TWI Industrial Member Report Summary 941/2009
C Nageswaran
Background
Line pipes are the arteries of the oil and gas industry, transporting the commodity from source to point of use. The high strength low alloy steel grade API-5L X65 has been used for line pipes widely and successfully for many years. For transporting highly corrosive hydrocarbon materials the use of corrosion resistant alloy (CRA) cladding is increasingly being used to prevent corrosion. The cladding is applied to the flow exposed inside surface of line pipe using two main methods: metallurgical bonding or mechanical lining.
The selection of CRA for the cladding depends on the service environment, in particular the temperature and concentrations of H2S, CO2 and chloride. A variety of alloys have been used including austenitic stainless Type 316L, Alloys 825 and 625. The selection of the CRA is critical and is often the result of extensive metallurgical tests in simulated service environments.
The non-destructive testing (NDT) of clad line pipes is complicated by the CRA and weld materials, weld configurations and processes. Conventional ultrasonic inspection makes use of single element probes that generate sound beams at specific angles. The echoes generated at discontinuities within the material are located and sized by assuming that the beam propagates at the specified angle, and that the sound pressure and energy behave predictably. This is true in apparently isotropic polycrystalline material such as carbon steels (including API-5L X65) where the properties (ie the elastic stiffness) remain ultrasonically constant regardless of propagation direction. However, the CRA and the weld material of clad line pipes are anisotropic and inhomogeneous, leading to severe distortion of the sound field. Subsequent interaction of a distorted sound field with discontinuities leads to erroneous interpretations, which could lead to false sentencing of the component or, more seriously, fail to find defects that threaten the integrity of the component.
This report is the follow-on to previous Core Research Programme (CRP) work reported by Nageswaran and Bird (2008). The previous work investigated the use of two novel phased array techniques, namely transmit-receive longitudinal (TRL) and time-of-flight diffraction (TOFD), for improving the detection and sizing of crack-like flaws in a thick section representative weld from the power industry. The techniques were evaluated against the best practice single element TRL probes and procedures specifically developed for the inspection of the critical component. The report concluded that 'The two phased array techniques (TRL and TOFD) were as capable as the best practice baseline technique designed for the detection of flaws ... Both techniques are ready to be adapted for application to industrial components' (Nageswaran and Bird, 2008). In addition, unlike the best practice technique using conventional probes, both phased array techniques were able to through-wall size crack-like flaws, which is important to perform effective engineering critical assessment (ECA) of components.
This report describes the work to develop and adapt the two phased array techniques (TRL and TOFD) for application to an oil and gas component. The specimens were two clad line pipe girth welds of different diameters and wall thicknesses (273.1mm outside diameter (OD), nominal 31.8mm wall thickness (WT) and 168.3mm OD, nominal 18.3mm WT). The line pipe material was API-5L X65 and Alloy 625 was used for both the cladding and weld. The gas tungsten arc welding (GTAW) process was used for deposition of the cladding (ie weld deposited metallurgical bonding) and for deposition of the nominal 60° included angle single bevel weld.
Since the component is commercially sensitive, only details relevant to the narrative of the report are released, at the request of the Industrial Member who donated the specimens.
Objective
Evaluate the inspection of clad line pipe girth welds using the phased array ultrasonic TRL and TOFD techniques to establish their capability to detect and size critical flaws.