This paper describes the problems involved in selecting a control system for a floating production system. The more commonly used control system options are defined and described at component level. Design criteria are reviewed indicating the effect of field size and layout. Functional requirements of the control system are described and related to their impact on the maintenance requirements. Conclusions are drawn about the solutions available and their impacts on production system efficiency.
Any floating production system may be classified according to a number of major system parameters such as production system size (single well, multi well) maintenance philosophy (divers, remote retrieval) and functional requirements. It is the intention of this paper to investigate the impact of these parameters and the selection and design of a suitable control system for a floating production system.
In preparing this paper the authors have addressed the problem of writing a detailed specification for a control system. Initially a number of questions would be asked about the floating production system and the specification developed.
It is proposed to describe these initial questions and the resultant trade-offs. As background to these discussions, a brief review is given of the principles behind subsea control systems. For the purposes of these discussions it is assumed that the specification being developed represents the most cost effective solution.
The problems of controlling a subsea valve can be solved in a number of ways, the choice being driven by the overall production system configurations and requirements.
The simplest solution is a direct hydraulic system (see fig. 1) where hydraulic fluid is pumped through a supply line to the valve actuator causing the actuator to move. As the length of supply line increases the energy required to move the fluid through the line becomes larger and the time taken to move the actuator increases.
The advantages of a direct system are its simplicity, reliability and hence, low maintenance costs. The disadvantage is a poor response time as the range increases.
An obvious way to overcome the disadvantages of a direct system is to keep the hydraulic supply close to the actuator and use a signal to control the supply. Three principal solutions of this sort are commonly used, discrete, sequenced and multiplexed electro-hydraulic.
A discrete hydraulic control system, as shown in fig. 2, uses a hydraulic signal to operate a pilot valve that allows hydraulic fluid from an accumulator to operate the actuator. The accumulator is trickle charged between operations at a rate greater than the average rate of discharge of the accumulator.