Refinement of the Drillpipe-Slip Mechanical Model (Russian)

Abstract

The PDF file of this paper is in Russian.

Slips, slip elevators, spiders, etc. have been widely used as oilfield service tools to grip and hold drill pipe, drill collars and casings by acting as a wedge. A pipe in a slip with a conical bearing surface is subjected to complex loads comprised of tensile, compressional, radial and bending stresses. To increase the holding force, the bearing surface of the gripping device in the form of a dies set is provided with teeth penetrating into the pipe body. This generates additional drag force on the contact surface that supplements the friction force.

This paper presents a new slip model, which uses the dependence of the drag force on the teeth penetration depth. The model is based on an improved dependence of the drag of wedge-shaped die teeth not only on the friction force in the "die-pipe" pair but also on the shearing force arising during teeth penetration in the pipe body. Equation parameters have been verified by multiple finite element (FE) calculations and experimental studies of dependence of the penetration force on the drag of a single tooth. The developed equations are used in the complete drillpipe-slip model. It accounts for pipe wall deformation under compressive stress generated by the slip and the drillstring weight and determines the active bearing length of the slip under load. The paper presents calculations of allowable areas for a slip of a specified length equipped with toothed dies based on the load carrying capacity and pipe strength.

The proposed model can be used for operative calculations of the slip carrying capacity, as well as for the development of new slip designs.