Abstract
Blow-out preventer (BOP) shearing is a critical element of well control affecting both safety and environment. BOP shear testing traditionally shows a wide range of actual pressures to shear for any given tubular size and grade. Universal predictor formulas are often dramatically conservative in order to cover outlier data in shear testing. The drilling industry needs improved theoretical understanding of the BOP shearing process. A correct constitutive model for material behavior is critical for theoretical modeling. The Extended Mohr-Coulomb (EMC) model is viable in studies of ductile fracture, but determination of the EMC coefficients for particular material can involve extensive specialized material testing. Theoretical work has focused on developing EMC coefficients with an intermediate level of theory involving cell modeling of void coalescence for high triaxiality (tensile dominated) stress conditions. With this method, EMC coefficients can be determined from standard material tensile tests. This method shows promise for practical application to prediction of BOP shearing of drilling and completions tubulars.
Material tests were performed on drill pipe remnants after full-scale BOP shearing tests were performed. This material test data was used with void coalescence theory to determine ECM coefficients which were then used in Finite Element Analysis (FEA) simulations of the full-scale BOP shears. The FEA used an element death technique to simulate pipe fracture and crack growth. In this initial phase of study, comparisons of theoretical predictions of maximum shearing force were mixed, but much was learned about the mechanism of BOP pipe fracture. Variation in properties from multiple material specimens taken from both adjacent locations and remote locations of the same drill pipes also indicated a larger than expected local variation in material properties. This fact, along with comprehension from the FEA that only a small zone of the drill pipe is involved in the shear initiation process suggests the cause of the wide variation which is observed in full-scale BOP shearing even when multiple shears are performed on a single pipe. Extensions of this study plan to investigate sensitivity of the theoretical model to characterization of the plastic strain hardening model, and will explore the use of a new void coalescence theory which has been developed for low-triaxiality (shear and compression dominated) ductile failure.
