Page 69 - Hydrocarbon Exploration and Production Second Edition

56 Drilling Systems and Equipment

The system described so far has been in operation for many decades. Several
disadvantages are apparent:

The requirement to add pipe after every 30 ft drilled is time consuming, results in
longer openhole times, longer drilling times and lower quality wellbores.
The rig floor is known to be one of the most dangerous areas of a rig, accounting
for a large percentage of lost time accidents.
The technology imposes limitations in terms of well trajectory and complexity.
For a modern mobile offshore drilling unit (MODU) day rates in excess of
500,000 US$ may be incurred. Hence any time savings constitute a considerable
potential for cost reduction. This, and the desire to improve the safety record of
drilling operations, has driven the automatisation of high-specification rigs, both
offshore and onshore. The areas which have been dramatically changed over the last
decade are as follows.

4.3.2. Top drive systems
Instead of having a rotary table in the rig floor the drive mechanism for the drill
string is mounted on guide rails and moves up and down inside the derrick. This
allows drilling in segments of 90 ft of pre-assembled pipe, significantly reducing
connection time. The more continuous drilling process also results in better hole
conditions and faster penetration rates. The latest rigs allow running of 120 ft
sections and are equipped with two derricks – one drilling the well and the other
used to concurrently pre-assemble drill strings (Figure 4.9).

4.3.3. Automated pipe handling
The manual labour on the rig floor has largely been replaced by a hydraulic system
which picks up pipe from the rack, transports it up to the rig floor and inserts it into
the drill string. The process is controlled by the rig crew manning a workstation
housed beside the rig floor (Figure 4.10).
Earlier on when we described the cutting action of the drill bit, we learned
about the drilling fluid or mud. The mud cools the bit and also removes the cuttings
by carrying them up the annulus, outside the drill pipe. At the surface, the mud runs
over a number of moving screens, the shale shakers (Figure 4.11), which remove the
cutting for disposal. The fine particles which pass through the screens are then
removed by desanders and desilters, the latter usually hydrocyclones.
Having been cleaned, the mud is transferred into the mud tanks, which are large
treatment and storage units. From there a powerful pump brings the mud up
through a pipe (stand pipe) and through a hose connected to the swivel (rotary hose)
forcing it down the hole inside the drill string. Eventually the cleaned mud will exit
again through the bit nozzles.
Originally, ‘mud’ was made from clay mixed with water – a simple system.
Today the preparation and treatment of drilling fluid has reached a sophistication
which requires specialist knowledge. The reason for this becomes clear if we
consider the properties expected.