Subsea Electric Control System

Ocean Industry, June 1992
Sigbjørn Sangesland, Anders Hägglin


It is possible to straighten the passage between platform and subsea completion and abandon the detour of hydraulics. Therefore an initiative regarding an all-electric control system for subsea production systems has been launched at the Department of Petroleum Technolgy at the Norwegian Institute of Technology (NTH). Laboratory test have proved that for instance simple electromagnets can be used to effectivly operate a conventional valve actuator.

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.

AREAS OF INTEREST
As remote completions are developed further and further away from a processing facility the cost of the control umbilical becoms a vital economical issue. Most current field development projects in the North Sea are regarded as marginal and will be produced as tie-backs to existing processing platforms.

Further problems are encountered when HP/HT (100MPa / 150 oC) wells are developed. In order to avoid well fluid to enter the control line, there is a requirement for the pressure of the hydraulic control fluid to be higher than the well pressure. This high pressure may cause problems and limitations for dynamic seals solenoid valves etc. Degrading hydraulic fluids caused by high temperature is also a predictable difficulty. The increasing distancies between subsea completions and the processing facility will lead the use of multiphase transportation technology which will inevitably expand the subsea production system. The need for flexible and easily extenable systems together with the fact that multiphase pumps will be electricly driven points toward all-electric control systems subsea.

Finally the leakage of hydraulic fluids represents both an economical and an environmental problem. The cost for replacing lost hydraulic fluid becoms substantial during the life cycle of a subsea installation. One solution to these problems is the use of electric systems instead of hydraulics. The use of electric actuators and controlls subsea 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 many 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 concept of unmanned platforms is dependent on good monitoring and reliable actuation of components. The unmanned platform is a possible place for initial trials of electric components before going subsea.

The research program at NTH was started to study an all-electric subsea control system. An electric system is thought as a suitable concept for control of subsea installations and solves some of the problems of future field developments. Work has concentrated on the development of an electric valve actuator. A prototype has been built and tested and several other concepts on subsea electric valve actuation have been evaluated.

The present control systems for subsea completions are all of them, in one way or another, depending on hydraulic pressure. When it can be shown that the reliability of an electric actuator is equal to a hydraulic actuator, together with the further advanteges of electric actuation like reliable and simple valve monitoring, no hydraulic leakage, no high pressure jumpers etc, it will be easy to gain interest for the development of an all-electric subsea control system.

DEVELOPMENT OBJECTIVES
To reduce costs when developing marginal oilfields at great distance from a platform, an all-electrical control system is an interesting alternative. It is also possible to increase per-formance of the subsea installation and to get a more efficient and capable control system, when using direct electrical power. Further, an electric control system is more flexible concerning extended developments and introduction of new equipment.

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 and further it can be installed from a comparatively smaller and less expensive ship. There is also a potential for simpler remote connections when using electric components.
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 performance
The performance of the subsea production system can be improved through qulitativly and quantitatively better monitoring capabilities and simple change out of components. 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 deterctors. 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 with for instance refineries. The room for improvment of the control of the production stream is large and with the introduction of an electric control system the possibilities of using this technology subsea will be introduced.

Less need for shutdown. The necessity of a shut down when making intervention will decrease due to reasons associated with the control system. A more simple change out of equipment is possible and further there will be an increased possibility of a fast repair operation due to the need of a not so sophisticated ship with good availability.

More efficient and capable control system
A subsea control system may become more efficient and capable through less and simple components, shorter responstime and increased monitoring capabilities when designing it all-electric.

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 components under water are assumed to be easier. Electric cables are cheaper than hydraulic ones and the installation cost is assumed to be lower. 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 possible number of input/output points will increase substantially. An electric computor based control system will efficiently monitor and control the production stream automatically, given the necessaryinput.

Flexibility in further development
Multiphase Pumping Technology. The introduction of electricly powered multiphase technology, makes it natural to use all-electric control systems. Flowline and pipeline valves used to separate pumps from the line will proberbly be electricly actuated since hydraulic power not necessary is present.

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 may decease the need for chemical injection concerning formation of hydrates.

TECHNICAL FEASABILITY

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, electrical actuation can be used on all the valves on a conventional subsea x-mas tree. The electric actuators for downhole safety valves, failsafe gate valves and choke valves have to be developed to be used in an all-electric subsea control system. Both rotary and linear electric actuators are in use onshore and the adaption to subsea use is on its way. 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.

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

The first step in the NTH project, towards an all-electrical subsea control system has been to look at the actuator of a choke valve. The choke valve was regarded as a non-critical element making it possible to integrate an electrically operated choke valve into a conventional electro-hydraulic control system. 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. The experience gained is now used for the development of further subsea valve actuators.

Electric connectors Among the most critical components in a subsea completion system are electrical underwater mateable connectors. To minimize the life cycle cost of a subsea system it must be composed of highly reliable components with a minimum need for maintenance.

The connectors may transmit signals or power, the later being more problematic than signal connectors. The power varies between 50 VA to 5 kVA. If, however subsea multiphase pumps are used, they have a power requirement from 0.5 MVA to 5MVA.

The conventional conductive connector rely on galvanic contact. The main advanteges of conductive connectors are their compactness, wide power range up to MVA and their large bandwidth from DC to MHz. However, for the higher power ratings extra precautions are necesary to avoid seawater ingress. Those precautions tend to increase the size and complexity of the connector. The risk of malfunction from ingress of seawater is the most important weakness of conductive connectors and the main reason for designers to choose inductive couplers.

An inductive coupler is an electric transformer with separable primary and secondary coils. Each half is encapsulated to protect the electric circuit from the seawater. This makes the coupler very reliable. It is however large in size and weight and have limited power and frequency range.

Capasitive couplings are being developed. They are based on the passage of high frequency current through two metal electrodes with enclosed seawater in between. They are simple and tolerant to seawater ingress but they restricted to low power ratings. To improve the power rating with todays technology the size of the coupling surface need a dramatic increase.
Improved conductive connectors that tolerate seawater ingress can in the future replace the conventional conductive connector and also be used in high power applications. If the transmission of data is superimposed on the power conductor, a reliable connection is made. Manufacturers of conductive connectors claim that their products are mateable/breakable under water but the offshore operators have yet to accept this claim.

Fibre optics
Fibre optical communication subsea has several advanteges compared to electrical systems. The concept of fibre optics offer immunity to electromagnetic interference and high speed of transmission. Further, low loss of signal and high bandwidth are useful in some applications, while in other small cable diameter and increased reliability might be of greater importance. When using separate wires for signal transmission and power distribution or when combining these signals, there is allways a risk for electromagnetic interference. The power consumption of the proposed multiphase pumps is at a magnitude that makes it difficult to superimpose data onto the electrical conductor and the general disturbans from electricly powered acticities offshore hampers the use of conventional signal wires.

High reliability can be obtained since electrical connector failurs, frequently observed in electrical subsea connectors, is not a relevant failure mode in fibre optic systems.

However, the fibre optic concept introduces further system complexity due to the transformation of signals when receiving and transmitting information. An additional problem is the mating and breaking of connectors under water since particles and biological growth increase damping and deteriorate the fibre.

A wet mateable/breakable fibre optical connector must be designed to avoid contamination of the optical endfaces, withstand hydrostatic pressure, align fibre ends with high accuracy and be easily connected by diver or ROV. Such connectors are starting to appear, however currently without field proven capabilities.

Combined Power and Signal Transmission
The 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 and baby alarms. 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. Further development of inductive couplers may help to simplify the system. Howerver, inductive power and signal couplers are difficult to combine in a single unit due to different requirements.

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 electrical 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 components from valves and pumps to flowlines and electric equipment. The low thermal conductivity of composits compared to metals improves 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.

Finally the composites consume less energy during manufacturing compared to most metal alloys. In line with international conventions on environmental protection in the 1990s, energy consumed during production and recoverble energy from the product when it served its time, will be integrated as part of the products lifetime costs and affects.

Fault-tolerant micro processors and computors
The utilization of computors and microprocessor in the offshore industry and in the subsea environment is increasing, making the dependance on high reliability and availability of those system a vital issue. Extreamly reliable computing has been limited to military, aerospace and communication applications in which the consequence of computor failure have a significant economical impact or loss of life.

The applications of fault-tolerant systems vary from telephone switching and transaction processing to long distant satelites and aircraft control. 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.

Reliabilty techniques have become more important in the offshore industry due to several factors. The use of computors 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 computor system operation, putting demands on the system to be more robust and tolerant to inadvertant user abuse. Further the repair cost increases with the complexity and variety of system configurations since the averege service engineer is exposed to fewer repeat situations, thus increasing the time to diagose. The position of the component on an offshore platform or on the seabed, increases the repair cost even more. Finally, 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 computor 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 computor based offshore and subsea control systems.