Some offshore wind turbine (OWT) support structures may fall short of required fatigue life expectations, according to a new Lloyd’s Register (LR) report. The new case study demonstrates how reliability-based inspection can help manage fatigue-driven risks in offshore wind turbine support structures.
The case study evaluated a North Atlantic offshore wind farm of sixty to seventy turbines (500–600 MW capacity). Offshore wind turbines are typically designed for 25 years of service, using a fatigue design factor of three — implying a minimum required fatigue life of 75 years. However, the study found that a critical joint in the jacket foundation would reach the end of its fatigue life after just 52 years, falling short of this design requirement.
Instead of redesigning the joint, the study took a reliability-based inspection (RBI) approach to identify and mitigate potential failure through targeted, risk-based maintenance.
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Determining structural safety levels
The study combined a S-N (Stress vs. Number of cycles) model, to estimate when structural safety drops below acceptable thresholds, with Fracture Mechanics (FM) crack growth analysis, to predict the probability of failure over time and inform inspection intervals. This approach incorporates inspection results via Probability of Detection (PoD) curves to allow inspection schedules to be dynamically updated, responding to real-world conditions and inspection findings.
The results suggest that the first inspection should be carried out around year nine. After that, depending on the inspection method, further inspections might be needed every year to maintain acceptable safety margins.
More advanced inspection tech needed
However, the case study highlights the limitations of current inspection methods. Visual and ultrasonic inspections were found to be less effective for fatigue-critical components. More advanced techniques, such as Eddy Current or ACFM, offer greater reliability and allow for longer inspection intervals, but only when operators were willing to adopt slightly lower safety thresholds.
While risk-based inspection (RBI) planning is effective in reducing in-service life costs and ensuring the longevity and safety for offshore wind turbine (OWT) structures, it requires expert input, reliable models, and software tools that can handle complex calculations. Ongoing research aims to refine the models and address the challenges during their application.
Reliability updating, especially when integrating PoD curves, requires complex modelling and precise calibration of parameters such as initial crack size and stress intensity factors, areas often underdeveloped in practice.
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Reliability-based methods
The study calls for wider industry collaboration to refine inspection standards, share real-time monitoring data to refine fatigue predictions, and adopt more flexible definitions of acceptable reliability where appropriate.
Kourosh Parsa, Global Head of Technology – Offshore and Subsea, LR: ‘Many offshore wind assets are designed to a standard fatigue factor, but real-world conditions often expose critical vulnerabilities. Our findings show that using reliability-based methods allows operators to focus inspections where the risks are greatest. By integrating sophisticated models and real-world inspection data, we can extend asset life, reduce costs and, most importantly, maintain safety.’
‘By focusing on the areas with the greatest risk, we cannot only help to manage fatigue-related issues more effectively — we’re also enabling developers and operators to make better-informed decisions that optimise asset life and performance,’ adds Manuel Ruiz, Head of Offshore Renewable Solutions.
The full case study can be downloaded here.
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