Dynamic response of multi-span offshore pipelines

• A semi-analytical method for multi-span pipeline analysis is developed.
• Effects of effective axial force and initial curvature are accounted for.
• The semi-analytical method is validated by means of detailed FEA.
• The influence of neighboring spans on dynamic response is investigated.
• Effects and consequences of neighboring spans on free span design are exemplified.

The dynamic response of offshore pipelines in free spans may be affected by the presence of adjacent spans, giving rise to complex mode shapes and modal stress distributions. Simplified and reliable methods for free span response calculations are widely applied in the pipeline engineering community, but existing methods are not valid for multi-spans. A semi-analytical method for multi-spans is developed in the present work, which provides a fast and accurate alternative to finite element modeling of potentially interacting spans. The proposed method includes effects of axial force and initial curvature due to static deformation. The impact on fundamental frequencies and associated modal stresses due to adjacent spans is firmly established based on large-scale parametric studies. It is further demonstrated that the influence of neighboring spans cannot be safely disregarded in calculations of fatigue damage and load effects from waves and currents.