Results are presented from an experimental study conducted to determine friction coefficient behavior in OCTG connections. The study was conducted using the OCTG friction and galling tester at Texas Tech University. The tester employs tubular pin and box specimen pairs fabricated from 73-mm (2 7/8 in.) diameter OCTG tubing. Friction coefficient was determined for test specimens made of material grades L80 and J55 at loads of up to 620 kN (140,000 lb) acting on a surface area of up to 645 mm2 (1 in.2) with sliding velocities of 0.53 and 1.78 cm/s (1 and 3.5 ft/min), and nominal surface roughnesses of 0.37, 1.2, and 1.5 micron (15, 47, and 60 μin.) Ra. An API Modified thread compound was used for all tests. When plotted versus the applied load, the friction coefficient behavior is seen to be highly nonlinear. In addition to load, the variables of sliding velocity, material, and surface roughness are shown to have an effect. Preliminary experiments show that the test surface geometry also has a significant effect on the friction coefficient.

A numerical analysis of the micromechanical behaviour of a granular material is described using a new program MASOM based on Cundall's discrete element method. In the analysis the individual grains which make up the material are taken to be deformable 2D polygons of arbitrary size and shape. Contact forces between the grains are calculated according to Mindlin's solution for frictional contact between elastic bodies. The material in each grain is taken to be linear elastic but limited by the fracture strength of the material. Fracture is permitted along any one of a number of candidate fracture planes if an associated compressive load tending to split the gain reaches a critical level. Fragments of fractured grains are carried until they become too small to track using the explicit time integration algorithm used to advance the solution. The MASOM program is able to consider a number of different classes of elements and different types of contact between the various classes. Thus, in addition to the granular material the program can also model containers and loading devices. The program is used to simulate uniaxial and triaxial compression tests for geological materials. The results are shown to give results for stress‐strain and stress difference versus pressure which are in qualitative agreement with test data. The numerical results reveal a very complex micromechanical behaviour in granular materials, including highly variable and rather unstable load paths and a very inhomogeneous load distribution within a representative sample of the material. A video of the response of a typical frictional material to applied loads shows an interesting localized effect near sample boundaries involving crowding together of grains which cannot be observed using conventional static field plots.

This paper presents a new equation for predicting the collapse of tubulars under external pressure. The development of the equation is based on a large number of non-linear finite element simulations of tubulars with different geometrical tolerances and mechanical properties. The simulation results for the collapse pressure and post-collapse geometry of the tubulars were verified with full scale physical tests. A nonlinear regression analysis was then performed on the data obtained from the simulations to optimize the parameters of the collapse equation. The new equation accounts for variations in the tubular diameter, wall thickness, ovality, eccentricity, and material elastic-plastic behavior.This new collapse equation provides significant advantages over existing formulas, mainly because it was developed based on non-linear mechanics solutions of the collapse problem as compared to equations derived from statistical analysis. In addition, the equation gives the true collapse pressure of the tubular, whether the tubular collapses elastically or after it has yielded. The new equation could be used to design tubulars based on manufacturing tolerances with respect to wall thickness, ovality, and eccentricity as well as material mechanical properties. This allows for design optimization which could account for significant cost savings, especially when designing expensive non-API tubulars such as corrosion resistant alloy tubulars. The new collapse equation presented in this paper provides accurate predictions of the collapse pressure of tubulars and accounts for tolerances in the tubular geometry and material elastic-plastic behavior. This new equation is simple and could be used in optimizing tubular design..References and illustrations at end of paper. 7