Influence of Axial Boundary Conditions on Free Spanning Pipeline Natural Frequencies

The effect of axial restraint (boundary conditions) on the natural frequency of a free spanning pipeline is examined in this paper. Theoretical calculation of the natural frequency of a straight pipeline with simple boundary conditions is a trivial task with exact solutions being available. A pipeline lying on the seabed however is neither completely straight and the interaction with the soil at the span shoulders create more complex boundary conditions. DNV-RP-F105 provides guidance on the calculation of free span boundary conditions with these increased complexities. The DNV recommended practice does not however take into account the effect of the axial restraint on the natural frequency. Results are presented in this paper for a range of axial stiffness combined with span out of straightness for a free spanning pipeline. The results presented show that the effect of axial restraint for moderately out of straight free spans can cause significant deviation in the calculation of the span natural frequency.