Shale shakers Suction line

2. Surveying Calculations
The principles used in surveying a wellbore are the same as those used in land
surveying.
2.1 Principles of Surveying
The basic principles of surveying can be illustrated by considering the two
dimensional system shown in Figure 2. The position (co-ordinates) of point, B
relative to the reference point A can be determined if the angle and the distance
AB is known. If the position of point A is defined as 0,0 in the X, Y co-ordinate
system the position of point B can be determined by the following equations:
Y B = AB Sin 
XB = AB Cos 
Hence the displacement of point B in the X and Y direction can be determined if the
angle and the linear distance between A and B are known. The position of a further
point C can be determined by the same procedure. The X and Y displacement of
C relative to the reference point A can be determined by adding together the X and
Y displacement of Point B to A and those of Point C to B. This process of defining
the position of a point relative to a specific reference point can be continued for any
number of points.

13 Subsea Drilling

The operations and equipment used to drill a well from a production platform are
almost identical to those used for a land well. A conductor is driven into the seabed
and the hole sections are drilled through wellhead and BOP equipment which is
similar to that used on land locations. The wellhead and BOP are located on the
lower deck of the platform. When the well has been drilled and completed the Xmas
tree (which is also similar to that used on land locations) is mounted on top of the
wellhead.
The type of wellhead and blowout prevention equipment used when drilling a well
from a mobile drilling rig will be quite different from that used on a platform based
operation. The equipment used in this case will depend on whether the operation
is being conducted from a floating drilling vessel (drillship or Semi-submersible)
or from a stable, Jackup drilling vessel. The vessel used will in turn depend largely
on whether the well is an exploration or development well and the water depth in
which it is being drilled.
When drilling from a Jackup, the drilling operations are very similar to platformbased
or land-based operations with a conductor being driven into the seabed and
conventional wellhead and surface BOP stack equipment being used. However, since
the Jackup will have to move off location when the drilling operation is complete
the casing strings must be physically supported at the seabed and it must be possible
to remotely disconnect the casing strings between the seabed and surface when the
operation is complete. The only alternative to this seabed support is to leave a ‘freestanding’
conductor on location but in most areas this is not a feasible alternative.
Seabed support for such wells is provided by a Mudline suspension (MLS) system.
The MLS system is a series of full bore housings and hangers run with the casing
strings and is discussed fully, later in this chapter.
When drilling with an MLS system the casing strings are temporarily extended
back from the mudline to surface and the conventional wellhead and BOP stack is
nippled up on top of these extension strings (just beneath the rigfloor). The MLS
system only provides physical support for the casing strings. All annulus sealing
and monitoring functions are provided by the wellhead at surface.
When the well has been drilled it is possible to convert the MLS system into a subsea
wellhead, such that the well can be completed subsea, although this is not a typical
application of MLS technology. These systems are generally used on development
drilling operations, where a platform is to be used for production purposes. The
operation is conducted as follows: a Jackup drilling unit and MLS system is used to
drill the wells; the wells are suspended and the tieback strings removed; and the rig
is moved away from the location. When the platform is complete it is installed over
the location and the wells are re-entered and re-connected, with extension strings, to
the lower deck of the platform and a conventional wellhead and Xmas tree system is
installed on top of the extension (tie-back) strings. This is known as a ‘pre-drilling’
operation.
When drilling from a floating vessel drillship or Semi-submersible (Figure 1) there
is always the possibility that, at some point during the drilling operation, the vessel
will have to disconnect from the well or even move off location due to bad weather.
The wellhead and all other BOP equipment are therefore situated on the seabed
with the drilling fluids being circulated back to the drilling vessel via a marine
riser. The BOP stack on the seabed is the primary well control device , in the event
of a kick. A hydraulic latch between the marine riser and the BOP stack ensures
that it is possible to close in the well, disconnect the marine riser from the top of
the BOP stack and move the rig off location safely at any stage during the drilling
operation. When the well has been drilled and the well is either suspended for later
completion or it may be completed immediately and a subsea Xmas tree installed
on the wellhead. We will assume that the well is to be completed immediately after
the drilling operations are complete.
The first part of this chapter will outline the operations and equipment used when
drilling and completing a well from a floating vessel, using a subsea wellhead