H
Bolting Regulations and Standards
The materials and strength of bolting for subsea drilling equipment are established by Bureau of Safety and Environmental Enforcement (BSEE) regulations, which include by reference standards, specifications, and recommend practices of the American Petroleum Institute (API), American Section of the International Association for Testing Materials (ASTM), American National Standards Institute (ANSI), American Iron and Steel Institute (AISI), International Organization for Standardization (ISO), National Association of Corrosion Engineers (NACE), and other similar agencies and bodies. This section includes brief descriptions of the content of U.S. federal government’s Code of Federal Regulations (CFR) Title 30, Chapter II, Subchapter B, Part 250 (30 CFR 250) and API documents that address bolted fasteners for service critical subsea equipment.
FEDERAL REGULATIONS
The U.S. federal government regulates offshore drilling and production operations through 30 CFR 250.1 Under 30 CFR 250, BSEE is authorized to regulate oil, gas, and sulphur exploration, development, and production operations on the Outer Continental Shelf (OCS). The API publishes standards, specifications, recommended practice and technical reports for petroleum and petrochemical
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1 U.S. Government Printing Office, 30 CFR 250—Oil and Gas and Sulphur Operations in the Outer Continental Shelf, https://www.gpo.gov/fdsys/granule/CFR-2011-title30-vol2/CFR-2011-title30-vol2-part250.
equipment and operations. The API is a national trade organization, but does not have regulatory authority. However, 30 CFR 250 incorporates by reference many API standards, specifications, and recommended practices.
It should be recognized that the difficulties in applying the U.S. CFRs to deepwater riser systems is that drilling rigs can work in other countries that have their own specific regulations and design standards that differ from those required when working in U.S. waters.
As a result of the Deepwater Horizon incident in 2010,2 Subpart G of 30 CFR 250 was revised to focus on, among other things, the performance of subsea safety critical equipment. Subpart G, however, does not address bolts on fasteners in particular. Rather, 30 CFR 250 incorporates by reference the following API standards, specifications and recommended practices:
- API Standard 53—Blowout Prevention Equipment Systems for Drilling Wells
- API RP 2RD—Design of Risers for Floating Production Systems and Tension-Leg Platforms
- API Spec. Q1—Specification for Quality Management System Requirements for Manufacturing Organizations for the Petroleum and Natural Gas Industry
- API Spec. 6A—Specification for Wellhead and Christmas Tree Equipment
- API Spec. 11D1—Packers and Bridge Plugs
- API Spec. 16A—Specification for Drill-through Equipment
- API Spec. 16C—Specification for Choke and Kill Systems
- API Spec. 16D—Specification for Control Systems for Drilling Well Control Equipment and Control Systems for Diverter Equipment
- API Spec. 17D—Design and Operation of Subsea Production Systems—Subsea Wellhead and Tree Equipment
- API RP 17H—Remotely Operated Tools and Interfaces on Subsea Production Systems
The content of the above API documents relevant to the subject of bolts will be discussed in the next section. Subpart G of 30 CFR 250 became effective on July 29, 2016. In addition, the API published several new documents specifically addressing bolts as a results of bolt failures are at the foundation of this National Academies of Sciences, Engineering, and Medicine study.
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2 NAE Report.
INDUSTRY STANDARDS, SPECIFICATIONS, AND RECOMMENDED PRACTICES
Although standards represent only minimum requirements, they must be met. An integral part of the design process should include an identification and review of all applicable, up-to-date standards that must be followed, as well as any other recommended or best practices.
This is particularly important because several organizations (i.e., API, ASME, NACE) may have promulgated many standards on the many components of a riser system, some of which over-lap or are in conflict, or have gaps.
The following standards, specifications, and recommended practices include requirements for connector bolts. Many of these documents were recently revised in response to the failures of connector bolts that motivated this National Academies study. Note that the API refers to the type of fastener bolts in this study as “closure bolting,” which are for flanges and other bolted connection on blowout preventer (BOP) equipment.
- API
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API Spec. 6A—Specification for Wellhead and Christmas Tree Equipment, 20th Edition, effective data April 1, 2011, Addendum 3 2013. It should be noted that Edition 20 of API Spec 6A does not include reference to API Spec 20E or 20F, such reference will be address in the next edition. Specifications for studs and nuts are included in Section 10.3. The API requires that the dimensions and thread pitch shall be in accordance with ASTM A193/A193M for studs and ASTM A194/A194M for nuts. The mechanical properties specified in Table 62 in API Spec 6A take precedence over those required by ASTM. Recommended torque is included in Annex D. Pre-load torques are based on a bolt axial stress of 50 percent of the specified minimum yield strength of the bolt steel. The API limits the maximum allowable tensile stress, SA, for closure bolting considering initial bolt-up, rated working pressure and hydrostatic test pressure conditions. The API specifies a safety factor of 0.83 for bolting stresses, which is based on the root area of the thread:
SA = 0,83 SbY
The bolting stresses should consider all loading on the closure, including pressure acting over the seal area, gasket loads and any additional mechanical and thermal loads.
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API Spec. 6A—Specification for Wellhead and Christmas Tree Equipment, 20th Edition, effective data April 1, 2011, Addendum 3 2013. It should be noted that Edition 20 of API Spec 6A does not include reference to API Spec 20E or 20F, such reference will be address in the next edition. Specifications for studs and nuts are included in Section 10.3. The API requires that the dimensions and thread pitch shall be in accordance with ASTM A193/A193M for studs and ASTM A194/A194M for nuts. The mechanical properties specified in Table 62 in API Spec 6A take precedence over those required by ASTM. Recommended torque is included in Annex D. Pre-load torques are based on a bolt axial stress of 50 percent of the specified minimum yield strength of the bolt steel. The API limits the maximum allowable tensile stress, SA, for closure bolting considering initial bolt-up, rated working pressure and hydrostatic test pressure conditions. The API specifies a safety factor of 0.83 for bolting stresses, which is based on the root area of the thread:
- API Spec. 16A—Specification for Drill-through Equipment
- API Spec. 17D—Design and Operation of Subsea Production Systems—Subsea Wellhead and Tree Equipment
- API Standard 53—Blowout Prevention Equipment Systems for Drilling Wells
- API RP 2RD—Design of Risers for Floating Production Systems and Tension-Leg Platforms
- API Spec. Q1—Specification for Quality Management System Requirements for Manufacturing Organizations for the Petroleum and Natural Gas Industry
- API Spec. 11D1—Packers and Bridge Plugs
- API Spec. 16C—Specification for Choke and Kill Systems
- API Spec. 16D—Specification for Control Systems for Drilling Well Control Equipment and Control Systems for Diverter Equipment
- API RP 16Q—Design, Selection, Operation, and Maintenance of Marine Drilling Riser System
- API RP 17H—Remotely Operated Tools and Interfaces on Subsea Production Systems
- API SPEC 20E—Alloy and Carbon Steel Bolting for Use in the Petroleum and Natural Gas Industries
- API SPEC 20F—Corrosion Resistant Bolting for Use in the Petroleum and Natural Gas Industries
The content of the above API documents relevant to the subject of bolts will be discussed in the next section. Subpart G of 30 CFR 250 became effective on July 29, 2016. In addition, the API published several new documents specifically addressing bolts as a result of the failures of bolts at the foundation of this study.
- ASTM Specifications
- ASTM A193/A193M, Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials for High Temperature or High Pressure Service and Other Special Purpose Applications. API Spec 20E incorporates specifications for bolt steel grades B7 and B7M. API Spec 6A refers to grade B7 and B7M. API Spec 17D refers to grade B7, B7M, and B16.
- ASTM A 194/194M, Standard Specification for Carbon and Alloy Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both. API Spec 20E incorporates specifications for bolt steel grades 2H, 4, 7, 2HM, and 7M. API Spec 6A refers to grade 2HM.
- ASTM A320/320M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for Low- Temperature Service. API Spec 20E incorporates specifications for bolt steel grades L7, L7M, and L432H. API Spec 6A
- refers to grade L7. API Spec 6A refers to grade L7, L7M, and L43. API Spec 17D refers to grade L7, L7M, and L43.
- ASTM A453/453, Standard Specification for High- Temperature Bolting, with Expansion Coefficients Comparable to Austenitic Stainless Steels. API Spec 20F incorporates specifications for bolt steel grade 660 and grade 660 Class D. API Spec 6A refers to grade 660. API Spec 17D refers to grade 660 Class D.
- ASTM A540, Standard Specification for Alloy Steel Bolting for Special Applications. API Spec 20E incorporates specifications for bolt steel grades B22 and B23.
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ASTM B633 (2011, 2007, 1998)—Standard Specification of Electrodeposited Coatings of Zinc on Iron or Steel. The QC-FIT report states:
This standard outlines different thickness classes with required salt spray test verification durations (See Appendix E, Table E.1 for coating finish types; ref. ASTM B633, 1998, 2007). Table E.2 specifies coating thickness classes based on the service condition (see ASTM B633, 1998, 2007, 2011). Section 6.4 recommends base metal alloys with an UTS value greater than 1700 MPa (247 ksi) should not be coated with zinc coating. The QC-FIT identified a concern about the manner that standards are applied within the supplier and manufacturer chains throughout industry. Table E.3 summarizes ASTM B633, the SC descriptions, and appropriate service conditions for each class (ASTM B633, 1998, 2007, 2011). The coating for the 2012 failed bolts manufactured 2007-2009 is a SC 2 class. SC 2 is for a moderate service condition, exposed mostly to indoor atmospheres, occasional condensation with minimum wear or abrasion. The recommended parts are tools, zippers, pull shelves and machine parts. The H4 connector bolts were coated to an SC 2 class and are used in marine subsea service blowout preventer (BOP) applications. According to GE, relevant API standards cannot be applied if a coating thicker than SC 2 is used.
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ASTM 849 (2007)—Standard Specification of Pre-treatment of Iron or Steel for Reducing Risk of Hydrogen Embrittlement. The QC-FIT report states:
ASTM B849 provides recommended guidance for stress relief, pre-bake heat duration of metals prior to electroplating. Table E.6 is an overview of recommended pre-bake durations and temperatures for high strength steels based on tensile strength (to be provided by customer) (Ref. 2007 ASTM B849). As seen in Table E.6, classes are based on the UTS values.
- ASTM850 (2009, 2004, 1998)—Standard Guide for Post Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement. The QC-FIT report states:
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ASTM B850 provides procedural guidance for post-baking, heat treatment duration for hydrogen stress relief of metals subjected to electroplating coating processes. Post-bake heat treatment is recommended for metals with a hardness value greater than >31 HRC and an UTS >145 ksi. The bolt design specification required a material hardness of 34-38 HRC, and a minimum UTS value of 145 ksi (ref. 2009 US Bolt MTR in 2013 Combined RCA Report, Appendix R page 335). Therefore per the 1998 edition for ASTM B850, the bolts were required to be post-baked from 2007 to 2009. If the design specification had clearly referenced ASTM B850, then the post-bake requirements would have been clear.
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ASTM F1941 (2010, 2007)—Standard Specification for Electrodeposited Coatings on Threaded Fasteners. The QC-FIT report states:
This specification covers application, performance and dimensional requirements for electrodeposited coatings on threaded fasteners with unified inch screw threads. It specifies coating thickness, supplementary hexavalent chromate or trivalent chromite finishes, corrosion resistance, precautions for managing the risk of hydrogen embrittlement and hydrogen embrittlement relief for high-strength and surface-hardened fasteners. The electrodeposited coating as ordered shall cover all surfaces and shall meet the requirements prescribed. Coated fasteners, when tested by continuous exposure to neutral salt spray shall show neither corrosion products of coatings (white corrosion) nor basis metal corrosion products (red rust) at the end of the test period. The coating thickness, embrittlement, corrosion resistance, and trivalent chromite finish shall be tested to meet the requirements prescribed.
- NACE Materials Requirement
- NACE MR0175 (2011, 2009, 2003)—Metals for Sulfide Stress Cracking and Stress Cracking Resistance Environments (Corrosion Standard for Materials for Use in H2S Containing Environments in Oil and Gas Production-2003 edition). NACE MR0175 specifies a maximum hardness of 32 HRC and minimum yield strength of 92,000 psi for subsea marine service. MR-0175 refers users to ASTM A193 grade B7M ASTM A320 grade L7M bolts and ASTM A194 grades 2HM and 7M nuts.
- NORSOK Standard
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NORSOK Standard M-001 – (2004) Materials Standard. Bolt materials are specified in Section 5.6 of M-001, “Bolting Materials for Pressure Equipment and Structural Use.” For bolts, the standard includes specifications for ASTM A193 grades B7, B16, and B8M; ASTM A320 grades L7 and L43. For nuts, the standard includes specifications for ASTM A194 grades 4/S, 7/S3, 2H, 7, 8M, and 8MA. In general, the standard states,
Carbon or low-alloyed bolting materials shall be used. Bolts with a diameter 10 mm shall be stainless steel according to ISO 3506-1, Type A4 (Type 316),
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NORSOK Standard M-001 – (2004) Materials Standard. Bolt materials are specified in Section 5.6 of M-001, “Bolting Materials for Pressure Equipment and Structural Use.” For bolts, the standard includes specifications for ASTM A193 grades B7, B16, and B8M; ASTM A320 grades L7 and L43. For nuts, the standard includes specifications for ASTM A194 grades 4/S, 7/S3, 2H, 7, 8M, and 8MA. In general, the standard states,
for metal temperatures below 60 °C if the stressed parts are exposed to humid saliferous environmental conditions (for nuts, see ISO 3506-2). If other bolting materials are required due to corrosion resistance or other reasons, the material shall be selected in accordance with the general requirements of this NORSOK standard. For sub-sea applications Alloy 625 shall be used when corrosion resistant bolts are required at ambient temperature, i.e. for conditions where the bolts are exposed to natural sea water and cathodic protection cannot be ensured. It shall be verified that the materials have acceptable mechanical properties at the design temperatures. Bolts used for sub-sea application shall have a maximum hardness of 300 HB or 32 HRC. The hardness shall be positively verified by spot hardness testing for each delivery, batch and size of bolts used.
With regard to bolt coatings, M-001 specifies,
Carbon steel and/or low alloy bolting material shall be hot dip galvanised to ASTM A153 or have similar corrosion protection. For submerged applications, where dissolution of a thick zinc layer may cause loss of bolt pretension, phosphating shall be used. For sub-sea installations the use of poly-tetra-fluoroethylene (PTFE) based coatings can be used provided electrical continuity is verified by measurements. Cadmium plating shall not be used.
It may be noted that API RP 17G also references the same ASTM standards as API Specs 6A, 16A, and 17D.
- Other
- Specification for Structural Joints Using High-Strength Bolts, Research Council on Structural Connections, August 1, 2014. This specification is not directly relevant to subsea bolts, but it does describe a technique for torqueing that provides for less variance, and it incorporates some useful human factors considerations for ensuring torqueing quality.
FLANGE BOLT DESIGN SPECIFICATIONS
Flange Bolt Preloading
There have been changes in the 2011 second edition of API Spec 17D from the 1996 first edition that appear to reflect improved good industry practices for bolting for subsea drilling equipment. API Spec 17D, Section 5.1.3.5 specifies that3
Closure bolting of all 6BX and 17SS flanges shall be made up using a method that has been shown to result in a stress range between 67% and 73% of the bolt’s material yield stress.
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3 API Specification 17D, ISO 1 3628-4, 2nd Edition, May 2011, Section 5.1.3.5, p. 19.
This stress range should result in a preload in excess of the separation force at test pressure while avoiding excessive stress beyond 83% of the bolt material’s yield strength.
This is a change from the 1996 first edition of API Spec 17D, in which the specified preload torque was “2/3 of the specified minimum yield stress.”4 Annex G of API Spec 17D contains the specified bolting preloads for L7, L43, B16, B7 or gr660 bolting steels, based on the 67 and 73 percent range of yield strength. Although Section 5.1.3.5 does not make it clear that the “material yield strength” refers to the minimum yield strength, this is clear in Annex G.
Regarding bolting tensile stress under service loading, API Spec 17D specified that
The maximum allowable tensile stress for closure bolting shall be determined considering initial bolt-up, rated working pressure and hydrostatic test pressure conditions. Bolting stresses, based on the root area of the thread, shall not exceed the limits given in ISO 10423.
The reference to ISO 10432 regarding the maximum stress at the thread root is a reference to Section 4.3.4 of API Spec 6A,5
The maximum allowable tensile stress, SA, for closure bolting shall be determined considering initial bolt-up, rated working pressure and hydrostatic test pressure conditions. Bolting stresses, based on the root area of the thread, shall not exceed the limit given in Equation (9):
where SY is the bolting material-specified minimum yield strength.
Bolting stresses shall be determined considering all loading on the closure, including pressure acting over the seal area, gasket loads and any additional mechanical and thermal loads.
API Spec 6A, however, specifies the preload torque for bolting to only 50 percet of the minimum yield strength of the bolting steel. API Spec 6A applies more specifically to land-based operations, which API Spec 17D is for subsea equipment.
Flange Bolt Material Specifications
API Spec 17D, Section 5.1.3.5 further specifies that6
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4 API Specification 17D, 1st Edition, August 1, 1996, Section 303.4, p. 22.
5 API Specification 6A, ISO 10423:2009 (Modified), 20th Edition, October 2010, Section 4.3.4, p. 28.
6 API Specification 17D, 1st Edition, August 1, 1996, Section 303.4, p. 22.
Closure bolting manufactured from carbon or alloy steel, when used in submerged service, shall be limited to 321 HBN (Rockwell “C” 35) maximum due to concerns with hydrogen embrittlement when connected to cathodic protection.
This is a consistent with the first edition of the 1996 first edition of API Spec 17D, which specified a maximum hardness of “Rockwell “C” 35” for carbon and alloy steels.7
