Online Response Test on Saturated Sand Seabed Underneath Structure Subjected to Ice and Seismic Loads
Authors: Hyodo Masayuki , Fujii Teruhisa , Fukuda Kenjirou , Kamezaki Kazuhiko , Yamauchi Yutaka 
Published: October 2008

In order to investigate the interaction between offshore structure and base ground subjected to seismic and ice loads, a substructure on-line dynamic testing system was developed. A dynamic response analysis by computer and a pseudo-dynamic loading test which obtains the restoring force of material experimentally are combined by a computer on-line data processing system. The analyses were carried out on an offshore gravity structure based on sand seabed subjected to ice load and earthquake motion by this method. Residual deformation of base ground were observed because of development of pore water pressure in the sand layer due to cyclic loads induced by drifting floe and earthquake. As a result, unique response behaviour of base ground and structure was observed in the present study.

Introduction

On April 12, 1986, the Molikpaq deployed in the Canadian Beaufort Sea suffered dynamic force induced by drifting floe1), 2). Although ice loading was considered as a quasi-static situation, much of the ice loading of the Molikpaq at significant load levels had been associated with cyclic, dynamic loading. Development of pore pressure was measured by piezometer installed in the sandfill core in it. This led to a research effort on the stability of base ground underneath the offshore structures in ice-sea. The similar platforms are constructed in Okhotsk near Sakhalin. In this area, not only ice load but also seimic force should be considered in practical design. For the purpose of investigating the interaction between offshore structure and base ground 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 the simple shear tests on real soil elements. In the present method, a computer analysis of dynamic response and a pseudo-dynamic loading test were combined with a computer on-line data processing system3). With this system, which was devised to analyze the overall response behaviour, the simple shear tests were adopted only for a few layers beneath the structure demonstrating complicated properties of restoring forces, while numerical models ware applied to the remaining layers of ground and super-structure. The analyses were carried out on an offshore gravity structure in ice sea based on sand seabed by this method. The ice load which was calculated by KARNA-NKK model4) was applied in order to simulate the given penetration speed of drifting floe against structure. At the same time, earthquake motion was input from the bottom of base ground. Degradation and residual shear deformation of base ground were observed because of developing pore water pressure in the sand layer due to cyclic loads induced by drifting floe and earthquake.

Stress Condition in Soil Element beneath Offshore Structure

The stress condition and behaviour of soil elements beneath the offshore structure which are expected during earthquake are shown in Fig. 1 and Table 1. The overburden stress Ðv' and earth pressure at rest Ðh' exist on the planes perpendicular to them before earthquake. When the earthquake happens, cyclic shear stresses exert on the both planes.