Landing Zone Guidance for Welds on In-Service Pipelines

Abstract Prior to making welds on an in-service pipeline for the purpose of installing full-encirclement repair sleeves or hot tap branch connections, it is customary to inspect the pipe wall in the zones where these welds will “land.” A recently completed Phase 1 of a joint industry project (JIP) at DNV [1] focused on developing guidance for these landing zones, both in terms of inspection procedures and acceptance criteria for indications discovered during inspection. The primary purpose of the pre-weld pipe wall inspection is to ensure adequate thickness to prevent burnthrough, where the welding arc causes the pipe wall to be penetrated, and to investigate for the presence of discontinuities — laminations in particular — within the pipe wall. Laminations are internal separations that create layers parallel to the pipe surface. The presence of a lamination can reduce the effective thickness of the pipe wall resulting in a risk of burnthrough even though the nominal wall thickness is considered to be sufficient. Recent trends in inspection practices have resulted in indications discovered during pre-weld pipe wall inspection being misidentified as laminations when they are actually small, isolated non-metallic inclusions that do not represent a pipe wall separation. This misidentification has resulted in the need to extend the length of full-encirclement repair sleeves, sometime by significant amounts, and relocate hot tap branch connections. Extending the length of repair sleeves increases the cost of repair activities, as well as the required extent of welding and time for which the ditch must be open. Long repair sleeves are also heavy, which can result in additional bending stresses in the pipeline at the sleeve ends. The ability to avoid this misidentification will decrease the cost of repair and increase the integrity of the repaired pipeline. The work in Phase 1 began with a review of current industry practices. Welding trials were then conducted on pipe materials with both true laminations and with indications that were reported to be laminations but were misidentified. Following welding, metallographic examination was carried out to identify any changes to the mid-wall features, such as the presence of lamellar tearing. The results of the review and the experimental work were used to develop recommended acceptance criteria to differentiate between inclusions and laminations. Lastly, recommended practices for the pre-weld pipe wall inspection were developed. This paper will present an outline of guidance that was developed. Additional welding trials are planned for Phase 2 of the JIP to further validate the guidance that has been developed to date.