The Role of Autonomous Subsea Control Systems in Subsea Production
Authors: Galletti A.R. , Franceschini G. 
Published: October 2008

ABSTRACT
This paper describes what is intended by autonomous control systems (ACS's), identifies their technical merits and limitations and illustrates the applications in which they may used advantageously. Various configurations of autonomous control systems are described for diverse applications, including the control of both subsea Christmas Trees and Emergency Shutdown Valves (ESV's). It is shown that ACS's, apart from their technical advantages, can have a great economical advantage in certain applications.

INTRODUCTION
With the growing interest in the industry for placing production systems on the seabed rather than on platforms, for a multitude of reasons which will not be covered in this paper, a great effort is being made in the research and development of-subsea systems.
For subsea wells that are placed in shallow water and relatively near to existing platforms, current technology is considered adequate to satisfy the needs of reliability, simplicity and low cost. However as the water depth and the distance from the surface facilities increases, so does the complexity and cost of the equipment. One of the principal components of a subsea system that is affected by these increases is the control equipment. This is mainly due to the problem of sending electrical and hydraulic power, as well as command signals, over .a large distance in water. In traditional systems this is always accomplished with an umbilical of some sort, which has a high cost that is directly proportional to its length, as well as technical problems such as the use of subsea mated connectors.
The principal aim of autonomous control systems is therefore to eliminate the need for the umbilical by making the system self-sufficient on the seabed.

GENERAL CONSIDERATIONS
Autonomous control systems can vary greatly, however they are characterized by four principal subsystems necessary to make the whole unit autonomous by eliminating the umbilical traditionally used. A description of these systems is given below:

Signal transmission.
To date hydroacoustic transmission has been used to satisfy the criteria of eliminating the umbilical. Radio links to cover at least part of the distance between the control stations and the subsea units have also been studied.

Hydraulic self sufficiency
Traditional systems all utilize an umbilical to provide the hydraulic power they need. In the absence of the umbilical, hydraulic self sufficiency must be attained and therefore a closed hydraulic circuit must be used. This requires a hydraulic system that has zero leakage to the external environment, minimal internal leakage and can maintain a high level of cleanliness during its life to assure the perfect functioning of the control valves. In addition a safety valve that automatically dumps the system in the event of loss of power must be included.

Power autonomy
An autonomous power source must be provided for the subsea unit. In this area extensive research has been performed on the most diverse concepts. However to date the most reliable form of energy supply is considered a retrievable battery pack.