Please cite this article in press as: C.T. Akdag, Behavior of closely spaced double-pile-supported jacket foundations for offshore wind energy converters, Applied Ocean Research (2016), http://dx.doi.org/10.1016/j.apor.2016.04.008 Applied Ocean Research xxx (2016) xxx–xxx Contents lists available at ScienceDirect Applied Ocean Research journal homepage: www.elsevier.com/locate/apor Behavior of closely spaced double-pile-supported jacket foundations for offshore wind energy converters Cihan Taylan Akdag ∗ Torbalı Vocational School of Higher Education, Geotechnics Program, Dokuz Eylül University, 35860 ˙ Izmir, Turkey a r t i c l e i n f o Article history: Received 28 September 2015 Received in revised form 30 January 2016 Accepted 17 April 2016 Keywords: Offshore wind energy converters Closely spaced piles Jacket foundation Numerical simulation Combined loading Load distribution a b s t r a c t As the offshore wind energy production units move to deeper waters the design of their foundations demand more creative and complex approaches especially for large turbines (i.e. ∼5–7 MW). In this arti- cle, a novel piled foundation alternative with closely spaced double piles at the edges of the jacket is studied for various pile spacing and lengths. A numerical parametric study was carried out to understand the effects of pile spacing and pile length on the behavior of the novel supporting system under monotonic combined loading in very dense sand. Prior to the analysis, the numerical model is validated in field tests with single-pile and double-pile configurations. The contribution of closely spaced double piles to the overall foundation response and load distribution among the piles were investigated. The response of the foundation was evaluated considering the horizontal load-head displacement, moment-head rotation, and initial stiffness of the soil-pile system. It is found that the response of the recommended foundation system is superior to that of a rather conventional system with single piles at the edges. The results indicate that the piles on the tension loading side evidently carried lower loads than those on the com- pression side. Moreover, the disposition of the piles is more important on the tension side, as the trailing piles carried considerably lower loads than the leading piles. It is found that the double pile system with a pile embedment length L/2 and a pile spacing of S = 5D, 6D provides better response, where L is the embedded pile length of conventional system and D is the diameter of pile. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The importance of renewable energy is significantly increas- ing, and the utilization of offshore wind energy has been growing. Within the next few decades, a vast number of offshore wind energy converters (OWECs) shall be erected to supply a large portion of our energy needs [1]. It is stated in the report of Euro- pean Wind Energy Association [2] that the next step for wind energy is offshore wind farms in deep waters, and projects of deep OWECs are being developed in Europe. In this respect, selecting and designing an appropriate OWEC foundation type is an essen- tial challenge for geotechnical engineers, as the foundation is the critical part of the design of OWECs. Therefore, the design of OWEC foundations has been the focus of several studies [3–5]. Gravity, monopile, and monopod suction bucket foundations are suitable for shallow and moderate water depths up to 25 m. Conversely, there is a need to enhance existing foundation systems for large wind turbines in deep waters (deeper than 30 m). Thus, innovative ∗ Corresponding author. E-mail addresses: taylan.akdag@deu.edu.tr, akdagcih@b-tu.de substructure concepts regarding multi-footing configurations have recently been under research and development [6,7]. Kim et al. [6] performed a parametric study to understand the group effect of tripod bucket foundations on the bearing capacity. The behavior of a three-bucket jacket substructure under long-term cyclic loading was investigated by Lupea et al. [7]. Shi et al. [8] stated that the jacket foundation is becoming progressively more interesting and is a good option for water depths between 30 and 80 m. Achmus et al. [9] indicated that for larger water depths, tripod and jacket foundations are more appropriate. However, the combined hor- izontal and vertical loading applied to individual piles of tripod and jacket foundations significantly increase with the water depth. In this regard, the foundation of wind turbines should ensure an adequate bearing capacity against such loading conditions in deep waters. Consequently, this study is focused on a novel supporting system of the jacket foundation for OWECs in deep waters. The con- ventional jacket foundation is supported by four piles positioned at the edges of the construction (Fig. 1a). For the recommended innovative foundation, the available jacket foundation system was supported using closely spaced double piles for each edge (Fig. 1b). To keep the symmetry of the foundation system on both the x- and y-axes, each leg was designed with closely spaced double piles http://dx.doi.org/10.1016/j.apor.2016.04.008 0141-1187/© 2016 Elsevier Ltd. All rights reserved.