Impacts of Atmospheric Turbulence on Wind Turbine Main-Bearing Function and Failures

The continued expansion of wind energy within nations' energy portfolios requires continued reductions in
the levelized cost of energy (LCOE), the ratio of financial cost to purchase and operate wind farms to financial
gain from the electrical power produced by the wind farm. Major contributors to the numerator include
replacement costs for premature component failure on the drivetrain, including, in particular, the main bearing
(Hart et al. 2020a). Whereas main bearing failure is likely not subsurface fatigue-related (Hart, et al. 2019) and
a number of potential mechanisms likely contribute, the dominant processes underlying premature main
bearing failures are not currently known (Hart, et al. 2019,2020a). Work has been undertaken to develop a
systematic approach to the study of main bearing loading and failure mechanisms (Hart 2020b); wherein, it is
demonstrated that a detailed understanding of the loads experienced by the main bearing, and their causal
mechanisms, is a necessary prerequisite to progress in this field. The proposed research programme centers on
the hypotheses that (1) the mechanisms underlying premature main bearing failures result from specific
repetitive time changes in the bearing load-zone, and (2) that these deleterious load-zone forcings are in
response to specific temporal characteristics in the moments and forces on the main shaft that result from the
passage of the energy-dominant atmospheric turbulence eddies through the wind turbine rotor plane.
Utility-scale wind turbines and wind farms respond to turbulence within the "atmospheric boundary layer"
(ABL), the 1-2 km region of the troposphere adjacent to the earth's surface. During the day, the structure of the
turbulence eddies transported within the ABL is driven by strong convection and strong shear, a coherent eddy
structure that varies systematically with the global stability state of the ABL (Khanna & Brasseur 1998, Jayaraman
& Brasseur 2014). As the eddies interact with rotating wind turbine blades, high temporal variabilities in
aerodynamic loads pass from the rotor hub to the main shaft in the form of torque, bending moment and axial
force fluctuations (Vijayakumar, et al. 2016) with three characteristic time scales (Nandi, et al. 2017) in main
shaft moments (Lavely 2017). The shortest of these is below 1 sec. with relative variability on order 50%
(Vijayakumar, et al. 2016). Lavely (2017) showed that, whereas main shaft torque fluctuations respond to rotoraveraged
horizontal winds, shaft bending moment fluctuations respond to time changes in spatial asymmetry in
horizontal velocity over the rotor plane, generated as turbulence eddies pass through the rotor.
If the above hypotheses are valid, it follows that the turbulence-generated deleterious load fluctuations on
the main bearing are likely driven by different classes of turbulence structure and loading response at different
time scales as atmospheric eddies pass through the rotor plane. Since ABL turbulence structure varies with
atmospheric stability, deleterious load characteristics change during the day and among seasons, as well as with
topography. Furthermore, within a wind farm the hypotheses can be extended to include potential deleterious
main bearing responses to combinations of atmospheric and rotor wake turbulence eddying motions due
especially to the generation of spatial asymmetries over wind turbine rotor planes.

This outward-facing doctoral training centre will create impact through knowledge enhancement and leadership development which will have significant benefit for society, people and the economy.

Societal Impacts:
A very large increase in renewable energy generation, mainly wind, wave and tidal, is expected in the coming years and decades to meet the UK Government and international obligations to reducing greenhouse gas emissions by at least 80 per cent by 2050 when compared to 1990 levels. In particular, the Offshore Wind Industry Council is proposing, under a Sector Deal, to deliver 30GW of offshore wind by 2030 and 50GW by 2050, whilst reducing the average price of electricity by 18%. The longer term societal and economic impacts arise from the difference that the CDT programme and its graduates make to the UK realising this medium-term and longer-term target. The societal impact of meeting these targets, over failing to meet them, can be calculated in avoided CO2, increased sustainability, security and resilience of the energy system in a safe, affordable and environmentally sensitive manner.

People Pipeline and Skills:
There is a widely recognised skills gap in renewable energy both in UK and Europe. Hence, the proposed CDT is timely contributing significantly to meeting the sector's skills demand by the provision of highly trained engineering leaders, expert in a broad range of wind and marine energy technologies and engineering. Most of the CDT graduates will be expected to take up posts in the growing commercial wind and marine energy sectors, and quickly rise to positions of leadership and influence. Some graduates will remain in the higher-education sector and develop academic careers providing much needed increased capacity and capability resulting in a positive impact through an expanded research-base and capability to deal with the inevitable research challenges of the sector as it develops further commercially.

Students will be mentored and encouraged to take a proactive role in creating impact with their research whilst observing Responsible Research and Innovation (RRI). All the Universities participating in this CDT proposal have explicit policies and resources in place to support knowledge exchange and impact and also public engagement. These support the students throughout their studies to engage in broader dialogue and deliberation and to be aware of the potential impacts and implications of their research.

Our CDT students will also engage in outreach activities and impact the wider community through the well-established Professional Engineering Training Scheme (PETS): this scheme is managed and directed by the students and provides opportunities to engage in outreach activities and to work with peers. e.g. PETS runs a schools and colleges programme wherein the students organise visits to schools and colleges to provide information about renewable energy and a basic introduction to the technology involved.

Economic Impact:
The low-carbon and renewable energy sector is estimated to increase five-fold by 2030, potentially bringing two million jobs to the UK. In particular, an ambitious Sector Deal for industry proposed by Government as part of its Clean Growth Strategy could see a total installed capacity of 30 GW of offshore wind by 2030 with the potential to create at least 50,000 jobs across the UK. If achieved, this would be a six fold increase from the current installed capacity and would make offshore wind the largest source of domestic electricity. To ensure resilience, it is also important to retain and develop the leading UK Wave and Tidal position. With the direct and indirect value added to the UK supply and installation chain in terms of job creation, intellectual property exploitation, and sales of wind, wave and tidal technology and services, the proposed CDT will make an important contribution through knowledge enhancement and leadership development.