Fracture Mechanics Assessment of Cracks in Areas of Large Scale Plasticity in Subsea HPHT Equipment

Abstract

Subsea HPHT components may be evaluated utilizing a fracture mechanics-based approach as per guidelines in API RP 17TR8 and ASME Section VIII Division 3. Typically, the assessment is performed based on methods described in API 579-1/ASME FFS-1 and BS7910. The analysis is performed to determine critical flaw sizes and estimate the fatigue life of a growing crack as a means of establishing inspection intervals for the equipment. In most cases, the assessment is based on a linear elastic fracture mechanics approach. The effect of plasticity is generally included via the use of a failure assessment diagram (FAD); however, even with this approach the effect of plastic strain around the crack is not explicitly considered. The assessment standards do not provide clear guidance for cases involving a crack which is located within a region of large-scale plastic strain. For these cases, API 579, Annex 9G.5 recommends utilizing a driving force method whereby the J-integral is directly evaluated from an elastic-plastic finite element model. This paper presents such an approach. A simplified representative geometry is considered for this study. A region of a stress concentration, such as is typically encountered near an internal radius is considered. Such a region can potentially show localized plasticity. J-integral is calculated by explicitly modeling a series of cracks of increasing depth through this zone of plasticity and the results are compared to the different methodologies described in API 579-1/ASME FFS-1 and BS7910. Cracks are modeled both completely and partially enveloped within the plastic zone. Results are summarized and compared, highlighting the key differences between different analysis approaches with the aim of identifying the most conservative assessment method for different crack sizes. Additionally, the effect of large-scale plasticity on the crack driving force is determined relative to similar conditions without plasticity. The results indicate that for cracks lying within the regions of localized plasticity, using an API579 Level 2 approach coupled with extracting elastic-plastic through wall stresses from an uncracked geometry may result in significant under prediction of the driving force. Conversely, extracting linear elastic stresses from an uncracked geometry may significantly over predict the driving force and may prove too conservative for determining acceptable crack sizes. This paper presents a comprehensive comparison of different analysis approaches used for evaluating cracks in subsea equipment. The results indicate that, for HPHT equipment with increased safety implications, a detailed elastic plastic fracture mechanics evaluation of the cracked geometry should be performed for cases in which localized plasticity is expected to occur.