Deepwater Riserless Mud Return System for Dual Gradient Tophole Drilling

Abstract The paper presents results of a large-scale field trial conducted from a deepwater semi submersible drilling vessel offshore Sabah, Malaysia, September 2008. A joint industry group comprising AGR Subsea, BP America, Shell, Norwegian Research Council’s DEMO 2000 program, and supported by PETRONAS undertook this work. The group set out to advance subsea mud return technology from its established commercial market of shallow water applications, 1800 ft (549 m) or less to deepwater depths and drilling requirements. Novel equipment and deployment methods were designed, developed, delivered, tested and proven on a demanding schedule. The technology is applicable to closed loop riserless tophole drilling, in water depths to ca. 5,000 ft (1500 m), as an alternative to seabed discharge of drilling fluid and cuttings. The system takes all returns back to the solids control equipment on the rig where recirculation of drilling fluid minimizes mud consumption thereby enabling the use of higher-quality fluids otherwise too costly to be continuously dumped. Anticipated improvements, confirmed by field trial results, are greatly improved control of shallow hazards and, a more-stable, nearly-gauge well bore with benefits of improved hole cleaning, allowing significantly deeper casing setting depths, and more effective primary cementing of surface casings with lower cement volumes. Mud logging of formation cuttings and associated drilled gases reveal geologic details of tophole intervals otherwise lost with seabed discharge methods. The technology facilitates drilling in sensitive subsea environments subject to strict zero-discharge rules, and in distant locations where resupply of drilling fluids and additives are impractical; particularly with MODUs that cannot store sufficient quantities of mud to self-support continuous dumping. The field trial was successful in 4657 ft (1419 m) water depth. Cost-effective use of high-quality drilling mud in tophole drilling was demonstrated. A near-gauge wellbore, detailed tophole mud logging data and effective primary cementing were obtained. The unique design and deployment of the deepwater mud return system are first-of-its-kind, and are described. The field trial results are significant as the first-ever application of subsea mud return technology to deepwater tophole drilling. Successful integration of topsides equipment and running procedures to the 3rd-generation semi submersible drilling vessel is an engineering and operational feat. Successful deployment and operation of the subsea equipment at true deepwater depths is a pioneering technical achievement. This technology finally opens the path for cost efficient deepwater dual gradient drilling.