High-Performance Bolting Technology for Offshore Oil and Natural Gas Operations (2018) / Chapter Skim
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2 Assessment of Critical Subsea Bolting System Design Elements
2 Assessment of Critical Subsea Bolting System Design Elements
Pages 23-60
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From page 23... ...
."1 1 . Armagost, Anadarko Petroleum Company, "Root Cause Failure Analysis, In Support of Im K proved System Reliability," presentation to the Workshop on Bolting Reliability for Offshore Oil and Natural Gas Operations, April 11, 2017.
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From page 24... ...
Appendix I is a tutorial on the design of a drilling riser. It begins with a discussion of the many forces that act on a drill riser system, and supplements the discussion in Appendix G, "Subsea Environmental Factors for Fastener Design." The appendix reviews the drilling riser system design process, operational analy sis, failure (weak point)
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From page 25... ...
The number of flange bolts and the size and strength of the bolts, studs, and nuts depends primarily on the flange size, seawater depth, and drilling or production fluid pressures and temperatures. Note that the threaded fasteners on the flanges and ram housing are of a variety of types, sizes, materials, and strengths.
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From page 26... ...
SOURCE: Bureau of Safety and Environmental Enforcement, Evaluation of Fasteners Failures -- Addendum, QC-FIT Report #2016-04, Office of Offshore Regulatory Programs, February 2016, https:// www.bsee.gov/sites/bsee.gov/files/qc-fit-nov-bop-bsr-bolt-report-7282017.pdf, pp.
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From page 27... ...
Operational Loads on Flange Bolts Because of the subsea environment, varied and dynamic forces act on the flanged connections in the subsea riser equipment and the threaded fasteners therein. Basic design considerations for bolted connection (i.e., flanges and hous ing)
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From page 28... ...
This apparent lack of concern is troublesome, since the committee had no access to statistical data or engineering studies regarding the effect of single bolt failures on overall flange connector reliability under a variety of loading and environmental conditions. As discussed later in this report, con sidering the past occurrence of multiple bolt failures in the same connector, the failure of any bolt in critical service warrants a full root cause analysis to reduce the potential for future failures.
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From page 29... ...
Fastener Material Selection Material and quality specifications are an important part of any design pro cess. Threaded fasteners for offshore applications are manufactured according to specifications6,7 including API Spec 20E, "Alloy and Carbon Steel Bolting for Use in the Petroleum and Natural Gas Industries," and API Spec 20F, "Corrosion Resistant Bolting for Use in the Petroleum and Natural Gas Industries."8,9 Each of these API specifications details the requirements for materials, properties, manufacturing, testing, inspection, and recording keeping.
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From page 30... ...
For example, fatigue loading does not appear to have been responsible for reported failures summarized in Appendix E Rather, the evidence of the several cases presented to the committee appear to be environmentally assisted intergranular brittle failure of bolting in flanged connections, and a few cases of what appeared to be ductile overload failures on a blind shear ram actuator housing.
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From page 31... ...
Subsea wellhead and Christmas tree bolts suffer additional and greater integrity risks, including: • Service in high-pressure salt water -- sometimes hypersaline • Exposure to H2S, CO2, hydrogen, and cathodic protection • Potential tensile forces coming from the rig through the riser • Potential bending moments caused by deep water currents The above four factors place significant and often uncertain environmental stress and loading on subsea bolts. This operating environment of subsea critical bolts raises questions about the appropriateness of the yield margin of safety im plied by API Spec 17D even though no failures due to inadequate yield margin of safety were reported to the committee.
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From page 32... ...
prior to further reduction by rolling to a form a billet (note that the difference between a bloom and billet is somewhat arbitrary and relates to size) .17 In continuous casting facilities, steel is solidified directly as a continuously moving billet with typical section sizes up to about 18 cm × 18 cm square.18 Recently con tinuous casting systems have been developed which produce jumbo blooms with larger section sizes up to about 45 cm × 61 cm19 and these systems may include an in-line forging system prior to rolling to billets.
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From page 33... ...
Hex head flange bolts and related connectors are produced by hot forging where control of thermal history (i.e., temperature and time) and forging param eters are critical to ensure the production of high quality, dimensionally accurate forgings.
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From page 34... ...
24 . Burgess, "Bolt Manufacturing -- A Look at Critical Operations," presentation at the Workshop L on Bolting Reliability for Offshore Oil and Natural Gas Operations, April 11, 2017.
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From page 35... ...
A recent root cause analysis of continuously cast bolt failures showed no direct relationship between crack initiation or growth by HE and the presence of banding.30 While banding typically is concentrated in the center part of rolled bars, hy drogen induced fractures typically initiate near surface in regions where banding is essentially absent. Advances in steel making casting technologies suggest that metrics other than reduction ratios would provide improved assessments of prod uct quality.
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From page 36... ...
Robust operating procedures are in place to minimize this possibility. There are two main failure modes relevant to this study of critical bolting in subsea BOP systems: • Ductile failure due to overload • Brittle failure modes, including intergranular failure due to environmentally assisted cracking or embrittlement Notwithstanding bolt failure, instances of loading beyond yield, or plastic deformation, can go unnoticed by visual inspections.
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From page 37... ...
In-Service Inspection BSEE currently requires a visual inspection by a remotely operated underwater vehicle (ROV) of a marine riser, wellhead and BOP system every 3 days if weather and sea conditions permit.32 This visual inspection cannot evaluate insipient bolt failures.
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From page 38... ...
The electrode potential of the metal surface is the parameter that is controlled during cathodic protection and the reduction reaction rate is related to the potential by the Tafel constant, which is in turn controlled by the type of metal and environment. A reasonable value for water reduction is an increase of an order 35 .
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From page 39... ...
The water reduction reaction rate is critical, since it controls the hydrogen production on the fastener surface, which in turns leads to hydrogen uptake into the fastener. Hydrogen uptake occurs prior to the atomic hydrogen combining on the metal surface to form hydrogen gas, which is released into the environment.
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From page 40... ...
First, the CP current and potential distribution vary with macro position on a structure and micro position in a crevice, thread root or crack as a function of the local potential and chemistry. In general, cathodically polarized confined spaces absorb less hydrogen while anodically polarized confined spaces may absorb more.43 As another example, over time the reduction reactions on a cathodic surface increases the local pH and calcareous deposits form on the surface modestly reducing hydrogen uptake.44 Ultimately the critical factor for a given environment, temperature, depth, flow and cathodic protection level is the cathodic current on the fastener.45 The hydrogen content varies critically with this factor and both the local and overall conditions are equally important.
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From page 41... ...
2. 53 Stress, Site Inspection, Metallurgical Examination, and Mechanical Damage Analysis, 2013, p.
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From page 42... ...
What is not disputed is that no report of an organized investigation of these returned H4 bolts has been provided to the committee or mentioned in any BSEE report. 55 Stress, Site Inspection, Metallurgical Examination, and Mechanical Damage Analysis, 2013, Ap pendix C
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From page 43... ...
The WC stud cluster failures motivated a reconsideration of the conclusions of the earlier investigations, if for no other reason than some studs exhibited virtually the same failure mode and features observed in the three previous cluster failures while possessing none of the attributes that were supposedly the cause of the earlier failures. The supposedly inadequate post electroplating heat treatment of the Discoverer India bolts, by an unmonitored lower tier subcontractor, was extensively discussed in the BSEE reports and the industry RCA for its "major role" in the three cluster failures mentioned previously.
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From page 44... ...
has been anno tated to include the average HRC stud hardness values for each bolt to illustrate the independence of hardness and fracture origin patterns and to highlight that bolts with hardness values significantly less than the industry-accepted value of 34 HRC also failed. As with the case of the Discoverer India bolts, the report on the Seadrill WC rig indicated that the industry "RCA also determined that the subcontracted ven dor's non-compliance to the QA/QC processes led to deviations from the OEMs manufacturing specification."68 And, again, the industry "RCA investigation at tributed the failure to non-conformances to the manufacturer's heat treatment material specifications, raw material specification, and quality control compliance impacting the fastener material properties."69 This time BSEE appeared to be less convinced and determined the RCA to exhibit "inconclusiveness" and stated "BSEE recommends that a more detailed investigation be performed by an independent third-party testing laboratory on behalf of the operator to determine the specific damage mechanism."70 64 BSEE, QC-FIT -- Summary of Findings, 2014, p.
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From page 45... ...
The industry RCA for the Seadrill WC failures specifically found "banding" in the stud microstructures that was attributed to using rolled bar from continuously cast ingots for the stud steel instead of the specified ingot cast material, "which can result in areas of localized high hardness and unexpected mechanical properties." However, based on metallographic cross sections of multiple secondary cracks which emanated from primary cracks and short cracks at thread roots indicating crack nucleation there does not appear to be any evidence in the micrographs of the cracks that the banding played any role in crack nucleation or propagation. Specifically, the micrographs showed that the cracks that exhibited irregular crack paths characteristic of intergranular fracture by HE "independent of the hard/soft banded regions." The report confirmed that the cracks progressed through or across the banded regions and showed no preference to propagate within these regions.
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From page 46... ...
Taken together, hydrogen uptake may be slightly lower. However, visual inspection of the fracture surfaces of all the Discoverer India bolts and Seadrill studs revealed calcareous deposits in the cracks which serve as a positive indicator of cathodic polarization into growing cracks.
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From page 47... ...
Unfortunately, because there is no regular inspection of bolts removed from service that have not failed, or monitoring for incipient cracks of bolts in service that have not failed, we are left with only the evidence of these few cluster failures. Something about their specific application caused the service cathodic polarization to enable fasteners to experi ence high hydrogen overpotentials and high enough stress states to enable HAC (i.e., brittle failure modes)
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From page 48... ...
Preload does not transfer to adjacent bolts sufficient to cause ductile overload. Moreover, in the flanges associated with the failed bolts being evaluated, the combined effect of a correctly selected bolt preload of all the bolts should "insulate" the remaining bolts from the bulk of the operational load cycles with the failure of a single bolt, or perhaps two.
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From page 49... ...
Bolts failed on several flanges over a period of time.75 Some flanges, but not all, exhibited cluster failures. Loading of the flanges was not uniform along the structure, and loads on the flanges was 75 e Th committee was not able to gain any information on the exact timing of bolt failures.
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From page 50... ...
The failure was interpreted to be due to external HE, which was well supported by a number of factors including brittle fracture appearance, delay time from load application until cracking and failure, high material hardness albeit within the ASTM A354 specification, a source capable of enabling significant levels of hydrogen production (i.e., a hot dip galvanized sacrificial anodic coating, which enable hydrogen production from the reduction of water when galvanically coupled to steel) , time for entry and diffusion of hydrogen to the fracture process zone at the root of threads, elevated applied stresses from loading, and, consequently, high tri-axial tensile stresses at thread roots.
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From page 51... ...
The relatively low threshold stress intensity for HAC in these steels at finite diffusible hydrogen concentrations enable by cathodic protection makes this scenario possible. Gross overload leading to ductile fracture over the entire bolt cross sections was not observed.
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From page 52... ...
. OPTIONS FOR IMPROVING THE SELECTION OF BOLTING MATERIAL PROPERTIES Currently material specifications for steels and CRAs used in the manufacture of threaded connectors are primarily detailed in the relevant API specifications (e.g., 20E or 20F)
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From page 53... ...
As the critical failures observed to date have been associated with environ mentally assisted cracking (primarily identified as hydrogen embrittlement) , in corporation of standard laboratory test methods to assess the susceptibility of candidate steels to hydrogen embrittlement and/or stress corrosion cracking should be required and the results would provide additional criteria for incorporation in standards for material selection and manufacture.
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From page 54... ...
the most likely and immediate means of mitigating bolt failures lies in material science, material specifications, and quality control. Finding: Oil and gas industry specifications and practice use torque as the value to be measured in tightening flange bolts.
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From page 55... ...
30 section 250 based on proactive assessment of risk areas. Finding: Current specifications for offshore fastener steels prohibit the use of continuous cast products, primarily because the existence of banding has been observed in steels which also failed in service by hydrogen embrittlement.
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From page 56... ...
This would include subjecting all fasteners, failed and un-failed, in these critical applications to a thorough post-operational inspection -- requiring a full dimensional check and metallurgic post-mortem, with root-cause analysis being performed when the equipment did not perform according to design. Finding: BSEE currently requires a visual inspection by ROV of a drilling riser and BOP every 3 days.81 Finding: BOPs are either sent to shore every 5 years for recertification or they are part of a Continuous Certification Program (CCP)
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From page 57... ...
Finding: There are several API documents that provide excellent techniques and guidance on various aspects of marine riser design, operations, inspection, and riser. These are: • API Spec 16F, "Specification for Marine Drilling Riser Equipment" • API RP 17G, "Recommended Practice for Completion/Worker Risers" • API RP 16Q, "Recommended Practice for Design, Selection, Operation and Maintenance of Marine Drilling Riser Systems" In particular, API RP 16Q, Chapter 6 (Riser Operations)
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From page 58... ...
There is a range of complicating factors that affect the electrode potential on the metal surface and, thereby, affect the rate of hydrogen production.83 Finding: The U.S. Navy and the French Navy have begun to utilize "low voltage" aluminum anodes to decrease anodic voltage, thus reducing the risk of HAC of their high-strength alloys.
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From page 59... ...
These investigations are necessary to definitively establish the origins of these cluster failures and to prove the effectiveness of mitigation strategies. Finding: Many of the critical failures observed to date have been associated with environmentally assisted cracking (primarily identified as hydrogen embrittle ment)
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From page 60... ...
Finding: Bolt designs, as currently specified in the oil and gas industry, utilize standard well-accepted thread designs. These thread designs can result in extremely high stress concentrations at the thread roots -- especially on the first thread root.
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