Horizontal Static Impedances for OWT Monopiles Based on Timoshenko Beam Theory

Abstract

Euler beam theory is always used for soil–pile analysis; however, its use is questionable for laterally loaded large-diameter monopiles. In this paper, the solution on the static impedances of monopiles under a combination of axial and horizontal loads was proposed based on three-dimensional continuous medium theory and Timoshenko beam theory. From the analysis results, the use of the Euler–Bernoulli beam theory leads to larger pile-head impedances, and the feature is more significant with lower aspect ratios and stronger pile-bottom constraints. The difference in the impedance from two theories may be up to 10%–43%. Consequently, Timoshenko beam theory is more appropriate for large-diameter monopiles, especially for monopiles with strong bottom constraints (such as rock-socketed monopiles). Meanwhile, the shear force and bending moment at the pile bottom have a considerable effect on the static impedances of large-diameter short monopiles, and only a consideration of the impedance contribution from the soil around the pile is insufficient. Furthermore, both the deflection and the bending moment of the pile increase due to the second-order effect of axial force, but the effect of the axial force on the impedance can be neglected for practical monopile designs. A simple and efficient approach to evaluate the impedance of large-diameter monopiles is desirable for designers, thus a simplified empirical equation for the pile-head static impedance was proposed for ease in calculating each component of static impedances for the large-diameter monopiles of offshore wind turbines (OWTs).