New Dual-Gradient Deepwater Drilling Systems-Principle, Method And Application

The paper presents the research on the new dual-gradient drilling (DGD) system, which is supported by the project of"Research on Deepwater Riser Engineering Methods and Trace Study of Dual-Gradient Drilling Technology"sponsored by CNOOC. Based on drilling engineering, mechanical engineering and ocean engineering theory, the principles of DGD system, the methods of multi-gradient drilling (MGD) system, and the multi-purpose undersea pump system are studied. The industrial application of DGD for South China Sea is described.The main work is summarized as follows:1. Principle of Managed Pressure Drilling System and Dual-Gradient Drilling System The principles of Managed Pressure Drilling systems such as continuous circulation system, equivalent circulating density reduction tool, pressurized mud cap drilling system, and controlled mud cap system are analyzed. The working principles of the DGD systems such as subsea mudlift drilling system, hollow glass spheres DGD system, riser dilution DGD system, and riser gaslift DGD system are described in detail. A two-phase flow model of riser gaslift is developed, which could be used to calculate the distribution of pressure, velocity and gas-bearing ratio in the riser, and provides a shortcut method to calculate the rate and volume of gas injection. Considering the feature and particular requirement of deepwater drilling, a program for deepwater DGD well profile design and optimization is developed by from-top-to–bottom method, which could be served as an instruction in engineering application.2. Multi-Gradient Drilling System and Its Key EquipmentA new MGD method based on the principles of DGD is proposed. The MGD method refers to injecting light weight solid additive(LWSA) to the wellbore annulus at one point or several points under the seabed mudline. Compared with the DGD, the MGD can maintain effectively wellbore annulus pressure safety between pore pressure and fracture pressure curves better, which is more suitable for the complex geology. Two conceptual models of the MGD system are presented respectively based on the hollow-sphere injection and riser dilution methods. The preliminary design and simulation for the key equipment of the system is introduced, which can provide a framework for further design and industry application.3. The New Multi-Purpose Undersea Pump SystemAccording to the principles of controlled mud cap system and subsea mudlift drilling system, the new multi-purpose undersea pump system (MPUPS) is presented firstly, which can be installed underwater or at mudline. MPUPS can be used to realize several drilling functions by control, such as DGD, controlled mud cap drilling, riser dilution drilling. The conceptual design of the MPUPS, the design and selection of the key equipment for MPUPS have been implemented. Detail design for the cylindrical interlayer shells structure form is performed, including ANSYS-based strength and stability assessment on the buoy cabin under the outside pressure and during the installation process, and FLUENT-based pressure distribution analysis on the surface of the buoy cabin in the water as well as the drag and the fluid resistance resulting from the buoy dynamic velocity & displacement response. The results indicated that the cylindrical interlayer shells structure form can meet the working requirements in deep water.4. Hydraulic Analysis of Multi-Purpose Undersea Pump Drilling SystemThe paper presents a model for temperature distribution in the DGD wellbore and a model for the bottom hole pressure, in which the temperature and pressure's effects on the density of the drilling fluid is considered. A program based on the models are written, which can be used to determined wellbore pressure distributions for the conventional drilling, riserless drilling, subsea mudlift drilling and undersea pump drilling, and has been verified with reference data. The main factors resulting in the"U-Tube"effect in the DGD are analyzed, and a drilling string value is designed for eliminating"U-Tube"effect. The DGD kick detection and well control methods have been investigated. The DGD simulation program is established, which can be applied to the well program and drilling simulation.5. Industrial Application of Dual-Gradient Drilling System for South China SeaThe paper suggests dual gradient top hole drilling (DGTHD) technology for the South China Sea based on casing drilling, which can shorten well construction time, and controlled shallow hazards. The concept design of DGTHD system and detail design of virtual riser system are performed. Moreover, the Monte-Carlo method and Controlled Interval and Memory (CIM) model can be adopted to simulate drilling process in South China Sea and to evaluate the drilling cost affected by multiple risk factors. Further more, a fault tree of the DGD system has been built up for qualitative and quantitative analysis on the possible failure mode of the DGD system based on the offshore engineering accidents' statistics database of the MMS and SINTEF, and some measures are presented to improve safety of the DGD system.