In-situ retrofit sets milestone in deepwater MPD evolution

MPD surface equipment was spotted on the rig prior to actual operations, to make piping and other preparations more accurate.

In an industry first, a deepwater dynamically positioned (DP) semisubmersible drilling rig was retrofitted expeditiously for full managed pressure drilling (MPD) functionality, while performing operations at sea in the Asia-Pacific region.

The in-situ installation of a semi-permanent MPD package on a deepwater rig, which was designed originally, and strictly, for conventional drilling methods, was unprecedented. Performing a major retrofit at sea, with limited shore-based support resources, and in parallel with ongoing rig operations, presents obvious safety and procedural difficulties. Extensive planning, and a detailed assessment of all associated operational risks, were conducted in advance of the integration project, to minimize, if not eliminate, all potential hazards associated with simultaneous operations.

As an added challenge, the deepwater drilling commitment required that the MPD integration be accomplished within a tightly constrained timeframe. To stay on schedule, all of the typical onshore support services—including systems integration testing (SIT) and certification—had to be carried out at the nearest available shore-based facility.

The objectives, along with the geological characteristics of the rapidly approaching deepwater drilling campaign, required that the rig be modified to enable the seamless transition from a conventional “open-to-atmosphere” circulating system to various MPD modes, when well conditions required such. Specifically, because the well program called for drilling through deep, karstified, fractured carbonate zones, there was an inordinate risk of severe-to-total circulation losses. Therefore, the rig needed the ability to switch to pressurized mud cap drilling (PMCD)—an MPD variant in which an annular fluid column (assisted by backpressure) is maintained above the vugular formation, with no returns to surface. Accordingly, as an additional part of the MPD-centric adaptations to well control policies, procedural modifications were developed specifically for operating in PMCD mode.

Fig. 1. The Weatherford rotating control device and associated MPD riser joint components, shown after integration into the rig’s riser system, and before being submerged into place.

Perhaps the biggest challenge encountered during the integration was the installation of a hard-piping network, to accommodate the distinctive flow regimes of an MPD system. Furthermore, because the MPD package had to fit within existing rig dimensions, the team needed to make a number of on-the-fly modifications to individual MPD components, including engineering a slimmed-down iteration of Weatherford’s patented below-the-tension-ring rotating control device (RCD), which enhances safety during MPD operations, Fig. 1.

The entire MPD rig integration project—from procurement to deployment, and including application-specific crew training—was completed within six months, which is noteworthy, considering that the average MPD rig-up takes more than one year. Moreover, along with reinforcing the absolute necessity of close collaboration between the operator, drilling contractor and MPD service provider, the project also provided keen insights to further accelerate the learning curve of continually evolving MPD technology.

EVOLVING DW STANDARD

The evolution of automated MPD as a mainstream, deepwater, well construction technique, which has been well-documented, can be attributed to its exceptional capacity to precisely control the annular pressure profile throughout the wellbore, and to deliver near-instantaneous kick detection and response.^2,3,4 These capabilities are paramount when drilling deep and ultra-deepwater wells, which are generally in areas characterized by extremely narrow, shifting pressure gradients that aggravate the well-control risks and nonproductive time (NPT) frequently accompanying severe-to-total lost circulation. Due to the demonstrated efficiencies and safety afforded by MPD, Brazil is among the deepwater theaters that mandate MPD, particularly for drilling exploration wells in the pre-salt zones.

Driving the increased use of MPD, in exceedingly tight deepwater drilling windows, is the capacity to immediately apply annular backpressure to control bottomhole pressures (BHP) and compensate for the annular pressure fluctuations that occur when the mud pumps are switched on and off. Furthermore, depending on the formation characteristics and well objectives, MPD offers the flexibility to easily transition between the primary operational variants used offshore, namely constant bottomhole pressure (CBHP), returns flow control and riser gas mitigation for HSE considerations, and PMCD mode.

The CBHP variant is used for drilling narrow or unknown mud-weight windows, with slightly overbalanced, near-balanced or slightly underbalanced mud density. With CBHP, surface backpressure (SBP) on connections is exerted, while in returns flow-control mode, there is no intent to hold surface backpressure on connections. This variant is employed primarily for early kick-loss detection and riser gas mitigation to reduce the frequency of false-positive kick alarms, identify and quantify wellbore breathing, provide positive diversion of surprise kicks from beneath the rig floor, and to isolate floor hands from otherwise open-to-atmosphere mud returns. As mentioned, transitioning to the PMCD variant has become compulsory when drilling severe-to-total loss zones with no mud or cuttings returns.

The Weatherford marine series RCD, featuring dual sealing elements, represents the core enabling technology for the continually evolving offshore MPD system. By sealing the top of the annulus, the RCD provides a pressure-tight barrier, which, together with the closed-loop configuration, diverts returning fluids from the rig floor. The latest-generation automatic MPD choke manifold, in turn, instantly applies the desired SBP to control mud flow and integrates the MPD system with the rig. In addition, the next-generation software that automatically integrates the surface and downhole pressure sensors in a real-time MPD data acquisition and process network. Therefore, the automated control system responds instantly to wellbore signals, to enable the driller to detect and contain incidents before they deteriorate into a well-control risk.

Not surprisingly, as MPD becomes the widely accepted standard for deepwater drilling, the demand for floaters outfitted to accommodate the closed-loop system has increased proportionally. While most of the latest-generation deepwater rigs are designed at the front end for MPD readiness, a considerable percentage of the existing global deepwater fleet will require a post-commissioning retrofit.

IN-SITU INTEGRATION

As Feasey, et al, pointed out, a number of possible bottlenecks must be addressed up front when retrofitting a deepwater rig, which was originally designed solely for conventional open-to-atmosphere drilling, for closed-loop MPD functionality.⁵ These include dimensional restrictions associated with the rotary table, diverter and upper riser package, as well as the selection of piping and associated components to deliver flow path flexibility and allow the seamless transition from one MPD variant to another. The holistic integration of all hardware, software and procedures also must be considered at the onset.

In other words, the modifications necessary for MPD system installations are difficult, even when performed on a deepwater rig still under construction, in an ideal environment. For this project, however, the issues were compounded appreciably. Because the project was the first conventional DP deepwater rig in the contractor’s fleet to be made MPD-ready, and the first-ever in-situ MPD integration of this kind, the learning curve had to be accelerated, demanding the utmost in collaboration between all parties involved. Additionally, owing to the tight timeframe for the upcoming Asia-Pacific deepwater well, the retrofit would largely have to be performed on location, and in conjunction with ongoing rig operations.

Among the operational and logistical challenges, one of the most obvious difficulties was installing an MPD package on a constantly moving, active rig. Thus, the other rig activities had to be taken into account, especially when tying-in to critical components.

Moreover, given the time constraints, the nearest accessible shore base was the only alternative site available for final assembly and testing. Like other aspects of the in-situ retrofit, the limitations of the shore-based facility required on-the-fly modifications to ensure that the MPD system was fully certified before it was shipped directly to the rig for installation.

A pre-job rig survey was performed to assess the feasibility of installing the deepwater MPD system, and making the necessary modifications. Integration of the MPD system required working within the basic existing rig dimensions.

During the survey, it was found that one of the rig’s existing telescopic joints would need to be modified to accommodate the integration of the MPD equipment. Specially fabricated crossovers from the riser and modified telescopic joint to the MPD equipment also
were required.

Ensco carried out engineering and riser analysis, primarily to assess the equipment interfaces and identify any limitations associated with integrating the deepwater MPD technologies into the rig’s existing riser configuration. Ensco, likewise, performed a separate drive off and riser analysis (DORA) specific to the targeted well plan, and the additional MPD equipment. Moreover, a finite-element analysis (FEA) was performed to factor in the exertion of backpressure and bending in the riser system for the MPD equipment.

FIT-FOR-PURPOSE MODIFICATIONS

While myriad aspects of the retrofit—such as modifying one of the rig’s telescopic joints and the hang-off points to support the weight of some 250 ft of additional hoses—were relatively straightforward, others required major modifications. One of the most complex of the integration challenges was the time-constrained procurement and complete in-situ installation of the hard-piping network, to enable switching from a conventional circulating configuration to an MPD system, with the flexibility to seamlessly transition between all the MPD modes.

Fig. 2. The MPD system surface piping (in red) was installed on the rig while deepwater drilling operations were performed to enable and facilitate multiple MPD flow-routing options.

At the onset, the automated control manifold and all associated deepwater MPD components were delivered to the rig on location, to accurately measure the interconnecting pipe work that would be required. Next, all new piping and associated connections in the moon-pool area were installed, specifically to accommodate MPD, including the link and standpipe connections for pumping into the annulus and providing ready access to the mud-gas separator and shale shakers. At the end of the successful project, an estimated 52,646 lb of new piping had been installed on the rig for use during MPD operations, Fig. 2.

Moreover, the integration required the installation of a modified version of the aforementioned below-tension-ring (BTR) RCD, which is critical for the safe deployment of the MPD technique on a DP deepwater rig. The BTR RCD affords the clearance necessary to maintain station-keeping and, more importantly, to deliver the unabated maneuverability essential for quick emergency disconnects. Specifically, the BTR RCD is made up as part of the marine riser and situated between the subsea BOP stack and the diverter just below the rig floor. With the MPD flowlines installed, the system demonstrated its capacity to effectively strip and positively divert annulus returns to the dedicated MPD choke manifold or other rig equipment while compensating for heave and DP rig movements. The BTR RCD technology also facilitates transitioning from conventional drilling to all of the MPD variants.

During the riser integration evaluation, Weatherford identified that the standard 19.375-in. OD of the BTR RCD bearing assembly, which functions well in 19.5-in. riser systems, would be ill-matched for the existing marine riser. Consequently, the BTR bearing assembly was redesigned with a reduced diameter that, along with modifications to the RCD body and all internal latch components, enabled the system to meet the Ensco rig’s telescopic joint and pass through an ID restriction of 19.1 in. Accordingly, the resulting MPD system was able to address multiple drill pipe sizes, without compromising the BTR RCD functionality, performance or rating.

SYSTEM TESTING, ACCEPTANCE

More than any other operating arena, deepwater operations demand the utmost in hardware and process integrity, which must be validated properly to meet the standards set in place by regulatory bodies and other pertinent certifications. Hence, despite less-than-optimal conditions at the resource-limited onshore support base, all critical components, including the lower telescopic joint and crossover, were inspected individually. Afterward, the entire system, including all MPD joint components, was assembled at the shore base, where it underwent a series of system integration tests. After a complete round of pressure- and function-testing sequences, the aggregate MPD system was certified as fit-for-purpose, and readied for deployment and installation on the rig.

On-board space constraints led to the use of phased array ultrasonic testing (PAUT), an advanced nondestructive testing (NDT) methodology, rather than X-ray diffraction (XRD), to confirm the metallurgical integrity of the newly installed piping network. Upon inspection, the American Bureau of Shipping certified the MPD piping system as meeting all applicable standards.

MPD-SPECIFIC TRAINING

Parallel with the retrofit initiative, Weatherford conducted extensive training that incorporated a specially developed core curriculum designed to fully indoctrinate rig crew members, and other pertinent stakeholders, in the unique intricacies of the MPD system. Specifically, the rig hands received sweeping and detailed instruction on the specialized technologies, and how their individual roles would change when the rig transitioned from conventional drilling to MPD operations. The extensive overview covered topics ranging from the differences in a closed-pressurized system, to early kick detection and fingerprinting, to handling and mitigating gas in the riser. Much of the course work focused on the idiosyncratic procedures and other elements of deepwater PMCD on a floater, including PMCD-specific riser and surface equipment, deepwater rig set-up, the system flow routing and well-control considerations and contingencies.

WELL-CONTROL CONSIDERATIONS

Throughout the retrofit process, the training team heavily emphasized that neither MPD nor the application-specific PMCD variant should be considered substitutes for Ensco’s accepted well-control practices. For example, training reinforced that the BTR RCD should be considered only as a diverter to be used in conjunction with—not as a replacement for—the annular and ram BOP stack.

Fig. 3. Innovative techniques were employed to enable the in-situ MPD rig integration initiative to proceed within time and cost constraints.

Consequently, well-control policy adaptations were formulated to account for the distinctive MPD/PMCD considerations, with discussion points centered on a number of concerns that needed to be addressed, pre-deployment. Kick modeling was performed to determine system limits and the peculiarities of narrow mud-weight windows and influx circulation.

OBSERVATIONS, KEY LEARNINGS

The initiative helped overcome many of the barriers to seamless MPD integration, while demonstrating the viability of performing an in-situ retrofit simultaneously with ongoing rig operations, Fig. 3. It also reinforced the value of adopting a proactive strategy to meet the steadily increasing demand for MPD-ready rigs, as more operators and nations mandate MPD for challenging deepwater and ultra-deepwater wells.

While not ideal in circumstance, the on-location retrofit provided some keen technical insights that are worth taking into account for future projects. As a case-in-point, consideration should be given to incorporating a bypass for the riser choke and kill, booster and conduit lines, thereby lessening system complexity and enabling full integration of the riser and MPD assembly.

Above all, the successful project reinforced the absolute requirement for the highest level of collaboration—between operators, drilling contractors and MPD service providers—to ensure that the respective expertise is captured and all objectives met. 

REFERENCES

1. Toralde, J.S.S, “RCD for DP drillship takes MPD deeper,” Drilling Contractor, July-August, 2011.
2. Hannegan, D. and A. Mahmood, “Offshore well integrity management with MPD tools and technology,” OTC paper 25723, presented at the Offshore Technology Conference, Houston, Texas, May 4–7, 2015.
3. Karnugroho, A. et al., “Managed pressure drilling technologies deployed for deepwater and HPHT well control in Indonesia,” World Oil, October, 2013.
4. Pavel, D. and G. Feasey, “Closed-loop drilling boosts deepwater MPD success,” E&P, October, 2012.
5. Feasey, G. et al., “Collaboration adds MPD functionality to deepwater rig.”