Basic Fluid and Pressure Forces on Oilwell Tubulars

Abstract

The object of this paper is to clearly and simply define the basic fluid and pressure forces on oilwell tubulars with respect to stresses, buoyancy, neutral point, and the effective buckling force denoted as the "fictitious force." The paper is designed as a preface to Lubinski and Hammerlindl. Introduction Conventional thought seems to be that an understanding of the basic fluid and pressure forces in oilwell tubulars is the domain of experts only. The objective of this paper is to disprove this by presenting a simple explanation of these basic forces. presenting a simple explanation of these basic forces. Part of the misunderstanding results from the belief that the problem is complex and difficult, when actually only an elementary knowledge of pressure and stress is required. In purely mechanical pressure and stress is required. In purely mechanical systems (i.e., no fluid or pressure), most engineers readily can determine and understand the forces associated with Hooke's law and buckling. However, with the addition of fluids, the engineer may become confused as to (1) whether buoyancy is a concentrated or distributed force, (2) why the neutral point is not at the point of zero axial stress, and (3) point is not at the point of zero axial stress, and (3) the nature of the nebulous force denoted as the "fictitious force" by Lubinski and Blenkarn. All of the above can be understood easily by observing the distribution of the principal stresses. This paper interrelates these stresses with buoyancy, the neutral point, and the fictitious force. point, and the fictitious force. The objectives of this paper are to give the practicing engineer a working knowledge of basic practicing engineer a working knowledge of basic fluid and pressure effects on oilwell tubulars and to increase the industry's ability to understand and advance the existing literature. Stress Distribution in the Absence of Fluid Fig. 1a illustrates a freely suspended tube of constant inside and outside diameter. In the absence of fluid, the axial stress at any point is equal to the weight below that point divided by the cross-sectional area of the tube at that point. Thus, the axial stress at a point a distance x from the lower end may be point a distance x from the lower end may be determined from (1) Fig. 1b shows the axial stress distribution in the absence of fluid, over the entire length.Throughout this paper, compressive forces are considered to be positive and tensile forces are considered to be negative, accounting for the minus sign in Eq. 1 and the equations that follow. Also, all equations pertain to a system of constant units.The radial stress at any point is obtained from (1) where p and p are the internal and external pressures, respectively, at the point of interest. pressures, respectively, at the point of interest. JPT P. 153