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Fatigue Performance of Locally Thickened and Machined Pipes

 
Published on 07 January 2012

Minimising the mismatch on assembly with conventional pipe ends can be a time consuming and costly job. Poorly matching pipe ends are invariably responsible for unpredictable fatigue performance in girth welds.

By locally thickening and machining the ends of the pipe, the need for pre-matching of pipe ends is greatly reduced, and improves the weld's fatigue performance.

V&M Tubes Germany commissioned TWI to evaluate the mechanical properties and fatigue performance of girth welds, made using locally thickened and machined pipe ends, so that it could compare these with the performance of girth welds on standard plain pipe ends.

The objectives set were five-fold. TWI was commissioned to;

  • Make a number of girth welds on plain end pipe of 273mm outside diameter and 28mm wall thickness.
  • Make a similar number of welds on pipe ends which have been locally thickened from 28mm to 32mm wall thickness and subsequently machined.
  • Inspect the girth welds using specified non-destructive techniques.
  • Carry out mechanical testing of girth welds in both types of pipes.
  • Perform full scale resonance fatigue testing of a selected number of welded pipe assemblies from each category.

The welding work was subcontracted and the welds were manufactured using the gas metal arc process and tested for their mechanical properties and their fatigue strength.

Upon completion of the testing TWI was able to tell V&M Tubes that all the manufactured girth welds met the acceptance criteria on the basis of radiographic and ultrasonic testing. Mechanical testing showed good toughness of the weld metal and very good cross weld tensile strengths well above the minimum specified values.

Full scale resonance testing at nominal stress ranges of 160MPa, 200MPa and 240MPa showed fatigue performance exceeding B1 design curve (DNV-RP-C203:2005) for girth welds on locally thickened and machined pipes, and C1 design curve for girth welds on plain pipe ends.

From the results it was possible to conclude that the girth welds produced using locally thickened and machined pipe ends performed better in fatigue than those welds produced on plain pipe ends.

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