Vertical wake deflection for floating wind turbines by differential ballast control

This paper presents a feasibility analysis of vertical wake steering for floating turbines by differential ballast control. This new concept is based on the idea of pitching the floater with respect to the water surface, thereby achieving a desired tilt of the turbine rotor disk. The pitch attitude is controlled by moving water ballast among the columns of the floater. This study considers the application of differential ballast control to a conceptual 10 MW wind turbine installed on two platforms, differing in size, weight and geometry. The analysis considers: a) the aerodynamic effects caused by rotor tilt on the power capture of the wake-steering turbine and at various downstream distances in its wake; b) the effects of tilting on fatigue and ultimate loads, limitedly to one of the two turbine-platform layouts; and c) for both configurations, the necessary amount of water movement, the time to achieve a desired attitude and the associated energy expenditure. Results indicate that – in accordance with previous research – steering the wake towards the sea surface leads to larger power gains than steering it towards the sky. Limitedly to the structural analysis conducted on one of the turbine-platform configurations, it appears that these gains can be obtained with only minor effects on loads, assuming a cautious application of vertical steering only in benign ambient conditions. Additionally, it is found that rotor tilt can be achieved in the order of minutes for the lighter of the two configurations, with reasonable water ballast movements. Although the analysis is preliminary and limited to the specific cases considered here, results seem to suggest that the concept is not unrealistic, and should be further investigated as a possible means to achieve variable tilt control for vertical wake steering in floating turbines.

Micro-Siting of Floating Turbines Through Multiple-Criteria Decision-Making

Multi-Criteria Decision Methods (MCDM) as a complement to current practices for identifying an initial compromise solution to the problem of wind turbine micro-siting are proposed. MCDM deals with multiple objectives in decision-making. The main goal of these methods is to choose among several alternatives using decision criteria previously defined. The use of MCDM guarantees the identification of the alternative that better performs than all the others according to the selected criteria. After a literature review on available methods for micro-siting of offshore wind turbines in a specific location, and an introduction to the MCDM, the usefulness and potential of MCDM in siting floating wind turbines is highlighted. The MCDM methods can add new parameters to the floating wind farms’ layout process. The results of this study support the potential role of these methods as crucial tools to technology developers and decision-makers.

Concept for a Wind-Yawing Shallow-Draft Floating Turbine

Offshore wind turbines are poised to become a vital part of the global energy landscape — particularly the floating types which give access to a much greater wind power resource. The design possibilities for floating turbines are so different from onshore and offshore fixed-bottom turbines, that a cost-reducing re-imagining may be justified. Apart from the expense of an offshore transmission cable and substation, the costs of hardware for offshore bottom-fixed wind turbines (CAPEX) are roughly twice as much as for onshore. Much of the extra cost can be attributed to the mass of the above- and underwater support and expensive installation. In this paper, we reconsider two aspects of present-day offshore turbines: (a) maintaining a land-turbine architecture (a slender tower with the rotor cantilevered from a yawing, equipment-filled nacelle); (b) seeking to minimize wave-induced motion and loads of the above-water plant. Our aim is an offshore floating design that is potentially less expensive than offshore fixed-bottom units. We outline the preliminary structural analyses that underlie a design focused on weight, cost reduction, and ease of manufacturing. This includes lattice towers, tubular hub and axle, and needle-roller bearings. The biggest concerns about the proposed lightweight system involve motions and forces induced by waves. Shallow-draft floats will follow the waves, leading to greater rotor translation and precession; and lattice towers may be subject to impact loads, ice buildup, and fouling. We present analysis of some of these motions and forces, along with resulting estimates for required structural weight as a preliminary investigation into the feasibility of this lightweight concept.

Computational Fluid Dynamics Analysis of Floating Offshore Wind Turbines in Severe Pitching Conditions

The unsteady aerodynamics of floating wind turbines is more complex than that of fixed-bottom turbines, and the uncertainty of low-fidelity predictions is higher for floating turbines. Navier–Stokes computational fluid dynamics (CFD) can improve the understanding of rotor and wake aerodynamics of floating turbines, and help improving lower-fidelity models. Here, the flow field of the NREL 5 MW rotor with fixed tower, and subjected to prescribed harmonic pitching past the tower base are investigated using blade-resolved CFD compressible flow COSA simulations and incompressible flow FLUENT simulations. CFD results are also compared to predictions of the FAST wind turbine code, which uses blade element momentum theory (BEMT). The selected rotor pitching parameters correspond to an extreme regime unlikely to occur without faults of the turbine safety system, and thus relevant to extreme aerodynamic load analysis. The rotor power and loads in fixed-tower mode predicted by both CFD codes and BEMT are in very good agreement. For the floating turbine, all predicted periodic profiles of rotor power and thrust are qualitatively similar, but the power peaks of both CFD predictions are significantly higher than those of BEMT. Moreover, cross-comparisons of the COSA and FLUENT predictions of blade static pressure also highlight significant compressible flow effects on rotor power and loads. The CFD analyses of the downstream rotor flow also reveal wake features unique to pitching turbines, primarily the space- and time-dependence of the wake generation strength, highlighted by intermittency of the tip vortex shedding.

Force Dynamics and Stationkeeping Costs for Multiline Anchor Systems in Floating Wind Farms With Different Spatial Parameters

While the offshore wind industry has shown a steady trend towards floating turbines, costs of these systems remain high. A multiline anchor concept may significantly reduce the high cost of floating wind, in which floating turbines share anchors. This work investigates the potential cost benefit of implementing a multiline anchor system relative to the conventional single-line anchor system over a range of spatial parameters. The OC4 DeepCwind semisubmersible platform is used to design catenary mooring systems for different water depths and turbine spacings. In all cases, the maximum anchor force in the 3-line anchor system is less than or equal to that of the single-line anchor system. Cost models for the mooring lines, anchors, installation and geotechnical site investigation are presented and discussed. In a 100-turbine farm, the multiline anchor system results in a 9–19% reduction in stationkeeping costs, with high and low estimates for the cost models additionally included. Larger reductions in the combined line and anchor cost result from mooring system configurations with smaller ratios of water depth to turbine spacing. Due to perimeter effects in the multiline configuration, larger cost reductions can be achieved for larger farm sizes.

Experimental Research for Stabilizing Offshore Floating Wind Turbines

Floating turbines are attracting increasing interest today. However, the power generation efficiency of a floating turbine is highly dependent on its motion stability in sea water. This issue is more marked, particularly when the floating turbines operate in relatively shallow water. In order to address this issue, a new concept motion stabilizer is studied in this paper. It is a completely passive device consisting of a number of heave plates. The plates are connected to the foundation of the floating wind turbine via structural arms. Since the heave plates are completely, rather than partially, exposed to water, all surfaces of them can be fully utilized to create the damping forces required to stabilize the floating wind turbine. Moreover, their stabilizing effect can be further amplified due to the application of the structural arms. This is because torques will be generated by the damping forces via the structural arms, and then applied to stabilizing the floating turbine. To verify the proposed concept motion stabilizer, its practical effectiveness on motion reduction is investigated in this paper. Both numerical and experimental testing results have shown that after using the proposed concept stabilizer, the motion stability of the floating turbine has been successfully improved over a wide range of wave periods even in relatively shallow water. Moreover, the comparison has shown that the stabilizer is more effective in stabilizing the floating wind turbine than single heave plate does. This suggests that the proposed concept stabilizer may provide a potentially viable solution for stabilizing floating wind turbines.

Large-Eddy Simulation-Based Study of Effect of Swell-Induced Pitch Motion on Wake-Flow Statistics and Power Extraction of Offshore Wind Turbines

In this study, the effects of ocean swell waves and swell-induced pitch motion on the wake-flow statistics and power extraction of floating wind turbines are numerically investigated. A hybrid numerical model coupling wind large-eddy (LES) and high-order spectral-wave simulations is employed to capture the effects of ocean swell waves on offshore wind. In the simulation, 3 × 3 floating wind turbines with prescribed pitch motions were modeled using the actuator disk model. The turbulence statistics and wind-power extraction rate for the floating turbines are quantified and compared to a reference case with fixed turbines. Statistical analysis based on the phase-average approach shows significant swell-correlated wind-velocity variations in both cases, and the swell-induced pitch motion of floating turbines is found to cause oscillations of wind-turbulence intensity and Reynolds stress, as well as an increase of vertical velocity variance in the near-wake region. Swells also cause periodic oscillation in extracted power density in the fixed turbine case, and the turbine pitch motion in the floating turbine case could further modulate this oscillation by shifting the phase dependence by about 180 degrees with respect to the swell-wave phase.

A Surrogate Modeling Approach for Fatigue Damage Assessment of Floating Wind Turbines

Fatigue analysis for floating wind turbines poses a novel challenge to calculation workflows if a probabilistic load environment is to be considered. The increased complexity of the structure itself as well as its interaction with the environment require a coupled and more detailed analysis with respect to resolution of environmental conditions compared to fixed bottom systems. Different approaches to address the computing challenge for floating turbines are possible to support engineering judgement and have been investigated in the past, with conservative binning on the one end of the accuracy scale and computation intensive Monte Carlo simulations on the other end. This study investigates the feasibility of regression based surrogate models based on radial basis functions. The investigation performed here is aligned with work performed in the H2020 project LIFES50+. Consequently, the considered system is the DTU 10MW Reference Wind Turbine installed on the LIFES50+ OO-Star Wind Floater Semi 10MW. The site under investigation is the LIFES50+ Site B (Gulf of Maine) medium severity representative site. Results show a similar convergence of lifetime fatigue load prediction as with Monte Carlo simulations indicating that this technique may be an alternative if a response model of the considered system is of interest. This may be interesting if damage loading is to be calculated at a different site and if a classification of met-ocean conditions is available.

PENELITIAN AWAL PENEMPATAN TURBIN PEMBANGKIT LISTRIK TENAGA ARUS LAUT (PLTAL) DARI DATA ARUS DAN MORFOLOGI DASAR LAUT DI SELAT BOLENG, NUSA TENGGARA TIMUR

Distribusi kecepatan arus di Selat Boleng sangat dipengaruhi oleh kondisi pasang surut, kedalaman dan bentukan morfologi dasar lautnya. Kecepatan arus bergradasi naik dari dangkal ke kedalaman yang lebih dalam. Dibagian selatan dan tengah selat distribusi kecepatan arus maksimum pada kedalaman laut antara 20 – 50 m, 50 – 100 m dan 100 – 180 m, masing-masing antara 0.5 – 2.0 m/det, 2.1 – 3.0 m/det dan di atas 3.0 m/det. Kecepatan arus maksimum terjadi pada saat kedudukan air pasang menuju pasang maksimum dan kedudukan air pada saat surut menuju surut minimum. Berdasarkan kedalaman laut, bentuk morfologi dasar laut dan distribusi kecepatan arus vertikal dan horizontalnya, maka lokasi penempatan turbin representatif adalah pada kedalaman 75 – 100 m yang terletak di sisi barat bagian selatan (area 1) dan tengah selat (area 2). Sedangkan di bagian utara selat (area 3) pada kedalaman 50 – 100 m. Kecepatan arus di lokasi ini pada saat pasang surut berkisar antara 1.5 – 3.1 m/dtk. Dari ketiga area ini paling representatif adalah area 2 dengan tipe turbin sistem pemberat (gravity base) dan turbin terapung. Kata kunci : pasang surut, kedalaman laut, morfologi dasar laut, kecepatan arus, lokasi turbin, Selat Boleng Current velocity distribution in the Strait of Boleng strongly influenced by tidal conditions, the depth of the sea and seabed morphology formation. Graded current velocity increased from shallow to deeper depths. In the southern and central strait at ocean depths between 20-50 m maximum current velocity distribution ranged from 0.5 - 2.0 m/s, depths 50-100 m maximum current velocity distribution ranged from 2.1 - 3.0 m/s and a depth of 100-180 m maximum current velocity distribution above 3.0 m/s. The maximum current velocity occurs when position of the flood toward the maximum flood and position of the ebb toward the minimum ebb. Based on the depth sea, seabed morphology and distribution of vertical and horizontal current velocity, the location of the turbine placement representative is on the west side of the strait at a depth 75-100 m, located in the southern part of the strait (area 1) and the middle of the strait (area 2). While in the northern part of the strait (area 3) at a depth of 50-100 m. Current velocity at the site in tidal conditions ranged from 1.5 - 3.1 m/sec. Of the three areas, the most representative area is the area 2 with a suitable turbine type is the type turbine with a ballast system (gravity base) and floating turbines. Keywords: tidal, depth sea, strait, current velocity, seabed morphology, turbine location, Boleng Strait