of facilities with shorter lengths of influenced ice field range.
7. Conclusion
During the winter of 1995, inflow of ice floes was not ac- tive, compared with normal years, and the ice floe approach period was intermittent. However, the relation between tension and the length of influenced ice field range and the resistance coefficient of ice plates, which could not be evaluated in 1994 in the facility's initial installation stage, could be examined, even for a limited period of time.
In 1996, 2 more spans on each side of the 6 existing spans are scheduled to be installed. Effective and accurate research is being planned to examine the relation between ice force and tension, in situations where the installation of 10 spans is nearing completion. It is important to proceed with statistical analyses using the accumulated data to estab- lish ice floe control methods.
References
1)A.Imaizumi, T.Ueda, K.Enoki, T.Teraya, H.Saeki, T.Sasajima: Design Method of Ice Boom. The 7th Cold Region Technology Conference(1991)
2)K.Kawai. F.Hara, K.Enoki. A.Nagano. S.Kumirnatsu. H.Saeki: Hydraulic Resistance Acting on Ice Floes, The 11th International Symposium on Okhotsk Sea & Sea Ice(1996) 3)T.Ueda, H.Saeki, T.Yamashita, Y.Muraki, K.Enoki: Fluid Force Acting on Floating Ice Floes and Their Drifting Velocity, Proceedings of Civil Engineering in the Ocean Vol.8(1992)
