AN ALL-ELECTRIC SUBSEA CONTROL SYSTEM

S. Sangesland, A. Hägglin, O.S. Haugerud and T.L. Jernström
Division of Petroleum Engineering and Applied Geophysics
Norwegian Institute of Technology
7034 Trondheim
Lerkendal 92, Trondheim

ABSTRACT
An all-electric control system is expected to be simpler and less expensive compared to a conventional electro-hydraulic control system. It is favourable to use when developing marginal fields at great distances from a processing facility and it also provides solutions to problems associated with high pressure and high temperature wells. Further it gives a higher degree of flexibility when expanding an existing system and when introducing new equipment. Finally, the removal of the hydraulic system removes the environmental and economical problems concerning the leakage of hydraulic control fluids.

INTRODUCTION
A lot of factors govern the choise of offshore production system . The oil companies still develop concepts for platforms in deeper waters to maintain the deck structure they are used to work from. These platform concepts will be prevailing for some time. However, the offshore oil and gas industry has a continuing need for field develop-ments to be costeffective. Subsea technology offers an acceptable solution to many of the problems involved with tie-backs to existing platform.

During the past eight years, 24 subsea completions have been installed in the Norwegian sector of the North Sea. In the future subsea technology will be used more commonly around the world and will increase its part of production of offshore hydrocarbons. It is expected that 250 wells are going to be completed subsea in the Norwegian sector of the North Sea between 1992 -2000 (ref. 1). There are different control systems for subsea completions all of them in one way or another depending on hydraulic pressure. Offshore field development in the nineties will encounter problems in the areas of distant marginal fields and high pressure / high temperature wells. Control systems for subsea compleations on distant marginal fields are expensive because of high costs for control umbilical with hydraulic lines. High pressure / high temperature wells reduce the possibility of using control system depending on hydraulic pressure due to problems with degrading control fluid at high temperature and equipment failure at high pressure.

At the Norwegian Institute of Technology / IPT a resarch program has been started to study an all-electric subsea control system. This system is thought as a suitable concept for control of subsea installations and solves some of the problems of future field developments.

In principle, electrical actuation may be used on all the valves on a conventional subsea x-mas tree. The first step towards an all-electrical subsea control system has been to look at one component, the actuator of a choke valve. The choke valve is regarded as a non-critical element which makes it possible to integrate an electrically operated choke valve into a conventional electro-hydraulic control system. This may be the first step towards an all-electric subsea control system.

CONTROL SYSTEM
A subsea control system is used to operate different kinds of valves on subsea installations. It also permit retrival of data by monitoring temperatures, pressures, leakage of hydraulic fluid and valve position. A control system comprises a lot of complex components and is considered the most critical part of a subsea installation. Selection of control system demands specification of field requirements such as depth, distance from processing facility, the technology profile of the field operator etc.

The costs of the control system including surface controls and control umbilical are high. Cost reductions in these areas are difficult to see and presumably requires something of a step in technology. One way to reduce the costs and simplify the control systems of an subsea installation is to use electrical power instead of hydraulic power to operate the valves.

The first subsea completions were controlled manually by divers. Through time the control systems have become remote controlled. First through direct hydraulics and later electro-hydraulics and today through multiplex electro-hydraulics. The simplest remotely operated system for control and monitoring of subsea system is direct hydraulic control. In this system each valve actuator is controlled through its own hydraulic line. The limitations are limited monitoring capabilities and distance limitations due to long response time and umbilical costs.

Multiplex electro-hydraulic systems have short response time and the monitoring capabilities are high. Multiplex electro-hydraulic control systems are quite complex and have often been a source of failure in subsea systems. The control valves converting the electric signal to hydraulic actuation is one source of failure. These systems have been used for distances up to 50 km. However, development of marginal fields far away from the host platform (30-150 km), may not be economical due to e.g. the high cost of the control umbilical.

When developing high pressure / high temperature wells (150 C/100 MPa) some serious problems may occur if hydraulic control is used. High temperature may cause the control fluid to degrade. In order to avoid well fluid to enter the control line, there is a requirement that the pressure of the hydraulic control fluid for the DHSV (Down Hole Safety Valve) must be higher than the well pressure. The high pressure may cause problems and limitations with dynamic seals, solenoid valves etc.

The leakage of hydraulic fluids represents both an economical and an environmental problem. The cost of replacing, storing and transportation of hydraulic fluid becoms substantial during the life-time of a subsea installation. It is also obvious that the oceans should not be exposed to further strain from pollution. An all-electric control system is expected to be simpler and less expensive compared to a conventional electro-hydraulic control system. It consists of few components and the control umbilical is simpler because no hydralic lines are required. A conceptual all-electric control system and a typical electro-hydraulic control system are shown in figure 1. There is no requirement to convert electric signals to hydraulic power and the hydraulic connectors and electro-hydraulic solenoid valves can be deleted. Change-out of components under water are assumed to be easier. Electric cables are cheaper than hydraulic ones and the installation cost is assumed to be lower. Electric actuators for downhole safety valves, failsafe gate valves and choke valves have to be developed and used in an all-electric subsea control system. The development of electronic components and direct electrical systems have reached a level were an all-electric control system for subsea use is feasible. Electric motors have been used in hard environments for a long time and high voltage can be used in water without danger.

The introduction of an all-electric control system solves the problems with high pressure/high temperature wells and leakage of hydraulic fluid and it has a positive influence on the over-all cost of the development of distant marginal fields. Further, the cables for power and signal transmission in an all-electric control system can be arranged to heattrace the flowlines. Heattracing of the flowlines may eliminate the need for chemical injection lines. An all-electric control system also have a high degree of flexibility in the area of electrical subsea- and flowline booster- pumps.

ACTUATION OF SUBSEA VALVES

Conventional actuated valves
For control of the production of hydrocarbones, the subsea system have a number of valves. Today all main valves are hydraulicly operated. In a subsea production system, the following types of valves are used:
- Downhole safety valve
- Gate valve - Choke valve

In an emergency (blow out) the downhole safety valve will close and stop the flow immediately. It is a fail safe close valve. For the control of the x-mas tree there are normaly 4-8 fail safe gate valves. These valves are operated during completion, workover and production shut-in of the well.

The choke valve is used to commingle the production subsea from wells with different pressure to reduce the number of flowline and risers.

Subsea chokes may also be required to fullfill gas-lift and water injection functions. There are two main types of choke valves, one is the fixed/rotating disc single stage choke valve and the other is the plug type single stage choke valve. Due to the risk of errossion this valve must be easy to replace. New developed actuators and choke valves can be installed and replaced with a remotely operated vehicle (ROV) or a remote operated tool (ROT).

Electric actuated valves
Several companies are looking into design and development of electric actuators, both for platform and subsea applications. Prototypes of electric actuators for down hole safety valves and gate valves have been developed and tested (Camco and AVA). However, this type of valves are still at the prototype stage.

DEVELOPMENT OF AN ELECTRIC CHOKE ACTUATOR
The typical actuator used for a choke valve is a hydraulicly operated stepping actuator. To open or close the valve, two cylinders are used to generate the required force to rotate the ratched wheel on the valve stem. During the development phase of the project, several concepts and components have been evaluated for electric actuation of a choke valve. For generation of force two concepts were preferred, one based on electromagnets and one based on electric motors. Both concepts are adapted to the low current and voltage available in conventional control systems.

Electric choke actuators may be applied in subsea systems using conventional electro-hydraulic control. The electric power and signal distribution unit can be integrated into the control pod and connected to different electric actuators. Still much work is needed before these items becomes "field proven".

Electric motor for actuation of choke valves
When using electric motors for actuation the steering characteristics and the capability of working in oil must be well proven. The components of the motor must be made of steel or other high corrosive resistant materials. In new motor design, low density materials is used to minimise the weight of the motor. Most of these materials have low corrosive resistance in sea water. This must be concidered when selecting the motor.

A bruchles DC-motor powered directly from the existing control pod where the available electric energy is low, 48 V and 10 A, has been chosen for this concept. In order to obtain the required torque a gear reduction have to be used.

Electromagnets for actuation of choke valves
The use of elektromagnets for actuation of choke valves gives a very simple and reliable concept due to high wear- and corrosive resistance. An electromagnet activated with high electric power is able to generate a powerful stroke. The stroke will have an advantageous effect on operating the valve under influence of the sticking effect. Short stroke length and no rotating elements gives the electromagnets high wear resistance. The materials in an electromagnet are principally steel and copper. The physically given choice of materials and the simple construction, gives the electromagnet high corrosive resistance.
In this concept a capacity bank is used to adapt the existing power supply and capacitors are used to gain a quick transformation of electric energy from the bank to the elektromagnet. To charge the capacitors, the electric current from the control pod is transformed to 200 V.

Laboratory tests
In the prototype stage of this project, two different types of electromagnets were developed, manufactured and tested. A conventional hydraulic actuated choke valve was rebuilt and the hydraulic cylinders replaced with electromagnets. Figure 2 shows the flowdiagram of an hydraulic and electric actuator. The choke valve with electric actuator is shown in figure 3.
A set up for laboratory tests of the electromagnets was built and force- and long time- tests were carried out, see figure 4.

The stroke and force of the magnet were adapted to the stroke and force of the hydraulic cylinders. The length of the stroke is about 15 mm and the design force is 5000 N for a choke actuator. With this length of stroke the energy needed to generate a force of 5000 N with the electromagnet was about 400 Joule .
The future work will be to optimize the function of the stepping actuator and the electromagnets. With a reduction in stroke length to 5 mm, the electromagnet will be able to generate the required force with an energy of about 100 Joule. This reduction will make it possible to use smaller electromagnets compared to the tested prototypes.

SUMMARY AND CONCLUSION
It is possible to straighten the passage between platform and subsea completion and abandon the detour of hydraulics. Therefore this initiative regarding an all-electric control system for subsea production systems has been launched. So far in the project we can summon the following conclusions:
- Test performed in the laboratory have proved that simple electromagnets can be used to operate a conventional choke actuator. In principle, electric actuation may be used on all valves on a conventional subsea X-mas tree.

- Using electric actuation of subsea valves solves problems encounted when completing marginal fields at great distance from existing infrastructure and high pressure/high temperature wells.

- Environmental pollution from leaking hydraulic fluid is avoided with an all-electric control system.

REFERENCES

/1/ Offshore & Energy Nr 2 1991

Fig.1, TYPICAL ELECTRO-HYDRAULIC CONTROL SYSTEM (A) AND AN CONCEPTUAL ALL-ELECTRIC

CONTROL SYSTEM (B).

Fig.2, DRAWINGS OF CHOKE VALVE WITH Fig.3, CHOKE VALVE WITH ELECTRIC ELECTRIC ACTUATOR. ACTUATOR (PROTOTYPE).

Fig.4, TESTRIG FOR ELECTRO-MAGNETS. Fig.5, ELECTRICAL CIRKUIT FOR CHOKE VALVE.