Dynamic stiffness of horizontally vibrating suction caissons

The promising potential for offshore wind market is on developing wind farms in deeper waters with bigger turbines. In deeper waters the design foundation configuration may consist of jacket structures supported by floating piles or by suction caissons. Taking the soil-structure interaction effects into consideration requires the prior estimation of the dynamic impedances of the foundation. Even though numerous studies exist for piles, only limited number of publications can be found for suction caissons subjected to dynamic loads. Therefore, the purpose of this study is to examine the dynamic response of this type of foundation using the finite element method (FEM) to account for the interaction with the soil. 3D numerical models for both the soil and the suction caisson are formulated in a frequency domain. The response of the soil surrounding the foundation is considered linear viscoelastic with hysteretic type damping. In addition, non-reflective boundaries are included in the model. Two different soil profiles are presented, one when the rigid bedrock is set close to the seabed and the other one when it is far away.
The dynamic impedances at the top of the foundation are determined and compared to existing analytical solutions suggested for piles. Relatively good agreement has been achieved comparing the numerical results with the analytical solutions. Then, the effect of the soil layer shear wave velocity on the dynamic stiffness coefficients is analysed. The results have indicated that increasing the stiffness of the soil stratum the dynamic impedances grow, while the damping reduces in the frequency range investigated.