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The Perfect Match: Laser Scanning and Fitness-for-Service

As seen in the June 2014 issue of Corrosion & Materials. Download the PDF version.
As seen in the July/August 2014 issue of Industrial Eye. Download the PDF version.

4Z and Quest Integrity Group have recently agreed to provide integrated services, combining the laser scanning, inspection and modeling services provided by 4Z with the fitness-for-service and remaining life assessment by Quest Integrity Group.

This new service is the first of its kind in Australia’s Industrial Sector.

Laser scanning is a highly effective and accurate inspection method for measuring changes in geometry of pressurised vessels, piping and pipelines. The laser scanning data is used to measure the deviations from normal geometry, such as those caused by corrosion, mechanical damage, and other structural distortions. The primary goal of the laser scanning analysis is to feed highly accurate geometric data of in-service vessels and components into advanced modelling and analysis software packages.

Figure 1 Laser scanning of a large pressure vessel

4Z experts provide 3D laser scanning services for vessels and components that enable both internal and external assessment of corrosion, displacement and other forms of structural damage. The laser data is taken and analysed for geometric deviations that meet or exceed the client’s performance indicators, such as wall thickness loss, pit depth, size and shape of structural defects. Once a problem area has been identified, the data can then be reverse engineered into a CAD model so that it can be imported for further analysis into Quest Integrity's proprietary software Signal™ Fitness-For-Service (Signal FFS) for conducting a Level 1 or 2 corrosion assessments.

Signal FFS provides rapid Level 1 and Level 2 fitness-for-service corrosion assessments and fracture mechanics analyses on fixed and rotating equipment such as piping, pipelines, pressure vessels and tanks. All analyses adhere to industry standards such as API 579/ASME-FFS or BS 7910.

The three levels of assessment vary in complexity and conservatism, with Level 1 being the least complex and most conservative. Level 2 assessments require more detailed input regarding operating conditions and material properties in order to provide a higher level of analysis. Level 3 assessments are the most advanced, requiring detailed data, computer analysis and a high level of technical knowledge and expertise in fitness-for-service assessment procedures. Once a Level 3 assessment has been carried out on a piece of pressure equipment employing laser scan data in a finite element analysis, the cost of subsequent assessments is reduced.

Laser scanning is highly advantageous if an assessment doesn’t comply with or pass the Level 2 assessment, for example, when the area of interest is in close proximity to a major structural discontinuity, or the geometry of the component is complicated and difficult to model accurately through finite element analysis (FEA). The resultant stress from the model can then be assessed against a fitness-for-service acceptability criteria for failure. Laser scanning data allows for a rapid assessment, and therefore a much faster return to service outcome or remediation planning. The natural corollary of this is that overall asset and safety management is significantly
improved.

The following example shows laser scanning applied to a pipe suffering from both internal and external corrosion. The pipe was 1320mm long, with an outer diameter of 635mm and a thickness of 24mm. Compared with traditional methods of conducting thickness grid measurements using a thickness gauge, laser scanning provides a more accurate thickness profile and enables future repeatability. The high level of accuracy also means that the level of conservatism employed during an assessment can be minimised, reducing unnecessary repairs and downtime.

The scanned thickness data gathered from Level 1 or 2 assessments can be imported into Signal FFS, enabling component assessment within a short timeframe. Figure 2 shows the input screen from Signal FFS which indicates the input parameters for the corroded conditions, such as nominal thickness and inside diameter. The thickness grid, as measured following the recommendations in API 579, is given at the bottom of the figure, indicating that the minimum measured thickness is 8.2mm compared to the nominal thickness of 13.5mm which equates to 39% wall loss.

 

Figure 2 Example of a SignalTM FFS assessment

Laser scans are compatible with all major FEA and modelling software. The images below illustrate the process of a Level 3 metal loss FEA assessment using a laser scanned model. In this case, it can be seen that the corrosion profile is captured by the scan (Figure 3). This level of corrosion profile modelling cannot be reasonably achieved through standard techniques.

Figure 3 Solid model constructed from laser scan data

In the following steps, corresponding material properties, boundary and loading conditions are applied, preceeded by the construction of the FEA mesh over the model (Figure 4). The simulation is then run and stress results extracted. Another advantage of FEA based analysis is that loading/operational parameters can be easily modified, allowing assessment of multiple operational scenarios quickly and easily.

Figure 4 Example of FEA mesh applied to model (top) and refinement around areas of interest (bottom)

 

Figure 5 Stress results visualized (top) and detailed view of high stressed location (bottom). Units in MPa.

Figure 5 shows an example of the stress results when the pipe is subjected to an internal pressure of 1 MPa. The magnitude and location of the peak stresses can be clearly seen and extracted for use in a Level 3 assessment. Besides metal loss assessments, the results can also be used for a variety of purposes, including but not limited to, evaluation of maximum allowable working pressure, cyclic fatigue analysis and leak before break analysis.More advanced forms of assessment such as coupled heat transfer-mechanical simulations and modal analysis are also made possible through the use of FEA.

With micron accuracy precision and reduction of inspection times by a factor of 10 times, laser inspection integrated with modern fitness-for-service and finite element analysis software is a rapid return to service method of analysis. The integration of 4Z laser scanning data with Quest Integrity Group’s fitness-for-service and remaining life assessment provides more accurate, cost effective and time efficient assessment that is not possible using currently available methods.

 

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