Fiberoptics speed controls, data on ultra-deepwater drilling multiplex

In deepwater, tripping the stack is very expensive and time consuming. Bringing the blowout preventer stack back up to the surface is necessary if there are problems in the installation or with the drilling controls system.

If a system has the reliability to allow the blowout preventer (BOP) stack and lower marine riser package (LMRP) to be run just once, then it will save the operator both time and money on any well program. The deepwater subsea controls system market has grown rapidly in the last few years, with a number of new design entrants.

ABB is the latest entrant in the deepwater subsea drilling controls market, but the firm's new multiplex (MUX) control design was chosen by R&B Falcon for the Peregrine IV (now called Deepwater Expedition) drillship. The vessel recently drilled a well offshore Brazil for Petrobras in a record water depth of 9,111 ft.

ABB says it has emphasized system reliability and redundancy. Mike Dyer, Drilling Control Systems Manager for ABB Offshore Systems, said that it had to merge two distinct drilling and production system technologies. The goal was to build a MUX that would offer an easier user interface with high-end diagnostics, while still providing redundancy and reliability during operation.

While the technology is in place for these control systems, Dyer said there is little experience in dealing with a deepwater blowout situation. He said the system had to be so reliable that a blowout situation would not occur from a control system failure. This meant using proven components and installing various redundancies.

Fiberoptic transmission

The subsea BOPs used on ultra-deepwater wells have very large through-bores, rated for 15,000 psi. To operate this equipment in deepwater requires a great volume of hydraulic fluid. It takes time to move this fluid. An electronic signal speeds the process by reaching the control system instantly, compared with waiting for a hydraulic pilot signal to travel almost two miles.

In deepwater, it can be difficult to get a hydraulic signal down from the rig to the seafloor with any speed. In water depths over 3,000 ft, Dyer said this could become a problem. To overcome this limitation, ABB opted for electro-hydraulics. This system sends a communication signal down fiber optic lines in the MUX cable, which also transmits the electronic power for the system. The signal activates the hydraulic powered mechanisms on the stack.

Dyer said there are a number of advantages to using fiber optics once an operator gets past the common fear that these fibers are delicate. "We use fiber optics because more information can be transmitted over a long distance quicker without a logjam," Dyer said.

Fiber optics communicates an almost unlimited variety of information, in addition to actuating valves or closing and opening the BOP rams. Dyer said the fiber optics in the MUX cable are sheathed in stainless steel tubes to protect them. There are two of these sheathed lines in each MUX cable, with four optical fibers in each.

Only two of the fibers are required for most operations communications. The additional fibers not only allow for redundancy, but also offer additional independent transmission lines for such things as video.

Interfacing system

ABB says it uses off-the-shelf computer components, such as a Microsoft Windows NT platform operating system, an Ethernet network, and existing software for the system interface. Typically, Dyer said that suppliers will not only build the controls system hardware, but also build the computer components and write the unique base software code for the system. The reason for this is to ensure all aspects of the system are optimized for the application.

Dyer said ABB opted to design for off-the-shelf components that were less expensive, but reliable and accessible.

System testing

The MUX continually tests all aspects of the system to make sure there are no surprises for the operator. If a component is found to be faulty or on the brink of failing, the operator is alerted. Using one of the consoles that operates the controls system, the operator can identify the component, access his inventory to see if a replacement part is on hand, and even see a picture of what the component looks like in the system.

If the needed part is not available on the vessel, the operator can order it, again from the same console, directly from ABB. This allows for fast and efficient maintenance. When responding to a call to perform MUX maintenance, Dyer said, an ABB maintenance person can take a print out of where the component is, how to replace it, and what it looks like. Using this type of interface minimizes the risks of error and prevents expensive downtime, Dyer said.