In order to investigate the interaction between an offshore structure and the foundation soil subjected to seismic and ice loads, a substructure online dynamic testing system was developed. A dynamic response analysis by computer and a pseudo-dynamic loading test which produces the restoring force experimentally are combined by a computer on-line data processing system. The analyses were carried out on an offshore gravity based structure on a sand seabed subjected to ice load and earthquake motion. Residual deformation of the foundation soil was observed because of development of pore water pressure in the sand layer due to cyclic loads induced by a drifting ice floe and an earthquake. As a result, the unique response behaviour of a soil foundation and structure was observed in the present study.
On April 12, 1986, the Molikpaq deployed in the Canadian Beaufort Sea was subjected to dynamic force induced by drifting ice floe (Jefferies et al. 1988, Kato 1997). Although ice loading was considered to be a quasi-static situation, much of the ice loading of the Molikpaq at significant load levels has been associated with cyclic, dynamic loading. Development of pore pressure was measured by piezometer installed in the sandffll core. This led to a research effort on the stability of the soil foundation underneath offshore structures in sea ice regions. Similar platforms are proposed for the Sea of Okhotsk near Sakhalin. In this area, not only ice load but also seismic force should be considered in practical design. For the purpose of investigating the interaction between an offshore structure and foundation soil subjected to ice load and seismic forces, a substructure on-line dynamic testing system was developed in this study by combining the response analysis with simple shear tests on real soil elements.
