Alexia Aubault

WindFloat: A floating foundation for offshore wind turbines

This manuscript summarizes the feasibility study conducted for the WindFloat technology. The WindFloat is a three-legged floating foundation for multimegawatt offshore wind turbines. It is designed to accommodate a wind turbine, 5 MW or larger, on one of the columns of the hull with minimal modifications to the nacelle and rotor. Potential redesign of the tower and of the turbine control software can be expected. Technologies for floating foundations for offshore wind turbines are evolving. It is agreed by most experts that the offshore wind industry will see a significant increase in activity in the near future. Fixed offshore turbines are limited in water depth to ∼30–50 m. Market transition to deeper waters is inevitable, provided that suitable technologies can be developed. Despite the increase in complexity, a floating foundation offers the following distinct advantages: Flexibility in site location; access to superior wind resources further offshore; ability to locate in coastal regions with limited...

WindFloat: A Floating Foundation for Offshore Wind Turbines—Part II: Hydrodynamics Analysis

WindFloat is a floating foundation for very large offshore wind turbines. This paper describes the hydrodynamic analysis of the hull, as well as ongoing work consisting of coupling hull hydrodynamics with wind-turbine aerodynamic forces. Three main approaches are presented in this paper: - The numerical hydrodynamic model of the platform and its mooring system; - Wave tank testing of a scale model of the platform with simplified aerodynamic simulation of the wind turbine; - FAST, an aerodynamic software package for wind turbine analysis with the ability to be coupled to the hydrodynamic model. These conference proceedings include two other papers presenting the design basis and main systems of this floating foundation [1], as well as structural analysis of the hull and mast [2].Copyright

Modeling of an Oscillating Water Column on the Floating Foundation WindFloat

This paper summarizes the theory behind the modeling that was performed to incorporate an oscillating- water-column type Wave energy Converter (WEC) into the WindFloat hull. The WindFloat is a floating structure supporting a very large (>5MW) wind turbine. By adding a WEC to the structure, the overall economic cost of the project can be improved by sharing both mooring and power infrastructure. A numerical model was developed using the diffraction-radiation code WAMIT and assuming as PTO equipment, a generic wells turbine. It is important to model the turbine accurately, to understand the power capacity of the device. Details on the modeling of the system are discussed and numerical results and compared against experiments as a validation of the model. The effect of coupling between the floating foundation of the WindFloat and the OWC is investigated thoroughly.

WINDFLOAT: A FLOATING FOUNDATION FOR OFFSHORE WIND TURBINES PART III: STRUCTURAL ANALYSIS

WindFloat is a floating foundation for large offshore wind turbines. This paper describes the structural engineering that was performed as part of the feasibility study conducted for qualification of the technology. Specifically, the preliminary scantling is described and the strength and fatigue analysis methodologies are explained, focusing on the following aspects: • the coupling between the wind turbine and the hull; • the interface between the hydrodynamic loading and the structural response. Prior to reading this manuscript, the reader is strongly encouraged to read the related paper, which focuses on the design basis for the WindFloat, and explores the requirements that must be addressed by the design teams in this new field. An additional paper in this series describes the hydrodynamic analysis and experimental validations.Copyright

Validation of Wave Run-Up Calculation Methods for a Gravity Based Structure

During the design of a Gravity-Base Structure (GBS) for harsh environments, it is essential to account for the maximum wave run-up in operational and extreme weather conditions. Linear diffraction theory and empirical correction factors are typically used in the early design phase of a project in which wave run-up is a concern. As the project nears final design, model tests are usually used to assess wave run-up and air gap requirements. This paper addresses the use of alternative methods for prediction of run-up around a GBS in approximately 100 m water depth. Results from a second-order diffraction code (WAMIT) and a fully nonlinear CFD program (ComFLOW) are compared to assess the importance of nonlinearities, which are shown to depend on incident wave steepness and wavelength. Extending diffraction theory to second-order significantly improves linear predictions and produces more realistic spatial patterns of maximum run-up. However CFD simulations are required to accurately predict run-up associated with very steep incident waves and highly nonlinear characteristics. In addition to regular wave computations, linear and second-order potential flow calculations are also compared against model test results for an irregular sea.Copyright

Design of a Point Absorber Inside the WindFloat Structure

This paper summarizes the modeling and testing that was performed to integrate a point-absorber type Wave-Energy Converter (WEC) within the WindFloat hull. The WindFloat is a floating structure supporting a very large (>5MW) wind turbine. By adding a wave-energy device to the structure, one can improve the overall economic cost of the project, since both the mooring system and power infrastructure are shared. For the device analyzed here, the modeling is first described and then the Motion Response Amplitude Operators (RAOs) are computed. From these motion responses, the theoretical mechanical power available is calculated. The power values depend on empirical coefficients that need to be confirmed through model testing in the lab. The hydrodynamic forces on each device are often dependent on the interference between the device and the hull, the mooring, and the non-linear effects which are challenging to model. Therefore, these forces are approximated using a Morrison-type formulation in the numerical models. The empirical values for drag coefficients, damping coefficients, and stiffness coefficients in this report are validated against model tests, which are also described.Copyright

Qualification of a Semi-Submersible Floating Foundation for Multi-Megawatt Wind Turbines

Recent trends in the wind industry point to the use of increasingly larger and more powerful machines with rated power ranging from 5 to 10MW exclusively designed for offshore use. Floating foundations offer greater flexibility in term of site selection for wind farms, and if properly designed, may result in comparable availability with equivalent offshore turbines on fixed foundations, while reducing the complexity and risks associated with offshore installation. The WindFloat platform is a semi-submersible platform with three columns fitted with a large horizontal water-entrapment plate at the base. The wind turbine and tower are fitted on one of the columns. The platform is designed to support commercially available multi-megawatt wind turbines with no hardware modification to the turbine. The qualification process followed for the development of a 150MW wind farm offshore Portugal is discussed. Because of economic constraints, optimization of the platform is essential to achieve project financial targets. A rational and comprehensive process was followed to optimize the system while maintaining the robustness required to survive in the offshore environment. The design process is based on a combination of advanced numerical analysis and scale model experimentation. Full-scale experimentation is ongoing. Selected design codes and industry standards are applied. The return period of extreme events is adjusted based on experience acquired by the wind industry. Because of the considerable aerodynamic loads generated by the wind turbine and their effects on platform motion, the ability to solve the combined aerodynamic and hydrodynamic problem is necessary. Additional factors, such as tower dynamics and turbine controls must also be taken into account. Development of a coupled hydro-servo-aero-elastic model constitutes a key element of the qualification process.

Parametric Optimization of a Semi-Submersible Platform With Heave Plates

The hydrodynamic responses of a semi-submersible platform are driven by its mass properties and geometric parameters, e.g. column size, spacing, draft and pontoon size. The mooring system also influences the platform responses. Heave plates added to the base of each column have been proposed to enhanced stability of semi-submersible platforms, particularly in the lower payload range. Optimization of a platform typically involves a compromise among a large number of factors including the structural weight, vertical, horizontal motion and rotations in operating and extreme sea-states, airgap, mooring size, etc. Optimization methods are reviewed. The complexity of the problem leads to the choice of a genetic algorithm presented herein. To allow systematic platform optimization assuming primary project parameters are given, i.e. payload, waterdepth, environmental conditions. A simplified hydrodynamic model is developed to capture the parametric sensitivity of the platform responses to primary design parameters.

Electrical Power Generation by Tidal Flow Acceleration

Hydropower is a significant contributor to the renewable power generation sector, but the energy in tidal currents is not commonly used to generate electricity. This is due to the relatively slow speed of tidal currents which does not allow for the economic development of underwater turbines in tidal regions. This paper investigates whether it is possible to increase locally the current speed in regions where the tidal current is normally not strong enough to generate significant power. The device proposed to increase current speed is composed of an arrangement of vertical walls made of poles supporting a thin membrane with suitable profile, referred to as Tidal Current Accelerating Structure or TCAS. Current turbines are to be placed in areas of accelerated flow to convert the current energy into electricity. In this paper, results of model tests that were performed to quantify the ability to increase current speed are discussed. It was found that the increase in flow velocity was not as significant as expected, probably due to interactions between the turbines and the current accelerating devices. Recall potential theory’s flow speed around a disk yields a velocity factor increase of 2 at 90 degrees from the stagnation point.© 2007 ASME

WindFloat Contraption: From Conception to Reproduction

This paper will discuss the very serious topic of the design of the WindFloat, a full-scale floating wind turbine. The importance of the fundamentals of hydrodynamics in achieving the desired performance cannot be overstressed. These will be discussed in this paper, together with some of the key considerations that entered into the design process. At the time of writing of this manuscript, a full-scale WindFloat prototype has been spinning for a few months, and the electricity it generated powered all the Christmas lights of Povoa de Varzim, a small town in the north of Portugal — whose inhabitants are not seeing a reduction in their electricity bill.The authors have chosen to disassociate the presentation — a progress report — from this manuscript, which should discuss a topic more appropriate to the permanent literature.© 2012 ASME