Effective stiffness method for rigid monopile foundations of offshore wind turbines and in-situ validation

It is widely accepted that the initial stiffness derived with the p-y methodology as prescribed by the American Petroleum Institute does not capture the true small-strain stiffness for rigidly behaving piles. We present an alternative method, capturing the 3D effects in the soil-pile interaction, in which the soil is characterized with in-situ seismic measurements. As the design of the foundations of offshore wind turbines often involves many expensive load simulations (load cases), the method also includes finding an effective 1D or Winkler stiffness yielding a similar pile response as in the 3D model. The method is exemplified for a real 5 m-diameter monopile embedded in stiff dense sand, in a near-shore wind farm in The Netherlands. The dynamic properties of this pile have been tested (prior to installation of the super-structure) with a unique measurement setup using a hydraulic shaker, exciting between 1 and 9 Hz. The response of this stand-alone pile is highly sensitive to the soil and allowed us to verify the effective 1D stiffness with much higher certainty as opposed to the usual situation including dynamic disturbances and uncertainties related to the interaction with the super-structure. The effective soil damping of the pile is estimated, and the performance of both the standard design (p-y) stiffness method and the proposed effective stiffness method is assessed in view of the measured strains and accelerations.