Flaw Acceptance Criteria for Thin Walled Offshore Pipelines
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During S-lay pipelay operations offshore, pipe joints are welded on the lay-barge firing line. On completion of the girth weld, Non-Destructive Testing (NDT) is performed to locate the existence of any welding flaws. Automated Ultrasonic Testing (AUT) technique is normally the principal NDT method, providing high Probability Of Detection (POD) and accuracy. Empirical limits for welding flaws used during production offshore are frequently classified as too conservative. Hence, they are often replaced by criteria derived from an Engineering Critical Assessment (ECA). The approach is particularly effective for EPIC projects, since all aspects are considered in a complete “fit-for-purpose” approach and limiting conditions for failure are avoided. The benefits from performing an ECA however decrease with thinner pipelines. It is therefore paramount that all installation and design parameters for thin walled pipelines during detailed design are carefully optimised so the benefits are maintained. The optimisation process is presented in this paper for two thin wall export pipelines. Maximum loads are extracted from a range of analyses performed for the two pipelines. The stress ranges are calculated for the installation condition, the temporary on-bottom condition and the operation condition. Geometrical effects at the welded joint (axial joint misalignment) along with corrosion allowance (where appropriate) are considered in the calculations. Peak and variable loadings for the design life are established. Both pipelines were designed and subsequently installed by Saipem Ltd. The flaw acceptance criteria for welding had been developed in-house. All limiting loads were considered, including operational data supplied by Company. This led to a highly successful installation campaign with minimisation of repairs offshore.
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