Determination of the Load Carrying Capacity of Damaged Pipes Using Local Approach to Fracture

Abstract

The subject of this study was the application of local approach to ductile fracture in order to estimate the integrity of damaged seam casing pipes for oil and gas drilling rigs. The experimental testing included tensile testing of specimens and a pressure test of a pipe with different levels of damage simulated by machined notches. In exploitation, such structures (i.e., pipes with local thin areas) can fail by the ductile fracture mechanism or by plastic collapse in the ligament. However, the majority of the procedures for determining their integrity are based on limit loads, i.e., plastic collapse criteria. In this work, a pipe subjected to internal pressure was modelled using the finite element method and local approach to fracture (the Complete Gurson Model - CGM), with the aim of determining damage development in the material (i.e., at the bottom of a machined defect) and of establishing the criteria for the maximum pressure that a damaged pipe can withstand. The results obtained using the micromechanical model are discussed and compared with several often used limit load expressions from the literature and a stress-based finite element criterion. It is shown that local approach can give appropriate results and represent failure criterion for pipes with local thin areas. [doi:10.2320/matertrans.M2011210]