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Detection of liner cracking in mechanically lined pipes

TWI has developed a technique to detect cracking in the liner of mechanically lined pipes (MPL) while simultaneously carrying out full scale resonance fatigue testing.

Project background

To meet an increasing demand for conveying corrosive fluids in the subsea oil and gas sector, MLP technologies have been developed.  These are cost-effective alternatives to more expensive options such as solid corrosion-resistant alloys or metallurgically clad pipes.

However, to qualify MPL, companies need to establish the fatigue performance of the liner before a carbon steel backing pipe is exposed to corrosive constituents.

Therefore, there is a need to develop a technique to detect cracking of the liner while carrying out full scale fatigue testing of MLP.

Work programme

Full scale fatigue testing was carried out on one 5.9m long MLP of nominal dimensions 323.85mm outer diameter and 18.9mm wall thickness (comprised of 15.9mm of carbon steel and 3mm of Inconel 625).  The test specimen contained one girth weld at mid-length with weld overlay on either side of the girth weld. The liner was welded to the weld overlay and extended to approximately 200mm from the specimen ends where it was sealed with a fillet weld.

A system for detecting leakage was developed using ‘wet detectors’ (absorbent material connected to wires, connected to the test machine trip circuit) fitted to two carefully positioned holes in the carbon steel pipe wall (holes located towards each pipe end).  To aid distinguishing between condensation and fluid from failure, a green colorant was also applied.

Figure 1
Figure 1

Achievements

Detection of liner cracking in MLP during full scale resonance fatigue testing was successfully carried out.  The test stopped once the thickness of the liner cracked and before the crack reached the pipe through thickness.

Cracking was first detected with water leakage, of green colour, from one drilled hole.  The hole was then plugged and the test re-started.  After a few cycles, the specimen tripped from the second drilled hole with water leakage of green colour.  The second hole was also plugged and the test re-started until through wall cracking occurred.  Cracking was indicated by the strain gauge readings (Figure 1).

The MLP was then cut and dye penetrant inspection was carried out on the clad overlay / liner transitions.  Cracks were detected in the liner on each side of the girth weld (Figure 2).

The next step will be for TWI to investigate this technique further by performing additional testing on various pipe sizes and liner thicknesses.

For further information about fatigue analysis, or to discuss your requirements, please email contactus@twi.co.uk

Figure 3
Figure 3
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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