Dominique Roddier

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 I: Design Basis and Qualification Process

This paper and the corresponding hydrodynamic and structural study paper (also in these proceedings) summarize the feasibility study conducted for the WindFloat technology. The WindFloat is a 3-legged floating foundation for very large 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 tower, nacelle and turbine. 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 approximately 30∼50m. Market transition to deeper waters is inevitable, provided suitable technologies can be developed. Despite the increase in complexity, a floating foundation offers distinct advantages: • Flexibility in site location. • Access to superior wind resources further offshore. • Ability to locate in coastal regions with limited shallow continental shelf. • Ability to locate further offshore to eliminate visual impacts. • An integrated structure, without a need to redesign the mast foundation connection for every project. • Simplified offshore installation procedures. Anchors are significantly cheaper to install than fixed foundations and large diameter towers. This paper focuses on the design basis for wind turbine floating foundations, and explores the requirements that must be addressed by design teams in this new field. It shows that the design of the hull for a large wind turbine must draw on the synergies with oil and gas offshore platform technology, while accounting for the different design requirements and functionality of the wind turbine.© 2009 ASME

Dynamic Modeling of Deepwater Offshore Wind Turbine Structures in Gulf of Mexico Storm Conditions

A Fourier spectrum based model of Gulf of Mexico storm conditions is applied to a 6 degree of freedom analytic simulation of a moored, floating offshore structure fitted with three rotary wind turbines. The resulting heave, surge, and sway motions are calculated using a Newtonian Runge-Kutta method. The angular motions of pitch, roll, and yaw are also calculated in this time-domain progression. The forces due to wind, waves, and mooring line tension are predicted as a function of time over a 4000 second interval. The WAMIT program is used to develop the wave forces on the platform. A constant force coefficient is used to estimate wind turbine loads. A TIMEFLOAT computer code calculates the motion of the system based on the various forces on the structure and the system’s inertia.Copyright

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.

Influence of the Reynolds Number on Spar Vortex Induced Motions (VIM): Multiple Scale Model Test Comparisons

There have been a number of publications on spar Vortex-Induced-Motions (VIM) model testing procedures and results over the past few years. All tests allowing full 6 DOF response to date have been done under sub-critical Reynolds Number conditions. Prior to 2006 tests under super-Critical Reynolds Number conditions had only been done with a fully submerged 1 DOF rig. Early in 2006, a series of Spar VIM experiments was undertaken in three different facilities: Force Technology in Denmark, the David Taylor Model Basin in Bethesda Maryland and UC Berkeley in California. The motivation of this work was to investigate the effect of Reynolds Number and hull appurtenances on spar vortex induced motions (VIM) for a vertically moored 6DOF truss spar hull model with strakes. The three series of tests were done at both sub and super-critical Reynolds Numbers, with matching Froude Numbers. In order to assess the importance of appurtenances (chains, pipes and anodes) and current heading on strake effectiveness, tests were done with several sets of appurtenances, and at various headings and reduced velocities. These experiments were unique and groundbreaking in many ways: • For the first time the issue of scalability of Spar VIM experiments has been addressed and tested in a systematic way. • For the first time the effect of appurtenances (pipes, chains and anodes) was systematically tested. • The model tested at the David Taylor Model Basin (DTMB) had a diameter of 5.8′ and a weight of 15,600 lbs. It is the largest spar model ever tested. Furthermore the DTMB tests series is the only supercritical spar VIM performed with a six degree of freedom (6DOF) rig. This paper describes the three model tests campaigns, focusing on the efforts made to ensure three complete geo-similar programs, and on the significant findings of these tests, effectively that the influence of Re is to add some conservativeness in the results as the testing scale is smaller.Copyright

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

Spar VIM Model Tests at Supercritical Reynolds Numbers

There have been a number of publications on spar Vortex-Induced-Motions (VIM) model testing procedures and results over the past few years. All tests allowing full 6 DOF response to date have been done under sub-critical Reynolds Number conditions. Tests under super-Critical Reynolds Number conditions have only been done with a fully submerged 1 DOF rig. Early in 2006, Chevron Energy Technology Company (CETC) completed a series of model tests to investigate the effect of Reynolds Number and hull appurtenances on spar vortex induced motions (VIM) for a vertically moored 6DOF truss spar hull model with strakes. Tests were done at both sub- and super-critical Reynolds Numbers, with matching Froude Numbers. In order to assess the importance of appurtenances (chains, pipes and anodes) and current heading on strake effectiveness, tests were done with several sets of appurtenances, and at various headings and reduced velocities. This paper addresses the challenges of performing spar VIM model tests at Super Critical Reynolds Numbers, and how they were resolved without the restrictions noted in earlier publications. Certain aspects of the effect of appurtenances and current heading on strake effectiveness and VIM response are discussed.Copyright