Subsea Equipment for Deep Water Drilling Using Dual Gradient Mud System

Abstract

Abstract Deepwater drilling presents both opportunity and challenge. One challenge that consistently presents itself is the difference in pressure, at the mudline, between the hydrostatic head of the mud in the riser and the formation pressure close to the mudline, which is, typically, defined by the seawater head. This pressure differential causes an operational difficulty that may, in some cases, prevent drilling the well to its target depth using conventional riser return drilling methods. The concept of Dual Gradient drilling, using a riser filled with seawater, has been proposed as a viable method of limiting this pressure imbalance and of facilitating the drilling of challenging deepwater wells. Over the years, especially in the ‘60’ s and ‘70’ s by major Oil companies, many different approaches to dual gradient drilling have been studied, but none have been commercially developed. During the past three years, three independent large-scale efforts have been initiated in attempts to provide a commercial dual gradient system to the industry. (Ref. 1). Several of these efforts include configurations that utilize a seawater filled risers. One of the major requirements of dual gradient drilling, using a seawater filled riser, is the need to create an interface between the drilling mud in the wellbore/wellhead and the seawater, or other fluid, in the riser. This can be simply a fluid/fluid interface, located generally above the wellhead in the riser, or a mechanical device to provide positive isolation of the two fluids. This paper will review the general concept of dual gradient drilling, while focusing on the development of a mechanical isolation method combined with a subsea pumping system for return of the drilling mud. In addition to reviewing the mechanical aspects of the hardware, operational methods and added value that the mechanical isolation system brings to deepwater drilling methods, will be discussed.