Structural safety – Studs and nuts Finn Kirkemo Statoil

Background API 6A allows use of low strength 2HM nuts with high strength studs L43 studs. Structural integrity of bolted connections with use of low strengths nuts raises is an issue that needs to be addressed A technical note from Statoil on studs and nuts has been issued distributed to API 6A and API 17D for comment. Statoil has also submitted this technical has also o ASTM and ASME for comment.

Main observations API 6A/17D requires only calculations of studs 2HM nuts have 5-25 % less structural capacity than L43 studs API 6A/17D allows 20 % less nut proof load than required by ASTM A320/A194 Example: Pressure test of connections with 2 ¼” stud to 83% of yield strength results in 100 % utilisation of 2HM nuts, i.e. a SAFETY ISSUE.

Recommendations Specify use heavy hex nuts with similar hardness as the studs. Proof load testing of nuts to follow the strength of the studs. Quality issues Traceability Sampling NDT, Dimension control