Page 213 - Drilling Technology in Nontechnical Language

204 Drilling Technology in Nontechnical Language Second Edition

Combined axial and internal forces. If a steel tube is in tension, it
has an increased resistance to burst and a decreased resistance to collapse.
Conversely, if a steel tube is in compression, it has a decreased resistance
to burst and an increased resistance to collapse. This is known as the
biaxial effect. The industry standard design tables for casing (published
by the American Petroleum Institute, API) that give strengths in tension,
collapse, and burst also include information on biaxial effects. Generally
when designing casing, the increased burst strength due to tension is
not allowed for (increasing the safety margin), but decreased collapse
resistance due to tension will be accounted for where it is relevant.

Corrosion. When acidic gases such as hydrogen sulfide (H S) or
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carbon dioxide (CO ) are present with water, steel components can become
2
seriously corroded. This is worse with high temperatures; corrosion rates
roughly double for every 32°C increase in temperature. It is also worse
with higher pressures and with higher concentrations of corrosive agents.
If H S, CO , and water are all present, corrosion resistance design will
2
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require a special study to determine the most cost-effective solution.
Special steels containing nickel or chromium can be used, but these
are much more expensive than plain carbon steels. These materials are also
softer than carbon steel, and the connections are easily damaged while
screwing joints together if they are spun too quickly or if they not exactly
aligned while turning.

Hydrogen sulfide is a particular problem for designing casings.
Hydrogen can enter the steel crystalline structure and causes hydrogen
embrittlement. The steel can break at well below expected failure loads
without warning if this has happened. The problem gets worse with higher
strength steels and at lower temperatures. Lower temperatures are found
at the top of the casing string, where the tensile load is highest. If H S
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is encountered in the well, it is very important that the correct grade of
steel is selected so as to avoid hydrogen embrittlement and failure at the
exposure temperature of the steel.

Connections. Most failures in casing (around 90%) occur at the
connection—the part that screws two joints together. This should not be
surprising; an extruded steel pipe is pretty strong, whereas a pipe that has
a thread cut on it must have decreased strength against some, if not all,
forces (fig. 9–4).

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