Advances in FMC based ultrasonic imaging IWEX for weld inspection in offshore wind structures

Scale effects in offshore wind power and welding challenges in ultra‑thick steels

There is a scale effect in wind farms that requires the use of increasingly larger wind turbines. This implies the manufacture of increasingly larger marine structures, resulting in a real challenge for the manufacturing industry to design, build and maintain steel colossi. Never before has the industry faced pieces of such size and weight.

The welding of these thermomechanical rolled steels more than 100 mm thick is a real challenge for today's industry, both in terms of the time required and the quality of the welds. Welding process with enormous heat inputs and narrow welding bevels are increasingly being used, which can result in weld defects.

Today, weld certification to guarantee the absence of weld defects requires inspection processes based on NDT (Non-Destructive Testing), which require time, highly specialized personnel, and the technologies. The international codes for quality welds require volumetric NDT such as radiographic examination or Ultrasonic Testing based in pulse-echo; Radiographic examination is not an option because it is an exclusive process for Radiation Safety requirements and does not allow for parallel work in workshops in 24/7 shift. The MUT (Manual Ultrasonic Testing) is an inefficient process with poor detectability and many errors can occur. The latest standards in quality welds requires recordable ultrasound technologies based on ToFD (Time of Flight Diffraction) and PAUT (Phased Array), which have been well-known in the industry for more than 20 years.

The objective of this paper is to introduce and presents the background and principles of IWEX(Inverse WavefieldExtrapolation) ultrasonic imaging that was applied during the manufacturing of Monopiles in a large Offshore Wind Project in comparison with other NDT Ultrasound Technologies (MUT, UTPA, ToFD and Tandem), with interesting result in terms of POD (Probability of detection), Inspection Speed, Evaluation process, false calls or misinterpretations.
 

Experimental methodology: comparative ultrasonic inspection of monopile welds

The method used in this project was to work on ultrasonic inspection of real monopile welds during the manufacturing process. Volumetric ultrasonic inspections were performed using different techniques (IWEX, ToFD, PAUT, Tandem, MUT), which allowed us to determine the inspection speeds, including the calibration, recording, and evaluation stages and the different PODs and the effect of the technologies on false calls.

These aspects are key to achieving profitability during a monopile fabrication project, since a dilatory UT inspection process, whether in recording or evaluation, or poor defect detection or an unreliable defect sizing system, can derail the fabrication project or incur penalties.

To carry out this study, certified personnel, calibrated equipment and inspection procedures were used in accordance with recognized codes.
 

Comparative results of ultrasonic NDT techniques for thick weld inspection

• MUT, are very slow, highly dependent on inspection personnel. Traceability of results is difficult due to the lack of records. The POD is low, and sizing is imprecise.
• ToFD, with good data acquisition speeds and a medium-high POD, presents the problem of interpreting results, as there are potential for false calls and misinterpretation of signals. Defect positioning must be complemented with MUT.
• PAUT, with its medium scanning speed and complex calibrations, has a high average POD, but requires multiple scans and different focal laws to cover the weld volume.
• IWEX, with its high average scanning speeds, requires fewer scans than previous techniques, thus reducing data recording time. It has a high POD and is easy to interpret, resulting in fewer false calls.
   

Conclusions on advanced ultrasonic imaging for monopile weld certification

The welding of thermomechanical rolled steels more than 100 mm thick is a real challenge for today's industry, both in terms of the time required and the quality of the welds. Welding process with enormous heat inputs and narrow welding bevels are increasingly being used, which can result in weld defects.

Selecting the most appropriate ultrasonic inspection technique for certifying the absence of welding defects is key to success during a monopile manufacturing project.

Ultrasonic inspections of metal welds are undergoing a revolution thanks to imaging techniques as IWEX, which allow for the detection of smaller welding defects, and for their better measurement and positioning within the weld volume. Furthermore, inspections with IWEX will take less time than before.
 

Learning objectives: optimizing NDT strategies in offshore monopile manufacturing

Monopile manufacturing projects are major challenges for the industry: the large dimensions and weight of the parts, rolling thick plates, welding, and painting. An often overlooked but key aspect is weld certification. The weld inspection process is time consuming and need high qualify personnel and high technology resources, sometimes is not available and becomes a bottleneck into production process.

Specialized NDT resources are required in monopile manufacturing organizations in order to select the most effective time and defect-detecting ultrasonic techniques to ensuring the profitability of a monopile manufacturing project.

This study provides clarity and important information for deciding on the most suitable ultrasonic techniques to ensure success in monopile manufacturing projects and avoid failures and costly penalties of time and money.