Experimental RAO’s analysis of a monolithic concrete spar structure for offshore floating wind turbines

Abstract

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.