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09305 Subsea Dissimilar Joints: Failure Mechanisms and Opportunities for Mitigation

Picture for 09305 Subsea Dissimilar Joints: Failure Mechanisms and Opportunities for Mitigation
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Product Number: 51300-09305-SG
ISBN: 09305 2009 CP
Author: Lee S. Smith, Mike F. Gittos and Viviane C. M. Beaugrand
Publication Date: 2009
Industry: Materials Selection and Design
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Dissimilar joints are a common part of subsea oil and gas systems where clad forged fittings are joined to clad or CRA pipe. Whilst the majority of these joints have given successful service, a small number have failed. In the current work, environmental testing was carried out on a dissimilar joint to an 8630M forging, buttered using alloy 625. Tests were monitored using a sensitive technique in order to capture the first moments of crack initiation and to determine the minimum threshold stress intensity factor Kth, below which there is no environmentally assisted cracking. Two distinct initiation mechanisms were identified; one dependent on hydrogen-charging in service (under cathodic protection) and one not. A detailed microstructural investigation, including chemistry, structure and strain, gave insight into the attributes of the ‘partially mixed zone’ (PMZ) immediately adjacent to the fusion boundary, on the weld side, in which one of the failure modes occurred. The chemical and microstructural characteristic of this PMZ are influenced by welding and postweld heat treatment (PWHT) parameters. Based on this investigation, the potential for mitigation of cracking susceptibility by materials selection or by modifying the welding/PWHT procedure is discussed.

Key words: Dissimilar joints, subsea, hydrogen embrittlement, cracking, microstructure
Dissimilar joints are a common part of subsea oil and gas systems where clad forged fittings are joined to clad or CRA pipe. Whilst the majority of these joints have given successful service, a small number have failed. In the current work, environmental testing was carried out on a dissimilar joint to an 8630M forging, buttered using alloy 625. Tests were monitored using a sensitive technique in order to capture the first moments of crack initiation and to determine the minimum threshold stress intensity factor Kth, below which there is no environmentally assisted cracking. Two distinct initiation mechanisms were identified; one dependent on hydrogen-charging in service (under cathodic protection) and one not. A detailed microstructural investigation, including chemistry, structure and strain, gave insight into the attributes of the ‘partially mixed zone’ (PMZ) immediately adjacent to the fusion boundary, on the weld side, in which one of the failure modes occurred. The chemical and microstructural characteristic of this PMZ are influenced by welding and postweld heat treatment (PWHT) parameters. Based on this investigation, the potential for mitigation of cracking susceptibility by materials selection or by modifying the welding/PWHT procedure is discussed.

Key words: Dissimilar joints, subsea, hydrogen embrittlement, cracking, microstructure
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