Phased array ultrasonic testing of ‎offshore wind bolted flange ‎connections

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Offshore wind will play a key role in the majority of countries' plans to accelerate towards NetZero ‎‎‎and low-carbon energy transitions. Using fasteners (bolts and nuts) as a joining strategy is a ‎‎common ‎practice in various sections of the Offshore Wind Turbines (OWT) which needs regular ‎‎Non-‎Destructive Testing (NDT). For example, the bolted connection between the monopile and the ‎‎‎transition piece is under an immense stress concentration, which can result in loosening and even ‎‎‎failure of the connection. The current procedure to test the bolts involves fixed permanent strain ‎‎‎gauges and/or ultrasonic methods (using single-element transducers) to ensure the specific preload is ‎maintained during the wind turbine operation. In the case of using the strain gauge, the ‎‎challenge is the ‎number of bolts used in turbines in a wind farm, which can result in thousands of ‎‎required strain ‎gauges, and then as a usual practice, only a very limited number of bolts can only be ‎‎monitored.‎
The ultrasonic stress measurement technology is based on the acoustoelasticity theory, the relationship ‎between ‎the acoustic wave velocity and material stress, and the change in the ultrasonic Time of Flight ‎‎(ToF) corresponded ‎to the change in the length of the bolt due to the tightening axial force. This ‎process will then rely on the ‎calibration procedure including measurement of the acoustoelastic ‎coefficient and also the ToF in the free-stress ‎bolt. In the traditional ultrasonic method, the operator ‎uses single-element transducers and assumes ‎any difference between the ToF of a bolt in service and ‎the calibration bolt corresponds to the stress (pre-load) ‎change. While this assumption can be true for a ‎brand-new bolt, similar to what is used in the lab for calibration, it ‎will ignore corrosion, defects, ‎ageing, creep, strain-hardening, fatigue and other material changes during the ‎service life. In this paper, ‎the Phased Array Ultrasonic Testing (PAUT) system will be used instead of the single-element ‎approach. The ‎‎advantage of the PAUT system over the single-element transducer is the possibility of ‎‎(I) ‎defect detection and (II) ‎stress measurement, simultaneously. Combining the defect detection and ‎stress measurement is critical, ‎otherwise, the ultrasonic stress measurement and calibration procedure ‎will be influenced by the possible defects. ‎Using the PAUT and an array instead of a single-element ‎transducer, will allow the detection of the possible ‎defects in some of the specific acoustic paths used ‎for the ToF measurement and then use alternative acoustic ‎paths for the stress measurement. ‎Furthermore, advanced post-processing algorithms like Total Focusing Method ‎‎(TFM) can allow the ‎possibility of focusing ‎on more threads which are usually critical points of concern in the ‎safety-‎critical bolts.‎ It should also be noted that the bolt material used for offshore applications is usually ‎marine-grade high-alloy steel and/or stainless steel which can result in a poor Signal-to-Noise Ratio ‎‎(SNR) corresponding to the austenitic microstructure and large grain noise. In this paper, Phase ‎Coherence Imaging (PCI) was used to improve the SNR value in the PAUT bolt testing. PCI is an ‎amplitude-free synthetic beamforming method, ‎which considers the phase dispersion at each discrete ‎image point. This allows‎ incoherent noises ‎resulting from side lobes, grating lobes, reverberations and ‎grain noise to be reduced.‎
The experimental setup included an M20 bolt tested by a 10 MHz 32-element array (Olympus, ‎USA) ‎and FIToolbox phased array controller (Diagnostic Sonar, UK). A washer-shaped load cell ‎‎(BoltSafe, ‎Netherlands) was used to verify the ultrasonic stress measurement results. The PAUT stress ‎measurement system could successfully detect the ToF variations caused by the bolt’s stress change ‎recorded by the load cell. For defect ‎detection, two Side-Drilled Holes (SDH) were produced between ‎the threads to quantify the scanning image and SNR. The Full Matrix Capturing (FMC) was then ‎imported into a Matlab-programmed TFM and PCI code. The application of these advanced post-‎processing algorithms resulted in a clearer scanning image, improved SNR value and detection of the ‎SDHs with a lower gain.‎
Original languageEnglish
Number of pages2
Publication statusPublished - 27 Jul 2023
Event50th Annual Review of Progress in Quantitative Nondestructive Evaluation - Sheraton Austin Hotel at the Capitol, Austin, TX, United States
Duration: 24 Jul 202327 Jul 2023
Conference number: 50


Conference50th Annual Review of Progress in Quantitative Nondestructive Evaluation
Abbreviated titleQNDE 2023
Country/TerritoryUnited States
CityAustin, TX
Internet address


  • ultrasonic stress measurement
  • phased array ultrasonic testing
  • total focusing method
  • phase coherence imaging
  • offshore wind turbines


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