Analysis on mechanical influencing factors of fatigue behavior for the packer on pipe ram in snubbing unit

Abstract

In snubbing service, the ram blowout preventer (BOP) is switched frequently, and the stress condition of the packer is very complicated. Cracks, rubber drop, and other fatigue failures often occur. The load characteristic was firstly analyzed in this paper based on the stress condition of packer in snubbing service. The mechanical property of rubber commonly used for packer was tested, and the performance parameters were obtained. Based on the crack initiation approach, taking the rubber matrix as the research object, the effective stress, the maximum Green–Lagrange strain and strain energy density were selected as damage parameters. The distribution contour of three damage parameters of the rubber matrix of the packer under different stress conditions and speeds of lifting or lowering pipe were obtained using finite element method (FEM). The effect of different stress conditions and speeds of lifting or lowering pipe on the fatigue behavior for the rubber matrix was analyzed. The result shows that the value of three damage parameters is relatively high on the upper area of the rubber matrix, causing the first occurrence of fatigue in the upper part, which agrees well with the oilfield application. The speed of the pipe does not change the location of fatigue point and the value of damage parameters, so that it has little effect on the fatigue behavior of the packer's rubber matrix. Finally, based on Miner linear damage rule, a calculation method for the fatigue behavior of the packer is presented.