Fracture mechanics assessments for pressure vessels are performed to determine critical flaw sizes and/or estimate the fatigue life of a growing crack as a means of establishing inspection intervals for the equipment. In most cases the evaluation is performed based on methods described in API 579-1/ASME FFS-1 and BS7910. With regards to the subsea HPHT components the most referred standards for evaluating crack like flaws are API RP 17TR8 and ASME Section VIII Division 3. The approaches described in these standards are mostly based on a linear elastic fracture mechanics. &nbsp;Even though plasticity can be accounted for by using a failure assessment diagram (FAD); however, even with this approach the effect of plastic strain around the crack is not explicitly considered.This paper presents an approach as per API 579, Annex 9G.5 which recommends utilizing a driving force method whereby the J-integral is directly evaluated from an elastic-plastic finite element model. The main goal is to study differences between the FAD approach with a linear elastic fracture mechanics calculation for the crack against the elastic-plastic J-integral based approach wherein the crack embedded in a plastic zone is modeled explicitly. Simplified representative geometries are considered for this study. The plasticity modeled is localized in the region of a stress concentration. Two scenarios for the plastic zone are considered a) crack present during initial loading with no residual plastic strain and b) load application following an unloading cycle with the presence of a residual plastic zone. Different crack sizes are considered for this comparison study ranging from small cracks completely embedded within the plastic region and larger cracks with partial embedment.The paper aims to identify the impact of the presence of a residual plastic strain on the crack response. Additionally, the effect of large-scale plasticity on the crack driving force is determined relative to similar conditions without plasticity. &nbsp;The paper presents comparison studies which highlight the key differences between different analysis approaches with the aim of identifying the most conservative assessment method for different crack sizes relative to plastic zones.

Evaluation of Cracks Embedded in Zones With Large Scale Plasticity in Subsea HPHT Equipment – Comparing FAD and Driving Force Approaches

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

ASME 2021 Pressure Vessels & Piping Conference (PVP2021)

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