An dynamic positioning system to rotate the ship’s bow

An efficient mooring system must maintain the vessel
position throughout the designated time, this is especially important to drill
ship, as any movement if the ship will disrupt the drilling process and damage
the pipe line which can potentially cause blowout. Therefore, a good mooring
system is vital to determine the success of a drillship.

There are a few types of mooring system available nowadays,
however, some does not apply to drillship, therefore these options will not be
discussed in this report. Two main mooring methods were compared and discussed
for their advantage and underlying problems. For early drillship configuration,
an 8 point fixed mooring systems were used, this generally provided a great
longitudinal stability when the bow was pointed in the more severe weather
condition. However, if the vessel was to face the prevailing weather at the
beam, it will affect its stability of the vessel significantly. Therefore,
increase the operational downtime when the weather hit the vessel on the
side.  

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Most drillship nowadays was built with a dynamic positioning
system to rotate the ship’s bow into the changing weather and improves the vessel’s
motion, although this does not improve their heave and roll motion because of
the larger water plane area compared to semisubmersible rig.

The usage of mooring lines is limited by the depth of the
ocean, therefore to identify the advantage of either system a set of variables
are to be compare against each other, such as operation downtime, cost
efficiency, and stability.

The time needed to for the system was relatively easy to
differentiate as DP system only need to receive signal from the satellite and
it would be ready for drilling operation, whilst mooring lines needed to be set
up prior to the drillship arrival. Therefore, the time needed for DP system to
be operational is negligible compared to the mooring line system.

Efficiency of either system is hard to justify as both have
the advantage over each other depending on the operation itself. Firstly,
installation comparisons, the cost of a DP system is higher than a mooring
system, however, costs from winches, anchors and chains are not inexpensive
either. Reliability also plays an important role in the comparison,
traditionally, mooring line was thought to be more reliable than a DP system as
it has the possibility of drive-offs. However, as technology advanced, the
possibility of major drive-offs that leads to a catastrophic incident is low as
1.11 per year(reference). Therefore, in the future, a DP system should be as
reliable, if not more reliable than mooring lines. Maintence cost is also a
major downside of a DP system as mooring lines are typically easier to repair
than thruster.

To conclude, mooring lines are more efficient when water
depth is not limited and for a longer drill time. However, the main use of
drillship is usually used for exploratory offshore drilling for some new oil
and gas wells. Therefore, the time usage will not be as long as development
drilling, so a DP system will have the ultimate advantage over mooring lines.