African Journal of
Environmental Science and Technology

  • Abbreviation: Afr. J. Environ. Sci. Technol.
  • Language: English
  • ISSN: 1996-0786
  • DOI: 10.5897/AJEST
  • Start Year: 2007
  • Published Articles: 991

Full Length Research Paper

An appraisal of high definition survey approaches in subsidence monitoring of crude oil storage tanks

Hart Lawrence
  • Hart Lawrence
  • Department of Surveying and Geomatics, Rivers State University, Port Harcourt, Rivers State Nigeria.
  • Google Scholar
Udeh Kenneth
  • Udeh Kenneth
  • Department of Surveying and Geomatics, Rivers State University, Port Harcourt, Rivers State Nigeria.
  • Google Scholar

  •  Received: 11 January 2020
  •  Accepted: 06 August 2020
  •  Published: 30 September 2020


Vertical and horizontal motion of the solid earth surface due to geological and geodynamical phenomena are generally consequences of terrain deformation. Variations as a result of the deformation affect structures, particularly large storage facilities that are steel and/or concrete in nature. The criticality of these phenomena underscores the need to carryout regular measurements and monitoring of its effect in all dimension. Over the years, methods and instrumentation for subsidence monitoring has evolved from the conventional to the recently high definition surveying approach with an increasing need to detect and analyse deformation changes with clarity at any given time. The aim of this work was to appraise the high definition surveying approaches in subsidence dynamics of crude oil storage facilities. Classical instrumentations and the terrestrial laser scanning equipment were deployed based on the principles of geodetic positioning and mapping for overlity, verticality and radial displacement parameters determination using the two approaches. This paper will provide spatial information in terms of point clouds, 2D and 3D models, vertical and radial displacement of the crude oil storage facility. The work will further demonstrate the optimal capability of high definition surveying approach in our quest to constantly manage the complexities associated with subsidence. Several crude oil storage facilities in addition to other private storage facilities all over the country, require a policy to ensure regular monitoring and analysis of these facilities especially with the trend of earth tremors being experienced in parts of the country.


Key words: Subsidence, high definition surveying, radial displacement, crude oil facility, geodetic positioning.


The advent of Terrestrial Laser Scanner (TLS) also referred to as High Definition Survey (HDS) with its current advancement in speed, has become a very useful survey equipment for various earth or near-earth based features that may be natural  and/or  man-made  such  as as-built surveys, 3-dimensional (3D) spatial point cloud generation, archaeological site mapping and modelling, terrain characteristics and associated changes (Hart et al., 2018).
Meanwhile   before   the   advent   of   these  new  HDS measurement, scientist and most geoscientists use the total station which basically is the improvement based on combination of the conventional theodolites and electronic distance measurement equipment (EDM). This compact equipment with in-built software enhanced the measurement capability of both angular and linear dimension of earth-based measurements. The use of total station for mapping and other surveys like deformation is still very relevant in today's world and can be used for subsidence monitoring especially on large storage crude oil tank farm.
The solid earth is under some form of stress and strain which results to ground movements. They can be attributed to natural processes or to some anthropogenic activities on the earth or near-earth surfaces. These movements can be slow or rapid depending on the magnitude of the force and/or load exerted on the solid earth though soft and compressible in most cases, it can also be as a result of the fluctuations associated with ground water and other geodynamical phenomena, (Crosetto et al., 2005). This development is synonymous to the Niger Delta Area of Nigeria where there are increase of oil and gas extraction on a daily basis in addition to the myriads of oil storage facilities spread within the area both by government and private holdings.
Large vertical cylindrical steel tanks widely used for crude oil storage generally consist of a thin bottom plate cylindrical shell and fixed or floating roof. These large tanks are susceptible to various types of settlement. The settlement components could be uniform settlement planer tilt or differential settlement. The uniform settlement and planer tilt causes rigid body deformation or rotation of the tank. Minimal differential settlement under the tank wall can induce large distortions along the tank top and high stresses at the tank base or in the top wind girder (Hart, et al, 2019). This work would showcase a classical case of the deployment of the TLS and conventional survey method in the monitoring of standards steel crude oil tank in the Niger-Delta area. The need for subsidence measurement and monitoring stems from the fact that slow movements related to load of structures on the solid earth has the potential of long-term damage and risk (Okeke, 2005). The aim of this work was to appraise the High Definition Survey approaches in the subsidence monitoring of crude oil storage. The objectives of the study were: 1) to determine the verticality of a crude oil tank using a 3-D model derived from laser scanning and 2) to determine overlity of a crude oil tank using a 3-D digital model derived from laser scanning output. The determination of the overlity, verticality and subsidence using conventional total station and digital level will provide the basis for the comparative analysis of the two approaches. The difference is that instead of measuring 'discrete points', the laser scanner measures 50000 points per second and with a point spacing of 0.1 at 100 m. This results in a very dense 'cloud'   of   data  points,  each  of  which  is  to  the  same accuracy as those measured by Total Station.


Study area description
The study area is located in the Bonny Island situated in the southern edge of River State in the Niger Delta of Nigeria near Port Harcourt which most appropriately is defined by the following coordinates in WGS84, 4° 26” 18.81 N,7° 9” 39.3E; 4° 25” 30.77 N,7° 10” 47.93 E; 4° 24” 39.45 N,7° 9” 51.26 E; 4° 25” 4.96 N,7° 8” 49.39 E. The study area as depicted in Figure 1 houses several crude oil storage facilities in varying shapes and sizes besides other oil and gas facilities.
Scope of the work
The scope of this work involves the deployment of contemporary laser scan equipment for scanning of crude oil tank 18 and show the processes involved from the in-situ checks to modelling. This is in addition to the conventional method (that is use of total station and level equipment) for subsidence monitoring of the same crude oil tank. The outputs of the two systems were reviewed both spatially and in 3-D for the laser scan, in order to generate the verticality, overlity and subsidence displacement from the two systems spatial and model output.
The principles of light detection and ranging (Lidar) operation in typical mapping operations
The principles of LIDAR are quite simple; in explaining these phenomena scientifically, a LIDAR instrument emits a rapid pulse of laser lights at a surface some at 150000 pulses per second. The constant speed of the laser light is known hence the LIDAR instrument can calculate the distance between the source and the target with high accuracy. The distance or range can be computed as follows (Okeke and Moka, 2004; Okeke, 2005; Casu et al., 2016):
The principles of the methodology will stem from traversing and differential levelling (that is Classical) and high definition survey methods based on laser scanning (that is modern). The fundamental principles of controls were deployed which include the establishment of six (6) GPS points within the Tank farm. This was followed by the extension of the control points to all the nine (9) tanks to be  monitored with the aid of a  Total  Station  and  a  Digital level instrument however, crude oil storage tank 18 was reviewed in this research. This process indicates deviations axially horizontally and vertically over a period of time in the event of loading or changes in the earth crust (Hart et al., 2018). The tank settlement surveys: subsidence, ovality and verticality were all based on the extension control points. The reference stations were set at BONGPS6. Position was then translocated to GPS-CP1 to CP6. The results of the in-situ process for control check as shown in Table 1 shows that the differences are within allowable limits for the use of the control stations.
Conventional approach-field procedure
Traversing was carried out using LEICA Total Station and was done in loops. This comprises three loops on the tank at three levels of the oil contents: low, middle and full levels. The traverse was based on the GPS points: GPSCP1, GPSCP2, GPSCP3, GPSCP4, GPSCP5 and GPSCP 6: and GPS extension controls around each tank. The tanks were marked 5m off the ground base at equal intervals. With the use of reflectorless total station the marked points at the base were bisected and tracked and its corresponding top of the tank was also tracked. Similarly, differential levelling was carried out using digital level which reads to four decimal places. The level was based on GPS pillars: BONGPS04 and BONGPS05 that proved in-situ and was extended to the tank site. The level was carried out on the already existing studs around the tanks. The level was done in two (2) loops-morning and evening at each oil level content of the tanks. That gave a total of six loops  for  each  of  the nine tanks and a ground total of 54 loops for the nine tanks. Adjustments of the traversing and levelling data were classical carried out to guide the processing and analysis. 
High definition survey approach  
The Leica Scan station C10 was deployed for this research. The crude oil tank was scanned from six vantage positions to cover and create an overlap as shown in Figure 2. A total of six scanworld was generated. According to Ezeomedo et al. (2017), they described a scan world as a single scan or collection of scans which are aligned to a common coordinate system. It also contains control spaces and model spaces. The control spaces usually carry the constraint information that is used for the registration of multiple scans. Also, the model space contains information from the database that has been modelled (Hirt, 2015). During the scanning, a combination of the different scanning methods like the known back station, the resection method and the intersection methods were used depending on the setup method most suitable at the station. The data output from the exercise which is known as the point cloud are very dense such that current scanners can collect anywhere between 200 and 10000 points per second (Francis et al., 2018).


The  vertical   check   in  Table  2  using  the  total  station showed a maximum deviation of 17’ 11” on stud 1 with minimum deviation of 3’ 26” on studs 12 and 17. Similarly, the mean radial displacement as shown in Table 3 ranges from 2 to 130 mm and graphically demonstrated in Figure 3. In another vein, Table 4 describes the mean deviation of load variation viz. 2.4000, 10.7000 and 15.5000 m of tank level.
Figure 4, depicts the axis of a cylinder that was fitted through the point cloud and the statistics showing the fit quality can be seen in the fit  quality Table 5. The  axis  of the cylinder when empty is shown in the image.
Table 6 describes the statistical information on the variability of the various levels of oil in the storage tank 18 as deployed by the scan models as shown in Figure 5 indicating the deviation as a function of the variation of the content level in tank 18. In the same vein, Figure 6 highlights the combination of the points of measurement using the classical (total station) and laser scan (high definition) techniques. The differentiation is in the colouration and markings.
Radial displacement
Figure 7 shows a comparison of the radial displacement at each stud as measured by the two survey instruments. As can be seen, there is a high degree of correlation. The average difference as measured by the two technologies was 6 mm. Figure 8 demonstrates the Radial Displacement Plots for every degree change, as derived from laser scan measurements and total station instrumentation.
The current methodology for computing the Tanks verticality using a Total station is to calculate  the  vertical
angle at 4 points (North-South and East-West). Laser scanning allows us to use the millions of surveyed points on the tank wall as shown in Figure 9, which we can 'best fit' a cylinder through. The axis of the cylinder indicates the overall tank verticality.
Tilt and subsidence
The mean of the level of the studs is done and then the deviation to the mean is computed for the three states (empty, mid, full). With Laser scanning, we take a strip of points around the base of the tank (which is concrete). This shows if the levels are changing, which would show subsidence. It also allows to see if the ground is tilting and   in   which direction. These  changes  are  shown  in Figure 10.


Laser scanning  facilitates  a  comprehensive  analysis  of storage tanks. Scanning allows us to monitor the entire tank shell for deformation rather than by the conventional surveying approach with a limited number of discrete points. Traditionally, storage tank survey is carried out using a total station or a simple measuring tape. While these    techniques     provide    the    necessary   position information, they are generally time intensive when multiple measurements are required. When using a measuring tape, measurements are prone to errors. 3D scanning removes these deficiencies by capturing thousands of points in the same time it takes to capture few points with a total station. Comparisons of millions of points between two scans over  an  extended  period  can highlight areas where change has occurred. Closer inspection or repairs could then be undertaken on the specific areas of concern to avoid costly failures and potential loss of assets. 3D modelling of the storage tank and its surroundings allows us to create a comprehensive data set that can be used to obtain direct measurements and   volumes  to   ensure   that   containment    dikes  for example, satisfy regulatory requirements. Areas of potential concern or failure can be quickly identified and quantified to reduce repair time.


The authors have not declared any conflict of interests.



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