BOP Control Systems

Operating principle

Pumps charge accumulator bottles with fluid against a nitrogen precharge. When a function fires, a valve routes stored fluid to the open or close side of that preventer, valve or connector. Usable volume is the fluid the bottles release between maximum operating pressure and the minimum pressure the function needs. Surface systems run a closed circuit on hydraulic oil. Subsea systems usually vent spent fluid to sea and run on water-based control fluid.

Architectures

• Surface systems, with an accumulator unit, pumps and a manifold worked from the driller's panel and a remote panel
• Discrete hydraulic subsea systems, which send a pilot signal down a hose bundle to each function in the pods
• Multiplex (MUX) subsea systems, which send electrical commands down armoured cables to solenoid valves in the pods

Every subsea system carries two redundant pods, usually called blue and yellow. Each pod must work every function on its own. Stack-mounted bottles supply the autoshear, deadman and ROV circuits.

Key specifications

Standards and certification

API 16D covers design, manufacture and testing of control systems for BOPs and diverters, including accumulator sizing. Addendum 1 to the 3rd edition, from 2023, requires that losing any single accumulator or bank costs no more than 25 percent of total main accumulator capacity. API Standard 53 sets response times. It asks a surface system to close each ram in 30 seconds or less, and a subsea system to close each ram within 45 seconds and each annular within 60 seconds. In US waters, 30 CFR 250.734 requires the subsea accumulator capacity to close each required shear ram, the ram locks and one pipe ram and to disconnect the LMRP, under MASP conditions.

Accumulator bottles are pressure vessels, so expect a certificate to ASME VIII or an equivalent code and periodic inspection under class or flag rules.

Failure modes and inspection

• Bladders and pistons in accumulators, which leak and let precharge drop
• Subsea plate-mounted valves and regulators, which wear and contaminate
• Hose bundles and jumpers, which age, kink and swell
• Solenoid valves, electronic modules and penetrators on MUX pods
• Pump packing, relief valves and pressure switches on the surface unit

Inspection should include precharge checks on every bottle, a drawdown test that proves usable volume, timed function tests from each panel and pod, and insulation and continuity tests on MUX cables and connectors.

Documentation

Ask for bottle certificates and their last inspection dates, precharge logs, drawdown and response-time test records, hose replacement dates and the pod service history. On a MUX system, get the software version, the hardware revision of each electronic module and any modification records.

Integration with the stack and rig

A control system is engineered for one stack. Function count, valve sizes and accumulator volume all follow the stack's operators, so a change to one preventer can mean a change to the control system. MUX systems also tie into the rig's power supply, the uninterruptible power units and the driller's cabin network. Confirm that the OEM still supports the electronics and software before you rely on a used MUX pod.

Refurbishment and recertification

A surface accumulator unit is the easiest used purchase, because you can test it on deck and move it between rigs. A refurbishment replaces bladders, seals and hoses and recertifies the bottles. Subsea pods should match the stack and system generation exactly, which makes a spare pod for a system already in your fleet the safest buy. Subsea control system OEMs include Cameron (SLB), NOV and HMH, and Koomey is a long-standing surface unit brand. See BOP stacks for the equipment these systems drive.