Managed-pressure drilling riser stack integration

As the deepwater prospects being drilled today address ever more complex conditions, the drilling industry needs to adjust its technology to address the challenges that arise.

The use of conventional "open to atmosphere" drilling methods cannot identify and respond to some of the well events that are encountered in time to prevent them from becoming an issue that in many cases can result in the need to abandon the well.  For example riser gas, which occurs when a gas influx entrained in an oil-based mud breaks out of solution as it is circulated to the surface. This typically occurs about 2000-3000ft below the drill floor, at which point the gas is above the blow out preventer (BOP) in the riser and beyond conventional containment.  The gas can be vented by using the rig diverter system, but this practice is implemented as the very last step in an emergency response and is uncontrolled. One advancement in rig technology used to try and regain control in a situation of gas in the riser is the riser gas handling (RGH) system. Although RGH offers some element of control, it still adopts a reactive approach that does not offer a solution designed to prevent the gas from ever entering the riser.

Closing the loop

Integrating a below-tension-ring (BTR) rotating control device (RCD) into the riser permits the closing and pressurization of the annular returns flow, a key component for MPD. Photo: Weatherford.

Another approach is to integrate a rotating control device (RCD) into the rig's upper riser package, enabling the drilling loop to be closed and pressurized. This integration allows for the installation of a managed pressure drilling (MPD) system that monitors mass flow (flow into and out of the well), providing precise early detection of a hazard, and introduces a means to employ swift preventative measures before a situation escalates into a potentially catastrophic scenario.

One example is detecting an influx wherein an unexpected overpressure zone is encountered, resulting in an influx of reservoir fluid into the wellbore. The MPD software detects the imbalance and calculates what the equivalent mud density should be to balance the well. Automated chokes react to apply surface back pressure, repressing the influx and allowing the driller the time to apply the safest means of circulating the influx out of the circulating system.  MPD is a proactive process that circulates drilling fluid within a contained, pressurized system geared towards riser gas prevention as opposed to conventional drilling circulating systems that are open to the atmosphere and require a reactive human response.  The main components of an MPD riser stack system are:

• A rotating control device (RCD), which creates a pressure tight barrier in the wellbore annulus thus enabling containment and diversion of returning fluids.
• An MPD annular preventer–necessary when replacing the RCD element during a constant bottomhole pressure (CBHP) MPD mode and allows for riser gas handling if drilling open to atmosphere (without the RCD element).
• A flow spool–required during RCD installation to divert returns flow to the rig via a flexible armored hose or gas when drilling in a conventional open to atmosphere setting (no RCD bearing installed).
• An MPD/diverter manifold–required to apply different MPD techniques.
• Surface control system(s),–required to operate the aforementioned components as part of the MPD operations.

Combined, all MPD riser stack components should be automatically controlled by a central intelligent control unit responding to the unique wellbore signals being monitored, including return flow, to enable immediate detection of changes in fluid flow.  This can provide instantaneous detection and response enabling drilling decisions to be made before they escalate to an uncontrollable level.  
Traditionally, the RCD is installed atop the BOP, but on a deepwater vessel, a more sophisticated RCD can be installed below the tension ring as an integral part of the marine riser system. Within the closed loop created by the RCD, changes in pressure are easily detected and effected, while pressure and mass-flow measurements provide real-time data that generates manual or automated changes in choke settings.  Manipulation of the MPD choke manifold leads to changes in annular backpressure at the surface, which immediately increases or decreases downhole wellbore pressure.

MPD in its various forms, presents the possibility to operate in any of the following modes:

• Pressurized mud cap drilling (PMCD), which can be used to drill formations where total circulation losses make conventional circulatory methods impossible.
• Constant bottomhole pressure drilling (CBHP), which for deepwater operations provides the precise wellbore pressure control required to navigate extremely narrow drilling windows between pore pressure and fracture gradient. The ability to dial in and hold a specific downhole pressure without changing mud weight provides a high degree of control and a first response to many pressure-related problems.
• Returns flow control.
• Dual-gradient drilling (DGD)

MPD-enabled rigs
An MPD riser stack, integrated with the purpose of MPD operations, will be the foundation for the future development of floating rigs able to effectively address the challenges of ever-deeper drilling horizons. It enables drilling modifications in three key segments:

• Riser gas handling
• Early identification of downhole flow anomalies such as a kick, loss, breathing or ballooning events.
• A proactive suite of managed-pressure drilling (MPD) methodologies that are bringing non-traditional solutions to complex deepwater drilling challenges such as dealing with uncontrollable losses or navigating narrow pore and fracture pressure windows. 

Despite the success of MPD systems, and the potential they present, their use aboard semisubmersibles and drillships is limited by a host of cost, personnel and deployment constraints on the equipment caused by older rig designs. Extending these MPD advantages to a broader scope of deepwater wells can offer safety, operational and economic rewards, however equipment deployment can be hampered by the inability of deepwater drilling vessels, originally built for conventional open-to-the-atmosphere circulating systems, to readily accommodate it.

RGH/MPD manifold, permanently built into the rig and designed to function as MPD. Photo: Weatherford.

Future developments
Fully realizing MPD benefits in deepwater applications requires a focused industry effort to develop guidelines, procedures and standards for equipment procurement, rig modification and design and perhaps most importantly adequate training for rig personnel.  In the move towards a degree of MPD readiness, modified rigs will have enhanced RGH systems but to go to full MPD capability more upgrades are generally required.  In newly built rigs these needs can be readily addressed; however in existing rigs they can be problematic.

Addressing the problem of MPD readiness will mandate a degree of cooperation across the spectrum of the industry. The most obvious means of achieving this would be for the E&P companies to create the demand by requiring drilling contractors to standardize the integration of a RGH system,  not for riser gas handling as the primary task, but capable of incorporating an RCD (MPD riser stack) for the greater purpose of MPD operations. The greater value to the industry will be an MPD-enabled rig capable of efficiently operating under the daily procedures required during MPD operations once the circulating loop has been closed by the installation of the RCD element. An excellent example of this approach was the design of a newly built drillship which incorporated MPD as part of its capabilities to operate offshore Angola where total mud losses frequently occur.  The success of the project was the result of a collaborative effort between operator, contractor and service company. The installation of a few minor components in the MPD riser stack and rig surface equipment, enabled the rig to rapidly transition from "open to atmosphere with RGH" to full MPD capability thus providing a step change in the rig's capabilities – a transition process that saved 12-14 months rig up time.

In future it may be appropriate to promote a system where increased industry access to MPD capabilities would be driven by mandates set by local and international governing bodies as part of their assessment of  applications for deepwater drilling permits. In lieu of such commitments, a coordinated effort as described is required between the E&P companies, drilling contractors and service companies to create an MPD infrastructure and commitment to it in order to facilitate the ability to drill what would otherwise not be possible.

About the authors

Written by  Guy Feasey and Neal Richard, Weatherford Monday, 09 June 2014 18:18

Guy Feasey is Weatherford's global business development manager. Neal Richard serves as a MPD technical manager for North America at Weatherford.