Engineering Critical Assessment (ECA) is required to develop production welding acceptance criteria, considering the anticipated installation and operational loads.
Unlike conventional carbon steel pipes, the production welding and NDT of CRA-clad or lined pipes may be very complicated. The key challenges are:
- Weld material may not overmatch the parent pipe, especially at elevated temperatures. This invalidates the conventional failure assessment diagram (FAD) approach to ECA.
- Large Hi/Loroot due to wall thickness misalignment and out-of-roundness at pipe ends. This results in high stress concentration factors and difficult-to-weld root pass.
- Inappropriate weld material selection due to insufficient preliminary ECA. This results in very small weld acceptance criteria or invalidates installation by reel-lay.
- Contractual obligations built on conventional weld requirements of overmatching weld and workmanship acceptance criteria.
- Utilised a comprehensive approach to ECA, from support provided on weld consumable selection and material testing to performing the ECA using 3D FEA based crack-driving force (CDF) approach.
- Where FEED phase involvement is applicable, preliminary ECA is carried out to facilitate weld consumable selection with specialist support in definition and review of the material testing program. These activities mitigate the risks of unsuitable weld material and unduly conservative characteristic properties used in ECA.
- 3D FEA based ECA is carried out for a range of parameters; e.g., various Hi/Loroot and flaw sizes, to allow optimisation of the tolerable flaw sizes and readiness to support weld acceptance decision making during the production welding campaign.
- Optimised tolerable flaw sizes for various scenarios to avoid a high weld rejection rate. This resulted in S-lay installation at an average of twice the normal lay rate.
- Following a weld material re-selection, installation by reel-lay was shown to be acceptable with achievable flaw acceptance criteria, avoiding costly late-stage changes to installation methodology.