Single-blade installation is a popular method for installing blades on bottom-fixed o↵shore wind turbines. A jack-up crane vessel is often employed, and individual blades with their roots equipped with mechanical joints and bolted connections are lifted to the tower-top height and mated with a pre-assembled hub. The final mating phase is challenging and faces significant risks of impact. Due to relative motions between the blade and the hub, substantial impact forces may arise and lead to severe structural damages at root connections, thereby causing delays in the installation task. The present paper considers a realistic scenario of the mating process and investigates the consequences of such impact loads. Here, a single-blade model with tugger lines and a monopile model were established using a multi-body formulation, and relative velocities under collinear wave and wind conditions were obtained. A three-dimensional finite element model was developed for the blade root with T-bolt connections, and an impact investigation was performed for the case in which a guiding connection impacts the hub. The results show severe bending and plastic deformation of the guide pin bolt together with failure of the adjoining composite laminate at the root connection.Based on the type of damage obtained for the di↵erent environmental conditions considered, this paper also discusses its consequence on the installation tasks and suggests onboard decision making in case of an impact incident. The results of this study provide new insights regarding the mating phase and can be utilised to establish response-based operational limits.
A study of Mg doping of AlxGa1−xN up to x∼50% using microstructural and electrical probes is reported. The viability of effective p-type doping is defined by a minimum concentration of Mg required to offset the background impurities and, more importantly, a maximum limit above which inversion domains and structural defects start to nucleate, accompanied by a rapid degradation of electrical transport. Resistivity of 10 Ω cm and free hole concentrations above 1017cm−3 are achieved for AlxGa1−xN up to x∼50% within an optimum window of Mg incorporation.
We report the growth of horizontally aligned arrays and networks of GaN nanowires (NWs) as resonant components in nanoelectromechanical systems (NEMS). A combination of top-down selective area growth (SAG) and bottom-up vapor-liquid-solid (VLS) synthesis enables flexible fabrication of highly ordered nanowire arrays in situ with no postgrowth dispersion. Mechanical resonance of free-standing nanowires are measured, with quality factors (Q) ranging from 400 to 1000. We obtained a Young's modulus (E) of approximately 338 GPa from an array of NWs with varying diameters and lengths. The measurement allows detection of nanowire motion with a rotating frame and reveals dual fundamental resonant modes in two orthogonal planes. A universal ratio between the resonant frequencies of these two fundamental modes, irrespective of their dimensions, is observed and attributed to an isosceles cross section of GaN NWs.
Vertically injected thin-film ultraviolet light-emitting diodes operating at 325 and 280nm are demonstrated. Low-temperature AlN interlayers allow crack-free growth of AlxGa1−xN with compositions up to x=0.53 on GaN-on-sapphire templates. The GaN layer allows laser-induced separation of the highly strained epi stack from the sapphire substrate with high yield. Cathode contacts are formed on nitrogen-face AlxGa1−xN (up to x=0.53) and allow vertical injection of current into the active region. Controlled roughening of the nitrogen-face AlxGa1−xN is also demonstrated through photoelectrochemical etching and results in >2.5× light extraction gain for 325 and 280nm devices.
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