Fabrication Challenges for the Aasta Hansteen SCRs and Flowlines
Authors: Jones Richard , Butterworth Christopher , Fryer Mark , Ferguson David , Cameron Neil , Spielbauer Paul 
Published: April 2016

Exploitation of the Aasta Hansteen field presented major challenges across several engineering disciplines. In addition to being the deepest development in the North Sea and involving the world's largest SPAR gas platform, the Aasta Hansteen project also required the fabrication and installation by reel-lay of steel catenary risers (SCRs) in heavy wall carbon steel and metallurgically clad pipe, together with mechanically lined (BuBi®) flowlines, which comprised the Subsea7 work scope.

Fabrication of the SCRs (12? × 25.6mm + 3mm 316L clad pipe, 14? × 28.6mm carbon steel (CS) pipe) and flowlines (12? × 19.7 + 3mm 316L BuBi® pipe) required the achievement of critical weld acceptance criterion especially in the fatigue sensitive zones for which no weld repairs were permissible. The maximum allowable flaw height as determined by Automatic Ultrsonic Testing (AUT) was 1.5mm. The weld solution for fabrication of the pipeline stalks at the spoolbase was based on the mechanised "Cold Metal Transfer"/Pulsed Gas Metal Arc Welding (CMT/PGMAW) processes for both carbon steel and clad/lined pipe. An extensive pre-production development programme was executed to ensure that the significant technical challenges were overcome whilst maintaining the use of cost effective welding solutions. Achievement of the exacting weld quality requirements and fatigue performance required step changes in welding and non-destructive examination (NDE) process technologies to be accomplished as follows: Development of optimised CMT and PGMAW waveformsInternal inspection using state of the laser/camera technologyAdvanced AUT using Phased Array probes together creep wave for near surface inspection

An extensive welding qualification programme was carried out which included some novel testing methodologies relative to current North Sea practices. For the carbon steel weld, workmanship weld acceptance criterion, based on an assumed toughness level and confirmed by project specific testing, were adopted in lieu of a conventional engineering critical assessment (ECA). Adequate fatigue performance was demonstrated by full scale fatigue testing of girth welds containing controlled flaws

In addition to welding and NDE technology improvements, a full pipe body demagnetisation facility was developed and installed in the Vigra spoolbase. Previous experience with clad/lined pipe had highlighted 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. The introduction of this new demagnetisation technology ensured that pipe material completely free of residual magnetism was delivered to the firing line thereby eliminating any risk of arc blow in production.

Following completion of the engineering activities, the fabrication campaign was successfully executed at the Vigra, spoolbase in Norway during early 2015. Fabrication performance exceeded both production and quality targets with repair rates for the SCRs below 0.5%

The project involved major improvements in welding and NDE technologies which enabled high quality welds to be manufactured with the use of cost effective welding solutions. The use of the full pipe body demagnetisation technology at the spoolbase also represented an industry first.