Effect of Non-homogeneity of Seabed Soil on Natural Frequency of Offshore Free Spanning Pipeline

Determination of natural frequency of free spanning pipeline is essential as it governs the allowable length and also fatigue life of the pipeline. From the literature survey, it is seen that a considerable amount of works have been studied to determine the effect of seabed soil on natural frequency of free span by the previous researchers. However, researchers have assumed that soil of the supports of free span is homogeneous. Well-used code DNVGL-RP-F105 (for free span analysis of offshore pipeline) also considers soil as homogeneous. But, recently, researchers state that non-homogeneity of soil has significant effects on dynamic soil stiffness. The objective of this paper is to determine the effect of non-homogeneity of soil on the natural frequency of free spanning pipeline. In this paper, a frequency analysis of the free spanning pipeline on homogeneous soil is carried out by Det Norske Veritas (DNV) guidelines and finite element modelling. A free spanning pipeline is analysed considering it on a non-homogeneous soil using soil mesh finite element modelling. The non-homogeneous soil model is designed with different layered soil combinations. Finally, it has been observed that finite element analysis software can be incorporated for pipe-soil interaction effectively. For layered soil profile, if the topmost soil stratum is thin, it has been found out that there is a significant variation in both inline and cross-flow natural frequencies compared to those of homogeneous soil having same topmost soil. Whereas if the topmost soil stratum is thick, it will not significantly affect in-line natural frequency, but cross-flow natural frequency tends to be same as that of homogeneous soil.