Page 121 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 97

1.4.22
Casing Design [ ]

Subsea wellhead. In deep water it may be decided to have the well
completed with the wellhead on the seabed. The wells may produce
through a manifold to a floating production station or into a pipeline.
Guideline subsea wellhead. Either a template is set on the seabed or
a temporary guide base run. Either way, posts are positioned on the
structure to allow guidelines from the rig to be latched. The drilling
assembly is attached to the guideline with soft rope, which breaks after
the bit has entered the slot. When the conductor is run, it is guided in
the same way.
An underwater camera system (either ROV or run on wireline)
allows entry to be visually monitored. The conductor (run on drillpipe)
locates in the guidebase; it may be latched in place depending on seabed
conditions. It is also possible to drill the conductor hole without a
guide, where the permanent guidebase is run attached to the conductor
to guide subsequent activities.
Guidelineless subsea wellhead. A template or reentry funnel may be
set on bottom or the well may be simply spudded straight into the
seabed. Care must be taken spudding; starting with very low drilling
parameters until the bit is a few feet below the mudline. Excess
WOB/RPM can easily cause the bit to “walk” across the seabed, espe-
cially if it is rocky. It may be necessary to wait for slack water in tidal
areas. High flow rates will wash out a large crater on a soft seabed. An
underwater camera system is necessary to monitor the operation.
A reentry funnel will be run fixed to the top of the conductor to
guide drilling assemblies and the surface casing string, if a template or
reentry funnel were not already run.
The following considerations are important in choosing a wellhead:

Type of system as outlined above
Maximum anticipated wellhead pressure and temperature
Conductor and casing sizes and connections
Wellbore fluids (e.g., H S, CO , or other conditions requiring
2 2
special service materials)
For a subsea wellhead, type of connector on top and type of
connector on the subsea BOP stack
Any other special requirements
Cost/availability

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