Abstract

Semisubmersible rigs have capacity for drilling in water depth ranging from shallow (>400 ft.) to ultra-deep water (>10,000ft). Station keeping and rig positioning over the well centre during drilling, completion and testing operation is achieved either by anchoring the rig in position via an 8 point or 12 point mooring system or by dynamic positioning (DP).

Semisubmersible rig development has progressed since the age of the 4th generation semis which is considered as the hinge or fulcrum generation consisting of a mix of old generation midwater rigs and new generation deep water rigs to the present. The rig evolution has progressed from the 4th generation rigs with water depth capability ranging from 1,500 ft. to 7,000 ft. and station keeping capabilities consisting of a mix of dynamic positioning and mooring to the most recent 7th generation semisubs which are mainly dynamically positioned. A comparative analysis of mooring and dynamic positioning will be performed in this paper.

A holistic and thematic approach will be adopted in this analysis and current operating and design guidelines as well as documented performances and limitations of vessels employing both type of station keeping methods will be reviewed. Important consideration will be given to rig-move efficiency, spudding-in, deck load requirements and motion characteristics. All the factors present significant operational opportunity to increase profitability or totally whittle it down especially in a deep water project. The potential value and efficiency of each factor will be evaluated

The station keeping drivers and degree of accuracy is driven by the type of project under consideration, either exploration, development or intervention project. This paper presents a comparative analysis of the risks, achievable efficiencies, operational limits and environmental constraints involved when employing a moored and DP rig for station keeping relative to the well objectives. Specifically, positional accuracy, ease of station keeping, ease of evacuation, riser design requirements and limitations and operational limits are some of the other key issues that will be addressed. A performance and commercial comparison of a dynamically positioned and moored semisubmersible rig on a 5 well exploration program will be used to highlight the value of each station keeping approaches

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