Some jacket structures after a period of operation in Vietnam shelf become more vibratory sensitive to wave load. Weakening of the base-structure connections may cause it. On the other hand, a geological property of the seabed soil foundation in the offshore region of Vietnam has not been investigated sufficiently. This report is aimed to present some results obtained recently by the authors in assessment of condition at the base-foundation connection for test model as well as existing offshore structures by an experimental study. Firstly, some results obtained in an experiment carried out for determining some properties of coral foundation are presented. Then, using the spring model of connection between the seabed and structure, the spring's stiffness has been determined from the measured on existing structure natural frequencies by the system identification approach.
The soil-structure interaction is an important aspect in response analysis of structure, especially for those operating in the offshore region where structural geology has not been sufficiently investigated. Monro et al (1985) have presented a comprehensive study on the soilstructure interaction in the dynamic response of an offshore structure. However, the authors were interested in rather gravity platform than the jacket structure. Such the problem for jacket structure on pile foundation was studied by Verkataramana (1989). In both the studies, the base-structure connection has been modeled as concentrated elastic springs, stiffness of which is determined from mechanical constants of soil and structure foundation by the assumed equations. A finite soilpile element based on the distributed spring model of soil piled structure has been developed and applied to analysis of jacket structure (Khiem et al, 1999). Karadeniz (1999) has developed a beam element for analysis of soil-structure interaction and pipeline.
