Relationships between lateral and rotational load transfer stiffnesses and soil modulus for the elastic response of monopiles

Abstract Monopiles supporting offshore wind turbines are generally of relatively low slenderness ratio and are subjected to lateral loads that emanate from high above the seabed at an eccentricity that in most cases exceeds the embedded length of the pile. The rotational stiffness is a critical aspect of their design, owing to tight tolerances on the cumulative rotation that is permitted by industry guidelines. From a practical perspective, most design calculations for the lateral response are performed using load transfer analysis. However, the low slenderness ratio and high moment loading relative to the horizontal load have prompted closer scrutiny of the load transfer formulations that have been used historically in the offshore oil and gas industry, since these were calibrated from the response of much longer piles under very different loading conditions. Recent industry-led studies have led to new recommendations, including the addition of lateral and rotational springs at the pile base and rotational load transfer springs distributed down the pile shaft. As a first step to evaluate the initial stiffness of such springs, this paper presents results from three-dimensional finite element analyses of a homogenous elastic soil, with the aim of relating values of spring stiffness to the elastic shear modulus of the soil. Effects of non-uniform soil stiffness are discussed and the gradually increasing benefits of including the additional spring terms are quantified.