THE INFLUENCE OF OPERATIONAL COSTS ON SUBSEA DEVELOPMENTS AND POSSIBLE COST REDUCTIONS FROM IMPROVED SUBSEA PRODUCTION CONTROL SYSTEMS.

Lerkendal 93, Trondheim
Anders Hägglin Chalmers University Of Technology
Sigbjørn Sangesland Norwegian Institute of Technology

ABSTRACT
The life-cycle cost for subsea installations in the North Sea is high. It is therefore necessary to develop more cost-effective systems. The main potential for cost reduction is however, an optimization of installation and intervention schedules offshore.

The design of former subsea installations shows a waste of resources that indicate that engineers have concentrated more on solving individual problems, than at looking at overall function and economy. A more analytical approach from the industry would make it possible to identify errors and insufficiencies at an early stage. For example a reduced number of remotely controlled functions on a subsea installation is probably cost-effective.

The capabilities of modern computerised control systems are finally working their way into modern offshore control rooms. The possibilities of doing the same subsea are evident but not jet fully accepted by the industry. In order to utilize the new control systems it is an advantege if all principle equipment is electric.
This paper gives an analysis of how the use of improved control systems on subsea systems has an influence on the effectiveness of subsea operations.

INTRODUCTION
The life-cycle cost for subsea installations in the North Sea is high. It is therefore necessary to develop more cost-effective systems. The main potential for cost reduction is however, an optimization of installation and intervention schedules offshore. This paper gives an analysis of how the use of improved control systems on subsea systems has an influence on the effectiveness of subsea operations.

The design of former subsea installations shows a waste of resources that indicate that engineers have concentrated more on solving individual problems, than at looking at overall function and economy. A more analytical approach from the industry would make it possible to identify errors and insufficiencies at an early stage. For example a reduced number of remotely controlled functions on a subsea installation is probably cost-effective and it can be shown that if a correction costs $ 1 on the drawing board it will cost $ 1000 to perform it after installation [1].

The capabilities of modern computorised control systems are finally working their way into modern offshore control rooms. The possibilities of doing the same subsea are evident but not jet fully accepted by the industry. In order to utilize the new control systems it is an advantage if all principle equipment is electric. A subsea control system is used to operate different kinds of valves on subsea installations. It also permit retrieval of data by monitoring temperatures, pressures, leakage of hydraulic fluid and valve positions. The control system comprises a lot of complex components and is considered a 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 attitude to control system technology varies from different oil companies and different areas of the world. In the North Sea, multiplexed electro-hydraulic control system may be the norm but world wide that system only accounts for 15% of the installed systems.

The control system of a subsea production system represents a substantial part of the cost of the system. Cost reduction in this area is difficult with the technology of today and requires a break through in technology.

During the nineties subsea production will be a standard procedure in hydrocarbon production offshore. The growing level of standardization shows an increased maturity in the industry. Today the oil companies have agreed to transfer more responsability to their contractors who then can work without the normal detailed specifications. This makes it possible for the contractor to use in-door knowledge and technology, which in its turn leads to more standardized systems. There are about 300 to 400 different subsea christmas trees installed when there is a need for only approximatly 10 [2].

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, subsea technology will be used more commonly around the world and will increase its part of production of offshore hydrocarbons. As the development of multiphase transportation and seabed separation continues and the distances between completions and platform increases the subsea production systems will expand. The horizontal drilling technology will also contribute to the number of installed subsea completions.

COST AND OPERATION ASPECTS

Comments on Areas for Cost Reductions
The area for most substantial cost savings is offshore operations. Further, the choice of intervention philosophy has the most overall influence on the subsea system. Finally, every subsea system is financially depending on efficient maintenance and repair procedures to maintain the production.

The best way of reducing maintenance cost is to avoid them or at least keep them as low as possible. A good, reliable and simple design based on standard equipment and procedures is a way to go. Further, it is vital to incorporate in the design the building of simple and effective methods of identifying damages and misfunctions and to make it easy to take measures against them.

The effectivness of the intervention system and its compatibility with the subsea installaiton is of course important. So is also the choice of necessary surface support. A control system can be made simpler by an increased use of ROVs in operation of valves. There is a general demand for certain valves to be remotely operated eg safety valves. There are valves used only during installation and intervention which can be operated by ROVs which will decrease the total complexity of the installation.

Substantial cuts in costs can be made through the use of smaller and specialized support vessels. The use of a drill rig is an "overkill" in several operations on subsea installations. Good design and proper planning should provide the necessary conditions for smaller and cheaper surface support vessels.

Preliminary Cost Distribution for a Subsea System
The cost of an average Norwegian subsea system can be distributed like this [3]:
Flowlines / Umbilicals / Cables 30%
Main Structure 8%
Production Equipment 15%
Control System 7%
Drilling and Completion 40%

The high cost for drilling and completion depends on vessel day rates and is not an easy task for cost reductions. Control system and production equipment contain few variables that can be effected. The main structure and flowlines / umbilicals / cables, give room for most cost reductions. 70% of main structure cost is the cost for installation. If more than 4% was spent on design, the cost could be reduced substantially [27]. Flowlines are harder to analyse due to the high cost for flexible lines which make up for the cost reduction in installation.

Life Cycle Cost
The life cycle cost of a subsea installation can be distributed as follows [4]:
Hardware 20%
Installation 15%
Operation 20%
Workover 10%
IMR, Inspection, Maintenance, Repair 35%

Offshore operations count for more than 50 % of the costs concerning the development and opeation of a subsea field. It is necessary to take operational aspects into account, not only when planning and performing the operations but most important when choosing intervention and design philosophy and when designing installation equipment. There is a lot to win in designing equipment easy to install and maintain. Whether a subsea installation should have diver or diverless intervention, be installed with or without guidelines, have vertical or horizontal access or whether it should have hydraulic or multiplex control system, has a strong influence on both installation and intervention procedures, eventhough it seams distant from a drawing table or a CAD-terminal.

At the detailed design of the installation equipment it is important to know the capabilities and limitations of the offshore vessel to be used. It is therefore necessary to specify the demands when the operator of offshore vessels is chosen. The detail design should take into account the very strong impetus from being able to perform the entire installation operation from only one vessel. The performance of any offshore operation depends highly on the experience of the people taking part. If there are repetitive steps in the operation that have been carried out before, they will probably be carried out successfully again. If it is a "first" there will be problems with all the subcontractors aboard, security routines, clearly defined responsabilities and communication routes. In a standardized operation things will run smoothly as everybody has done it before.

CONCEPT OF ELECTRIC CONTROL
In the early days of the offshore industry equipment control was 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 eliminated. However, the offshore industry did not distinguish the difference and a whole decade was lost. Now, however there seems 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 the many sections of an operation through the use of microprocessors-based controllers, each of which are really small specialized computors. Any failure of one does not adversely affect the operation sections controlled by others. If using two in a redundant arrangenment where each section only requires 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.

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 use of electric actuators and controls 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 AND INCREASED OPERATION PERFORMANCE OF IMPROVED CONTROL SYSTEMS

Reduced costs
Less expensive components. The hydraulic lines represent a substantial part of the cost of a subsea development compared to a power cable. The lines are also vulnerable during installation and the termination is a usual source of problems. Further components might prove to be less expensive as the electric industry is used to standardisation and has 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 opposed to scheduled maintenance, making it possible to postpone intervention to a convenient opportunity.

Increased production performance
The production performance of the subsea system can be improved through qualitatively 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 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 with for instance refineries. The room for improvement 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 responsetime 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 responsetime. The response 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 computer based control system will efficiently monitor and control the production stream automatically, given the necessary input.

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 probably 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 will decrease the need for chemical injection concerning formation of hydrates. [5]

CONCLUSIONS
Cost reductions when using an all-electric control system are evident, but are they substantial? Possibilities 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.

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. 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 disintegrate. Not necessaryly through convincing people that if theyput their lives 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.

REFERENCES
1 Inderberg, O., “The Standardization and Simplification of Subsea Development and Equipment-Cost Incentivesì, proceedings from UTC 90, Bergen.

2 Sangesland, S., “Subsea Production Technology 1989ì, Petroleum Technology at Norwegian Institute of Technology, Trondheim.

3 Suul, G., “Potential Cost-Reduction of Subsea Statelitesì, proceedings from UTC 90, Bergen.

4 ABB Oil and Gas, Report published by “Swedish National Industrial Board “ (SIND).

5 Ahlen, C. H., “Electric Heat Tracing of Submarine Pipelinesì, proceedings from International Offshore and Polar Engineering, Edinbourgh, 1991.