Elastic rocking vibration of an offshore Gravity Base Foundation

• Dynamic rocking impedances of a disc on saturated soil under seawater are studied.
• Both permeable and impermeable discs are considered.
• The impedances are used to study rocking vibration of a Gravity Base Foundation.
• The existence of seawater half-space can alleviate its dynamic amplitudes.
• A permeable bottom surface can also alleviate its dynamic amplitudes.

Gravity Base Foundation is one of the main foundations in offshore wind turbine structure, and the foundation rocking stiffness mostly controls the structure fatigue life. This study models the rocking vibration of a rigid disc foundation on a poro-elastic seabed, covered by a compressible inviscid seawater half-space. The coupled rocking problem is solved using the 3-dimensional elastic wave theory. The behavior of seawater and soil are depicted with the Euler equations and the Biot's theory, respectively. The contact surface of the disc foundation with the soil and the water is assumed to be smooth. The solutions are expressed in Fredholm integral equations of the second kind and solved numerically. The obtained dynamic impedances are used to study the dynamic behavior of Gravity Base Foundations of offshore wind turbines, which are very sensitive to dynamic moments. It is found that foundation with a permeable surface has much smaller dynamic amplitudes, compared to the one with an impermeable surface. The existence of seawater half-space will also alleviate the dynamic amplitudes, but decreasing the natural frequency of the foundation.