Deepwater DP Drilling Rigs on Shallow Water Wells, Can it Work?

Abstract

Abstract The steady influx of new build rigs into the Gulf of Mexico has flooded the market with 10,000ft plus rated rigs, forcing smaller rigs to forage elsewhere. This influx is clearly driven by the industry's move to deeper and deeper waters. The effect of these changing priorities has led to a shortfall in rigs specifically tailored to shallow-medium water depths. In some cases cost and schedule constraints and rig availability have prompted contractors and operators to consider deepwater DP rigs on shallower water wells. Paramount to the success of this practice is safe rig operation, management of unplanned DP system failure (i.e. drift-off) and maximizing drilling operability. Operability is primarily dependent on the prevailing environment, water depth and riser tension. While new rigs offer many advantages in terms of high tension capacity and better station keeping, this can be outweighed by heavier subsea hardware (e.g. LMRP, BOP), larger vessel excursions in extreme environments and poorly optimized tensioner and anti-recoil performance at lower tensions. This paper discusses the challenges associated with operating deepwater rigs on shallow water wells; looking specifically at tension optimization, operating envelopes and drift off events. A comparison study is presented between the drift off and operability scenarios of a new generation deepwater rig and an older generation rig suited for shallower waters. Sensitivity studies are also presented to environmental directionality and magnitude and the effect on the structural and mechanical limits of the riser and tensioning system.