This paper describes effects of surrounding ice sheets on foundation design of gravity based structures while an earthquake occurs. Extensive ice tank tests were performed to simulate an offshore structure placed in moving or non-moving ice sheets. We gave a sinusoidal vibration to a model foundation to represent an earthquake. The test results showed that moving ice sheets affect foundation loads and an interaction between structure and surrounding ice sheet may mitigate the foundation loads.
The current design codes for offshore structures do not handle ice and seismic load as combined loads. CSA (1992) regards that a seismic load is rare event and ice load is not considered as accompanied load. API (1995) notes that special consideration is given to an effect of ice when an offshore structure is placed in seismic area. The seismic load governs foundation designs when an offshore structure is located in an earthquake area like the Sea of Okhotsk. Offshore Sakhalin Island may be the first sea area where both ice and seismic loads are considered. The offshore Sakhalin area is covered with sea ice approximately a half year and it is probable that an earthquake will take place while sea ice surrounds the structure. It is quite important to clarify the effect of surrounding ice sheet to ensure safe operations. Miura et at. (1989, 1994) considered the effect of a non-moving ice sheet on the sliding stability using numerical analysis. On the other hand, little research for the case of moving ice floe has been done and the affect of ice sheet with some velocity is not fully understood. To clarify the effect of surrounding ice sheets, we performed vibration tests in ice tank as a first step to observe basic phenomena and obtain verification data to a mathematical model.
