Experimental and numerical methods for the evaluation of wave loads on offshore platforms in extreme sea conditions

Abstract

The aim of the research was to characterise the hydrodynamic surge, heave and pitch loads of a floating semi-submerged tension leg platform for supporting wind turbines. A series of experimental measurements on a 1/100th scaled model were undertaken and the results were compared to those obtained from a numerical RANSE model. Relevance and innovation consisted in using a model strictly close to reality during experimental tests, and in using non-linear waves both in the wave tank and with the state-of-the-art RANSE solver. The full-scale geometry of the structure considered consisted of four cylindrical bodies joined to a central large cylinder (5 meters diameter, 20 meters length) which supported the wind turbine tower by means of eight tubular connections (3.3 meters diameter, 12.3 meters length). Each body was composed of one cylinder (7.5 meters diameter, 20 meters length) with a further larger cylindrical body (9 meters diameter, 7.8 height) to increase the stability. The study concentrated on the prediction of non-linear hydrodynamic forces acting on the complex structure. The scaled model was tested in a water wave maker in a fixed condition. Three separate load cells were used to measure the forces and moment components. A preliminary study was dedicated to the verification of the second and third order Stokes waves obtained with the wave maker by a sampling of the waves measured in the tank for each of the reported test condition. The period of the water waves ranged between 6 seconds and 1.15 seconds, while the full-scale wave height varied between 3.37 to 10.7 meters. The numerical RANSE model was based on a special non-structured (trimmed type) mesh grid with anisotropic refinement in the three cardinal directions to ensure a good convergence property with a limited expense in terms of total number of cells, while representing the various geometrical details of the structure (such as the connections and the cylinders). Experimental results were compared in a scientific graphic form with RANSE values. Finally, a common nondimensionalized engineering law for predicting these forces was identified versus the wave slope (k*a) and the Keulegan-Carpenter number (KC).