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CALCULATION OF OIL TANK VOLUME AND REPORT GENERATION SYSTEM WITHTRIM AND LIST CORRECTIONS

Ming-Sen Hu and Chia-Rei TaoDepartment of Military Meteorology, Air Force Institute of Technology, Kaohsiung, Taiwan, R.O.C.

E-mail: [email protected]; [email protected]

IMETI 2015 N5007_SCINo. 16-CSME-50, E.I.C. Accession 3936

ABSTRACTThe capacity of ship’s oil tanks is usually designed as a tabled form in order to obtain oil volumes byusing the measured ullage heights. However, the tank walls easily deform or distort due to long-term heavyloading. This phenomenon may cause serious errors that the carrying capacity in oil tanker does not matchwith the values of the tabled form. In this paper, we perform an oil tank volume calibration project that aimsto develop a tank volume calculation and report a generation software with trim and list corrections. Thecurrent internal specification for each tank is measured first, and then all specification data measured canbe input to this software system to calculate each tank’s volume. These calculated results will be verifiedby actual delivery volume tests. This software system has been applied to the Der-Yun Oil Tanker of CPCCorp. The result shows that the overall error of calibrated volume for all tanks is under 0.1%. It is provedthat this system highly improves the correctness of the vessel’s carrying capacity.

Keywords: tank calibration; volume calculation; trim/list correction.

CALCUL DU VOLUME DU RÉSERVOIR D’UN PÉTROLIER ET SYSTÈME DE GÉNÉRATIONDE RAPPORTS DE CORRECTION DE L’ASSIETTE ET DU GÎTE

RÉSUMÉLa capacité du réservoir d’un pétrolier est habituellement calculée à partir d’un format établi pour obtenirle volume de pétrole en utilisant les mesures de hauteur de jaugeage. Toutefois, les parois du réservoir sontfacilement déformées ou distordues par les chargements lourds sur le long terme. Ce phénomène peut causerdes erreurs graves, faisant que la capacité du réservoir ne correspond pas aux valeurs dans le format établi.Nous présentons dans cet article un projet de calibration du volume d’un réservoir de pétrolier dans le butde développer un calcul de volume, et un logiciel de génération de rapports avec correction de l’assietteet du gîte. Pour commencer, pour chaque réservoir, les spécifications internes réelles sont mesurées, etensuite toutes les données de spécifications sont entrées dans le système pour calculer le volume pour chaqueréservoir. Les résultats obtenus seront vérifiés par les tests de volume à la livraison. Ce système est appliquéchez Der-Yun Oil Tanker of CPC Corp. Les résultats obtenus montrent que le taux d’erreur globale de lacalibration de volume pour tous les réservoirs est en-dessous de 0.1%. Il est prouvé que ce système amélioreconsidérablement la justesse du calcul de la capacité de transport d’un pétrolier.

Mots-clés : calibration de réservoir; calcul du volume; correction de l’assiette et du gîte.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016 835

1. INTRODUCTION

An oil tank is primarily used to store and carry petrochemical products. Because of its massive volume, itis difficult to apply direct methods to measure the volume (or mass) of the tank. Generally, it is an essentialprinciple that the measured value of the ullage level combines the temperature and pressure to calculate thevolume. The other method is to measure the sounding and cross reference the volume table of the tank togenerate the corresponding values [1, 4, 5]. However, because of long-term heavy loading, the oil tank wallseasily distort or deform [3, 11]. This phenomenon may cause serious errors that the carrying capacity in theoil tank does not match with the values of the tabled form. The errors affect the oil business transactions,which require accurate volume measurements of the associated products. Therefore, the volume calibrationsof the tanks must be implemented.

In this study, we took the Der-Yun tanker (Chinese Petroleum Corporation, CPC) as an example. TheDer-Yun tanker fits in 12 oil tanks separated on the Port side (left-hand side) and Starboard side (right-handside), and each side has six tanks. Each tank has a serial number (Tank No.) respectively from 1–5P, SLOP-P and 1–5S, SLOP-S etc., as shown in Fig. 1. All tanks are separated by trapezoid metal bulkheads, andequipped with metal sloping plates on their flanks. These devices are used to strengthen the tank structures[2, 9]. Every tank has sounding tubes, suction pipes, inlet pipes, and metal crew ladders, etc. In termsof the calculation of the tank’s volume, first, measure the liquid surface level (UTI ullage) and then crossreference the volume table to convert the corresponding values [2, 4]. However, the oil tank distortion maycause errors on the actual oil volume and the value obtained from volume table (allowance value <0.3%). Itseriously affects the accuracy of the oil tanker’s capacity, and results in the burden of oil volume revision.

The traditional ASTM methods include liquid calibration by meter, calibration by linear measurementand calibration by vessel drawings and so on [6, 12]. The internal electro-optical distance ranging (IEODR)method [8] is a new method proposed recently. The IEODR method is applied to ground-based verticalcylindrical tank’s volume calibration originally, but some studies have noted that it also has a good accuracyon the ship’s oil tank [7]. In this article, we have made a special research project by implementing therevised IEODR method on volume calibration of the Der-Yun tanker. A tank volume computation andreport generation software system with trim and list corrections were developed. First of all, we measurethe current internal specification for each tank, and then input all measured specification data to the softwaresystem to calculate the volume of each tank. These calculated results were verified with the actual deliveryvolume tests. Finally, the new volume tables are generated based on the computation output to control theerror range under 0.1%.

Fig. 1. The vessel oil tank disposition of CPC Der-Yun tanker.

836 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 5, 2016

Fig. 2. The measured specification of no. 3P and 3S cargo tank.

Fig. 3. The procedure of tank volume calibration.

2. TANK VOLUME CALIBRATION PROCESS

In this study, the volume calibration procedure of the Der-Yun tanker shown in Fig. 3 is explained in thefollowing steps:

1. The current specification measurement of tanks: Measure the specification parameters of each tank inall directions by laser theodolite or rangefinder [13, 14]. These parameters include the lengths fromthe right to the left, the lengths from the front to the rear, the height from the top to the bottom, thelength and height of sloping plate, the length and height of each bulkhead, position of inlet and outletpipe and the dimensions of various obstacles inside etc. Figure 2 is the measured specification of no.3P and 3S oil tanks.


Fig. 4. The cross section of liquid level gauging.

2. Build up all measurement data: All of the measured data were classified and input to the softwaresystem for archiving to the file.

3. Tank volume computation: Calculate the volume amount in every millimeter (mm) vertically basedon the structures of the various oil tanks.

4. Delivery oil volume test: Before the oil tanker unloads the oil products in the harbor, we measured theliquid surface level (ullage) of each tank and calculated the corresponding oil volume in the softwaresystem. On the other hand, the liquid volumes through the flow-meter were recorded as referencevalues. Then we compared the calculated volumes with the reference values for all the tanks. If theoverall error is under 0.1%, it indicates that the result meets our requirements. Then go to step (6). Ifthe overall error is above 0.1%, move to step (5) and adjust the tank error parameters.

5. Tank error parameters adjustment: Carry out the tank specification measurement manually. Many ex-isting error parameters must be considered in the volume calculation due to the limitations of manualmeasurement. The major considerations are the error parameters for tank length and width. Theseparameter values may be adjusted by comparing the calculated volume with the flow-meter value ofeach tank. Through the error parameters adjustment, errors between the calculated volume and thereference volume may be reduced.

6. Tank reports generation: There are three tables generated automatically for each tank in the system:the ullage volume table, the trim correction table and the list correction table.

3. CALCULATION OF TANK VOLUMES

The cross-section structure of liquid level gauging is shown in Fig. 4. From cross-section view of Fig. 4,the highest part is near the middle of the oil tank, and then decreases gradually toward both the end sides.LHeight is the height of left sidewall. LBase is the baseline. UM max is the maximum height of measuringullage. Um is the measured ullage. Sh = UM max−Um is the actual sounding height. The actual oil volume


Table 1. The algorithm of portside tank slice volumes in 1 mm thickness.

(a) (b)

Fig. 5. Two ullage tapes gauging construction. (a) Ullage detector hangs vertically and points to the center of theearth; (b) Ullage detector points to tank bottom through a sounding tube.

TankVolumeSh may be obtained from the summation of slice volumes as shown in Eq. (1). Table 1 showsthe calculation algorithm of portside tank slice volume with height ∆h within 1 mm thickness.

The LCDiff refers to the height difference from the left sidewall and the central line, the UCDiff is theheight difference from the gauge point to the central line, the TLength and TWidth respectively are thetank length and width, the TLError(∆h) and TWError(∆h) refer to the error adjusting parameters for tanklength and width with height ∆h, and the SlopePlateVolumeInMM(∆h), ObstacleVolumeInMM(∆h) andBulkheadVolumeInMM(∆h) represent separately the computing function of the height (∆h), 1 mm is theslice volume of the sloping plate, the obstacle slice volume and bulkhead slice volume. Consequently, thetank volume at Um ullage can be obtained from the volume of tank body under height Sh by subtractingthe sloping plates volume, obstacles volume, bulkheads volume, and finally adding the error adjustmentvolumes.


Table 2. The trim correction formula.

4. TRIM AND LIST CORRECTION

When a tanker is not on an even keel at the time of gauging, the vessel’s trim or list correction must betaken into account to accurately determine the oil volumes on board. As far as trim and list correction areconcerned, some recent studies employ a framework of measurement that hangs the ullage detector verticallyin the tank and points directly to the center of earth [5], as shown in Fig. 5(a). Yet this research involved aframework of measurement that inserts a ullage detector into a sounding tube and points to the bottom ofthe tank, as shown in Fig. 5(b). We proposed the trim correction formula and list correction formula to dealwith issues occurring within various ullage ranges.


The trim angle φ of a vessel is defined as the difference of the fore draft mark MF and the aft draft markMB, i.e., φ = MF −MB. It may be divided into two cases: “Case 1: φ < 0” indicates that the aft has alarger draft mark and “Case 2: φ > 0” indicates that the fore has a larger draft mark. Table 2 shows thetrim correction formula for these two cases, where L is the length between fore and aft draft marks andT = |MF −MB|. The wedge region in the formula refers the area where the liquid is only in contact withthree tank walls. It can be divided into upper wedge region and lower wedge region depending on thecalculated UT value in expression (2) or (3). Also if UT > UM max in Case 1, it is in the lower small wedgeregion; or if UT < 0 in Case 2, it is in the upper small wedge region. UT 2 in Table 2 represents the correctedullage for the trim while the gauge falls in wedge regions. Utilize the trim correction formula as shown inTable 2. The value of Um can be revised as UT first, and then the actual sounding height Sh =UM max−UT

can be calculated.The list angle θ of the tank measured by the inclinometer is shown in Table 3. It may be divided into

two cases: “Case 1: θ < 0” indicates the ship tilts to starboard side and “Case 2: θ > 0” indicates the shiptilts to portside. Besides, the gauge points of portside and starboard side are not located at the center of thetanks. They are all near the centerline of the vessel. So we need to define the portside formula and starboardside formula for each case. Table 3 shows the list correction formula for the abovementioned four cases.Similarly, it needs to consider the upper (small) wedge regions and lower (small) wedge regions for eachcase as the trim formula in Table 2. The UL2 in Table 3 represents the corrected ullage for list while thegauge falls in wedge regions. Through the list correction formula in Table 3, the Um can be revised as UL

first, and the actual sounding height Sh =UM max−UL can be calculated.

5. SOFTWARE SYSTEM DEVELOPMENT

The tank volume calculation and report generation software system has been developed in this research. Theinitial screenshot is shown in Fig. 6. The user may start from selecting the desired tank number. Next, inputthe UTI ullage, the trim difference, and the list angle etc. Then press the “Level calibration and volumecalculation” button. In accordance with the input parameters, the system will calculate the corrected heightand the corresponding tank volume automatically.

(a) (b)

Fig. 6. The screenshot of system execution. (a) Trim correction and volume calculation. (b) List correction and volumecalculation.


Table 3. The list correction formula.

For example, Fig. 6(a) demonstrates that when the trim difference is 0.52 M, the ullage height is correctedfrom 1910 to 1926 mm, and the tank volume is calculated as 769.263 M3. Figure 6(b) shows that when thelist angle is 1.5 degree and tilted to portside, the ullage height is corrected from 2480 to 2400 mm, and thetank volume is calculated as 744.013 M3. The rest of page frames in the execution screen may allow users toarchive various report files, to print the tank volume tables, trim correction tables, and list correction tables,and to output the wedge region volume tables.


Table 4. The calibration coefficient table of the Der-Yun tanker (test date: 12 December 2014).Tank UTI Ullage Trim Diff. List Corrected Tank PD Difference Error (%)No. (mm) (m) Angle Height (mm) Volume (M3) Meter (M3) KLS1P 2390 1.27 0 2420 519.421 1044.343 0.378 0.0361S 2330 1.27 0 2360 525.32P 1900 0.52 0 1916 766.378 1534.316 1.325 0.0862S 1910 0.52 0 1926 769.2633P 2480 0 1.5 2400 744.013 1481.013 1.061 0.0723S 2520 0 1.5 2440 738.0614P 2020 0.9 0 2050 778.079 1565.998 –0.867 –0.0554S 1980 0.9 0 2010 787.0525P 1970 2.03 0 2023 585.224 1167.627 1.129 0.0975S 1970 2.03 0 2022 583.532

Total 6796.323 6793.297 3.026 0.045

6. VOLUME CALIBRATION RESULT ANALYSIS

In this research, the tank volume tests and calibrations on the Der-Yun tanker were carried out for 16 timesfrom July 2014 to February 2015. When the Der-Yun tanker stayed in the port, we measured the ullage of10 oil tanks on board from no. 1P to 5S, and recorded the tank volumes through a flowmeter (as referencevolumes). The SLOP-P and SLOP-S tanks were excluded because their capacities were small enough tobe ignored. The tank volumes were able to be calculated according to the measured ullage in the softwaresystem. Later, we compared the errors of the calculated volumes and their cross-reference volumes. If theoverall error of these ten tanks is under 0.1%, that indicated that the calculated results meet the requirementsof the study. If the overall error is above 0.1%, we have to adjust the tank error parameters, and thenrecalculate the tank volumes until the results conform to the project requirements, The results are shown inthe procedure flow of Fig. 3.

Table 4 represents the calibration coefficients of the Der-Yun tanker volumes test on December 14, 2014.As the table shows here, the Corrected height column was obtained from the UTI ullage column with trimcorrection and list correction. The Tank Volume column was calculated on the basis of the corrected height.The PD Meter field is the total flow-meter volume of portside and starboard side tanks. The Differencecolumn is the difference (error) of the sum of portside and starboard side tank volumes minus the PD metervolume. The Error (%) column is the error percentage of the above difference. The following is an instanceof volume calibration. The sum of no. 1P and 1S tank volumes is 1044.721 (= 519.421+525.3) M3. Thedifference between the sum volume and the flow-meter volume is 0.378 (= 1044.721−1044.343) M3. Theerror percentage is 0.036% (= 0.378/1044.373). The overall error is 3.026 (= 6796.323− 6793.297) M3

and its error percentage is 0.045% (= 3.026/6793.297). This overall error percentage is under 0.1%. Theresult agrees well with the project requirement.

After the calculations in the Der-Yun oil tanker was undertaken, the tank volume tests and calibrationworks for 16 times, and the volume tables of all tanks were reestablished. In order to analyze the volumecalibration performance in this project, statistical comparison of the total errors between the new tables andcurrent tables is shown in Table 5.

Table 5 shows the statistics of overall errors in tank volume tests done 16 times. The “Cur. Table Volume”refers to the total volume of the 10 oil tanks from the current volumes based on each tank’s Um (marked asA), “New Table Volume” refers to total volume of the 10 oil tanks from the newer volume table basedon each tank’s Um (indication is B), “PD Meter Volume” is the total volume of the 10 oil tanks throughthe flowmeter (indication is C), “Cur. Diff.” refers to A-C, “Cur. Error (%)” is the current total error


Table 5. The statistical comparison of the overall tank volume errors.No. Test Cur. Table New Table PD Meter Cur. Diff. New Diff. Cur. Error New Error

Date Volume Volume Volume (M3) (M3) (%) (A – C)/ (%) (B – C)/(M3) (A) (M3) (B) (M3) (C) (A – C) (B – C) C × 100% C × 100%

1 2014/7/27 6650.59 6666.006 6665.6 –15.01 0.406 –0.225 0.0062 2014/8/3 6863.84 6881.903 6880.9 –17.06 1.003 –0.248 0.0153 2014/8/24 6816.74 6839.819 6840 –23.26 –0.181 –0.340 –0.0034 2014/10/16 5982.37 5999.996 6000 –17.63 –0.004 –0.294 0.0005 2014/11/4 6622.53 6638.969 6637.111 –14.581 1.858 –0.220 0.0286 2014/11/9 6836.91 6856.903 6857.16 –20.25 –0.257 –0.295 –0.0047 2014/11/15 6846.66 6865.662 6861.34 –14.68 4.322 –0.214 0.0638 2014/11/19 6724.41 6740.558 6742.81 –18.4 –2.252 –0.273 –0.0339 2014/11/26 5989.68 6002.991 5999.54 –9.86 3.451 –0.164 0.058

10 2014/11/28 5082.62 5092.476 5088.02 –5.4 4.456 –0.106 0.08811 2014/12/5 6008.71 6021.317 6020.31 –11.6 1.007 –0.193 0.01712 2014/12/11 6838.44 6856.087 6847.5 –9.06 3.171 –0.132 0.04613 2014/12/14 6781.21 6796.323 6793.297 –12.087 3.026 –0.178 0.04514 2015/2/3(1) 1972.14 1957.524 1957.4 14.74 0.124 0.753 0.00615 2015/2/3(2) 4453.97 4456.307 4456.6 –2.63 –0.293 –0.059 –0.00716 2015/2/3(3) 6845.48 6863.96 6867.4 –21.92 –3.44 –0.319 –0.050

Fig. 7. The analysis curves of the overall volume errors. (a) Analysis curves of (A – C) and (B – C). (b) Analysiscurves of |Cur. Error (%)| and |New Error (%)|.

percentage ((A – C)/C×100%); “New Diff.” refers to B-C, “New Error (%)” is the new total error percentage((B – C)/C×100%). From the statistical values, they show that the overall volume obtained from presenttables is close to the the actual flow-meter overall volume. Also its overall error percentage is already closeto 0.3%. Three test results had surpassed 0.3%. Thus it is understood that the present volume tables wereerroneous. The overall errors of the new table (B – C) are less than the overall errors of the present table(A – C). The overall error percentages of the new tables are under 0.1%. The result is in keeping with thedemand of the project.

Figure 7(a) shows the analysis curves of the current overall volume error (A – C) and new overall volumeerror (B – C). For the most part, the “A – C” curve is below the zero level, the absolute mean value of|A – C| is 16.339 M3, but the “B – C” curve is very close to the zero level, its absolute mean value is1.744 M3. Figure 7(b) shows the analysis curves of the absolute values of current volume error percentage|Cur. Error(%)| = |(A – C)/C × 100%| and new volume error percentage |New Error(%)| = |(B – C)/C ×100%|. The distribution of these two curves is similar to Fig. 7(a). The mean value of |(A – C)/C × 100%|


is about 0.25%, and the mean value of |(B – C)/C × 100%| is about 0.03%. We compare these two meanvalues; the latter value has been reduced to one-eighth of the former value (0.25% : 0.03% = 8.3 : 1).

7. CONCLUSIONS

The purpose of this study is to solve the problems of the oil tank volume table error by developing a softwaresystem in a research project. A modified IEDOR method has been successfully applied in this research. Thevalidity of the software system has been verified for 16 times by comparing with the actual volumes carriedby the Der-Yun oil tanker (CPC). The corrections have been made by adjusting volume error parameters aftereach verification operation. The goal of this step is to achieve the volume calculation error to 1/1000. Usingthe modified IEDOR method, higher volume calibration accuracy is obtained than in the other methods byadjusting tank error parameters iteratively. Finally, the new volume tables, new trim correction tables andnew list correction tables for each tank have been created. On the other hand, we found that the mean errorpercentage of new volume tables can reach 0.027% after 16 verifications. It is only one-tenth of the existingmean error percentage (0.291%).

REFERENCES

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