Gaining Peace Of Mind: Laser-based gas detector technology eliminates false alarms, improves worker safety
- Published: Monday, 25 May 2015
- Author: Maurice Smith
Thirteen years into operations at its massive offshore Terra Nova oil production facility, Suncor Energy was running into two big problems. The oil was unexpectedly souring, creating a potential safety problem since the facility’s gas detection system was not set up to detect hydrogen sulphide (H2S). And its existing monitoring system was proving unreliable partly due to the harsh environment in which the platform operates, leading to false alarms and unnecessary shutdowns that were costing the consortium that owns the facility $5 million to $10 million a year.
Suncor is the operator of the Terra Nova floating production, storage and offloading (FPSO) vessel, located on the Grand Banks about 350 kilometres from St. John's. It is jointly owned by Suncor (37.675 per cent), ExxonMobil (19 per cent), Statoil (15 per cent), Husky Energy (13 per cent), Murphy Oil (10.475 per cent), Mosbacher Operating (3.85 per cent) and Chevron Canada (one per cent).
The FPSO is one of the largest ever built—at 292 metres long approximately the size of three football fields laid end to end and standing 18 storeys high from the keel to the helideck. The oilfield, in the Jeanne d'Arc Basin, is the second largest producing oilfield off the East Coast. Its produced gases are separated from the oil and re-injected into the reservoir for production support and possible future extraction. The vessel can store 960,000 barrels of oil and accommodates up to 120 personnel.
Short of the Arctic, it’s hard to imagine a more remote and inhospitable location for oil extraction. Terra Nova is in iceberg alley, necessitating measures to avoid damage from icebergs—it is a double-hulled, ice-reinforced vessel with a dynamic positioning system that enables it to change to more favourable headings in high winds and storms. It is situated in one of the foggiest areas of the planet, shrouded in fog more than 200 days a year, and is hit by snow squalls, freezing rain, gale-force winds and storm waves exceeding 25 metres in height.
Such conditions created numerous challenges for the vessel’s original gas detection system—based on both infrared line-of-sight (LOS) detectors and point detectors—including malfunctions, the need for frequent maintenance and false alarms that caused numerous production shutdowns.
In a recent paper detailing the switchover to a new laser-based system by representatives of Suncor, Senscient and GexCon US, the authors report that during periods of fog or snow many LOS detectors would report a blocked beam fault condition and, depending on the situation, some would falsely detect gas. A high level of maintenance was required, temporary portable detectors were often deployed and three to four production trips per year were experienced due to false gas detection.
“During a trip initiated by gas detection, emergency shutdown valves are closed quickly, equipment is tripped and gas inventory is sent to the flare system. Shutting down the plant in this manner has potential damaging effects on plant equipment such as generators and gas compressors. The FPSO experienced damage to process equipment during emergency shutdowns several times prior to the installation of the laser-based gas detectors, resulting in prolonged outages and significant repair costs,” says the paper. The trips resulted in production deferments of approximately 50,000–100,000 barrels of oil per year.
In 2010, the field began producing toxic H2S, forcing an upgrade from the existing gas detection system that did not incorporate dedicated H2S detection. A multi-disciplinary team with representation from safety, risk analysis, operations, instrumentation and controls engineering was tasked to come up with a new gas detection solution.
SOLUTION SEARCH
Suncor began to investigate and trial various gas detection technologies while simultaneously building a gas dispersion model for the FPSO and using this analysis to complete a comprehensive gas detection evaluation and optimization study. A detailed analysis of the facility based on computational fluid dynamics (CFD) modelling was performed, and more than 1,400 gas leak scenarios were simulated and used in the evaluation, detector selection process, optimization and overall design of the upgrade to the gas detection system.
Among the aspects analyzed were the simulation of a range of realistic gas leak cases, simulating explosions to establish dangerous cloud sizes for each module of the FPSO, benchmarking a range of detector designs and layouts, arriving at a recommended optimum system for each module, and ensuring sufficient performance for both toxic and hydrocarbon gas detection.
The Suncor team’s extensive search led it to the selection of Senscient’s Enhanced Laser Diode Spectroscopy (ELDS) technology, which combines toxic and flammable gas detection in a single LOS detector while minimizing maintenance and increasing reliability, as a replacement system.
“The main advantage is that we can detect both flammable and toxic gases using open path technology, and the existing technology they were using couldn’t detect sour gas, and their reservoir was souring,” says Rajat Barua, Senscient chief executive officer and one of the authors of the paper.
“And secondly, the existing technology was giving them tremendous problems in adverse weather conditions, when they had fog, sleet, snow, mist, rain, because the existing technology they had relied on [used] infrared radiation, which is absorbed by water. Hence in those conditions, first of all they would lose their detection coverage, and secondly they were getting false alarms, which was very costly to them because it was causing the shutdown to their platform—that was a huge detrimental impact they were having to their operations in lost time and lost production.”
ELDS uses a transmitter-receiver configuration to detect and measure gas concentrations at specific target gas absorption wavelengths over distances of up to 200 metres. The transmitter uses highly reliable, solid-state laser diode sources similar to those used in telecommunications applications to generate a laser beam. The receiver measures absorbance changes when a combustible or toxic gas passes through the laser beam, according to Senscient. It uses harmonic fingerprinting to detect small fractional absorbances and eliminate false alarms.
“A harmonic fingerprint is a specific set of harmonic components introduced by target gas absorption where the relative amplitudes and phases of the components are known and specific to the target gas absorption line that is being scanned,” states the paper. “Using a small retained sample of target gas inside the transmitter, the temperature and wavelength modulation currents applied to the transmitter’s laser diodes are actively controlled to lock the lasers such that absorption by target gas produces specific harmonic fingerprints. The relative amplitudes and phases of the harmonic components in a harmonic fingerprint are so specific and unique that only absorption by the specified target gas produces a signal with the desired harmonic fingerprint.”
Noise, absorption by atmospheric gases and coherent interference effects don’t produce signals with the harmonic fingerprint, enabling the system to eliminate false alarms. The gas reference cell also enables remote, on command, electronic functional testing of the gas detector either locally or from a control room under any conditions. ELDS units are programmed to conduct a validity test every 24 hours and the results are automatically logged. “The presence of a gas reference cell is an innovation that improves reliability and reduces maintenance, which eliminates the need for technicians to carry cylinders of hazardous gases through the FPSO in order to test gas detectors,” it states.
The laser-based sensors also have a minimum detection threshold that is much lower than the older infrared-based detectors, providing up to a fivefold increase in sensitivity without experiencing any drifting and related false alarms. This increases the detectable volume and makes it more likely that any given detector will be exposed to detectable gas before the flammable volume reaches a dangerous size.
Stringent performance testing of the ELDS system began in 2011 at the Terra Nova onshore distributed control system (DCS) simulator. Tested in simulated fog using water mist, in direct water spray on the device lenses, with plastic of various types placed in the beam path and with snow placed over the lens to approximately one-inch thickness, the system performed without issue.
A dual methane and H2S ELDS detector was installed on the FPSO in May 2011 for a 24-month trial period and, other than a loose mounting bracket, performed without incident and created no spurious trips. The decision was made in 2013 to replace, over two years, all 141 infrared LOS detectors with 158 ELDS detectors, with the extra units to provide additional coverage as recommended by a GexCon assessment.
Prior to the upgrade, the Terra Nova FPSO experienced three to five plant trips, more than 100,000 fault indications and 20–25 unrevealed failures per year. Afterward, “the performance of the upgraded system has been exceptional,” the paper states.
The number of faults/blocked beam indications fell from 3.82 per day to 0.14 per day. Not counting two ELDS detectors with alignment issues that caused most of the faults, and one faulty infrared detector that accounted for almost half of that system’s failures, the overall fault rate was 1.1 per detector day with the infrared system (due mainly to weather, dirty optics and alignment), compared to 0.025 for the laser system.
“Some things are harder to quantify, like improved safety, but certainly the most apparent saving is in production. By eliminating the false alarms, which was costing them $5 [million] to $10 million a year, by simply eliminating the false alarms, we have increased the production and hence the revenue for Terra Nova by $5 [million] to $10 million a year,” Barua says.
The requirements for technical response and troubleshooting also fell dramatically, from 234 maintenance work orders per year from 2009 to 2013 (not including unrecorded callouts to clean detectors due to snow, rain and mist) to just 13 maintenance work orders from June 2013 to July 2014 for issues related to the laser-based devices.
ONSHORE USE
Founded in 2004, Senscient developed and commercialized the ELDS technology over a period of almost 10 years. “The technology itself was developed between 2004 and 2008, when the [research and development] was done, and between 2008 and 2011 the company received all of the approvals and certifications—with safety devices there are a lot of approvals required—and we have been in commercial mode since 2011,” says Barua.
There are now over 1,300 sensors in operation around the world, in 30 countries, with several installed in facilities in the Gulf of Mexico and the North Sea, he says. (While Barua is based in Houston, the company’s headquarters and research and development team are in the U.K.)
In addition to the oil and gas sector, which represents about 75 per cent of the company’s business, the sensors are also used in other industries, including the semiconductor, agriculture and environmental monitoring industries.
Onshore oil and gas facilities are also moving to laser-based gas detectors. In western Canada, Calgary-based Spartan Controls is distributor of Senscient sensors. “Within oil and gas, one of the big gases of interest tends to be hydrogen sulphide, and western Canada has a lot of sour gas production, so there is a lot of interest there,” Barua says.
Jim Hueston, Spartan Controls manager, fire and safety, says the company has been selling Senscient gas detectors for just over a year. “I would say we have sold over 100 in the first year,” he says. “And we have pretty solid orders for the next couple of years.” A couple of major heavy oil and oilsands producers are adopting the technology as is an agricultural company.
“The big benefit is that there are no false alarms, there is no calibration required and there are no sensor changes, which are some of the biggest costs for our customers. And it’s an industrialized packaged device, so it’s meant for cold, nasty, windy, dirty environments. If it is delicate, it’s not going to last.
“Another benefit is it’s really quick to install. One of the most difficult parts of setting up an open path detector is alignment, and I was recently able to align one in under two minutes. Some of the older ones from different manufacturers have taken an hour to do the full alignment,” Hueston says.
While it is more costly upfront, the system is cheaper over the long run, he says. “These cover a large area—they can replace a number of point detectors, and since they don’t require maintenance, the cost of ownership is actually quite small versus looking at a number of point detectors.”
But one of the greatest values is less quantifiable, he says—worker safety and peace of mind. “One of my customers actually had a leak, and they had a lost-time incident because of it, and the workers—and probably the management—had lost faith in the gas detection that was there. They replaced it with the Senscient laser-based open path, and they extensively tested it for the first six or eight months, and they were very happy. So at the end of the day not only were the workers happy with it, but I think the management was happy that they had found a solution. They are responsible for those workers, and you want to make sure that you are providing an environment that is safe, and so I think peace of mind for all the stakeholders was one of the big things they got out of it.”
Going forward, Hueston says point detectors will continue to have a place, but new technologies like laser-based detectors will gain ground. “I truly believe that in a couple of years from now, we are going to see a step-change in gas detection. We are going to see a whole lot more open path detection, intermingled with point detection and ultrasonic detection, because I think that combination is what is going to provide the best coverage for customers. Both the open path detection and ultrasonic detection are gaining traction, and I think the use of point detection is going to decline.”
CONTACT FOR MORE INFORMATION
Rajat Barua, Senscient
Tel: 281-467-6735
Email: [email protected]
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