1 - 2010-10-19

Safety and quality issues – studs and nuts

Presentation API 6A Winter meeting Feb. 2011

2 - 2010-10-19

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

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Example of 2HM nut failure during shell test

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Possible failure modes

Stud stripping Nut stripping

Stud tension failure

Not acceptable failure mode in design codes Assumed failure mode in design

codes

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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.

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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.

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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.

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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

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Non-linear FEA analysis, 1 ¼”

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Non-linear FEA analysis of 2 ¼”

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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

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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

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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

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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

Thank you