A well operator requested the inspection of coiled tubing made from Duplex stainless steel. Duplex steels are often used in velocity strings, as they can remain for long periods of time inside wells, and this material is able to withstand the harsh conditions. However, this material proves more difficult to inspect due to the fact that it often contains a higher percentage of chrome (usually around 16 percent) and is therefore typically nonmagnetic — preventing the possibility of a typical coiled tubing inspection by means of a Magnetic Flux Leakage (MFL) tool.
The customer’s particular coiled tubing section to be inspected consisted of CRA-8019 (Cr19) duplex stainless steel, with a chrome component of 19 percent. The diameter of the coil was 1.5 inches (38.1 mm), and the wall thickness was 0.109 inches (2.77 mm).
ROSEN coiled tubing inspections are typically performed utilizing the Automated Coiled Tubing Inspection Monitoring (ACIM) tool, which utilizes Magnetic Flux Leakage (MFL) technology and mechanical sensors. However, in the case of this inspection, it had been unclear if this method would be feasible due to the unknown magnetic quality of the steel. Further investigation was required.
ROSEN’s ACIM tool is a nondestructive testing solution that functions by magnetizing the wall of the pipe, then measuring the magnetic leaks, or areas where metal loss is present, in real time.
In order to evaluate the capability of this technology for this application, a physical test was performed. The client supplied a section of new duplex stainless steel Cr19 pipe to ROSEN engineers. Artificial flaws were created along the pipe, including drilled holes, external longitudinal (axial) notches and external transverse notches. The test pipe was then scanned with the ACIM tool, which was calibrated normally for carbon steel.
Through this test run, it was discovered that, indeed, the combinations of Cr (19.5 – 21.5 percent) and Ni (1.0 – 3.0 percent) magnetized the pipe wall just enough for the ACIM tool to hold a strong enough magnetic current. All artificial metal loss anomalies were positively identified, which confirmed the suitability of the technology for this application.
As carbon steel and Duplex steel (e.g. Cr19) have a different permeability, it was likely that the internal flaws would interact with the technology differently, ultimately being less detectable. The results of the test, as well as the information on the potential for variation in permeability, were discussed with the customer. The customer determined that the primary goal was to detect external flaws, and, therefore, the inspection could be carried out, according to the test conditions and with the ACIM tool.
The inspection of the coiled tubing was successfully completed within the agreed time frame. External flaws were successfully identified during the inspection, as expected, and data quality was to the satisfaction of the customer.
This was the first inspection the customer was able to achieve on coiled tubing made from Duplex steel Cr19. This inspection provided the customer with valuable information on the integrity status of the asset. Additionally, this inspection enabled the customer to be in compliance with the requirements of gas well operation. Furthermore, it helped the operator to achieve safe, efficient and optimal operation.