Wind energy. Methods for computation of wave forcing and the resulting motion of a slender offshore floating structure

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

9 REFERENCES

Objectives and Topics

This thesis investigates and compares different methods for calculating wave-induced loads and the resulting platform motions of an OC3 Hywind floating offshore wind turbine. The primary research goal is to evaluate the applicability and accuracy of Morison’s equation against more advanced potential theory-based simulations (SESAM and FAST) for floating structures.

• Comparison of Morison’s equation (MATLAB) with potential theory (FAST, SESAM)
• Hydrodynamic analysis of the OC3 Hywind spar-buoy system
• Calculation of wave excitation loads and platform response motions
• Evaluation of time-domain vs. frequency-domain simulation approaches
• Impact of structural mass and buoyancy on platform dynamics

Excerpt from the Book

Summary of Chapters

1 INTRODUCTION: Outlines the motivation for offshore wind energy and sets the scope of this thesis regarding the comparison of simulation methods for the OC3 Hywind model.

2 STATE OF THE ART: Provides the theoretical foundation regarding regular and irregular waves, hydrostatic stability, and the hydrodynamics of rigid bodies.

3 FLOATING WIND TURBINE MODEL: Details the structural, hydrodynamic, and mooring system specifications of the OC3 Hywind spar-buoy used for all subsequent simulations.

4 MATLAB: Describes the implementation of a modified Morison formulation in the time domain to estimate wave forces acting on the floating structure.

5 SESAM: Explains the modelling process and hydrodynamic analysis of the wind turbine structure using the commercial software suite SESAM in the frequency domain.

6 FAST: Details the hydrodynamic module HydroDyn within the FAST code, emphasizing time-domain simulations and the inclusion of radiation and diffraction terms.

7 COMPARISONS OF THE METHODS: Evaluates the differences between the calculated excitation loads and response motions across the three utilized methods.

8 SUMMARY AND CONCLUSION: Synthesizes the findings and provides recommendations for future improvements in the hydrodynamic modelling of floating offshore wind turbines.

Keywords

Floating offshore wind turbine, OC3 Hywind, Wave forces, Platform motions, Morison's equation, Diffraction theory, Radiation loads, Hydrodynamics, SESAM, FAST, MATLAB, JONSWAP spectrum, Response amplitude operator, Time domain, Frequency domain

Frequently Asked Questions

What is the core focus of this thesis?

The work primarily focuses on comparing different numerical methods for calculating wave-induced loads and the resulting motions of floating offshore wind turbines, specifically the OC3 Hywind spar-buoy.

Which simulation methods are compared?

The thesis compares the modified Morison formulation (implemented in MATLAB), the potential theory-based frequency-domain analysis (using SESAM), and the time-domain coupled aero-hydro-servo-elastic simulation (using FAST).

What is the primary objective regarding the software comparison?

The main objective is to determine how well simpler models like Morison's equation approximate the loads and motions of a floating structure compared to higher-fidelity diffraction-based models.

Which hydrodynamic theories are employed?

The study utilizes both the Morison equation for slender structures and linear potential (diffraction/radiation) theory for large-volume structures, incorporating specific methods like Wheeler stretching for kinematics.

What constitutes the main body of work?

The main body covers the theoretical background of wave loads, the specific design parameters of the OC3 Hywind model, and the implementation details for MATLAB, SESAM, and FAST simulations.

Which keywords define this research?

Key terms include Floating offshore wind turbine, OC3 Hywind, Wave forces, Platform motions, Hydrodynamics, and specific software like SESAM and FAST.

Why are Morison forces generally found to be higher than those computed by FAST?

The analysis suggests that Morison's equation often overestimates forces because it neglects wave-radiation damping and diffraction effects, which are significant for floating structures.

Does the thesis conclude that SESAM is accurate?

The thesis concludes that while SESAM provides useful engineering approximations, it is considered uncertain in its accuracy for floating structures due to its reliance on user-defined properties and frequency-domain assumptions, advocating for time-domain analysis like FAST for more reliable results.