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Qualification of Reeled Mechanically Lined Pipes for Fatigue Service

TWI carried out a programme of work focused on the qualification of mechanically lined pipes (MLPs) for dynamic applications, in the process demonstrating that MLPs reeled at atmospheric pressure are suitable for fatigue services.

Full-scale resonance fatigue testing and non-destructive examinations, such as dye penetrant inspection (DPI) and eddy current testing (ECT), were carried out on reeled MLPs. The specimens had a nominal outer diameter of 8.625in (219mm), a total wall thickness of 32.1mm and contained alloy 625 liners, clad overlay and girth welds.

Background

To meet an increasing demand for conveying corrosive fluids in the subsea oil and gas sector, TWI Member company Technip has designed MLP that enables reel-lay installation under atmospheric pressure without the risk of wrinkling of the corrosion-resistant alloy (CRA) lining. The work carried out by TWI involved additional tests designed to qualify these new reelable MLPs and provide further evidence that a high fatigue performance can be reached without breaching the CRA liner. Each specimen was 5m long and made of machined clad overlay welds on either side of a central CRA girth weld, with a liner on each side of the clad overlay weld.

Figure 1 Inspection tool set up for DPI inspection with spray can and camera.
Figure 1 Inspection tool set up for DPI inspection with spray can and camera.

Aims and methodology

The objectives were to carry out resonance fatigue testing on six MLPs, which were previously subjected to reeling at atmospheric pressure, in order to qualify the fatigue performance to DNV fatigue curves (DNV, 2012). Additional tests were carried out using DPI and ECT at the interface between the clad overlay weld and the liner, 2.5m from the pipe end, to detect cracks.

The pipes were fatigue tested at ambient temperature, at a frequency of approximately 30Hz, in TWI’s fatigue testing machine. The specimens were pressurised to achieve an axial mean stress of 90MPa and two specimens were tested at 80MPa, 120MPa and 175MPa stress ranges. DPI and ECT were developed and a dedicated inspection tool was built to allow the inspections (Figure 1). DPI was undertaken prior to fatigue testing, after reaching the DNV class D target curve and on test completion. ECT was carried out after reaching the DNV class D target curve when suspicious indications were found with DPI. Only one indication, of 2mm, was detected in one pipe and all fatigue test results reached the DNV class D target curve (Figure 2).

Looking ahead

A similar programme including resonance fatigue testing combined with DPI and ECT will be performed on six new MLPs. A new ECT is currently being developed to determine subsurface flaws which could initiate from the outer surface of the liner.

References

DNV, 2012: ‘Fatigue Design of Offshore Steel Structures’, DNV-RP-C203, October 2012.

IOSPE2015-TPC-0481: ‘Qualification of Reeled Mechanically Lined Pipes for Fatigue Service’, 2015.

For more information, please email contactus@twi.co.uk

Figure 2 Fatigue test results with DNV Class D and C SN curves.
Figure 2 Fatigue test results with DNV Class D and C SN curves.
Avatar Emilie Buennagel Principal Project Leader - Fatigue Integrity

Emilie joined TWI in August 2010 after spending more than three years in the engineering and technology division of a large materials testing company in the UK. Previously she completed her MSc studies at the University of Orleans in France, including two work placements at Nexans in Germany.

Emilie brought to TWI experience of mechanicals testing and metallurgy, primarily for the aerospace industry. In her previous employment she was responsible for managing projects and developing new test programmes. At TWI she manages a range of projects, including failure investigations, fatigue assessments and resonance fatigue testing of full-scale pipe.

Among the research projects Emilie has been involved with has been a core research programme investigation comparing approaches to design fatigue assessment, with reference to a pressure vessel designed to an old standard. A published paper based on this research is available on the TWI website.

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