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the drilling mud and rock cuttings to be isolated from the water
column and circulated back to the rig. Another complication in
offshore drilling is that, since the sea is seldom calm, the rig is
constantly heaving and rolling in the swell. Tensioners and motion
compensators allow the riser and drill string to remain relatively
stationary, and keep a constant weight on the drill bit while the rig
moves with the swell. But what of the winds and currents that tend
to move the rig off the drilling location? It is easy to imagine the
problems that even a small amount of drifting would cause when
the drill pipe is extended thousands of metres into the sea bed. The
industry has adopted two solutions to this problem. The first
involves an elaborate system of 8 to 12 anchors, each weighing up
to 20 tons. Mooring lines comprised of heavy steel chains can
extend a mile or more, depending on water depth [4].
A second system that is being increasingly used is the
dynamic positioning system or DPS. The DPS can either
supplement or replace the anchors with a computer controlled
system of propellers or thrusters.
Through an array of motion sensors, the computer can
immediately detect any movement of the rig and then apply just
enough power to the appropriate thrusters to compensate and keep
the rig exactly on location. This type of system makes it possible
to drill in much deeper water, where anchors would not be
feasible. Wells can now be drilled in water depths exceeding 2000
metres. Another important advantage of DPS is that it allows the
drilling rig to quickly disconnect the riser and move off the
location in case of an emergency (such as the approach of an
iceberg). As you can imagine, the retrieval of several twenty-ton
anchors would be quite time consuming and would require a lot
more lead time when making the important decision to leave the
drilling location. The significance of such a decision becomes
apparent when one realizes that it costs in the neighbourhood of
$250,000 per day to operate a rig on the Grand Banks. In this kind
of operation, any time lost becomes very expensive.

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