Consideration for Design of Drilling Conductors for the New Generation of Deepwater, Harsh Weather Jackups

Abstract

ABSTRACT As the next generation of harsh weather jackups for the North Sea push the operating water depth limits from 300 feet (91m) upwards to 500 feet (150m), the environmental criteria also increase. At these more severe depth and environmental conditions the increased dynamics of both platform and drilling conductor will increasingly affect drilling operations. When drilling in water depths greater than several hundred feet the conductor must be tensioned to resist buckling and increase its stiffness, the amount of tension being proportional to water depth and environmental conditions. The tensioning device is usually placed at the top of the conductor and just below wellhead/BOP stack, thus creating a point of lateral fixity. As currents and waves act on the tensioned but unstayed conductor, the wellhead/BOP stack rotates about the point of support to suit the structural deflection of the conductor below. At some limiting sea state, the top of the BOP stack no longer lines up with the rotary table and drilling must be suspended. If the top of the BOP stack is also laterally fixed (or hydraulically snubbed), a bending momement is induced in the BOP stack, which under severe storm conditions, can exceed the structural limit of the BOP stack connections. A frequency domain computer program was used to parametrically analyze the dynamics of the system and to calculate the maximum stress levels in the various components. The paper examines over 75 combinations of physical configuration, water depth, environmental criteria, drag coefficient, conductor diameter, vertical tension and joint stiffness. Appropriate graphical representation of the results are presented. Based on the analysis, the authors conclude that the design of the drilling conductor for winter storm conditions in the North Sea and for water depths greater than 300 feet will be significantly more complex than present shallow water designs. An ongoing analytical program is postulated to study composite configurations, fatigue effects, and drilling downtime.