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Effect of Low Frequency on the Fatigue Response of Wellhead Extension Joint Welds Exposed to Simulated Seawater with Cathodic Protection

This work considers wellhead extension joint welds for high-pressure deepwater Gulf of Mexico prospects with a design shut-in tubing pressure (SITP) above 15000 psi. The structural welds joining 75K 8630M low pressure wellhead housing (LPWH) forgings to API 5L Grade X80M PSL 2 conductor casing are expected to be fatigue-sensitive due to water depths approaching 6000 ft. and substantial estimated weight of a 20K BOP stack. Cyclic loading is expected to be from wave‑induced motions (WIM) and vortex‑induced vibration (VIV). The wellhead extension joints extend 10 to 15 ft. above the mudline and will be exposed to seawater as well as cathodic protection when the wellhead is connected to either the BOP stack or the tubing head spool and subsea tree.The present work considered both S-N and fracture mechanics based designs. Fatigue curves such as those in BS (British Standard) 7608 are not yet proven for structural steels with yield strengths in excess of 400 N/mm2 (58 ksi) in the presence of cathodic protection and fatigue crack growth laws such as those in BS 7910 for steels in a marine environment are only applicable to materials with yield strengths less than 600 N/mm2 (87 ksi). Materials in the subject assemblies may have yield strengths in excess of 690 N/mm2 (100 ksi) so both S-N and fatigue crack growth rate (FCGR) testing in a simulated seawater with cathodic protection environment were determined to be appropriate. This paper describes the environmental fatigue testing of welds and the relationship between the test data established fatigue curves and published literature. Tests were performed in a 3.5% NaCl solution at 40 °F with pH adjusted to 8.2 and an applied potential of -1100 mV Ag/AgCl. Frequency scans were conducted in several weld microstructures to assess growth rate sensitivity of each microstructure to cyclic frequency and the fatigue response at the low end of the frequency range was explored in greater detail.

Product Number: 51319-13071-SG
Author: Adam Rowe
Publication Date: 2019
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