Experimental low frequency mooring analysis of a floating offshore wind turbine

Abstract

Previous research into floating offshore wind turbines consistently shows some manner of discrepancy between numerical and experimental simulations when it comes to low frequency forces and motions. A part of solving this issue is the generation of accurate data for the calibration and validation of the numerical models, needed because of the coupled analysis introduced due to the relatively large wind force. The goal of this research thus became to generate accurate experimental data focussed on the low natural frequency of the mooring system in surge direction. This data had to be accompanied by an uncertainty assessment and should encompass multiple mooring systems for a good comparison.
In order to achieve these goals experimental model tests were set up using bichromatic wave sets in a long towing tank. The bichromatic wave sets allowed for the creation of beating patterns, targeting the specific low frequencies on and around the surge natural frequency via the difference frequencies. The long towing tank reduced the amount of reflections and clutter in the tank. Together these factors ensured accurate excitation of model at the desired frequencies.
The model used was a 1:96 semi-submersible model with three buoyancy columns supporting a central tower column. The depth in the tank being 1.25 m at model scale ensured a relatively deep testing environment. Two different mooring systems were tested, each with a different surge natural frequency but with the same semi-taut fibre rope-chain lay-out. A comprehensive measurement system consisting of force transducers in the mooring lines and wind force device and a camera tracking system enabled accurate measurement of both the force and motion of the model. Experiments revolved around examining the influence of the difference frequency in the wave excitation on the low frequency behaviour of the mooring system and floater.
From the lead mooring line force time series, it could be observed that during the surge natural frequency test the mooring line had triple the force compared to the wave tests with the monochromatic components, showing a clear effect of the bichromatic beating pattern. From the tests it also became clear that mooring system 1 had rear lines which were slack during wind and wave excitation. These slack lines had a number of effects on the behaviour of the model during the tests, such as increasing the mooring line force and surge response.
The tests thus show that the low frequency response comes from the difference frequency in the bichromatic wave sets, as was the goal. The heightening of the response around the natural frequency also shows that getting close to this frequency amplifies the effect. The comparison between the test results and the results from the OC5 project show that this is likely also the case there. At the same time the surge response does not show this heightened response. Which is likely due to the non-linear stiffness of the mooring system.
Furthermore, it was seen that the slack lines have a very large impact on the force in the mooring lines, especially on the lower frequencies and should thus be avoided at all times. The damping ratio as a parameter for the low frequency response is both physically and experimentally (excluding the slacked lined system) consistent and shows potential for development.