Thermal Stratification and Turbulent Wind Loading on Floating Wind Turbines

Abstract

Offshore wind is a promising clean energy alternative to fossil fuels. The wind resource at sea is considerably stronger than on land, and the offshore wind turbines (OWTs) tend to be larger. Currently, more than 160 commercial offshore wind farms (OWFs) are in operation worldwide, of which 2% are floating wind farms (FWFs). Bottom-fixed OWTs are mainly installed close to shore and in shallow water, where operation, maintenance and substructure costs are lower than for floating solutions. However, some countries have deep water close to the coast, which makes the development of floating offshore wind turbines (FOWTs) an attractive alternative (e.g. in Japan, Norway, and the west coast of the USA). The floating offshore wind industry is expected to continue to reduce costs, making it competitive with bottom-fixed developments in the near future. The atmospheric inflow to wind turbines includes mesoscale and microscale processes. For a FOWT, the floater introduces additional degrees of freedom compared to a bottom-fixed foundation, thus extending the dynamic response of FOWTs to a wider frequency interval. The natural frequencies of the floaters are in the range of the microturbulence frequencies influenced by the thermal stratification of the atmosphere (atmospheric stability). The design standards for wind turbines do not usually take atmospheric stability into account. However, atmospheric stability should be considered in the design of state-of-the-art FOWTs to improve estimates of the wind loads, structural response and power production. This thesis investigates the impact of atmospheric stability on the FOWT response and fatigue loads through numerical simulations. The two study cases include the 5MW OC3-Hywind spar wind turbine (WT) and the 5MW OC4 DeepCwind semi-submersible WT. The wind inflow is generated using different turbulence spectral models: Davenport-Kaimal, Uniform Shear, Pointed-Blunt, and Højstrup Spectral Model with Daven port co-coherence. The variable atmospheric stability is simulated using the Pointed-Blunt Model and the Højstrup Spectral Model with Davenport co-coherence. The effects of single-point and two-point spectra on the two WTs are assessed separately and then jointly. A single wave loading scenario with a fixed significant wave height and peak period is adopted. In both study cases, the simulated fatigue damage associated with the tower top twisting and the side-side bending moment is found to increase in an unstable atmosphere compared to a near-neutral atmosphere, as is the yaw and sway of the floaters. In a near-neutral atmosphere, the difference in the spatial turbulence characteristics (co-coherence) in the two spectral models (Uniform Shear Model and the Davenport-Kaimal Model) is also seen to affect the response of the OC3-Hywind and the OC4 DeepCwind. For example, the tower side-side bending moment is found to be greater by up to 27% in a less coherent turbulent flow. This emphasises the need to base the design of FOWT on lateral co-coherence models derived from offshore field measurements at separations relevant to FOWTs, which are currently scarce. The simulated response of the OC3-Hywind to atmospheric stability using the Pointed-Blunt Model is compared to the observed response from the full-scale measurements of a 6MW spar WT at Hywind Scotland. The simulated floater pitch and yaw are found to be consistent with the measured responses. This highlights the importance of incorporating atmospheric stability into the design of FOWTs. In addition, analyses of wind measurements at a coastal site, Vindeby, are carried out with a focus on turbulence characteristics. These characteristics are then compared with measurements from an offshore platform, FINO1, from which the Pointed-Blunt model is derived. The spectral turbulence characteristics observed at Vindeby (≤ 45m above sea level (asl)) show a reasonable agreement with the characteristics observed at FINO1 (≤ 81.5m asl). This thesis recommends the inclusion of atmospheric stability in the design of state-of-the-art FOWTs, to obtain representative wind loading. Future wind measurements to include heights above 100m asl, and measurements to obtain information on lateral co-coherence are encouraged to provide relevant design data. Lateral co-coherence, which is representative of offshore sites with relevant separation distances for modern FOWTs, has been partially addressed in the COTUR (COherence of TURbulence with lidars) project. However, lateral co-coherence in the MABL remains a topic for further study. Keywords: Floating wind turbine, Atmospheric stability, Turbulence model, Spectral turbulence characteristics, Co-coherence, Floater response, Fatigue load