# Analysis of a Gyroscopic-Stabilized Floating Offshore Hybrid Wind-Wave Platform

^{*}

## Abstract

**:**

## 1. Introduction

_{2}null emissions, as far as RES are concerned, sources other than wind, solar, and hydropower should be further developed and encouraged. Ocean energy electrical generation is a technology that has not yet reached grid parity, but is nowadays approaching the commercialization stage. It is commonly referred to wave, tidal, and thermal energy sources [1]. Even if tidal range is currently the most mature sea technology, the wave resource seems to be the most interesting among ocean energy resources because of its wide potential. In these, always changing, energy production circumstances, offshore power extraction is becoming more and more interesting, aiming at taking advantage of stronger resources than onshore ones and at reducing land consumption.

## 2. System Models

- Fincantieri Sea Flower floater
- NREL 5-MW wind turbine
- Omnidirectional Mooring system
- Gyroscopic conversion

- the platform pitch motion is equal to the delta motion of the gyroscope system. Thus, it is used as input to the gyroscope subsystem, in order to evaluate its performance and power production.
- the torque exerted by the Power Take Off (PTO) of the WEC is used as input to the hull subsystem, considering it as the reaction of the gyroscope system on the whole platform, that then is part of the force and torque balance that is executed inside the hull subsystem. The structural stability depends on wave, current, wind loads, and the contribution of the mooring system and the gyro system.

_{exc}is the excitation force of the floating platform; F

_{moor}is the force exerted by the moorings; F

_{WT}is the force exerted by the wind turbine; and F

_{gyro}is the force unloaded by the gyroscopic harvester. In the following, the loads for each system are presented.

#### 2.1. The Sea Flower Floater Hydrodynamic Model

#### 2.2. Wind Turbine

#### 2.2.1. Aerodynamic Forces

#### 2.2.2. Control System

- Controller of the generator torque, which has the aim of maximizing the power production below the rated wind speed.
- A full-span rotor-collective blade-pitch controller, designed to adjust the generator speed above the rated wind speed.

#### 2.2.3. Generator-Torque Controller

#### 2.3. Moorings

- the model of the catenary system is quasi-static, therefore dynamic actions are neglected, and the only effect of mass is weight, since inertia is neglected. Additionally, all kinds of damping sources are neglected (e.g., frictions at connection points, at ropes inner, and also with seabed and hydrodynamic resistances);
- the ropes of polyester have an elastic behavior only; and
- during the computation, all the mooring lines are assumed to be rectilinear in every instant. It is justified by considering that the rope part of the line is predominant in length and kept in tension by the two chain sections, so that the catenary curves formed by the three segments are close to linearity, since the angle between the lines and seabed is very low. In typical floater motions, the lines do not vary significantly in their configuration due to their large length.

#### Dynamics Modeling

#### 2.4. Gyroscopic Harvester

#### 2.4.1. Dynamic Model

_{g}is the inertia of the gyroscope around its spinning axis u, and I

_{g}is the inertia around the other two axes.

#### 2.4.2. Extractable Power

- ${P}_{d}$ is the average power absorbed from the system through the damper;
- $J$ is the moment of inertia of the flywheel around its spinning axis ${z}_{1}$;
- $\dot{\varphi}$ is the constant angular velocity of the flywheel around axis ${z}_{1}$ (therefore $J\dot{\varphi}$ is the angular momentum of the gyro);
- $\omega $ is the wave frequency;
- ${\delta}_{0}$ is the angle of pitching; and
- $c$ is the damping factor.

- $J\dot{\phi}$ is the angular momentum of the gyroscope;
- ${\delta}_{0}$ is the pitching amplitude of the floater; and
- ${\epsilon}_{0}$ is the amplitude of the oscillation on the PTO shaft

#### 2.4.3. The Gyro Unit

#### 2.5. Case Study Site

## 3. Results

^{2}reached when the significant wave height was 6.25 m. However, Figure 19 shows that the WEC integration allowed an important percentage reduction of the acceleration detected at the nacelle of the wind turbine, with a reduction greater than 15%. This means a significant reduction of the mechanical stress on the turbine itself, which could also provide an interesting improvement to the useful life of the wind turbine.

## 4. Conclusions

- stabilization of the floater, which could reach reductions in the hull pitch motions up to −37% in favorable conditions (i.e., the case study ones);
- reduction of the acceleration at the turbine nacelle, in the order of −10% in the case study wave conditions; and
- improved power production, coming from both the WEC electrical outcome and the slight increase of the wind turbine power. It can reach values of +120%, when the mean wind speed is low (and thus the wind turbine power is also low), around 3.4 m/s, and 128 gyros are installed, with respect to the configuration without gyroscopes.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 15.**Variation of the power produced by the whole platform as the number of gyros increases for each wind speed.

**Figure 21.**Percentage difference between the power produced by the whole offshore platform and the wind turbine.

Platform diameter | 63 m |

Platform draft | 12 m |

Floater mass | 15,000 t |

Rating | 5 MW |
---|---|

Rotor orientation, configuration | Upwind, three blades |

Control | Variable speed, collective pitch |

Drivetrain | High speed, multiple-stage gearbox |

Rotor, hub diameter | 126 m, 3 m |

Cut-in, rated, cut-out wind speed | 3 m/s, 11.4 m/s, 25 m/s |

Cut-in, rated rotor speed | 6.9 rpm, 12.1 rpm |

Overhang, shaft tilt, precone | 5 m, 5°, 2.5° |

Rotor mass (hub mass) | 110 t (56.78 t) |

Nacelle mass | 240 t |

Tower mass | 250 t |

Overall mass | 600 t |

Hub inertia about rotor axis | 115.926 kg m^{2} |

Hub CM coordinates in shaft CS | (0 m, 0 m, −5.0191 m) |

Nacelle CM coordinates in nacelle CS | (1.75 m, 0 m, 1.9 m) |

Tower height | 77.6 m |

Distance from nacelle base to rotor axis | 2 m |

Distance from rotor axis to tower base | 90 m |

Rated wind speed | 11.4 m/s |

Rated rotor speed | 12.1 rpm |

Rated generator speed | 1173.7 rpm |

Rated generator torque | 43,094 Nm |

Rated mechanical power | 5.30 MW |

Rated electric power | 5 MW |

Number of Mooring Lines | 6 | |

Angle between adjacent lines | 60° | |

Seabed depth | 50 m, 200 m | |

Mooring leg composition | rope-chain-rope | |

Unstretched lengths (200 m depth) | 100-700-300 m | |

Chain | Type | Studlink, R3 |

Diameter | 90 mm | |

Mass per unit length | 182 kg/m | |

Min. breaking load | 6647 kN | |

Axial stiffness | 699 MN | |

Rope | Type | Polyester |

Diameter | 160 mm | |

Mass per unit length | 16.8 kg/m | |

Min. breaking load | 7112 kN | |

Axial stiffness | 59.3 MN |

Symbol | Quantity | Value | |
---|---|---|---|

$\dot{\phi}$ | Flywheel maximum speed | 1000 | rpm |

J | Flywheel moment of inertia | 7.5 × 10^{3} | kg m^{2} |

m_{g} | Flywheel mass | 1 × 10^{4} | kg |

$\dot{\epsilon}$ | Generator maximum speed | 25 | rpm |

T_{ε} | Generator rated torque | 50 | kNm |

P_{ε} | Power electronics max power | 130 | kW |

d_{b} | Distance between bearings | 1.476 | m |

P_{R} | PTO rated power | 50 | kW |

Mean Wind Speed [m/s] |
---|

0 |

3.40 |

6.17 |

12.99 |

25.84 |

Mean Wind Speed [m/s] | Gyro Power [kW] | WT Power [kW] | Energy Platform Power [kW] | Variation [%] | ||
---|---|---|---|---|---|---|

0 Gyros | 128 Gyros | 0 Gyros | 128 Gyros | |||

3.4 | 0 | 400 | 250 | 250 | 650 | +155 |

6.17 | 0 | 400 | 1013 | 1013 | 1413 | +39.5 |

12.99 | 0 | 400 | 4242 | 4242 | 4642 | +9.4 |

25.84 | 0 | 389 | 4479 | 4479 | 4868 | +8.7 |

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## Share and Cite

**MDPI and ACS Style**

Fenu, B.; Attanasio, V.; Casalone, P.; Novo, R.; Cervelli, G.; Bonfanti, M.; Sirigu, S.A.; Bracco, G.; Mattiazzo, G.
Analysis of a Gyroscopic-Stabilized Floating Offshore Hybrid Wind-Wave Platform. *J. Mar. Sci. Eng.* **2020**, *8*, 439.
https://doi.org/10.3390/jmse8060439

**AMA Style**

Fenu B, Attanasio V, Casalone P, Novo R, Cervelli G, Bonfanti M, Sirigu SA, Bracco G, Mattiazzo G.
Analysis of a Gyroscopic-Stabilized Floating Offshore Hybrid Wind-Wave Platform. *Journal of Marine Science and Engineering*. 2020; 8(6):439.
https://doi.org/10.3390/jmse8060439

**Chicago/Turabian Style**

Fenu, Beatrice, Valentino Attanasio, Pietro Casalone, Riccardo Novo, Giulia Cervelli, Mauro Bonfanti, Sergej Antonello Sirigu, Giovanni Bracco, and Giuliana Mattiazzo.
2020. "Analysis of a Gyroscopic-Stabilized Floating Offshore Hybrid Wind-Wave Platform" *Journal of Marine Science and Engineering* 8, no. 6: 439.
https://doi.org/10.3390/jmse8060439