Page 103 - Drilling Technology in Nontechnical Language

94 Drilling Technology in Nontechnical Language Second Edition

Drilling for surface casing

With a floating drilling rig, surface hole is drilled with returns to the

seabed. No diverter is run. If shallow gas is encountered, the rig can, if
necessary, drop the drillstring and move off location, away from the gas

flow. The gas will flow to the sea, and the gas plume will disperse as it
rises. Any current will move it away from the rig. If a very strong flow was

encountered in shallow water, the rig might be endangered by the gas flow
destabilizing the rig, causing it to capsize.
As before, the BHA is guided to the template using rope on the guide
wires. The surface hole is generally not logged. Of course, no cuttings
samples are possible because they are dispersed on the seabed around the
template. As this is a development well, the geology is known, and so the
surface hole terminates at a predetermined depth, where the formation is
known to be strong enough to hold pressure if a kick is taken.
The surface casing is run and lowered into place using drillpipe. At the
top of the casing is screwed a special tool that contains the cement plugs,
and it is cemented in a very similar manner to a liner in a land well.
On top of the surface casing is a subsea wellhead housing, which is a
section of thick-walled pipe. Inside is a profile that the next casing hanger

lands on.
On the outside of the wellhead housing is a profile onto which the

blowout preventers are latched (fig. 4–8).
Special underwater blowout preventers are set on top of the well after

setting surface casing (fig. 4–9).
After testing the subsea BOP on the rig to ensure it functions correctly
and holds pressure, large diameter pipe (called riser pipe) is connected
to the top of the BOP and used to lower the BOP into the sea. Hydraulic
hoses and electrical cables are secured down the outside of the riser to
connect the BOP to the rig. This allows control of the BOP functions and
transmits status information back to the rig. The BOP is lowered down to
the template, guided by the guide wires. At the top of the riser is a special

joint called a telescopic joint (figs. 4–10 and 4–11). This fastens under the
rig, and it opens and closes to allow for the up-and-down movement of the
rig once the BOP connects to the wellhead housing. The BOP is lowered so

that the hydraulically controlled latch goes over the wellhead outer profile,
and then the latch is closed and forms a pressure-tight seal.

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