Dimensional Closure for Fatigue-Critical Tubulars

For fatigue-critical risers and offshore tubular systems, dimensional conformity is more than a manufacturing inspection issue. End mismatch, ovality, wall-thickness variation, weld profile, local alignment, end preparation and installation-induced geometry can influence local stress concentration, fit-up behaviour and fatigue performance.

This white paper develops an As-Built Geometry Evidence Chain for buyer-side and EPC control of fatigue-critical tubular packages. The proposed framework connects the structural and fatigue design basis to manufacturing measurements, end-condition control, fit-up, transport and installation activities, and final as-built evidence.

Drawing on publicly available DNV guidance and attributed industry examples, the paper considers the complementary roles of DNV-ST-F201, DNV-RP-C203, DNV-ST-F101 and DNV-ST-N001 in providing system, fatigue, pipeline and marine-operation context. These documents are treated according to their respective purposes rather than as interchangeable sources of dimensional acceptance criteria.

A central proposition is that measured geometry forms part of the structural evidence for a fatigue-critical system. A design assumption that cannot be preserved, measured or reconstructed through manufacture and installation is not fully closed from a project-assurance perspective.

The paper therefore recommends translating fatigue-sensitive design assumptions into measurable manufacturing and fit-up requirements before award. Relevant variables may include global and end ovality, wall thickness, straightness, internal and external mismatch, weld and transition geometry, counterbore or machining dimensions, and other project-specific features that affect structural or installation performance.

The framework deliberately avoids proposing universal dimensional tolerances, mismatch limits or fatigue acceptance values. Such numerical requirements must come from the governing structural, riser, pipeline and fabrication basis for the specific project. Instead, the paper focuses on maintaining a controlled relationship between design assumptions, measurement method, datum definition, measurement uncertainty, production evidence and engineering disposition.

Manufacturing operations such as forming, sizing, expansion, heat treatment, counterboring, machining and repair are treated as controlled processes where they can alter fatigue-relevant geometry. The evidence chain also extends beyond factory final inspection where transportation, handling, fit-up or installation may change the final configuration.

An Execution Decision Point is introduced for consequential geometric decisions, including acceptance of measured geometry, machining or repair of an end, release of a joint for assembly, and acceptance of the final as-built condition. The decision is tied to the governing design basis, traceable dimensional evidence, affected product identity, responsible engineering authority and documented disposition.

This paper forms No. 11 of the From Mill to Project Independent Research Series — Phase II. It is an independent, manufacturer-neutral practitioner research paper based on publicly available information and author synthesis. It does not replace project-specific fatigue assessment, structural analysis, fabrication requirements, installation engineering or responsible engineering judgement.

Author-written abstract, reproduced from the authoritative Zenodo DOI record.