Fabrication Challenges for the Aasta Hansteen SCRs and Flowlines

摘要

Exploitation of the Aasta Hansteen field presented major challenges across several engineering disciplines.rnIn addition to being the deepest development in the North Sea and involving the world’s largestrnSPAR gas platform, the Aasta Hansteen project also required the fabrication and installation by reel-layrnof steel catenary risers (SCRs) in heavy wall carbon steel and metallurgically clad pipe, together withrnmechanically lined (BuBi®) flowlines, which comprised the Subsea7 work scope.rnFabrication of the SCRs (12" × 25.6mm + 3mm 316L clad pipe, 14"× 28.6mm carbon steel (CS)rnpipe) and flowlines (12" × 19.7 + 3mm 316L BuBi® pipe) required the achievement of critical weldrnacceptance criterion especially in the fatigue sensitive zones for which no weld repairs were permissible.rnThe maximum allowable flaw height as determined by Automatic Ultrsonic Testing (AUT) was 1.5mm.rnThe weld solution for fabrication of the pipeline stalks at the spoolbase was based on the mechanisedrn"Cold Metal Transfer"/Pulsed Gas Metal Arc Welding (CMT/PGMAW) processes for both carbon steelrnand clad/lined pipe. An extensive pre-production development programme was executed to ensure that thernsignificant technical challenges were overcome whilst maintaining the use of cost effective weldingrnsolutions. Achievement of the exacting weld quality requirements and fatigue performance required steprnchanges in welding and non-destructive examination (NDE) process technologies to be accomplished asrnfollows:rn1. Development of optimised CMT and PGMAW waveformsrn2. Internal inspection using state of the laser/camera technologyrn3. Advanced AUT using Phased Array probes together creep wave for near surface inspectionrnAn extensive welding qualification programme was carried out which included some novel testingrnmethodologies relative to current North Sea practices. For the carbon steel weld, workmanship weldrnacceptance criterion, based on an assumed toughness level and confirmed by project specific testing, werernadopted in lieu of a conventional engineering critical assessment (ECA). Adequate fatigue performancernwas demonstrated by full scale fatigue testing of girth welds containing controlled flawsrnIn addition to welding and NDE technology improvements, a full pipe body demagnetisation facilityrnwas developed and installed in the Vigra spoolbase. Previous experience with clad/lined pipe hadrnhighlighted the susceptibility of these pipe materials to residual magnetism which can cause severe arc instabilities or ‘arc blow’ during welding. This can result in reduced productivity and loss of weld quality.rnThe introduction of this new demagnetisation technology ensured that pipe material completely free ofrnresidual magnetism was delivered to the firing line thereby eliminating any risk of arc blow in production.rnFollowing completion of the engineering activities, the fabrication campaign was successfully executedrnat the Vigra, spoolbase in Norway during early 2015. Fabrication performance exceeded both productionrnand quality targets with repair rates for the SCRs below 0.5%rnThe project involved major improvements in welding and NDE technologies which enabled highrnquality welds to be manufactured with the use of cost effective welding solutions. The use of the full pipernbody demagnetisation technology at the spoolbase also represented an industry first.