Safety and quality issues – studs and nuts

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

2. Background • ASTM A320 require studs of Grades L7 and L43 to be equipped with nuts confirming to Grade 4 or Grade 7 of ASTM A194, i.e. high strength nuts to be equipped with high strength studs • ISO 10423/API 6A allows for use of low strength nuts of type ASTM A194 2HM with high strength studs like ASTM A320 Grade L7/L43 • The background for ISO 10423/API 6A allowing for use of low strength nuts (2HM) to high strength studs (L7) is unknown (introduced in API 6A 1985, 2H 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 6A, API 17D, ASTM, ASME and ISO for comment. • SC17 decided 18. January 2011 to monitor SC6 and present the 2HM issue at the SC6 February 2011 meeting

3. Example of 2HM nut failure during shell test

4. Possible failure modes Stud tension failure Stud stripping Nut stripping Not acceptable failure mode in design codes Assumed failure mode in design codes

5. 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.

6. ASTM A320/A194 requirements • Hardness requirements, HBmin • High strength assembly (L43/7): Stud L43: 250 Nut 7: 248 • Low strength assembly (L7M/7M: Stud L7M: 200 Nut 7M: 159 • Proof load test load • High strength assembly (L43/7): 1,4 x tensile capacity of L43 stud • Low strength assembly (L7M/7M): 1,5 x tensile capacity of L7M stud • No requirement for proof load of nuts when d > 1 ½ in.

7. API 6A 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 (L43/7): 1,4 x tensile capacity of L43 stud • Low strength assembly (L7M/7M): 1,5 x tensile capacity of L7M stud • “High” strength assembly (L43/2HM): 1,5 x tensile capacity of L7M stud • No requirement for proof load of nuts when diameter is greater than 1 ½ in. • API 6A proof load test of nuts for “high” strength assemblies with 2HM nuts is ≈20% less than required by ASTM A320/A194.

8. Calculation of 2HM nut capacities • API 6A 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) • Non-linear 2D FEA performed for 2 ¼” and 1 ¼” stud/nut assemblies • Nut capacity formulas established based upon formulas in VDI 2230 Part 1 and non-linear FEA • FEA results validated with 1 ¼” L7/7 tests and 2 ¼” 2HM nut testing • Calculation model also validated with 1 3/8” and 1 ½” 2HM nut testing

9. Non-linear FEA analysis, 1 ¼”

10. Non-linear FEA analysis of 2 ¼”

11. Results of calculations • Limit load calculations • 2HM nuts have 5 % to 25 % less structural capacity than L43 studs for ½” to 4” size • Example. 2 ¼” nut is utilised to100 % structural capacity when the stud is utilised to 83 % of structural capacity. I.e. for pressure shell test where ISO 10423/API 6A 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 2HM/7M nuts should be increased from 159 to minimum 200, which is the minimum hardness for ASTM A320 Grade L7M studs to fulfil proof load testing • Minimum HB of 248 for Grade 4/7 applied to L7/L43 is ok for proof load testing

12. Conclusions • API 6A/ISO 10423 • Nuts is assumed to have higher structural capacities than studs, however, calculations shows that 2HM nuts has less 5% to 25% less structural capacity than L7/L43 studs, hence, derating have to be considered • Nut proof load test of nuts for “high” strength assemblies (L7/L43 studs) with 2HM nuts is ≈20% less than required by ASTM A320/A194 • ASTM A194 • Minimum hardness of nuts should be the same as for studs, i.e. the 2HM/7M nut minimum HB hardness should be 200 when used with L7M 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 2HM shall not be used in combination with high strength studs like L7/L43 – STRUCTURAL INTEGRITY AND SAFETY CONCERN

13. API 6A – 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

14. 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