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Fatigue testing girth welds in hydrofluoric acid solutions

Acidizing of oil and gas wells is one of the most established and widely used stimulation methods to improve productivity of wells. The process involves pumping acid into a wellbore or geological formation, to dissolve restrictive material or formation rock and enhance the flow-paths or channels for oil or gas to the well-bore. Hydrofluoric acid (HF) in combination with hydrochloric acid (HCl), also known as mud acid, is typically used for dissolution of sandstone formations. Even in the presence of corrosion inhibitors, these highly corrosive acids have a strong influence on the corrosion fatigue performance of pipelines.

For the first time at TWI, a programme of fatigue endurance and fatigue crack growth rate (FCGR) testing was undertaken in order to investigate the effect of an HF-containing proprietary solution on the fatigue performance of C-Mn steel girth welds.

The following objectives were proposed for tests in the acidizing solution containing corrosion inhibitor in order to determine:

  • the effect of loading frequency on FCGRs
  • the FCGR acceleration factor by comparing data obtained in air
  • the environmental knock down factor (EKDF) by comparing fatigue endurance of the girth welds to data obtained in air.
Figure 1. Special PPE worn by the technical staff
Figure 1. Special PPE worn by the technical staff

Work programme

Due to the corrosivity and toxicity of the HF-containing test solution, additional precautions were made. All relevant staff and personnel undertook a special training course on safe handling of HF. Special PPE was worn by technical staff (Figure 1), emergency protocols established and a dedicated shower was installed prior to the start of the project.

In corrosive media, the loading frequency is known to influence FCGRs.  A series of frequency scanning tests were conducted to determine the saturated loading/plateau frequency where a decrease loading frequency does not further increase the FCGR. FCGR “Paris-law” tests were carried out to determine the enhancement of the FCGR compared to that in air. These tests were conducted on single edge notched bend (SENB) specimens and the direct current potential drop (DCPD) method used to monitor crack length.

Fatigue endurance tests were conducted in both air and the HF-containing environment to produce S-N curves.  A test machine equipped with a polyvinylidene fluoride (PVDF) vessel for the environmental tests is shown in Figure 2. Comparison of the fatigue lives in both environments resulted in an EKDF, which was vital for the fatigue design of pipelines subjected to these acidizing environments.

Project outcomes

A strong frequency dependence of FCGR was observed in this proprietary HF-containing solution. FCGRs were significantly higher in this medium compared to the data obtained in air.  The Paris law constants derived from the testing were useful in engineering critical assessment to determine weld flaw acceptance criteria. Fatigue lives were considerably lower in the acidizing solution and the EKDF obtained was useful for design purposes. The client was very satisfied with the outcomes of the project and provided excellent feedback.

For further information please email contactus@twi.co.uk.

Figure 2. A 600kN fatigue endurance test machine equipped with a PVDF environmental chamber
Figure 2. A 600kN fatigue endurance test machine equipped with a PVDF environmental chamber
Avatar Kuveshni Govender Principal Project Leader – Fatigue Integrity Management

Kuveshni joined TWI in 2012 as a Senior Project Leader in the Fatigue Integrity Management section. She manages projects covering the fatigue performance of welded and unwelded components across a wide range of industrial sectors. Her main area of work concerns corrosion fatigue, including fatigue endurance and fatigue crack growth rate testing, in sour, sweet, seawater and other corrosive environments. Kuveshni is also involved in failure investigations of structures within multidisciplinary teams.

Prior to joining TWI, she worked as a Postdoctoral Research Associate in the Materials Performance Centre, University of Manchester where she was involved in projects concerning corrosion issues experienced in the nuclear industry, viz. stress corrosion cracking and corrosion fatigue. Kuveshni obtained a PhD in Materials Chemistry in 2003 from the University of Manchester and has 25 publications.

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