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Field testing demonstrates the ability to reduce the maximum reverse torque and rate of torque increase. During normal operation, rollers in the bearings are aligned and rolling at the expected or design speed while they are in the load zone. As they leave the load zone, the rollers slow down and can misalign within the tolerance of the cages. As they approach the load zone again, they are gradually brought back into alignment and smoothly accelerate to support the load.
In a rapid load zone reversal, the unloaded rollers opposite the normal load zone are suddenly loaded while in this misaligned state. This rapid loading increases the load concentration of the rollers in the middle of the inner race. Although load reversals are common to many types of equipment, torque reversals of the magnitude and rapidity common in wind turbines is rare in other equipment.
And helical gearing commonly used in wind-turbine gearboxes adds additional subsurface stress and strain. With helical gearing, every cylindrical bearing will see high axial loads simultaneous with the radial loads impacting misaligned rollers. Cylindrical rollers may slip axially and damage the raceway surface, further decreasing bearing life through pitting and spalling.
Sliding can also result in scuffing the raceway, causing microscopic cracks that let subsurface entry of water or aggressive oil additives to initiate hydrogen induced WEA. If the radial impact load is so high that the roller breaks through the oil film, it can create mixed friction sufficient to prevent slipping axially. Axial subsurface stresses will be added to the Hertzian stresses from the radial impact, magnifying the peak stresses.
The strain rate would be magnified as well. The potential to exceed a threshold where WEA microstructural alterations could form, explains why WEA damage is common in wind-turbine gearboxes and rare in most other bearing applications. Tapered-roller bearings can also be damaged by the affects of load reversals. Those used on high and intermediate-speed shafts typically are not preloaded due to thermal constraints, and can see damaging axial movement and impact loads.
High-resolution torque reversal recordings of wind turbines reflect the rate of change in torque. The steeper slope indicates significantly higher strain rates. Axial cracking issues in bearings were not a prominent failure mode until larger megawatt and multi-megawatt class wind turbines were put in service. It was not a common failure mode of earlier, smaller turbine models where the failure mode was more commonly bearing surface deterioration from pitting and scuffing.
The issue of axial cracking grew along with turbine size. To understand why, it is important to zoom in on the field data to look at the first quarter of a second of the worst torque reversals that were recorded on three different size wind turbines. The graph Comparison of torque reversals recorded on three different sizes of wind turbines compares the relative torque magnitude and the rate of torque change during rapid reversals. High-resolution torque monitoring equipment recorded torque reversals that approximated the turbine rated torque at the high-speed shaft on 0.
The torsional natural frequencies of the turbines during aero braking were all similar with oscillation periods about 0. Higher torque loads of the larger turbines can be accommodated in the sizing of the bearings but the oil film thickness will not increase. Any roller misalignment easily causes higher local-contact stress during a torque reversal. The difference in the rate of increase of the reverse torque during the torque reversal may be an even bigger factor, as it directly relates to the strain rate in the raceway as the rollers impact.
Both the higher stress and the faster strain rate increase the instantaneous plastic deformation energy. In a typical power curve, pitchable blades let modern wind turbines perform close to an ideal power curve. Blade orientation and control lets turbines maximize the energy generation. There is a threshold in plastic deformation energy where the instantaneous heat created causes the microstructural alteration of the base material.
Could the increasing strain rate of larger turbines be exceeding that threshold? During rapid load zone reversals, could the rapidly increasing load on the rollers create plastic deformation fast enough to transform a microscopic sliver of the bearing steel to a super-hard ferrite WEA inclusion?
Certainly all the elements for WEA microstructural alterations are simultaneously present for an instant. The torque reversal takes up the gear backlash and rapidly impacts the idling rollers on the unloaded side of the bearing. The misaligned rollers can make the stress concentration and the plastic deformation under the rollers worse and initiate mixed friction contact.
Simultaneous reversal of the axial load due to the helical gearing causes surface traction and additional stress at the inner-raceway subsurface. The resulting high-strain rates and plastic deformation can explain the creation of the WEA microstructural transformation.
Because the WEA sliver is perfectly placed under the raceway surface, one microscopic sliver would be enough to initiate the WEA damage that would result in an axially cracked, failed bearing. All it takes is one moment to exceed the combination of high load and strain-rate threshold.
That event could come from one severe e-stop, a combination of an e-stop with a wind gust, high-wind shutdown, control malfunction, or sensor failure. Every doubling of the torque magnitude, with the same natural frequency, may effectively quadruple the instantaneous deformation energy that causes WEA damage.
Rapid and severe impact loading of the rollers on the bearing raceway can cause stress-induced WEA damage, significantly shortening the life of bearings and gearboxes. There are three ways to address the problem:. It is a systematic, collaborative approach effective for assessing even the most challenging forms of cracking. This approach does not advocate employing the fanciest or most expensive crack detection technology; instead, it suggests an added-value approach to ensure that objectives and needs are understood.
It includes pre-inspection elements that answer critical questions to allow for optimal system selection. The framework continues as a flexible guide through the entire process from inspection to integrity, ultimately resulting in a proper threat management plan.
It is modular and adaptable, ensuring a common understanding and allowing operators to choose which elements are relevant to them in reaching their objectives and make the decisions needed for safe and efficient pipeline operation. Find out more about our Framework. In-line inspection, of course, is a major part of the pipeline integrity framework.
ROSEN uses the latest generation of crack detection technologies. Using liquid-coupled ultrasonic or dry-coupled electromagnetic acoustic technologies supported by the unwavering magnetic flux leakage technology, RoCD provides reliable crack detection and accurate crack sizing.
The technology also establishes appropriate baseline standards for the successful and effective management of pipeline integrity. Basically, any of the above-mentioned technologies can be modified to detect and size circumferential cracking as well.
Since the determination of the axial load is also important, supporting technologies like axial stress detection and bending strain analysis are recommended. Circumferential Crack Detection Services are used frequently but not as often as axial crack detection services. Collecting the data is half the battle — using it properly and gaining the most valuable information from it is the trick. Operators have to make the inspection data work for them. This includes proper reporting and analysis along with further assessments of the data.
Close collaboration of expert data evaluators and senior integrity engineers with extensive experience in dealing with cracks in pipelines ensures credible results and that efforts are focused on the critical areas. Properly visualizing data in reporting software based on fully analyzed data covering the entire pipeline provides easy asses to the information at hand and is best for reviewing potentially harmful anomalies.
A ranking of possible crack features identified by the inspection system allows the operator to make decisions. It highlights failure risks, recommends further field investigation and identifies pipeline compliance requirements. Once the preliminary in-line inspection results become available, it is directly possible to calculate indicative defect failure pressures to ensure that immediate integrity threats are identified and prioritized.
In combination with a susceptibility analysis, and ILI data evaluator confidence, the results will be used to drive the selection of sites for initial in-field investigations.
The process ultimately prioritizes features that need immediate attention and identifies where further field verification is necessary. For a complete crack assessment, final results from in-line inspections, any testing and in-field work are combined, and the features assessed, to determine the impact on the immediate and future integrity of the pipeline. The future integrity assessment considers fatigue and environmental e.
SCC growth mechanisms where applicable. A summary of all previous activities — including root cause analyses and metallurgical testing — provides a comprehensive list of mitigation and repair actions. Using the chosen assessment method e. The output informs the minimum sizing requirements of the ILI system to locate critical pipeline cracks. It also highlights the impact of conservative assessment inputs, such as an assumed fracture toughness.
This can be extended to a Critical Crack Defect Manual, which defines acceptance curves, response times and pressure reduction requirements to assist in-field decision-making. A root cause analysis can range from diagnosing the types of cracking present on a pipeline to a full investigation of crack-induced failures. Materials, corrosion and welding experts experienced in all conceivable types of cracking are on hand to accurately diagnose the cracking type.
State-of-the-art laboratory testing is available to support investigations when necessary. To ensure a comprehensive integrity management strategy, the consequence of failure must be combined with the threat of cracking to determine overall risk. A key and unique input is our approach to susceptibility modelling: it starts with industry good practices but is continuously modified and developed for each pipeline based on the results of ILI and field verification activities to produce a detailed bespoke model.
It is also possible to develop a completely new model. What Is Axial Axial is a private deal network serving professionals who own, advise, and invest in North American lower middle market companies. The buy-side granularly describes their criteria and interests. The sell-side reviews each buy-side profile, selects who to engage, and can tailor each message and attachment. Reputation Data and Content Marketing Axial shines a light on the lower middle market like never before.
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