Pipe girth welds under plastic straining

Strain-based design and fracture assessment of pipelines are becoming increasingly important because large pipeline networks tend to be placed in challenging environments where they might undergo large strains due to ground movement. For example, existing strain capacity prediction methods in the public domain were evaluated using small-scale fracture mechanics testing and large‑scale pipe testing.

Overview

In the last decade, extensive efforts were made by industry and researchers to develop strain-based design and assessment methods, due to the need to install pipeline networks in challenging environments where the pipelines are likely to undergo plastic straining. In these circumstances, strain-based design and fracture assessment methods should be considered. However at the current time, there is no unified strain-based design/assessment approach that is universally recognised by the oil and gas industries. This study, therefore, will contribute to developing a satisfactory approach for strain-based design and assessment of pipelines.

Objectives

To measure the fracture toughness of pipe girth welds using SENT specimens
To determine crack driving force and strain capacity of pipe girth welds using large-scale testing
To compare the existing strain-based fracture assessment methods with the large scale test results

Figure 1. Notch position employed in the pipe test

Figure 2. Pipe installed in tension machine

Solution

The pipe studied was seamless to API 5L PSL2 Grade X65. Tensile testing was carried out to determine the stress-strain curves of parent and weld metals.

Fracture toughness was determined using 2BxB SENT specimens using a multiple specimen method to derive CTOD resistance curves (R-curves) at room temperature. The tests were compliant with BS 8571.

For the purposes of testing, the pipes were cut to a length of 2000mm. The girth weld was located in the middle of the pipe length and the weld cap at the designated notch location was ground flush with the original pipe surface and etched to reveal the weld. This procedure enabled the notch to be located along the weld centreline and also facilitated fitting of the knife edges close to the notch mouth, for subsequent instrumentation with clip gauges, as illustrated in Figure 1.

A single electrical discharge machining (EDM) notch was inserted at 4 o’clock in the pipe, which was subjected to tension only. Figure 2 shows the installation of the pipe test in the tension machine. Before this, the pipe was instrumented with strain gauges and linear variable differential transformers (LVDTs), as shown in Figure 3.

The existing strain-based methods were divided into five groups:

Group 1: Modification to existing stress-based approaches
Group 2: Extension to the EPRI scheme for fully plastic J estimate
Group 3: Reference strain method
Group 4: Strain capacity method
Group 5: Others

The strain capacity predicted using the methods in each group was compared with the large-scale test results. Figure 4 shows that most of the existing methods are conservative compared with the test results.

Conclusion

In this study, the pipe sample was subjected to axial load only and attained a maximum load plateau at a strain of 5.07%; this was defined as its tensile strain capacity, and crack tip opening displacement (CTOD) of 4.03mm. Most of the strain-based methods conservatively predicted the strain capacity for pipes under tension only.

Figure 4. Comparison of strain capacity predicted using the existing strain-based methods with the large-scale pipe test result

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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