Eólica | Wind Power FuturEnergy | Septiembre September 2017 www.futurenergyweb.es 38 de simulación basadas en el dominio del tiempo, que consideren todos los fenómenos que afectan al sistema, como la aerodinámica, la dinámica estructural, la hidrodinámica, las estrategias de control y la dinámica de las líneas de fondeo, todas ellas de manera integrada. Estas herramientas de simulación se utilizan para calcular las cargas que son empleadas para dimensionar estructuralmente los diferentes componentes del aerogenerador, por lo que su precisión es fundamental para optimizar el aerogenerador flotante. Además, el coste computacional debe ser razonable para realizar un proceso de diseño eficiente, más aún si consideramos los miles de casos de diseño que la normativa de aerogeneradores marinos exige para su certificación [1]. Este artículo se centra en un proyecto concreto de desarrollo y validación de un código dinámico para líneas de fondeo de aerogeneradores flotantes. El código se acopló a un simulador de aerogeneradores flotantes y se cuantificó el impacto que tiene considerar la dinámica de los fondeos en la computación de las cargas del aerogenerador. Esto permite seleccionar el adecuado nivel de complejidad de los modelos de aerogeneradores flotantes y optimizar el coste computacional. Los detalles de este estudio pueden consultarse en [2]. Desarrollo del código y validación experimental El código de simulación para líneas de fondeo desarrollado por CENER se denomina Offshore Platform Anchoring System Simulator (OPASS) y está basado en el Método de los Elementos Finitos. Para optimizar la eficiencia computacional se ha utilizado una formulación lumped mass. El código se validó mediante experimentos en el tanque de agua del École Centrale de Nantes. Los ensayos consistieron en sumergir una cadena con un extremo anclado en el fondo y el otro extremo (fairlead) suspendido a la altura de la lámina de agua, que se excitó con movimientos armónicos horizontales de diferentes frecuencias en el plano de la catenaria, como muestra la Figura 1. El código fue capaz de predecir la tensión medida (Figura 2) y los movimientos de la cadena incluso en los casos de mayor frecuencia de domain simulation tools capable of accounting for all the phenomena that affect the system including aerodynamics, structural dynamics, hydrodynamics, control strategies and mooring line dynamics. These simulation tools are able to calculate the loads that are used for the structural dimensioning of the different wind turbine components, meaning that their accuracy is critical for optimising the floating wind turbine. In addition, the computational cost has to be reasonable to carry out an efficient design process, particularly when taking into account the thousands of design cases required by offshore wind turbine guidelines to achieve certification [1]. This article describes a specific research project and the validation of a mooring line dynamics code for floating wind turbines. The code was coupled to a floating wind turbine simulator and was used to quantify the impact that mooring line dynamics have to take into account when calculating wind turbine loads. The conclusions provide criteria to select the correct level of model complexity for floating wind turbines and optimise the computational cost. The details of this study can be found in [2]. Code development and experimental validation The simulation code for mooring lines developed by CENER is called Offshore Platform Anchoring System Simulator (OPASS) and is based on the Finite Element Method. To optimise computational efficiency, a lumped mass approach was used. The code was validated via experiments performed at the École Centrale de Nantes wave tank. The tests involved submerging a chain with one end anchored to the bottom of the tank and the other end (fairlead) suspended at the still water level. The suspension point was excited using a prescribed periodic motion at different frequencies in the plane of the catenary, as shown in Figure 1. The code was able to predict the measured tension (Figure 2) and the movements of the chain, even in cases with the highest excitation frequency (period of 1.58 s), where the line totally loses tension and suddenly recovers it in a snap load. Impact of mooring line dynamics in design loads The code was coupled to the FAST wind turbine integrated simulator [3] and was used to carry out an extensive assessment of the effect of mooring line dynamics on ultimate and fatigue loads. The three models of floating wind turbine shown in Figure 3 were considered, representing the three main existing typologies: spar, semi-submersible and tension leg platform (TLP). The models are public and a detailed description of each can be found in [4], [5] and [6]. Each one of these platforms supports the same 5 MW NREL Baseline wind turbine, based on the assumption that they are situated in the same location, close to the Irish coast (52º 10’N, 11º 45’W), at a depth of 200 m. According to the IEC 61400-3 [1] guideline, more than 20,000 cases were launched and processed to calculate the loads, using the dynamic mooring line model and comparing it with a simpler quasistatic model. The effect of mooring line dynamics depends on platform type and is heightened in those components that are closer to the platform. Figura 1. Vista lateral de la configuración del ensayo | Figure 1. Side view of the test configuration Figura 2. Comparativa de tensiones medidas y calculadas para diferentes periodos de excitación. Figure 2. Comparison of measured and calculated tensions for different excitation periods.
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