(linear distance between fixed structures), consists of 14 fixed structures, with 13 ice boom spans located between them.
There are 2 types of ice boom forms: the special type e- quipped with a net hanging beneath it and the normal type with no net. Although a normal type ice boom is effective up to a certain flow velocity, ice plates will turn around the ful- crum of an ice boom, and flow through underneath. To pre- vent this, the special type was prepared with a net equipped to increase the critical flow velocity. According to results from the model experiments conducted by Enoki et al., the critical flow velocities of both types vary depending on the form (size and thickness) of ice plates to be controlled. In the case of the facilities studied, when thickness of ice plates which may affect aquaculture facilities in the lagoon were determined to be 2.0m or more, the critical flow velocity was 1.4m/sec for the special type and 1.0m/sec for the normal type. Based on the estimated flow velocities, it was decided (Section B), and the normal type for the 6 spans on the to install the special type for the 7 spans in the center sides, 3 spans on either side (Section A and C). Both types of booms are designed to transfer the combined forces of wind and flow (environmental forces), which act on ice plates during ice floe control, to the fixed structure through the tension of the main wires.
Fig. 1 The location of the ice barrier
Fig. 2 The structure of the ice booms (Special type)
3. Design Method and Calculation of Tension
The environmental forces acting on the main wires of ice booms, load strength per unit length W, make ice booms draw a parabola on a plane and generate tension T at the main wires, corresponding to the angle θ at the fulcrum.1)
Design calculations were conducted for 2 models: where ice plates cover the water surfaces (ice floe approach situation) and where no ice plates exist and wave forces act on ice booms (wave situation). Although there may be inter- mediate states where ice floes approach the outside sea, waves in this status are known to be attenuated by ice plates. Therefore, it was decided that no external force other than those found in the models existed. As the facility's con- struction materials are designed for ice floe approach situa- tions, this section describes the environmental force calcula-tion process and the results obtained in this situation.
Fig. 3 Transmission of load
3.1 Environmental force of ice plates
Fig. 4 Calculation flow
(1) Load strength applied by flow τ wt(tf/m)
where
wo: unit weight of sea water (= 1.03t/m 3)
Cw: resistance coefficient of ice plates (at time of design: Cw=0.01 1, based on Mcphee's survey in the Arctic Ocean)
