Multiplex Subsea control systems are often compared purely on a price basis against Direct Hydraulic Systems and Sequence Hydraulic Control Systems, usually by groups at the study phase of a project Inexperienced In the Construction, Commissioning and Operation of Subsea Systems.
The actual cost difference on larger Subsea Systems, between all but very simple systems In minimal. The theoretical cost savings are transferred to other areas such as more complex and numerous umbilicals, hardware requirements such as J-tubes, weight and installation costs.
This is a very poor trade off against system operability, and the maintainable data retrieval available from a multiplex control system so essential to good reservoir management.
This paper cites experience of a simple direct hydraulic satellite well control system, and goes on to compare the design, Commissioning and Operability of three marginal Subsea Developments using multiplexed control systems: Texaco Highlander, Sun oil Balmoral, and Amerada Hess Ivanhoe/Rob Roy.
The authors, having been directly involved at all stages of Subsea Design of these fields, have been able to bring together the facts to dispel concerns on the commitment to a valuable tool so essential for the future of Subsea Development.
While many papers are presented on theoretical applications of subsea control systems and components most people concerned with the actual design, installation and commissioning of the system are usually unable to allocate the tune for the preparation of papers and their subsequent presentation.
During the past few years the authors have spent a great deal of tune offshore installing, hooking up and commissioning a number of very successful developments.
Following the completion of the Ivanhoe/Rob Roy development during the last quarter of 1989 we specifically allocated time to talk about our own experiences after previous involvement In four major subsea control systems to present the case for MULTIPLEX CONTROL SYSTEMS - THE HEART TO THE OPERABILITY OF SUBSEA DEVELOPMENTS
Direct hydraulic systems have a number of attributes, they are simple to use, understand, maintain and have no electrics underwater. This last attribute should therefore make for a very reliable system.
The fundamental principle that a hydraulic force exerted In one direction exerts an equal force In all other directions works very well on applications where the force can be contained radially, for example, with a low expansion hose, used over short distances, and where the actuated swept volume of a cylinder is small.
On a simple tree system with only a moderate distance between the control panel and the actuated valve, direct hydraulic valve actuation can be satisfactorily executed up to a distance of 4 km (13,000f t). There are however several limitations associated with direct hydraulics.
The limitations on direct hydraulic systems are:
Extended ESD response tunes (acceptable or not ?)
Hydraulic fluid viscosity
No well data IS available
On/Off control does not suit remote choke actuation