This paper presents results of an experimental study on 3-dimensional scour at submarine pipelines with uniform sediments under a unidirectional steady current. A dimensional analysis is first conducted to identify all the important non-dimensional parameters. Laboratory experiments are then conducted to study the development of a 3-dimensional pipeline scour hole under different sets of environmental conditions. The results are recorded and the corresponding propagation velocities of the free span calculated. The effects of four parameters on the propagation velocity are studied through the conduct of several groups of experiments; each of which exclusively focuses on one particular parameter. Moreover, the scour pattern under different combinations of environmental conditions is discussed to obtain an improved understanding on the mechanism of scour hole propagation at the span shoulder of pipelines