Bolting Reliability for Offshore Oil and Natural Gas Operations Proceedings of a Workshop (2018) / Chapter Skim
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2 Panel 1: Subsea Fastener Design Requirements
2 Panel 1: Subsea Fastener Design Requirements
Pages 8-30
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From page 8... ...
He thanked the National Academies for inviting him and emphasized GE's commitment to protecting the environment and the safety of oil and gas industry workers around the world. Noting that his own son is a mechanical engineer who 8
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From page 9... ...
Failure Mechanisms and Options for Mitigation Turning to potential failure mechanisms, Haeberle introduced the work of API's Multi-Segment Task Group on Bolting, which formed in 2015 and issued a report in 2016 that included detailed mapping of potential failure mechanisms, existing API specifications and industry standards that work well to mitigate the risk of failure, and items and actions other API and ASTM International committees should examine to further reduce failure risks (see Figure 2.1)
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From page 10... ...
In the context of this change, Haeberle noted that introducing lower maximum hardness values for bolts without reducing the pressure rating or load bearing capacity of certain existing components may be challenging. The API task group also created detailed failure maps, which include a failure's contributing factors, mitigating factors, and process elements, plus recommen
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From page 11... ...
, provides anufacturers with m material processing parameters and helps end users take appropriate recautions in p FIGURE 2.2 Environmentally assisted cracking (EAC) results from a combination of a susceptible material, environmental conditions that can cause EAC, and a tensile stress that exceeds the threshold to cause cracking in the susceptible material.
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From page 12... ...
The characteristics of certain nickel-based alloys could make them more effective for subsea applications, but they need to have properly defined limits to mitigate risks. Besides HE, the API task group also examined general stress corrosion crack ing.
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From page 13... ...
Wrapping up, Haeberle reiterated that bolt failure on subsea drilling equipment can result in major or minor losses of control or containment and offered a quick recap of the API task group's 2016 report, which identified multiple failure mechanisms, mitigating factors, and areas of further study. In particular, the task group identified areas where mitigations in current specifications are either nonexistent or insufficient, as well as key knowledge gaps with regard to defining the limitations on low alloy steels and CRA bolting materials.
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From page 14... ...
Offshore drilling risers are extremely large and complex structures that vary greatly in size and include multiple flanges and bolts. Offshore oil production equipment relies on the same materials and fasteners.
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From page 15... ...
Most fastener components are also protected with CP via anodes, some of which are placed very close to flanges. Choosing the Right Materials The criteria for selecting bolt materials focus mainly on mechanical properties, such as strength and toughness; corrosion resistance, particularly with CRAs; and resistance to EAC, such as stress corrosion cracking, HE, or sustained load cracking.
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From page 16... ...
High-strength steels combined with CP have been found to be susceptible to stress corrosion cracking and HE. In fact, their susceptibility to cracking increases as their yield strength increases.
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From page 17... ...
This principle is true of the 18 fastener materials currently in use, Esaklul said. Precipitated hardened nickel alloys have some potential but are much more expensive than low alloy steels.
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From page 18... ...
Other variables to consider include CP protection, zinc coating, and anode interactions. Despite few failures, Esaklul emphasized that alloys for subsea use must adhere to materials specifications and quality assurances for heat treatment, cold work, and maximum hardness to ensure EAC resistance.
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From page 19... ...
Causes of Hydrogen Embrittlement As discussed by other speakers at the workshop, bolt failures can happen through a variety of mechanisms. Ductile failure can result from overload, fatigue, or the use of insufficient materials.
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From page 20... ...
But even in those harsher North Sea conditions, drilling in relatively shallow water does not require high-strength bolts, and there have not been many bolt failures. As other speakers mentioned, deeper water brings new challenges.
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From page 21... ...
Kane noted that while one alloy, A286, was most resistant to HE in NASA testing, it was susceptible to chloride stress corrosion cracking. This example illustrates that while HE is a primary concern, hydrogen stress cracking, sulfide stress cracking, and liquid metal embrittlement should not be discounted as potential mechanisms of failure.
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From page 22... ...
Evolution of Bolt Design For Rolls-Royce Naval Marine, fatigue is considered to be the predominant design challenge for underwater bolts, even more so than corrosion. Stefansson described a unique bolt design used by Rolls-Royce, made of K-monel, a nickel al loy.
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From page 23... ...
The second-generation design was developed in the late 1980s, when propellers got bigger and blades got heavier, adding demands that exceeded the capabilities of Morgrip bolts. For this second-generation design, Rolls-Royce chose an Inconel alloy for the material, which has a yield strength of 150 ksi.
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From page 24... ...
A Rigorous Design Process Stefansson offered an overview of the Rolls-Royce bolt design process. The process begins with the propeller's particulars, such as power, RPM, and diameter.
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From page 25... ...
He began with a brief overview of the panel presentations: Haeberle detailed API's standards to mitigate bolt failure; Esaklul covered the material characteristics required for successful fastener performance; Kane described the environmental conditions that bear on bolt performance; and Stefansson discussed the design and testing process for undersea bolts in the context of ship propellers. Several trends emerged in the discussion.
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From page 26... ...
Material Quality Clyde Briant, Brown University, asked Haeberle to expand on his remarks about the preparation of the steel and whether enhancing quality could increase hard ness maximums. Haeberle remarked that the manufacturing equipment, the bolt designs, and many of the specifications in use today are from the 1970s and 1980s, though they have been revised significantly over the decades.
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From page 27... ...
He asked if there was a plan to replace those bolts, or whether the specification only addressed new bolts. Haeberle replied that while some bolt failures from BSEE's QCFIT report were zinc electroplated, not all subsea bolts are.
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From page 28... ...
The topic was further discussed in Fleece's presentation on Day 2 of the workshop. Torquing Hilbert asked if it is true that ductile failures are much rarer, and whether this is because there is enough yield strength in the material.
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From page 29... ...
Stefansson noted that impressed currents diminish as they travel farther from their source and can't protect anything inside a closed structure. Kane mentioned that CP is usually used to protect the upper BOP stacks, and anodes protect systems closer to the bottom of the ocean floor, but we don't necessarily know how well these systems coexist.
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From page 30... ...
Examples of root causes, he explained, might include poor manufacturing processes, improper heat treatment, or the quality of the material. Brahimi stressed the need to be able to control these procedures to ensure uni formity and asserted that the emphasis of failure investigations should be on root causes rather than mechanisms of failure.
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