Stresses in Casing Under Cyclic Thermal Loading
The stress behavior of the casing can best be described by considering a typical stress-temperature change diagram for casing during a steam injection-production cycle as shown in Fig. 4.27. In the following discussion the term ‘casing’ refers to both the pipe body and the coupling which are considered to be indistinguishable. The initial stress in the casing is zero.
Path 1-2 represents the elastic portion of the compressive stress due to an increase in temperature, AT. For a plain carbon steel, the compressive stress generated by thermal expansion is about 200 AT (psi). Point 2 represents the yield point of the casing (either pipe body yield or joint yield in compression). If the compressive stress at the maximal casing temperature does not exceed that at point 2, the casing will return to zero stress as the wellbore cools. The temperature corresponding to the stress at point 2 is designated ATyp, the temperature at which the yield point is reached.
As the temperature exceeds ДTyp, the stress temperature curve follows the path 2-3 because the casing is able to absorb only a small part of the thermal expansion forces by stress increase. Instead, most of the expansion forces above yield point are dissipated through permanent deformation (plastic flow) of the casing.
During cooling, casing initially behaves elastically. The stress-temperature relationship is represented by path 3-4 which is parallel to 1-2 but offset by a change in temperature ATX. Path 2-3 is not reversible because irreversible changes occur in the structure of the casing as it yields. The elastic portion of the stress increase is recoverable and a zero stress is reached when the casing temperature has decreased by the amount ATmax — ATyf, = ATX. Thus, the casing temperature is higher than the initial casing temperature by ATX at zero stress (neutral point).
As the casing cools below ATX (zero stress), thermal contraction forces similar to the expansion forces encountered in the heating cycle cause the pipe to be in tension. The resulting tensile stress is approximately 200 ATX psi. Casing failure at the coupling will occur if this tensile load exceeds the joint fracture or pullout strength during cooling process. Three types of failure have been observed:
1. Tensile failure in the last engaged pipe threads.
2. Tensile failure by pin-end jumpout.
3. Compression failure by closing off the coupling stand-off clearance.
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ДТ а О, а |
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ДТ,
О
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N |
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Г |
г
Fig. 4.28: Rotationally symmetric pipe under pressure and temperature.