References of "Mai, Anh Quang"
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See detailRisk based inspection and maintenance planning of miter gates
Dang Van Thuong, ULiege; Morato Dominguez, Pablo Gabriel ULiege; Mai, Anh Quang ULiege et al

Conference (2019, September 30)

Navigation locks play a significant role in inland waterway infrastructures. They allow vessels to transfer from one water level to another. Because of its low cost, high transport capacity, fuel ... [more ▼]

Navigation locks play a significant role in inland waterway infrastructures. They allow vessels to transfer from one water level to another. Because of its low cost, high transport capacity, fuel efficiency and environmental friendliness, it is expected that inland navigation transportation will grow and that locks will become larger and of more importance in the future in countries like Vietnam. Miter gates are widely used in navigation locks thanks to its economical and aesthetic aspects. Due to variations of water levels during operation, miter gates are subjected to fatigue damage caused by cyclic loading. The nature of fatigue damage is complicated because fatigue cracks can develop from various unexpected initial defects coming from the manufacturing process. Inspection and repair of developed cracks are costly because the gate needs to be put out of service. The risk-based approach for inspection and maintenance planning is based on pre-posterior decision analysis concerning the two basic decision rules about inspection and repair. This paper discusses the methods to update the failure probability of welded joints considering crack inspection data by using Dynamic Bayesian Network. This method is useful for effective computation deterioration modeling, and failure probabilities can be updated rapidly. Risk-based inspection planning combines the updated probabilities with specific costs of failure, inspection, repair and annual discounting rate in order to estimate the operation and maintenance costs, which is a substantial part of the total expected costs during the service life. The methodology is then applied to a specific case where the operation, maintenance and failure costs of a miter gate are estimated for two sets of heuristic decision rules with respect to the time of inspection: “inspections performed at regular time intervals” and “inspection performed when a certain annual probability failure threshold is reached”. It is assumed that any crack size which is equal or larger than the detectable size (detected during inspection) is repaired. The result shows that the heuristic method based on an “annual probability failure threshold” leads to a lower operation, maintenance and failure cost for the specific case study. [less ▲]

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See detailPrediction of remaining fatigue life of welded joints in wind turbine support structures considering strain measurement and a joint distribution of oceanographic data
Mai, Anh Quang ULiege; Morato Dominguez, Pablo Gabriel ULiege; Rigo, Philippe ULiege

in Marine Structures (2019), 66

Reassessing the remaining fatigue life of the wind turbine support structures becomes more and more crucial for operation, maintenance, and life extension when they are reaching the end of their design ... [more ▼]

Reassessing the remaining fatigue life of the wind turbine support structures becomes more and more crucial for operation, maintenance, and life extension when they are reaching the end of their design service life. By using measured oceanographic and strain data, each year, remaining fatigue life can be updated to adapt the operation to real loading conditions. Previous works have not put attention to address the complexity of offshore loading combinations and as-constructed state of the structure in estimating structural responses for fatigue behaviour to stochastically predict the remaining fatigue life. The present paper links the oceanographic data to fatigue damage by using measured strain, and uses the Bayesian approach to update the joint distribution of the oceanographic data. Consequently, the failure probability of the support structure can be updated and so the predicted fatigue life. The year-to-year variation of the 10-min mean wind speed, the unrepresentativeness of measured strain, the measurement uncertainty, and corrosion are considered together with uncertainties in Miner′s rule and S–N curves. The present research shows that the real oceanographic data can be used to adjust the predicted remaining fatigue life and eventually give decision support for the wind turbine operation. [less ▲]

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See detailPOMDP based Maintenance Optimization of Offshore Wind Substructures including Monitoring
Morato Dominguez, Pablo Gabriel ULiege; Nielsen, Jannie S.; Mai, Anh Quang ULiege et al

in Proceedings of the 13th International Conference on Applications of Statistics and Probability in Civil Engineering (ICASP13) (2019, May 27)

Sequential decision making under uncertainty is a complex task limited normally by computational requirements. A novel methodology is proposed in this paper to identify the optimal maintenance strategy of ... [more ▼]

Sequential decision making under uncertainty is a complex task limited normally by computational requirements. A novel methodology is proposed in this paper to identify the optimal maintenance strategy of a structural component by using a point-based Partially Observable Markov Decision Process (POMDP). The framework integrates a dynamic bayesian network to track the deterioration over time with a POMDP model for the generation of a dynamic policy. The methodology is applied to an example quantifying whether a monitoring scheme is cost effective. This complex decision problem comprised of 200 damage states is solved accurately within 60 seconds of computational time. [less ▲]

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See detailReliability assessment of aging miter gates in the presence of corrosion and fatigue
Dang Van Thuong, ULiege; Mai, Anh Quang ULiege; Morato Dominguez, Pablo Gabriel ULiege et al

Poster (2019, February 18)

Aging Hydraulic steel structures, especially lock gates suffer from structural deterioration with fatigue and corrosion, resulting in a reduction of their resistance. A prolonged exposure to these hazards ... [more ▼]

Aging Hydraulic steel structures, especially lock gates suffer from structural deterioration with fatigue and corrosion, resulting in a reduction of their resistance. A prolonged exposure to these hazards results in a reduction of structural resistance which can lead to failure. This research presents an approach to compute the reliability of aging miter gates in the presence of corrosion and fatigue. First order reliability index method is used to calculate reliability of the structure. An example of an aging miter gate is used for demonstration. [less ▲]

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See detailPoint-based POMDP Risk Based Inspection of Offshore Wind Substructures
Morato Dominguez, Pablo Gabriel ULiege; Nielsen, Jannie Sonderkær; Mai, Anh Quang ULiege et al

in Caspeele, Robby; Taerwe, Luck; Frangopol, Dan M. (Eds.) Life Cycle Analysis and Assessment in Civil Engineering: Towards an Integrated Vision (2019)

This article presents a novel methodology to select the optimal maintenance strategy of an offshore wind structural component, providing a flexible and reliable support to decision-making and balancing ... [more ▼]

This article presents a novel methodology to select the optimal maintenance strategy of an offshore wind structural component, providing a flexible and reliable support to decision-making and balancing inspection, repair and failure costs. The procedure to create a “Point-Based” Partially Observable Markov Decision Process (POMDP) is described and a calibrated fracture mechanics model is introduced to assess the deterioration of the structure. The methodology is then tested for a tubular joint through a 60-states POMDP, obtaining the optimal maintenance policy in low computational time and in good agreement with common Risk-Based Inspection (RBI) methods. [less ▲]

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See detailUpdating Failure Probability of a Welded Joint Considering Monitoring and Inspection - For Offshore Wind Turbine Substructures
Mai, Anh Quang ULiege

Doctoral thesis (2018)

Reliability assessment of existing offshore wind turbine (OWT) support structures taking advantage of historical data from inspection and monitoring is crucial for the optimisation of maintenance and life ... [more ▼]

Reliability assessment of existing offshore wind turbine (OWT) support structures taking advantage of historical data from inspection and monitoring is crucial for the optimisation of maintenance and life extension. In this thesis, the crack inspection data and the monitoring data are separately considered for updating failure probability of welded joints. Concerning crack inspection data, this thesis aims at finding advantages of using the fatigue assessment diagram (FAD) in the failure criteria for failure probability estimation and updating. The crack inspection results (no detection or detected) and possible immediate interventions (repair normally or perfectly) are considered. Failure probabilities are calculated using the FAD and then compared with those obtained from the usual critical crack size criteria. The simulation-based approach is used to calculate and update the failure probability. Crack depth and length are simulated simultaneously. The crack propagations are calculated using a bi- linear Paris’ law with stress-range value varying over time. Uncertainties come from the crack growth parameters, initial crack sizes, fracture toughness, yield and ultimate strengths, FAD formula, stress intensity factor, stress-range values, and the detectable crack size. By combining the fracture toughness with the crack size in the failure criteria, the results show that the estimated failure probability of the welded joint is significantly increased in comparison to the case where only critical crack size is considered in the LSF. In comparison with the failure criteria which includes both the critical crack size and fracture toughness, the FAD approach gives similar reliability results when the applied peak tensile stress is small. However, when the applied peak ten- sile stress is high (the ratio between applied peak tensile stress and yield strength is more than 65%), the FAD approach predicts higher failure probability values. The uncertainty in FAD does not significantly affect the predicted failure probability of the joint as compared to the uncertainties in the ultimate and yield strength. This is because the latter affects directly the cut-off location of the FAD curve where the plastic failure is defined. The FAD approach can be used to update failure probability considering crack inspections and intervention actions. The calculation shows that reliability of the welded joint increases when no crack is detected, or when a crack is detected and repaired. In comparison with a perfect repair, a normal repair assumption significantly reduces the reliability of the joint. With regard to the monitoring data, the research questions how to effectively incorporate the measured strain and the oceanographic data (wind, wave) for updating failure probability of a welded joint in fatigue failure mode. To answer the question, the monitoring data is used to update the characteristics of a random variable in the LSF. Consequently, the updated random variable is then used in a reliability analysis to obtain the updated failure probability. The limit state function (LSF) is based on the Miner’s rule and solved using the first order reliability method (FORM). The random variable used to update failure probability is the joint distribution of wind speed, wave height and wave period. The monitoring data consists of strain, 10-minute mean wind speed, significant wave height, and mean wave period. Fatigue damage is summed up from all load combinations, i.e. from all the discretized components of the joint distribution of wind and wave. The measured strain data is used to calculate fatigue damage in each load combination. The probability of each load combination is calculated us- ing its joint distribution, which in turn can be updated using monitoring data. The 10-minute mean wind speed is assumed to follow a Weibull distribution and can be updated using Bayesian approach. Assuming that the scale parameter is a nor- mally distributed random variable with unknown mean and standard deviation, the predictive distribution of this random variable becomes a student’s t-distribution. The proposed methodology has been applied to a monopile support structure of a wind farm in Belgium. The measured strain is used to find the potential hot-spot location. Stresses are derived at the hot-spot location for fatigue analyses. It is assumed that stress-ranges in each load combination follow a Weibull distribution. The stress-range distribution parameters are found by performing least squares fitting method on the fatigue damage. The results show that the Weibull distribution is generally not very good for fitting stress-ranges in each wind-speed bin for the considered data. However, the inte- grated fatigue damage for the considered load combination is quite accurate since it is the objective of the fitting procedure. The main influence on the remaining fatigue life is the magnitude of stress-ranges at the hot-spot. So the stress conentration factor, the interpolating factor (for example to obtain stresses at under water locations), the correction factor for corrosion effects (if any), and their uncer- tainties should be estimated with great care. The measured 10-minute wind speed data has a significant effect in adjusting the predicted probability of failure and eventually the remaining fatigue life. The duration of strain measurement should be long enough to be combined with oceanographic data. Longer strain measurement duration, greater number of parameters in the oceanographic data can be considered for the failure probability updating. [less ▲]

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See detailUsing SCADA in updating failure probability of a monopile support structure
Mai, Anh Quang ULiege; Weijtjens, Wout; Christof, Devriendt et al

in EAWE PhD Seminar 2018 Proceedings (2018, September 18)

Assessment of existing offshore wind turbine support structures is crucial for a condition-based inspection planning and life extension decision. This paper aims at utilizing SCADA as new information to ... [more ▼]

Assessment of existing offshore wind turbine support structures is crucial for a condition-based inspection planning and life extension decision. This paper aims at utilizing SCADA as new information to update failure probability of monopile support structures in fatigue failure mode. The SCADA was collected from a Belgian offshore wind farm. It is assumed that fatigue loading of the monopile support structure comes mainly from wind action. One-year data of measured strain is used together with the concurrently measured 10-minute wind speed to built the limit state function. Fatigue-critical strain history is derived from the data at three measuring locations on the same circumference. The 10-minute mean wind speed is considered as a random variable and used to calculate the fatigue damage. Fatigue damage is calculated using Miner's rule. The year-to-year variation of the 10-minute mean wind speed, the unrepresentativeness of the measured strain, and the measurement uncertainty are considered together with uncertainties in Miner's rule and the S-N curve. The Bayesian approach is used to update the 10-minute mean wind speed distribution considering measurement data. Consequently, the failure probability of the support structure can be updated. The results show that after three years of measurement, there is no significant differences in the calculated failure probability between cases where the updated 10-minute mean wind speed distribution and the design distribution are used. The load factor applied to the measured stress (e.g. stress concentration factor, corrosion effects, or interpolation factor for unmeasured locations) significantly affects the calculated failure probabilities. In conclusion, the wind measurement data can be used to predict failure probability of the monopile support structure given that a limited concurrently measured strain data is available. Further research is needed to combine the SCADA with inspection data (crack, corrosion) for optimising the inspection plan or a life extension purpose. [less ▲]

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See detailUpdating the failure probability of miter gates based on observation of water levels
Dang Van Thuong, ULiege; Mai, Anh Quang ULiege; Morato Dominguez, Pablo Gabriel ULiege et al

in Structural Integrity (2018, September 05)

Hydraulic steel structures, especially lock gates play a significant role in keeping navigation traffic uninterrupted. After a few decades of operation, many of the welded joints may suffer various ... [more ▼]

Hydraulic steel structures, especially lock gates play a significant role in keeping navigation traffic uninterrupted. After a few decades of operation, many of the welded joints may suffer various degrees of deterioration, primarily due to fatigue. To economically combining crack inspection with a scheduled maintenance of the movable parts of the gate, it is valuable to predict inspection time of the welded joints using the historical operations of the gate, i.e. the variation of water levels. Updating failure probability of welded joint is mature in the offshore industry, but it is rarely applied for inland navigation lock gates where the contribution to fatigue failure comes from the variation of water pressures during operation of the lock gates. The scope of this paper is to predict the inspection time of a welded joint using the observed water levels from the operational history. The updating of the failure probability is done for three inspection techniques, considering annual probability and repair decisions. The results show the effects of critical annual probability and the probability of detections (PODs) on the update failure probability for a welded joint. [less ▲]

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See detailFatigue Assessment Diagram in Updating failure probability
Mai, Anh Quang ULiege

Poster (2016, December 05)

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See detailOMAE2016 Presentation:Updating failure probability of a welded joint in OWT substructures
Mai, Anh Quang ULiege

Conference (2016, June 22)

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See detailUpdating Failure Probability Of A Welded Joint In Offshore Wind Turbine Substructures
Mai, Anh Quang ULiege; Sørensen, John D.; Rigo, Philippe ULiege

in The 35th International Conference on Ocean, Offshore and Arctic Engineering (OMAE) (2016, June)

For offshore wind turbines, the support structure contributes with a significant part to the Levelized Cost Of Energy (LCOE). LCOE is the total cost to build and operate a whole offshore wind turbine ... [more ▼]

For offshore wind turbines, the support structure contributes with a significant part to the Levelized Cost Of Energy (LCOE). LCOE is the total cost to build and operate a whole offshore wind turbine structure over its lifetime divided by the total energy output of the wind turbine over that lifetime. That total expected cost of operation & maintenance may be lowered by application of reliability- and risk-based maintenance strategies and updating of the reliability based on e.g. inspections performed during the design lifetime. Updating the reliability (or alternatively the failure probability) of a welded joint can theoretically be done using Bayesian updating. However, for tubular joints in offshore wind turbine substructure when considering a two dimensional crack growth and a failure criterion combined brittle fracture and material strength, the updating is quite complex due to the wind turbine loading obtained during operation. This paper considers how the reliability (or the probability of failure) of welded steel details can be updated in the case where the fatigue failure is modelled by a fracture mechanics approach and a Failure Assessment Diagram (FAD) is used to define a limit state equation. Besides, it is shown how the probability of failure of fatigue critical joints can be updated for various inspection scenarios. A two dimensional bi-linear model is considered for the crack growth. Calculation of the crack depth and the crack length are coupled. The stress intensity factor is calculated following the sophisticated procedure in BS 7910:2005. The initial crack size, the yield and ultimate strengths of steel, the fracture toughness, the stress intensity factor, and the stress-ranges are considered as uncertain and modelled by random variables. The probability of detection is used to account for uncertainty in crack inspections. By using Monte Carlo simulations, stress-range histories are generated randomly and together with the calculated stress intensity factors used to check the limit state condition using the FAD approach. The probability of failure updating is done for three inspection scenarios: No crack detected; Crack detected and repaired; Crack detected but not repaired. The paper presents illustrative results for offshore wind applications for the three scenarios, including comparisons of application of the FAD approach for modelling the limit state equation with a ‘conventional’ approach where it is assumed that failure happens when crack-depth reaches the plate thickness or the stress intensity factor is equal to or larger than the fracture toughness of the material. Finally, it is discussed how application of the updating procedure can be used for inspection planning for offshore wind turbine support structures, and thus also for reducing the required safety factors at the design stage. [less ▲]

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See detailFatigue Reliability of joint connections in support structure of offshore wind turbines
Mai, Anh Quang ULiege

Master of advanced studies dissertation (2014)

This study is my initial attempt to investigate the evolution of fatigue reliability of welded joints in offshore wind turbine support structures. This information is crucial to update the inspected data ... [more ▼]

This study is my initial attempt to investigate the evolution of fatigue reliability of welded joints in offshore wind turbine support structures. This information is crucial to update the inspected data (crack sizes, probability of crack detection) to make the plan for the next inspection and maintenance of existing offshore structures in general. The thesis is limited to tracing back the fatigue reliability of welded joints from their design information. The later work for inspection planning will be continued in the framework of my Ph.D thesis. On the basis of the results of this research, it can be concluded that for the fatigue reliability problem, FORM and SORM gives results with high accuracy compared to MCS and that the design point found by FORM/SORM are really the global minimal reliability index. The assumption in dealing with cumulated damage shows a good agreement with MCS. This is the foundation for using a simplified LSF in reliability calculation of accumulative fatigue problem. [less ▲]

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See detailDesign Monopile Foundation of Offshore Wind Turbines
Mai, Anh Quang ULiege

Master's dissertation (2012)

Design optimization is crucial to the development of the offshore wind turbine industry. This time consuming process is better to be done with a number of input parameters that is as short as possible ... [more ▼]

Design optimization is crucial to the development of the offshore wind turbine industry. This time consuming process is better to be done with a number of input parameters that is as short as possible. Whether the foundation pile part can be neglected in the design optimization process of an offshore wind turbine structure is a question need to be answer. In order to see the importance of the presence of the foundation pile in dynamic behavior of the whole structure, dimensions of the foundation pile must be determined basing on requirements in ultimate limit state and serviceability limit state in current design standards. Afterward, the differences in dynamic behavior between a fixed- at-seabed tower model and a tower with foundation model must be observed. Beam nonlinear Winkler Foundation model in addition to gapping and non-gapping behavior in pile-soil interface were used to model the foundation. With the chosen offshore wind turbine project of 7MW and 115m high to seabed, a foundation pile with a penetration length of 26m, diameter of 6m and wall thickness of 8cm had been found. The dynamic behavior of the two models showed that it was not on the safe side if the foundation was neglected in design optimization process. And that the internal damping of the soil was the most important factor in behavior of the structure. These results will be useful for reconsidering parameters in design optimization process of monopile offshore wind turbines as well as choosing suitable methods to solve dynamic equations in the optimization procedure. [less ▲]

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