18 S C A N D I N A V I A N O I L - G A S M A G A Z I N E N O . 1 / 2 2 0 1 5

Prior to H2S production, the mostsignificant issue had been per-formance during adverse weatherconditions. During periods of fogor snow many detectors wouldreport a blocked beam fault condi-tion and, depending on the situa-tion, some would falsely detect gas.As a result, Suncor would frequent-ly be forced to assign techniciansto clean the lenses and return theunits to service.

The assessment team involved per-sonnel from safety, risk analysis,operations, instrumentation andcontrols engineering. Based on theneed to upgrade the gas detectionsystem to detect H2S and the LOSdetector performance issues, sever-al projects were initiated.

The first initiative was to build agas dispersion model for the TerraNova FPSO and use this analysisto complete a comprehensive gasdetection evaluation and optimisa-tion study. In parallel, Suncorbegan to investigate and trial

various gas detection technologiesthat could address the problemswith the existing system and detectboth flammable and toxic gasreleases.

Evaluation of the GasDetection SystemDetailed analysis based on compu-tational fluid dynamics (CFD)modelling was performed for theTerra Nova FPSO by GexCon. Theoverall objective of the analysiswas to test and optimise systemperformance and suggest improve-ments for the existing system anddetector layout.

The CFD model FLACS was chosenin the present study because itallows the simulation of the rele-vant toxic and flammable gaseousreleases, the modelling of thedetailed geometry within the facili-ty, the simulation of both naturaland forced ventilation within thefacility, the evaluation of gas con-centration measured by line andpoint detectors, the evaluation of

danger potential for the dispersioncloud, and the ability to performparametric sensitivity and optimi-sation studies. The geometry of theTerra Nova FPSO was based on acomputer-aided design (CAD)model.

The quantitative evaluation of theGDS in the process modules wasbased on testing the performancefor a large set of realistic gas leakscenarios. The ventilation condi-tions were determined by the windspeed and direction, the heading ofthe FPSO, large-scale effects frommodules and buildings upwindand downwind of the area of inter-est, as well as smaller-scale effectsfrom buildings, walls, equipmentand piping inside the area of inter-est itself. In aggregate, over 1,400gas leak scenarios were simulatedand used in the evaluation, detec-tor selection process, optimisationand overall design of the upgradeto the gas detection system.

Selecting the Right GasDetector TechnologySince it commenced operation in2002, the Terra Nova FPSO hasrelied primarily on LOS detection.Therefore, Suncor’s preferredoption was to determine if LOStechnology existed that couldreplace its existing detectors andprovide coverage for both flamma-ble and toxic gas leaks. This wouldminimise structural modificationsrequired to mount new units,avoid having to run new cablesand trays, thereby minimisingconstruction time and cost. Anextensive search eventually led theSuncor team to lab test, field trialand ultimately select the EnhancedLaser Diode Spectroscopy(ELDS™) technology. This tech-nology, developed by Senscient,had the capability of combiningtoxic and flammable gas detectionin a single LOS detector whilst sig-nificantly improving the perform-ance and reliability of the FPSO’soverall gas detection system.

T E C H N O L O G Y G A S D E T E C T I O N

The original Terra Nova gas detection system was designed to utilise a combination of point detectors and line-of-sight (LOS) detectors based oninfrared (IR) technology, with the bulk of coverage being provided by the LOStype. Since the facility first started production in 2002 there have been on-goingissues with the reliability of the LOS detectors. By 2010 parts of the Terra Novafield were beginning to produce hydrogen sulphide (H2S) in the well fluids. Astoxic gases were not expected in the original Terra Nova design, the fixed gasdetection system did not incorporate dedicated H2S detection. A multi-disciplinary team was assembled to assess and upgrade the overall gasdetection system on the Floating, Production Storage and Offloading (FPSO).

BY RAJAT BARUA AND ED SHARPE

Laser-Based Gas DetectionTechnology on Terra Nova FPSO

After the field began producing hydrogen sulphide in 2010, the Terra Nova FPSO underwent a complete decommissioning andupgrading of its gas detection system (photos: Senscient)

Eliminating False Alarms and Improving Safety Performance –

3_18_OilGas_1-2_2015_ark1_Layout 1 05.02.15 12.16 Side 18

In February 2011, a trial ELDS™unit was acquired and subjected toqualification testing at the TerraNova onshore distributed controlsystem (DCS) simulator. Over aperiod of two weeks, stringent per-formance tests were conducted onthe device, under four conditionsthat were known to generate prob-lems for the existing IR detectors:simulated fog using water mist,direct water spray on the devicelenses, plastic of various typesplaced in the beam path, and snowplaced over the lens (approximate-ly 2.5 cm thick). The test resultsshowed no issue with performancefor all four conditions.

Using a transmitter-receiver con-figuration, ELDS™ systems detectand measure gas concentrations atspecific target gas absorptionwavelengths over distances of up to200 metres. One of the key tech-niques that enables ELDS™ todetect small fractional absorbancesand eliminate false alarms isHarmonic Finger- printing. AHarmonic Fingerprint is a specificset of harmonic components intro-duced by target gas absorptionwhere the relative amplitudes andphases of the components areknown and specific to the targetgas absorption line that is beingscanned.

Implementation of theRetrofit and UpgradeOnce onshore testing was complet-ed, a dual Methane-H2S ELDS™detector was installed on the TerraNova FPSO, in May 2011, in placeof an existing IR LOS device thatwas known to cause issues in thepast. The distance between thetransmitter (Tx) and receiver (Rx)for this installation was approxi-mately 34 metres. During a trialperiod that lasted approximately24 months, the ELDS™ detectorperformed very well, with zero spu-rious trips. The only issue identi-fied was a loose mounting bracketthat resulted in misalignment, aproblem that was easy to identifyand resolve. Following the success-ful offshore trial, a decision wasmade by Suncor to replace all

existing IR LOS gas detectors onthe Terra Nova FPSO with theELDS™ technology. In aggregate,141 IR LOS detectors were slatedfor replacement. A recommenda-tion from the study by GexConadded another 17 ELDS™ detec-tors to provide for additional cover-age. Therefore, the overall retrofitand upgrade plan called for thedecommissioning or 141 IR LOSand the installation of 158ELDS™ detectors.

Retrofitting the FPSO while it wason location, in full operation,required significant planning. Athorough work package was devel-oped which provided a plan for thedecommissioning of the existingIR detectors, installation of thenew ELDS™ detectors and train-ing of personnel. Change out ofthe LOS detectors commenced in2013, and is scheduled to be com-pleted int 2015.

Impact on SafetyPerformanceThe performance of the upgradedsystem has been exceptional. Priorto the upgrade, the Terra NovaFPSO experienced 3-5 plant trips,more than 200,000 fault indica-tions and 20-25 unrevealed fail-ures per year. Data from the TerraNova data historian, maintenancemanagement system and lost pro-duction tracking register wasanalysed to quantify this performance.

A review of the maintenance man-agement system showed that therewas an average of 21 unrevealedgas detector failures per year from2009 to 2013. The signal these IR

devices were sending to the controlsystem indicated full health, how-ever, they failed to respond to afunction check. The ELDS™detectors address this problem bycompleting a daily diagnosticcheck and signalling a fault ifthere is an issue. As of December31, 2014, Terra Nova had notrecorded a single unrevealed fail-ure with the laser-based gas detec-tors. Startup and shutdown of afacility introduce risks that are notpresent when a plant is in steadystate operations. By reducing the

number of spurious trips, the timespent in startup or shutdownmodes is reduced, thereby decreas-ing the risk to the facility.

The laser-based sensors installedon the Terra Nova FPSO have aminimum detection threshold thatis much lower than the older IR-based detectors. Instead of a hydro-carbon detection threshold of 1LELm/2.5 LELm (low/highalarm), the laser-based detectorscan be set to a threshold of 0.2LELm/1 LELm (low/high alarm),without experiencing any driftingand related spurious alarms. Theeffect of this five-fold increase insensitivity is that the detectablevolume of any gas cloud is larger.Increasing the sensitivity of detec-tors increases the detectable vol-ume and makes it more likely thatany given detector will be exposedto detectable gas before the flam-mable volume reaches a danger-ous size. Furthermore, the H2Scontent on Terra Nova means

T E C H N O L O G Y G A S D E T E C T I O N

Terra NovaThe Terra Nova oil field is situated in the Grand Banks, approximately 350 kilometres east-southeast of the city of St. John’s in the Canadian province of Newfoundland and Labrador andis the second largest offshore oil field in Canada. The total recoverable oil reserves in the fieldare estimated by the Canada-Newfoundland Labrador Offshore Petroleum Board (C-NLOPB) tobe 419 million barrels. The owners of the Terra Nova field are: Suncor Energy (37.675%),ExxonMobil (19%), Statoil (15%), Husky Energy (13%), Murphy Oil (10.475%), MosbacherOperating Ltd. (3.85%), and Chevron Canada (1%). Suncor Energy is the operator of the TerraNova field.

Production from the field began in 2002, through the use of a FPSO vessel. This was the firstdevelopment in North America to use FPSO technology. One of the largest FPSO vessels everbuilt, the Terra Nova FPSO is 292 metres long and 46 metres wide. The Terra Nova FPSO canstore 960,000 barrels of oil and accommodate up to 120 personnel while producing.

S C A N D I N A V I A N O I L - G A S M A G A Z I N E N O . 1 / 2 2 0 1 5 19

Key areas/modules of the Terra Nova FPSO (illustrations: Senscient)

Terra Nova FPSO FLACS geometry model

19_34_OilGas_1-2_2015_ark2_Layout 1 05.02.15 12.05 Side 19

even small leaks can be haz-ardous, thus increased sensitivity isan important tool to ensure workersafety.

A comprehensive dispersion analy-sis based on worst case H2S con-centrations in the productionstream was performed for theentire FPSO. This study revealedthat hazardous levels of H2S couldoccur in the port half of moduleM04, the area of the process facili-ty where regeneration of glycol andtreatment of gas and producedwater takes place. Consequently, adedicated H2S detection system wasdeveloped and installed in the M04module utilising laser-based LOSdevices.

Impact on Maintenance andOffshore OperationsA review of performance data fromthe Terra Nova data historian

shows dramatic improvement overthe IR devices in spurious tripavoidance. The key metricreviewed was the number offaults/blocked beam indications.

Performance data wasreviewed from January toDecember of 2011 for theIR-based detectors as theFPSO was off station formaintenance for portionsof 2012 and 2013. Datafrom June 2013 to June2014 was used to capturethe performance of detec-tors that had been changedto laser-based systems.

With such a significantdecrease in the number offaults, the requirement fortechnician response andtroubleshooting has beendrastically reduced. From2009 to 2013, an average of234 maintenance workorders per year was gener-ated, relating to IR LOSdetectors being in fault.Note that this number doesnot include the significantnumber of maintenance

call outs during conditions of lim-ited visibility (rain, fog, mist andsnow) to clean detectors as thesecalls are not tracked. From June2013 to July 2014, there were only13 maintenance work orders created relating to issues withlaser-based devices.

Impact on ProductionHistorically, the Terra Nova FPSOhas experienced three to four pro-duction trips per year due to falsegas detection. These trips haveresulted in production defermentsof approximately 50,000 to100,000 barrels per year. During atrip initiated by gas detection,emergency shutdown valves areclosed quickly, equipment istripped and gas inventory is sent tothe flare system.

Shutting down the plant in thismanner has potential damagingeffects on plant equipment such asgenerators and gas compressors.The FPSO experienced damage toprocess equipment during emer-gency shutdowns several timesprior to the installation of thelaser-based gas detectors, resultingin prolonged outages and signifi-cant repair costs. Conversely, therehas not been a single instance offalse gas detection with the laser-based gas detection technology.

The experience from Terra Nova demonstrates that imple-mentation of advanced laser-based

gas detection technology can resultin a significant improvement insafety, reliability and processuptime in industrial facilitieswhere the threat of a toxic or flammable gas leak exists. n

20 S C A N D I N A V I A N O I L - G A S M A G A Z I N E N O . 1 / 2 2 0 1 5

The Authors:

Rajat Barua is President and CEOof Senscient. Prior to his currentrole, he worked at Schlumberger,Shell, Cameron International andLime Rock Partners. He started hiscareer as a field engineer and has17 years of experience in the ener-gy industry in roles that span oper-ations, finance, management,sales and marketing. Mr Barua is agraduate of McGill University andHarvard University.

Ed Sharpe has 10 years of experi-ence in the oil and gas industry,and is the process automation leadwith Suncor Energy, supportingactivities on the Terra Nova FPSO.He is a graduate of MemorialUniversity of Newfoundland with adegree in electrical engineeringand is a TUV Functional SafetyProfessional.

ELDS™ systems use a transmitter-receiver configuration todetect and measure gas concentrations at specific target gasabsorption wavelengths over distances of up to 200 metres

Gas release simulation resulting in a hazardous flammablecloud scenario (1,250 m3) in the power generation module(M09) – concentration contours from 50% LEL (lower explo-sive limit) depicted in deep blue, to UEL (upper explosivelimit) depicted in dark red

T E C H N O L O G Y G A S D E T E C T I O N

ELDS™ systems installed in field operations

T 1 2

19_34_OilGas_1-2_2015_ark2_Layout 1 05.02.15 12.06 Side 20