ABSTRACT
Waves are one of the most significant environmental concerns, and wave loads must be systematically analyzed for designing and maintaining offshore structures (Agarwal & Manuel 2011, Marino et al. 2013). In the past decade, a significant amount of research concerning wave forces acting on offshore structures has been carried out (Forristall 2000, Longuet-Higgins 1963, Li et al. 2011). Since the emerging of offshore wind energy, the focus of these studies has been placed upon calculating wave loads acting on the foundations of offshore wind turbines (Roald et al. 2013, Myrhaug & Holmedal 2010, Marino et al. 2011), in which most studies employed linear wave models. Considering the nonlinearity of irregular waves in shallow waters where tripile offshore wind turbines are commonly seated, this study presents the analysis of irregular nonlinear wave force on a tripile foundation. The analysis was carried out using the open-source program HydroDyn. The outline of the paper is arranged as follows: Section 2 illustrates the fundamental equations of second-order nonlinear irregular waves. The flowchart of calculating
1 INTRODUCTION
Tripile foundations are composed of a three-legged jacked structure connected to a monopile in the upper part to support superstructures (4COffshore 2016). They are a newly-emerging foundation concept for offshore wind turbines (see Figure 1). According to ‘The European offshore wind industry-key trends and statistics 2015’, a report by the European Wind Energy Association, by 2015, tripile foundations account for 1.7% of the sub-
second-order nonlinear wave force in HydroDyn is outlined in Section 3. In Section 4, nonlinear wave force on the tripile foundation is calculated and compared to the results of the linear wave model. The influence of incident wave angle on the wave force and surface elevation is also studied. Finally, Section 5 concludes the study.
