An Assessment of Crude Oil Leakage Beneath a Tank in a Refinery in the Niger Delta Sub-region Nigeria, Through Geotechnical Techniques

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A substantial amount of crude oil was alleged to have leaked beneath a Tank at a refinery situated within the Niger Delta Sub-region of Nigeria. Our geotechnical consultancy firm was engaged to find a method of estimating the amount of crude oil lost through leakage beneath this Tank. A geotechnical technique that involved a very careful manual drilling of Standard Penetration Test (SPT) holes within the confinement of this 64-meter diameter tank to depths of 20 meters was carried out. The manual drilling was undertaken to avoid any incidences of fire outbreak that may arise from the use of petrol engines commonly used in percussion drillings in the presence of the flammable crude oil remnants within this crude-oil drained tank, during project execution. This technique was used to retrieve crude-oil contaminated soil samples within the x4No.borings carried out within the crude oil tank. Applying the “4-Phase Concept of Air/Crude Oil/Water/Soils” after Otunyo (1993), Teme and Otunyo (1999), an estimated Total Crude Oil Equivalent of about 545.44 US Barrels of crude oil, was found to have leaked beneath the crude oil Tank. Soil profiles encountered during the boring with their engineering properties included (i) Brownish Lateritic Clayey Sands (SC); with friction angle (ϕu) of = 8o; unconfined Cohesive strength (Cu) of 14.4 kPa and average bearing pressures (q u (ave.) ) of 566.54 kPa; (ii) Darkish well-graded Sands (SW) with friction angle (ϕu) of = 34o; unconfined Cohesive strength (Cu) of 0.00 kPa and average bearing pressures (q u (ave.) ) of 129.83 kPa; (iii) Brownish Lateritic Silty Sandy Clays (SM-SC) with friction angles (ϕu) of = 8–14o; unconfined Cohesive strength (Cu) of 24.8-26.60 kPa and average bearing pressures (q u (ave.) ) of 151.472 kPa and (iv) Reddish-brown lateritic Silty Clays (ML) with friction angles (ϕu) of = 4–8o; unconfined Cohesive strength (Cu) of 28.6–35.20 kPa and average bearing pressures (q u (ave.) ) of 179.012 kPa. Additionally, a 20-Ton CPT Machine was employed to evaluate the soil bearing capacities surrounding the almost 49-year old Crude Oil Tank. The values of the maximum soil bearing pressures at 6-equidistant positions (60o) outside the tank varied from 2300–3500 kg/cm2 [225.56 – 343.25] kPa, while the maximum sleeve-frictions varied from 100.00–420.00 kg/cm2 [96.177–41.19] kPa, at depths varying from 1.00 m to 16.80 m. This paper describes the detailed methodology of planning, execution and computations of the various parameters involved in this novel study.