Subsea Bop Control Systems
1: Essential Knowledge
As the indirect hydraulic control system is the most common system in use on semi-
submersible rigs, the examination focuses exclusively on this system. Therefore, it is necessary to be conversant with this type of control. In particular, the examination concentrates on the following:
- Position of equipment in the control system. You should be familiar with the equipment and be able to located specific items by name. You should also be able to determine where individual items are located such as SPM valves, shuttle valves, pod selector valves etc.
- Identifying which components are duplicated in the control system and which components have no duplication. e.g. There are two SPM valves for each function (one in each pod) but only one shuttle valve.
- The sequence of events required to operate a particular function on the BOP.
- Knowledge of the pressure of fluids in particular systems. Rig air pressure, manifold pressure, accumulator pressure, annular closing pressure, pilot fluid pressure and power fluid pressure should all be familiar to you.
- Remember that pilot signals go to both pods every time whereas power fluid goes only to the active pod.
- Sizing of accumulators should be understood. You should understand the reasons for locating the accumulators at the subsea stack. Understand the difference between float and bladder type accumulators.
- Methods used for trouble shooting problems such as leaks and malfunctions.
Every component in a blowout preventer assembly is operated hydraulically by moving a piston up and down or back and forth. Thus the function of a BOP control system is to direct hydraulic fluid to the appropriate side of the operating piston and to provide the means for fluid on the other side of the piston to be expelled.
On land, jack-up or platform drilling operations the control of the BOP is easily achieved in a conventional manner by coupling each BOP function directly to a source of hydraulic power situated at a safe location away from the wellhead. Operation of a particular BOP function is then accomplished by directing hydraulic power from the control unit back and forth along two large bore lines to the appropriate operating piston.
This system uses the minimum number of controlling valves to direct the hydraulic fluid to the required function. It also enables the returning fluid to be returned to the control unit for further use.
For subsea drilling operations, it is necessary to control larger, more complex BOP assemblies which are remotely located on the seabed. In this instance, direct control cannot be applied since the resulting control lines connecting the BOPs to the surface would be prohibitively large to handle. Reaction times would also be unacceptable due to the longer distances to the BOP functions and the consequent pressure drop.
In order to overcome these problems indirect operating systems have been developed. There are two types — hydraulic and multiplex electro-hydraulic of which the indirect hydraulic system is by far the most common.
INDIRECT HYDRAULIC SYSTEM
This reduces the size of the control umbilical by splitting the hydraulic control functions into two:
- Transmitting hydraulic power to the BOP down a large diameter line.
- Transmitting hydraulic signals down smaller lines to pilot valves which in turn direct the operating power fluid to the appropriate BOP function.
The pilot valves are located in control pods on the BOP stack. In order to provide a complete back-up of the subsea equipment there are two control pods — usually referred to as the blue and the yellow pods.
No attempt is made to recover the hydraulic power fluid once it has been used to operate a function since this would increase the number of lines required in the umbilical. Instead the fluid is vented subsea from the control pod.
As greater water depths were encountered the problems of umbilical handling and reaction times became significant. In order to overcome them the hydraulic lines controlling the pilot valves were replaced by separate electrical cables which operate solenoid valves. These valves then send a hydraulic signal to the relevant pilot valve which in turn is actuated and directs power fluid to its associated BOP function.
The time division multiplexing system provides simultaneous execution of commands and results in a relatively compact electrical umbilical. This typically consists of four power conductors, five conductors for signal transmission and additional back-up and instrumentation lines. With the armoured sheath the umbilical has a resulting diameter of some 1.5 inches with a weight of about 3 lb/ft in air.