Tesi - Hydraulic response of a spar-type floating wind turbine for varying wind speed and pitch control

Department of Industrial Engineering

Master Degree in Energy Engineering

Hydraulic response of a spar-type floating wind turbine for varying wind speed and pitch control

Supervisor Candidate

Prof. Giorgio Pavesi Simone Guazzotti

Co-supervisor

Prof. Luca Martinelli, Prof. Piero Ruol

Academic year 2023-2024

Graduation date 04/07/2024

Contents

1 Introduction 7

1.1 Offshore wind future scenario . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2 Offshore wind in Europe and World . . . . . . . . . . . . . . . . . . . . . . . 9

1.3 Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.4 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 Offshore wind turbine 13

2.1 Bottom fixed foundation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Floating wind foundation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.1 Spar buoy turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2.2 Semi-submersible turbine . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.3 Tension-Leg-Platform (TLP) . . . . . . . . . . . . . . . . . . . . . . . 18

3 Floating wind turbine model 19

3.1 Reference turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.2 Wind turbine model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2.1 Thrust force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2.2 Solid Edge model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.3 Environmental conditions and external forces considered . . . . . . . . . . . . 27

3.3.1 Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.3.2 Wind loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.3.3 Waves and current loads . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3.4 Mooring system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4 Stability of the floating platform 35

4.1 Ship theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.1.1 Ship motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.1.2 Hydrostatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.2 Stability for a floating wind turbine . . . . . . . . . . . . . . . . . . . . . . . . 39

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4.2.1 Heeling and restoring moments . . . . . . . . . . . . . . . . . . . . . 39

4.2.2 Relative wind speed and negative aerodynamic damping . . . . . . . . 40

5 Hydrodynamic model of a spar wind turbine 43

5.1 Hydrostatics stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.2 Uncoupled pitch motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.2.1 Added moment of inertia . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.2.2 Natural period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.2.3 Hydrostatic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.2.4 Dynamic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.3.1 Floating wind turbine’s motion over time . . . . . . . . . . . . . . . . 47

5.3.2 Estimation of the power generated . . . . . . . . . . . . . . . . . . . . 48

5.3.3 Comparison between 1 sec and 10 sec time-step simulations . . . . . . 51

6 Conclusions 55

6.1 Limitations and possible improvements . . . . . . . . . . . . . . . . . . . . . 56

6.1.1 Aerodynamic modeling: BEM theory and CFD . . . . . . . . . . . . . 56

6.1.2 Advance simulation tools for floating wind turbines . . . . . . . . . . . 56

6.1.3 Influence of hydrodynamic forces and mooring systems . . . . . . . . 56

6.1.4 Platform motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

A MATLAB codes 59

A.1 Resolution of the uncoupled pitch motion differential equation . . . . . . . . . 59

A.2 Addition of a noise signal to wind speed time series . . . . . . . . . . . . . . . 61

A.3 Calculation of the wind force and moment on the tower . . . . . . . . . . . . . 62

A.4 Power estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

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List of figures

1.2 Offshore wind resource map, generated by Energy Sector Management Assistance Program (ESMAP) with data from Global Wind Atlas. . . . . . . . . . . 9

1.3 Italy offshore wind resource map . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.4 Copenhagen Offshore Partners’ projects in Italy . . . . . . . . . . . . . . . . . 10

2.2 Main primary principles of achieving stability of a FOWT . . . . . . . . . . . 15

2.3 Triangle method showing different ways to achieve stability . . . . . . . . . . 15

2.4 Spar-type wind turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.5 25MW WindFloat Atlantic wind farm, located off the coast of Viana do Castelo (Portugal), made of three 8.4 MW turbines [Ocean Wind (OW) website] . . . . 17

2.6 SBM TLP floating wind [SBM offshore] . . . . . . . . . . . . . . . . . . . . . 18

3.1 Graphical representation of 8MW Hywind Tampen turbine, taken as reference. 19

3.2 Design parameters Hywind Tampen turbine, taken from [6] . . . . . . . . . . . 20

3.4 Graphical interface of the MATLAB software . . . . . . . . . . . . . . . . . . 23

3.5 Torque, power and pitch angle under different wind conditions . . . . . . . . . 24

3.6 Structural layout of composite materials within a typical blade cross section [10] 25

3.7 Solid Edge model of the blade turbine . . . . . . . . . . . . . . . . . . . . . . 26

3.8 Scheme of the external forces acting on the floating wind turbine . . . . . . . . 27

3.9 Two years wind speed time series and the time interval considered for the analysis 28

3.10 Correlation between modelled and measurement wind gust . . . . . . . . . . . 30

3.11 Modelled wind speed with local variability signal and measurement wind speed linearly interpolated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.12 Detailed view of the first two hours of wind time series . . . . . . . . . . . . . 31

4.1 6 degree of freedom of a ship and floating wind turbine . . . . . . . . . . . . . 36

4.2 Floating object in a stable and unstable position . . . . . . . . . . . . . . . . . 37

4.3 Shift of centre of buoyancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.4 Righting moment and wind heeling moment curves . . . . . . . . . . . . . . . 40

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5.1 Inclination angle associated to the pitch motion of the floating wind turbine platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2 Tangential velocity of the platform at nacelle height associated to the pitch motion 48

5.3 Estimation of the power generated of the floating wind turbine considered and the hypothetical bottom-fixed turbine over the period analysed of five days . . 49

5.4 Detailed time-scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.5 Power difference between floating wind turbine and bottom fixed in kW h (a) and in % respect to the power generated by the bottom-fixed turbine at the corresponding time (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.6 Inclination angle for 1s and 10s time-step simulations . . . . . . . . . . . . . . 51

5.7 Nacelle velocity for 1s and 10s time-step simulations . . . . . . . . . . . . . . 52

5.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.9 Comparison between 1s and 10s time-step simulation regarding the power estimated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

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Chapter 1

Introduction

Climate change is one of the most critical challenges of our time. According to the Intergovernmental Panel on Climate Change (IPCC), global temperatures have increased by 1.09°C above pre-industrial levels due to human activities [8]. Not to overcome the threshold of temperature rise of 1.5°C, set by the Paris Agreement (2015), global net human-caused CO2 emissions need to decrease by about 45% from 2010 levels by 2030, reaching net zero around 2050 [14]. Despite the scientific evidence of the ongoing climate change, energy-related carbon dioxide (CO2) emissions have increased 1.3% annually, on average, over the last five years [15].

In response to these urgent challenges, the global energy transition has become a critical focus. According to International Renewable Energy Agency (IRENA) previsions, electricity is expected to become the dominant energy vector, increasing from 20% to almost 50% of final consumption by 2050. This implies gross electricity consumption to be more than double. Renewable power will be able to account for 86% of global power demand. Renewable energy supply, increased electrification of energy services can deliver 75% of needed reductions to energy-related CO2 emissions [15].

Offshore wind energy, in particular, offers a promising solution to the acceleration of the global energy transition. Offshore wind farms can harness stronger and more constant winds than those at land. This is due to the absence of land obstacles and lower surface roughness. More constant winds allows the supply of energy to be more consistent and decreases the mechanical stress to which the structures are subjected. Offshore wind energy faces fewer limitations compared to onshore wind, which are constrained by public acceptance, logistical issues and require vast and windy land areas. Compared to onshore wind, the visual and noise impacts are also reduced. Moreover the installation of offshore turbines is significantly easier than onshore turbines.

Despite the advantages, offshore wind turbine are more costly. To withstand larger wind load, the turbine require more robust structural design. The cost of foundation structure, installation and maintenance, transmission and grid integration due to the offshore location represents a further contribution to total cost.

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1.1 Offshore wind future scenario

Offshore wind capacity is expected to rapidly expand over the coming decades. Considering the current plans and targets set by countries, the future offshore projection of offshore wind capacity estimated by IRENA are presented in Tab.1.1. Technological advancements are a key driver of this growth. Thanks to research and technological advancements, the offshore wind industry is shifting towards larger turbines and exploring new sea areas with higher depths and distances from the coast.

As it is possible to see in fig. 1.1a, the offshore wind industry experienced a massive development in the period 2000-2019. The Levelized Cost of Energy (LCOE) experienced a decrease of 59% in the period 2010-2022, from USD 0.197/kilowatt hour (kWh) to USD 0.081/kWh [11], as shown in fig. 1.1b. Moreover, the integration of offshore wind farms with other renewable energy sources and advanced energy storage systems is expected to optimize grid stability and reliability.

Table 1.1: Global offshore wind future development

(a) Offshore wind turbine development trend (b) LCOE

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1.2 Offshore wind in Europe and World

Until now the offshore market at commercial scale is guided by bottom fixed turbines. The majority of offshore turbines are installed in the North Sea, the Baltic Sea and the Yellow Sea (China). These three sea has in common two fundamental features: vast ocean areas with strong wind and shallow waters, with average depth between 45m and 90m.

Due to its potential high energy output and its scalability, offshore wind is suitable to supply electricity to densely populated coastal areas. The wind resource map fig. 1.2 provide an estimate of mean annual wind speed until 200 km from the shore and at a hub height of 100 m.

Figure 1.2: Offshore wind resource map, generated by Energy Sector Management Assistance Program (ESMAP) with data from Global Wind Atlas.

Italy

The development of FOWTs in Italy began with a small demonstrator off the coast of Apulia in 2007. In 2022 ’Beleolico’ become the first offshore wind farm, composed of ten 3 MW bottom-fixed turbines. Due to Tyrrhenian and Ionian Sea’s deep waters, Italy is suitable mainly for floating turbines, fig. 1.3.

In the following years several projects are expected to be developed with Copenhagen Offshore Partners, for a total of 3 GW of capacity. [22] The Italian Ministry for Environment and Energy Security has approved the Environmental Impact Assessment (EIA) of the 250 MW 7SeasMed floating offshore wind project. It will be located approximately 35 kilometers off the coast of Marsala (Sicily) and it will consists of 21 semi-submerged wind turbine of 12 MW each. [21]

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Figure 1.3: Italy offshore wind resource map

Figure 1.4: Copenhagen Offshore Partners’ projects in Italy

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1.3 Challenges

Offshore wind energy holds immense potential, but its deployment faces several significant challenges that needs to be addressed to ensure its viability and efficiency.

1.3.0.1 Technical factors

Offshore wind turbines require an accurate and comprehensive engineering design. The structure is subjected to harsh marine conditions with regards to saltwater corrosion, intense wind, marine currents and waves. The stability issue is obviously particularly relevant for floating wind turbine and it will be discussed in depth in the following chapters.

The grid integration of large power plants located in a remote location is also a challenge. Energy storage systems and advanced grid management techniques and needed to ensure stability and reliability when integrating large amount of intermittent power into the grid.

1.3.0.2 Financial factors

The capital investment for offshore wind farms is substantial, driven by the costs of installation, maintenance and transmission infrastructure. The high financial risk and long payback time could be a further obstacle. However, economies of scale, technological advancements and incentives for renewables are steadily reducing these costs. Several financing mechanisms like public-private partnerships (PPA) and green bond are used to attract investments.

1.3.0.3 Environmental factors

The presence and installation of offshore wind farm can have an impact on the marine environment and ecosystem. Noise from pile drilling and hammering, changes in water flow and the physical presence of turbines can affect marine life. Multi-use concepts for offshore wind power plants are going to be developed to enhance the efficient use of ocean space. The goal is to integrate offshore wind with different energy sources like wave energy converters (WECs), tidal and solar energy, energy storage and hydrogen production, marine protected areas, fishing and shipping activities.

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1.4 Thesis outline

The scope of this thesis is to evaluate the stability of a spar-type floating turbine. This evaluation is conducted through a one-degree-of-freedom analysis, focusing on the platform’s pitch motion under varying wind speed conditions. The reference turbine for this study is the Hywind Tampen wind farm in Norway, specifically utilizing the Siemens Gamesa SG-8MW-167 DD turbine model.

Wind data for the analysis is sourced from the Norwegian Meteorological Institute at the location corresponding to the Hywind Tampen wind farm. The external forces considered in this study include the loads exerted by the wind on the tower and the rotor. The latter is derived using a Blade Element Momentum (BEM) theory approach. By analyzing the platform’s inclination over time and its relative movement, this thesis estimates the power and energy generated over a period of five days. A more accurate analysis was carried out with a time-step of Δt = 1 sec and compared with the 10 sec simulation.

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Chapter 2

Offshore wind turbine

According to the water depth and seabed morphology, bottom fixed turbines can be installed through different kind of foundations.

2.1 Bottom fixed foundation

Shallow waters (up to about 30 m):

• Consist of a single large-diameter steel tube driven deep into the Monopile foundation seabed. Thanks to its simplicity and cost-effectiveness, it is the most common foundation type. Monopiles are generally limited to shallower waters due to structural limitations and the challenges posed by dynamic loads from waves and currents. The installation on the seabed is carried out through hydraulic hammer or more innovative and less noisy methods like high frequency vibrations and screw-pile [24].
• Use their own weight to stay anchored at the seabed and Gravity base structures (GBS) to resist the overturning moment. Differently from monopiles, GBS can be installed in several soil types such as clay, sand, and rock. The seabed needs to be flat in order to ensure that structure is perfectly vertical. To achieve the optimal conditions, the seabed can be leveled through dredging.

Medium water depth (from 30 m to about 80 m):

• Is a three legged underwater structure, in which three steel tubes are Tripod foundation arranged in an equilateral triangle and joined together with a central vertical pile. Each leg of the tripod is anchored to the seabed using piles driven into the ground. It is crucial to ensure that the loads are evenly distributed among the three legs.

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• Are lattice-steel structures which drew inspiration, knowledge and Jacket foundations expertise from oil platform industry. Jackets distribute loads through their multiple legs, reducing the stress on individual piles. These structures are anchored to the seabed by suction anchors or traditional pile hammered into the soil at each corner of the jacket. Jacket structure are more expensive to fabricate and install compared to other foundation concepts.

(a) Monopile (b) Gravity-based (c) Tripod

(d) Jacket

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2.2 Floating wind foundation

Floating offshore wind turbines (FOWTs) enable to reach depth until 1000 m and thus to exploit a much larger sea area and harness even stronger and more stable winds [7]. This represents an opportunity for countries and regions with significant seabed drops, such as Japan, China, the United States and Europe (Mediterranean Sea and Portugal). FOWTs require higher investment and present more technical chal