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Engineering Critical Assessment of Gas Lifting Piping

Overview

Iced up gas lift and kill lines operated by the client were susceptible to brittle fracture. Therefore, TWI carried out mechanical testing and engineering critical assessment (ECA) to advise the fracture resistance properties and the maximum allowable flaw sizes at the welds. This gave confidence to the client in understanding the material behaviour of the gas lift and kill lines, and assisting them on inspection and repair plans.

Gas lift and kill piping systems of 2” and 4” were used to inject gas into the well to enhance the production of the reservoir.  Initially, the well injection pressure was 188 barg, but due to reservoir depletion the well pressure dropped to 50 barg.  This resulted in the formation of ice downstream of the choke valve, due to the Joule-Thomson effect, as shown in Figure 1. The Charpy energy reported for parent material and weld metal of the 4” section was found to be low and the client had concerns regarding the risk of brittle fracture. ECA and fractography were carried out to determine the susceptibility of brittle fracture and to provide the maximum allowable flaw size.

Objectives

  • To measure tensile and fracture toughness properties for the 4” section of the pipelines to provide input for ECA
  • To carry out fractography on the tested tensile and fracture toughness specimens
  • To determine the maximum allowable surface breaking flaw sizes for girth welds in the 4” piping system
Figure 1. Iced up gas lift lines
Figure 1. Iced up gas lift lines

Solution

Samples were provided by the client, then because the weld dimension is too small to be able to extract all-weld tensile specimens, hardness tests were performed at ambient temperature: Figure 2 shows the map of hardness indentations. This was then used to estimate the yield and tensile strength of the weld metal. Four tensile specimens of parent material, transverse to the girth weld, were tested at the lowest operating and ambient temperatures respectively. Fracture toughness tests were carried out at the lowest operating temperature using SENB specimens, with notches at the weld centre line and heat affected zone respectively.

The tensile properties and fracture toughness data obtained, together with the geometric and stress information, were fed into ECAs which were carried out using TWI’s CrackWISE®5 software shown in Figure 3.

Conclusion

The maximum allowable sizes of internal circumferential surface-breaking flaws were delivered to the client. As a result of the allowable flaw sizes being large, it is unlikely that brittle fracture will occur at the time of assessment. An inspection schedule was recommended so that the piping system can be operated safely to avoid fracture.

Figure 2. Map of hardness indentations
Figure 2. Map of hardness indentations
Figure 3. CrackWISE®5
Figure 3. CrackWISE®5
Avatar Dr Guiyi Wu Principal Project Leader, Asset and Fracture Management

Guiyi's work focuses on fracture mechanics based assessment and fitness-for-service assessment to various standards, and includes material characterisation testing, finite element analysis, and residual stress prediction and measurement. He manages TWI’s ECA software: CrackWISE® and also supervises MSc and PhD students at NSIRC. Guiyi is particularly interested in the research areas of additive manufacturing, strain-based fracture analysis, probabilistic fracture mechanics, and residual stress prediction and measurement. He sits on the BS 7910 and R6 residual stress group committees, serves as a reviewer for the ASME conference and journal papers, and has authored or co-authored more than 20 peer reviewed conference and journal papers.

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