Over the last few years the technology involved in control of subsea completions has shifted dramatically from the direct hydraulic approach in favour of the multiplexed electrohydraulic control system.
The reasons for this trend are many and varied, but by far the most significant is the ability to obtain analogue information from subsea, effectively providing a window on the subsea world. Another important facility provided by this method is that of the ability to reconfigure sequenced actions as the role of a well slot is changed.
But what has this to do with an integrated subsea and topside control system? Prior to the more general acceptance that a multiplexed electrohydraulic control system is the way to operate subsea completions, it was not possible to integrate the two
The development of platform-based process control systems is changing as rapidly today as it has done throughout the comparatively short history of offshore field development. Even today the extent to which the offshore process is automated is a long way short of comparable process plants onshore in refineries, pharmaceutical plants and the food industry.
A number of the early platforms had virtually no automated control except for some small subsystems, such as localized control of power generation At this time it was common for a substantial amount of monitoring to be done offshore and for the information to be relayed back to an onshore installation where supervisory control was carried out. Systems were later installed on platforms but still only operated in the role of gathering information for the operator and as a central point from which he could control the processes They still did not carry out any direct control without operator intervention. These systems are now used extensively on many different applications and are universally known as supervisory control and data acquisition, or SCADA, systems.
The local automated control of small subsystems has been increasing over the years but, until the comparatively recent availability of proven large distributed control systems, overall control of platform processes has still been an operator function. However, with the trend towards reducing the number of people working offshore for both safety and economic reasons, the role of computer based systems in control of all the platform processes is increasing.
Early subsea completions were controlled directly by hydraulics from the surface. This technique is a perfectly sound way of operating a small number of independent subsea completions. However, as the complexity of subsea installations increased, so the control regime became more sophisticated, moving through sequenced hydraulic control to electrohydraulic control when reliable subsea electrical connections became available.
The operator interface to the early electrohydraulic control systems was very similar in operation to early process-plant control systems, and was based on large mimic panels containing pushbuttons and indicators. This, of course, required some electronic intelligence to convert operator actions into electrical signals understood by the subsea electronics. In addition, the surface equipment then had to convert the signals received from subsea into a format comprehensible to the operator.