This paper discusses the principles involved in calculating subsea electrohydraulic system performance. Particular emphasis is placed on the control umbilical, which is shown to be the dominant factor governing response A simple yardstick method for estimating umbilical performance is described, this method is compared with a computer model and also with experimental results Finally, the computer simulation of complete electrohydraulic systems is described and its use in the optimization of component selection is demonstrated
In recent years the trend in configuration for subsea wellhead production control systems has been for remote electrohydraulic control of Christmas tree valve actuators and the downhole safety valve (DHSV). The electrohydraulic directional control valves are located subsea in a control pod, local to the wellhead, as shown in Fig. 1. An umbilical carries the hydraulic and electrical links between the platform and wellhead.
The GEC Engineering Research Centre has recently undertaken the task of modelling several such systems in conjunction with the Offshore Projects Group of GEC Avionics. In this contribution the way in which both simple approximations and computer simulations can be used to assess system behaviour is discussed, comparisons with experimental data are made, and some case studies are presented.
For modelling purposes, a subsea electrohydraulic control system can be broken down into three component blocks
the topsides hydraulic power unit (HPU);
the subsea directional control valves, Christmas tree valve actuators, and DHSV.
Recently there has been a requirement for increasing distances between the platform and wellhead and umbilical lengths of 20km or more are now in prospect As umbilicals grow longer, their behaviour becomes the dominant factor in the performance of the control system and the ability to understand this behaviour becomes increasingly important.
For the purposes of this study the term ‘long umbilical’ is taken to refer to an umbilical over 5 km in length. The umbilical hydraulic supply hose has two dominant hydraulic properties.
The inherent compliance of the supply hose means that effectively it acts as a hydraulic accumulator or capacitor, providing storage of pressurized fluid
The extremely long length and relatively small diameter of the supply hose gives it a high resistance to flow
The stored energy in the umbilical hydraulic hose can be used to meet the demands of subsea valve actuator operations, reducing the number of subsea accumulators necessary. The difficulty in making use of this energy is that it is distributed along the length of the umbilical. Because of the umbilical's resistive properties, there is a time delay before it can be made available to satisfy a flow requirement such as that created by the opening of a tree valve. In addition, there is an even greater time delay before the surface HPU, usually consisting of a pump and accumulator set, experiences the flow demand resulting from a tree valve movement and begins to deliver flow into the surface end of the umbilical to cater for this demand.