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

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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.

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Avoid Potential Repeated Failure (Cracked Gas Compressor line) Through Proper Material Assessment

Product Number: MPWT19-14239

Author: Ahmed Elsharkawi, Amro Hassanein

Publication Date: 2019

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The heat treatment condition of industrial materials is a critical parameter for material evaluation and its fitness for intended service. Proper heat treatment will produce desired mechanical and physical properties, while absence or improper heat treatment may lead to major failure with huge production, Environmental, Health, and Safety (EHS) impacts. We hereby explain an actual case for cracked gas compressor (CGC) 5th stage discharge line caustic stress corrosion cracking (SCC) that caused unplanned plant shut down and resulted in noticed financial and production loss.
The proven root cause is absence of normalization. Emergency piping batch is received and supposed to be normalized to avoid failure recurrence. Many discrepancies extracted from submitted material certificates, also many physical signs observed on the material itself raising doubts about received material compatibility. Using advanced Positive Material Identification (PMI) device to verify the chemical composition of the received material, results show that the material is questionable.
As a precise test to verify heat treatment condition, microstructure analysis test (metallography) conducted to confirm normalization condition of the material, the resulted grain structure size and growth confirm that one of the received pipes has improper or absence of normalization.
The material rejected as it is proven and confirmed that it is not normalized as per the requirements to avoid further potential of hydrocarbon leakage due to improper material specification. Rejecting the material eliminate the potential of having repeated failure, in addition to 600,000 Saudi Riyal (SR) cost saving as material cost. A common recommendation shared with concerned parties to consider metallography as a mandatory test to be submitted with heat-treated material test certificate (MTC).
Key words: Heat treatment, microstructure, normalization, grain structure, failure, metallography