This report evaluates the influence of dynamic motions, more specifically earthquake motions, on the stiffness of offshore soil. As this stiffness reduces over the course of an earthquake, the structural response of offshore technology is affected. The focus of the investigation presented is on the influence of this seismically triggered stiffness degradation on the simplified design procedure of jack-up vessels, a vessel used to install and maintain offshore structures such as wind turbines, in the offshore area of Japan and the East and South Chinese Sea.
A literature study was conducted to outline the shortcomings in the current screening process of a jack-up vessel, as well as to get acquainted with some geotechnical concepts. These concepts were used to develop a soil model with a simplified structure on top. Earthquake motions of different intensity were applied to the base of this soil deposit and the response of the structure was compared to the response of a three-dimensional jack-up model, in which the soil was represented by linear springs and dashpots.
The results show that for lower intensity levels, defined as the earthquake’s level of peak ground acceleration, there is barely any degradation in the soil. For earthquakes with a peak ground acceleration of around 0.1g, there sometimes is degradation in the soil, however, almost never significant enough to alter the structural response. For more intense earthquakes, there is a clear degradation in the soil deposit, which affects the response of the jack-up vessel. For the case presented in this research project, an equivalent stiffness degradation factor is found for these more intense earthquakes, so that soil stiffness degradation can be implemented in the simplified design procedure of a jack-up vessel. Furthermore, the results imply that linking the earthquake’s signal intensity to both the peak ground acceleration and the Arias intensity level will lead to a more accurate subdivision of the equivalent stiffness degradation factors