Many subsea developments have made use of E/H multiplexed control systems. This paper examines the most significant of these applications highlighting their main features. The evolvement of E/H control systems is followed by examining the lessons learnt on these projects and by assessing the main critical items.
Subsea Control Systems are considered by many people the most critical part of any subsea installation. The complexity of functions which must be performed by subsea control systems is very often extremely high. In large systems for the control of templates, the number of components which make up a control system exceeds, by far, all other subsea components put together. Subsea Control Systems differ substantially from control systems for land or for platforms since they are designed for life in excess of 10-15 years without any maintenance. Nevertheless, in case of failure, they must be capable of being replaced independently from the rest of the subsea installation. This creates the need for mechanical, hydraulic and electrical connections which must be capable of being made and broken underwater, very often by ROVs or running tools, without any loss of capability.
Since the first installations in the early seventies, subsea control systems have evolved considerably. Multiplexed electro-hydraulics (E/H) systems have become more and more acceptable and in the North Sea, this trend has now become the norm. This has been achieved by a parallel development on the technical and on the economical side. Subsea control systems have become more reliable, thus creating confidence in the operators. Reliability has been achieved by simplifying the subsea part of the system, by re-designing parts which had caused problems, by developing new components, by acquiring experience in the testing and operation of the system. At the same time the "total" cost has come down considerably: The concept of total cost is important when comparing different types of control systems, e.g. direct hydraulics, piloted hydraulics, multiplexed E/H, subsea pumps etc. It must include all the costs which will be incurred by the oil company: development, cost of the hardware testing, installations, cost of umbilicals, maintenance. In addition, the uptime of the system, information which allows optimisation of reserves, life of the system etc should also be considered. When all these factors are considered, E/H multiplexed control systems are the most effective way of controlling subsea completions.
This paper looks at various types of control systems, at some of the most important projects and at their main features, and at the most critical components. It tries to describe how control systems have evolved by applying the lessons learnt on actual installations and show where the industry is at the moment and how it could progress from here.
The paper deals only with "wet" systems and does not examine other applications of control systems, such as BOP, direct control of subsea trees etc. These systems are not substantially different from any other "above water" control systems and therefore will not be discussed here.
