Control of subsea wellheads has evolved from the earliest diver operated wellheads to systems involving sophisticated electro-hydraulic techniques. With the increase in technological sophistication comes an increase in cost which limits the number of subsea wells that are candidates for such control systems.
Several types of hydraulic control systems have been developed with the aim of reducing both the complexity and the cost of control systems for subsea production wellheads. Recent innovations have resulted in further simplification and the increased use of direct hydraulic control systems.
This trend toward simplification will be reviewed along with the reasons for the trend. Response times of a typical direct control system will be analyzed.
The earliest control systems employed on subsea production wellheads were in fact not control systems at all. Opening and closure of the tree valves was done by a diver. This severely limited the depth at which such trees could be installed and also provided slow response time.
As the water depths in which subsea wellheads were placed increased, the need arose for control systems that could be operated by field personnel located on the surface facility (platform, floating production vessel or shore base). Piloted hydraulic control systems were developed in response to this need. The principal part of a piloted system was a hydraulically actuated 2 position, 3 way control valve with spring return and nominal port sizes of 1/16"-1/4"diameter.
The piloted system typically incorporated a set of pilot valves located on the tree and used a hydraulic control hose bundle to provide hydraulic communication between the surface facility and the subsea tree. The tree valves were controlled by application of hydraulic pressure through one of the control hoses. This resulted in a shift of the subsea pilot valve, which caused pressured hydraulic fluid to be directed from the subsea accumulator to the appropriate tree valve actuator. This type control system was an extension of similar systems used, then and now, for subsea BOP controls.
As offset distances between the subsea wellhead and surface facilities increased, the response time of the piloted hydraulic system lengthened considerably due to the expansion characteristics of hydraulic hose. Additionally the cost of the hydraulic umbilical became significant as each tree function required one hydraulic control line. This, coupled with the desire of operators to obtain wellhead status and operating parameters, caused the development of several versions of electro-hydraulic control systems, both hardwired and multiplexed.
Each major control system vendor developed electrohydraulic control systems, some with the support of major oil companies. These systems controlled the tree valves by electrically shifting solenoid operated pilot valves which directed hydraulic fluid to the tree valve actuators. Electrical signals were sent either by means of a hardwired system or via a multiplexed communication system. These systems were typically designed for use in a 'clustered', environment, such as a subsea multi-well manifold, or for remote distances of 5,000 ft. or more offset. Similar systems continue their development cycle today, however they have not proved as cost effective and simple to operate as some operators desired for single well satellite applications.
