Page 83 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 59

Casing Design [ ]
1.4.7

(stress) is a linear scale and the X axis (number of cycles) is a loga-
rithmic scale.
An example of an S-N curve is shown below. In this example,
60,000 psi would cause almost immediate failure (10 cycles), 30,000
psi would fail at 800 cycles and 20,000 psi would allow an infinite
number of cycles before failure (see Fig. 1-10).

Example S-N curve
60,000

50,000

40,000
Stress, psi 30,000

20,000

10,000
0
10 100 1000 10000 100000 1000000
Number of cycles to failure
Fig. 1-10 Example of an S-N Curve

S-N curves should be available from the product manufacturer.
If there is a notch in the steel (such as a slip mark) then the stress
concentrates at the end of the notch and can be much higher than
would be calculated simply by looking at the load and the remaining
cross-sectional area. This notch is known as a “stress raiser” and is very
significant in fatigue failures since the actual stress is far higher than
the average stress. The concentration of stress at the tip is increased by
increased notch length and decreased notch tip radius. If a crack is
carefully filed out and the edges smoothed, the stress will not concen-
trate and fatigue life will be extended, though the load required to fail
the steel will still be reduced due to the smaller cross-sectional area
supporting the load. In a corrosive environment, there is no lower
fatigue S-N limit.
While examining a drillstring fatigue failure, an initial crack or
defect can often be recognized that may be rusty on the faces. The steel
tends to hold for some cycles while the stress causes localized work

59