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Safety and quality issues – studs and nuts Presentation API 6 A Winter meeting Safety and quality issues – studs and nuts Presentation API 6 A Winter meeting Feb. 2011 1 - 2010 -10 -19

Background • ASTM A 320 require studs of Grades L 7 and L 43 Background • ASTM A 320 require studs of Grades L 7 and L 43 to be equipped with nuts confirming to Grade 4 or Grade 7 of ASTM A 194, i. e. high strength nuts to be equipped with high strength studs • ISO 10423/API 6 A allows for use of low strength nuts of type ASTM A 194 2 HM with high strength studs like ASTM A 320 Grade L 7/L 43 • The background for ISO 10423/API 6 A allowing for use of low strength nuts (2 HM) to high strength studs (L 7) is unknown (introduced in API 6 A 1985, 2 H was used before 1985). • 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 6 A, API 17 D, ASTM, ASME and ISO for comment. • SC 17 decided 18. January 2011 to monitor SC 6 and present the 2 HM issue at the SC 6 February 2011 meeting 2 2010 -10 -19

Example of 2 HM nut failure during shell test 3 2010 -10 -19 Example of 2 HM nut failure during shell test 3 2010 -10 -19

Possible failure modes Stud tension failure Stud stripping Nut stripping Not acceptable failure mode Possible failure modes Stud tension failure Stud stripping Nut stripping Not acceptable failure mode in design codes 4 2010 -10 -19 Assumed failure mode in design codes

Why thread stripping is not acceptable? • Tensile failure of the stud is easily Why thread stripping is not acceptable? • Tensile failure of the stud is easily detected. • The initiation of stripping failure, though, is difficult to identify because the stripping develops gradually, some tension remains in the stud, and there is little or no visible damage. • Since replacement of damaged fasteners is essential for sound joints, tensile failure of the stud is desirable during overtightening during make-up or overloading in service. • Strongest stud to fail before weakest nut; checked by nut proof loading. 5 2010 -10 -19

ASTM A 320/A 194 requirements • Hardness requirements, HBmin − High strength assembly (L ASTM A 320/A 194 requirements • Hardness requirements, HBmin − High strength assembly (L 43/7): Nut 7: − Low strength assembly (L 7 M/7 M: • Stud L 43: 250 Stud L 7 M: 200 Nut 7 M: 159 248 Proof load test load − High strength assembly (L 43/7): − Low strength assembly (L 7 M/7 M): • 6 1, 4 x tensile capacity of L 43 stud 1, 5 x tensile capacity of L 7 M stud No requirement for proof load of nuts when d > 1 ½ in. 2010 -10 -19

API 6 A requirements • Stud design capacity is based upon limit load • API 6 A requirements • Stud design capacity is based upon limit load • The nut limit load shall be higher than the stud limit load (only studs are checked) • Stud allowable loads: − 83 % of limit load for hydrostatic test • Proof load test load for diameter up to and including 1 ½ in. − High strength assembly (L 43/7): 1, 4 x tensile capacity of L 43 stud − Low strength assembly (L 7 M/7 M): 1, 5 x tensile capacity of L 7 M stud − “High” strength assembly (L 43/2 HM): 1, 5 x tensile capacity of L 7 M stud • • 7 No requirement for proof load of nuts when diameter is greater than 1 ½ in. API 6 A proof load test of nuts for “high” strength assemblies with 2 HM nuts is ≈20% less than required by ASTM A 320/A 194. 2010 -10 -19

Calculation of 2 HM nut capacities • API 6 A and ASME VIII Div. Calculation of 2 HM nut capacities • API 6 A and ASME VIII Div. 2/3 limit load − Worst case dimensional tolerances nut/stud − Coefficient of thread friction equal to zero − Minimum specified yield strength 60 ksi (HB 159 ≈ Su = 80 ksi) • • Nut capacity formulas established based upon formulas in VDI 2230 Part 1 and non-linear FEA • FEA results validated with 1 ¼” L 7/7 tests and 2 ¼” 2 HM nut testing • 8 Non-linear 2 D FEA performed for 2 ¼” and 1 ¼” stud/nut assemblies Calculation model also validated with 1 3/8” and 1 ½” 2 HM nut testing 2010 -10 -19

Non-linear FEA analysis, 1 ¼” 9 2010 -10 -19 Non-linear FEA analysis, 1 ¼” 9 2010 -10 -19

Non-linear FEA analysis of 2 ¼” 10 2010 -10 -19 Non-linear FEA analysis of 2 ¼” 10 2010 -10 -19

Results of calculations • Limit load calculations − 2 HM nuts have 5 % Results of calculations • Limit load calculations − 2 HM nuts have 5 % to 25 % less structural capacity than L 43 studs for ½” to 4” size − Example. 2 ¼” nut is utilised to 100 % structural capacity when the stud is utilised to 83 % of structural capacity. I. e. for pressure shell test where ISO 10423/API 6 A allows 83 % utilisation of the stud, the nut has 100 % utilisation. NOT ACCEPTABLE and this is a HSE ISSUE. • Proof load testing − Minimum HB for Grade 2 HM/7 M nuts should be increased from 159 to minimum 200, which is the minimum hardness for ASTM A 320 Grade L 7 M studs to fulfil proof load testing − Minimum HB of 248 for Grade 4/7 applied to L 7/L 43 is ok for proof load testing 11 2010 -10 -19

Conclusions • API 6 A/ISO 10423 − Nuts is assumed to have higher structural Conclusions • API 6 A/ISO 10423 − Nuts is assumed to have higher structural capacities than studs, however, calculations shows that 2 HM nuts has less 5% to 25% less structural capacity than L 7/L 43 studs, hence, derating have to be considered − Nut proof load test of nuts for “high” strength assemblies (L 7/L 43 studs) with 2 HM nuts is ≈20% less than required by ASTM A 320/A 194 • ASTM A 194 − Minimum hardness of nuts should be the same as for studs, i. e. the 2 HM/7 M nut minimum HB hardness should be 200 when used with L 7 M studs • General − Studs shall be equipped with heavy hex nuts with a grade of steel or minimum hardness similar to that of the studs. − Low strength nuts like 2 HM shall not be used in combination with high strength studs like L 7/L 43 – STRUCTURAL INTEGRITY AND SAFETY CONCERN 12 2010 -10 -19

API 6 A – Quality issues of closure bolting • No traceability between heat API 6 A – Quality issues of closure bolting • No traceability between heat treatment batch and material certificate • No NDT (MPI/DP/UT/RT) • No dimensional control • Low sampling rate, e. g. hardness testing of Grade 7 nuts: 1 of 800 • No proof load testing for nuts when d> 1 ½” • The studs (nuts) are the governing structural component for all API BX type flanged connections and not the flange • Not consistent quality requirements of fasteners compared with forged flanges/components as fasteners are the governing (critical) structural component in the API BX type flanged connection 13 2010 -10 -19

Thank you Presentation title: Safety and quality issues – studs and nuts Presenters name: Thank you Presentation title: Safety and quality issues – studs and nuts Presenters name: Finn Kirkemo Presenters title: Technical Advisor E-mail address: fkir@statoil. com, tel: +47 901 27 901 www. statoil. com 14 2010 -10 -19