Fast Response Retrofitable Ultra Deepwater BOP Control System
Authors: Childers Mark A. , Buckley Margaret , Curtiss Jason P. 
Published: December 2007

Industry standards (API and MSS) for subsea blowout preventers (BOPs) requires ram BOPs to close within 45 seconds and annular BOPs within 60 seconds; however, it is an industry goal to close these critical functions as fast as is practical (1). Most existing subsea BOP control systems are Piloted All-Hydraulic. Directional control valves in the control pods on the subsea BOP stack are controlled via hydraulic pulses (pressure up and/or bleed off) through 3/16-inch ID pilot hoses. The directional control valves shift time is heavily dependent on the hose length, among other factors. Thus the pulse time to shift the valves is fairly constant regardless of the water depth, given that the umbilical length remains constant for a given outfitting of a Mobile Offshore Drilling Unit (MODU). Calculations and empirical data suggest that a typical subsea Piloted All-Hydraulic Control System, even with the faster "pressure-bias" type modification, has trouble meeting desired closing times when functioning through approximately 5,500 ft or more of control umbilical. In ultra deepwater depths (over 5,000 ft), electronic coded signal or Multiplex (MPX) systems have been used almost exclusively to solve reaction times; however, they have certain disadvantages. MPX system are very expensive, complicated, time consuming to troubleshoot and repair, and are impractical to economically retrofit to existing MODUs and BOP stacks.

A newly patented (2) hybrid Electro-Hydraulic (EH) Control System has been developed that is readily retrofitable to Piloted All-Hydraulic Control Systems for deepwater upgrades. It utilizes the existing driller's and toolpusher's panels, pump/accumulator unit, UPS, most of the rig's electrical wiring, existing rig top side hydraulic umbilicals, both subsea pods, pod retrieving frames and, in all likelihood, requires no modifications to the BOP stack or its frame. The new concept centers around running electrical lines to critical solenoid pod functions (rams, annulars and possibly connector release) that are very time dependent and hydraulic pilot lines to all other non-critical time dependent functions. A module consisting primarily of solenoids and hydraulic/electrical connectors is packaged to fit on top of the existing all-hydraulic control pod. The overall "kit" is designed to be placed aboard the MODU and BOP stack during a long rig move or short non-shipyard installation.

The EH Control System uses existing field proven technology and components. It can be used with or without guidelines. The subsea control pods can be retrieved on guidelines or via ROV with guidelineless drilling systems. This is very advantageous compared to existing deepwater MPX systems in that the marine riser and LMRP does not have to be retrieved in the event of an unplanned repair. The umbilical (less weight and OD than standard all-hydraulic umbilicals) can be run on guidelines or on the OD of the marine riser. The umbilicals have ROV remote mateable make-and-break junction boxes on the pods. The overall response time for critical functions of the EH Control System is as fast as a MPX system at a fraction of the capital and installation cost. It is technically simple and can be serviced easily by any qualified rig electrician. The EH Control System is very compatible with the Slim Riser concept, is retrofitable to any standard Piloted All-Hydraulic Control System, compactable with new build project, and can replace a MPX system.