Session: 09-02-07 Wind Energy: Fatigue Analysis

Submission Number: 180483

Enhancing Floating Offshore Wind Turbine Levelized Cost of Electricity: via Fatigue Stress Reductions 

 

Enhancing floating wind levelised cost of electricity via fatigue stress reduction

Jack Batley (AMC), Jean-Roch Nader (AMC), Jon Gumley (AMOG), Jack McDonald (AMOG)

Developing cheap and reliable offshore wind turbines has long been an industry goal for renewable energy generation. Offshore turbines offer access to more consistent and higher average wind speeds compared to land-based turbines. Traditional fixed-bottom offshore turbines are a proven technology, however, are limited to installation in relatively shallow water depths. Floating offshore wind turbines provide several advantages, such as being able to be installed in much greater water depths and further offshore than their fixed counterparts. This flexibility and more persistent wind resource for floating systems comes at the expense of a more complex dynamic system of a floating platform and its mooring and dynamic cable systems, resulting in a higher lifetime Levelised Cost of Electricity (LCOE) compared to fixed-bottom.

This study investigates a novel operational strategy to reduce the lifetime LCOE for floating offshore wind turbines. The strategy aims to mitigate structural fatigue through selective turbine shutdowns during highly‑damaging sea-states that are otherwise within the normal operating limits of the unit. By optimising operational time rather than relying solely on design improvements, the research offers a new approach to improving the long-term economic performance of floating wind farms.

The analysis focused on the 15 MW National Renewable Energy Laboratory reference turbine mounted on the semi‑submersible VolturnUS-S platform and was performed using fully coupled time-domain analysis in OrcaFlex. Environmental conditions representative of the Bass Strait off Northern Tasmania, an emerging offshore wind development area, were used. Wind speed, significant wave height, spectral peak wave period, and directional occurrence data were combined to generate over 4315 individual sea-states. Each sea-state was simulated for both operational and parked turbine states to capture the full range of fatigue responses. Fatigue damages were evaluated through a rainflow cycle counting algorithm.

The analysis focused on the tower base as a fatigue-critical location, with a baseline estimated life expectancy of approximately 31 years assuming standard operational limits. Fatigue damage rates were found to be most severe in sea-states with wave periods of 5‑7 s coinciding with the natural frequency of the platform pitch response, exciting a resonant response which increases tower base bending moments. In contrast, the fatigue damage rates in parked conditions were an order of magnitude less due to the reduction of aerodynamic thrust forces and subsequently the tower bending moments, confirming the potential for extending life expectancy through targeted shutdowns.

Economic analysis was performed by integrating the fatigue results into a development economics model that accounted for capital expenditure (CAPEX), operation and maintenance costs, and decommissioning costs with varying discount rates between 3 and 7%. By selectively choosing whether a turbine enters a parked mode during the most damaging sea-states, the relationship between loss of energy production and fatigue reduction was quantified. Selectively parking the turbine in conditions which, if it was in normal operation, represent the top 5% most damaging sea-states provided the most cost-effective balance. A predicted improvement of 50% to fatigue life was achieved, corresponding to a reduction in LCOE of 8% when assuming a discount rate of 3%.

The case study analysed here demonstrates that limiting operation in high fatigue damage sea-states offers a practical and economical approach for increasing the commercial viability of floating offshore wind. This strategy can offer structural fatigue benefits with minimal impact to the overall energy production, and the subsequent cost benefits that come from either reduced CAPEX or life extension. Beyond its economic implications, this research supports broader industry objectives of sustainable and resilient renewable energy generation and represents a step toward more advanced adaptive control schemes floating wind platforms that will further improve the viability of the technology.

Presenting Author: Jack Mcdonald AMOG Pty Ltd

Presenting Author Biography: Jack is a Lead Engineer in the Maritime team at AMOG consulting, based in Melbourne, Australia. Graduating from Monash University with Honours after studying a Bachelor of Aerospace Engineering, Jack has over 7 years of experience in offshore dynamic analysis of floating systems, including Floating Offshore Wind Turbines.

Authors:

Jack Batley Australian Maritime College, University of Tasmania
Jean-Roch Nader Australian Maritime College, University of Tasmania
Jack Mcdonald AMOG Pty Ltd
Jon Gumley AMOG Pty Ltd