Effect of residual stress on the integrity of a branch connection

A new connection to an existing gas pipeline was made by hot-tapping, welding directly onto a pressurised pipeline. The welds were not post-weld heat treated, causing significant residual stresses. The critical weld had residual stresses determined by neutron diffraction using ANSTO's residual stress diffractometer, Kowari. The maximum measured residual stress (290 MPa) was 60% of the yield strength. The magnitudes of errors from a number of sources were estimated.An integrity assessment of the welded branch connection was performed with the measured residual stress values and with residual stress distributions from the BS 7910 and API 579 analysis codes. Analysis using estimates of residual stress from API 579 overestimated the critical crack size.

► Residual stresses were measured by neutron diffraction in a thick section, non post-weld heat treated ferritic weld.
► There is little published data on these welds.
► The work compares the measured residual stresses with code-based residual stress distributions.