Subsea oil and gas production systems can be subject to Hydrogen Induced Stress Cracking (“HISC”) depending on the material, cathodic protection and other factors. A failure in this kind of systems can lead to safety issues as well as environmental hazards and high repair costs. The analysis of recent failures has led to the recognition of HISC as a very important issue related to local stress and strain. This has necessitated the extensive use of Finite Elements Methods for the analysis of all system components. Since HISC is a recent issue, there are very few cases of such assessments reported in the literature. This paper is based on the assessment of the susceptibility of subsea piping manifolds of Duplex stainless steel to Hydrogen Induced Stress Cracking, which was conducted during the Skarv project by General Electric Oil & Gas. A variety of cases consisting of different loads and configurations were considered to give a broad assessment using a recently developed code (DNV-RP-F112-October2008). This work has led to the development of a set of procedures and models for the assessment of the entire system which is described in the current paper. The proposed methodology is useful for both design purposes and also for the verification of parts, which, if found to be non-compliant, would require redesign. In general, parts that were determined to be non-compliant using a linear assessment were found to be compliant through non-linear analysis, in fact 3D plastic analysis leads to a redistribution of stress and strain and hence, to lower values. “Cold creep” was not considered since the levels of stress and strain were considered to be low enough to avoid this phenomenon. As a consequence of this experience, a new methodology was developed, which is able to speed up the analysis process and to predict local stresses from only pipe elements. The latter permits the use of a linear assessment for bends, T junctions and weldolet even with misalignment and erosion, avoiding the need to perform 3D analysis. The second part of the paper describes this method.
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