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Welding of Internally Clad X65 and X70 Pipes for Pre-Salt Subsea Oil Applications

Extraction of Brazilian pre-salt subsea oil reserves, located under 2000 m of water and up to 5000 m under the seabed, including a salt layer, require materials that can withstand higher pressures, temperatures, H₂S, and CO2 content than traditional oil extraction. X65 pipe internally clad with nickel-based Alloy 625 is currently considered, as it provides the necessary corrosion resistance. The pipes will be reeled after welding on site, meaning that no post weld heat treatment can be employed. During reeling, the pipe must yield before the weld does, resulting in a strength requirement. The goal is to join the pipes using a high deposition weld process that requires no post weld heat treatment (PWHT) with a filler material that overmatches the base metal strength by 100 MPa. The weld consumables considered for this application include low alloy steel, nickel-based superalloys, and super duplex stainless steel.

Low alloy steel meets the strength requirement but is susceptible to solidification cracking when deposited directly atop Alloy 625. Nickel-based superalloys will ideally achieve the required strength, possibly without precipitation of γ′ and γ′′, but are also susceptible to solidification cracking. Super duplex stainless steel meets the strength requirement without defects, however concern exists over embrittling phases and resistance to hydrogen induced cracking. While each approach has potential concerns, they are being tested to ascertain if the issues can be avoided and successful welds can be made. 

This project is a collaboration between three students, each with their own approach. Parameter development is performed in-house with automated welding for all test welds using Cold Metal Transfer (CMT), pulsed gas metal arc welding (GMAW-P), and a hybrid between the two, CMT+Pulse. Metallurgical characterization is performed with optical microscopy, scanning electron microscopy, electron back scatter diffraction, and mechanical testing. Bead-on-plate experiments were performed to assess material compatibility and groove welding is underway. Once defect free groove welds are accomplished, mechanical testing will be performed to verify strength requirements.

 

Industry Sponsors: Petrobras, Vallourec, Acute Technologies, Voestalpine Bohler

Faculty: Boian Alexandrov (OSU)

Graduate Students: Graciela Penso, Evan O'Brien, Emerica Suma

Industry Contacts: Diego Garcia, Carolina Vilas Boas, Tim Thompson, Hermann Weber