The relevance of the interaction between the structural stiffness and the wind turbine dynamics are well known by the industry from decades of expertise. However, the novel floating concepts are commonly designed by tools that disregard the individual structural stiffness of the elements which compose the floating platform. Usually, these elements are treated as rigid body elements in the design process. However, their flexibility can be relevant in the long time structural life assessment, particularly for the fatigue assessment. Then, a structural analysis that includes the dynamic effects over the deformation of the structure is required. The paper proposes a model that integrates the instantaneous deformation of the structure by a nonlinear dynamic Finite Element Analysis (FEA) computation in a fully coupled Offshore Wind Turbine (WT) model in the time domain.
A 3D hydro-aero-servo-elastic coupled numerical model in time domain for structural analysis of floating or bottom fixed structures has been developed at the UPC-BarcelonaTech. The code integrates the forces exerted by the waves and currents as well as the aerodynamic loads, including the wind turbine, and the mooring system. For the hydrodynamic loads the Morison’s equation is used to compute waves and current forces. Both Airy (regular and irregular) and Stokes 5th order wave theories are implemented. The mooring system can be computed in a quasi-static way or considering their fully dynamics. The aerodynamic loads are computed with AeroDyn standalone module from NREL, which is coupled to the FE model. The structure is discretized with one-dimensional beam finite elements, in which the formulation considers small strains as well as large motions under the implementations of the Euler beams theory. With this approach, the dynamic interaction between the wind turbine and the structure, as well as the effects on the internal forces are implicitly coupled in the formulation.
A comparison of the Windcrete concrete SPAR concept experimental Response Amplitude Operators (RAO) against the numerical results are presented, as well as some results from simulations under normal operation conditions.
The design of a new solution from the early stages to the fully commercial units needs to achieve and overtake the different technology readiness levels (TRL). The floating platforms for offshore wind turbines (FOWTS) are not an exception. Moreover, there are some handicaps for these technology solutions like the high initial investment costs, the low number of available offshore wind turbines for full scale prototypes, or the big differences of the construction and installation between the initial smaller prototypes and the final designs in a commercial stage.
Windcete is a monolithic concrete spar platform, including both the tower and the floater in a unique concrete member of which a proof of concept was developed in the KIC Innoenergy AFOSP project. The monolithic characteristic means that joints are avoided, thus the fatigue resistance is increased since weak points are driven out. The whole structure is in compression state by the use of active reinforcement, and it is designed to avoid traction at any point during the span life of the platform. Furthermore, the use of concrete allows to reduce the Capital Expenditure (CAPEX) because its low cost, and also reduces the Operational Expenditure (OPEX) due to the its high resistance against the marine environment.
This article highlights the scalability of the concept through the main dimensions, construction and installation processes for three different Windcrete units. These units are predesigned for different TRL to compare the evolution from an initial prototype to commercial units for a wind farm.
A 3D aero-hydro-servo-elastic coupled finite element method (FEM)
numerical model in time domain for structural analysis has been
developed at the UPC-BarcelonaTech (FloaWDyn). The code integrates
the environmental forces from the waves and currents as well as the
aerodynamic loads, including the wind turbine, and the mooring
system. As a part of the FloaWDyn numerical code validation, in the
paper is presented the calibration of the numerical model accordingly to
the DeepCwind experimental scaled model, as well as some simulations
of the Phase 2 of the OC5 project, which include the free decay tests
and some coupled wind and waves tests. The simulations are finally
compared to the results obtained in the experimental campaign at
Campos, A.; Molins, C.; Gironella, X.; Trubat, P. Proceedings of the Institution of Civil Engineers. Maritime engineering Vol. 169, num. 2, p. 49-63 DOI: 10.1680/jmaen.2014.24 Data de publicació: 2016-06 Article en revista
Significant research efforts are now being directed at floating offshore wind turbines. The main challenge posed by floating wind turbines is the high construction and installation costs of the substructure, which make it too expensive for commercial exploitation in the current energy market. With the aim of achieving a cost-effective floating platform for offshore wind turbines, a new concept of a monolithic floating spar buoy is hereby presented. The monolithic concrete structure includes both the tower and the floater, built as a continuous single piece. This new concept offers a significant cost reduction during the construction phase and also while in operation, because the platform becomes almost free of maintenance during its lifetime. The main dimensions and the hydrostatic and hydrodynamic properties are presented, including a basic structural assessment of the platform to ensure its structural integrity. The construction and installation processes are presented, taking into account the special requirements of the monolithic design. Finally, a cost comparison between a steel and an equivalent concrete platform design has been performed, showing a material cost reduction larger than 60% in the case of the concrete design.
Nowadays the offshore wind energy market is clearly oriented to be extended around the world. Bottom fixed solutions for supporting offshore wind turbines are useful in shallow waters which are available in a limited extent unless a continental shelf exists. Considering the Oil & Gas background knowledge, move from bottom fixed solutions to floating solutions is not a
technical challenge, but the cost of each structure in terms of industry profit is currently the main issue for its commercial implementation. That point has induced huge research efforts on the topic.
Recently, a new concept consisting of a monolithic concrete SPAR platform was experimentally and numerically studied in the framework of the AFOSP KIC-InnoEnergy project (Alternative Floating Platform Designs for Offshore Wind
Towers using Low Cost Materials). The studies comprised a set of hydrodynamic tests performed in the CIEM wave flume facility at UPC, with a 1:100 scaled model assuming Froude similitude.
The whole test campaign includes free decay tests, RAO’s determination, regular and irregular waves with and without wind mean force. For the determination of the platform RAO’s, a set of 21 regular waves trains with periods ranging from 0.8s up to 4.8s were applied. The 6 DOF motions of the platform were measured with an infrared stereoscopic vision system.
In this paper, a summary of pitch and heave RAO’s tests will be presented with the main objective to calibrate and validate the accuracy of the Morison-based numerical model for floating wind turbine platforms developed at the Universitat Politècnica de Catalunya.
Because the wave flume spatial constraints, both Airy and Stokes wave theories are necessary to reproduce the correct wave kinematics. The numerical model includes both theories and a comparison between them has been done, checking the validity range of each one. The simulations revealed a reasonable good agreement with the experimental results, as well with the computed RAO’s in commercial software.
Preliminary studies of a concept consisting of a monolithic concrete SPAR platform were presented in 2014. The studies were performed in the framework of the AFOSP KIC-InnoEnergy project (Alternative Floating Platform Designs for Offshore Wind Towers using Low Cost Materials) showing significant costs reduction. The experimental phase of the project was developed during 2014.
The experiments comprised a set of hydrodynamic tests performed in the CIEM wave flume facility at the Universitat Politècnica de Catalunya (UPC), with a 1:100 scale model assuming Froude similitude. The complete experimental campaign included free decay tests, a set of 22 regular wave trains of different periods to determine the RAO’s and another set of 21 regular and irregular wave trains in conjunction with a mechanical wind device, simulating the mean thrust force exerted by the wind turbine.
To adjust the weight of the whole system, a set of adjustable weights inside de scale model were designed assuring such properties, particularly the
pitch/roll inertia. The scaled model of the mooring system was carefully studied because the constraints in width of the flume facility. A mechanical wind device
was also specifically designed to ensure an averaged force at the top of the model, simulating the effect of the mean rotor thrust force.
A detailed description of the methodology for the experimental campaign and a summary of the experimental results are presented.
One of the main aspects when testing floating offshore platforms is the scaled mooring system, particularly with the increased depths where such platforms are intended. The paper proposes the use of truncated mooring systems to emulate the real mooring system by solving an optimization problem. This approach could be an interesting option when the existing testing facilities do not have enough available space. As part of the development of a new spar platform made of concrete for Floating Offshore Wind Turbines (FOWTs), called Windcrete, a station keeping system with catenary shaped lines was selected. The test facility available for the planned experiments had an important width constraint. Then, an algorithm to optimize the design of the scaled truncated mooring system using different weights of lines was developed. The optimization process adjusts the quasi-static behavior of the scaled mooring system as much as possible to the real mooring system within its expected maximum displacement range, where the catenary line provides the restoring forces by its suspended line length.
One of the main aspects when testing floating offshore platforms is the scaled mooring system, particularly with the increased depths where such platforms are intended. When the existing testing facilities do not have enough available space, the use of truncated mooring systems with a similar behaviour to the real mooring system could be an interesting option. As part of the development of SPAR platform made of concrete for FOWTs a station keeping system with catenary shaped lines was selected. The test facility available for the planned experiments presented an important width constraint. Then, an algorithm to optimize the design of the scaled truncated mooring system by using different weights of lines was developed. The optimization process adjusts the quasi-static behaviour of the scaled mooring system as much as possible to the real mooring system within its expected maximum displacement range, where the catenary line provides the restoring forces by its suspended line length.