Unlike fossil fuels (e.g. oil, coal and natural gas), wind energy is a renewable energy resource. Since winds at sea are stronger and more consistent than onshore winds, the demand for offshore wind turbines has increased over the last years. As energy can be produced more efficient in deeper water, several floating offshore wind turbine constructions, such as the OC3 Hywind spar-buoy, have been proposed. The design of floating wind turbines depends on the simulation of the system behavior caused by exciting forces.
This thesis deals with the comparison between different methods for calculating wave forces and resulting platform motions of a floating offshore wind turbine. On the one hand, wave exciting loads computed with Morison’s equation are compared to the hydrodynamic forces simulated by the open source code FAST on the basis of the diffraction theory. On the other hand, response motions of the floating structure are simulated by the commercial offshore software SESAM in the frequency domain and compared with the motions calculated by FAST in the time domain.
Table of Contents
1. Introduction
2. Floating Offshore Wind Turbine Model
3. Calculation of Wave Loads
4. Modified Morison Formulation
5. SESAM
6. FAST
7. Comparison of the Methods
8. Summary
9. Conclusion and Outlook
Research Objectives and Topics
The primary objective of this thesis is to provide a comprehensive presentation and comparison of different numerical methods used for calculating wave forces and the resulting motion response of floating offshore wind turbines (FOWT). By examining various modeling approaches, the work investigates the discrepancies in load prediction and dynamic behavior when applying different hydrodynamic theories to a slender floating structure.
- Analysis of hydrodynamic load computation using Morison's equation and diffraction theory.
- Evaluation of the OC3 Hywind spar-buoy model as a floating platform.
- Comparative assessment of frequency-domain (SESAM) and time-domain (FAST) simulations.
- Investigation of wave excitation, radiation, and damping effects on platform stability.
- Validation of numerical simulation reliability for floating offshore structures.
Excerpt from the Book
Calculation of Wave Loads
Diffraction Theory (D/L > 0.2)
Wave Excitation: Loads acting on the body when it is restrained from oscillating and incident waves are present. Consists of Froude-Krilov forces and Diffraction forces.
Wave Radiation: Loads acting on the floating body when it is forced to oscillate and no incident waves occur. Consists of Added mass and Wave Damping.
Chapter Summary
1. Introduction: Provides background on global energy demand and the growing significance of offshore wind energy as a renewable resource.
2. Floating Offshore Wind Turbine Model: Introduces the OC3 Hywind spar-buoy system, including its structural dimensions and design origin.
3. Calculation of Wave Loads: Explains fundamental hydrodynamic theories, focusing on Diffraction Theory and the components of wave excitation and radiation.
4. Modified Morison Formulation: Details the application of Airy theory and Wheeler Stretching to adjust Morison's equation for wave kinematics.
5. SESAM: Describes the software suite used for finite element structural engineering and hydrodynamic design, including WADAM and DeepC modules.
6. FAST: Discusses the NREL-developed tool for analyzing coupled dynamic responses and aerodynamic loads on wind turbines.
7. Comparison of the Methods: Contrasts the results between MATLAB-based Morison implementations, SESAM, and FAST simulations regarding platform motion and force.
8. Summary: Recaps the analytical scope and the simulation methods applied to investigate FOWT performance.
9. Conclusion and Outlook: Discusses the findings regarding accuracy discrepancies and suggests future improvements for hydrodynamic modeling.
Keywords
Floating Offshore Wind Turbine, FOWT, Hywind, Wave Loads, Morison Equation, Diffraction Theory, SESAM, FAST, Hydrodynamics, Frequency Domain, Time Domain, Response Amplitude Operator, Added Mass, Wave Damping, Structural Dynamics
Frequently Asked Questions
What is the core focus of this research?
The work focuses on comparing numerical methods for calculating hydrodynamic wave forces and the dynamic motion of slender floating offshore wind structures.
Which specific structural model is utilized?
The research utilizes the OC3 Hywind spar-buoy model, a standard deep-water floating substructure for offshore wind energy.
What is the primary goal of the comparison?
The goal is to determine how different modeling techniques (e.g., Morison formulation vs. diffraction theory) influence the calculated load and resulting motion responses of the platform.
Which scientific software tools are compared?
The study primarily compares custom calculations in MATLAB, the DNV SESAM suite, and the NREL FAST tool.
How is the hydrodynamic analysis structured?
The analysis covers both frequency-domain approaches, used by SESAM, and time-domain simulations, as performed by FAST.
Which parameters characterize the findings?
Key parameters include wave excitation forces, added mass, wave radiation, and the resulting response spectra for surge, heave, and pitch motions.
Why is Morison's equation considered insufficient for FOWTs?
Morison's equation typically neglects wave-radiation damping and diffraction loads, making it less accurate for complex floating structures compared to diffraction-based models.
What are the main takeaways regarding SESAM's accuracy?
The study notes that while SESAM provides valuable insights, its reliance on frequency-domain assumptions makes it less suitable for modeling transient effects compared to time-domain simulations.
How does the platform motion differ between FAST and SESAM?
The research highlights significant differences in response spectra, specifically noting larger motions for pitch and heave in FAST simulations compared to SESAM.
- Quote paper
- Olga Glöckner (Author), 2014, Comparison of methods for the computation of wave forcing, Munich, GRIN Verlag, https://www.grin.com/document/425814
