DIVERLESS SUBSEA SYSTEMS-CONCLUSION NEW FEATURES ON ROVS AND CONTROL SYSTEMS ADD FLEXIBILITY AND CUT COSTS

Robert H. Rothberg, Johnce E. Hall
Amoco Production Co.
Houston

Larry D. Douglas
Oil Industry Engineering Inc.
Houston

Kerry G. Kirkland
Consulting Engineer
Houston

William S. Manuel
Manuel Designs Inc.
Houston

Subsea maintenance with remotely operated vehicles (ROVs) has a substantial role in defining diverless subsea operations. System complexity, and hence also cost and reliability, are affected by the types of maintenance interfaces available.

This concluding part of a three-part series covers Amoco's development of ROV tooling that incorporates variable buoyancy, a vertical running tool, and an electrohydraulic power package that includes a horizontal torque tool.

In the development of a diverless subsea production system (DSPS), Amoco also has concentrated on designing new control systems and ROVs that can remain subsea for extended periods of time.

ROVS

The inventory of ROV work packages available to a subsea designer has a large impact on the size and placement of equipment modules as well as the type of maintenance interfaces that are required. Amoco has focused on the 50 + hp work class ROV.

AU work package interfaces required for a complete subsea system implementation have been tank tested using ROV tooling. Work has also progressed on a resident ROV concept, designed for a reliable 1 year immersion and aimed at minimizing the number of remotely controlled hydraulic valves on a subsea system.

The resident ROV also provides direct readout of underwater gauge and valve position indicator status, reducing telemetry requirements.

MAINTENANCE ROV

Standard drilling support vehicles have rapidly progressed technologically over the last 10 years, but continue to be used primarily for inspection and limited manipulative functions. While horsepower and reliability have made great gains, the actual task capability has remained limited.

The DSPS team determined that any deepwater production facility would be heavily reliant upon ROVs for installation, operation, and manipulation of the seafloor components; therefore, effort was focused on the development of additional tooling packages and operating enhancements that would greatly expand the capability og present day ROV systems.

The maintenance ROV (MROV) was developed to meet these requirements and is comprised of two primary elements: the tool interface package (TIP) including the torque tool and hot line stab tool and the variable buoyancy package (VBP).

The system (Fig. 1) presents the operator with the capability to select a vehicle of opportunity, for relatively quick modification using the Amoco MROV components. A supplemental foam block offsets the weight of the TIP and VBP add-on's.

At the work site, all maintenance interfaces use a single docking point with latching capability to reduce operator effort in acquisition and station keeping. The latching mechanism is failsafe in the event of vehicle power loss.

The design of the interface "bucket" includes a dual taper guidance technique. The variable buoyancy package provides 300 lb of variable ballast that has proven suitable for all tooling requirements encountered in the DSPS program to date.

The use of variable ballast eliminates the need for transfer weights common in early ROV tooling configurations. Extension to 600 lb has been investigated and is readily accomplished within the same package size. The VBP also houses a vertical running tool used to latch and release packages having a standardized fishneck profile looking up.

The tool interface package (TIP) is a self-contained electrohydraulic control system that powers torque tool, manipulator, hot stab, and video functions.

Hot stab interfaces are standardized with respect to the location of the torque tool; however, a manipulator-held whip line stab can also be used.

The TIP can be packaged to serve as a general purpose electrohydraulic power and control device. The TIP has been identified as the driver for the MIR tool associated with the short jumper flow line connection method, described in Part 2 of this series (OGJ, Mar. 22, p. 95).

With the tooling package, the ROV can actuate and override tree and manifold valves, install umbilical terminations and instrument packages, and assist in the installation of flow line jumpers and pipeline spool pieces. The MROV concept has been fabricated and demonstrated in a test tank in conjunction with many of the actual prototype control components.

RESIDENT ROV

The resident ROV (RROV) was conceived as a possible solution to real time monitoring and operation of a remote seafloor installation.

In essence, the function of the human land lease pumper is duplicated by a long-term immersion vehicle. Daily surveillance of gauges, well test valve manipulations, and general inspection would be performed.

A hydrophone has also been included to permit listening for sounds associated with normal production, and an ultraviolet light source can be included to highlight hydrocarbon leakage.

While remote electrohydraulic control systems have been recommended and used for such development schemes, they do not provide the eyes and ears that a production operator would prefer.

Concerns over ROV reliability-industry records indicate dive time between maintenance of just over 11 days-would initially discourage the pursuit of such a concept. However, the present electrohydraulic ROV systems are designed for high power, high capability, high mission flexibility, and frequent maintenance.

By changing the design to have minimum capabilities to meet the long-term operation needs of a subsea system, a 1-year mission is feasible.

The RROV system (Fig. 2) fulfills such a mission, providing real time video signals of the seafloor system, as well as valve override, operation, and full time inspection capability.

To provide these features without the high maintenance aspect of conventional ROVS, the RROV has been designed as an all-electric, simplistic vehicle incorporating a minimum of special functions. The high reliability components are designed for long-term submergence.

All electronics are removed from the vehicle and all power switching is done from the surface.

The vehicle is little more than five thruster motors, a torque tool, and a camera potted directly to a tether. Lamp and camera life have been addressed through redundancy.

In the case of a long distance offset from the host facility, such as 20 miles, the RROV would require a fiber-optic umbilical in combination with power and signal conductors.

While the RROV has not been developed to the same level of completeness as many of the other components of the DSPS, ROV design and manufacturing companies indicate confidence in the capability to manufacture such a vehicle.

Evaluations have been completed on tether management, system block diagram-level reliability, high reliability component identification, and numerous other feasibility issues.

At present, the RROV continues to offer the promise of permitting at least partial offset of the cost of electrohydraulic control components for each well.

CONTROL SYSTEMS

To reduce costs, Amoco examined several new control system designs. Umbilical terminations and instrument packages were two redesigned elements. Also, a "party line" sequenced hydraulic control system suitable for a multiwell production system was developed that includes a triple pilot "and/or" logic and a local safety supervisory shut-in capability.

UMBILICAL TERMINATIONS

The most prevalent hydraulic umbilical termination methods have nonpressure balanced stab connections that require a high locking force. In addition, first generation systems tended to "roll" pressure seals out during separation operations.

These drawbacks, in combination with a proprietary design and a virtual sole source situation, stimulated the development of the Amoco J-box and umbilical termination (Fig. 3).

This system uses pressure-balanced hydraulic stabs to alleviate pressure end loads and reduce structural weight. Seawater excluders are included in the stabs and receptacles for use during unmated periods, but a check function has not been required.

The Amoco design provides the capability for an ROV system to install and remove the umbilical termination, unassisted by surface manipulations. A byproduct of this design is the ability to replace seals by recovering the seal plate without disturbing a previously installed umbilical. This seal plate also offers the capability of reprogramming the seafloor hydraulic system by simply rerouting tubing on the seal plate.

Equipment such as filter cartridges and flow regulation devices that are subject to maintenance requirements can be included in the seal plate.

Testing has also been completed using the seal plate to mate and demate conductive electrical coupler halves along with the hydraulic functions. In this mode, the seal plate can serve as an electrical junction plate or can be expanded to serve as a control module base.

The tested junction box is rated for 10 functions, but the mating tolerances were based on expansion to a larger 24 function layout.

Work has also been performed to combine the capabilities of the MROV torque tool and the features of the junction box into a horizontally mated umbilical connection (Fig. 4). A small electrical umbilical jumper connection package is also available for installation by the ROV.

INSTRUMENT PACKAGES

Instrumentation is a control function that has not received much attention by the subsea industry. Traditionally, if instrumentation were required, it was placed permanently on the tree structure or in the control pod as a first step toward maintainability.

Subsea instrumentation failure has seldom justified shutting in production to effect repairs; therefore, it has been considered a disposable feature.

In the DSPS work, instrumentation was felt to be important enough to be made more accessible and utilitarian than in previous systems.

Several types of instrument packages developed for the DSPS provide the capability for pressure monitoring, either by ROV visual inspection or by electrical transducer monitoring. The depth-compensated pressure gauges in the gauge package are specifically designed to allow precise reading by the ROV in an environment where ordinary gauges are difficult to see, let alone read precisely (Fig. 5).

Other instrument packages offer control isolation and diagnostic capabilities. Each of these instrument packages is designed as a modular component conveyed by the MROV, and can be maintained without disturbing the primary tree or control equipment.

The packages are based on a standardized profile and feature a latching mechanism powered by the MROV torque tool. Syntactic foam is used to lighten the package weight for conveyance on the MROV torque tool.

HYDRAULIC CONTROL

One desire has been to reduce the size and complexity of subsea control systems and the number of control umbilicals without compromising seafloor control capability. Part of that capability includes the need to assure a rapid safety shutdown in the event of equipment damage or failure.

The predominant historical approach uses a multiplexed electrohydraulic control system to achieve low response time and to reduce the number of seafloor control umbilicals. In the case of a single well, pressure-sequenced hydraulic controls have been used to simplify the control umbilical requirements.

Sequenced systems have fallen into disfavor because of their inherent limitation to a specific sequence of operations. Sequenced systems have also been difficult to implement on a multiple well basis for the same reason.

A concept called the "party line system" has been developed as an alternate approach to sequential controls. The system uses varying control pressure on a sense line to activate various modes of operation and to bring a selected well off production for test or other functions without disturbing the remaining wells.

The party line system allows a well or wells to follow the individual function commands being sent down the hydraulic control umbilical.

An individual well may be taken off the party line and placed on its own safety supervisory control loop for continued production, unaffected by the control operations occurring on the party line header system.

In a representative party line (Fig. 6), all wells tied to the party line header system will receive the same tree function commands. One mode select control line for each well then determines whether a well will follow the commands passed via the party line control headers or will produce under its local safety system.

While the circuit uses pressure levels to command the operating mode, the steps are very large compared to historical sequenced control systems.

The well may also be individually commanded to shut in using the mode select line. In its simplest form, the control umbilical bundle consists of the main supply, one mode select line per well, and any number of tree function command lines.

The party line features allow a reduction in the number of hoses in the control bundle, thus offering a potential cost savings in that area. The heart of the party line system is a triple pilot control valve actuator developed under Amoco funding (Fig. 7).

The triple pilot control valve allows multiple signals and combination modes to activate different circuits in the hydraulic system. The triple pilot actuator is effectively a combined "and/or" logic device. These logic functions can be performed by existing hardware, but at a substantial increase in component count and loss of reliability.

LOCAL SHUT DOWN

A local safety feature is needed when multiwell production is commingled into a pipeline system that is rated for less than shut-in wellhead pressure. The hi-lo safety supervisory control system (Fig. 6) has been extensively tested for repeatability of set points under conditions of varying time between actuations.

This time variation addressed the "stiction" issue and demonstrated the reliability of the safety system. Stiction is a form of static friction that occurs when a seal resides in one position for an extended period and then is shifted, potentially requiring a higher force.

Both spring-set and surface hydraulic pressure-set control valves were used in the testing. Long-duration cycle tests on the sequence system and party line logic components have been completed.

For multiwell systems, a monitor feature was developed to trigger a surface alarm in the event of a local shut down.

ON-GOING EFFORT

The DSPS program has represented a major undertaking for Amoco Production Co. and has offered many spin-off benefits for subsea technology in shallow water applications.

Limited financial resources have been focused in key areas such as flow line connection technology and ROV tooling. Other ongoing work areas include umbilical improvements, chemical injection distribution, and subsea choke designs.

Effort is now focused on reducing the number of umbilical lines through chemical injection distribution subsea, and on investigating alternative umbilical materials to thermoplastic hoses.

The subsea choke work area has led to the development of a prototype fail-closed, ROV-adjustable choke to be integrated with a safety shutdown system. The fail-closed choke aids in protecting the underwater safety valve (USV) during shutdown and is presently being upgraded for remote adjustment and ROV-assisted maintainability.

ACKNOWLEDGMENTS

The work performed on the diverless subsea production system has been a team effort involving numerous contractors and subcontractors that include Ocean Systems Engineering, Cooper Oil Tool, FMC Corp., Oil Industry Engineering, and Gilmore Valve Co. The authors wish to thank each team member for its respective contributions to the program described in this article.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.