Influence of vertical shear stresses due to pile-soil interaction on lateral dynamic responses for offshore monopiles

摘要

It has recently been pointed out that the vertical shear stresses at pile-soil interface play an important role in static responses of offshore monopiles with large diameters, and traditional p - y curves cannot fully capture this influence. However, the influences of the vertical shear stresses on the dynamic response of monopiles are still not clear. This is the main focus of this study. The coupled horizontal and rocking vibration of a monopile embedded in a fully saturated poroelastic seabed is studied in this paper. The focus of the study is the difference between the responses with and without the vertical shear stresses on the pile. The monopile dynamic response is studied by the integral equation method, with both 3D elastodynamic theory and Euler-Bernoulli beam theory. Selected numerical results for highlighting the influence of the vertical shear stresses on the dynamic contact load distributions, pile displacements, and lateral dynamic impedance functions are examined for different length to diameter (l/d) ratios as well as poroelastic materials and frequencies of excitation. These results confirm that exclusion of the vertical shear stresses will lead to potentially very conservative design for monopiles with l/d 6 in soft soil, and monopiles with l/d 10 in medium to stiff soils. The computed dynamic responses of the monopile when the vertical shear stress is ignored can be up to 30% larger than the case when the vertical shear stress is considered.