Chuong 5b hydraulics cont d

Well Drilling Engineering Drilling Hydraulics (cont’d) Dr DO QUANG KHANH 10 Drilling Hydraulics (cont’d)         Effect of Buoyancy on Buckling The Concept of Stability Force Stability Analysis Mass Balance Energy Balance Flow Through Nozzles Hydraulic Horsepower Hydraulic Impact Force READ: ADE, Ch HW #: ADE # 4.14, 4.15, 4.16, 4.21 Fh Buckling of Tubulars F h - Fb l Neutral Point l Slender pipe suspended in wellbore Partially buckled slender pipe Neutral Point Fb Buckling of Tubulars • Long slender columns, like DP, l Neutral Point Neutral Point Fb have low resistance to bending and tend to fail by buckling if • Force at bottom (Fb) causes neutral point to move up • What is the effect of buoyancy on buckling? • What is NEUTRAL POINT? What is NEUTRAL POINT? • One definition of NEUTRAL l POINT is the point above which there is no tendency towards buckling • Resistance to buckling is Neutral Point Neutral Point indicated, in part, by: The Moment of Inertia  4 I  d n  d   in  64 Consider the following: 19.5 #/ft drillpipe Depth = 10,000 ft Mud wt = 15 #/gal PHYD PHYD = 0.052 (MW) (Depth) = 0.052 * 15 * 10,000 = 7,800 psi Axial tensile stress in pipe at bottom = - 7,800 psi What is the axial force at bottom? Axial Tension: FT = W1 - F2 FT FT = w x - P2 (AO - Ai ) At surface, FT = 19.5 * 10,000 - 7,800 (5.73) = 195,000 - 44,700 = 150,300 lbf At bottom, FT = 19.5 * - 7,800 (5.73) = - 44,700 lbf Same as before! F2 Stability Force: Ai FS = Aipi - AO pO FS = (Ai - AO) p (if pi = pO) At surface, FS = - 5.73 * = At bottom, FS = ( - 5.73) (7,800) = - 44,700 lbs THE NEUTRAL POINT is where FS = FT Therefore, Neutral point is at bottom! PIPE WILL NOT BUCKLE!! Compression 44,700 Tension 150,300 FS FT Zero Axial Stress Neutral Point Depth of Zero Axial Stress Point = 150,306 = 7,708 10 ft 19.5