Petroleum Engineering 406
Floating Drilling
Lesson 9c
Wellheads and Casing
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Lesson 9c - Wellheads and
Casing
 Temporary and Permanent Guide Bases
 Subsea Cementing
 Casing Seals
2
Wellheads and Casing
A subsea wellhead, just like a land
wellhead:
 Must support the BOP’s while drilling
 Must support the suspended casing
while cementing, and
 Must seal off between casing strings
during drilling and production
operations.
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Wellheads and Casing, cont.
In floating drilling, the casing hangers,
casing seals and cementing heads
differ from land and platform
operations in the following manner:
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Wellheads and Casing
1. The first and second casing strings
are cemented with returns to the
seabed.
2. Casing is run with the last joint madeup on a casing hanger and
permanently suspended prior to
cementing. Mud returns flow
through fluting in the hanger.
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Wellheads and Casing
3. Usually, cementing plugs are
located at the wellhead and
released remotely. The
cementing string from the vessel
to the wellhead is drill pipe.
4. Casing seals are run and set
remotely.
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Wellheads and Casing
5. Special test tools are required for
remotely testing the casing seals.
6. Wear bushings are essential for
protecting the wellhead.
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8
Depth BML
240 ft
1,100 ft
4,100 ft
8,600 ft
10,100 ft
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10
Permanent
Guide
Structure.
Temporary
Guide Base
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12
Utility
Guide Frame
Hole Opener
Temporary
Guide Base
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Fracture gradient at
100 ft. below seabed
(Santa Barbara
Channel).
Why drill with returns to the seafloor?
With this low fracture gradient it is difficult to
drill with returns to the surface.
No shallow gas would be expected at this
depth below the mudline.
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Shallow
Gas
Blowout
Gas
reduces
buoyancy!
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Gas in the
Water Column
Typical specific gravity variations in a
blowout boil have increasing effect
nearer the water’s surface.
Fortunately for a semi-submersible, the
rig’s primary flotation members are
situated below the zones where specific
gravity has been reduced the most.
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Gas in the
Water Column
If there is sufficient length to the mooring
system cables/ chains, the rig will be
pushed off location and out of harm’s way.
However, the plume can also cause the rig
to list, which reduces its freeboard and
makes it more susceptible to capsizing.
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Fig. 4-9.
Sub-sea
cementing
system.
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Fig.44-10.
Typical sealing
arrangement for
subsea wells.
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Metal-to-Metal Casing Annulus Seal
 Assures maximum seal over
extended periods, even in highpressure holes
 Eliminates dependence on seal
materials that deteriorate or “cold
flow”.
 Available on systems up to 15,000
psi pressure integrity.
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1. Actuating force is transferred to the
2. Resilient compression element
which expands, forcing the
3. Metal seal lips into contact
with the surface of the
4. Wellhead housing and the
5. Casing hanger
Upper Metal
Seal Lips
Resilient
Compression
Element
Lower Metal
Seal Lips
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Casing Hanger and Pack-off
Assembly
Single trip installation
The pack-off seal assembly is run
simultaneously with the casing hanger
body. All operations - installing the casing
hanger, cementing the casing string and
actuating and testing the pack-off seal are
performed in a single trip of the running
string.
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Large Flow-By Areas
Large flow-by areas can handle most
drilling fluid applications with a minimal
drop in pressure.
Deep 2" wide flow-by slots in the casing
hanger body, and ample porting through
the pack-off nut assembly, provide clear
passage for cuttings and mudcake
without plugging.
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Shallow Gas Plan
After the rig is properly anchored the
following steps will be followed as
there is a potential for shallow gas in
this area:
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Shallow Gas Plan
1. Leave mooring line pawls or stoppels
unset until the 20” casing has been
set and cemented.
2. Mooring winches will be manned while
the 8 1/2” pilot holes for the 30” and
20” casings are being drilled.
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Shallow Gas Plan
3. Mooring winches will be manned
while the 8 1/2” pilot holes for the
30” and 20” casings are being
opened up or under reamed.
4. The moonpool and seafloor will be
observed for gas bubbles until the
20” casing is set and cemented.
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