VIV Fatigue Assessment of Deepwater Multi-span Pipelines

Submarine pipeline is the lifeline of the ocean oilfield production system, and the free spans of which often occur vortex-induced vibrations (VIV) in current flow. These vibrations are one of the main reasons for fatigue damage of pipelines. For a long period of time the research of VIV of ocean pipelines mostly aimed at single spans, however, actually on the complex topography seafloor, multi-spans are quite familiar. Compared to single spans, the dynamic vibration response of multi-spans are much more complex. During VIV response analysis, if a multi-span is simply divided into several single spans, there must be great error in the calculation results. Thereby, the main work of this paper is to find proper method for multi-span VIV response and fatigue analysis.Aimed on multi-spans of ocean pipeline, this paper analyses the characteristic of multi-spans VIV, and introduces two kinds of calculation method for VIV response. One of the methods is based on DNV response modal, combined with mode analysis using ABAQUS FEM software; the other method is based on conversation of energy, adopting experiential lift coefficient to predict the VIV response of multi-spans in frequency domain. It is proved through case study that the results are almost the same using these two methods, which means these two methods are both reliable.In this paper, the VIV fatigue life is calculated according to Miner linear cumulative damage theory, selecting proper S-N curves. At the same time, a considerable research has been done to analyses the influence of some parameters for VIV, such as the span shoulder length, the length between adjacent spans and the feature of sea soil, and especially the nonlinear boundary condition of pipe soil interaction. A time and frequency domain calculation method is used to predict the VIV response of multi-spans with nonlinear boundary condition.