This paper presents an approach to locate the position of an unmanned mobile inspection platform at the bottom of an oil storage tank. The proposed approach uses 3-range and 1-bearing sensors mounted on the platform and the known tank contour for localization. The effects of the uncertainties in the measurement systems on the errors in the platform locations have been, investigated. The results indicated that the approach is comparable to the trilateration technique in terms of positioning errors. The work constitutes important first steps towards the development of an unmanned submersible mobile platform to simultaneously locate and map the structural defects in the base of a filled oil tank.