Subsea drilling BOP controls ready for ultra-deepwater
Subsea modules consist of an electronics package and hydraulic controls (courtesy ABB Seatec Division). A MUX being lowered into the Gulf of Mexico (courtesy of Hydril). [7,110 bytes]
Electro-hydraulic systems prepared to operate in 12,000-ft depths
Rick Von Flatern
Subsea modules consist of an electronics package and hydraulic controls (courtesy ABB Seatec Division).
But better technology practiced by a leaner industry, growing hydrocarbon demand from emerging industrial nations that dampened the impact of price fluctuations, and deepwater deposits of hitherto unimagined proportions, have caused contractors to dust off subsea control technology in anticipation of drilling programs in record-setting water depths for at least the next five years. In doing so, they have rekindled what is arguably the most competitive sector of the industry, involving no more than a handful of companies chasing a predicted hundreds of millions of dollars of investment during the next 3-5 years.
Mulitplex subsea drilling controls (MUX) are electro-hydraulic systems for remotely operating BOPs. Until the advent of deep and ultradeep drilling, hydraulic lines were run directly to the seafloor BOPs from the surface where functions were controlled from panels located on the rig floor, rig office, or other area of the rig. But the efficiency of such an arrangement decreases with length and water depth. At about 4,000 ft depths, function signals transmitted through a hydraulic umbilical increases the response time at the BOPs.
So, as it has in so many facets of the offshore industry, deepwater physics has forced subsea control manufacturers to innovation. In the case of MUX controls, the solution was to transfer the command signal from hydraulic impulses to more reliable, faster electronic ones. And while hydraulic fluid is still supplied from the surface through long umbilicals, with all accumulators distributed between the surface and the seafloor, the signal to activate the valves is relayed instantly to the hydraulic pilot valve, which in turns directs the pressured hydraulic fluid to perform any of 60-100 functions on the BOP body.
Electronics and evolutionWhat signals are sent, when they are sent, what functions they perform, and how those operations are implemented are the crux of the competition among the few companies involved in the complex technology. Manufacturers speak of their products in terms of generations, though unlike similar designations used for drilling vessels, the distinctions are not so standardized throughout the industry.
Ray Dextraze has served as an independent subsea controls consultant for such companies as Sonat, Sedco, and Smedvig. He places the first generation MUX in the mid-1970s. By today's standards it was a simplistic hydraulic system operated through electronic impulse. Shaffer's incorporation of then-revolutionary microprocessing into the system in the early 1980s, said Dextraze, defined the next, second generation. Then, and for more than a decade, a total lack of demand silenced the subsea control industry as operators steered clear of the sky-high cost arena that is deepwater drilling.
In 1995, Transocean, having gained possession of the Discoverer Seven Seas with its Sonat buyout, sought to upgrade its MUX system and awarded Hydril the contract. Included in that contract was a PC-based system. Dextraze calls this change from PLC-(programmable logic controllers) based systems a generational one.
That distinction is one upon which experts disagree. And indeed, the choice between PLC- and PC-based systems seems more a philosophical one than an argument made in hard-and-fast technical terms. Proponents of PCs claim an advantage exists in that they can specifically program their software for the unique demands of individual subsea BOP installations.
Others agree with Cameron Controls sales manager David Wiley, whose company uses PC-based surface electronics based on standard PLCs, that there is not such a sharp distinction between the two systems. Cameron's MUX, he said, "all run on PLCs, which are essentially dedicated PCs with large I/O capacity."
Joe Roche is controls marketing services manager for Hydril, a MUX manufacturer offering imbedded microporcessors. "We believe that imbedded controllers, which reflect many years of progress, are best suited to the mission," he said. "The system was designed specifically to operate a BOP, as opposed to a broad range of jobs. It features a standard software package and compact electronics. The software package is easily configured for the application."
It's decision to incorporate what Roche describes as the "latest microprocessor technology" into their MUX system, has led Hydril to claim its system, placed July 21, 1997, in 6,000-ft Gulf of Mexico waters to be the first of its new generation.
PLCs versus PCsBut others, including competitors Shaffer and ABB Seatec, are opposed to any notion that PCs represent an evolutionary step in MUX technology, saying PLCs are easily programmed to carry out any series of events and that PCs are an unwarranted complication. "We approach from the idea of using field-proven technologies," said ABB Seatec's Iain Duncan, whose system was placed offshore Brazil eight months earlier and was drilling its fourth Petrobras well, also in about 6,000 ft. "Everyone does the same thing. The idea is to push a button, fire a solenoid to fire the pilot valve which fires the valve on the BOP."
Industrial-based PCs and PLCs, he contends, are sufficiently sophisticated for those tasks and have been designed to have the flexibility of off-the-shelf availability, a feature whose value is also touted by Shaffer's marketing director, Reid Nuttall. "PLCs use off-the-shelf components and software for availability," Nuttall said. "We use the electronics that are available from other industries which means support will continue through the years as the buyers of the technology are more than just the subsea industry. That means there is a high install base that will continue to be supported by the [electronics] industry."
Nuttall said his company learned their lesson about unique software and hardware from earlier experiences programming and re-programming the Ocean Alliance. Five programmers worked on the original of that purpose-designed program. But construction delays meant the rig took four years to complete. By then, the five programmers had moved on and were unavailable to make final alterations to the program and were not available to service the system during its years of service.
Minor function changes became time-consuming, expensive, and complicated. As a result, Shaffer has outsourced much of its electronics hardware and all of its software. And the program is no longer altered by code writers, but through a form of automatic programming that reads a cause-and-effect schedule and writes the necessary commands.
Among the most important pieces of technology imported to MUX systems from outside the petroleum industry has been fiber optics. While all manufacturers continue to provide standard wiring for those operators still leery of fiber optics, all have expressed a preference for fiber optics as their wiring medium of choice because of its ability to carry more than one signal, e.g. a command signal and a diagnostic inquiry, on the same channel.
Fiber optics have other advantages that make it a perfect fit for deep water. "Fiber optics are actually cheaper as depth extends," Nuttall said. "And though some would argue the point, it is not particularly difficult to make connections or splice."
And fiber optics have a feature particularly attractive to drilling rigs - they do not suffer interference, or electronic noise, generated by nearby cables as does standard copper wire. In the wire-heavy environment of drilling rigs that can be a considerable advantage.
Redundancy, fault toleranceTo retrieve and re-run a deepwater riser can easily require more than a week. It is downtime during which rigs, at day rates are more than $100,000, are not making any headway. It is little wonder, then, that reliability is a prime concern of controls manufacturers. To that end, each uses one of two redundancy tactics - triple modular (TMR), or dual hot standby.
- Triple modular: TMR means three isolated parallel systems are consulted on each diagnostic and each command decision made by the system. Power runs to each module, each of which has two complete and independent control systems, on three lines. Each command and each diagnostic decision is "voted" upon by the three systems. When two agree, it is acted upon.
- Dual hot standby: This process means that two independent systems are operating on each pod, each of which has two complete systems within them. When one system goes down, the other automatically takes over without interruption, or as Cameron's Wiley puts it - "bumpless." Hot standby refers to the fact that even as the first level of the system is essentially "in charge," the backup is actually also running and fully functional. It then simply and seamlessly assumes the duties of the prime power source.
"The design goal is that no one point of failure is so critical that it will shut down the operation," said Roche of his company's system of redundancy and fault tolerance. "There are redundant electronics in each control pod such that each has an A and B control package. That is backed up by an electro-hydraulic system that is hardwired and totally independent of the MUX system."
Necessary for true reliability is problem prevention, that is, spotting a situation ripe for a problem before it manifests itself. To that end, the diagnostic arm of the system continually tests and reports on each function on the BOP, sending signals every fraction of a second. For example, in the case of Hydril, the system sends a signal that checks whether a solenoid would move, should it be given the signal to do so.
Making connectionsThe basic components of the submerged subsea control pods are a chamber, housing the electronics package, and a bank of hydraulic valves which initiate the actual BOP functions. Making the connections between electrical pod and solenoid, which is the moving part that mechanically changes the hydraulic valve position, has been traditionally accomplished through connections encased in pressurized electrolyte fluid baths in order to isolate the connections from hydrostatic pressure and water invasion - the mortal enemies of subsea connections.
Isolating the connections in a sealed bath also affords contractors, who historically distrust electrical connections in such adverse conditions, a method for testing the connections on the surface.
"In some people's philosophy," Dextraze said, "if it cannot be tested, it doesn't work."
But manufacturers are overcoming this long-standing industry bias and are installing systems with wet connectors exposed to the water and pressures at the ocean floor. These connectors have been used by the US Navy's submarine service for some time and manufacturers have successfully completed surface tests.
Recovery timeDespite the attention to redundancy and the latest electronics, manufacturers must still face the reality that failures are inevitable. The pod's hydraulic sections, after all, still consist of complex, tight-tolerance, sealed, moving parts that, in order to meet industry standards, must be able to move sufficient amounts of fluid quickly enough to complete BOP operations in 60 seconds or less.
In response to that reality, manufacturers have been seeking ways to move the pod between the sea floor and the surface more quickly and without moving the riser. Most commonly available from all suppliers is recoverable pods disconnected from the BOP module and guided to the surface by ROVs. The option is, in fact, available and in demand among some operators and contractors. But the practice has its doubters. "The technology is there," Duncan said. "But whether it can actually be done is another matter entirely."
Duncan and others, many of whom discourage buyers from incorporating the ROV retrieval option in their systems, are wary of them because of the path the pods must follow to and from the surface. Shearing currents alter direction 180 degrees in a short period of time, and it is not hard to imagine the multimillion dollar pod being slammed into and debilitating itself, and the even more expensive riser.
Also, there is the question of reconnecting the pod from the surface. There are no guidelines as in shallower waters, and lining up funnel guides can be tricky in the currents along the ocean bottom.
Still, Roche says his company is ready to provide the option because some operators and contractors choose it. "We can hot stab into the lower marine riser package," he said.
Duncan said his company is working on an entirely different approach to recovering pods but as it is in the development stage for a particular customer he is not at liberty to discuss details.
Statoil and others are also requiring an additional system option - a closed loop for the hydraulic system so that none of the operating fluid is vented to the sea. While the request sounds extravagant, given the fluid is organic and the volumes vented at a mile below the surface are quite small, it is not a technical problem. Like ROV recoverable systems, all manufacturers offer the option with varying degrees of enthusiasm.
Generations to comeSubsea drilling controls are not optional. They offer the only way to safely drill at the extreme depths to which the industry is irrevocably committed. And while competition for a portion of the limited number of lucrative contracts is fierce, the level of demand and short lead-time will likely assure each supplier an opportunity to deploy its system.
And despite their impressive level of sophistication, researchers are not standing still. Future MUX systems, according to ABB Seatec's Duncan, will have significant advantages over those currently available.
The next generation of subsea controls, he said, will have to be more reliable. Watching the technology evolve, as well as how each philosophy fares in a fiercely competitive business, should prove one of the more interesting offshore oil industry venues for the opening of the 21st Century.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.