Hydrodynamic analysis of semi- submersible floating platform combined with wave energy converters

Treball de Fi de Màster Master’s in Renewable Energy Hydrodynamic analysis of semisubmersible floating platform combined with wave energy converters MEMÒRIA Escola Tècnica Superior d’Enginyeria Industrial de Barcelona Author: Shreya Tushar Chavan Director: Oriol Gomis-Bellmunt Call: 2nd October 2024 2 | P a g e O c t o b e r 2 0 2 4 Hydrodynamic performance and analysis of SSP combined with WECs 3 | P a g e Abstract This study investigates the performance of a hybrid floating platform equipped with Point Absorber Wave Energy Converters (PAWECs). It provides an in-depth analysis of the platform's dynamics, WEC efficiency, and overall power production capabilities. A thorough literature review on floating offshore wind turbines, wave energy converters, and hybrid systems forms the theoretical basis. Metocean data from a selected site in the Irish Sea is analyzed to identify key factors affecting performance. A comprehensive design of the Hybrid Floating Wind-Wave Platform (HFWWP) is developed using SolidWorks, and a hydrodynamic analysis is performed using Ansys AQWA to model the platform’s response to waves. These simulations are validated against experimental data. Time-domain analysis using WEC-Sim is carried out to simulate power generation under varying wave conditions. Key findings include a significant reduction in pitch motion with the integration of WECs, which improves platform stability. The Response Amplitude Operator (RAO) results showed that the addition of WECs dampened surge and pitch motions, enhancing overall platform stability. Additionally, the optimal Power Take-Off (PTO) damping coefficient was identified, maximizing energy production across different sea states. The research concludes that hybrid floating platforms hold strong potential as a sustainable and efficient renewable energy solution. 4 | P a g e O c t o b e r 2 0 2 4 Hydrodynamic performance and analysis of SSP combined with WECs 5 | P a g e Table of Contents Abstract ............................................................................................................................. 3 Abbreviations ..................................................................................................................... 7 List of Figures .................................................................................................................... 8 List of Tables ..................................................................................................................... 9 1. Introduction ................................................................................................................ 11 1.1 Motivation .................................................................................................................. 11 1.2 Scope ...................................................................................................................... 12 1.3 Objectives ............................................................................................................... 13 2. Theoretical background ............................................................................................. 14 2.1 Theoretical foundation ............................................................................................... 14 2.1.1 Wave mechanics and wave energy ..................................................................... 14 2.1.2 Potential flow theory and Linear (Airy) wave theory ............................................. 15 2.1.3 Wave energy ....................................................................................................... 17 2.1.4 Hydrodynamic diffraction ..................................................................................... 19 2.1.5 Boundary Element Method (BEM) ....................................................................... 20 2.1.6 Numerical modelling ............................................................................................ 20 2.1.7 Equation of motion .............................................................................................. 20 2.2 Background and state of the matter ........................................................................... 21 2.2.1 Offshore Renewable Energy ............................................................................... 21 2.2.2 Floating platforms ................................................................................................ 23 2.2.3 WECs .................................................................................................................. 25 2.2.4 HWWFP .............................................................................................................. 26 3. Methodology .............................................................................................................. 27 3.1 Reference site ........................................................................................................... 27 3.2 Design parameters .................................................................................................... 30 3.2.1 Orientation and coordinate system ...................................................................... 30 3.2.2 SSP..................................................................................................................... 31 3.2.3 WECs .................................................................................................................. 32 3.2.4 HWWFP .............................................................................................................. 33 3.3 System properties ...................................................................................................... 34 3.4 Power generation ...................................................................................................... 36 6 | P a g e O c t o b e r 2 0 2 4 3.4.1 BEMIO ................................................................................................................ 37 3.4.2 Simulink model .................................................................................................... 37 3.4.3 PTO .................................................................................................................... 38 3.4.4 Inputs .................................................................................................................. 39 4. Results and discussion .............................................................................................. 41 4.1 Hydrodynamic diffraction results ................................................................................ 41 4.1.1 Validation for SSP ............................................................................................... 41 4.1.2 RAO .................................................................................................................... 42 4.1.3 Hydrodynamic coefficients and forces ................................................................. 45 4.1.3.2 Hydrodynamic diffraction .................................................................................. 47 4.2 Time domain analysis .............................................................................................. 49 5. Conclusion ................................................................................................................. 52 6. Limitations and recommended future works ............................................................... 53 Acknowledgements ......................................................................................................... 55 References ...................................................................................................................... 56 Appendix A ...................................................................................................................... 60 Hydrodynamic performance and analysis of SSP combined with WECs 7 | P a g e Abbreviations BEM – Boundary Element Method CAD – Computer-Aided Design DOF – Degree of Freedom EU – European Union GW – Giga Watt HD – Hydrodynamic Diffraction HFWWP – Hybrid Floating Wind and Wave Platform JPD – Joint Probability Distribution MW – Mega Watt PAWEC – Point Absorber Wave Energy Converters PTO – Power Take Off RAO – Response Amplitude Operator SS – Sea State SSP – Semi-Submersible Platform SW – SolidWorks SWL – Sea Water Level SWL – Surface Water Level WEC – Wave Energy Converter Wec-SIM – Wave energy converter SIMulator 8 | P a g e O c t o b e r 2 0 2 4 List of Figures Figure 1. Simple representation of ocean wave [17] ...................................................... 15 Figure 2. Governing equations for WEC hydrodynamic modelling [19] ........................... 15 Figure 3. Relative amounts of energy as a function of wave period in ocean waves[17] . 19 Figure 4. Hydrodynamics of a semi-submerged body ..................................................... 21 Figure 5. Global distribution of ocean energy activity [24] ............................................... 23 Figure 6. Classification of Offshore Wind Structures [38] ................................................ 24 Figure 7. DeepCwind floating platform ............................................................................ 25 Figure 8. Classification of PAWECs[57] .......................................................................... 26 Figure 9. Site location ..................................................................................................... 28 Figure 10. Joint probability distribution between Significant wave height and Peak period. ....................................................................................................................................... 29 Figure 11. Coordinate system ......................................................................................... 31 Figure 12. Structures of SSP .......................................................................................... 31 Figure 13. WEC dimensions ........................................................................................... 33 Figure 14. HWWFP layout .............................................................................................. 34 Figure 15. Meshed HWWFP ........................................................................................... 36 Figure 16. Optimal PTO damping coefficient for WECs .................................................. 39 Figure 17. Natural modes for SSP .................................................................................. 42 Figure 18. Surge RAO for SSP and HWWFP ................................................................. 43 Figure 19. Heave RAO for SSP and HWWFP ................................................................. 44 Figure 20. Pitch RAO for SSP and HWWFP ................................................................... 44 Figure 21. Surge component of radiation damping for SSP and HWWFP ....................... 46 Figure 22. Heave component of radiation damping for SSP and HWWFP ..................... 46 Figure 23. Pitch component of radiation damping for SSP and HWWFP ........................ 47 Figure 24. Surge component of hydrodynamic diffraction forces for HWWFP and SSP .. 48 Figure 25. Heave component of hydrodynamic diffraction forces for HWWFP and SSP . 48 Figure 26. Pitch component of hydrodynamic diffraction forces for HWWFP and SSP.... 49 Figure 27. Forces and moment acting on SSP for different SS ....................................... 50 Figure 28. SSP surge, heave and pitch for different SS .................................................. 50 Figure 29. Power generation from WEC 1 for different SS .............................................. 51 Figure 30. Power generated by each WEC for SS3 ........................................................ 52 Figure 31. HWWFP Simulink model................................................................................ 61 Hydrodynamic performance and analysis of SSP combined with WECs 9 | P a g e List of Tables Table 1. Design parameters for SSP ............................................................................... 32 Table 2. Design parameters for WEC ............................................................................. 32 Table 3. Design parameters for HWWFP ........................................................................ 33 Table 4. Properties of SSP and WEC .............................................................................. 35 Table 5. Mesh details ...................................................................................................... 35 Table 6. Simulation parameters for HD ........................................................................... 35 Table 7. Input parameters for Wec-SIM .......................................................................... 40 Table 8. Sea states definition .......................................................................................... 40 Table 9. Comparison of AQWA with experimental results of 3 DOF motion natural periods ....................................................................................................................................... 42 16 | P a g e O c t o b e r 2 0 2 4 ∇∙𝑉=0 ( 3 ) Where, ∇ is the gradient operator and Φ= Φ(x,y,z,t) is velocity potential function. By substituting (2) in (3), we arrive at the Laplace equation, 𝛻²𝛷 = 0 ( 4 ) The components of velocity in Cartesian coordinates are, 𝑢= 𝜕𝑢 𝜕𝑥,𝑣= 𝜕𝑣 𝜕𝑥 ,𝑤= 𝜕𝑤 𝜕𝑥 ( 5 ) The velocity must satisfy the conversation of mass equation, which is given by: 𝜕𝑢 𝜕𝑥+𝜕𝑣 𝜕𝑦+𝜕𝑤 𝜕𝑧=0 ( 6 ) From equations (4), (5) and (6) we get the Laplace equation in Cartesian coordinates, 𝛻2𝛷 =𝜕2𝛷 𝜕𝑥2+𝜕2𝛷 𝜕𝑦2+𝜕2𝛷 𝜕𝑧2 = 0 To solve the Laplace equation for a regular wave, as mentioned in equation (1), boundary conditions need to be defined, which describe the wave’s characteristics. The kinematic boundary condition represents the motion of the free surface, and states that a fluid particle at the surface should always remain at the water surface [21]. The equation below is a mathematical representation of the kinematic boundary condition of a fluid particle at 𝜂: {𝜕𝜙 𝜕𝑧=𝜕𝜂 𝜕𝑡 𝑎𝑡 𝑧 = 0 {𝜕𝜙 𝜕𝑧=0 𝑎𝑡 𝑧= −𝑑 Where −𝑑 indicates a water depth below the surface water level. Hydrodynamic performance and analysis of SSP combined with WECs 17 | P a g e In addition to the kinematic boundary condition, the water surface must also satisfy a dynamic boundary condition. This means that the pressure on the surface of the water must be the same as the pressure of the air above it. This condition is based on Bernoulli's equation, which deals with the forces acting on the water surface. The momentum balance equation, which is based on Newton's second law of motion, describes the relationship between the forces acting on the water surface and its acceleration. 𝜕(𝜌𝑢) 𝜕𝑡 +𝜕𝑢(𝜌𝑢) 𝜕𝑥 +𝜕𝑣(𝜌𝑢) 𝜕𝑦 +𝜕𝑤(𝜌𝑢) 𝜕𝑧 = 𝐹𝑥 Ignoring non-linear terms and simplifying yields the linearized Bernoulli equation for unsteady flow 𝜕𝜙 𝜕𝑡+𝑝 𝜌+𝑔𝑧 = 0 In terms of velocity potential, the linearized Bernoulli equation for unsteady flow can be rewritten as 𝜕𝜙 𝜕𝑡 + 𝑔𝜂 = 0 Linear wave theory is the simplest approach to solving the equation for waves. It's a basic approximation of the velocity field. In deep water, linear wave theory assumes that the water depth is much larger than the wavelength. This means that when at the water's surface (𝑧=0), the kinematic and dynamic boundary conditions can be simplified. 𝜕𝜙 𝜕𝑧 − 𝜕𝜂 𝜕𝑡=0 2.1.3 Wave energy Figure 3. shows the wave energy spectrum, which illustrates the distribution of wave energy across different wave periods. The wave energy spectrum typically shows a peak in the gravity wave band, indicating that most of the wave energy is concentrated in waves with periods between 1 second and 30 seconds. The site selected for the analysis in this study, falls between this period range. In linear wave theory, which assumes small-amplitude waves, the total wave energy can be divided into two components: kinetic energy and potential energy [22]. Kinetic energy is the energy associated with the motion of the water particles. For a wave, the kinetic energy is related to the velocity of the water particles and potential energy is the energy stored in 18 | P a g e O c t o b e r 2 0 2 4 the wave due to the elevation of the water surface above its mean level. These are given by: 𝐸𝑘= ∫1 2 𝑧 = 𝜂 𝑧 = −𝑑 𝜌(𝑢2+𝑤2)𝑑𝑧=1 4𝜌𝑔𝑎2 𝐸𝑝= ∫ 𝜌𝑔𝑧 𝑑𝑧 𝑧 = 𝜂 𝑧 = −𝑑 =1 4𝜌𝑔𝑎2 Significant wave height 𝐻𝑠 is a statistical measure used to describe the average height of the highest one-third of waves in a wave record. It provides a representative value for the wave conditions in a given area. It is a widely used metric to describe the severity of wave conditions. It provides a clear and concise way to communicate the wave height. It is more commonly used than mean wave height. It is defined as the average height of the highest one third of the waves: 𝐻𝑠= 1 𝑁3 ⁄ ∑𝐻𝑗 𝑁3 ⁄ 𝑗=1 Under Linear wave theory [23], the kinetic and potential energy are the same. Thus, by replacing 𝑎 with 𝐻𝑠, we get the total energy produced by waves: 𝐸𝑇= 1 16𝜌𝑔𝐻𝑠2 ( 7 ) The wave power level, 𝑃, per width unit in a wave in terms of the significant wave height (𝐻𝑠) and the energy period (𝑇𝑒) can be given as follows: 𝑃= 𝜌𝑔2𝐻𝑠2𝑇𝑒 64 ( 8 ) Hydrodynamic performance and analysis of SSP combined with WECs 19 | P a g e Figure 3. Relative amounts of energy as a function of wave period in ocean waves[17] 2.1.4 Hydrodynamic diffraction Hydrodynamic diffraction involves the interaction of waves with structures, leading to wave scattering and the generation of forces on the structure. The analysis is based on the theory mentioned in 2.1.2. Hydrodynamic coefficients, including added mass and radiation damping, are calculated to represent the inertia and energy dissipation due to wavestructure interaction. Wave excitation forces and moments are also determined. The added mass and damping are the imaginary and real parts, respectively, of the radiation wave potential, 𝜑𝑟𝑘, and are given by: 𝐴𝑗𝑘= 𝜌 𝜔 ∫𝐼𝑚 [ 𝑥 𝑆0𝜑𝑟𝑘 (𝑋)] 𝑛𝑗 𝑑𝑆 𝐵𝑗𝑘= −𝜌∫𝑅𝑒 [ 𝑥 𝑆0𝜑𝑟𝑘 (𝑋)] 𝑛𝑗 𝑑𝑆 Where, 𝑋 is the space dependent potential term, 𝑛𝑗 is the 𝑗𝑡ℎ unit normal vector of the body surface pointing outwards, and 𝑆0 is the wetted surface of the body in still water. The term 𝑗=(1,6) is the notation for the conventional six DOFs. The complete derivation of these terms can be found in [18]. 20 | P a g e O c t o b e r 2 0 2 4 2.1.5 Boundary Element Method (BEM) The Boundary Element Method (BEM) is extensively used in hydrodynamics to analyse the interaction between fluid and structures, such as ships, offshore platforms, and wave energy converters. BEM reduces a 3D problem to a 2D surface problem by focusing on the boundaries of the domain rather than the entire volume. The method transforms the governing partial differential equations into integral equations over the boundary of the domain. This is particularly useful for problems involving infinite or semi-infinite domains, such as wave propagation. The problem is divided into diffraction and radiation components. The Boundary Element Method (BEM) is used to solve the boundary value problem, with the structure’s surface discretized into panels. Hydrodynamic coefficients, including added mass and radiation damping, are calculated to represent the inertia and energy dissipation due to wave-structure interaction. Wave excitation forces and moments are also determined. In ANSYS AQWA, the structure is meshed, and the hydrodynamic diffraction analysis is performed using specific solver settings. 2.1.6 Numerical modelling Time domain analysis examines how a signal or system's output changes over time, providing direct insights into its dynamic behaviour and transient responses. In contrast, frequency domain analysis transforms time-domain signals into their frequency components, revealing the signal's frequency content and aiding in spectral analysis, resonance identification, and filter design. For analysing a hybrid wind-wave floating platform, both time domain and frequency domain analyses were essential. Time domain analysis was crucial for observing the platform's dynamic response to time-varying wave conditions. Frequency domain analysis focused on understanding the frequency characteristics of the platform and its interaction with waves. This analysis was particularly useful for identifying resonant frequencies, and stability of the platform. To achieve the best results, a combined approach was used. Starting with frequency domain analysis to identify key resonant frequencies and optimize the design for energy capture, followed by time domain analysis to evaluate the platform's performance under realistic, time-varying wave and wind conditions and to fine-tune control systems. 2.1.7 Equation of motion The equation of motion for a floating body in hydrodynamics typically involves the balance of forces and moments acting on the body. This equation accounts for the body’s inertia, hydrodynamic forces, and external forces such as gravity and buoyancy. The general equation of motion for a floating body is given by: Hydrodynamic performance and analysis of SSP combined with WECs 21 | P a g e 𝑚𝑋󰇘= 𝐹𝑒𝑥𝑡(𝑡)+ 𝐹𝑚𝑑(𝑡)+ 𝐹𝑟𝑎𝑑(𝑡)+ 𝐹𝑝𝑡𝑜(𝑡)+𝐹𝑣(𝑡)+ 𝐹𝑚𝑒(𝑡)+ 𝐹𝐵(𝑡)+ 𝐹𝑚(𝑡) ( 9 ) where 𝑋󰇘 is the (translational and rotational) acceleration vector of the device, 𝑚 is the mass matrix, 𝐹𝑒𝑥𝑡(𝑡) is the wave excitation force and torque (6-element) vector, 𝐹𝑚𝑑(𝑡) is the mean drift force and torque vector, 𝐹𝑟𝑎𝑑(𝑡) is the force and torque vector resulting from wave radiation, 𝐹𝑝𝑡𝑜(𝑡) is the Power Take Off (PTO) force and torque vector, 𝐹𝑣(𝑡) is the damping force and torque vector, 𝐹𝑚𝑒(𝑡) is the Morison Element force and torque vector, 𝐹𝐵(𝑡) is the net buoyancy restoring force and torque vector, and 𝐹𝑚(𝑡) is the force and torque vector resulting from the mooring connection. 𝐹𝑒𝑥𝑡(𝑡), 𝐹𝑟𝑎𝑑(𝑡), and 𝐹𝐵(𝑡) are calculated using hydrodynamic coefficients provided by the frequency-domain BEM solver, ANSYS AQWA. How these forces act on an actual floating body can be seen in Figure 4. Figure 4. Hydrodynamics of a semi-submerged body 2.2 Background and state of the matter 2.2.1 Offshore Renewable Energy Offshore renewable energy, such as wind, wave power and tidal, is crucial for reducing carbon emissions and achieving sustainable energy goals. Many countries are leveraging the vast oceans to generate clean energy. Figure 5. highlights the global landscape of ocean energy development, with Europe as a key player and emerging markets showing 22 | P a g e O c t o b e r 2 0 2 4 increasing interest in this renewable energy source. The majority of ocean energy projects are concentrated in Europe, particularly in the North Sea and Atlantic regions. This reflects the strong focus on renewable energy development and the favourable geographical conditions for ocean energy in these areas [24]. As of 2020, global installed offshore wind capacity exceeded 34 GW, with Europe accounting for over 70% of this capacity. Leading countries in offshore wind deployment include China, Denmark, the UK, and Germany. Amidst the COVID-19 pandemic, governments worldwide have set ambitious targets for offshore wind development, aligning with the Paris Agreement's goal of limiting global temperature rise to 1.5°C [25]. Currently, tidal barrage projects dominate the current market, followed by wave energy, while other ocean technologies remain in research stages. European countries and Australia lead in ocean energy development, with a focus on tidal stream and wave energy. The European Commission aims for at least 1 GW of installed capacity for wave and tidal energy in the EU by 2030 and 40 GW by 2050 [26]. Beyond Europe, countries like China, Japan, and the Republic of Korea are emerging as key players in ocean energy innovation [27]. Since this study focuses on the site in the Northen Seas, it is worth noting that countries along this corridor, Belgium, Denmark, Germany, Ireland, France, Luxembourg, Netherlands, and Sweden have set a non-binding agreement for priority offshore grid corridor Northern Seas offshore grids (NSOG), and Ireland aims to increase generation to 13 GW by 2040 [28]. Hydrodynamic performance and analysis of SSP combined with WECs 23 | P a g e Figure 5. Global distribution of ocean energy activity [24] 2.2.2 Floating platforms The transition from onshore to offshore wind energy started with the commercial installation of a wind farm, Vindeby, built by Denmark in 1991, with a total capacity of 5MW [29]. Since then, the offshore wind industry has evolved tremendously and has been embraced due to its potential benefits over onshore wind. Offshore wind farms have several advantages over onshore wind farms. They have less environmental impact, allowing for larger turbines and faster spinning blades. This leads to higher electricity production and lower costs. Offshore wind farms can produce up to 50% more electricity than onshore farms because of stronger and more consistent winds. While land-based wind farms are limited in size, offshore farms can be much larger, with capacities exceeding 100 MW. This shift towards offshore wind has led to advancements in wind energy technology [30-32]. Within offshore wind turbines, there are two main types: fixed-support and floating as seen in Figure 6. Fixed-support turbines can be either monopiles or jackets. Monopiles are single tower-like structures embedded in the seabed, while jackets have a lattice structure with three or four legs. Fixed-support turbines are limited to a maximum seabed depth of 60 meters, which is a problem because most of the world's offshore wind energy is found in deeper waters. These turbines are also limited in size, with the largest rotor diameter currently being 80 meters [33]. Floating wind structures allows the industry to enhance the capacity and efficiency of the offshore wind energy sector. With these benefits in mind, multiple projects have been planned, and deployed, for areas with deeper waters such as 24 | P a g e O c t o b e r 2 0 2 4 Fukushima FORWARD project in Japan [34, 35], the Hywind project in Scotland demonstrating a spar-type substructure [36] and the WindFloat Atlantic in Portugal using a semi-submersible floating structure[37]. Figure 6. Classification of Offshore Wind Structures [38] 2.2.2.1 SSP The design principles and technologies developed for oil and gas production in deep waters have been adapted and refined for use in the renewable energy sector. Existing oil rig platforms inspire many of the foundational engineering concepts used for SSPs. Semisubmersible platforms offer several benefits, including the ability to carry heavy loads, function in various water depths, and be moved after they are no longer needed. Because they are partially submerged, these platforms are very stable in rough seas. They are also strong and cost-effective[39]. Numerous companies and institutions are researching and developing semi-submersible floating platforms for offshore wind applications, and multiple of these platforms have been deployed for commercial use. Principle Power rolled out its 4th generation of WindFloat design and achieved an operational track record of exceeding 200,000 hours since its installation in 2011[40]. X1 Wind is a Spanish company specializing in floating wind technology. They have developed a unique floating platform design called PivotBuoy, and completed testing of this prototype in 2023 [41]. OCG-Wind platform, developed by Archer Wind is another example of the commercialisation of SSPs [42]. Hydrodynamic performance and analysis of SSP combined with WECs 25 | P a g e University of Maine designed a SSP, named DeepCwind platform, Figure 7., which was extensively researched and tested, with the help of multiple collaborators, intending to develop a robust SSP as a foundation for floating platforms[43]. This platform has exhibited great results during its testing and experimentation phase. Quality data from these experiments is publicly available for researchers to use this platform for individual research, as well as validation[44-47]. This platform has been used for this study. Figure 7. DeepCwind floating platform 2.2.3 WECs WECs are widely classified based on several criteria, primarily focusing on their operational principles and the type of wave energy they harness. The classifications often overlap because WECs can be designed to fit multiple criteria. For example, a point absorber can be used both nearshore and offshore, and an oscillating water column can be classified by its operational principle and its location. The diversity in WEC designs and their adaptability to different environments and wave conditions make it challenging to fit them into a single, rigid classification system. Instead, the various classification methods provide a flexible 32 | P a g e O c t o b e r 2 0 2 4 Parameter Value Depth of platform base below SWL (total draft) 20 m Elevation of main column (tower base) above SWL 10 m Elevation of offset columns above SWL 12 m Spacing between offset columns 50 m Length of upper columns 26 m Length of base columns 6 m Depth to top of base columns below SWL 14 m Diameter of main column 6.5 m Diameter of offset (upper) columns 12 m Diameter of base columns 24 m Diameter of pontoons and cross braces 1.6 m Table 1. Design parameters for SSP 3.2.3 WECs The external geometry of the WEC is similar to a truncated cone. This was designed based of the points discussed in section 2.2.3. Table 2. Shows the design parameters for the WEC. The WEC is hollow, and the linear PTO is placed within the WEC. The PTO system is positioned towards the bottom of the device and the lowers the CM Parameter Value Depth below SWL (draft) 6 m Elevation above SWL 6 m Diameter (maximum) 12 m Diameter (minimum) 3 Table 2. Design parameters for WEC Hydrodynamic performance and analysis of SSP combined with WECs 33 | P a g e Figure 13. WEC dimensions 3.2.4 HWWFP Two vertical pontoons are added between the upper and the lower pontoons, which will guide the heaving motion of the WECs. Figure 14. shows the layout of WECs. The distance Parameter Value Distance from centre of upper column 16.5 m Distance between centres of WECs 17 m Gap between WECs 4 m Table 3. Design parameters for HWWFP 34 | P a g e O c t o b e r 2 0 2 4 Figure 14. HWWFP layout 3.3 System properties As mentioned in section 2.1.4, BEM is used in AQWA and a meshing is performed for the same. The mesh statistics are mentioned in table 4. BEM does not mesh solid bodies, so all solid bodies from SD are converted to surface bodies. The maximum element size for meshing has a limit in AQWA, which also limits the maximum allowable frequency to be considered in simulations. Although, the simulation is run for the frequencies for the data obtained at the reference site, and the upper limit of the range is within the maximum allowable frequency. A separate “Natural modes” solution is run to validate the natural frequencies of the SSP. Further, to perform HD in AQWA, the simulation parameters needed to be set up. A simulation was run for the SSP and HWWFP to obtain, and compare, coefficients like added mass, radiation damping and wave excitation forces. The parameters are mentioned in table 5. It should be noted that for SSP simulation, the total mass properties, which include the 5 MW wind turbine, were used so as to validate the study results with the actual results. Properties Value Total mass of SSP 1.3958E+7 kg Displacement SSP 1.3917E+4 m3 SSP CM location below SWL 8.07 m SSP roll inertia about CM 1.3947E+10 kg-m2 Hydrodynamic performance and analysis of SSP combined with WECs 35 | P a g e SSP pitch inertia about CM 1.5552E+10 kg-m2 SSP yaw inertia about CM 1.3692E+10 kg-m2 Mass of each WEC 17700 kg WEC CM location below SWL 3 m WEC roll inertia about CM 312924.78 kg-m2 WEC pitch inertia about CM 314084.35 kg-m2 WEC yaw inertia about CM 316647.54 kg-m2 Table 4. Properties of SSP and WEC ANSYS AQWA has a limit on the total number of elements for HD, which is 40,000. Given the dimensions of the structure, the generated mesh is coarse, and this directly affects the accuracy of the results. Table 5. Shows the mesh statistics for SSP and HWWFP. The element size had to be reduced in HWWFP after adding the 6 WECs. Figure 15. displays the meshed HWWFP. Parameter Value SSP mesh element size 0.55 m SSP mesh elements 35761 HWWFP mesh element size 0.58 m HWWFP mesh elements 39375 Table 5. Mesh details All the parts were assigned their own local coordinate system at the centre of mass within the meshing component because local coordinate systems help in accurately defining the geometry and orientation of parts, especially if they are not aligned with the global coordinate system. Applying boundary conditions, loads, and constraints can be more straightforward when using a local coordinate system. This is particularly useful for complex structures or assemblies, such as this study. Most of the values for the environment of the simulation are default, values need to be changed before solving the simulation. Some values are defined, as mentioned in table 6. Parameter Value Water depth 200 m Water density 1026 kg/m2 Acceleration due to gravity 9.81 m/s2 Table 6. Simulation parameters for HD 36 | P a g e O c t o b e r 2 0 2 4 Figure 15. Meshed HWWFP At this step, the model is ready to be simulated in HD AQWA. All of the above sections can be summarized as pre-processing steps in a workflow for hydrodynamic analysis. 3.4 Power generation In this study, the focus is on analysing the wave energy conversion aspect of a wind-wave hybrid platform, specifically excluding the wind generation component. The simulation process involves using WEC-Sim to evaluate the performance of the WECs under various sea conditions. These sea states are representative of the conditions at a specific reference site, which has been analysed to provide realistic wave height and period data. The simulations cover a comprehensive range of sea states to ensure that the performance of the WEC is thoroughly evaluated under different wave conditions. WEC-Sim performs time-domain analysis, solving the equation (9) for the WEC system in six DOFs. The hydrodynamic coefficients, such as added mass and radiation damping, are derived from BEM based potential flow solvers. These coefficients are essential for accurately modelling the interaction between the WEC and the waves. Hydrodynamic performance and analysis of SSP combined with WECs 37 | P a g e 3.4.1 BEMIO BEMIO (Boundary Element Method Input/Output) in WEC-Sim is essential for processing hydrodynamic data from BEM solvers like AQWA. It converts this data into a format usable by WEC-Sim, which is .h5, calculates impulse response functions (IRFs), and performs state space realization for efficient time-domain simulations. BEMIO ensures accurate and reliable input data, enhances computational efficiency, and supports multiple BEM solvers, making it a crucial component for accurate and efficient modelling of wave energy converters. WEC-Sim scales the hydrodynamic coefficients according to these equations: |𝐹𝑒𝑥𝑡(𝜔)           |= |𝐹𝑒𝑥𝑡(𝜔)| 𝜌𝑔 𝐴(𝜔)        = 𝐴(𝜔) 𝜌 𝐵(𝜔)        = 𝐵(𝜔) 𝜌𝜔 𝐾ℎ𝑠      = 𝐾ℎ𝑠 𝜌𝑔 Where 𝐾ℎ𝑠 is linear hydrostatic restoring coefficient. The MATLAB code to obtain the BEMIO file can be found in Appendix A. 3.4.2 Simulink model The Simulink modelling for the hybrid wind-wave platform involved several key components to accurately simulate the system’s dynamics. The model included an “Active Method: Input File” block, which specified the input file containing the simulation parameters, such as wave characteristics and PTO settings. Constraint blocks were used to represent the constraints applied to the system, such as mooring lines or fixed points, which limited the movement of the WEC components. Body blocks, labelled from body (1) to body (7), represented different parts of the platform, including the floating structure and WEC components. Each body block included hydrodynamic properties and mass characteristics essential for simulating the physical behaviour of the system. PTO blocks simulated the energy conversion mechanisms, converting mechanical energy from the WEC into electrical energy. The connections between these blocks represented the physical and data interactions within the system, ensuring that the dynamics of the platform and its components were accurately modelled. The SSP was connected to the seabed through a 6 DOF constraint 38 | P a g e O c t o b e r 2 0 2 4 block and the WECs were connected with a translational PTO since there is only a heaving motion DOF for the WEC. This setup allowed for a comprehensive simulation of the wave energy conversion process. The Simulink model can be found in Appendix A. 3.4.3 PTO A Linear PTO is employed in this study, which is available in wec-Sim. In the context of a heaving point absorber WEC, the device moves vertically with the rise and fall of ocean waves. A linear PTO directly captures this vertical (or heaving) motion, as opposed to rotational PTO systems that convert wave motion into rotational energy. This linear PTO system is simple spring-damper mechanism. The spring component in the PTO stores energy from the wave-induced motion, much like a conventional spring stores potential energy when compressed or stretched. In the case of a wave energy converter (WEC), the spring absorbs and stores energy during the heaving motion of the device. When the WEC moves due to a wave, the spring compresses or stretches, and then releases that energy as the wave subsides. The damper, on the other hand, dissipates energy, typically converting it into a useful form, such as electrical energy. The damper resists the relative motion between the WEC and the PTO, and this resistance helps to extract energy from the system. The force exerted by a Power Take-Off (PTO) system modelled as a springdamper is calculated based on the combined contributions of the spring and damper components. The total PTO force, 𝐹𝑝𝑡𝑜 , is the sum of the forces from the spring and damper, which depend on the displacement and velocity of the system, respectively. The general equation is: 𝐹𝑝𝑡𝑜= −𝐾𝑝𝑡𝑜𝑋− 𝐵𝑝𝑡𝑜𝑋󰇗 Where 𝐾𝑝𝑡𝑜 is spring stiffness, 𝐵𝑝𝑡𝑜 is damping coefficient, 𝑋 is displacement of WEC from equilibrium position and 𝑋󰇗 is the rate of displacement. In many WEC systems, the 𝐾𝑝𝑡𝑜 is often considered as zero in the PTO system, especially for linear PTO designs. It is done to ensure the WEC moves freely with the waves, maximizes energy absorption through pure damping, simplifies the system's dynamics, and leverages the natural buoyancy of the device as a restoring force. By focusing only on damping, the PTO can efficiently convert the wave-induced motion into usable energy without interference from spring-like forces [68]. The energy extracted from the waves is proportional to the relative motion, and the damping coefficient determines the balance between excessive resistance and optimal energy absorption. Equation (14) represents the analytical approach to find the optimal damping coefficient for a single heaving body [61]. Hydrodynamic performance and analysis of SSP combined with WECs 39 | P a g e 𝐵𝑝𝑡𝑜= √((𝑚+𝐴3,3)𝜔2−(𝐾𝑝𝑡𝑜+ 𝐹𝐵))2 𝜔2+𝐵3,32 ( 14 ) Figure 16. shows the plot obtained using the above equation. The respective values of optimal PTO damping coefficients are incorporated while defining the sea states mentioned in Table 8 Figure 16. Optimal PTO damping coefficient for WECs 3.4.4 Inputs Wec-SIM gives a lot of freedom with the simulation inputs. All the input parameters as defined in a single input file. The details of this MATLAB script is available in Appendix A. The wecSimInputFile.m is essential for setting up the simulation environment in WEC-Sim. It ensures that all necessary parameters and properties are correctly defined, enabling accurate and efficient simulations. By organizing the simulation setup in a structured manner, this file facilitates the integration of various components, such as wave conditions, body dynamics, and PTO systems, into a cohesive model. The key parameters defined for this study are given in table 7. Parameter Value Fixed time step 0.04 s End time 180 s 40 | P a g e O c t o b e r 2 0 2 4 Ramp time 150 s Wave class Irregular Wave spectra ‘JS’ PTO stiffness 0 Table 7. Input parameters for Wec-SIM A fixed time step was preferred due to the simplicity and consistency. Since the reference site for the model simulation is in the deep seas, an irregular wave class was chosen. Irregular waves better represent the complex and random nature of real ocean waves compared to regular waves, which are idealized and less representative of actual sea conditions. As for the wave spectra, JONSWAP (JS – Joint North Sea Wave Project) was selected. JS is specifically designed to model the energy distribution of waves in deep sea states. Using the JS spectrum allows for a more accurate representation of the wave energy environment, leading to better predictions of the WEC’s performance. The JS spectrum is widely accepted and used in the marine and offshore industries for wave modelling. Its use in this simulation aligns with industry practices, ensuring that your results are credible and comparable to other studies [69]. Once the WEC model is constructed, the SimMechanics 6DOF multi-body solver performed the simulation by summing forces from time domain modules at each time step and advancing the simulation in time using a 4th-order Runge Kutta integration scheme [70]. To evaluate the power performance of the WEC system, three representative sea states were selected based on data from the reference site. Each sea state is characterized by its significant wave height and peak wave period. These parameters correspond to the dominant conditions at the site and allow for the performance analysis of the WECs under realistic operating scenarios. The range of the SS is within the range of the reference site and can be referred to in Table 8. Sea State Wave period Wave height B_pto SS1 7.5 s 1.7 m 2.81E+06 (N/m) SS2 10 s 2.2 m 3.90E+06 (N/m) SS3 11.2 s 3.5 m 4.83E+06 (N/m) Table 8. Sea states definition Hydrodynamic performance and analysis of SSP combined with WECs 41 | P a g e 4. Results and discussion This section presents the outcomes of the simulations and analyses. It discusses the hydrodynamic performance of the hybrid floating platform, comparing the Response Amplitude Operator (RAO) results for different motions (surge, heave, and pitch). The discussion highlights key findings, such as the reduction of pitch motion due to the integration of WECs and the overall improvement in platform stability. The power performance analysis is also covered, detailing how different wave conditions and damping values impact energy production. 4.1 Hydrodynamic diffraction results This section displays, and discusses the results obtained from HD AQWA. Comparative plots for SSP and HWWFP are generated. Analysing the frequency response for surge, heave, and pitch is more common than for sway, roll, and yaw due to the dominant nature and operational significance of these motions. Surge, heave, and pitch typically have more pronounced effects on the stability and performance of floating structures, directly impacting vertical and longitudinal stability, which are critical for marine operations. These motions also experience significant restoring forces due to buoyancy and gravity, making them essential for stability and resonance analysis. Additionally, heave and pitch affect vertical displacement and angular tilt, crucial for operations like energy extraction in wave energy converters and stability in floating wind turbines, while surge impacts forward and backward movement, important for mooring and station-keeping. Therefore, focusing on these motions helps in designing structures that can withstand wave-induced forces, maintain operational efficiency, and ensure safety and comfort for personnel on board. All the results are plotted for the range of 4 to 17 s (0.37 to 1.571 rad/s), which covers the dominant range for the reference site of 6 to 13 s. Additionally, the data is analysed for the 0° wave direction, which is the wave heading towards positive X. 4.1.1 Validation for SSP The model developed for SSP is simulated in HD AQWA to obtain the natural modes between the range of 0.01 Hz to 0.1 Hz. The results showed 3 peaks within this range, as mentioned in table 7 and figure 16. The model is validated with the experimental values from [15], for 3 degrees of freedom (DOF) heave, roll and pitch, before adding the WECs to the platform. DOF Experiment (s) Study (s) Heave 17.5 17.24 48 | P a g e O c t o b e r 2 0 2 4 Figure 24. Surge component of hydrodynamic diffraction forces for HWWFP and SSP Figure 25. Heave component of hydrodynamic diffraction forces for HWWFP and SSP Hydrodynamic performance and analysis of SSP combined with WECs 49 | P a g e Figure 26. Pitch component of hydrodynamic diffraction forces for HWWFP and SSP 4.2 Time domain analysis Given that SS3 has the highest wave period and significant wave height among the selected conditions, it is expected that the resulting hydrodynamic forces on the SSP are greatest in this state. As seen in Figure 27 and Figure 28, the surge, heave forces, and pitch moments exerted on the SSP are notably higher for SS3 compared to other sea states. This is because the larger wave amplitudes and longer periods associated with SS3 generate stronger interactions between the waves and the platform, leading to increased forces and moments. In particular, the heaving forces are plotted with negative values in the figures, reflecting the fact that these forces are measured at the centre of mass of the platform, which is located below the SWL. The negative sign indicates the downward direction of the forces relative to the SWL. Despite the significant wave heights and energy in SS3, the SSP's vertical displacement in heave motion stabilizes after the initial transient phase. Following the initial displacement, the variation in the heave motion remains within 1 meter, even for SS3. This indicates that the SSP's design effectively dampens large vertical movements, maintaining stability in extreme sea states. 50 | P a g e O c t o b e r 2 0 2 4 Figure 27. Forces and moment acting on SSP for different SS Figure 28. SSP surge, heave and pitch for different SS Hydrodynamic performance and analysis of SSP combined with WECs 51 | P a g e Figure 29. Power generation from WEC 1 for different SS Figure 29. shows the power generation from WEC 1. The graph demonstrates the potential for significant power generation from wave energy, even under moderate sea states. The variation in the power and forces validate the dynamic behaviour of the WECs. Despite the fluctuations, WEC 1 demonstrates the ability to generate significant amounts of power, particularly in more energetic waves. This indicates that even in moderate sea states, where the wave energy is lower than in more extreme conditions, WEC 1 is capable of consistently capturing and converting wave energy into usable power. 52 | P a g e O c t o b e r 2 0 2 4 Figure 30. Power generated by each WEC for SS3 In Figure 30., the power generation capabilities of each WEC are plotted, highlighting how the layout of the WECs impacts their energy capture. WECs 1, 5, and 6 consistently produce higher power output compared to the others. This can be attributed to their position within the array, as shown in Figure 14., where WECs 1 and 6 are located at the outermost edges, directly facing the incoming waves. These positions allow them to absorb the most energy from the wave front, leading to enhanced power generation. WEC 5, positioned centrally, also benefits from the surrounding wave interactions, further increasing its output. This analysis underscores the importance of WEC array positioning in maximizing energy capture efficiency. 5. Conclusion The conclusion summarizes the major findings of the study, emphasizing the potential of hybrid floating platforms for renewable energy generation. It reflects on the implications of the research, particularly regarding the stability and efficiency of such systems in offshore environments. The conclusion also outlines possible future research directions, including the exploration of economic feasibility and additional experimental validations. The overall purpose of this study was to understand the ongoing development in the field of ocean renewable energy sector and contribute to its research. This work can be considered a part of the exploration going on for hybrid wind and wave systems. As mentioned in section 1.3, the two main objectives were to understand the impact of the integration of WECs with SSP in terms of stability and power generation. This was done using a combination of frequency domain and time domain analysis. The methodology Hydrodynamic performance and analysis of SSP combined with WECs 53 | P a g e explicitly mentions the detailed steps taken to numerically model the HWWFP system. The conclusion of the study is as follows: • While both the SSP and the HFFWP exhibit similar surge and heave responses at higher frequencies, the HFFWP demonstrates improved pitch damping, suggesting that the WECs effectively reduce rotational motion. The platform's natural frequency for heave is evident in its pronounced heave motion at lower frequencies. • The results of radiation damping for the hybrid wind-wave platform show that the addition of WECs has a positive impact on its hydrodynamic behaviour. While the surge damping is similar for both the SSP and the HFFWP at lower frequencies, the HFFWP exhibits higher surge damping at higher frequencies. In terms of heave and pitch damping, the HFFWP consistently demonstrates higher values compared to the SSP, indicating that the WECs contribute to improved platform stability and control. • While the overall diffraction forces decrease with increasing frequency for both systems, the HFFWP consistently experiences higher forces, particularly in the surge and heave directions. The pitch diffraction force for the HFFWP is slightly lower at higher frequencies compared to the SSP. This suggests that the WECs may have a beneficial effect on reducing the platform's rotational motion. • Along with the response analysis, it is important to note that planning the layout of the WECs also has a significant impact on the power generation capabilities. These findings validate the importance of optimal WEC placement in maximizing power generation, as supported by the data in reference [70]. 6. Limitations and recommended future works The study primarily focused on short-term hydrodynamic performance and power generation efficiency. The study evaluated the platform's performance under a limited range of sea states, primarily focusing on moderate wave conditions. Extreme conditions, such as those encountered during storms or hurricanes, were not explored. This limits the understanding of the platform's robustness and survivability under more challenging conditions. Simulations should be expanded to include extreme sea states, such as those experienced during storms and rogue waves. Understanding the platform’s behaviour in such conditions will help improve its resilience and guide the design of fail-safe mechanisms to protect the structure under adverse conditions. Experimental testing in wave basins with scale models is also recommended to validate these simulations. Deploying prototypes in offshore environments would provide valuable data on platform stability, energy generation, and maintenance needs. Such trials would also reveal practical 54 | P a g e O c t o b e r 2 0 2 4 challenges and opportunities for further technological improvements, accelerating the path toward commercial deployment. Hydrodynamic performance and analysis of SSP combined with WECs 55 | P a g e Acknowledgements To my parents and Riya, your steadfast support throughout these past years has been the foundation of my academic journey. Your faith in me and your ongoing encouragement have been priceless, and for that, I am truly thankful. I would also like to express my sincere appreciation to my supervisor, Oriol Gomis- Bellmunt, for his invaluable guidance. Finally, to my friends, your unwavering support and frequent check-ins have been a source of strength during the most challenging moments. Your encouragement has been invaluable. To each of you, I offer my heartfelt thanks. 56 | P a g e O c t o b e r 2 0 2 4 References [1] F. Z. Rebecca Williams, "GLOBAL OFFSHORE WIND REPORT 2023," GLOBAL WIND ENERGY COUNCIL, 28 August 2023 2023. [2] IRENA, "Floating offshore wind outlook," International Renewable Energy Agency, Abu Dhabi, 2024. [3] V. Masterson, "Wave energy: can ocean power solve the global energy crisis?," March 22, 2022. [4] A. Felix et al., "Wave Energy in Tropical Regions: Deployment Challenges, Environmental and Social Perspectives," Journal of Marine Science and Engineering, vol. 7, no. 7, p. 219, 2019. [Online]. Available: https://www.mdpi.com/2077-1312/7/7/219. [5] R. Pelc, "Renewable energy from the ocean," Marine Policy, 2002 2002, doi: https://doi.org/10.1016/S0308-597X(02)00045-3. [6] S. H. Salter, "Wave power," Nature, vol. 249, no. 5459, pp. 720-724, 1974/06/01 1974, doi: 10.1038/249720a0. [7] A. Myhr, C. Bjerkseter, A. Ågotnes, and T. A. Nygaard, "Levelised cost of energy for offshore floating wind turbines in a life cycle perspective," Renewable energy, vol. 66, pp. 714-728, 2014. [8] D. Roddier, C. Cermelli, A. Aubault, and A. Weinstein, "WindFloat: A floating foundation for offshore wind turbines," Journal of renewable and sustainable energy, vol. 2, no. 3, 2010. [9] C. M. Wang, T. Utsunomiya, S. C. Wee, and Y. S. Choo, "Research on floating wind turbines: a literature survey," The IES Journal Part A: Civil & Structural Engineering, vol. 3, no. 4, pp. 267-277, 2010/11/01 2010, doi: 10.1080/19373260.2010.517395. [10] H. R. Ghafari, A. Neisi, H. Ghassemi, and M. Iranmanesh, "Power production of the hybrid Wavestar point absorber mounted around the Hywind spar platform and its dynamic response," Journal of Renewable and Sustainable Energy, vol. 13, no. 3, 2021, doi: 10.1063/5.0046590. [11] J. S. Rony and D. Karmakar, "Coupled Dynamic Analysis of Hybrid Offshore Wind Turbine and Wave Energy Converter," Journal of Offshore Mechanics and Arctic Engineering, vol. 144, no. 3, 2021, doi: 10.1115/1.4052936. [12] J. S. Rony and D. Karmakar, "Coupled dynamic analysis of hybrid STLP-WEC offshore floating wind turbine with different mooring configurations," Journal of Ocean Engineering and Marine Energy, vol. 9, no. 4, pp. 623-651, 2023/11/01 2023, doi: 10.1007/s40722-023- 00287-w. [13] L. Castro-Santos, E. Martins, and C. Guedes Soares, "Cost assessment methodology for combined wind and wave floating offshore renewable energy systems," Renewable Energy, vol. 97, pp. 866-880, 2016/11/01/ 2016, doi: https://doi.org/10.1016/j.renene.2016.06.016. [14] A. H. Slocum, J. M. Kluger, and S. Mannai, "Energy Harvesting and Storage System Stabilized Offshore Wind Turbines," 2019 Offshore Energy and Storage Summit (OSES), pp. 1-6, 2019. [15] J. J. A. Robertson, F. Wendt, A. Goupee, H. Dagher, "Definition of the OC5 DeepCwind Semisubmersible Floating System " NREL, 2013. [16] D. O’Donnell, J. Murphy, and V. Pakrashi, "Comparison of Response Amplitude Operator Curve Generation Methods for Scaled Floating Renewable Energy Platforms in Ocean Wave Basin," ASME Letters in Dynamic Systems and Control, vol. 1, no. 2, 2020, doi: 10.1115/1.4049169. Hydrodynamic performance and analysis of SSP combined with WECs 57 | P a g e [17] J. Bosboom, Oscillations of the ocean water surface. Delft University of Technology, 2021. [18] "Ansys Theory Manual." Ansys https://ansyshelp.ansys.com/public/account/secured?returnurl=/Views/Secured/corp/v 242/en/aqwa_thy/aqwa_thy.html (accessed July 26, 2024). [19] B. Guo and J. V. Ringwood, "Geometric optimisation of wave energy conversion devices: A survey," Applied Energy, vol. 297, p. 117100, 2021/09/01/ 2021, doi: https://doi.org/10.1016/j.apenergy.2021.117100. [20] A. H. Techet, 2005. [21] L. S. Sugar, "PERFORMANCE OF A NEAR SHORE OSCILLATING WAVE SURGE CONVERTER WITH VARIABLE FLAP CONFIGURATIONS " Master of Science in Mechanical Engineering, Department of Engineering East Carolina University 2021. [22] D. Apsley, "Waves: Linear Wave Theory," Hydraulics, 2024. [Online]. Available: https://personalpages.manchester.ac.uk/staff/david.d.apsley/lectures/hydraulics3/Wav esLinear.pdf [23] Falnes, J , Author and Perlin, M , Reviewer, "Ocean Waves and Oscillating Systems: Linear Interactions Including Wave-Energy Extraction," Applied Mechanics Reviews, vol. 56, no. 1, pp. B3-B3, 2003, doi: 10.1115/1.1523355. [24] "INNOVATION OUTLOOK OCEAN ENERGY TECHNOLOGIES," IRENA, 2020. [Online]. Available: https://www.irena.org/- /media/Files/IRENA/Agency/Publication/2020/Dec/IRENA_Innovation_Outlook_Ocean_ Energy_2020.pdf [25] "Offshore renewables: An action agenda for deployment, International Renewable Energy Agency," IRENA, Abu Dhabi, 2021. [Online]. Available: /- /media/Files/IRENA/Agency/Publication/2021/Jul/IRENA_G20_Offshore_renewables_20 21.pdf [26] "Boosting Offshore Renewable Energy for a Climate Neutral Europe," ed. Brussels: European Comission, 2020. [27] D. Magagna, R. Monfardini, and A. Uihlein, "Ocean energy in Europe," International Marine Energy Journal, vol. 1, pp. 1-7, 08/30 2018, doi: 10.36688/imej.1.1-7. [28] R. W. Ricardo, "NSOG_non-binding_offshore_goals_final." [Online]. Available: https://circabc.europa.eu/ui/group/8f5f9424-a7ef-4dbf-b914- 1af1d12ff5d2/library/5dbd6168-e529-4604-a4d0-6c2c7cfb4b62/details [29] "Vindeby Offshore Wind Farm." [Online]. Available: https://tethys.pnnl.gov/wind-project- sites/vindeby-offshore-wind-farm#description [30] V. N. Dinh and B. Basu, "On the modeling of spar-type floating offshore wind turbines," Key Engineering Materials, vol. 569, pp. 636-643, 2013. [31] I. IEA, "Energy technology perspectives 2017," Catalysing Energy Technology Transformations, 2017. [32] C. L. Archer and M. Z. Jacobson, "Evaluation of global wind power," Journal of Geophysical Research: Atmospheres, vol. 110, no. D12, 2005. [33] T. Asim, S. Z. Islam, A. Hemmati, and M. S. U. Khalid, "A Review of Recent Advancements in Offshore Wind Turbine Technology," Energies, vol. 15, no. 2, p. 579, 2022. [Online]. Available: https://www.mdpi.com/1996-1073/15/2/579. [34] M. Karimirad, Offshore Energy Structures: Springer Cham, 2014. [35] "Fukushima Floating Offshore Wind Farm Demonstration Project." [Online]. Available: https://www.fukushima-forward.jp/english/ [36] "Hywind Scotland." [Online]. Available: https://www.equinor.com/energy/hywindscotland [37] "WindFloat Atlantic." https://www.windfloat-atlantic.com/the-wind-farm/#project (accessed.