Video: API 6A and 17D Bolting Stress Analysis and Fatigue Consideration
- J. I. Lim (Cameron, a Schlumberger Company) | P. Bunch (Cameron, a Schlumberger Company) | Y. Han (Cameron, a Schlumberger Company)
- Document ID
- Offshore Technology Conference
- Publication Date
- Document Type
- 2017. Copyright is retained by the author. This presentation is distributed with the permission of the author. Contact the author for permission to use material from this video.
- 4.2 Pipelines, Flowlines and Risers, 4.5.6 Subsea Production Equipment, 4.2.3 Materials and Corrosion, 4 Facilities Design, Construction and Operation, 4.5 Offshore Facilities and Subsea Systems
- Fatigue Life Cycle, Finite Element Analysis, Fracture Mechanics, Stress Range, Bolted Connection Assembly
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API 6A and 17D, specifications provide the design basis for surface and subsea production equipment bolting in the oil and gas industry. These documents provide both design stress allowables and material requirements for bolting. API defined bolt preload requirements are 0.5 times the material minimum yield strength and 0.67 times the minimum yield strength for API 6A and 17D, respectively. The maximum allowable stress in the bolt due to pressure, thermal and external loading is defined to be 0.83 times the minimum yield strength based on the nominal stress in the bolt at the minimum root diameter.
Given these design criteria this paper evaluates the peak stresses in the threads for both principal stresses and combined component stresses and correlates them to the nominal stress in the bolt to define a stress concentration factor (SCF) value based on bolt size. Three bolt sizes, 1", 2" and 3" were evaluated where each bolt was modeled as a single bolt assembly simulating a nut to bolt connection, a flange thickness and a threaded API body. Each design was evaluated using B7 bolt, 2H nut and API 75K body material properties. The finite element analysis (FEA) considered both elastic and elastic-plastic material properties for comparison of results. Using the results of the FEA, stress range vs. cycles, S-N, and fatigue characteristics of the bolt assembly were evaluated based on the nominal bolt stress range vs. the root radius stress range in the highest stressed threads. The S-N fatigue life predictions for bolting are evaluated based on mean stress and peak stress ranges in the bolt threads. As an alternate to S-N evaluation, the stress distribution at the root radius of the thread was input into the fracture mechanics analysis as a weight function for calculating crack tip stress intensity. The analysis is based on elastic stress distribution in the threads and an assumed intial defect size based on nondestructive examination (NDE) inspection. Issues of defining material fatigue properties for loading conditions of bolt threads and the environment of saltwater with cathodic protection (CP) are addressed.