Session: 04-04-03: Rigid Pipelines III

Submission Number: 157228

A Segment Test Validation Program for Reeling ECA of Partially Over-Matching CRA Girth Welds 

A segment test validation programme for reeling ECA of partially over-matching CRA girth welds is presented. Reeling ECA for partially over-matching CRA girth welds is based on FEA and utilises a tear-fatigue approach that accounts for the blunting limit in JR curves during pipe spooling and reeling. Fatigue crack growth occurs by low cycle fatigue in each strain cycle in reeling, and by tearing of the flaw, but the latter only if the crack driving force exceeds the blunting limit.

Conventional ECA references the FAD concept in BS7910 and is limited to weld strength that at least even-matches the pipe material. For partially over-matched strength girth welds, the reeling ECA methodology is based on an FE approach in which the weld geometry misalignment and material strength mismatches are explicitly modelled.

Fatigue crack growth for low cycle fatigue in reeling cycles is calculated using Paris’ Law. If the crack tip opening displacement (CTOD) due to straining of the pipe in reeling is less than blunting limit, tearing is not initiated. However, if the CTOD exceeds the blunting limit than tearing is initiated and must be included in the total tearing growth.

In this test programme, segment specimens were machined from welded pipe pups composed of carbon steel backing pipe with a 3mm thickness layer of corrosion resistant alloy on the inner circumference. Two welding procedures were selected and assessed for the segment test programme.

Pre-test ECA was done based on the FE-based reeling ECA approach to determine a sensible initial flaw size to be introduced in the segment specimen. The initial flaw size was selected to significant and measurable total flaw growth the end of the test. The target flaw growth in the test exceeded 1mm. A test that had immeasurable or indeterminate flaw growth would, of course, be inconclusive.

Large sized EDM notches were introduced at the weld central line, from the external cap.  Segment tests involved 5 strain cycles, from zero strain to the target strain. The maximum final flaw growth was measured using a scanning electron microscope (SEM).

Post-test ECA was done with the FE-based reeling methodology and it incorporated explicit modelling of the segment specimen, and included test specific data and the test setup.

The ECA-by-FEA approach is completely validated by the excellent agreement between measure flaw growth and calculated flaw growth from post-testing ECA. The detailed comparison demonstrated a marginal over-prediction of flaw growth by ECA-by-FEA. Such conservatism of the ECA-by-FEA approach was observed in each of the two welding procedures selected for the segment tests.

Presenting Author: Lingjun Cao Subsea 7

Presenting Author Biography: Dr Lingjun Cao started working on offshore pipeline ECA from 2007. Lingjun joined Subsea7 in 2010 and has been with Subsea7 for the last 15 years. He has primarily worked on offshore flowline/riser ECA, and currently holds the position of ECA team manager. Besides working with conventional ECAs, Lingjun has worked extensively on FE-based ECA for partially over-matching or fully under-matching girth welds under large plastic strain and high pressure/high temperature conditions. Lingjun has extensive experience of ECAs for CRA welds in metallurgically clad pipes, as well as the ECA of overlay welds in mechanically lined pipe.