Experiments were conducted on a 1:80 scaled column of the WindFloat semi-submersible floating offshore wind turbine platform. The structure was forced to oscillate at frequencies of up to 6 Hz and at various amplitudes to create a parameter space much larger than previously reported. The hydrodynamic coefficients of the column with a hexagonal heave plate were compared to the base case of a column with a circular heave plate. Results show remarkably similar behavior between the two cases over the range of parameters tested, but quite distinct from published data on square plates. At low Keulegan-Carpenter number, the hexagonal plate showed a slightly higher added mass, but the difference narrowed down with increasing KC. An opposite trend was noticed for the damping coefficients. Overall the maximum difference in damping was about 8%. The paper presents some of the challenges in experimenting over a large parameter range, and also analyzes the trends in data over the range. It is expected that the presented data will be of use with engineers attempting to use heave plates for stabilizing wind turbine platforms in range of wave and wind conditions to maximize wind energy generation efficiency.
In this work a wind tunnel with an open jet configuration is investigated for use in offshore wind turbine testing. This study characterizes the open-jet wind-tunnel using measurements of the velocity field detailing mean velocities and turbulence intensities with and without a scaled wind turbine. Measurements have been taken downstream to evaluate the expected area of turbine operation and the shear zone. The effects on the flow due to the wake and turbine blockage have also been identified. Additionally, the combination of honeycomb and screens necessary to produce a homogeneous flow at the desired velocity with low turbulence intensity has been identified. This work provides a useful data set that will be used as a benchmark to evaluate the benefits of recirculating wind tunnels. The data set has resulted in identifying conditions that would prevent producing the desired flows. The data set has also resulted in recommendations concerning the shape of the wind tunnel sections at the University of Maine’s wind-wave (W2) facility to minimize its interactions with the turbine wake expansion, turbine blockage, and the turbine associated wake shear zone.
Tanker vessels used for offshore oil production and storage are kept at station by turret mooring systems, enabling the vessel to weathervane in the direction of dominant environmental loads. The disruption of heading equilibrium for a turret-moored tanker was predicted by experiments and numerical studies. A vessel was observed to lose control in head sea condition, wherein for wavelength from 0.73 < λ/L < 2 (L-ship length) the model drifted to a large angle of 45–60 degrees (Thiagarajan et al. 2013). Previous numerical analyses conducted by the authors identified that this heading drift reduced remarkably in the presence of wind. This finding is confirmed by an experimental study and reported in this paper. A geometrically scaled down version of a turret-moored FPSO at 1:120 scale of a prototype VLCC was tested at the Alfond W2 Wind & Wave Ocean Engineering facility of the University of Maine. This lab is a unique facility equipped with a high-performance wind machine over a multidirectional wave generator, and can create regular or random sea-states with wind speeds up to 7 m/s. The tests reported here were conducted with regular waves under two wind speeds (12 and 25 m/s full scale). It was observed that the presence of an initially bow wind can minimize the heading instability. The reason for this observation is described by analyzing the effect of the wind induced moments on the equilibrium condition. Free-decay tests were also conducted to investigate the contribution of the wind damping to the total damping. Measured results show that in the presence of wind, the damping values are higher than those estimated due to hydrodynamics only. It also has been discussed that this wind induced damping on FPSOs, can result in smaller heading angles. From this study, it is concluded that the wind can play a large role in the station-keeping dynamics of the moored-tankers.
The design of a 1/15th geometrically scaled wave tank model of the Azura™ commercial-scale wave energy device is presented. The objectives of the wave tank tests, conducted at the University of Maine Harlod Alfond Wind/Wave Ocean Engineering Lab (W2), included verification of the Azura’s energy capture in irregular waves, evaluation of performance in survival wave conditions, and testing of two advanced control algorithms. Due to the difficulty in properly Froude Scaling a hydraulic system, the model used a direct-drive rotary motor/generator power takeoff (PTO), with the dynamics of the hydraulic PTO included via a hardware-in-the-loop simulation. This PTO implementation led to additional design requirements being imposed on the model drivetrain. In addition to the model PTO design, the instrumentation design, structural design, and test plans are presented. The resulting model and PTO achieved a high level of controllability, and accurately emulated the dynamics of the hydraulic PTO of the full-scale Azura prototype.
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