Fit-for-service assessment of large ductile cast iron components by fracture mechanics

Siemens Gamesa employs ultrasound testing to ensure that the components of its wind turbines are completely reliable. There are certain challenges that arise in conducting inspections of larger components, which are typically made of ductile cast iron – a material with excellent toughness-to-price ratio. With ductile cast iron, it is important to test for porosity defects but also to assess whether the porosity detected is large enough to be significant. To optimize current assessment methods and exploit the full potential of advanced ultrasound testing, Siemens Gamesa’s Technology team has joined forces with fracture mechanics experts from the Technical University of Denmark (DTU) in the Fast Track project – an initiative supported by Innovation Fund Denmark.

 
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Large components – significant cost savings

Optimizing production costs is one of the main drivers of increased competitiveness in the wind energy sector. With wind turbines increasing in size, large components are becoming even larger. This is certainly the case for large ductile cast iron components such as the rotor hub, the main shaft, and often the nacelle itself. They are expensive to produce, and very expensive to replace if they are disqualified in an inspection. When it comes to components with a mass of 50 tons or more, accepting or disqualifying one single part carries an associated cost equal to that of a mid-size car. On the other hand, the structural integrity of these load-bearing parts is essential for the safe operation of the wind turbine, so a detailed quality assessment of each component is an absolute necessity.

New ultrasound testing technology – new opportunities

Traditional ultrasound testing technology is a non-destructive technique useful for detecting the presence of porosity defects in metallic components, but it provides only very limited information about the exact location and size of detected porosities. Because of this limitation, Siemens Gamesa’s Technology team implemented phased array ultrasound testing (PAUT). This alternative method can detect any existing porosity and estimate its size and exact location more accurately and efficiently than traditional testing. There remained one weakness, however. The new system can identify even small defects, but these need to be properly assessed to determine if they are large enough to pose a real risk to the component’s integrity. Because if they do not, the component need not be disqualified.

The Fast Track Project: an easy access to the right experts

Looking for answers, Siemens Gamesa’s Technology team turned to a team of fracture mechanics experts from DTU Mechanical Engineering. They joined forces to use the framework of the Fast Track project to improve current assessment methods and exploit the full potential of advanced ultrasound testing. This close collaboration also included DNV GL, a global quality assurance and risk management company, to ensure that the developed solution would comply with certified expectations and practices.

Assessing components with fracture mechanics simulations

The team began by identifying several benchmark scenarios that accomplish the project goal. They also designed the right software tools and methods for assessing any ultrasonically observable porosities in sufficient detail. At all times, the project emphasized a balance of theory and practice, to ensure an accurate and practical assessment that can be implemented by the industry and also received well by certification entities. Additionally, the project tackled a range of important questions. For example:

  • How can detected porosities be simplified in shape so as to be represented in a simulation?

  • How are loads applied?

  • Which fracture mechanical parameters should be used?

  • How can conservatism be ensured in the assessment?

Apart from identifying appropriate software and theoretical methods, the Fast track workflow developed by the team demonstrates an efficient analysis scenario specific to large ductile cast iron components of the type that Siemens Gamesa engineers need to assess. The Fast Track project has yielded numerous benefits, not the least of which is the fact that Siemens Gamesa and DTU have mutually expanded their knowledge and skills through effective collaboration.

 
Figure 1 shows the developed workflow when reviewing defects from UT scans

Figure 1 shows the developed workflow when reviewing defects from UT scans

 

About Fast track

The Fast Track project is based on a systematic model for partnering between enterprises like Siemens Gamesa, Hempel, Terma, Elplatek, FORCE Technology, and knowledge institutions such as DTU, Aalborg University, and Teknologisk Institut. The project’s philosophy is simple: putting the right people across technology fields together to allow for effective solutions within material engineering. 

The Fast Track project supports a range of material research and development initiatives by providing companies across Denmark with research assistance and equipment. A dedicated team of experts ensures implementable solutions, developed on a fast track timeline of less than six months. The project’s goal is to raise awareness of advanced equipment facilities in Denmark, promote industry competitiveness, and create more jobs related to advanced material processing in the country.