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string assembly is lowered through the rotary table and landed in the PGS, which

has already been set on the spider or moonpool beams below the rotary floor and

through which the 30-in. casing joint has been lowered. The 30-in. housing joint is

handled using a housing running tool. Lockdown is accomplished by bolting down

a split lock plate that secures the 30-in. housing to the PGS. The four vertical posts

on the permanent guide structure guide subsequent tools and stabilize equipment

for the BOP stack when it is run and landed on the subsea wellhead.

Once a casing string is run and landed in place, circulation is established to

clean the hole. Then cement slurry is pumped, under pressure, down through the

casing, through the casing shoe, and up into the casing/hole annulus. The liquid

cement inside the casing string is displaced with a calculated volume of water or

drilling fluid to place the cement in the correct position where it sets (becomes

solid) in approximately three to four hours. Usually the 30-in. and 20-in.
casing/hole annulus is cemented back up to the mud line. Besides sealing off the

upper formation zones, the cement provides a strong pile section to support the
heavy weight of the BOP stack when it is attached to the wellhead.

If formation conditions are favorable, the 13% in., 9% in., and 7-in. casing
annuli may be cemented up only a few hundred feet inside the shoe of the previous

casing string. This, of course, increases the amount of recoverable casing when and

if the well is abandoned. The composition and formation pressures of the various

zones encountered, as well as government regulations, determine exactly how

much cement is set in each annulus.

The set cement seals each annulus to prevent migration of gases or fluids to

other zones of the subsoil geologic structure. Cement is also used to shut off highly

permeable zones (potential lost-circulation zones), high-pressure zones, or other

problem zones. Because of these possible problems, the weight of the cement

slurry must be controlled closely to avoid placing large hydrostatic loads on the

formation. Cementing protects the casing against corrosion from subsurface

mineral waters and electrolytic action from the outside. Cementing also helps
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