With the development of Subsea Oil and Gas fields at ever increasing water depths, greater demands are placed upon the control systems used with workover/completion riser systems This paper reviews the various types of control systems previously used and considers developments to meet the operational requirements for use in water depths up to 3000 metres from both moored and dynamically positioned vessels
Workover control systems have become increasingly complex over the years, driven both by the need to meet ever-stricter regulatory requirements both from a safety and environmental perspective, and the requirements for operating in ever-deeper waters Early equipment utilised simple direct hydraulic systems, operated from manual hydraulic control valves located on a small control panel supplied from the rig hydraulic system These systems have now developed to the complex workover control system we see today operating in the North Sea Continental Shelf, utilising fail safe piloted hydraulic systems and, in some cases, provided with remote electrically operated panels and multi-level sequenced shutdown systems Figure 1 shows a typical combined hydraulic power unit and control panel.
These systems, however, still do not provide the level of performance required for operating in harsh, deepwater environments from a dynamically positioned vessel This has led to the development of electro-hydraulic multiplex workover control systems to provide the level of performance required
To provide a better understanding of the limitations of existing systems the three basic types of hydraulic control system are briefly reviewed below and comparisons in response time given for a typical hydraulically actuated subsea gate valve.
Direct Hydraulic Systems have traditionally been used in very shallow water depths, typically less than 200 metres or where the speed of response is not critical Direct systems have the advantage of being simple in operation but require larger hydraulic hoses within the umbilical, up to half-inch diameter being required to maintain a reasonable response time for high volume functions such as hydraulic connectors Direct hydraulic control is typically used for operation of tree valves, wellhead and flowline connectors Since hydraulic power is provided directly from the surface this control method cannot be used for subsea failsafe functions requiring hydraulic energy to operate For example release of the EDP hydraulic connector
Piloted Hydraulic Systems allow an improvement to response time since the hydraulic supply for the function is provided from locally installed subsea accumulators Smaller diameter hoses can be used within the umbilical since only a small volume of fluid is required to operate the piston within the pilot control valve Piloted systems are capable of giving adequate response times at water depths in excess of 1000 metres Piloted control also allows failsafe operation for shear rams, cutter valves and EDP connectors using the locally mounted subsea accumulators to provide the hydraulic power required.