Concept Alternatives and Feasibility Analyses of Dual Gradient Drilling Riser Systems
Authors: Smith Ken L , Bhalla Kenneth , Huey David , GANPATYE ATUL , Ma Kai-tung 
Published: May 2013

Abstract
This paper summarizes the concept development work aimed at designing a safe, practical, and economically feasible Dual Gradient Drilling (DGD) riser system for deployment in ultra-deepwater environments. Three (3) concept alternatives were shortlisted for evaluation after an initial screening of more than twenty (20) potential solutions. The benefits and the risks of the three alternatives are identified, and the anticipated functional constraints associated with each alternative are highlighted. The paper reviews several technical issues on the basis of which the concepts were evaluated and scored, and discusses the technological developmental efforts required to address these issues. A methodology is developed to rank the alternatives based on the results of the technical analyses, comparison of costs, benefits, and risks, and the determination of potential functional constraints. The effort documented here is a testament to the fact that even the most challenging design issues can be successfully addressed by efficient team work and comprehensive engineering.

Introduction
Dual gradient drilling is a managed pressure drilling technique that may extend the economic viability of ultra-deepwater drilling and production, in a manner that can be as safe as conventional drilling technology. It is especially beneficial in wells characterized by a narrow margin between pore pressures and fracture pressures. The dual gradient drilling approach offers the potential to reach the well total depth with a fewer number of casing strings, potentially resulting in a relatively larger wellbore as compared to the conventional single gradient drilling. This is made possible by the utilization of two mud column gradients in the wellbore during drilling, namely, sea water gradient from the rig floor to the mudline, and a heavier mud gradient from the mudline to the desired drilling depth. Specially designed components are required to sustain the two gradients near the mudline and to return the cuttings back to the rig floor. Since 1996, under the auspices of the SubSea MudLift Drilling (SMD) Joint Industry Project (JIP), three such components essential for dual gradient drilling operations, have been developed; these include the MudLift Pump (MLP), the Subsea Rotating Device (SRD), and the Solids Processing Unit (SPU). The detailed functional aspects of these components have been described in several publications in the past [1–3].
The MLP is the principal component that enables the DGD operations; it pumps the drilling fluid (mud) and cuttings back to the rig floor with the help of diaphragm pumps driven by sea water (or power fluid). The SRD, consisting of rotating seals, maintains the interface between the sea water density fluid in the drilling riser and the drilling mud, and diverts the mud to the MLP through the SPU. The SPU is a device that reduces cuttings to a size that can be managed by the MLP. The proposed options revolve around the integration of these components in a technologically and economically feasible drilling system. The study commenced with more than twenty different alternatives being subjected to preliminary evaluation; of these three alternatives were shortlisted for detailed analysis. The broad concepts underlying the three options are illustrated in Figure 1.