Unlike fossil fuels (for example 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
1.1 Background
1.2 Outline
2 STATE OF THE ART
2.1 Regular Waves
2.1.1 Description
2.1.2 Linear Wave Theory
2.1.2.1 Basic equations
2.1.2.2 Boundary conditions
2.1.2.3 Wave kinematics and pressure
2.1.3 Stretched Airy Theory
2.2 Irregular Waves
2.2.1 Description in the Frequency Domain
2.3 Hydrostatics of Floating Structures
2.3.1 Static Stability
2.4 Hydrodynamics of Rigid Bodies
2.4.1 Coordinate Systems
2.4.2 Diffraction Theory
2.5 Hydrostatic- and Dynamic Loads on Floating Structures
2.5.1 Fundamentals
2.5.2 Forces and Moments
2.5.3 Radiation and Diffraction Loads
2.5.4 Wave Excitation Loads
2.5.5 Hydrostatic Loads
2.6 Floating Structures in Waves
2.6.1 Coupled Equations of Motion
2.6.2 Motions in Regular Waves
2.6.2.1 Response amplitude operator
2.6.3 Motions in Irregular Waves
3 FLOATING WIND TURBINE MODEL
3.1 OC3 Hywind
3.1.1 Tower and Platform Structural Properties
3.1.2 Floating Platform Hydrodynamic Properties
3.1.3 Mooring System Properties
4 MATLAB
4.1 Morison Forces
4.1.1 Morison Forces due to Regular Waves
4.1.2 Morison forces due to Irregular Waves
5 SESAM
5.1 GeniE
5.1.1 The Modelling Process
5.2 HydroD
5.2.1 Coordinate System
5.2.2 Panel Model
5.2.3 Mass Model
5.2.4 Analysis Preparation
5.2.5 Wadam
5.2.5.1 Global Response Analysis in Wadam
5.2.6 Postresp
6 FAST
6.1 Basic Assumptions
6.2 Hydrodynamic Module (HydroDyn)
6.2.1 Diffraction Problem
6.2.2 Radiation Problem
6.3 Hydrodynamic Results
6.3.1 Regular Waves
6.3.2 Irregular Waves
7 COMPARISONS OF THE METHODS
7.1 Comparison of Excitation Loads
7.2 Comparison of Response Motions
8 SUMMARY AND CONCLUSION
Objectives & Scope
This thesis aims to investigate the wave-induced loads acting on an OC3 Hywind spar-buoy and to analyze the resulting motions of its support platform. By comparing different numerical methods—specifically Morison's equation in MATLAB, diffraction theory via SESAM, and the fully coupled aero-hydro-servo-elastic simulations in FAST—the research evaluates how these approaches model hydrodynamic interactions, platform responses, and the overall dynamic behavior of floating offshore wind turbines.
- Comparison of Morison's equation, potential theory, and numerical time-domain simulations.
- Analysis of hydrodynamic load computation and resulting floating structure motions.
- Evaluation of steady-state and transient responses in both regular and irregular wave conditions.
- Investigation of the OC3 Hywind platform's hydrodynamic properties and mooring system influence.
- Benchmarking FAST, SESAM, and MATLAB for offshore wind turbine engineering applications.
Extract from the Book
2.1 Regular Waves
The essential parameters of regular waves are discussed and illustrated in chapter 2.1.1. Then, basic equations, boundary conditions and wave kinematics according to the linear wave theory are presented in chapter 2.1.2. Since the linear wave theory neglects the kinematics above mean sea level, the so-called Wheeler Stretching method is introduced, which enables the calculation of the kinematics up to the free water surface (see section 2.1.3).
2.1.1 Description
In general, the spatial and temporal development of the sea state can be examined by the linear or non-linear wave theory. The first-order theory is based on the assumption of small wave steepness, which is defined as the ratio between wave height H and wavelength L, and considers the movement of the water particles on orbits. Figure 2.4 depicts the change of orbits depending on the water depth h. For wave steepness H/L greater than 1/50, waves with a finite amplitude behave nonlinearly. In this case, non-linear wave theories should be applied (EAK, 2002). This thesis basically deals with the first-order theory.
Summary of Chapters
1 INTRODUCTION: Outlines the motivation for renewable energy, the development of offshore wind turbines, and the scope of this research regarding numerical simulations of FOWT dynamics.
2 STATE OF THE ART: Provides the theoretical foundations of wave theory, hydrostatics, and hydrodynamics of rigid bodies, including definitions of boundary conditions and motion equations.
3 FLOATING WIND TURBINE MODEL: Details the structural, hydrodynamic, and mooring properties of the OC3 Hywind spar-buoy system used as the baseline for this study.
4 MATLAB: Covers the application of Morison’s equation to calculate wave forces on the platform in both the time and frequency domains.
5 SESAM: Describes the design and hydrodynamic analysis workflow within the DNV SESAM software suite, focusing on panel model generation and frequency-domain response analysis.
6 FAST: Introduces the open-source code FAST and the HydroDyn module for time-domain simulation of fully coupled aero-hydro-servo-elastic models.
7 COMPARISONS OF THE METHODS: Analyzes and compares the results from the different computational methods, focusing on excitation loads and response motion spectra.
8 SUMMARY AND CONCLUSION: Summarizes key findings, discusses differences between the used methodologies, and provides recommendations for future research.
Keywords
Floating offshore wind turbine, OC3 Hywind, Wave forces, Platform motions, Morison’s equation, Linear wave theory, Wheeler stretching, Hydrodynamics, Potential theory, SESAM, FAST, MATLAB, Response amplitude operator, JONSWAP spectrum, Mooring system
Frequently Asked Questions
What is the fundamental focus of this thesis?
This work focuses on comparing different numerical methods to compute wave forces and the subsequent platform motions of a floating offshore wind turbine (FOWT), specifically the OC3 Hywind model.
Which specific themes are central to the study?
The central themes include the application of Morison's equation, linear diffraction theory, and fully coupled time-domain simulations to estimate the hydrodynamic response of floating structures.
What is the primary goal of this research?
The primary goal is to evaluate the accuracy and consistency of different computational approaches (MATLAB, SESAM, and FAST) in predicting the loads and movement of an OC3 Hywind spar-buoy in varied sea states.
Which scientific methods are utilized?
The study employs linear wave theory, Morison’s equation, the panel method for diffraction analysis, and numerical simulation of coupled equations of motion in both time and frequency domains.
What content is covered in the main section?
The main sections cover the theoretical background of wave physics, the specification of the turbine model, the simulation processes in each program (MATLAB, SESAM, FAST), and a comparative analysis of the resulting hydrodynamic loads and response spectra.
Which keywords best characterize this work?
The most important keywords include Floating offshore wind turbine, OC3 Hywind, Wave forces, Platform motions, Morison’s equation, and Hydrodynamics.
Why are results from FAST and SESAM compared?
Comparing them helps identify how different assumptions—such as steady-state vs. transient responses or frequency-domain vs. time-domain calculations—affect the predicted stability and motion of the structure.
Does the work address resonance?
Yes, the thesis performs an eigenanalysis of the OC3 Hywind platform to ensure that the excitation frequencies of the waves do not lead to significant resonant responses in the platform motion.
How is the Wheeler stretching method applied?
It is used in the MATLAB calculations to modify the kinematics of the linear wave theory up to the instantaneous free water surface, rather than stopping at the mean sea level.
- Citar trabajo
- Olga Glöckner (Autor), 2014, A comparison of methods for computation of wave forcing, Múnich, GRIN Verlag, https://www.grin.com/document/425816
