Multiplex BOP system utilizes updated technology, decreases riser disconnect time

Pete Fougere
Transocean Offshore Inc. Houston

Joseph R. Roche
Hydril Co. Houston

• Transocean's Discoverer Seven Seas is poised to lower the multiplex BOP system on top of the stack. The BOP system is positioned beside the moon pool next to the derrick (Fig. 1). [28,499 bytes]
• The driller's panel mimics the BOP stack and other functions. Its location allows the toolpusher or driller easy access during the drilling operations (Fig. 3). [18,845 bytes]
• The central control unit is a modern day, computer-controlled version of the hydraulic manifold. Input from the driller's panel is directed through the control unit, which processes, directs, and transmits them through the MUX cable to the pods (Fig. 4). [15,726 bytes]
• This multiconductor, armored MUX cable can transmit commands to a pod positioned on the BOP stack. Each of the two pods on the BOPstack is linked to the surface with separate, redundant cables (Fig 6). [24,590 bytes]
• This high-temperature, high-pressure probe is positioned in the choke line of the lower marine-riser package. It provides circulating fluid data during well-control operations (Fig. 7). [21,775 bytes]

A new multiplex (MUX) blowout preventer (BOP) system provides quicker riser decoupling and speeds shear and pipe ram closing for deep and ultra-deep water operations.

The system uses the latest advances in software, microelectronics, and materials technology.

To cope with ever-increasing water depths, operators, contractors, and manufacturers are developing faster-reacting, more-reliable, blow-out control systems with expanded functional capabilities.

In a recent refurbishment, Transocean Offshore Inc. refitted the Discoverer Seven Seas (DSS) drillship with the new MUX BOP control system (Fig. 1) [28,499 bytes], designed and developed by Hydril Co.

It was introduced into Gulf of Mexico service July 19, 1997, in 5,808 ft of water for Chevron Corp. and was successfully tested and operated without any problems. Subsequently, the DSS drilled a well for Exxon Corp. in 5,838 ft of water.

MUX advantages

MUX BOP communication systems respond faster than hydraulic systems. This is because electronically coded commands travel faster through conductive cables than hydraulic signals do through hose bundles.

Quicker response times are imperative for dynamically positioned, deep and ultra-deep drilling operations where the only connection between the floating drill rig and the BOP on the seabed is the riser. Emergency conditions can warrant a rapid well shut-in and riser disconnect so the rig can immediately move off location.

Hydraulic response time is further slowed by hose expansion along a hydraulic umbilical, which for deep and ultra-deep operations are thousands of feet in length. This extends the waiting time between initiation and completion of a functional command.

With a MUX system, there are no hydraulic fluid conduits in the communications cable. Instead, a separate supply line runs along the riser and ties in to the subsea accumulator bank.

In addition, MUX cables have a distinct advantage over hydraulic bundles because a single MUX cable allows several commands to alternately share a common conductor. Hydraulic cables only allow a single command. This enhanced capability results in reduced MUX umbilical size and improved reliability.

Deepwater control systems

BOP control systems operate equipment used to control pressure during well control operations. A typical subsea BOP stack includes: four ram BOPs, two annular BOPs, a hydraulically operated wellhead connector, a hydraulically operated lower marine riser connector, and an array of choke and kill valves.

An individual operation such as closing a ram, latching a connector, or opening a kill valve is called a "function." Functions are controlled by hydraulic signals that operate valves. Hydraulic fluid preparation consists of mixing, pressurizing, surface storage, subsea transportation, subsea storage, and pressure level regulation.

A deepwater, MUX control system is configured as follows (Fig. 2) [343,640 bytes]:

1. A biodegradable concentrate is mixed in exact proportion with clean water and stored in a separate reservoir. This mixture provides lubricity and combats bacteria.

2. Electrically driven pumps utilize the reservoir fluid and elevate the fluid pressure/discharge into the surface accumulator banks where the power fluid is stored.

3. As the pressurized liquid enters each surface accumulator bottle, compressed nitrogen gas maintains a charge on the power fluid so that it is energized and ready at all times to operate the chosen functions. The accumulators are thick-walled, high-capacity bottles that are manifolded together. Each bottle has an internal rubber bladder that has been precharged with nitrogen gas.

4. The control panels display illuminated push buttons arranged to physically mimic the BOP stack and other functions. The driller's panel is located for convenient access by the driller on the rig floor (Fig. 3) [18,845 bytes]. Auxiliary panels may also be placed in the toolpusher's office and other locations.

5. The central-control unit serves as a tie-in between the driller's or toolpusher's panel and the MUX cable reel (Fig. 4) [15,726 bytes]. It houses the central processing units, software applications, and other components that control communications and functions between the surface and the pods. The central control unit replaces the hydraulically controlled manifold valves used on conventional systems (Fig. 5) [89,804 bytes].

6. MUX cable reels that carry a multiconductor, armored cable provide power and communications paths from the central control unit to each subsea pod (Fig. 6) [24,590 bytes]. These cables replace the conventional hydraulic umbilicals that carried pilot signals (functional commands) and power fluid to the pods. The MUX cables cost $10/ft, while conventional, hydraulic-pilot umbilicals cost $70/ft.

7. For redundancy, there are two control pods mounted on the lower riser package on the BOP stack. Each pod can operate all subsea functions. Electrically operated solenoid valves in the pods direct hydraulic pilot pressure to operate the SPM valves. Regulators in the pods match the operating pressure for each group of functions. While one SPM valve directs power fluid to the desired function, a corresponding SPM valve allows the displaced fluid to vent.

8. Typically, a rigid conduit auxiliary line integral to the marine riser carries most of the power fluid subsea.

Engineered components

Elements of the new MUX system minimize downtime. Its design is engineered to reduce single point failures subsea. Dual-electric and transformer packages are provided on each subsea pod. The use of field-proven components and thoroughly tested components add to the confidence level.

Qualification component testing, simulated environmental system testing, and predeployment pressure testing all contribute to reduced downtime.

In addition, standalone systems including electrohydraulic and acoustic systems provide additional critical function redundancies outside of the basic MUX system.

The use of corrosion and galling-resistance materials reduce operating-system costs, and stainless steel alloys decrease the occurrence of costly pod and component overhauls. Solid materials have replaced electroless-nickel plating and moly-coating because these surfaces are susceptible to scaling and pitting. Ceramic coatings have supplanted paint systems, and electro-polishing of manifolds promises to reduce maintenance time.

Additional drillship features

The DSS was built in 1976. In 1987, it set a world record water drilling depth of 7,520 ft which it held for 9 years. The DSS has been outfitted with special features to enhance its deep-water, well-control capability. This includes a high-pressure, high-temperature probe (Fig. 7) [21,775 bytes] installed in the choke line at the lower marine riser package (LMRP). This probe provides circulating-fluid data during well-control operations.

The ship also has an auto-shear capability in case a catastrophic event results in the parting of the riser or the lower-marine riser package. If this situation occurs, the control system will automatically close the shear rams and safely cut the pipe in the BOP in order to seal-off the well pressure below. This is the first fail-safe system of this type used in the industry.