Incorporation of Handheld LIBS into A Pipeline Repair Workflow

  • AINDT Firtst gas LIBS for pipeline maintenance workflow

    Overview
    Introduction
    Carbon Equivalents (CE) Testing
    The SciAps Z200C+ Handheld LIBS
    Approach
    Results and Discussion
    Conclusions
    References

    Overview

    Gas pipelines form a critical part of infrastructure in the supply of natural gas in countries such as New Zealand. Management of the integrity of this asset is an ongoing process that can have serious ramifications if neglected. Where repairs are required welding is often necessitated. To ensure long-term stability of the weld repairs, carbon equivalent (CE) testing and thermal analysis tests must be carried out to formulate the optimal repair procedure. Recently, First Gas have incorporated handheld LIBS into the CE determination procedure which allows for more rapid and portable performance and with it improved economic benefits.

Introduction

First Gas is responsible for maintaining over 2500km of high-pressure gas transmission pipeline and 4800km of gas distribution pipeline that provide natural gas supplies throughout the North Island of New Zealand. Welding on the pipeline is common to facilitate new connections or to carry out repairs. Being able to achieve the welding process in the safest and most cost-effective manner is a focus of First Gas and other companies tasked with asset management of pipelines.

Carbon Equivalent (CE) Testing

Carbon equivalent (CE) testing is a critical step based on the materials’ composition that must be carried out when welding is required. CE testing is performed on ferrous alloys to evaluate the weldability of steels and to avoid weld hardening and hydrogen cracking susceptibility in the heat affected zone (HAZ). With sections of pipeline approaching 60 years old, datasheets on some pipe is not available. Thus, testing of older sections is required, while newer sections are also tested to ensure they are consistent with specifications.

Traditionally this is carried out in a lab using techniques such as Spark Optical Emission Spectroscopy (OES). More recently, Laser Induced Breakdown Spectroscopy (LIBS) has become a viable alternative with the advantage that it is available in a handheld configuration that is ideal for field/on site use.

Spark OES and LIBS work in a similar manner. They both vaporise a small quantity of material thus generating a plasma characteristic of the elements present in the parent material. As the names suggests, Spark OES uses an electrical discharge, while LIBS uses a laser. A spectrometer looks at the colours/wavelengths in the plasma which can be translated to composition, with quantitative analysis made possible using calibration curves.

The SciAps Z200C+ Handheld LIBS

The SciAps Z200C+ is the first handheld LIBS on the market capable of quantitatively measuring content. With a sensitivity of 80ppm. it is sensitive enough to be able to distinguish alloys such as 316 stainless steel (0.03 to 0.08%C) from 316L (<0.03%C). Designed with CE testing in mind, the Z200C+ can complete a chemical composition measurement and CE calculation in a matter of seconds, leaving on a small burn scar in the surface, typically smaller than a spark OES.

In this paper we outline a methodology using a handheld LIBS) that is capable of accurately quantifying the presence of all elements needed for CE determination. The LIBS-based methodology provides several advantages such speed of analysis, the ability to perform measurement on site non-destructively and the follow-on benefits to workflow efficiency.

Approach

  1. Remove any coating or oxidation from steel to reveal bare metal surface using a grinder with abrasive disk
  2. Once the Z200C+ is warmed up calibrate using CRMs (Certified Reference Materials)
  3. For each analysis location, perform a minimum for 4 measurements (so an average value can be calculated), with each test consisting of 4 to 5 shots
  4. Re-check the calibration of the Z200C+ to ensure consistency of measurements

Results and Discussion

The compact nature and handheld form factor enable the Z200C+ to be easily incorporated as an in-field instrument. It has also been shown to be able to perform under inclement weather conditions. These factors are all advantages over more conventional techniques for CE testing such as spark OES.

Test sites 1435 and 1441 both had 5 measurements taken, while test site 1446 was the average of 4 readings. After the surface had been prepared properly, it took less than 15 seconds to complete each individual measurement.

Figure 3 shows the reports generated by the Z200C+ based on averages of the multiple tests at each site. These reports can be exported from the instrument through a number of methods, such as Bluetooth or Wi-Fi. At the top it provides measurement reference details. In large text, the instrument has analysed the alloy to be a 1010 carbon steel with a certainty of around 90% for all sites tested. For the main alloying elements, a bar graph is provided, that shows the allowable composition range for alloy 1010. The small arrow indicates where the instrumentmeasured the composition to be in relation to the alloy grade specifications. The CE has also been calculated and determined based on the International Institute for Welding (IIW) protocol.

Test site 1435

Test site 1441

Test site 1446

Figure 3. Chemical composition analysis generated by the Z200C+ LIBS device.

The SciAps Z200C+ alloy identification is consistent, for all three sites, identifying the pipeline alloy to be a 1010 plain carbon steel based on measurements taken. The identification is based on chemical analysis performed by the instrument and matched to an in-built database.

Analyses of CRMs before and after the pipeline measurements shows excellent consistency with their respective data sheets. This indicates that the LIBS calibration is accurate and that the calibration is not drifting during the measurement period. This also implies that the handheld LIBS is capable of producing data that is equivalent to spark OES.

The Z200C+ LIBS device also conveniently calculates the CE using an in-built algorithm. Figure 4 provides a comparison of the CE measurements from each test site. The data shows excellent consistent and low variability between each measurement.

Figure 4. Comparison of CE measurements for the 3 test sites.

To date, First Gas have been using the Z200C+ for 24 months and have gained solid experience and confidence in the LIBs tool.

Conclusions

CE testing using the SciAps Z200C+ has proven to be a fast and reliable technique. As such, New Zealand’s First Gas, responsible for the nation’s  natural gas transmission network has included LIBS analysis for CE determination of steel pipes into their asset management workflow where it has been found to be an excellent alternative to spark OES. Furthermore, its handheld configuration brings efficiency gains with measurements able to be carried out in situ and subsequent weld repairs able to be effected immediately thereafter, thus streamlining the asset management workflow.

References

  1. Weldability Of Materials – Carbon Manganese And Low Alloy Steels – https://www.twi-global.com/technical-knowledge/job-knowledge/weldability-of-materials-carbon-manganese-and-low-alloy-steels-019, accessed August 27, 2019
  2. Carbon Equivalent Formulae In Relation To Hydrogen Cracking – https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-the-difference-between-the-various-carbon-equivalent-formulae-used-in-relation-to-hydrogen-cracking accessed August 27, 2019.

Authors

Peter Airey1, Cameron Chai1, Hayden Handley2 and Steve Nicholls2.

  1. AXT PTY LTD
  2. First Gas

This paper was first published in the “Industrial Eye”, the official jounal of the Australian Institute for Non-Destructive Testing, May/June 2020 issue.
The information was also presented at the AINDT SA Trade Expo and Conference, 4-6 November 2019.

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