tankradar on lpg/lng tanks provides safety and … conferences/2000/data...tankradar on lpg/lng...

Title page

TankRadar on LPG/LNG Tanks Provides Safety andMaintenance-free Operation

By Johan Sandberg and Tomas Akerstrom, Saab Marine Electronics

Section 1:

Precision Radar Level Gauging

On LPG and LNG storage tanks

Johan Sandberg

Radar technology is quickly replacing old level gauging technology. The non-contact microwavetechnology provides safety, high accuracy, high reliability, superior longevity and low cost of purchasingand ownership.

Background:Level gauging has in the past been dominated by mechanical systems like floats, servo driven displacer,pressure sensors and capacitance probes. All these technologies require active sensor parts to be insertedin to the storage tanks. Moving parts are in contact with the liquids measured. Typically, mechanical andintrusive systems will require regular maintenance to perform reliably. Cost for spare parts, calibrationand tank entry makes these old technologies expensive to maintain. Malfunctions in intrusive gaugingsystems can lead to incorrect Inventory assessment and over fill situations.

Mechanical gauges:

Picture 1. Typical servo displacer gauge on a pressurized tank.

Servo driven displacer gauges has been used since the early fifties. The servo gauge is an electro-mechanical devise. The displacer is suspended in a thin wire. The wire is wound up on a drum with onegroove per turn, keeping the diameter constant. An electric motor turns the drum moving the displacer upor down. As the displacer hits the liquid surface the tension in the wire changes. A weighing balance

detector senses this tension-change. The servo control circuit will maintain a constant wire strain andmakes the displacer follow the liquid surface as the level changes. See picture 2.A servo gauge contains many moving parts. When the tank is agitated or the liquid is boiling theservomotor is in frequent movement. Maintenance and calibration is needed to keep the unit in goodcondition.

The servo gauge can be isolated from the tank by a ball valve. A calibration chamber above the ball valveis used to calibrate the unit. When inserted in the tank the displacer is maintained in place by a still pipe.

Picture 2. The main components in a servo level gauge.A complicated mechanical design with many moving parts.

Picture 3. A servo tank gauge under maintenance.

Since it is difficult to determine if the displacer is fully wound up above the ball valve it is possible tobreak the wire by closing the valve. If this happens the displacer and the wire will fall down to the bottomof the tank. A new displacer and wire needs to be fitted and calibrated.

Capacitance sensors consist of multiple, segmented probes. The probes are connected by wring to theelectronics outside. Tank entry is needed if any probe with the probes occur.

Radar Technology

Radar level gauging has been available for industrial applications since the mid 1970 s. The technologyhas gained a large market share over the existing intrusive gauging methods.A radar level gauge consists of a microwave transceiver and an antenna made for the application. A lowpower microwave signal (< 0.5 mW) is sent from the antenna towards the liquid surface. The liquidreflects the microwaves back to the antenna. The transmitted and received signals are compared and thedistance to the surface can be measured within fractions of a millimeter. The radar gauge has no movingparts and no part of the unit is in contact with the liquid. Basically any liquid can be measured. Radargauges can with proper design be used for level gauging on any hydrocarbon from Methane to Asphalt atthe same high accuracy.

Picture 4. Typical Radar Tank Gauge installation on a fixed roof tank.

How Radar works

Picture 5. The Radar equation

Picture 6. The strength of the radar reflection is depending on the dielectric constant of theliquid. Both LPG and LNG have typically a very low reflectivity.

When a wave travelling in a medium encounters an interfacetowards another medium with different properties reflection willtake place.

Part of the wave will be reflected back and part of the wave willcontinue through the interface.

The magnitudes of the reflected and transmitted waves dependon the physical properties of the two media.

E Ereflected incident= ?Γ

Γ =−

+

µε

µε

µε

µε

2

2

1

1

2

2

1

1ε - permittivityµ - permeability

RADAR

PPtt

PPrr

GGΓΓRR

In decibel notation:

Pr - received powerPt - transmitted powerG - antenna gainλ - wavelengthR - distance to surfaceΓ - reflection coefficientL - loss

P P G D Lr t R= + ? + + +2 ΓG > 0DR < 0Γ < 0L < 0All properties expressed in decibels.

LR

GPP

t

r ?Γ?√↵

?= 22

2

8πλ

Radar physics

A radar level gauge transmits and receives microwave signals at the same time. The strength of thereflected signal depends on many factors. The distance, the antenna size, the atmospheric attenuation andthe properties of the reflecting liquid will have influence on the ratio between transmitted and receivedsignals. When the microwaves are guided in a pipe the distance effect will be smaller compared with freespace propagation.

Picture 5 and 6 describes the mathematical model of the radar conditions in a typical radar level gaugeapplication. The strength of the radar reflection depends of the dielectric properties of the liquid. Liquidpetroleum gas and LNG typically has a very low dielectric constant and provides poor reflectivity. In sucha situation with both weak reflectivity and long distances it is most important that the radar receiver isvery sensitive.

Product Dielectric constant (liquid phase)Butane 1.4Propane 1.6Methane 1.7Diesel 2.0Water 80.0

Picture 7. Real life factors have an effect on radar level gauging. The distance factor is much lesswhen a still pipe is used to guide the microwaves. The still pipe also reduce the effect from thesurface conditions.

Any radar level gauge must have some signal return. The following factorsaffects the strength of that returned signal:

1) Distance attenuation. Long distance gives weaksignal

2) Antenna size. Large antenna gives narrow beamand larger receiving area.

3) Surface conditions. Waves and turbulencereduces the returned energy.

4) Dielectric constant. Oil gives bad reflection.Water gives good.

5) Foam. Can in the worst case eliminate the echo.

6) Vapor and dust. Can influence depending on thefrequency of the radar. At 10GHz it is no problem.

7) Dirt on the antenna.

Technical solutions:

Picture 8. A Radar Tank gauge for LPG/LNG tanks.

Picture 8 describes a Radar Tank Gauge for pressurized tanks used for LPG or LNG storage. Theelectronics and the microwave transceiver are located in the Flameproof Transmitter Head. Themicrowaves are transmitted through a pressure seal and a ball valve into a still pipe. The still pipe reachesall the way to the bottom of the tank. The cone at the top of the pipe provides a smooth transition from the21mm wave-guide in the radar gauge to the still pipe diameter of 100mm. The radar unit receivestemperature information from external RTD sensors. The built in pressure sensor monitors the tankpressure. All measurement data is transmitted to a readout unit (PC or display) through a common fieldbus.

VerificationPin

Picture 9. A TankRadar gauge on a LPG sphere.

To produce commercial precision radar gauges the design of the microwave components, signalprocessing and antennas is of greatest importance. When properly designed, the radar gauges will exceedthe demands for custody transfer accuracy.

Safety is #1 when designing a radar level gauge for gas application. A radar unit must be isolated from thetank atmosphere by a heavy duty, microwave transparent window. An optional ball valve is fittedunderneath the pressure window. The electronics can be removed at any time. Se picture 8.In atmospheric tanks verification of a radar tank gauge is easy to perform. Simply compare the readingswith a calibrated hand tape. In a pressurized tank this is not possible.

With an ingenious design using in-tank references, verification can be made inside the tank while thegauge is in normal operation.By installing a small, ( �1mm) verification pin in the still-pipe (see picture 9) the verification of the radargauge can be done automatically with no interruption of normal level gauging.The pin will produce a small radar echo but let most of the microwaves to pass down to the next pin andthe liquid surface. The reflection from the pin will be compared with the actual pin position verifyinglevel gauging performance inside the active range.

When LPG is stored at ambient temperature the pressure is high which results in high vapor density abovethe liquid surface. High vapor density will have a small effect on the microwave propagation velocity. Forinstance in a butane tank at 20°C the pressure is about 2 Bar. At this condition the vapor influence is0.26%. In worst case this means 26 mm deviation on a 10 meter distance.By measuring the pressure the vapor density is calculated. Knowing the vapor density the radar gauge cancalculate the deviation. The radar gauge has a built in pressure sensor. With vapor compensation the finalaccuracy is within 1-2 mm.

The calculation and correction for vapor influence on the radar wave is simple and basically consists ofusing the viral polynom for determination of amount of hydrocarbon in vapor phase,from pressure, temperature and vapor pressure tables (stored in the gauge). Debey s lawwill transfer the density of hydrocarbons in vapor phase to effect on dielectric constant,I.e. the dielectricity of the vapor phase will adjust the calculation of the velocity of theRadar wave. This method has been used for the last 12 years in radar based level gaugingon LPG tanks, and has gained acceptance worldwide.The API (American Petroleum Institute) standard chapter 3.3 states and describes the suitability for radargauges on LPG storage.

With information of level, temperature and pressure other interesting tank data can be extracted. Since thevolume of the tank is known both the liquid and gas volume can be calculated. Based on this and knowingthe properties of the gas the liquid equivalent of the product in gas phase can be calculated as well.

The LPG radar gauges are installed on spheres and bullet shaped tanks for various LPG products. Seepicture 11, 12 and 13.

Picture 10. Radar tank gauging can be implemented in pressurized tanks of differentshapes like bullet tanks or spheres.

Picture 11 and 12. A TankRadar installation on aLPG sphere in Finland.

Picture 13. A Saab Radar on a LPG sphere in Tahiti.

Radar vs. intrusive systems

Picture 14. A TankRadar LPG gauge replacing a mechanical servo gauge

Most non-radar level gauges have parts of the unit inside the tank and in contact with both liquid andvapors. Sooner or later these parts will require maintenance. A displacer can be fitted outside the tank butthe broken displacer will remain in the tank along with a wire. A capacitance sensor must be removed forrepair. This can only be done when the tank is out of service. Broken gauges mean no level gauging. Thislack of gauging can stretch over a long time period leaving the tank operator to rely on secondary back-upgauging.Radar provides superior gauging availability. If any failure occur the MTTR (Mean Time to Repair) isless than 30 minutes. Any repair is done outside the tank. The built in system with verification pin makesit is possible at basically any time, to verify the gauging performance inside the tank. Normal levelgauging is not interrupted when the verification process is running.

LNG applications

Picture 15. A radar tank gauge on a LNG tank.

LNG level gauging provides interesting challenges. The product is cold -160°C. The gauging distance isoften long, more than 50 meters. Level reading reliability is very important.Radar level gauging offers excellent solutions for LNG. The accuracy is high and the reliability isoutstanding. Radar needs no active parts inside the tank. All serviceable parts are out side. Only a stillpipe is needed inside the tank.Compared with capacitance based systems and mechanical servo gauges, the radar has superior reliabilityand gauging availability. There is virtually no need for maintenance, calibration or repair.

TankRadar gauges has been in successful operation at LNG storage tank farms in Yokohama, Japan since1993.

The main reason for choosing TankRadar as the LNG level gauge in Japan were:• High accuracy — better than 10 mm over the entire measuring range (custody transfer accuracy)• High reliability — There must be no need to open the tank for repair• Long life — the TankRadar shall perform for 50 years• Low cost

The instrument accuracy of the TankRadar is +- 1mm. The MTBF is better than 70 years.The purchasing cost of the TankRadar is considerably lower than alternative solutions (capacitancesystems).

Picture 16. Inside an LNG tank. Looking uptowards the top.

Picture 17. Pipe package including the radar stillpipe. A 100 mm diameter 60 meter long pipe is used.

Picture 18

TankRadar installation on in-ground LNGstorage tank.

Conclusion

There are obvious benefits from using Radar level gauging on liquefied gases. The nature of the storagetanks are such that the tanks are difficult and costly to enter. All gauge parts are be outside the tank.

With proper design of the microwave receiver the weak echo from the liquid surface can be detected evenat long distances. Using the right methods of modulation and signal processing the gauging accuracy bebetter than one millimeter. By measuring the gas properties (pressure or temp) it is easy to correct for anyvapor effect.The Radar can be verified by using simple in-tank references. Verification of the gauge performance canbe done at any time without interfering with the normal gauging.The radar has no moving parts and requires no maintenance. The MTBF is empirically calculated toexceed 70 years in the field.

Tank Radar is built to last a long time providing high accuracy and availability over its entire life.TankRadar has a long track record of proven performance on hundreds of LPG tanks in refineries andterminals world wide.

Radar has also proven to perform well on LNG providing excellent accuracy low cost of ownership.

[ paperGT2000_2.doc, Johan Sandberg ]

Precision Radar Level gaugingon LPG and LNG storage tanks

The Saab TankRadar History

1976Firsttankerdelivery

1980World leaderin levelgaugingfor tankers

1984First onshoredelivery

1996Worldleaderin the oilindustry

1997Two newsystems:LPG/LNG gaugefor tankers andTankRadar Profor the chemicalindustry

1999TankRadarREX forinventory tankgaugingapplications

Broad range of knowledge

Tank gauges with highsensitivity for toughapplications in thechemical industry.

Tank gauging systems forrefineries and tank terminalswith demands for highaccuracy in a wide range ofapplications.

Equipment for the steelindustry, measuring pellets andlevels in blast furnaces.

Measuring of wave height.Saab WaveRadar on oil rigsmeasures up to 30 metreswave height.

Broad knowledge inspecialty radar level gaugingapplications

FinanceQualityEnvironment

QQuality managementsystem, Certified byDet Norske Veritas

Environmental managementsystem, Certified byDet Norske Veritas

Triple AISO 9001ISO 14001

Saab - Certified supplier

For the highest credit rating inDun & Bradstreet s creditsystem

Market Trend:Radar replaces old technology

Old mechanical gauges on LPG:

¥ Moving parts¥ Displacer¥ Intrusive¥ Contact with the liquid¥ Large isolation valve¥ Calibration out side

Mechanicaltechnology

¥ Moving parts¥ Wear & Tear¥ Maintenance¥ Calibration¥ Spare parts¥ Cost

Old Mechanical Gauges

¥ Non contact

¥ No moving parts

¥ No maintenance

¥ High reliability

¥ High accuracy

¥ Long life

micro waveRADAR

Radar Tank Gauging

TankRadar systems

¥ All pressurized tanks¥ Spheres, bullets¥ LPG: Butane, Propane etc¥ LNG @ -160¡C¥ 150 - 600 Psi¥ > 60 m (200 ft) range

Radar for LPG/LNG

Quartz Reference

Microwave module

MetrologicalSeal

Flame proof Housing

DigitalSignal Processor

DigitalField bus

Wave Guide

Tank Connection(antenna)

VerificationPin

Range> 60 m

In-tankverification

Custodytransferaccuracy

¥ Spheres

¥ Bullets

¥ LPG

¥ Condensate

¥ LNG

¥Remotereadout

¥Remotediagnostics

¥Local readout

Install & Forget

Typical LPG installation

¥ Sphere tank

¥ 20 meter range

¥ Butane

¥ 2 - 5 Bar¥ Temperature

inputs

TankRadar for LNG

TankRadar installed on in-groundLNG storage tank

Inside an LNG tankequipped with TankRadar gauge

LNG tank pipe-clusterincluding still pipe for radar level gauging

TankRadar on LPG & LNG

¥ High reliability¥ Long range¥ High accuracy¥ No moving parts¥ No maintenance¥ Long life¥ Low cost

Section 2: Radar tank gauging for marine CTS in LNG applications

Contents:

Brief history of radar tank gauging.................................................................................................................................28LNG application considerations .....................................................................................................................................28Field experience from LNG vessels applications...........................................................................................................29System performance and comparative advantages.........................................................................................................30Acknowledgements .........................................................................................................................................................32

Brief history of radar tank gauging

Since 1975 more than 40.000 radar level gauges have been delivered to tankers and shore based storagetanks. The shore-based applications cover primarily chemical industries, tank terminals and refineries.Radar gauges for storage tanks are available with an accuracy of < ±1 mm and are approved for custodytransfer applications. Over 1500 tankers have radar level gauging systems from Saab Marine Electronicsinstalled and today our market share is 50% for crude -, product - and chemical tanker newbuildings.

First LNG tanker installation/testing took place in August 1997 onboard Khannur, which has since beenoperating on its trade with a Saab TankRadar in tank # 5 certified for CTS.

Full ship installations certified for LNG tanker CTS took place October 1999 with Saab retrofitting"Delta" (ex Southern) for Argent Marine Operations and Autronica retrofitting "Century" for Bergesen.Both ships took their first LNG loading at the end of 1999.

LNG application considerations

Current level gauging systems for gas tankers uses intrusive methods (sensor element in contact withliquid), and so far all installations have been duplicated per tank with these systems, to enablemeasurements while waiting for repair/replacement, which in most cases involves emptying, heating upand gas freeing.

Any competitive marine LNG gauging method (radar) will have to comply to similar installationrequirements, i.e. gauging inside the pipe tower (Moss-type tanks) or along the tripod structure(Membrane-type tanks). Using a still pipe for such radar gauging is incidentally very advantageous, sincethis means no disturbances from tank interiors.

The marine LNG radar gauge is intrinsically safe and has only the stainless steel conical adapter to the still

pipe in the tank vapor. A permanent microwave transparent pressure seal enables the radar electronics tobe fast and safely replaced in case of any failure during operation. Thus, one reliable radar may replacecurrent duplicated sensors.

The temperature measurements are done using a sealed inerted thermowell in each tank with insertion oftypically five steel mantled cable sensors from outside (on dome top). This design has been used by us formore than fifteen years in about 15,000 ship tanks, and is now also used for the cryogenic application.Thus, the temperature measurement installation requires no in-tank cabling and any sensor can be replacedfrom top of the tank anytime.

Field experience from LNG vessels applications

Following successful installations and tests of Saab TankRadar on shore LNG tanks in Japan 1993 to1996, discussions for marine verification test procedure for CTS acceptance continued with NKKK in1997. The procedure was used both for the Khannur single tank installation and for the first FATs (factoryacceptance tests) and deliveries of the first full ship tank installation of Delta and Galeomma (ex. Arzew)in 1999. For LNG/C Khannur, a "Certificate of Accuracy" and a custody transfer approval were given byNKKK, and the ship has been in trade for Osprey since with the system performing and operating to thefull satisfaction of the operator and the crew. The number of cargo transfers using the system as CTS arenow more than 40.

On Khannur tank # 5, the radar still pipe was lowered and installed into a wider pipe for the existing float.On the first full ship installation, LNG/C Delta, which took place in Sept-Oct 1999, the pipes wereinstalled through the existing 6" tank penetration of the removed nitrogen bubbler system and supportedto pump pipes in the tank.

Extensive logs and follow-ups of Khannur operation reveal that any "accurate" comparison with thesecondary (capacitive) system for that ship has not been feasible.

LNG/C Delta, since January 2000 trading regularly between Bonny, Nigeria, to Europe has been followedwith great interest. This ship has certificate for primary CTS for both the Saab TankRadar and theFoxboro CT4 upgraded system. The Logs from Delta comparing Saab radar and the capacitive system hasrevealed no suspicions whatsoever on the excellence of radar performance. A sistership, LNG/C Arzew,now renamed Galeomma has been installed and certified with Saab TankRadar in the same way and wasset into operation in July this year on trade from Oman to US and Europe.

The trading so far has directed these LNG vessels with Saab TankRadar CTS to the following terminalswith full acceptance of the radar gauge CTS reports:Terminal Operator Ships No of transfersDampier, Aus NWS Khannur > 9Arzew, Alg Sonatrach Khannur > 8Everett, Ma, USA Distrigas Khannur > 13Lake Charles, La, USA Trunkline Khannur, Galeomma > 4 + 1Zeebrugge, B Distrigas NV Delta gastrialRas Laffan, Qatar QatarGas Delta, Khannur 1 + 3Pyeong Taek, Korea KoGas Delta 1Montoir, Fr Gas de France Delta 1Bonny, Nigeria NLNG Delta 6Huelva, Sp EnaGas Delta, Khannur 4 + 2Marmara Ereglisi, Tu Tupras Delta 1Qalhat, Oman Oman LNG Galeomma gastrial & 1 loading

System performance and comparative advantages

Technical Data:• Gauging speed 4 tanks/sec.• Ullage gauging range 0 - 45 m

i.e. from 20 mm above bottom to 100% full• Accuracy (sounding) < –5 mm• Temperature gauging -165¡C to +55¡C• Accuracy (temperature) including sensor:

< – 0.2¡C @ -165¡C to -145¡C,< – 0.3¡C @ -145¡C to -80¡C,< – 1.5¡C @ -80¡C to +50¡C

• Pressure measurement 0.8 -1.4 bar(abs)• Accuracy (pressure) < 0,5% (<3 mbar) LNG• Start-up time < 10 sec. per tank• Verification Pin at 2 m radar range (= just above 100% full)

Attenuator on bottom (~ level 20 mm)

Advantages over current LNG gauging techniques:

- Radar technology is proven to highest reliability and lowest maintenance on marine vessels- No equipment inside the tank that can break or degrade in operation.- All gauge electronics are intrinsically safe and sealed off from tank, located on the tank top, and can bereplaced at any time.- One common tank penetration handles level, temperature and pressure.- No costly full size ball valve for still pipe is needed, since the radar head incorporates a certified (smallsize) ball valve in the wave guide between the pressure seal and the conical adapter. This enables thepressure sensor to be verified or replaced with the tank in operation.- Verification of measurement repeatability can be made under full operational conditions, at any level,using the verification pin in the top part of the pipe as the "upper verification point".- At maximum radar range, i.e. bottom level, and at empty occasions (not necessarily gas free), theresidual echo from the flat array attenuator enables verification of the "lower verification point".- By radar gauging, the measuring accuracy for custody transfer calculations of LNG on gas tankers isenhanced to < – 5 mm uncertainty at all levels.

Advantages over competitive radar systems for LNG:

- No pipe joints used for verification.Saab uses a patented verification pin above 100% levelThere is fundamental physics behind the interference between two closely located radar echoes (Methaneliquid level & e.g. the discontinuity of pipe joint) implicating that when the liquid level is in the vicinityof (typically < 200 mm) such a "disturbance" in the pipe, the liquid level estimation cannot be made withfull accuracy. A maximum uncertainty of otherwise 2-5 mm maximum will in these regions more likelybe 20 - 50 mm.Saab has extensive experience from testing discontinuities in still pipes in LNG tanks (since 1993) and theonly way to avoid excessive uncertainties within the tank fill range is to manufacture the pipes & jointswithout any disturbances and locate any "verification means" outside the operation level range.

- Correction for radar propagationSaab determines the correction from actual vapour temperature, pressure and the knowledge ofcomposition and physical properties of the concerned gases. An alternative system solution based upon"continuos measurement verification in real time" of the intermediate pipe joint echoes needs not only

vapour temperature and pressure for correction of propagation velocity, but also relies on the calculationof physical lengths of pipe segments in the vapor phase. It is well known from LNG operation that spotvapor temperatures can differ as much as 30…C from the average. This implies possible great "calibration"errors.

- Lowest measurable levelSaab provides a bottom mounted flat array attenuator (patent pending), which allows for reliablemeasuring to 20 mm level and with full accuracy (+/- 5 mm) down to at least 40 mm level. Thisperformance is independent of tank type.Due to the development work of the low profile attenuator device, its patent application and thedevelopment of how to attach it safely to the two membrane types, only the latest retrofit Galeomma hadthe flat array attenuator installed. For Delta, where the radar still pipe was centered over a knuckle crossin the Mark I type tank, a slightly higher uncertainty in the lower region below 150 mm had to beaccepted.

Use of any other "specially designed reflection plate" (e.g. as is used for LPG tanks) provides seriouslimitation to measure very low levels. It is obvious from such a design that a practical lowest level is atleast 100 mm or higher. Competitive systems using capacitive technology states lowest measurable levelas 35 mm, which is now matched by Saab radar both for LNG and LPG tanks.

Clarification of the need for accurate LNG measurement to at least this low level can be done for twocases:a) GT&T membrane tanks (regardless if this tank has stainless steel or invar membrane):The normal marine operation of these tanks includes emptying to very low levels, well below 100 mm,and still leaving enough liquid volume on the big flat area bottom for the cooling on the ballast trade.Furthermore, the radar still pipe (stainless steel) will shrink abt 100 mm in length from installationtemperature to cryogenic condition. Due to the fact that it is fixed to the tank top, the low end will raisesame distance, and the lowest measurable level would be very much higher up using a "specially designedreflection plate" fixed to the pipe lower end (at about pipe center line crossing the reflection plate).b) Moss Aluminum tanks:The same amount of left liquid for the cooling during ballast trade may in this case possibly mean areduced requirement on measurable lowest level. Probably more than 100-200 mm liquid will be left ineach tank after unloading, but this amount of LNG will be boiled-off and consumed (abt 0,15% volumeper day) and at next loading, there is a (CTS) need for measuring of the level at beginning of loading. Itis very unlikely that a system with a "specially designed reflection plate" fixed to the pipe lower end, willsatisfy this situation, even if taken into account that the pipe low end distance from bottom basicallyremains the same from installation to cryogenic condition (tank & pipe in same material = Al).

- Pipe dimensionThe smaller the diameter (used at same microwave frequency, X-band or 10 GHz, as Saab and others) themore critical are irregularities, such as ventilation holes and pipe joints, for the undisturbed propagationof radar in the pipe. Saab has optimised the pipe size to (inner dia) 107 mm with 10 mm ventilation holeseach half meter and with non-interfering joints for several reasons:a) it avoids the uncertainty of level estimation from the ambiguity caused by several detectablemicrowave modes; eliminating all discontinuities within the measuring range avoids the phenomenon"mode conversion" at e.g. pipe joints. For accurate level gauging the interference between differentpropagation modes is disastrous.b) 107 mm pipe will allow for undisturbed use of big enough (10 mm dia) ventilation holes at 500 mmseparation as well as reasonably specified flanged jointsc) using a wider pipe, the resistive attenuation of the waveguide will be acceptable also at the maximumranges of this application (45 - 50 m)

d) any hydrocarbon wetting (=lining) of the pipe inside, e.g. as thin as 0.01 mm will deteriorate accuracyfar more in a smaller diameter pipe.e) in a significantly over-dimensioned waveguide, i.e. with diameter several times greater than thewavelength (3 cm), the required relative accuracy (abt 0.0001) will be easier met than in a smallerwaveguide with one-mode propagation. The accuracy (here= stability) of pipe diameter and radarfrequency are much less critical for a 107 mm pipe than for a 50 mm pipe.

- Subsequent verifications of level measurementsThe Saab use of a) verification pin above 100% level and b) a bottom mounted specially designed planararray attenuator, means that a subsequent verification can be done a) any time with the verification pin,and b) with sufficient accuracy at empty tank condition using the echo from the planar array attenuator.The use of pipe joint reflections for verification, means practically (unless a pipe joint located above100% is in use) that verification using an "upper verification point" (ref to ISO/DIS 13689.2) may bepossible, but dependent of fill level. With the use of a plain 45 degree reflection plate at the low pipe end,it is not likely that a verification using a "lower verification point" can be done with sufficient accuracyeven at empty tank conditions.

Acknowledgements

We would like to express appreciation to the following companies having encouraged and challenged usin the development and application of radar for liquefied gas gauging: Tokyo Gas, Osprey Maritime(Europe) Ltd, Shell International Trading and Shipping Co Ltd, SIGTTO, NKKK, GT&T, Moss MaritimeA/S and several others that has brought their extensive knowledge and experience to us in order toaccomplish this achievement.A special thanks to Mr Bjarne Thygesen, who gave full support from first step.

1

Saab Marine Electronics AB

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Radar in LNG/LPG gauging

Saab TankRadar¤ on LNG/LPG TanksProvides Safety and Maintenance-free

Operation

¥ Johan Sandberg, Sales Manager, Saab Tank Control Div.¥ Tomas k erstr m, Technical Manager, Marine Division


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Section 2

¥Radar tank gauging for marine CTS in LNG applications

4


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Custody transfer & Marine Productsexperience base

¥ Inventory Tank Gauging for shore tanks¥ Tank Gauging & Cargo Handling for tankers¥ CTS for liquefied gas tankers (LNG)¥ Industrial tank gauging

5


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Saab Marine Electronics world-wide

Marine service Marine sales rep

STC sales & service rep

Head office & manufacturing

6


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Saab Marine Electronics in short

¥ Market leader in tank gauging on shore and marine vessels¥ Turnover: 56 MUSD (1999)¥ 290 employees, of which 200 in Sweden¥ Represented in 82 countries by 90 agents/reps¥ Fully owned by the Saab group, turnover (1999) >2000 MUSD,

~18000 employees


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Shaping the futureShaping the futurein tank gaugingin tank gauging

Vision

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Radar tank gauging in retrospect¥ 1976 First marine vessel system installed by Saab¥ 1980 SUM-21 no 1 level gauge on tankers¥ 1983 Shore tank testing began by Saab¥ 1985 1st shore & 2nd marine gauge generations in use,

and 1st radar competitor on the market¥ 1991 2nd generation shore tank gauge, TRL/2,

also used for LPG tanks & test LNG (1993)¥ 1994 3rd generation marine system, G3, introduced¥ 1996 1st LNG gauge in operation (Tokyo Gas),

and the Marine liquefied gas gauge introduced¥ 1999 3rd generation shore tank gauge, Saab Rex,

and LNG/C Delta in CTS operation with G3 radar

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Saab TankRadar¤ installations

¥ Installed gauges world-wide > 40000� gauges on tankers, all types > 20000� gauges in liquefied gas applications > 1000

� shore tanks LNG gauges 6

� marine LNG gauges 13

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LNG application considerations

¥ Performance -- best in market

¥ Reliability -- no service or calibration in years

¥ Non-intrusive -- no delicate components in the tank

¥ Intrinsically safe -- of course

¥ Verifiable -- with tanks in operation

¥ Repairable -- with certified spares

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TankRadar CTS Gauge unitPressure seal

Ball valve Pressure sensor

Temperature sensors

Thermowell

Protective hose for cable

Electronic box

Antenna cone

Still pipe

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Pipe for inerting the thermowell

Temperature sensors

Thermowell

Tank dome penetration

Protective hose for cable

Electronic box

Antenna cone

Still pipe

Ventilation hole(s)

Verification pin

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TankRadar G3 CTS for Gas tankers

Work Station Printer Work Station Level Unit Radar TankGauge

Redundancy Box I/O Box

Interface toother computer Still pipe and

thermowell

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Bottom attenuator

Patent pending

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Traded terminals, Khannur:(total so far > 40 transfers)Dampier, Aus.Arzew, Alg.Everett, MA, USLake Charles, LA, USRas Laffan, QatarHuelva, Spain

Traded terminals,Delta & Galeomma :(total so far > 20 transfers)Ras Laffan, QatarQalhat, OmanPyeong Taek, KoreaMontoir, FrBonny, NigeriaHuelva, SpainMarmara, Turkey

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System performance

¥ Gauging speed 4 tanks/sec.¥ Types of cargo acc to IMO IGC¥ Accuracy (sounding) –5 mm over gauging range 0 - 45 m¥ Temperature gauging -165¡C to +55¡C

¥ Accuracy (temperature) incl. sensor: – 0.2¡C @ -165¡C to145¡C – 0.3¡C @ -145¡C to -80¡C – 1.5¡C @ -80¡C to +50¡C

¥ Pressure measurement 0.8 -1.4 barabs LNG 1.0 - 8 barabs LPG

¥ Accuracy (pressure) < 0.5% (< 3 mbar LNG)¥ Start-up time < 10 sec. per tank¥ Verification pin at ~2 m range (above 100% level)

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Advantages of Saab TankRadar¤ for LNG - 1¥ Easy retrofit installation using existing pipe/penetrations¥ One common tank penetration for level and temperature¥ Easy installations in Moss & Membrane -type tanks,¥ Measuring very close to bottom, accomplished by planar

array attenuator (patent pending) mounted on bottom¥ Verification/certification at installation easily accomplished¥ Verification easy during operational conditions

� By means of verification pin (patented)� By means of verifiers (test cable lengths 20% and 80% level)� By means of attenuator echo repeatability at empty tank, ~ bottom

¥ Excellent performance on LNG, no disturbance of boiling

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Advantages of Saab TankRadar¤ for LNG - 2

Feature Saab

Lowest measurable level 20 mm

Accuracy over total tank height ± 5 mm over 45 m

In-tank verification points

Level correction method Using gas and tank physical data

Calibration Not needed after installation

In-tank items Only a → 107 mm pipe

Serviceable during operation All parts

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Installation on LNG/C Delta & Galeomma

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Alternative temperature sensor installation

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Factory Acceptance Testing

¥ A Saab TankRadar CTS has been ordered for HHI #1295/96for Shell Nigeria a year ago

¥ The first ship system is completed, assembled and under FATat Saab in Goteborg this week

tankradar on lpg/lng tanks provides safety and … conferences/2000/data...tankradar on lpg/lng...

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