Electric Control of Subsea Production Systems - A Feasible and Cost-effective Way to the Future

Subsea 92, London
Anders Hägglin Chalmers University of Technology, Göteborg
Sigbjørn Sangesland Norwegian Institute of Technology, Trondheim

ABSTRACT
A subsea control system comprises a lot of complex components and is considered a critical part of a subsea installation. An all-electric control system is expected to be simpler and less expensive compared to a conventional subsea control system.

This paper gives a review of potential areas of interest for an all- electric subsea control system. Electric control 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 eliminates the environmental and economical problems concerning the leakage of hydraulic fluids.

The paper also discusses development objectives such as reduced costs, increased production performance and a more efficient and capable control system.

Finally the technical feasibility of certain key elements are discussed such as electric valve actuators, electric connectors, combined power and signal transmission and fibre optics.

INTRODUCTION
An all-electric subsea 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 eliminates the environmental and economical problems concerning the leakage of hydraulic control fluids.

The attitude to control system technology varies from different oil companies and areas of the world. In the North Sea, multiplexed electro-hydraulic control system may be the norm but world wide such systems only account for 15% of the installations.

The use of electric actuators and controlls under water is not a new concept. Electric thrusters on ROVs have been used for years and at far greater depths than current subsea completions. Onshore the refinary industry is using electricly actuated valves in some applications. Linear electric actuators are used e.g. on the space shuttle providing substantial force and are also developed for subsea use on multiphase pumps.

The development of electronic components and electric systems have reached a level were an 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. Cost reductions when using an all-electric control system are evident, but are they substantial? Possibillities of an increased performance of a subsea production system and a more capable control system when relying on electric power are obvious but are the improvements considerable? Developments in coming years will answer these questions. The forefront of technology on key elements such as data transmission and connectors have come a long way and will soon not be considered as problem areas.

CONCEPT OF ELECTRIC CONTROL
In the early days of the offshore industry equipment control were largely pneumatic with some hydraulic. When the electronic central-computer control systems began to emerge in the 1960´s the offshore industry was reluctant to their use because of the dramatic effects from a central computer failure.
With the advent of the distributed control systems (DCS) the risk of total disfunctioning of an entire control system from a central failure was eliminnated. However, the offshore industry did not distinguish the difference and a hole decade was lost. Now, however there seem to be a rush towards the DCS technology on new or refurbished platforms and the remaining field to conquer is the subsea installations. A DCS provides for localized control of many sections of an operation through the use of microprocessor-based controllers, each of which are really small specialized computers. Any failure of one does not adversely affect the operation sections controlled by others. If using two in a redundant arrangement where each sections only require one for control, no failure of a processor can harm any part of the operation. These processors cost so much less than a main frame computer that the concept of redundancy has become a practical reality.

AREAS OF INTEREST

Unmanned platforms
The North Sea will see small reservoirs developed as tie-backs using subsea completions and unmanned platforms. The concept of unmanned platforms have recieved increased attention from North Sea operators lately. This is due to the potential cost reduction from demanning, but perhaps more important is the safety aspect and the high rate of fatal accidents. However, without the recent advances in technology regarding instrumentation and communication equipment, the concept of unmanned platforms would not be considered reliable enough for most applications.

Electric control systems gives more efficient monitoring and more reliable control of active components. The onshore refinery industry is already using electricly actuated and controlled valves in some applications. New digital systems have increased capabilities for remote control, data aquisition and fault detection. Operated together in an in-field network using line-of-sight microwave transmission, electronic devices offers additional benefits in efficiency and safety.

Distant marginal fields
Most current findings in the North Sea are regarded as marginal and some of them distant. Only 10% of the approximately 450 undeveloped hydrocarbon findings are within 5 km from the nearest platform and the mean distance is 15 km. The economics of the more distant fields, representing 3.5 billions of barrels, may be improved with effective solutions from new technologies such as horizontal wells and multiphase production, combined with reusable installations giving a necessary standardization.

Multiplex electro-hydraulic control systems have been used for distances up to 50 km. However, 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 electric power instead of hydraulic power to operate the valves and electronic devices to monitor the system.

High pressure/high temperature wells
When developing HP/HT wells (70 MPa / 150 oC ) some serious problems may occur if hydraulic control is used. High temperature may cause the control fluid to degrade causing failure in valve actuation. High pressure can lead to well fluid entering the control line and therefore 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. These are two reasons for the high priority given to the development of an electricly actuated DHSV by certain oil companies. The high pressure may also cause problems and limitations with dynamic seals, solenoid valves etc. HP discoveries are not common but e.g. the Central Graben area is an interesting example. Due to the small market no real incentive exists on part of the manufacturers to develop new technology leaving the Operators to pay the bill. This means that North Sea HP wells must allow high flowrates due to high costs, harsh safety requirements and proberbly limited completion life.

Since 1984 23 HP/HT wells have been drilled in the Norwegian sector. A futher 23 wells, deeper than 4000 m, are also defined as HP/HT regardless of actual pressure or temperature. During this period 213 wells were drilled in the Norwegian sector.

Extension of installations
In the early years of the next century the substantial part of all UKCS findings will be accessible to tie-back production through multiphase transportation. The corresponding distances in the Norwegian sector are somewhat longer, but eventually those distances will be considered mangeable. This future increase in flow distances using multiphase technology combined with an extended use of subsea completions and down hole pumps will call for flexible installations, easy to develop further.

Down Hole Pumps
Down hole pumps for maintaining and increasing production from offshore wells are used on UKCS but this far not in the Norwegian sector. They have a short life, typical MTBF is one year, but the technology is improving. Typical power requirements range from 100 kW to 750 kW. The number of pumps required will increase as the producing wells gets older.

Multiphase Pumping Technology
The multiphase devices demands high electric power and further installation of valves to make it possible to seal of the components. These valves will be electricly actuated since no hydraulic power necessary is close by. Projects such as Poseidon, VASPS and KBS discusses electric power demands for pumps between 800 kW and 2 MW. The future number of installations is hard to predict due to different conceptual possibilities to produce marginal fields.

Heat tracing. The cables for power and signal transmission in an all-electric control system can be arranged around a flowline to heat trace the flowlines through induction of a magnetic field. Heat tracing of the flowlines will decrease the need for chemical injection concerning formation of hydrates. The electric power can be used to run electric equipment on a subsea installation, however not while heat tracing since the frequency of the AC current then will be 300-400 Hz. The general size of pipe is 8"-14". To maintain a temperature of 25 oC the power requirement is 50-100 W/m. A 150 - 300 mm2 cable will manage to transfer 1-2 MW over a distance of 20-40 km.

Leakage of hydraulic fluid
The cost of mineral oil based hydraulic fluids is high. To replace the fluid due to degradation from high temperature or long time use represents both an economical and an environmental problem. The cost of storing and transportion of the hydraulic fluid becoms substantial and it is also obvious that the oceans should not be exposed to further strain from pollution.

The introduction of water based control fluids has eliminated the need for a return line and provided the industry with a product with low viscisity, the last being impotant over long distancies. However the cost of re-filling the system is high when fluid, transportation and storing is considered, even if the price of the fluid is only 1/5 of the mineral oil. The environmental issue concerning disposition of the water based fluid could also be a problem in the future.

DEVELOPMENT OBJECTIVES

Reduced costs
Less expensive components. The hydraulic lines represents a substantial part of the cost of a subsea development compared to a power cable. The lines are also vulnerable during installation and the terminations is a usual source of problems. Further components might prove to be less expensive as the electric industry is used to standardisation and have a vast experience from installations in demanding environments. Reduced installation costs. The installation of an electric power cable is an operation where large operational experience exists. There is also a potential for simpler remote connections due to the generally smaller size of conductive connectors.

Reduced maintenance costs. Due to the increased monitoring capabilities there is a possibility for maintenance procedures based on monitoring as oposed to scheduled maintenance, making it possible to postpone intervention to a convinient opportunity.

Increased production performance
Better control and monitoring capabilities. A number of monitoring instruments are currently being developed for subsea use based on electronic components, for example sand and slug detectors. A platform such as Statfjord C has about 20000 instruments connected to its control system. The number of input/output points found on offshore installations are however low compared to for instance refineries. With the introduction of an electric control system the possibilities of using this technology subsea will be introduced.

More efficient and capable control system
Simpler control system. An all-electric control system 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 electric components under water is assumed to be easier. Shorter responstime. The respons on a command is immediate in an electric control system. Together with an improvement of the monitoring capabilities this will make the subsea system easier to control and the production more stable.

Increased monitoring capabilities. The increase in possible input/output points and an electric computer based control system will efficiently monitor and control the production stream automatically, given the necessary input. Further it will be possible to exactly diagnose the condition of both the control system it self and the production installation, giving room for condition based maintenance.

Flexibility in further development
The introduction of electricly powered multiphase technology and down hole pumps will be easier with an all-electric control system, which is presumed to be easily expanded. Heat tracing of the flowlines for control of the formation of hydrates will further expand the control system.

TECHNICAL FEASIBILITY

Electric valve actuators
Electrical valve actuation is used within the refinery industri, where good performance has been obtained on low and medium pressure installations. This technology has made it possible to monitor installations with increased automation and less manitenance. In principle, electric actuation can be used on all valves on a conventional subsea x-mas tree. Both rotary and linear electric actuators are in use onshore and the adaption to subsea use is on its way.

An electricly actuated down hole safety valve (DHSV) has highest priority in the oil companies at the moment. Several oil companies and valve manufacturers are looking into design and development of electric DHSVs, both for platform and subsea applications. Prototypes of electric actuators for DHSVs and gate valves are being developed and tested.

Electric connectors
Among the most critical components in a subsea completion system are electrical underwater mateable connectors. The connectors transmit signals or power, the later being most problematic. There are no proven subsea connectors for a higher voltage than 6 kV, however developments are promising.Conventional conductive connectors rely on galvanic contact. Their main advantege is their compactness, wide power range and their large bandwidth from DC to MHz. However, for the higher power ratings extra precautions are necesary to avoid seawater ingress. Improved conductive connectors that tolerate seawater ingress will in the future replace the conventional conductive connector and also be used in high power applications. Manufacturers of conductive connectors claim their products to be mateable under water but the offshore operators have yet to accept this claim.

Fibre optics
Fibre optical communication has several advanteges compared to electric systems. It offer high speed of transmission and immunity to electromagnetic interference. Further, low loss of signal and high bandwidth are useful in some applications, while in other small cable diameter might be of greater importance.

Interference from proposed multiphase pumps makes it difficult to superimpose data onto the electric conductor and the general disturbans from electricly powered activities offshore hampers the use of conventional signal wires.

An additional problem is mating of fibre connectors under water since particles and biological growth increase damping and deteriorate the fibre. Connectors are starting to appear, however currently without field proven capabilities.The fibre optic concept introduces further system complexity due to the transformation of signals when receiving and transmitting information. Further, the magnitude of data is so low that only a fraction of the transmitting capacity of the fibre will be used. If no dramatic change occurs, the use of optical fibres is an overkill if only the ammount of data is concerned.

Combined Power and Signal Transmission
Use of combined power and signal transmission is conventional technology onshore and is now accepted for subsea use. To superimpose data on power conductors is common technology in e.g. telephones. The offshore industry has been slow in taking up this concept, but today working systems becomes available from several manufacturers.

This technique improve long range communications and simplifies distribution systems. The Troll Oseberg Gas Injection project (TOGI) on the Norwegian part of the North Sea, proved the long range capability (50 km) of the system even if the simplicity and cost reduction still are to be seen. However, 50 km is not enough and perhaps the subsea suppliers should look towards the telecommunication industry, who are crossing oceans.

Tests have been performed on 150 km cables and encouraging results have been obtained, making it clear that noice is the limiting factor and careful attention must be paid to this point. If disturbancies are to severe the transmission must be made on fibre optic lincs.

Sensors
Sensors applied in subsea production systems are in principle topside equipment. However, as depth increases all service should preferably be done by ROVs. Production and system fuctions requireing monitoring at seabed level are pressure, temperature, valve position, sand detection, hydrocarbon leakage, flow and errosion/corrosion. All these sensorns are based on electronic technology.

Cables
The manufacturing and laying of electric cables on the seafloor must be regarded as conventional technology. However, there are several technical and operational criterions to consider when deciding type and size of the cable and the transmitting voltage. Infield transmitting of power will be done through AC current. DC systems are not cost-efficient for small power needs.

For subsea use power requirements are low (1 kW), medium (500kW) or high (10 MW). The low power users are the control electronics, measuring devices and the motors and valve actuators that need only a few watts. The medium power users are down hole pumps that need a few kilowatts and the high power users are the different kind of multi phase devices that may require as much as a few megawatts.

For infield use and over moderate distancies the transmitting voltage will be 6,12 or 24kV giving a smaller cable and possibilities to heat trace the flowlines through induction. For longer distancies and very high power requirements other techniques and values will apply.

Choice of materials
With the 1992 Norwegian Petroleum Directorate "Regulations Concerning Process and Auxiliary Systems" the possibility of a wide use of composite materials offshore is introduced.

Composites have low electric conductivity and with proper selection they will be idle to almost all forms of chemical and environmental corrosion. The problems regarding galvanic corrosion when using electric power and signals is evident and the use of composites can make the choice of materials in some applications easier.

In subsea production systems, composites will be used in valves, pumps, flowlines and electric equipment. The low thermal conductivity of composits compared to metals improve insulation and the need of chemical injection to prevent hydrates. Composites may also reduce the need for chemical injection of corrosive inhibitors and bicides, strongly effecting the overall complexity of a subsea control system.

Fault-tolerant micro processors and computers
Utilization of computers and microprocessor in the offshore industry and in the subsea environment is increasing, making dependance on high reliability and availability a vital issue. Extreamly reliable computing has been limited to military, aerospace and communication applications in which the consequence of computer failure have a significant economical impact or loss of life. The offshore subsea mode of application is the Long life-postponed maintenance type. This is equivalent to satelites and distant weather service stations. In the first case repair is not possible and in the second case there is a desire to postpone maintenance to a scheduled opportunity.

The use of computers in harsher environments where temperature and humidity vary and the power supply may fluctuate and the electromagnetic interference is evident, is a reality today. The typical user is a novice operator concerning computer system operation, putting demands on the system to be more robust and tolerant to inadvertant user abuse.

The position of the component on an offshore platform or on the seabed, increases the repair cost and as the number of components increase so does the overall failure rate, requireing fault-tolerant systems to keep the failure rate at an acceptable level.

Fault tolerance is becomming as fundamental to computer design as logic synthesis and future designs will be compared by their reliability and tolerance to failure. This is established technology which will be considered and applied in all microprocessor and computer based offshore and subsea control systems.

Field-bus transmission of data
The technology of digital control signal transmission on a twisted pair of wire between machines or process equipment is well developed, however without an acknowleded standard for module connection. With an increasing number of sensors, as automation is spreading, the number of wires to each of those sensors quickly escalates into large and vulnerable cables.

A field-bus manages the physical transmission to and from all components that the process or manufacturing requires on a single twisted pair of wire. An international standard commission is trying to negotiate a common field-bus system. There are a few systems available but they are not compatible. To suit different field bus systems, manufacturers of sensors and actuators are providing there products with adapter modules.The available field bus systems manage to control about 200 connected devices and wire lengths of up to 2 km. The rate of transmission is between 30 kbit/s - 1 Mbit/s depending on application.

The development of the sensors is very fast and as electronic circuits now can be manufactured in small numbers to a low cost, the prices are going down. Further, the design of the circuits can now be made incorporating sensor with all necessary transmission electronics in one miniaturized processor.

CONCLUSIONS
There is a market potential for an all-electric control system on subsea production installations. The difficult part today is not to state that money will be saved, but to say how much.

Small and distant findings will be developed and the existing infrastructure on the North Sea will be used for many years to come. The hereditary reluctance towards electric and electronic systems in the offshore industry will eventually disintigrate. Not necessaryly through convincing people that if they lay their lifes in the hands of electronics each time they fly, they might as well use the same equipment on their subsea installations. But rather through the simple fact that electric control is both proven and cost-efficient, however not yet in the subsea application.