Ultrasonic testing is one of the most common non-destructive testing (NDT) methods for periodic inspection of crude oil storage tanks. Ultrasonic phased array system can effectively and efficiently image different types of defects in the structures through direct contact method. However, the sludge remained on the tank floor gives rise to difficulties on detecting the defects in the floor plate using existing techniques. Therefore, this paper focuses on the development of novel phased array testing methods without requiring complete removal of the sludge. The appropriate experimental apparatus and its parameters used for through-sludge testing are selected based on the acoustic characteristics of the oil sludge composition. Furthermore, a novel phased array full matrix capture method is explored based on firings of multi-element aperture. The total focusing method (TFM) is studied by considering the existence of sludge. The experimental results from the sludge thickness study demonstrate that the TFM imaging technique can more accurately measure the change in thickness of the metal sample at the submillimeter scale. Additionally, high resolution imaging of side drilled holes in size of 1 millimetre can be realized on the samples covered with sludge. Last, the effect of aperture size on the detectability is investigated. Consequently, the results obtained from different sludge experiments show good reliability and measurement consistency of our proposed methods. It delivers great potential to accurately locate and quantitatively evaluate the corrosion defects frequently found in the tank floor.