Laser-based gas detection technology and dispersion modeling used to eliminate false alarms and improve safety performance on Terra Nova FPSO Filippo Gavelli, Ph.D., P.E. Head of Dispersion Consulting GexCon
Introduction and background Terra Nova is the second largest offshore oilfield in Canada. Total recoverable reserves are estimated at 419 million barrels. Production began in 2002, through the use of a Floating, Production, Storage and Offloading (FPSO) vessel. Owners of the Terra Nova oilfield 38% 10% 19% 4% 15% 1% 13% 2
Location of the Terra Nova field 3
Terra Nova FPSO one of the world s largest vessels 4
The Terra Nova FPSO on a sunny day 5
The Terra Nova FPSO on a foggy day 6
Advent of sour gas in 2010 Original gas detection system designed to utilize point and LOS detectors based on infrared (IR) technology (predominately LOS). 2002 onwards issues with reliability for LOS; during fog or snow IR detectors would report a fault condition and some would falsely detect gas. Terra Nova FPSO remote facility. Gas detection = complete ESD Based on the need to upgrade the gas detection system to detect H 2 S (2010) and the IR performance issues, two projects were initiated. 1. To build a gas dispersion model and use this analysis to benchmark and optimize the safety performance of the gas detection system. 2. To investigate and trial gas detection technologies that addressed problems with the existing system and could detect flammable and toxic gas releases. 7
Gas dispersion simulation modeling CFD (FLACS) was used to simulate 1,480 leak scenarios and used in the evaluation, optimization, detector selection and design of the upgrade to the system. To optimize performance a diverse set of leak scenarios was simulated. Leak scenarios were selected that resulted in flammable and toxic clouds in all areas. Leaks that yielded challenging clouds were chosen: test the system. Safety criteria for flammable gas: A confirmed gas alarm must be triggered before a leak reaches a hazardous level Safety criteria for toxic gas: Low alarm must be triggered within 30 seconds for any leak scenario where the volume with an H 2 S concentration above 10ppm exceeds 100m 3 or 100ppm exceeds 10m 3. 8
Dimensioning cloud size varied by module Module Dim. Cloud Size Main Deck 88 m 3 Separation 332 m 3 Prod. Water 406 m 3 Water Inj. 604 m 3 Power Gen. 695 m 3 9
Geometry model of the FPSO using FLACS 10
Gas dispersion simulation of a gas leak scenario 11
Simulation of a gas release 12
Simulation of a gas release 13
Simulation of a gas release main deck 14
Optimization of the gas detection system To optimize performance, a diverse and challenging set of leak scenarios was simulated. Leak scenarios were selected that resulted in flammable and toxic clouds in all areas. Leaks that yielded challenging clouds were chosen. 15
Optimization GDS water injection module 16
Undetected scenario water injection module IR layout 17
Same scenario now detected using laser LOS technology Note: Detector is (barely) impinged. With lower set points (more sensitive detector) the scenario is detected. 18
M02 worst undetected Response of critical detector Exposure curve for detector impinged by scenario shown on previous slide. With old set point, no alarm is given. New set point provides detection. 19
Implementation of the retrofit and upgrade Preferred option was LOS technology that could replace its existing detectors to minimize structural modifications required to mount new units. Following an offshore trial (2011-13), a decision was made to replace all existing IR LOS gas detectors with ELDS TM technology. Overall retrofit and upgrade plan called for: 1. the decommissioning of all 141 IR LOS 2. the installation of 158 ELDS TM detectors. Retrofitting the FPSO while it was in full operation, required significant planning. A thorough work package was developed for decommissioning, installation and training of personnel. 20
Installation of an ELDS TM system on the FPSO 21
Detector layout on main process deck 22
Results impact on safety Significant impact using CFD performance based design and ELDS TM technology Data from the Terra Nova data historian, maintenance management system and lost production tracking register was analyzed to quantify performance. 1. Sour gas detection Benefits two-fold by utilizing the performance based CFD detector design and Senscient s dual laser Methane - H 2 S units. Minimized installation, retrofit and wiring costs (dual laser) Alarm triggered within 30 seconds for leaks where: H 2 S > 10ppm for 100m 3 or H 2 S > 100ppm exceeds 10m 3. 23
Results impact on safety 2. Detector five-fold increase in sensitivity could be employed in CFD design IR set at 1 LELm / 2.5 LELm (low / high alarm) to avoid drifting/false alarms Laser detectors can be set at 0.2 LELm / 0.5 LELm (low / high alarm) 3. Elimination of unrevealed gas detector failures IR had an average of 21 unrevealed failures per year ELDS TM has not recorded a single unrevealed failure. 4. Combining voting across fire zones improved detection performance Improved gas detection system design 5. Increased availability of detection safeguards During adverse weather conditions offshore personnel would place inhibits on IR detectors 24
Results impact on maintenance 1. Average of 234 maintenance work orders per year generated relating to IR LOS detectors being in fault. (not including minor maintenance calls) 2. Over comparable period 13 maintenance work orders for laser-based devices (minor re-alignment issues). 3. Automated calibration and daily testing reduced operating costs eliminates need for technicians to carry cylinders of hazardous gases no sensor replacement 4. Bluetooth connectivity eliminates need for scaffolding or ladders to make physical connections 25
The performance of the upgraded system has been exceptional. Thank you for your attention fgavelli@gexcon.com