This paper addresses the navigation problem for an autonomous robot designed to inspect sewerage. The robot, which is about half size of the sewer diameter, must keep its orientation in the entirely dark sewerage. The only a priory information known is the geometrical shape of the sewer. This implies a strong geometrical constraint on the environment the robot can expect. In the paper we worked out an active vision system to be used on-board of the sewer robot. The system has two components: (a) an optical camera and (b) a laser cross hair projector generating an ideal cross. The print left by the laser cross hair projector on the pipe surface is snapped with the camera. The image of the 'cross trace' on the camera plane is two intersecting quadratic curves. The shape of the curves gives a clue about a direction the robot looks. In this paper ewe investigate the curves that are the images of the laser 'cross traces' as they are seen on the camera plane for a simulated environmental model. We show how the shape of the curves viewed by the camera depends upon particular camera/laser relative position and orientation, assuming a cylindrical sewer pipe. We give a strategy how to align the robot with the sewer axis on the basis of curve images.