TECHNOLOGY GAS DETECTION Eliminating False Alarms and Improving Safety Performance – Laser-Based Gas Detection Technology on Terra Nova FPSO The original Terra Nova gas detection system was designed to utilise a combination of point detectors and line-of-sight (LOS) detectors based on infrared (IR) technology, with the bulk of coverage being provided by the LOS type. Since the facility first started production in 2002 there have been on-going issues with the reliability of the LOS detectors. By 2010 parts of the Terra Nova field were beginning to produce hydrogen sulphide (H 2S) in the well fluids. As toxic gases were not expected in the original Terra Nova design, the fixed gas detection system did not incorporate dedicated H 2S detection. A multi-disciplinary team was assembled to assess and upgrade the overall gas detection system on the Floating, Production Storage and Offloading (FPSO). BY RAJAT BURUA AND ED SHARPE After the field began producing hydrogen sulphide in 2010, the Terra Nova FPSO underwent a complete decommissioning and upgrading of its gas detection system (photos: Senscient) Prior to H 2S production, the most various gas detection technologies danger potential for the dispersion Selecting the Right Gas significant issue had been per - that could address the problems cloud, and the ability to perform Detector Technology formance during adverse weather with the existing system and detect parametric sensitivity and optimi - Since it commenced operation in conditions. During periods of fog both flammable and toxic gas sation studies. The geometry of the 2002, the Terra Nova FPSO has or snow many detectors would releases. Terra Nova FPSO was based on a relied primarily on LOS detection. report a blocked beam fault condi - computer-aided design (CAD) Therefore, Suncor’s preferred tion and, depending on the situa - Evaluation of the Gas model. option was to determine if LOS tion, some would falsely detect gas. Detection System technology existed that could As a result, Suncor would frequent - Detailed analysis based on compu - The quantitative evaluation of the replace its existing detectors and ly be forced to assign technicians tational fluid dynamics (CFD) GDS in the process modules was provide coverage for both flamma - to clean the lenses and return the modelling was performed for the based on testing the performance ble and toxic gas leaks. This would units to service. Terra Nova FPSO by GexCon. The for a large set of realistic gas leak minimise structural modifications overall objective of the analysis scenarios. The ventilation condi - required to mount new units, The assessment team involved per - was to test and optimise system tions were determined by the wind avoid having to run new cables sonnel from safety, risk analysis, performance and suggest improve - speed and direction, the heading of and trays, thereby minimising operations, instrumentation and ments for the existing system and the FPSO, large-scale effects from construction time and cost. An controls engineering. Based on the detector layout. modules and buildings upwind extensive search eventually led the need to upgrade the gas detection and downwind of the area of inter - Suncor team to lab test, field trial system to detect H 2S and the LOS The CFD model FLACS was chosen est, as well as smaller-scale effects and ultimately select the Enhanced detector performance issues, sever - in the present study because it from buildings, walls, equipment Laser Diode Spectroscopy al projects were initiated. allows the simulation of the rele - and piping inside the area of inter - (ELDS™) technology. This tech - vant toxic and flammable gaseous est itself. In aggregate, over 1,400 nology, developed by Senscient, The first initiative was to build a releases, the modelling of the gas leak scenarios were simulated had the capability of combining gas dispersion model for the Terra detailed geometry within the facili - and used in the evaluation, detec - toxic and flammable gas detection Nova FPSO and use this analysis ty, the simulation of both natural tor selection process, optimisation in a single LOS detector whilst sig - to complete a comprehensive gas and forced ventilation within the and overall design of the upgrade nificantly improving the perform - detection evaluation and optimisa - facility, the evaluation of gas con - to the gas detection system. ance and reliability of the FPSO’s tion study. In parallel, Suncor centration measured by line and overall gas detection system. began to investigate and trial point detectors, the evaluation of 18 SCANDINAVIAN OIL-GAS MAGAZINE NO. 1/2 2015 TECHNOLOGY GAS DETECTION In February 2011, a trial ELDS™ existing IR LOS gas detectors on unit was acquired and subjected to the Terra Nova FPSO with the qualification testing at the Terra ELDS™ technology. In aggregate, Nova onshore distributed control 141 IR LOS detectors were slated system (DCS) simulator. Over a for replacement. A recommenda - period of two weeks, stringent per - tion from the study by GexCon formance tests were conducted on added another 17 ELDS™ detec - the device, under four conditions tors to provide for additional cover - that were known to generate prob - age. Therefore, the overall retrofit lems for the existing IR detectors: and upgrade plan called for the simulated fog using water mist, decommissioning or 141 IR LOS direct water spray on the device and the installation of 158 lenses, plastic of various types ELDS™ detectors. Key areas/modules of placed in the beam path, and snow the Terra Nova FPSO (illustrations: Senscient) placed over the lens (approximate - Retrofitting the FPSO while it was ly 2.5 cm thick). The test results on location, in full operation, showed no issue with performance required significant planning. A for all four conditions. thorough work package was devel - oped which provided a plan for the Using a transmitter-receiver con - decommissioning of the existing figuration, ELDS™ systems detect IR detectors, installation of the and measure gas concentrations at new ELDS™ detectors and train - specific target gas absorption ing of personnel. Change out of wavelengths over distances of up to the LOS detectors commenced in 200 metres. One of the key tech - 2013, and is scheduled to be com - niques that enables ELDS™ to pleted int 2015. detect small fractional absorbances and eliminate false alarms is Impact on Safety Harmonic Finger- printing. A Performance Harmonic Fingerprint is a specific The performance of the upgraded set of harmonic components intro - system has been exceptional. Prior Terra Nova FPSO FLACS geometry model duced by target gas absorption to the upgrade, the Terra Nova where the relative amplitudes and FPSO experienced 3-5 plant trips, devices were sending to the control number of spurious trips, the time phases of the components are more than 200,000 fault indica - system indicated full health, how - spent in startup or shutdown known and specific to the target tions and 20-25 unrevealed fail - ever, they failed to respond to a modes is reduced, thereby decreas - gas absorption line that is being ures per year. Data from the Terra function check. The ELDS™ ing the risk to the facility. scanned. Nova data historian, maintenance detectors address this problem by management system and lost pro - completing a daily diagnostic The laser-based sensors installed Implementation of the duction tracking register was check and signalling a fault if on the Terra Nova FPSO have a Retrofit and Upgrade analysed to quantify this there is an issue. As of December minimum detection threshold that Once onshore testing was complet - performance. 31, 2014, Terra Nova had not is much lower than the older IR- ed, a dual Methane- H2S ELDS™ recorded a single unrevealed fail - based detectors. Instead of a hydro - detector was installed on the Terra A review of the maintenance man - ure with the laser-based gas detec - carbon detection threshold of 1 Nova FPSO , in May 2011, in place agement system showed that there tors. Startup and shutdown of a LELm/2.5 LELm (low/high of an existing IR LOS device that was an average of 21 unrevealed facility introduce risks that are not alarm), the laser-based detectors was known to cause issues in the gas detector failures per year from present when a plant is in steady can be set to a threshold of 0.2 past. The distance between the 2009 to 2013. The signal these IR state operations. By reducing the LELm/1 LELm (low/high alarm), transmitter (Tx) and receiver (Rx) without experiencing any drifting for this installation was approxi - Terra Nova and related spurious alarms. The mately 34 metres. During a trial The Terra Nova oil field is situated in the Grand Banks, approximately 350 kilometres east- effect of this five-fold increase in period that lasted approximately southeast of the city of St. John’s in the Canadian province of Newfoundland and Labrador and sensitivity is that the detectable 24 months, the ELDS™ detector is the second largest offshore oil field in Canada. The total recoverable oil reserves in the field volume of any gas cloud is larger. are estimated by the Canada-Newfoundland Labrador Offshore Petroleum Board (C-NLOPB) to Increasing the sensitivity of detec - performed very well, with zero spu - be 419 million barrels. The owners of the Terra Nova field are: Suncor Energy (37.675%), rious trips. The only issue identi - ExxonMobil (19%), Statoil (15%), Husky Energy (13%), Murphy Oil (10.475%), Mosbacher tors increases the detectable vol - fied was a loose mounting bracket Operating Ltd. (3.85%), and Chevron Canada (1%). Suncor Energy is the operator of the Terra ume and makes it more likely that that resulted in misalignment, a Nova field. any given detector will be exposed problem that was easy to identify Production from the field began in 2002, through the use of a FPSO vessel.