subsidence monitoring

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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 1 (1): 48-51

48

Geospatial Settlement Monitoring of Above Oil Storage Tank

1R. Ehigiator Irughe,

2Jacob Odeh Ehiorobo

and

O.M. Ehigiator

3

1Siberian State Academy of Geodesy, Novosibirsk, Russia.

2

Faculty of Engineering, University of Benin, Faculty of Engineering, Benin City.3Faculty of Basic Science, Benson- Idahosa University, Benin City, Nigeria.

Corresponding Author:R. Ehigiator Irughe

______________________________________________________________________________________________

Abstract

Tanks used for storing crude oil in tank farms are usually cylindrical and made up of concrete foundation As a result of

age, loading and off loading of crude oil from the tanks, geological condition of the site and other environmental factors,

the foundation upon which these tanks are constructed are subjected to movement particularly settlement at the tankfoundation. The safety of the tanks becomes of paramount importance as any catastrophic failure will adversely affect

the environment. At the Forcados Tank Farm, there are ten tanks currently used for crude oil storage. In this study, onlytank 209 was used as case study scenario for the Subsidence monitoring of the tanks foundation under loading. In order

to monitor these tanks, studs were attached to the base of the tank at equal intervals. Measurement for subsidence was

carried out on the studs position from primary Geodetic controls located within the tank farm in 2000, 2003 and 2004

respectively. The result of the study revealed that maximum subsidence of 20.39mm occurred at stud 12 and minimum of5.49mm occurred in stud 1 for tank 209. A conclusion that the tank foundation is not stable.

_______________________________________________________________________________________________

Keywords: tank farm, bundwall, subsidence geodetic monitoring

______________________________________________________________________________________________

ITRODUCTIOThe area features meandering Creeks and mangrove

swamp Fig. I. The annual rainfall in this area ranges from

3,000mm to 3,900mm. Temperature ranges from

230c around July to between 350c around February and

March. Tanks at the Forcados farm were constructed

between 1967 and 1970. There are ten crude oil tanks

each 21m high and diameter 76.2m (Ehigiator,

2005). Others are two emulsion tanks, and continuoushydration tanks. Storage tanks used by most oil

companies in Nigeria are cylindrical in shape, Oil

production from Delta, Edo and Bayelsa State are

delivered at the terminal and processed before being

exported.

Figure. 1: Map showing location of Forcados terminal by

the Bight of Benin, Nigeria

Forcados Tank Farm

The aim of any monitoring project is to determine if there

is any movement taking place in the structure. There are

ten storage tanks at the Forcados terminal whose normal

diameter is each 76.2m and height 21m with storage

capacity of 500,000 bbl. (Ehigiator, 2005)

As a result of age, geological formation of the tank farm

site, non uniform settlement of tank foundation, loadingand off loading, temperature of the crude leading to stress

and strain on tanks membrane, primary and secondary

settlement of sediments, the tanks tend to undergo radial

deformation or out of roundness.

In addition shell out of roundness may result in gaps

between the tanks shell and subsequent hydrocarbon

emission into the environment. As a safety measure,

each tank is surrounded by a bund wall which measures

150m and at a height of about 12m.

The aim of this large space is for the wall to

accommodate oil spill in case of any displacement

resulting from earth movements or spill from the tank.

It is to be noted that certain terrain types are not at rest,

but are slowly moving due to primary formation of

sediment at the AKATA Formation (the marine

environment where oil and gas are formed) and

secondary settlement of sediment at the AGBADA

formation (High temperature environment where oil and

gas are trapped). As a result of the imperceptible but


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existent movements, the relative locations of points that

are in such areas also change (Ehigiator, 2005)

Monitoring of the tanks foundation will help in

identifying and quantifying deterioration which may be

caused by applied load, from various sources such as

physical, chemical, climatic condition, soil types and

proximity to the sea where the ground water level is very

low (Ehigiator, 2010). The determination of settlement

around the foundation is of importance especially when

the rate of settlement is excessive or uneven (Ehigiator,

2010). For the subsidence monitoring precise levelling

method is used (Ehiorobo, 2000) have shown that precise

levelling method is much more accurate for evaluating

vertical movement than for instance GPS method.

Structural Monitoring Of Crude Oil Storage TankMeasurement of vertical movements was carried out by

Periodic levelling using Geodetic levelling instrument

with telescopic staves by 3 wires method at three

different oil levels, that is, at 3m, 10m and 19m

respectively. The tanks under study were monitored foryear 200, 2003 and 2004. The Geodetic monitoring

system consisted of a primary geodetic control network

established in 1990 by an indigenous Survey company,

Geodetic Positioning Services limited.

In 2002 a more dense control network was established

around the tanks for monitoring for subsidence under

hydrostatic pressure within the tank farm, tank verticality,

ovality and roof gap measurements were also based on

the control points. For the monitoring purposes, studs

were attached to the base of the tanks at equal interval.

Additionally, three monitoring points were established

around each tank in addition to the other bench marks

within the bond walls. This arrangement is to allow for

the detection of any movement of points within the bond

wall and at a reasonable distance from the tank. Levelling

to the studs were carried out in the mornings and evening

periods of the day. This is to allow for the elimination of

midday heat effects of the sun which is likely to cause

uneven expansion of the tanks. Using the established

geodetic control points, repeated levels and other

measurements were carried out in 2000, 2003 and 2004

for the tanks.

RESULT AALYSIS

LEVEL NET Software was used in the processing of thelevel network for each of the tanks monitored from epoch

to epoch. The results for tank settlement monitoring for

tanks 209 for year 2000, 2003 and 2004 at three-oil level

are presented in the figures below.

3.84

3.85

3.86

3.87

3.88

3.89

3.9

3.91

3.92

3.93

3.94

2004 TANK 9 SUBSIDENCE

OIL LEVEL: 3.543m

OIL LEVEL: 10.043m

OIL LEVEL: 19.337m

Figure 1. The results for tank settlement monitoring for tanks 209 for year 2004 at three-oil level


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OIL L EVE L : 3 .0 EVEL : 1 0 . EVE L : 18 .3 3 1 m

S T UD 1 3 . 91 07 5 3 .8 98 75 3 .8 87 25

S T UD 2 3 . 91 62 7 3 .9 05 09 3 .8 92 61

S T UD 3 3 . 91 02 5 3 .8 99 38 3 .8 87 59

S T UD 4 3 . 92 15 2 3 .9 00 71 3 .8 98 22

S T UD 5 3 . 91 03 8 3 .8 89 91 3 .8 87 41

S T UD 6 3 . 90 41 3 3 .8 83 97 3 .8 81 09

S T UD 7 3 . 89 82 7 3 .8 80 01 3 .8 77 49

S T UD 8 3 . 91 06 3 3 .8 91 68 3 .8 86 64

S T UD 9 3 . 91 82 7 3 .8 97 15 3 .8 94 88

S T UD 1 0 3 . 92 01 0 3 .8 99 85 3 .8 93 63

S T UD 1 1 3 . 92 53 2 3 .9 04 25 3 .8 99 11

S T UD 1 2 3 . 91 51 3 3 .9 00 55 3 .8 96 95

S T UD 1 3 3 . 92 36 5 3 .9 09 11 3 .8 97 26

S T UD 1 4 3 . 93 36 5 3 .9 19 44 3 .9 05 91

S T UD 1 5 3 . 92 63 5 3 .9 13 42 3 .9 02 10

S T UD 1 6 3 . 91 84 6 3 .9 05 83 3 .8 93 88

S T UD 1 3 . 91 07 5 3 .8 98 75 3 .8 87 25 3.84000

3.85000

3.86000

3.87000

3.88000

3.89000

3.90000

3.91000

3.92000

3.93000

3.94000

2 0 0 3 TA N K 9 S U B S I D E N C E

Figure 2. The results for tank settlement monitoring for tanks 209 for year 2003 at three-oil level

S T U D 1 S T U D 2 S T U D 3 S T U D 4 S T U D 5 S T U D 6 S T U D 7 S T U D 8 S T U D 9 S T U D 1 0 S T U D 1 1 S T U D 1 2 S T U D 1 3

O / L: 3. 08 6 3 91 7. 6 3 92 2. 85 3 91 6. 57 3 93 0. 55 3 91 9. 98 3 91 3. 07 3 90 7. 36 3 92 2. 05 3 92 6. 21 3 92 7. 25 3 93 1. 38 3 91 9. 77 3 92 7. 33

O / L: 11 .5 80 3 90 3. 81 3 90 8. 9 3 90 3. 9 3 91 6. 66 3 90 5. 69 3 89 5. 6 3 89 4. 29 3 91 0. 59 3 91 5. 42 3 91 6. 7 3 92 0. 17 3 90 7. 7 3 91 6. 69

O / L: 20 .2 26 3 88 7. 56 3 89 1. 75 3 88 6. 91 3 90 1. 01 3 89 0. 53 3 88 4. 09 3 88 2. 37 3 89 0. 55 3 89 8. 25 3 89 8. 02 3 90 3. 56 3 88 8. 22 3 89 7. 89

A p p e n d i x 7 A

3 8 5 0

3 8 6 0

3 8 7 0

3 8 8 0

3 8 9 0

3 9 0 0

3 9 1 0

3 9 2 0

3 9 3 0

3 9 4 0

3 9 5 0

2 0 0 0 T A N K 9 S U B S ID E N C E

Figure. 3 The results for tank settlement monitoring for tanks 209 for year 2000 at three-oil level

DISCUSSIOS AD RESULTS

The results from subsidence monitoring for Tank 209 in

2000, 2003 and 2004 show that the largest settlement

occurred at stud 12 between the period with a value of

14.57mm between 2000 and 2003 and 16.43mm between

2003 and 2004. The lowest subsidence occurred at Stud 1

between 2003 and 2004 with a value of 5.39mm. In all

areas the rate of subsidence was fairly evenly spread

around the tank for the period of investigations.

There appear to be an uneven movement of the ground

below this tank. The results are however within an

acceptable limit hence the tank cannot be said to be at

risk. Further investigations are however ongoing to

access the stability of the ground below this tank.

COCLUSIOS

Monitoring of tanks and tanks foundation helps in

identifying and quantifying deteriorations which may

lead to tank failure. The history of tank disaster

throughout the world reveals that problems often arise

undetected due to inaccurate evaluation of foundation

defects. For any tank monitoring programme to be

effective, the equipment used for the monitoring must

be precise and of the highest quality. The monitoring

personnel must be experienced in not only data capture

but also the analysis of the acquired data.

In carrying out monitoring for subsidence at the tank base

at the forcado tank farm, any noticeable difference or

settlement in excess of 5cm means that such a tank

should be put out of use. Measurements carried out for


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tank 209 however reveal no abnormal ground subsidence

at the base of the tanks.

REFERECES

Ehigiator, Irugbe R. 2005. Environmental safety and

monitoring of crude oil storage tanks at the Forcados

terminal. M. Eng Thesis, Department of Civil

Engineering, University of Benin, Benin City.

Ehigiator et al., 2010. Structural deformation analysis of

cylindrical oil storage tank using geodetic observations.

Geo -Siberia International Exhibition and scientific

conference, Novosibrisk Russia Federation. 15-22.

Ehigiator et al., 2010. Determination of the ovality of

crude oil storage tanks using least square. Advanced

materials research. A TransTech series, Switzerland. 34-

40.

Ehiorobo O. J. 2000. 3D Control Baseline forDeformation studies of the Ikpoba Dam Journal of

Engineering Sciences and Application Vol. 2. 68 87.

Ehiorobo O. J. 2000. Evaluation of Geodetic heights for

Monitoring for subsidence at the Ikpoba River Dam

.Journal of Engineering Sciences and Application Vol. 4.

8 21.

Kassar M and Becker J.M. 1999. Error source in high

precision levelling How to minimize their effects on

heights, FIG Commission 5, March 15-17 Gavle Sweden

subsidence monitoring

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