Abstract
Large-diameter monopiles are the predominant foundation solution for supporting offshore wind turbines. They are conventionally designed using soil reaction curves developed for long slender piles used for supporting offshore oil and gas platforms (e.g., the API p-y model). However, due to the difference in the length/diameter ratio and the resulting soil mechanisms, the use of p-y curves alone can lead to significant under-prediction of the lateral stiffness and capacity of monopiles. To overcome the shortcoming, the authors have previously proposed a conceptual two-spring framework, i.e., the so-called ‘p-y + MR-θR’ model, to capture the monopile response in soft clay under lateral loading. The framework uses distributed p-y springs to consider the lateral soil resistance along the pile above the rotation point (RP) and a single moment-rotation (MR-θR) spring attached at the RP to capture the entire soil resistance below the RP, i.e., the distributed resistance along the pile, base shear and base moment at the pile tip. The proposed p-y and MR-θR springs were curve-fitted to the results of 3D numerical analyses. However, as the stress-strain response and the shear strength profile inevitably influence the p-y and MR-θR springs, the applicability of the empirical formulations to soil conditions other than those examined is uncertain. This study proposes an enhancement to the ‘p-y + MR-θR’ framework, in which the p-y and MR-θR springs are not tied to a specific soil and strength profile but fundamentally linked to the properties that can be measured directly in the site investigation and laboratory. This extension is achieved through analytical analyses and an extensive parametric numerical study. The predictive capabilities of the model are demonstrated by back-analyses of finite element analyses and centrifuge model tests. The proposed model provides practising engineers with a simple yet powerful approach to use site-specific soil reaction curves in the design of monopiles embedded in soft clay.
Highlights
Monopiles are the dominant foundation solution for supporting offshore wind turbines (OWTs)
Gmax can be determined through a resonant column test or bender elements incorporated in a soil element test. γfp literally stands for the plastic shear strain at failure and should be determined by curve-fitting that provides the best fit to the stress-strain curve measured in the soil element shear test
The lateral soil resis tance above the rotation point is represented by distributed p-y springs, while the soil resistance below the rotation point is lumped into a concentrated MR-θR spring
Summary
Monopiles are the dominant foundation solution for supporting offshore wind turbines (OWTs). To resist large lateral load and bending moment resulting from wind, waves and current, pile diameter (D) in the range of 6–8 m is common nowadays. Extra-large monopiles with diameter greater than 10 m are being considered in newer projects. The embedded length-to-diameter ratio (L/D) of monopiles is typically in the range of 4–8 (Doherty and Gavin, 2012; Murphy et al, 2018) in competent soils. In soft seabed conditions, such as offshore China, the L/D ratio can be as high as 10 or more (Lai et al, 2020)
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