With the use of a unique deepwater managed-pressure drilling (MPD) system to successfully drill fractured carbonates in the Makassar Straits of Indonesia, Transocean’s GSF Explorer continues to push the boundaries of offshore drilling.

Installation of the first integrated MPD system in a marine riser below sea level provided a flexible solution that enhanced drilling capabilities in the difficult formation and improved safety and efficiency through early kick detection, riser gas handling, MPD, and pressurized mud cap drilling – an MPD variant used in total lost circulation conditions.

MPD components
Weatherford’s deepwater MPD system is installed above the intermediate flex joint in the riser and below a standard slip joint. As a result of this configuration, the riser can be used in a conventional manner with full-bore access to the well. The entire system is installed through the rotary table when the riser and BOP are deployed. The 40-ft (12-m) MPD system provides riser gas handling and early kick detection as soon as the BOP is connected to the well.

The riser MPD assembly used on the GSF Explorer comprises three main components: the flow spool, riser annulus, and rotating control device (RCD).

When mounted below the tension ring on a floating rig, the RCD creates a positive seal around the rotating drillpipe and tool joints to positively divert annulus returns. (Images courtesy of Weatherford International Ltd.)

The flow spool provides the connection for the flowlines from the top of the riser to the MPD manifold. Two 6-in. flowlines are connected at the moonpool to allow returns to the shale shakers and mud pits.

A 21 1/4-in. subsea annular BOP is installed above the flow spool. The annular BOP allows riser gas handling. If a kick is detected in the riser, the annulus can be closed to provide controlled handling of a riser influx through the flow spool and back to the surface.

The Weatherford Model 7875 Below Tension Ring Seashield RCD is installed on top of the annular BOP in the MPD joint in the riser. This RCD provides pressure control for annulus gas containment and drilling operations. Its principle use is to provide an annular seal around the drillpipe during drilling and tripping operations.

The inside profile of the RCD includes a hydraulic latch assembly to receive, retain, and release the bearing seal assembly. With the bearing assembly removed, the 2,000-psi RCD system is capable of handling the full-size 18/4-in. BOP tools.

This RCD currently is the only one in the world that can be installed in a deepwater marine riser and support the riser tension requirements while conforming to API 16 RCD drill-through specifications. The ability to put the RCD in tension with the standard riser allows it to become a standard component of the riser and enables installation below the conventional rig slip joint.

Installing the RCD below the tension ring and slip joint allows deepwater drilling operations to be conducted conventionally as in a standard deepwater drilling configuration, yet it provides early kick detection and riser gas handling at any time during operation. The body of the RCD has a subsea-rated hydraulic stab plate mounted so hydraulic hoses and electrical connections can be made while running the assembly below the water line.

The closed-loop Microflux MPD control system minimizes risk to personnel and the environment by accurately leveraging proprietary algorithms to detect minute fluid influxes and losses with a degree of precision that is unattainable using conventional equipment.

A specially designed “buffer” manifold mounted close to the MPD choke manifold provides the connection between the MPD flow spool in the riser and the MPD choke manifold to the flowline. This manifold provides all of the circulation requirements for the operation and also incorporates the pressure relief system for the riser.

The Microflux MPD Choke Manifold is a specially designed 5,000-psi manifold equipped with dual chokes. It is instrumented with a Coriolis mass flowmeter and precision quartz pressure sensors, which provide the early kick detection system.

A high-rate two-phase separator is installed behind the MPD manifold to allow gas handling. This additional high-rate gas separator is installed to avoid tying into the existing mud gas separator.

System functionality
A main requirement for the system was to provide early kick detection. This is achieved by directing return flow through the MPD choke manifold and Coriolis meter. The configuration allows backpressure to be applied as soon as a kick is detected to control the well.

If a kick is not detected until the gas is inside the marine riser, the system allows the surface annulus to be closed. Once it is closed and circulation is stopped, the subsea BOP can be closed to contain the gas in the riser, making it possible to use either the MPD manifold or rig choke manifold to circulate gas out in a controlled manner.

Glomar Explorer operations
The deepwater MPD system is field proven. In one case, it was used to measure pore pressures in carbonates while drilling with an underbalanced fluid. Wellbore pressure was controlled at all times during drilling, connections, and tripping operations.

Kick detection has been successful at very low volumes. Early in its application, the system detected a 2-bbl kick while drilling in carbonates. The MPD system controlled the kick and managed wellbore pressures.

The upper part of the carbonate structure was drilled using the constant bottomhole pressure (BHP) MPD methodology without encountering losses. Losses were observed once the first fractures were encountered and increased as more fractures were exposed.

The MPD mode was changed from constant BHP to a pressurized mud cap drilling system when losses could no longer be managed with the mud supply. To continue drilling despite total losses, an oil-based cap fluid was placed in the annulus to control annular pressures, while seawater was pumped down the drillpipe. The system allowed the well to be drilled to its targeted depth safely. It also allowed formation pressures to be measured in real time and logs to be run safely.

The use of a deepwater MPD system to drill fractured carbonate structures where total circulation losses are experienced opens new opportunities to access difficult-to-reach resources. The highly flexible system provides multiple methodologies and capabilities that can be applied according to well conditions and objectives. In the drillship installation, the MPD system enabled drilling operations in otherwise undrillable conditions while improving safety and efficiency.

Offshore