The subsea control system operate the subsea BOP stack. Every component of BOP stack operated hydraulically by moving piston up and down or back and forth. Thus the function of BOP control system is to direct the hydraulic fluid to the appropriate side of operating piston and to provide the means for the fluids on the other side to be expelled.
On land, jack-up or platform drilling rig the operations of control of the BOP is achieved in a conventional manner by coupling each BOP function to a source of hydraulic power situated at a safe location away from the wellhead. So each BOP function is performed or accomplished by directing the 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 return to control unit for further use.
In subsea the operation becomes much difficult, now it is necessary to control the BOP functions which are located in shallow or deep level seabed. In this conventional method of control lines can’t be applied
since the resulting control lines connecting the BOP to surface would be prohibitively large to handle.
Reaction time would also be unacceptable and the consequent pressure drop.
That is why, for subsea BOP control systems, indirect operating systems have been developed. There are two types,
1. Indirect Hydraulic System (most common) 2. Multiplex Hydraulic System
Indirect Hydraulic System:
In this system the size of control umbilical is reduced by splitting the hydraulic control functions into two,
Transmitting hydraulic power to the BOP down a large diameter line.
Transmitting hydraulic signals down the 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 yellow pods.
No attempt will be made to recover the hydraulic power fluid once it has separate electric cables which operate solenoid valves. These valves then send a hydraulic signal to the relevant pilot valve which in turns 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 consider four power conductors, five conductors for signal transmission and additional backup and instrumentation lines. With the protective coating umbilical has a resulting diameter of 1.5 inches with a weight of 3 lb/ft in air.
Acoustic System:
In addition to either of the primary control methods mentioned above, the subsea BOP stack can also be equipped with an acoustic emergency back-up system. In principle this is similar to the other two systems, but with the hydraulic or electric commands to the pilot valves being replaced with acoustic signals.
Being a purely back-up system the number of commands is limited to those which might be required in an absolute emergency.
Figure 8.4
Figure shows the general arrangement. Fluid used to operate the functions on the BOP stack is delivered from the hydraulic power unit on command from the central hydraulic control manifold. This contains the valves in the subsea control pods and which are operated either manually or by solenoid actuated air operators.
In this way the manifold can be controlled remotely via actuators from the master electric panel (usually located on a rig floor) or from an electric mini-panel (located in a safe area). The system may include several remote mini-panels if desired. An electric power pack with battery back-up provides an independent supply to the panels via the central control manifold.
The pilot fluid is sent to the subsea control pods through individual, small diameter hoses bundled around the larger diameter hose which delivers the power fluid. In order to provide complete redundancy for the subsea portion of the control system there are two independent hydraulic hose bundles and two independent control pods.
The hydraulic hose bundles (or umbilical) are stored on two hose reels, each of which is equipped with a special manual control manifold so that certain stack functions can be operated whilst the stack is being run. Hydraulic jumper hose bundles connect the central hydraulic control manifold to the two hose reels. Each umbilical is run over a special sheave and terminates in its control pod.
For repair purposes each pod along with its umbilical can be retrieved and run independently of the BOP stack. In order to do this, the pod and umbilical is run on a wireline which is usually motion compensated. In some designs of control system, the umbilical is run attached to the riser in order to give it more support and reduce fatigue at hose connections. The pod is still attached to a wireline for retrieval purposes. The design has the advantage of not having to handle the umbilical whenever the pod is pulled but has the disadvantage of requiring more subsea remote hydraulic connections.
Guidance of the pod is provided by the guidance frame and guidance wires as shown.
Figure 8.5 Subsea Stack and Choke Manifold Arrangement
Choke Line Friction Losses (CLFL)
If shut-in casing pressure (SICP) is held constant until kill rate is achieved, BHP will be increased by an amount equal to CLFL. To accomplish constant BHP, a method must be used while bringing the mud pump to kill rate.If CLFL is not accounted for, casing pressure varies from SICP at pump start up to SICP + CLFL with the pump at kill rate. This results in BHP increasing by an amount equal to CLFL.
Another method in case Driller don’t have CLFL reading is keeping the Kill Line gauge constant while bringing the pump up to speed eliminates the effect of CLFL. No pre calculated CLFL information is required.
There are four recognized methods of recording choke line friction losses at slow circulating rates of 1 - 5 bbl/min.
First Method:
Record the pressure required to circulate the well through the marine riser with the bop open. Record the pressure required to circulate through a full open choke.
CLFL = First pressure reading – Second pressure reading 700 - 500 = 200 psi
Second Method: