8. BOP CONTROL SYSTEM
8.2 HYDRAULIC CONTROL UNIT
Rule 32: The hydraulic control unit with the master control panel shall be easily accessible, on the escape way and not at the rig floor, in a non-classified (non-hazardous) area. It shall be adequately protected against dropped objects and fire.
Onshore, it shall be installed at least at 15 m from the wellhead.
If possible, the hydraulic control unit will not be located in direct view of the BOP or wellhead.
8.2.1 ACCUMULATORS VOLUME AND PRESSURE
8.2.1.1 Surface BOP
RULE 33:For surface BOP, the total accumulator volume shall be equal to the volume required for the full closure and full opening of all preventers (rams and bag) plus all remote operated valves with a residual pressure which does not drop below the nitrogen pre-charge pressure plus 200 psi, the pumping systems being off.
If shear rams are installed, the residual pressure of the accumulators after having closed the hang-off pipe rams and sheared the drill pipe string shall be at least equal to the manufacturer’s recommended pressure, the pumping systems being off.
This accumulator shall be split into at least two bottles banks. Each multi-bottle accumulator banks shall have valve for bank isolation.
8.2.1.2 Subsea BOP
Note: Some times subsea accumulators are installed on BOP stack in order to improve closing performance times. In that case these subsea accumulators are part of the primary accumulators.
Rule 34: For subsea BOP, the total primary accumulators volume shall be equal to one and haft times the volume required for the full closure and opening of all preventers (rams and bags ) plus all remote operated valves with a residual pressure which does not drop below the nitrogen pre-charge pressure plus 200 psi, the pumping systems being out of service.
In addition, the residual pressure of the total primary accumulators after having closed the hang-off pipe rams and sheared the drill pipe string shall be at least equal to the manufacturer’s recommended pressure, the pumping systems being off.
In case subsea accumulators are part of the primary system, the total primary accumulators volume shall include the volume of the subsea accumulators on the BOP stack.
In case of dedicated subsea accumulators for emergency back up system, the volume of the dedicated subsea accumulators for emergency back up system shall be equal to one and half times the volume required to close the hang-off pipe rams, to shear the drill pipe string, to close a second set of pipe rams, to activate the ram locks and disconnect the lower marine riser
package with a residual pressure which does not drop below the nitrogen pre-charge pressure plus 200 psi, the pumping systems being off.
In addition, the residual pressure of the dedicated subsea accumulators for emergency back up system after having closed the hang-off pipe rams and sheared the drill pipe string shall be at least equal to the manufacturer’s recommended pressure, the pumping systems being off.
The surface accumulator shall be split into at least four bottles banks.
Subsea accumulators (dedicated to the emergency back up system or part of the primary accumulators), if any, shall be split into at least two bottles banks.
Each subsea multi-bottle accumulator banks shall have ROV operated valves for bank isolation.
An example of accumulator volume calculation is given in appendix 3.
Rule 35: If emergency back up system (acoustic, dead man system or equivalent) is installed, it should have its own dedicated and independent pilot circuit. The dedicated subsea accumulators for emergency back up system shall be connected in such a way as fluid is not lost if the supply line(s) from the rig is cut.
In case of emergency back up system, dedicated subsea accumulators installed on the BOP stack are mandatory. These dedicated subsea accumulators providing energy to the pilot and the power circuit of the emergency back up system are not used during primary BOP functions.
8.2.2 CLOSING TIME PERFORMANCES
Rule 36: For surface installations, the BOP control system shall be capable of closing each ram BOP within 30 seconds. Closing time shall not exceed 30 seconds for annular BOPs smaller than 18 ¾ inches nominal bore and 45 seconds for annular preventers of 18 ¾ inches nominal bore and larger. Response time for choke and kill valves (either open or close) shall not exceed the minimum observed ram BOP close response time.
For subsea installations, the BOP control system shall be capable of closing each ram BOP in 45 seconds or less. Closing time shall not exceed 60 seconds for annular BOPs. Response time for choke and kill valves (either open or close) shall not exceed the minimum observed ram BOP close response time. Time to unlatch the lower marine riser package shall not exceed 45 seconds.
The required pressure for shearing the DP shall be obtained in less than two minutes.
8.2.3 PUMPS SYSTEM
Rule 37: For the pump system of the BOP control system the minimum requirement is two pump systems, driven by independent power sources:
either 1 electrical pump system plus one air pump system
or 2 electrical pump systems completely independent regarding the electric network (cables, distribution boxes)
In addition, each pumping system shall be designed to fulfil, independently of the other source, the most stringent case:
Either charging of the accumulator from the pre-charge pressure to the rating pressure of the accumulator in less than 15 minutes,
Or closure of the annular preventer on drill pipe with correct sealing pressure and the opening of the hydraulic choke valve in less than 2 minutes.
In case of two electrical pump system one electrical pump system shall be powered by the emergency generator.
Pumping shall start automatically when pressure in the accumulator drops below 90% of the maximum operating pressure.