Nonlinear Pitch Decay of a Floating Offshore Wind Turbine Structure

Abstract

Model tests were conducted on three generic floating wind turbine systems in 2011, and reported in a series of papers at OMAE 2012. These tests were conducted at the MARIN facility in the Netherlands, by a consortium of universities, government research organizations and industry. As part of the testing program, decay tests in platform pitch were conducted with and without wind. It was found that for spar and semi-submersible type structures, resonant pitch motion was damped due to wind in storm sea conditions. The nonlinear decay motion of a floating wind turbine platform is modeled using a one degree-of-freedom nonlinear oscillation equation about a mean offset angle. Attention is paid to the turbine thrust coefficient and its variability with respect to oncoming flow speed, which in turn is affected by the structure pitch motion. The equation of motion reveals that the mean offset position has an important role in the stiffness, damping and consequently the natural period of pitch motion. Several important dimensionless parameters are introduced. The paper discusses a simple thrust model for an offshore wind turbine based on rudiments of blade element theory. Using the simplified thrust coefficient formulation, the increase in platform pitch damping due to wind is formulated. Experimental data reported from prior tests described above show good agreement with the theoretical model.

Experimental and numerical studies on the low-frequency responses of a spar-type floating offshore wind turbine

“…It is found that the second-order forces were small for the OC3-Hywind spar, but clear second-order LF responses were seen, due to the wave excitation at resonant frequencies. Thiagarajan et al (2013) studied the nonlinear decay motion of a spar FOWT and compared with model test results. In particular, the nonlinearity of the pitch motion is analyzed in detail in the study.…”