Engineering Critical Assessment of Thin-Walled Offshore Pipelines

Since the introduction of automated ultrasonic testing (AUT) in the late 1990s, acceptance levels for fabrication flaws in girth welds have in the main been established using fracture mechanics. The specific advice required for engineering critical assessment (ECA) where pipeline installation methods involve plastic strain, is already available in company specifications, and general standards and guidance. Although each subsea development is typically comprised of a number of pipelines with widely ranging functions, materials and geometry, the combination of small diameter and thin-wall linepipe is normally associated with MEG antifreeze pipelines and gas lift lines. In 2005, a thin wall was considered to be 9.9 mm while projects currently being engineered for installation in 2008–10 push the limit down to 6.9 mm. Despite the existence in some standards of a lower limit on pipe wall thickness below which ECA is no longer required — arguably 13mm in the case of DNV OS-F101 — assessments are nonetheless often performed for thinner-wall flowlines. Such analyses are required in order to provide case-specific confirmation of prescriptive workmanship-based acceptance criteria. These limits on fabrication flaws are not necessarily arbitrary and may originally have been derived from some form of general fracture mechanics study, although certainly not for thin pipes. Set against the backdrop of some recent projects on the Norwegian continental shelf (NCS), including Ormen Lange and Tyrihans, this study presents the background to the mechanical testing and assessment methodologies developed to overcome the challenges posed by thin-wall flowlines. Brittle fracture is not expected, either in the pipeline or during fracture mechanics testing, especially with constraint-matched single edge notched tension (SENT) specimens. This is primarily due to crack tip stress fields being dominated by plane stress conditions, more than is the case in thicker sections. The resulting material behaviour is characterised by ductility and is controlled by strength properties, not by fracture toughness. Specimen dimensions scale according to wall thickness and can be sufficiently small to make handling and testing difficult; the correspondingly small loads and attachment of instrumentation requiring specialised test equipment and laboratory procedures in order to obtain accurate and reproducible results. Pipes with low D/t values — typical for plastic strain installation — place even more stringent limitations on testing as specimen thickness is further restricted by pipe curvature. The paper covers the particular aspects of testing and analysis in the ECAs for installation we perform for thin-walled pipe. The estimation of crack driving force using finite element (FE) simulations is also discussed where important differences are noted between tolerable flaw dimensions from these and BS7910-based assessments. We even tentatively argue for the qualified use of existing fracture mechanics SENT data obtained on similar linepipe grades and weld procedures. Comparative evaluation can be made of material and weld tensile properties to ensure equivalence. Although WPQ testing, notched cross-weld tensile tests and segment testing are all that are required, limited fracture mechanics testing may also be performed to calibrate against the reference R-curves should further confidence be necessary.