COSTA

Artículos científicos

• Hybrid testing system development for single point mooring lines FOWTs by O. Pires-Jiménez, J. Azcona-Armendáriz, Carlos Casanovas, Ignacio Castelló.
• OF2: Coupling OpenFAST and OpenFOAM for high-fidelity aero-hydro-servo-elastic FOWT simulations by G. Campaña-Alonso, R. Martín-San-Román, B. Méndez-López, P. Benito-Cia, J. Azcona-Armendáriz.
• High fidelity Computational Fluid Dynamics models comparison in the simulation of full scale floating platform under free decay tests by M. Gil-Liberal, J. Fuertes, A. Torres, J. Armañanzas, J. leon, G. Campaña, B. Mendez.

Comunicaciones en congresos

• EERA DEEPWIND24. 17-19/01/2024. Trondheim, Noruega.

Floating offshore wind turbine (FOWT) energy has been consolidated as a key strategic priority for the energy transition. That is because of the enormous wind potential open sea, which provides a vast surface with low environmental, acoustic and visual impact. That is especially useful in light of the growing occupation of wind farm areas on land. In particular, FOWT technology becomes indispensable in very deep areas where structures attached to the seabed are not viable. Navarre, with a strong industrial fabric in the renewable energy sector, is deeply committed to this technology.

One of the most significant challenges with this technology is controlling rotation around the vertical axis (yaw) of the entire floating platform. The torque generated by the rotor of the aerogenerator throws the whole system out of alignment, and the inherent low rigidity of the floating structure is not sufficient to compensate for the effect. The consequences are critical. Dealignment in regards to the wind increases structural loads and fatigue, lowers energy production and, in extreme situations, can completely destabilise the assembly.

The COSTA project was born with the goal of resolving that fundamental problem that limits the development of floating offshore wind turbines. Their main proposal is to develop a control strategy that acts on the pitch of the rotor blades to generate restorative moments, counteracting yaw torque.

To achieve that solution, COSTA is first focusing on exceeding the current limitations of the simulation models and test methods. Those tools are not currently precise enough to represent situations where the floating system experiences large rotations, which impedes tackling the yaw problem effectively. The project seeks to improve the models and techniques so floating aerogenerator designs can be optimised. All the improvements developed for the models, tests and controls were evaluated using a public aerogenerator model using key performance indicators (KPIs) to quantify the impact on lowering the cost of floating offshore wind energy.

The control tasks, initially focused on single turbine aerogenerators, were modified so they could be used with double turbine floating aerogenerators, due to the needs of an industrial client. On these platforms, that frequently do not have an individual yaw mechanism on the turbines and use a single point mooring (SPM) system that lets the entire platform rotate to line itself up with the wind, a coordinating control strategy has been developed.

The coordinating control is a superior control hierarchy whose primary goal is to ensure that both turbines stay turned into the wind to maximise energy production. The strategy is based on creating a difference in aerodynamic push between the rotors. That is achieved by modifying the rotation speed reference that is sent to the individual controllers for each turbine, which in turn varies the pitch setting of the blades or the torque of the generator. In that way, a turbine increases its push while the other one reduces it, which creates a moment that tends to align the platform with the wind.

Control Results:

• The uncontrolled yaw drift can be avoided.
• However, there can still be significant dealignments.
• This improvement is achieved at the cost of intensive use of blade pitch, which presents challenges for adapting it to a real case.
• Consequently, this line of research has been determined to be a promising one to follow.

In regards to the fluid-structure interaction co-simulations:

• Simulations of the interaction between the platform and waves were done with OpenFOAM (CFD) for both fixed platforms with 6 degrees of freedom and an SPM mooring system.
• Coupling Anchoring Systems with CFD Codes: MoorDyn, a model that represents the dynamic effects of anchor lines, has been coupled using OpenFOAM. This coupling is crucial for a precise representation of mooring forces, because MoorDyn considers effects like inertia, damping, buoyancy and hydrodynamic forces.
• Comparison of Simulation Tools: Comparisons were done between OpenFOAM and XFlow to simulate pitch heave cases on the DeepCWind platform. The results of both codes compared satisfactorily under diverse conditions.
• Simulation of Extreme Movements: OpenFOAM has proven to be able to simulate an extreme 30º rotation on the Z axis with a converged simulation, validating its functionality under these extreme conditions.
• Hydrodynamic-Aeroelastic Coupling: OpenFOAM (for the hydrodynamic simulation of the platform with CFD) has been coupled with OpenFAST from NREL (for the simulation of the functioning of the aerogenerator). The coupling makes it possible to exchange forces and moments between the aerogenerator and the platform at every step in time, obtaining detailed information for all the components of the floating aerogenerator-platform system.

Lastly, the COSTA project improved the in channel tests at scale for floating aerogenerators with SPM. The improvement in those methodologies was demonstrated in a test at scale at CEHIPAR in Madrid.

In synthesis, COSTA is laying the groundwork for overcoming the complex challenges of floating offshore wind turbines by developing innovative control strategies and substantially improving the test and simulation tools, which is fundamental for lowering the cost of this emerging technology that is vital for the energy transition.