Not only the driving for offshore wind energy capacity of 12 GW by Korea’s Renewable Energy 2030 plan but also the need for the rejuvenation of existing world-class shipbuilders’ infrastructures is drawing much attention to offshore wind energy in Korea, especially to the diverse substructures. Considering the deep-sea environment in the East Sea, this paper presents detailed modeling and analysis of spar-type substructure for a 5 MW floating offshore wind turbine (FOWT). This process uses a fully coupled integrated load analysis, which was carried out using FAST, a widely used integrated load analysis software developed by NREL, coupled with an in-house hydrodynamic code (UOU code). The environmental design loads were calculated from data recorded over three years at the Ulsan Marine buoy point according to the ABS and DNVGL standards. The total 12 maximum cases from DLC 6.1 were selected to evaluate the structural integrity of the spar-type substructure under the three co-directional conditions (45°, 135°, and 315°) of wind and wave. A three-dimensional (3D) structural finite element (FE) model incorporating the wind turbine tower and floating structure bolted joint connection was constructed in FEGate (pre/post-structural analysis module based on MSC NASTRAN for ship and offshore structures). The FEM analysis applied the external loads such as the structural loads due to the inertial acceleration, buoyancy, and gravity, and the environmental loads due to the wind, wave, and current. The three-dimensional FE analysis results from the MSC Nastran software showed that the designed spar-type substructure had enough strength to endure the extreme limitation in the East Sea based on the von Mises criteria. The current process of this study would be applicable to the other substructures such as the submersible type.

Thick airfoils are conventionally adopted in the blade root region of a wind turbine to ensure structural safety under extreme conditions, despite the resulting power loss. To prevent this loss, a passive flow control device known as a vortex generator (VG) is installed at the starting point of the stall to control the flow field near the wall of the suction surface. In this study, we used computational fluid dynamics (CFD) to investigate the aerodynamic characteristics induced as a result of the shape and layout of the VG on a multi-MW wind turbine blade. The separated and vortical flow behavior on the suction surface of the wind turbine blade equipped with VGs was captured by the Reynolds-averaged Navier–Stokes (RANS) steady-flow simulation. The parametric sensitivity study of the VG shape parameters such as the chord-wise length, height, and interval of the fair of VGs was conducted using thick DU airfoil on the blade inboard area. Based on these results, the response surface method (RSM) was used to investigate the influence of the design parameters of the VG. Based on the CFD results, the VG design parameters were selected by considering the lift coefficient and vorticity above the trailing edge. The maximum vorticity from the trailing edge of the selected VG and the lift coefficient were 55.7% and 0.42% higher, respectively, than the average. The selected VG design and layout were adopted for a multi-MW wind turbine and reduced stall occurrence in the blade root area, as predicted by the simulation results. The VG improved the aerodynamic performance of the multi-MW wind turbine by 2.8% at the rated wind speed.

The importance of a reliable blade root connection has grown due to the higher-gravity-induced edgewise loads on the blade root that resulted from the recent increased size and weight of a wind turbine rotor blade. To avoid the loosening of a bolt joint connection or even consecutive blade failures, the stress concentration factor (SCF) at the bolt thread root that is sensitive to fatigue should be understood comprehensively. In this work, two-dimensional and three-dimensional finite element (FE) analysis methods were used to determine the SCF at the bolt threads both between an insert and a M42 bolt used for a large offshore blade, and between a M42 bolt and a nut. The effect of various geometric parameters on the SCF were also investigated, which included shank diameter, nut height, nut type, and relief cone. Results showed that the decreased diameter of a M42 bolt shank diameter was the dominant design driver in reducing the stress concentration factor by 40%, from 3.94 to 2.32. The round nut type was also a recommended factor to be implemented to connect bolts and inner pitch bearing with an additional 10% SCF reduction. The relief cones applied to bolt threads and insert threads also contributed to the reduction of SCF to 2.01, a 49% reduction in total. This work not only provides guidelines by which to choose the proper geometry of the bolt and nut for a large blade, but also could be beneficial in designing bolted joint connections of segment or modular blades.

Recently, several countries have made commitments to move to a net-zero emission by the year 2050 in a response to climate change. Among various renewable energy systems to realize the target, wind energy system has been gaining much attention as a favorable alternative source to fossil fuel energy. In particular, many floating offshore wind turbines (FOWT) are expected to be installed because of vast installation resources without water depth limit conditions, stable and strong wind resources, relatively low constraints on noise emission, and space restriction compared to onshore wind turbines. In this study, a 10 MW superconducting floating offshore wind turbine was modeled with a 1/90 scale ratio and was experimentally tested at the Ocean Engineering Widetank of the University of Ulsan. The model calibration of the scaled model was performed with free decay test and showed a good correlation with simulation results calculated from FAST V8 of NREL. The motion characteristics of the 10 MW superconducting FOWT semi-submersible type platform was investigated under regular waves and irregular waves through the comparison of model test data and simulation results. The study on the motion characteristics of the model showed that the simulation considering the 2nd order wave effects to hydrodynamic forces and moments provided better accuracy close to the model test data.

This paper outlines a novel segment test methodology for wind turbine rotor blades. The segment test mainly aims at improving the efficiency of the fatigue test as a future test method at Fraunhofer Institute for Wind Energy Systems (IWES). While resulting in reduced testing times, target fatigue bending moments over the intended test area have to be matched within an acceptable range of overload. The numerical simulation reveals that the proposed segment testing has a significant time savings of up to 43 % and 52 % for 60 and 90 m blades. When compared to the experimental data of a 60 m full-length blade fatigue test, the proposed methodology also shows better test quality over the intended area and better certifiable regions over the outboard area.

In this paper, a simple but effective trailing edge flap system was proposed. This preliminary concept uses a more practical and stable actuation system, which consists of a motor-driven worm gear drive and a flexible torsion bar. The flexible torsion bar is designed to be easily twisted while keeping bending rigidity as a support, and the worm gear drive not only provides high torque to overcome aerodynamic forces on the flap area and the torsional rigidity of the support bar, but also acts as a brake to avoid instability due to the high torsional flexibility of the support bar. A preliminary level design study was performed to show the applicability of the new trailing edge flap system for wind turbine rotor blades or helicopter blades. Doi: 10.28991/HIJ-2020-01-03-01 Full Text: PDF