Free-span occurs normally in pipeline at uneven seabed, dynamic seabed or pipeline crossing. The analysis of free-span, including static analysis and dynamic analysis, is an important subject in the study of pipeline integrity management. Static analysis of free span for subsea pipeline is to evaluate the stress distribution of spanning pipeline in the ultimate conditions, and qualify the stress with design codes in the engineering analysis. However, dynamic analysis of subsea spanning pipeline is much complicated due to VIV fatigue. In 2006 DNV-RP-F105 suggested a methodology of dynamic analysis for long spanning pipeline with multi-mode responses, but the fatigue analysis method for multi-modes is not detailed. In addition, the fatigue analysis of multi-spanning pipeline is not clear. The gap between the continuous two spans, and the pipe-soil interaction control the fatigue damage of the multi-spanning pipeline. In this paper, a VIV fatigue analysis method for multi-spanning pipeline is suggested based on VIV analysis. In this method, Abaqus FE model is developed first to obtain the stress distribution and the natural frequency of each vibration mode for spanning pipeline on seabed in different configurations with three multi-spans, and then the fatigue analysis of VIV is carried out for the spanning pipeline based on DNV-RP-F105. An example of fatigue analysis for a multi-spanning pipeline is presented; finally, several sensitivity analyses demonstrate the effects of key parameters on the VIV fatigue.

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