A wellhead housing was removed from an abandoned subsea well, which on examination revealed extensive damage due to corrosion and erosion to the internal surface. The equipment in question was an 18 3/4-in.-diameter wellhead body rated at 15,000 lb/in. 2 and manufactured from AISI 8630. Owing to the damage caused by corrosion during production, the inside surfaces of the wellhead contained areas that were under the minimum wall thickness. The manufacturer claimed that a procedure was available to restore the wellhead housing to its original integrity in two stages. The first stage concerned bead-on-plate trials to establish the mechanical properties at specified PWHT temperatures and times. Depending on the results, the second stage would involve qualifying butt joint weld procedures for repairing the internal damage to the wellhead housing. Material selected for the first stage involved removing a 12-in. section from a wellhead forging. Welding processes used for the bead-on-plate trials involved shield metal arc welding (SMAW), submerged arc welding (SAW), and gas tungsten arc welding (GTAW). Procedures to be used for the bead-on-plate trials were based on the parameters selected for welding the full penetration butt joint weld procedures for qualifying the buttering technique. The segment removed from the well-head forging was split into four test blocks identified as A, B, C, and D. Each test block was PWHT at the following tempeatures; B: 677 C deg/6 h, C: 670 C deg/7 h, D: 670 C deg/10 h. Block A was retained to verify the mechanical properties in the supplied conditions. Although the investigated yield strength was lower than the necessary 85,000 lb/in. 2, the manufacturer was confident that the material properties and PWHT applied for the weld procedures would be sufficient to achieve the demanded values. But as the operator insisted, the manufacturer could not substantiate that the repairs were in accordance with ASME IX. It was shown that the prolonged PWHT time causes a drop in tensile and yield strength. Subsequent metallography revealed extensive carbide precipitation after PWHT. Based on this results it is recommended that tighter control on PWHT and stress-relieving should be applied.


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    Title :

    Postweld heat treatment is critical to refurbishing a wellhead housing


    Contributors:
    Still, J.R. (author) / Blackwood, V. (author)

    Published in:

    Welding Journal, New York ; 83 , 10 ; 34-38


    Publication date :

    2004


    Size :

    5 Seiten, 10 Bilder, 3 Tabellen, 2 Quellen



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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