Abstract

A multi-cylinder TLP design concept of a floating structure is presented for supporting a 10 MW Wind Turbine (WT). The combined effects of water waves and wind on the floating offshore wind turbine are investigated. A design-oriented solution method for the coupled dynamics of the floating structure in the frequency domain is presented and the corresponding RAO's are calculated. The hydrodynamic characteristics are determined using matched axisymmetric eigen-function expansions, while the linearized loading contributed by the WT (gravitational, inertial and aerodynamic) is estimated based on a reduced order model. In addition, time domain hydro-servo-aero-elastic simulations of the coupled floating structure are performed considering wave excitation described by a white noise spectrum whereby the effective RAO's are computed from the corresponding time signals. Comparison between the two methods is performed at the RAO's level for various wind speeds.

Introduction

Renewable energy technologies can help countries meet their policy goals for secure, reliable and affordable energy to expand electricity access and promote development. Offshore wind farms are at the beginning of their commercial deployment stage. They have higher capital costs than onshore wind farms, but this is offset to some extent by higher capacity factors (Butterfield et al. 2007). Ultimately, offshore wind farms will allow a much greater deployment of wind in the longer-term.

The concept of a floating wind turbine existed since the early 1970s, but the industry only started researching it in the mid-1990s (EWEA, 2013). In this respect the main challenge concerns the design of the supporting structure, meaning the floater and the moorings. In 2009, Statoil installed the world's first large scale grid connected floating wind turbine, Hywind (Jonkman & Musial, 2010) in Norway, with a 2.3 MW turbine. The second large scale floating system, WindFloat, developed by Principle Power in partnership with EDP and Repsol, was installed off the Portuguese coast in 2011 (Carmelli et. al. 2009). Currently, offshore wind farms are using the three main types of deep offshore foundations, adapted from offshore oil and gas industry: Spar Buoys (Mazarakos & Mavrakos, 2016; 2018a; Mazarakos et. al. 2019a), Semi-submersibles (Mazarakos, 2010; Mazarakos & Mavrakos, 2018b) and Tension Leg Platforms (Sclavounos & Wayman, 2006; Mazarakos et. al. 2014; Mazarakos & Charalambous, 2018; Mazarakos & Mavrakos, 2017). Barge-type floaters (e.g. Ideol's Moonpool Floater) are quite relevant too (Guignier et. al. 2016).

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