An offshore pipeline often has many free-span formations; it mainly occurs due to entrenched marine pipes being laid on an uneven seafloor of the seabed, wave flow scouring or residual stress of the pipeline. The determination of the fundamental natural frequency of the free-span offshore pipelines is an important part of the dynamic analysis them. The fundamental natural frequency impacts the design of an offshore pipeline; thus, determining the fundamental natural frequency is crucial. A free-span pipeline natural frequency depends on pipeline geometry, seabed soil characteristics, the property of transporting material, etc. This paper deals with the effect of C-phi seabed soil (mixed soil) characteristics on the fundamental natural frequency. In this study, silty clay with sand seabed has been considered, which is referred from Sheyang Offshore Project in Yancheng City Jiangsu Province China. A generalised analytical method, already developed by the author, has been used for the analytical study. The numerical study is conducted by using commercial FEM software ABAQUS for validation purposes on a homogeneous seabed soil. Finally, natural frequency analysis has been performed, and a comparison of results has been provided in this paper. It has been found out that generalised analytical method gives the natural frequency of the free span pipeline without compromising any accuracy. This analysis also shows that free span pipelines rested on the silty clay with sand seabed soil gives higher natural frequency than stiff clay soil but lower than very stiff clay soil