A modified Nonlinear Foundation Beam model for half-rigid monopiles with Timoshenko representation and compatible soil springs in sand

Abstract

There is a large number of monopile foundations for offshore wind turbines falling in the type of half-rigid pile, and are recommended to be analyzed by the p-y method in design. However, the p-y method currently adopted in oil and gas industry for slender piles has proved to be inapplicable for monopile-soil interaction, with overestimated ultimate lateral resistance and underestimated initial stiffness predicted. Thus, factoring in the bending resistance provided by shaft friction and end bearing forces for large diameter monopiles, a modified Nonlinear Foundation Beam (NFB) method is proposed in this paper to predict the monopile response in sand, where a beam element considering sectional shear stress is adopted, and another three sets of soil springs are incorporated with the p-y soil spring. Unified relationships of the proposed soil springs are also proposed with varying sand and pile properties. The modified NFB method is validated by published test results with diameters ranging from 2 m to 6 m, and L/D ratios from 5.08 to 8.29, proving its applicability for analysis of half-rigid monopiles.