Relating the Stiffness of Load-Transfer Functions for Monopile Foundations to Elastic Soil Modulus

Estimating the stiffness of large diameter monopiles used to support offshore wind turbines is an important step in the design process. This is generally done using load-transfer functions in the form of independent (Winkler) springs to represent the relationship between the net soil resistance at any depth adjacent to a pile (p) and the lateral deflection of the pile at the same depth (y). Recent studies for monopiles, which have low length to diameter ratios and high moment loading, have led to recommendations for additional moment-rotational, or m-θ, springs distributed along the shaft of the monopile, as well as base lateral and rotational springs. A significant challenge in applying these new recommendations is to relate the load-transfer functions to conventional soil properties. A simple ‘Step 0′ design approach has derived relationships between the initial stiffness values of the load-transfer functions and the elastic modulus of the soil at a given depth. In this paper, the accuracy of these relationships is examined for a more complex, layered soil profile that includes realistic but abrupt changes in soil modulus. It shown that once a soil modulus profile is established, a reasonably accurate estimate of the stiffness of a monopile can be obtained, sufficient for initial design calculations, without the need for more onerous 3D FEA.