ARTICLES

Download PDF    References

1    2    3    4   5

Development Trends

The significant cost associated with the design, manufacture and installation of subsea control systems provides incentives to improve the existing systems and to invest in research for new systems. (2) Basically, there are two schools of thought as to how to improve control of subsea completions, which are characterised by the development of:

• the Subsea Powered Autonomous Remote Control System (SPARCS); and
• the Integrated Control Buoy.

These systems were developed with the aim of reducing overall cost of control systems by removing the need for umbilical and topside equipment required for conventional systems.

SPARCS

SPARCS (3) has been developed by Kvaerner FSSL to provide a low-cost control system solution for marginal oilfields (see Figure 7). It is a completely self-contained power generation/control system located at the wellhead. Communication with the topside facility is via acoustic signal transmission, and electric and hydraulic power required for valve operations and monitoring functions is generated by a subsea turbine-driven generator. The system comprises two groups of components: a surface controller and a subsea control unit.

Figure 7: SPARC System

The surface controller is located on the host platform and consists of an operator interface console, acoustic telemetry system and the required cables and power supply. The acoustic telemetry system includes an acoustic transponder, transmitter/receiver and interface to a directional hydrophone that can either be clamped into position or, alternatively, deployed in a wireline system.

The subsea control unit is mounted near the well and provides all the control and data-monitoring functions. Its major components are as follows:

• a control module mounted near the wellhead and housing valves and controllers;
• a hydraulic control module incorporating solenoid control valves;
• a hydraulic power unit including pumps, motors, accumulators and filters fitted with dual supply lines. The hydraulic power unit is a closed-loop system so all leakages and venting flows are returned to the reservoir, which is fully pressure- compensated to ambient pressure. Hydraulic fluid is water-based.
• the electrical power supply generated by turbo- electric generator installed in the flowline spool of the water injection. It converts the kinetic energy of the fluid into electrical energy and thus provides continuous power to the control unit. An alternative power supply is provided by a thermoelectric generator clamped to a production flowline;
• a battery system is required to provide back-up power for periods of peak power demands. It is installed and operated in an oil-filled environment and uses flooded liquid electrolyte lead acid cells. A power conditioner system provides the interface between the subsea generators and the battery system. It also makes sure that the control unit has priority over all other system power requirements; and
• the acoustic transponder system, which acts as a ‘slave’ to the surface unit, i.e. it operates on command from the surface unit. It may, however, initiate transmission if a malfunction of any subsea-located equipment in the subsea unit occurs.

The system design range is 10km. Planned maintenance intervals for the system vary between two years (hydraulic fluid replacement and battery replacement) and five years (hydraulic filters, hydraulic motor, charge accumulators, transponder and electric generator).

The items with the highest research and development content, including the thermoelectric generator and the subsea hydraulic power unit, have all been tested successfully.

Integrated Control Buoy

The Integrated Control Buoy (ICB) (4) has been developed by John Brown Engineers and Constructors (see Figure 8). The buoy houses all auxiliary systems required to operate and monitor the well performance. Full control of the subsea production system is obtained from the host platform via radio link to the buoy. The concept combines proven systems (buoy control and monitoring, dynamic riser and radio communication) in a new configuration that utilises a moored buoy connected via a dynamic riser (umbilical) to the subsea production system. Buoy technology is field-proven and widely used in oceanographic and meteorological data-gathering. The control system on the buoy is fully autonomous, i.e. it will control the subsea production system without requiring a continuous communication link to the host platform. Well parameters are measured and analysed by the buoy control system and any required action is taken autonomously, i.e. without explicit command from the platform.

Figure 8: Integrated Control Buoy

The system is designed to control and monitor all x-mas tree functions and critical subsea process parameters. All required safety back-up such as emergency shutdown and fire and gas systems are also provided.

The dynamic riser (multi-bundle type comprising hydraulic hoses, chemical injection hoses and instrument/signal cables) provides the link between the buoy and the subsea control system, distributes hydraulic and electric power and control to various valves and carries signals from monitoring devices.

Radio communication with the platform is via line- of-sight microwave link and takes place at regular intervals, controlled by the buoy. Typical information transmitted is the well status (temperature, pressure, valve positions and hydrocarbon leakage, etc.) and on-board buoy status (fuel level and battery voltage level, etc.).

A cost study carried out comparing this system with conventional systems for a single satellite well 15km from host showed substantial potential cost savings. The conceptual design of the system has been completed and the system was installed in summer 1995.

Conclusion

This article provides a short, and by no means exhaustive, review of subsea control systems in use today. The evolution of the system from a simple direct hydraulic control system to more advanced electrohydraulic systems has taken a long time. The reason is the conservatism of the industry caused, to some extent, by the necessity to meet stringent government regulations. It is a major challenge toand receive acceptance of a new concept in the offshore oil and gas industry. To even be considered, the system needs to be ‘proven’ in other applications or, alternatively, it needs to offer a significant cost-saving potential. The SPARCS and the Integrated Control Buoy systems presented in this article bring new elements and technologies into well- established fields and offer potential large savings; however, it is likely to be some time before these systems will be at a technological level where they can be fully adapted by offshore oil and gas developers.

1    2    3    4   5