Christian Cermelli

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

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

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.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE STABILIZING EFFECTS OF A WATER-ENTRAPMENT PLATE ON A DEEPWATER MINIMAL FLOATING PLATFORM

The stabilizing effects of a water-entrapment plate at the keel of a small three-legged semi-submersible platform are determined using laboratory experiments and time-domain simulations. Motion predictions were carried out in the time-domain using coupled-analysis between the vessel and its mooring, linear diffraction-radiation theory, and an empirical wave-viscous interaction model. Model tests were conducted at the U.C. Berkeley Ship Model Testing Facility to determine the validity of the numerical model.Copyright

Design and Installation of a Tension Moored Wind Turbine

This paper describes the design of the floater and the mooring system for a small wind turbine. The engineering basis and the hydrodynamics calculations are described, as well as the installation and commissioning sequences.Copyright

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