Numerical Analysis of Large-Diameter Monopiles in Dense Sand Supporting Offshore Wind Turbines Sheikh Sharif Ahmed 1 and Bipul Hawlader 2 Abstract: Large-diameter monopiles are widely used foundations for offshore wind turbines. In the present study, three-dimensional finite-element (FE) analyses are performed to estimate the static lateral load-carrying capacity of monopiles in dense sand subjected to eccentric loading. A modified Mohr–Coulomb (MMC) model that considers prepeak hardening, postpeak softening, and the effects of mean effective stress and relative density on stress–strain behavior of dense sand is adopted in the FE analysis. FE analyses are also per- formed with the Mohr–Coulomb (MC) model. The load–displacement behavior observed in model tests can be simulated better with the MMC model than with the MC model. On the basis of a parametric study for different length-to-diameter ratios of the pile, load–moment capacity interaction diagrams were developed for different degrees of rotation. A simplified model, based on the concept of lateral pres- sure distribution on the pile, is also proposed for the estimation of its capacity. DOI: 10.1061/(ASCE)GM.1943-5622.0000633. © 2016 American Society of Civil Engineers. Author keywords: Monopiles; Finite element; Dense sand; Modified Mohr–Coulomb model; Lateral load; Offshore wind turbine. Introduction Wind energy is one of the most promising and fastest-growing renewable energy sources around the world. Because winds are steadier and stronger in offshore than in onshore environments, and because there is less visual impact in offshore, a large number of offshore wind farms have been constructed and are under construc- tion. The most widely used foundation system for offshore wind tur- bines is the monopile, which is a large-diameter 3- to 6-m hollow steel-driven pile with a length-to-diameter ratio of less than 8 (LeBlanc et al. 2010; Doherty and Gavin 2012; Doherty et al. 2012; Kuo et al. 2011). Monopiles have been reported to be an efficient solution for offshore wind turbine foundations in water depths up to 35 m (Doherty and Gavin 2012). The dominating load on an off- shore monopile is the lateral load from wind and waves, which acts at a large eccentricity above the pile head. To estimate the load-carrying capacity of monopiles, the p–y curve method recommended by the American Petroleum Institute (API 2011) and Det Norske Veritas (DNV 2011) is widely used. A p–y curve defines the relationship between mobilized soil re- sistance (p) and the lateral displacement (y) of a section of the pile. The reliability of the p–y curve method in monopile design has been questioned by a number of researchers (Abdel-Rahman and Achmus 2005; Lesny and Wiemann 2006; Achmus et al. 2009; LeBlanc et al. 2010; Doherty and Gavin 2012). The API and DNV recommendations are a slightly modified form of the p–y curve method proposed by Reese et al. (1974), which is based mainly on the field test results of two 610-mm-diameter flexible slender piles. However, the large-diameter offshore monopiles behave as a rigid pile under lateral loading. Moreover, in the API recommendations, the initial stiffness of the p–y curve is inde- pendent of the diameter of the pile, which is also questionable. Doherty and Gavin (2012) discussed the limitations of the API and DNV methods in calculating the lateral load-carrying capacity of offshore monopiles. Monopiles have been installed successfully in a variety of soil conditions; however, the focus of the present study is to model monopiles in dense sand. Studies have been performed in the past under both static and cyclic loading conditions (e.g., Achmus et al. 2009; Cu ellar 2011; Ebin 2012); however, cyclic loading is not discussed further here, because it is not the focus of the pres- ent study. To understand the behavior of large-diameter monop- iles in sand, mainly three different approaches have been taken in recent years, namely, physical modeling, numerical modeling, and modification of the p–y curves. LeBlanc et al. (2010) reported the response of a small-scale model pile under static and cyclic loading installed in loose and dense sands. Centrifuge tests were also conducted to understand the response of large-diameter monopiles in dense sand subjected to static and cyclic lateral loading at different eccentricities (e.g., Klinkvort et al. 2010; Klinkvort and Hededal 2011; Klinkvort and Hededal 2014). Møller and Christiansen (2011) conducted 1g model tests in satu- rated and dry dense sands. Conducting centrifuge tests using 2.2- and 4.4-m-diameter monopiles, Alderlieste (2011) showed that the stiffness of the load–displacement curves increases with di- ameter. A comparison of results of centrifuge tests and the API approach showed that the API approach significantly overesti- mates the initial stiffness of load–displacement behavior. To match test data, Alderlieste (2011) modified the API formulation by introducing a stress-dependent stiffness relation. However, the author recognized that the modified API approach still underesti- mates the load at small displacements and overestimates it at large displacements and therefore recommended further studies. It should also be noted here that small-scale model tests were con- ducted to estimate the lateral load-carrying capacity of rigid piles and bucket foundations (e.g., Prasad and Chari 1999; Lee et al. 2003; Ibsen et al. 2014). However, contradictory evidence of 1 Dept. of Civil Engineering, Memorial Univ. of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5. E-mail: ssa725@mun.ca 2 Associate Professor, Dept. of Civil Engineering, Memorial Univ. of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X5 (correspond- ing author). E-mail: bipul@mun.ca Note. This manuscript was submitted on June 4, 2015; approved on November 3, 2015; published online on February 23, 2016. Discussion pe- riod open until July 23, 2016; separate discussions must be submitted for individual papers. This paper is part of the International Journal of Geomechanics, © ASCE, ISSN 1532-3641. © ASCE 04016018-1 Int. J. Geomech. Int. J. Geomech., 04016018 Downloaded from ascelibrary.org by The University of Asia Pacific on 02/25/16. Copyright ASCE. For personal use only; all rights reserved.