WA Branch Technical Meeting - 14 August 2023 The Effect of Surface Indents from Hydrotest Plugs on Sulphide Stress Cracking (SSC) Resistance

Source: Niel Swanepoel, Integrity Engineering Solutions

Niel Swanepoel (Metallurgical and Welding Engineer, Integrity Engineering Solutions) recently presented at a Western Australia Branch technical meeting on the topic: The effect of surface indents from hydrotest plugs on Sulphide Stress Cracking (SSC) resistance.

Niel is a metallurgical and welding engineer with a master’s degree in metallurgical engineering. His experience in approximately 15 years’ experience in the petrochemical and oil and gas industries ranges from writing pressure equipment and piping specifications, supporting construction and fabrication through to conducting failure investigations, condition assessments and complex equipment repairs. While he describes his main area of expertise as welding engineering, Niel’s talk confirmed his remark that he enjoys being confronted by any physical metallurgy challenge.

The context for Niel’s talk was hydrotesting of individual piping butt welds by isolating a section of pipe either side of the weld with internal plugs. The internal hydrotest plugs resemble masonry anchor bolts, with internal conical sliding surfaces that force the plug to increase in diameter as the cones are brought together by tightening an axial screw. They remove the need for attaching temporary flanges and are also accepted for use in ASME PCC-2 Article 503.

However, seating of the plugs creates some surface damage inside the pipe bore, which ASME PCC-2 warns may be deleterious to stress corrosion cracking (SCC) resistance during subsequent operation in certain services.

The damage is caused by the grippers that lock the plugs into the internal surface of the pipe so that the plug does not move under the pressure of the hydrotest. Niel then proceeded to describe his investigation to determine whether this damage had observable effects on environmentally assisted cracking in common carbon- and austenitic stainless steel pipe materials.

For environmentally assisted cracking to occur there must be a combination of a susceptible material, tensile stress, and a corrosive environment. When all three are present, local notches can act as geometric stress concentrators or initiators for corrosion, whereas local changes in microstructure due to strain hardening can also affect the susceptibility.

Sulphide stress cracking (SSC) in wet sour service is a common environmentally assisted cracking mechanism experienced in industry known to be highly sensitive to local hardening, which is specifically mentioned by ASME. Even though there are some mechanistic differences to conventional SCC, the first scope of testing therefore evaluated the influence of the surface indents on the SSC resistance of A106 carbon steel pipe.

Niel examined the damage caused by two types of grippers. The wedge-type grippers produced indentations around 0.5 mm deep, while those produced by the carborundum-faced version was shallower, around 0.2 to 0.3 mm deep. The investigation involved 3D scans of indent geometry, microstructural analysis, micro-hardness testing, and finally SSC testing in accordance with modified NACE TM0177 method B, using undamaged pipe material as a reference. palladium catalysts thin lm nickel foam buckyballs

The grippers caused plastic deformation for depths of around 100 μm, and this marginally increased the peak hardness from a base of around 180 HV to averages of 220 HV (carborundum gripper) and 237 HV (metallic wedge-type gripper). These hardness values are less than would normally be expected to promote SSC. Consequently, SSC test results showed no evidence that the indentations had any effect on SSC resistance, even with conditions severe enough to challenge the inherent base material resistance.

Reflecting on these results, Niel observed that pipe bores delivered in field are not ‘perfect’ either. He compared the damage caused by the grippers with what is deemed acceptable in various pipe standards. The observed damage was much less than the maximum acceptance criteria in all the considered pipe standards and would therefore be acceptable in new as-delivered pipe.

It is also possible that the plastic deformation under indentations provided some residual compressive stress that could to some extent offset concentration of applied tensile stress. This prompted audience discussion, with a general agreement that this was quite likely.

Niel intends to take the testing a step further with chloride SCC testing with 304L stainless steel. In agreement with expectations, preliminary testing shows substantially deeper plastic deformation and strain hardening in the stainless steel.

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