Deepwater Challenges & Solutions

The BTR RCD is a key component of a deepwater MPD system, containing annular flow and redirecting it to help form a closed-loop circulating system. (Photos courtesy of Weatherford International)

In deep water, one of the hazards that closed-loop drilling systems help mitigate is the threat of riser gas.

Closed-loop drilling (CLD) systems allow drilling operations to be performed in a closed environment instead of the conventional method of keeping drilling fluid returns open to the atmosphere.

The ability to close or restrict the flow of circulation returns and control and maintain pressure on the well while simultaneously rotating and continuing to drill is only possible with a rotating control device (RCD). An RCD creates a pressure-tight barrier in the wellbore annulus that allows fluid returns to be contained and diverted, forming a vital line of defense against drilling hazards. In a deepwater drilling environment, one of the hazards that CLD systems help mitigate is the threat of riser gas.

Previous studies on deepwater wells have revealed that due to the use of oil-based fluids, gas kicks that are unintentionally entrained in the return mud flow are unlikely to break out of solution until they reach a depth of 610 m to 915 m (2,000 ft to 3,000 ft) below the drill floor. At this point subsea BOPs will no longer be able to contain them. The conventional practice of dealing with gas in the riser is to use the rig diverter system to vent it, but there is minimal control and considerable risk involved. It is for this reason that handling of gas in the riser on a deepwater rig is complex and challenging.

To mitigate this risk using CLD systems, an RCD is installed on top of the rig marine riser together with an additional annular BOP and flow spool directly below it. This combination provides a system that is already in place to safely divert fluids containing gas away from the rig floor and toward an automated managed-pressure drilling (MPD) choke manifold system.

Riser gas handling applications require an automated MPD choke manifold with Coriolis mass flow meters and high-precision pressure sensors that enable higher sensitivity and greater data resolution. An in-line gas chromatograph and a high-rate mud/gas separator provide gas characterization and gas handling capabilities, respectively. In addition to safely handling gas in the riser, this system allows drilling operations to be conducted with surface pressure and advanced kick and loss detection and control capabilities.

Closed-loop drilling systems for DP drilling vessels

The RCD and the MPD joint are shown during the installation process.

CLD systems only recently have been used in deepwater environments and more recently in dynamically positioned (DP) drilling vessels. A major technical hurdle for this application was cleared when the industry's first RCD made up below the tension ring and integral to the riser package of a DP drillship was developed.

The Weatherford SeaShield Model 7875 Below-Tension-Ring (BTR) RCD was first deployed in 2010 and has since been used on at least five deepwater wildcat wells in Indonesia.

In Indonesia, the BTR RCD was installed above the intermediate riser joint and below a standard slip joint about 43 m (140 ft) below the rig floor and 12 m (40 ft) below sea level. Hydraulic and electrical connections below the water line are made via a subsea-rated hydraulic stab plate. The BTR RCD is the first rotating head designed and field tested to support riser tension requirements of as much as 3 million lbs. and is certified to the drill-through specifications of API 16 RCD, the industry standard for RCDs. Using this standard, the RCD has been rated to a static and dynamic pressure ratings of 2,000 psi and 1,000 psi (at 100 rpm), respectively. Prior MPD operations aboard floating vessels have been configured with a surface RCD above the water line and the tension ring. Because the new RCD is made up below the tension ring, no modifications are required to the riser's telescoping slip joint or the rig's mud returns system.

In the deepwater CLD set-up, an MPD annular BOP is installed below the BTR RCD. The MPD annular BOP is used in conventional drilling operations when the RCD sealing element is not installed to shut the well in and facilitate CLD riser degassing operations. The MPD annular BOP also can be used when the RCD sealing element needs to be replaced. The MPD flow spool is installed below the MPD annular BOP and is connected to flexible hoses, which act as the primary flow lines for MPD operations.

The other major component of the deepwater MPD system is the Microflux Control Automated MPD Choke Manifold. It is a specialized 5,000-psi manifold equipped with dual chokes and instrumented with a Coriolis mass flowmeter and precision pressure sensors. The system enables the management of annulus pressure throughout the wellbore and in the marine riser. Adding annular pressure control at surface and controlling pressure in the marine riser will assist in controlling gas breakout. The gas can be kept in solution by applying pressure on surface and will then expand after exiting the riser and passing through the automated MPD choke manifold system. The expanded gas is then diverted and managed accordingly through the high-rate MPD mud/gas separator system.

Deepwater CLD systems and riser gas risk mitigation

The CLD system for deepwater applications on a DP vessel takes both a proactive and reactive approach on mitigating the risk of an event involving gas in the drilling riser. The proactive approach uses the early kick detection and control capabilities of the CLD system to drastically cut back the incidence of reservoir gas entering the oil-based mud system at depth and dissolving into it without being detected, only to come out of solution later on when already in the drilling riser and above the subsea BOPs.

Both testing and field deployment have proven the system's ability to detect flow anomalies within its defined range earlier and faster than conventional systems. The automated MPD choke manifold system is operated in automatic mode and will immediately detect a kick and close in on its choke to increase the bottomhole circulating pressure and control the well.

In the six rank wildcat deepwater wells drilled with the CLD system, there have been no instances recorded where formation gas has broken out of solution above the BOPs and inside the riser.

Furthermore, the CLD system control algorithms detected at least five flow anomalies, kept them to a minimal volume, and circulated them safely out using the rig well control equipment.

In the unlikely event that an influx does make it above the subsea BOPs and into the riser, the CLD system setup can be used to circulate the influx out of the well in a controlled manner.

In the deepwater CLD system deployed in the field, there has so far been one instance when an influx was circulated out through the riser using the system after being detected. After closing in on the subsea BOPs, the MPD annular BOP also was closed, and the fluid system above the subsea BOPs was circulated out through the automated MPD choke manifold and the high-rate mud gas separator. The mud in the riser was then weighted up, and the subsea BOPs were opened and the rest of the well circulated to a higher mud weight with the MPD choke manifold exerting and controlling backpressure to avoid subsequent fluid gain.