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01015 BRITTLE FRACTURE IN AN UPPER TREE CONNECTOR SYSTEM AT MENSA - An Analysis

Picture for 01015 BRITTLE FRACTURE IN AN UPPER TREE
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Product Number: 51300-01015-SG
ISBN: 01015 2001 CP
Author: Robert Mack and Steve Norton
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Mensa is a gas field with three subsea wells that are located in 5300 feet (1600 m) of water in the Gulf of Mexico. On January 2, 1998, the outer housing (OH) of the upper tree connector of the Mensa A-1 wellhead failed in service in a completely brittle manner and directly caused the immediate shut-in of the A-1 well. All safety systems worked as designed; therefore, a minimal amount of hydrocarbons were released to the environment. The fracture initiated at the root of two diametrically opposed keyways in the OH and propagated rapidly, rendering the OH into three large pieces. The cause of the failure was the extremely poor fracture toughness of the forged, quenched and tempered, AISI 4140 OH. The Charpy impact toughness of the material at room temperature was determined to be 2.5 to 4.0 ft-lbs (3.4 to 5.4 J); the fracture mode was 100% brittle. Therefore, the flaw tolerance of the OH was extremely poor. Contrary to the actual properties of the A-1 OH, information in the material certificates indicated very good impact properties as measured using quality test coupons (QTC) per API 6A. The results of the failure analysis of the OH, and the results from the evaluation of other OH's purchased for Mensa and in service at the time of the A-1 failure are presented in this paper. The results from this study strongly suggest that "small" QTC samples are inadequate as a QA/QC tool to assure the fitness-for-service of large, low alloy steel forgings. Therefore, it is proposed that API Standard 6A be revised to specify better QA/QC processes to determine the quality of all steel forgings. Such improvements could prevent failures like the one that occurred at Mensa. Keywords: Mensa, subsea, wellhead, connector, Charpy impact, brittle, failure, gas, production, well, heat treatment, API, QTC, quality control.
Mensa is a gas field with three subsea wells that are located in 5300 feet (1600 m) of water in the Gulf of Mexico. On January 2, 1998, the outer housing (OH) of the upper tree connector of the Mensa A-1 wellhead failed in service in a completely brittle manner and directly caused the immediate shut-in of the A-1 well. All safety systems worked as designed; therefore, a minimal amount of hydrocarbons were released to the environment. The fracture initiated at the root of two diametrically opposed keyways in the OH and propagated rapidly, rendering the OH into three large pieces. The cause of the failure was the extremely poor fracture toughness of the forged, quenched and tempered, AISI 4140 OH. The Charpy impact toughness of the material at room temperature was determined to be 2.5 to 4.0 ft-lbs (3.4 to 5.4 J); the fracture mode was 100% brittle. Therefore, the flaw tolerance of the OH was extremely poor. Contrary to the actual properties of the A-1 OH, information in the material certificates indicated very good impact properties as measured using quality test coupons (QTC) per API 6A. The results of the failure analysis of the OH, and the results from the evaluation of other OH's purchased for Mensa and in service at the time of the A-1 failure are presented in this paper. The results from this study strongly suggest that "small" QTC samples are inadequate as a QA/QC tool to assure the fitness-for-service of large, low alloy steel forgings. Therefore, it is proposed that API Standard 6A be revised to specify better QA/QC processes to determine the quality of all steel forgings. Such improvements could prevent failures like the one that occurred at Mensa. Keywords: Mensa, subsea, wellhead, connector, Charpy impact, brittle, failure, gas, production, well, heat treatment, API, QTC, quality control.
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