GRAPHICAL METHOD, CASING COLLAPSE AND BURST DESIGN
Since the Casing Collapse and Casing Burst loads vary linearly with depth, a plot may be made using the Example On The Previous Post.
A casing collapse design line is drawn on a graph of depth versus pressure by using the hydrostatic pressure of 73 pcf mud at 10,400 ft. of 5,272 psi and zero hydrostatic pressure at the surface. The appropriate design factor of 1.125 is applied to the hydrostatic pressure and a line is then drawn (see below).
Similarly, the maximum burst load line is drawn on the same graph by connecting the burst load points of 2,300 psi at 10,400 ft and 6,533 psi at the surface. The burst design line is established by multiplying 2,300 and 6,533 psi by the burst design factor of 1.1 or 2,530 psi at 10,400 ft and 7,186 psi at the surface and drawing a line between these two points.
The first section of pipe is selected based on the casing collapse requirement at the setting depth. In this example 53.5# C-95 has a casing collapse rating of 8960 psi which is off the chart. The casing collapse rating of the next weaker section is plotted on the appropriate collapse design line and the changeover depth read at the intersection on the graph. A vertical line for the first section is drawn from the casing setting depth to the changeover depth and a horizontal line is drawn from the intersection of the second casing collapse rating plotted on the design line to the collapse rating of the first section. Subsequent segments are similarly determined.
Concurrently burst ratings are plotted and vertical and horizontal lines are drawn
Above the cement top and when the casing is in tension, the casing collapse ratings are reduced by the effect of tension on collapse.
At changeover depths above the cement top, the axial stress is calculated. Where the pipe is in tension, a percent of rated collapse is read from Table below based on the Axial Tension. Using the percent of rated collapse multiplied by the changeover depth adjusts the depth to the correct depth. The casing collapse design factor at the bottom of the weaker section then is calculated to determine if the casing collapse design requirements are sufficient. If the depth is not correct, the design factor calculated times the depth used will adjust the changeover point to the correct depth. By repetition the correct depth will finally be selected. If the pipe is not in tension, plot the collapse rating of the next weaker section in collapse on the design line and continue the design as before.
As the design continues upward from the bottom a depth will be encountered where casing collapse no longer controls the design. Above this depth the design will be controlled by burst or tension. If burst controls the design, the burst ratings of the casing are plotted on the burst design line and the burst loads are read from the burst load line at the corresponding depth. Changeover depths are read directly from the graph. If tension is controlling the design, the changeover depth is calculated directly. The changeover depth is calculated by using the tension rating divided by 1.6 and subtracting the buoyed weight of the pipe below; from this remainder divide by the buoyed weight per foot of the pipe used to determine the footage of pipe to be used.