Bolting Reliability for Offshore Oil and Natural Gas Operations Proceedings of a Workshop (2018) / Chapter Skim
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3 Panel 2: Subsea Fastener Design Quality
3 Panel 2: Subsea Fastener Design Quality
Pages 31-59
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From page 31... ...
HYDROGEN EMBRITTLEMENT AND HIGH-STRENGTH STEEL FASTENERS Salim Brahimi, Industrial Fasteners Institute Salim Brahimi is the director of engineering technology at the Industrial F asteners Institute, which represents and advances the interests of the North A merican fastener industry and offers technical expertise, standards development, training on topics such as torque tension and the friction coefficient, and research support. Brahimi is also president of IBECA Technologies Corporation and research manager at the McGill Hydrogen Embrittlement Facility, and was recently appointed chair of ASTM International's subcommittee on Fasteners, F16.
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From page 32... ...
Some fasteners are used in self-drilling applications, where their high surface hardness makes them particularly susceptible to HE. This variety of criteria means that bolt design requires a range of expertise, and when failures occur, a broad spectrum of information is needed in order to pinpoint a root cause.
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From page 33... ...
is an indication that the material was treated improperly, Brahimi said, noting that in his experience, improper processing has been the root cause of many failures. To emphasize this point, Brahimi detailed several fastener failures: • In 2013, anchor rods on the San Francisco-Oakland Bay Bridge failed.
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From page 34... ...
A small change in their application could shift the "stress" circle of the diagram toward the others, resulting in HE, but they generally function well. HE in Subsea Fasteners Brahimi then turned to the role of HE in subsea bolt failures.
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From page 35... ...
Many failures are misdiagnosed because this mapping isn't conducted, and HE or stress corrosion cracking is labeled the "cause of the failure" as opposed to the "root cause." HE is a failure mechanism, Brahimi emphasized, not a root cause. In Brahimi's experience, the root cause is either a design issue in susceptible material or a manufacturing issue in nonsusceptible material.
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From page 36... ...
Microstructure Brahimi noted that his research uses materials created via ideal, tightly con trolled manufacturing processes, resulting in a uniform microstructure. While the industry may assume that all fasteners are made this way, the unfortunate reality is that they are not, and when bolts fail it is usually because of an imperfect procedure affecting the uniformity of the microstructure, Brahimi said.
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From page 37... ...
McCarthy pointed out that those two characteristics are different, and Brahimi agreed but added that they are complementary. The bending test measures a loss in strength, and a slow strain rate test measures a loss of ductility.
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From page 38... ...
Brahimi answered that he would still expect a 100 percent acceptance, but would want to look very closely at the fracture surface in the microstructure to check its reaction. An unnamed participant asked how a bolt below 39 HRC, and thus not sus ceptible to IHE, would behave when CP is applied.
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From page 39... ...
Once we identify, rank, and eliminate the variables that cause critical failure, we can test and improve noncritical variables. Instead of removing plating variables, for example, MacMoy posited that it would behoove the industry to standardize testing, enforce rules, and create strict regimes meant to "cover our bases" to minimize mounting risk.
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From page 40... ...
In addition to removing variables in fastener materials, MacMoy suggested supply chain vendors should be held to a higher standard. Comparing engineers to the armed services generals who direct strategy and vendors and suppliers to the captains and colo nels who provide on-the-ground intelligence, MacMoy suggested it would be best for engineers to reduce their reliance on testing and instead encourage vendors to increase their technical authority and abstract reasoning.
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From page 41... ...
Fastener Failures Amaya emphasized that bolt failure is an industry-wide problem that affects all manufacturers and all operators. Subsea bolt failures have made the news several times over the years, most recently in 2016 when several bolts failed on a BOP during routine maintenance.
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From page 42... ...
. Most bolt failures in subsea applications fail due to brittle fractures, Amaya said.
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From page 43... ...
Of these two categories, the high-strength fasteners L7, B7, and L43 raise the greatest concern, Amaya explained, since they are the most susceptible to HE from the sacrificial anode CP system. In addition, the higher the hardness of a carbon alloy steel component, the more susceptible it is to HE due to the distortion of the lattice structure.
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From page 44... ...
While this microstructural problem is rarely produced, the fact that it can happen at all means that it is a possibility for which we must test, so in addition to hardness testing, metallography is conducted to ensure there is no martensitic banding after continuous casting. In that example, before they identified the true root cause, investigators first asked whether the error could have been introduced via heat treatment.
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From page 45... ...
Hydrogen atoms generated by CP can penetrate the material there, get trapped, and potentially lead to HE if the stress level increases. Part of what makes failure analysis so difficult is that it takes just the right combination of material and processing factors for a bolt to fail.
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From page 46... ...
The issue of galvanic coatings that dissolve when put into a subsea seawater environment, Amaya explained, is that as they dissolve they are an added source of hydrogen. With hydrogen generated by the CP plus the additional hydrogen generated by the galvanic dissolution of the coating, conditions may arise where for a brief time a great deal of hydrogen is being pumped into a highly stressed area.
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From page 47... ...
A close look at DNV GL failures in the North Sea shows that 9 percent have been fastener failures. Among those, 65 percent were brittle fractures, 25 percent were fatigue failures, 5 percent were corrosion failures, and the remaining 5 percent were unidentified.
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From page 48... ...
Pointing again to the principles of "highly reliable organizations," Moghissi noted that such organizations are willing to add safety factors or extra performance components despite the cost or inconvenience. In the oil and gas industry, because we still don't know the distribution of unsafe materials, we are not yet in a position to add safety factors.
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From page 49... ...
Because there are so few failures, data sharing is crucial to enable a widespread, thorough understanding of these underlying issues. A better understanding of distributions, how they aggregate, and their probabilities could lead to risk-informed decision making that could be more successful and meaningful than just "meeting a spec." PANEL 2 DISCUSSION: SUBSEA FASTENER DESIGN QUALITY John Scully moderated a discussion following the workshop's second panel on Subsea Fastener Design Quality.
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From page 50... ...
After a discussion about critical ity, participants discussed particulars around standards and specifications, bolt design and manufacturing, testing, and safety factors. Participants and panelists debated the influence of environmental factors versus the components and manu facturing of the material itself.
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From page 51... ...
Bennett expressed disagreement with some presenters who stated that hardness should be a focus, and asked the panel why API's 20E has hardness specifications when, as several people pointed out, it is an imprecise measurement and the specifications could include errors. Building on this point, Moghissi said that focusing on one characteristic oversimplifies the problem and ignores exceptions that have the potential to cause catastrophic failures.
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From page 52... ...
Subsea bolts are much larger, and the compressive stress effects are dramatically less than in a small bolt. Casting Participants discussed the continuous casting process to identify opportuni ties for testing and tracking quality in a given "heat." In response to a question by McCarthy, Goin defined a heat as the contents that are melted together in one pot.
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From page 53... ...
Also, rolled threads don't add compressive residual stress that can increase a bolt's fatigue limit. For aerospace's critical applications, bolts are thread rolled after heat treatment to increase the fatigue limit and counteract tensile stress in service.
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From page 54... ...
The specifications also require that bolt manufacturers check the properties of the material they buy, even after heat treatment. Asked if testing can be representa tive if it covers only one piece of a much larger body of material, Amaya replied that it should be, because furnaces have calibration requirements manufacturers have to meet yearly.
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From page 55... ...
Scully described how loading can increase hydrogen concentration and crack growth and lower threshold stress. As a result, slow strain tests, slow step rising load tests, and step loading tests have yielded very different loading rate results.
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From page 56... ...
Adding CP reduces the fastener strength and changes the safety factor. Brahimi noted that limiting subsea fastener strength introduces that safety fac tor by default, although it may not always be necessary to do so.
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From page 57... ...
Details about material susceptibility, stress, and safety factors have been discussed, but there's a great need to qualify and quantify the data, such as hydrogen charging, in the real environment. In his view, a test conducted in a beaker does not yet fully replicate the conditions at 10,000 feet below sea level.
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From page 58... ...
James Jennings pointed out that the microbiological environment is important to consider, too. He cited an example in which the Navy traced subsea bolt failures to microbes that interacted with the lubricants on the bolts.
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From page 59... ...
Moghissi has advocated for similar tests in the oil and gas industry, and 20E attempts to address manufacturing processes, but there is room for improvement. CP also needs to be more closely examined to understand the maximum amount of hydrogen a particular hardness can handle, he said.
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