Concrete fatigue experiment for sensor prototyping and validation of industrial SHM trials

In this paper, preliminary results from a concrete fatigue experiment using a custom built machine are demonstrated. A pre-cracked concrete member is instrumented with bespoke metallic-bonded and epoxy-bonded fiber Bragg grating (FBG) displacement sensors, retrofitted over the crack. Fatigue loading is applied to the beam, with cycle magnitudes replicating results from a previous industrial trial concerning structural health monitoring (SHM) of a wind turbine foundation. Results are compared to an FEM model for verification. The new metallic-bonded crack displacement sensor design is compared in performance with the traditional epoxy-bonded design. Both sensors were sufficiently resilient under dynamic loading to successfully undergo 105 cycle fatigue test. The sensors display a linear relationship with respect to one another; however, from the initial thermal characterization of the devices between 20 and 65 °C, the epoxy-bonded sensor exhibited considerable drift with every subsequent temperature cycle while the metallic-bonded construction was stable within the experimental error. The set up can be used over a long term to validate in situ results from distributed SHM sensors and for initial testing of sensors and data analytics strategies prior to any future field installations

Design and demonstration of a low-cost small-scale fatigue testing machine for multi-purpose testing of materials, sensors and structures

Mechanical fatigue testing of materials, prototype structures or sensors is often required prior to the deployment of these components in industrial applications. Such fatigue tests often requires the continuous long-term use of an appropriate loading machine, which can incur significant costs when outsourcing and can limit customization options. In this work, design and implementation of a low-cost small-scale machine capable of customizable fatigue experimentation on structural beams is presented. The design is thoroughly modeled using FEM software and compared to a sample experiment, demonstrating long-term endurance of the machine. This approach to fatigue testing is then evaluated against the typical cost of outsourcing in the UK, providing evidence that for long-term testing of at least 373 hours, a custom machine is the preferred option

Deterioration of cracks in onshore wind turbine foundations

Cracks can occur in reinforced-concrete onshore wind turbine foundations due to factors such as the use of substandard concrete mix, mistakes in foundation design or multi-stage concrete pouring under challenging weather conditions. Cracks are routinely identified via above ground inspections and follow-on examination of excavated underground surfaces and are repaired, for example with resin injection and grouting. Their impact on the structure or the efficacy of the repair are often unknown as crack degradation during normal operating conditions is unexplored. In this work, sub-surface cracks in an onshore wind turbine foundation have been instrumented with fibre-optic based strain sensors in an attempt to determine severity and magnitude of deterioration over time. Here we determine cracks monitored show a small magnitude of deterioration over the initial 9-month period after sensor installation, suggesting that repair is not required. We propose a novel methodology for the classification of the types of deterioration evident in cracks as “reactive”, “permanent” and “behavioural”, and demonstrate methods to extract these types of deterioration. Such methods will continually be developed over time as further knowledge of crack behaviour is gained to determine appropriate limits and identify the optimal time to repair

Life extension for wind turbine structures and foundations

This thesis explores economic and technical lifetime extension considerations of wind turbine generators. The research unveils that onshore support structures will likely have suffcient fatigue reserves left, beyond the initial design life, to support an extended operation. However, suitability is strongly dependent on the difference between the designand encountered load profile, asset maintenance activities, operational knowledge, and the policy environment. A lifetime extension decision model is developed and input parameters are scrutinised revealing the feasibility to replace the entire drive train of a wind turbine and yet, be profitable when exposed to a non-subsidised environment. The designed decision modelis further applied based on operational data of two wind farms in the UK to derive anasset specific lifetime extension strategy. In addition, a methodology and field trial is presented to monitor operational foundationstresses based on optical sensor networks aimed at reducing conservative designassumptions, enabling the reuse of foundations for greater rated turbines as well as tosupport the lifetime extension decision-making. Furthermore, this thesis provides guidance on how to evaluate and obtain the strategic lifetime extension decision-making at an early stage (year 10-14) by means of a fatigue analysis of tower strain measurements. Fatigue tower findings in conjunction with operational data of an asset are subsequently assessed to define the turbine's/wind farm'sunique economic lifetime extension boundaries.This thesis explores economic and technical lifetime extension considerations of wind turbine generators. The research unveils that onshore support structures will likely have suffcient fatigue reserves left, beyond the initial design life, to support an extended operation. However, suitability is strongly dependent on the difference between the designand encountered load profile, asset maintenance activities, operational knowledge, and the policy environment. A lifetime extension decision model is developed and input parameters are scrutinised revealing the feasibility to replace the entire drive train of a wind turbine and yet, be profitable when exposed to a non-subsidised environment. The designed decision modelis further applied based on operational data of two wind farms in the UK to derive anasset specific lifetime extension strategy. In addition, a methodology and field trial is presented to monitor operational foundationstresses based on optical sensor networks aimed at reducing conservative designassumptions, enabling the reuse of foundations for greater rated turbines as well as tosupport the lifetime extension decision-making. Furthermore, this thesis provides guidance on how to evaluate and obtain the strategic lifetime extension decision-making at an early stage (year 10-14) by means of a fatigue analysis of tower strain measurements. Fatigue tower findings in conjunction with operational data of an asset are subsequently assessed to define the turbine's/wind farm'sunique economic lifetime extension boundaries

Field Demonstration of Real-Time Wind Turbine Foundation Strain Monitoring

Onshore wind turbine foundations are generally over-engineered as their internal stress states are challenging to directly monitor during operation. While there are industry drivers to shift towards more economical foundation designs, making this transition safely will require new monitoring techniques, so that the uncertainties around structural health can be reduced. This paper presents the initial results of a real-time strain monitoring campaign for an operating wind turbine foundation. Selected reinforcement bars were instrumented with metal packaged optical fibre strain sensors prior to concrete casting. In this paper, we outline the sensors’ design, characterisation and installation, and present 67 days of operational data. During this time, measured foundation strains did not exceed 95 μ ϵ , and showed a strong correlation with both measured tower displacements and the results of a foundation finite element model. The work demonstrates that real-time foundation monitoring is not only achievable, but that it has the potential to help operators and policymakers quantify the conservatism of their existing design codes

Wireless concrete strength monitoring of wind turbine foundations

Wind turbine foundations are typically cast in place, leaving the concrete to mature under environmental conditions that vary in time and space. As a result, there is uncertainty around the concrete’s initial performance, and this can encourage both costly over-design and inaccurate prognoses of structural health. Here, we demonstrate the field application of a dense, wireless thermocouple network to monitor the strength development of an onshore, reinforced-concrete wind turbine foundation. Up-to-date methods in fly ash concrete strength and maturity modelling are used to estimate the distribution and evolution of foundation strength over 29 days of curing. Strength estimates are verified by core samples, extracted from the foundation base. In addition, an artificial neural network, trained using temperature data, is exploited to estimate distributed concrete strengths using only sparse thermocouple data. Our techniques provide a practical alternative to computational models, and could assist site operators in making more informed decisions about foundation design, construction, operation and maintenance

Preliminary characterization of metal-packaged fiber Bragg gratings under fatigue loading

This paper presents preliminary results for metal-packaged fiber Bragg grating strain and temperature sensors designed specifically for structural health monitoring in civil engineering applications. The laboratory experiments show that the in-house manufactured metal-packaged sensors are sufficiently resilient, under dynamic loading, to successfully undergo a million cycle fatigue test without adverse deterioration in performance. In addition, from the thermal characterization of the devices, a conclusion can be drawn that the metal-packaged sensors offer superior performance over sensors assembled using epoxy bonding. These early results are very promising, inspiring confidence in the adopted methodologies, and giving the mandate to proceed with more detailed laboratory testing to evaluate reliability and lifetime of the transducers in the future work

Lifetime extension of onshore wind turbines : a review covering Germany, Spain, Denmark, and the UK

A significant number of wind turbines will reach the end of their planned service life in the near future. A decision on lifetime extension is complex and experiences to date are limited. This review presents the current state-of-the-art for lifetime extension of onshore wind turbines in Germany, Spain, Denmark, and the UK. Information was gathered through a literature review and 24 guidelinebased interviews with key market players. Technical, economic and legal aspects are discussed. Results indicate that end-of-life solutions will develop a significant market over the next five years. The application of updated load simulation and inspections for technical lifetime extension assessment differs between countries. A major concern is the uncertainty about future electricity spot market prices, which determine if lifetime extension is economically feasible

Crack monitoring of operational wind turbine foundations

The degradation of onshore, reinforced-concrete wind turbine foundations is usually assessed via above-ground inspections, or through lengthy excavation campaigns that suspend wind power generation. Foundation cracks can and do occur below ground level, and while sustained measurements of crack behaviour could be used to quantify the risk of water ingress and reinforcement corrosion, these cracks have not yet been monitored during turbine operation. Here, we outline the design, fabrication and field installation of subterranean fibre-optic sensors for monitoring the opening and lateral displacements of foundation cracks during wind turbine operation. We detail methods for in situ sensor characterisation, verify sensor responses against theoretical tower strains derived from wind speed data, and then show that measured crack displacements correlate with monitored tower strains. Our results show that foundation crack opening displacements respond linearly to tower strain and do not change by more than ±5 μm. Lateral crack displacements were found to be negligible. We anticipate that the work outlined here will provide a starting point for real-time, long-term and dynamic analyses of crack displacements in future. Our findings could furthermore inform the development of cost-effective monitoring systems for ageing wind turbine foundations

Field Demonstration of Real-Time Wind Turbine Foundation Strain Monitoring

Onshore wind turbine foundations are generally over-engineered as their internal stress states are challenging to directly monitor during operation. While there are industry drivers to shift towards more economical foundation designs, making this transition safely will require new monitoring techniques, so that the uncertainties around structural health can be reduced. This paper presents the initial results of a real-time strain monitoring campaign for an operating wind turbine foundation. Selected reinforcement bars were instrumented with metal packaged optical fibre strain sensors prior to concrete casting. In this paper, we outline the sensors’ design, characterisation and installation, and present 67 days of operational data. During this time, measured foundation strains did not exceed 95 μ ϵ , and showed a strong correlation with both measured tower displacements and the results of a foundation finite element model. The work demonstrates that real-time foundation monitoring is not only achievable, but that it has the potential to help operators and policymakers quantify the conservatism of their existing design codes