CC BY
269
УДК 528.48: 528.235
Ашраф А. Бешр, Р. Эхигиатор-Иругхе
СГГА, Новосибирск
О.М. Эхигиатор
Нигерия
АНАЛИЗ СТРУКТУРНОЙ ДЕФОРМАЦИИ ЦИЛИНДРИЧЕСКИХ РЕЗЕРВУАРОВ ДЛЯ ХРАНЕНИЯ НЕФТИ С ИСПОЛЬЗОВАНИЕМ ГЕОДЕЗИЧЕСКИХ ИЗМЕРЕНИЙ
Разработка эффективных методов выявления и анализа деформаций вертикальных стальных цилиндрических резервуаров является актуальной задачей, поскольку ее успешное решение вносит важный вклад в обеспечение надежности, долговечности и безопасности эксплуатации резервуаров. Представлена методика выполнения измерений и анализа наблюдений для определения деформаций вертикальных цилиндрических резервуаров.
Ashraf A. Beshr, R.A. Ehigiator-Irughe SSGA, Novosibirsk O.M Ehigiator Nigeria
STRUCTURAL DEFORMATION ANALYSIS OF CYLINDRICAL OIL STORAGE TANK USING GEODETIC OBSERVATIONS
The main purpose of structural deformation monitoring scheme and analysis is to detect any significant movements of the structure. An effective approach is to model the structure by using well-chosen discrete points located on the surface of the structure which, when situated correctly, accurately depict the characteristics of the structure. It can then be said that any movements of those points represent deformations of the object. Large, aboveground oil storage tanks are examples of structures that must be routinely surveyed to monitor their stability and overall integrity.
This paper outlines the procedure of geodetic monitoring system of circular oil storage tanks and presents the analysis of the resulted observations to determine the values of their deformation.
Any movements of the monitoring point locations (and thus deformations of the structure) can be detected by maintaining the same point locations over time and by performing measurements to them at specified time intervals enabling direct point displacement comparisons. A common approach for this method is to place physical targets on each chosen discrete point to which measurements can be made. However, there are certain situations in which monitoring the deformations of a large structure using direct displacement measurements of targeted points is uneconomical, unsafe, inefficient, or simply impossible. Reasons for this limitation vary, but it may be as simple as placement of permanent target prisms on the structure is too difficult or costly.
To obtain correct object point displacements (and thus its deformation), the stability of the reference stations and control points must be ensured [1]. The main
conclusion from the many papers written on this topic states that every measurement made to a monitored object must be connected to stable control points. This is accomplished by creating a reference network of control points surrounding a particular structure.
A relatively methodology for structural data analysis of monitoring structural deformation system of large oil storage tanks is presented in this paper. This method of deformation analysis utilizes a combination of coordinates of several discrete points located on the surface of the tanks and its accuracy for different epochs of observations.
The studied circular oil storage tanks are located in local governmental area of delta state in Nigeria, which forms a common boundary with the Bight of Benin by the Atlantic Ocean. Tanks were constructed between the period 1967 and 1970. Storage tanks, which are used by most oil companies in Nigeria, are cylindrical in shape. There are ten crude oil tanks each one of them has 21m high and diameter 76m.
As a result of tanks age, geological formation of the soil around tanks in this area in Nigeria, non uniform settlement of tanks foundations, loading and off loading of oil and temperature of the crude will cause stress and strain for tanks membrane and settlement of sediments. The tanks tend to undergo radial deformation or out of roundness. So, monitoring the structural deformation of these circular oil storage tanks must be done by using accurate geodetic observations and analysis methods.
To develop a reliable and cost effective monitoring system of any tank of the studied ten storage oil tanks, deformation monitoring scheme consists of measurements made to the monitored object from several monitoring stations that are referred to several reference control points (assumed to be stable). To obtain correct object point displacements (and thus deformation), the stability of the monitoring stations must be ensured. This is accomplished by creating a reference network of monitoring stations surrounding a particular structure (Fig. 1).
Monitoring the structural deformation for one of these tanks is presented in this paper. The circular cross section of the studied tank (tank № 2) is divided into 20 points, as shown in figure 1. These monitoring points are suited at the outer surface of the tank and placed at the same level 2.0 m from the tank base. For each epoch of observations at any time k, the measurements are done at three oil levels 3m, 10 m and 19 m.
The methodology of monitoring structural deformation of oil storage tank utilizes a combination of using intersection process (geodetic surveying with angular and distance measurements) by using total station Sokkia SET1 130R and leveling by using precise automatic level with parallel plate micrometer.
The survey stations (control points and monitoring stations) are located planimetrically by total station traverse while their elevations are determined using precise leveling.
The monitoring stations are connected to the existing control networks at Forcados terminal. However, it is important to state that the monitoring stations, surrounding the studied tank, were first established in 1999 by Geodetic
Positioning Services Limited. All recent control established were referred to the control established in 1999 after confirming their integrity. National Nigerian Geodetic Control specifications for 2nd order deformation study were followed.
B. M. 1
im II
1 -A
//
/
Z*-
B. M. 2 X
B. M. 3
Monitoring points on Occupied stations by tank surface total station
Control points
Figure 1. Structural deformation monitoring system for circular oil storage tank
Calibrations of the used total station were done to confirm the specified accuracy for measuring distances and angles. For carrying out leveling during the process of monitoring, some precautions were done for example conducting a two peg test before levelling; all survey lines were leveled independency in opposite directions; all survey stations are to be change points and back sights and foresights are to be equidistant to within 30cm maximum, sighting distance is 40m.
The horizontal coordinates (X, Y) of monitoring points (points 1, 2, 3... 20, Fig. 1) were calculated by using formulae of intersection in dependence on the angular and linear measurements from total station to these points. For example for point 1 (fig. 1), the coordinates can be calculated by using the following formulae
X^ cot aB + XB cot aA + YA - YB Cot aA + Cot aB
Y =
Y cot
aB + YB cot
aA + Xa
XD
Cot aA + Cot a
where: XA, YA, XB, YB - coordinates of stations A, B; aA, aB - horizontal angles measured from stations A, B.
The elevations of all monitoring 20 points were calculated by using precise leveling. The invar staff is fixed vertically at the monitoring points on the outer surface of the tank. Left and right readings on the staff at each point were taken.
Structural analysis is required to determine whether significant movements are occurred between the monitoring campaigns or not. Geometric modeling is used to analyze spatial horizontal and vertical displacements of the monitoring points on the outer surface of the tank.
Point displacements Aj are calculated by differencing the adjusted coordinates of this point J for the most recent survey campaign (k+1), from the coordinates obtained at reference time (k), as following:
(2)
" X (K+1) - XJK > 'AXJ
II ■-3 < Y( k+1) -YJ K' = AYj
Z ( k+1) - ZJK > AZj
1 vK+\ v K+1 7K+I
where: X, ,Yr ,Zr
- the adjusted coordinates at time tk+1; Xj, Yj , Zj
the
adjusted coordinates at time tk; K=1,2,...,m (m - number of epochs of observations); J=1,2,.,n (n - number of monitoring points (in this case n=20)).
Each movement vector has magnitude and direction expressed as point displacement coordinate differences. These vectors describe the displacement field over a given time interval. Displacements that exceed the amount of movement expected under normal operating conditions will indicate possible abnormal behavior. Comparison of the magnitude of the calculated displacement and its associated survey accuracy indicates whether the reported movement is more likely due to survey error [1].
|Dj |< (Ej)
where: Dj - the magnitude of the displacement for point J, which can be calculated as following:
DJ = V (AXj )2 + (AYJ )2 + (AZJ )2 (3)
But EJ - the maximum dimension of combined 95% confidence ellipse for point J, it can be calculated as following [2]:
Ej = 1.96 ^(m-)2 + (m£ )2 (4)
V ' v AJ
, K+1
where: mA3 - the standard error in position for the most recent survey; mAj
- the standard error in position for the (initial) or reference survey.
Then if |Dj|< Ej - the point isn't moved; else |Dj |> Ej - the point is moved.
So the resulting coordinates of monitoring points must be converted into meaningful engineering values by using the suggested analysis method. Point displacements in horizontal and vertical components are calculated individually by differencing the adjusted coordinates between two epochs of observations (between 2003 and 2004; between 2003 and 2008). The comparison of the magnitude of the calculated displacement and its associated surveying accuracy will be done for both two periods of time (one year (from 2003:2004) and five years (from 2003: 2008)). This comparison indicates whether the reported movement is more likely due to surveying error or not. One of these comparisons is presented in table 1.
Analysis the results in table 1 show that, in this period of time (from February 2003 until August 2004) all the monitoring points are moved in horizontal direction except point (STUD 4) because the difference in horizontal component exceeds the expected surveying error at these points. For vertical direction, all the points are moved except points (STUD 16, STUD 20, STUD 1, STUD 12, STUD 14 and STUD 4).
In the other hand, analysis the results in the period of February 2003 to October 2008 show that all the monitoring points on the tank surface are moved from their positions. In horizontal components the values of deformation ranged from 4.01 mm to 103.94 mm but in the vertical components the deformation values ranged from 0.2 mm to 23.27 mm.
Table 1. Comparison the magnitude of the calculated coordinate differences and its associated surveying accuracy (in the period from February 2003 until August
2004)
Point For horizontal com ponents For vertical component
AX, mm AY, mm 77 horiz. EJ = Movement or not AZj mm ,= mm Movement or not
,jAXj +AYJ mm
l9MmAXj + mAY2 mm
STUD 6 -16 -65 66.94 7.46 Yes 12.03 3.83 Yes
STUD 16 27 20 33.60 10.00 Yes 2.88 4.92 No
STUD 7 -96 -56 111.14 7.20 Yes 30.64 4.29 Yes
STUD 17 14 0 14.00 10.00 Yes 5.00 4.93 Yes
STUD 8 14 -1 14.04 8.29 Yes 30.88 2.11 Yes
STUD 18 24 16 28.84 9.94 Yes 5.54 4.81 Yes
STUD 9 -26 -19 32.20 8.61 Yes 28.45 2.16 Yes
STUD 19 -27 -11 29.15 9.81 Yes 7.33 4.58 Yes
STUD 10 14 -5 14.87 8.87 Yes 24.38 2.67 Yes
STUD 20 31 5 31.40 9.62 Yes 0.96 4.24 No
STUD 11 13 -2 13.15 9.13 Yes 11.52 3.24 Yes
STUD 1 -16 10 18.87 9.40 Yes 0.06 3.81 No
STUD 12 -21 22 30.41 9.38 Yes 11.26 3.76 Yes
STUD 2 46 -5 46.27 9.15 Yes 12.56 3.29 Yes
STUD 13 -15 7 16.55 9.61 Yes 0.32 4.21 No
STUD 3 -20 -16 25.61 8.88 Yes 10.48 2.70 Yes
STUD 14 -14 3 14.32 9.79 Yes 3.48 4.56 No
STUD 4 -1 -3 3.16 8.63 No 1.13 2.20 No
STUD 15 12 16 20.00 9.93 Yes 8.89 4.79 Yes
STUD 5 6 15 16.16 8.34 Yes 4.44 2.05 Yes
Conclusion
Based on the presented analysis, the following results can be summarized:
1. The proposed surveying monitoring technique of large circular oil storage tanks can provide valuable deformation data of the structural members and movements of buildings;
2. The suggested technique of analysis the structural deformation observations can be used to identify and determine the values of deformation for any structure between any two epochs of observations;
3. The studied oil tank (tank № 2) has great deformation values which reach up to 104 mm in horizontal component and 25 mm in vertical, so structural solution must be founded to solve these problems.
References
1) Gairns, C. Development of semi-automated system for structural deformation monitoring using a reflectorless total station/ C. Gairns // M.Sc. Thesis. - Department of Geodesy and Geomatics Engineering - University of New Brunswick, 2008. - 118 pp.
2) Vanatwerp, R. L. Engineering and design: deformation monitoring and control surveying/ R. L. Vanatwerp// Engineer manual. - U.S Army corps of engineering. EM 1110-11004. - Washington. - U.S, 1994. -141 pp.
3) Ashraf, A. Beshr. Accurate surveying measurements for smart structural members/ Ashraf A. Beshr// M.Sc. Thesis. - Mansoura university. - Mansoura. - Egypt, 2004. - 194 pp.
4) Ehigiator-Irughe, R. Environmental safety and monitoring of crude oil storage tanks at the Forcados terminal/ R. Ehigiator-Irughe// M. Eng Thesis.- Department of civil engineering, university of Benin, Benin City. Nigeria. - 2005. - 281 pp.
© Arnpaty A. Eernp, P. Эхигиатор-Hругхе, O.M. Эхигиатор, 2010
