The research intends to evaluate the variation in the resistance and the lift of a torpedo shaped AUV brought about by the wall effect inside the pipe as it moves out of the axis inside a water pipeline. Movement of an AUV at the axis of a pipe causes minimum resistance and lift forces, but when the AUV moves at a position parallel with the axis of the pipe (out of axis of the pipe), the hydrodynamic forces especially the lift force changes. The AUV must be able to move a float inside the pipe and perform non-contact inspection. In water pipes having limited diameters, there is the wall effect. The added resistance and the lift have to be calculated accurately, which is a necessary requirement for the determination of the vehicle speed, power demand, control, range and duration of the operation. According to the findings of this paper, when moving at the center of pipe the ratio of AUV diameter to pipe diameter is equal to 12. This value can be considered for the determination of" the critical pipe diameter" which gives zero resistance. The results of this study can be applied for torpedo movement inside the torpedo tube. The analysis is performed by the Flow Vision (V.2.3) software based on the CFD method and solving the RANS equations.