An efficient mooring system must maintain the vesselposition throughout the designated time, this is especially important to drillship, as any movement if the ship will disrupt the drilling process and damagethe pipe line which can potentially cause blowout. Therefore, a good mooringsystem 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 bediscussed in this report. Two main mooring methods were compared and discussedfor their advantage and underlying problems. For early drillship configuration,an 8 point fixed mooring systems were used, this generally provided a greatlongitudinal stability when the bow was pointed in the more severe weathercondition. However, if the vessel was to face the prevailing weather at thebeam, it will affect its stability of the vessel significantly.
Therefore,increase the operational downtime when the weather hit the vessel on theside. Most drillship nowadays was built with a dynamic positioningsystem to rotate the ship’s bow into the changing weather and improves the vessel’smotion, although this does not improve their heave and roll motion because ofthe larger water plane area compared to semisubmersible rig. The usage of mooring lines is limited by the depth of theocean, therefore to identify the advantage of either system a set of variablesare to be compare against each other, such as operation downtime, costefficiency, and stability.The time needed to for the system was relatively easy todifferentiate as DP system only need to receive signal from the satellite andit would be ready for drilling operation, whilst mooring lines needed to be setup prior to the drillship arrival. Therefore, the time needed for DP system tobe operational is negligible compared to the mooring line system.
Efficiency of either system is hard to justify as both havethe advantage over each other depending on the operation itself. Firstly,installation comparisons, the cost of a DP system is higher than a mooringsystem, however, costs from winches, anchors and chains are not inexpensiveeither. Reliability also plays an important role in the comparison,traditionally, mooring line was thought to be more reliable than a DP system asit has the possibility of drive-offs. However, as technology advanced, thepossibility of major drive-offs that leads to a catastrophic incident is low as1.11 per year(reference). Therefore, in the future, a DP system should be asreliable, if not more reliable than mooring lines.
Maintence cost is also amajor downside of a DP system as mooring lines are typically easier to repairthan thruster.To conclude, mooring lines are more efficient when waterdepth is not limited and for a longer drill time. However, the main use ofdrillship is usually used for exploratory offshore drilling for some new oiland gas wells. Therefore, the time usage will not be as long as developmentdrilling, so a DP system will have the ultimate advantage over mooring lines.