GEOG 4891 Lecture Notes - Lecture 5: Casing String, Cylinder Stress, Polyethylene Terephthalate
Dead weight of casing string causes axial load which tends to fail casing longitudinal besides it lowers collapse resistance of casing string. Failure starts from the root of the last perfect thread just because the wall thickness of casing is the least there. Axial stress has no effect on elastic collapse, but on plastic failure. Fa = ymin aj where aj is root thread area and can be calculated by: Aj = (t-h)(de t - h) here h=0. 071 (by api standard) or aj value can be obtained from table 2. 15 for all sizes and weights of casing. Collapse pressure (pc) must be corrected (pcc) due to axial loading (w): Pcc = (pc / k) [(sqrt(k 2 3w 2) w] where k = a*yave. Plain-end area (a) is obtained from table 15, k from table 16. Question: calculate the collapse resistance of 16-in 55-lb f-25 casing & 5. 5-in 17-lb j-55 casing.