Characterization of Measured VIV for Free Spanning Pipelines

As the search for oil and gas takes place on increasing water depths, the traditional production systems based on fixed or floating platforms becomes increasingly costly. A good alternative to the traditional production systems are subsea to shore solutions, where pipelines transport the product to a land terminal for processing. One of the main challenges of documenting pipeline integrity in subsea to shore solutions is related to fatigue induced by vortex induced vibrations (VIV). Rough and uneven seabed introduce free spans that can be short or long, have small or large clearance and can have a large variety of boundary conditions at the shoulders. Hence, there is a need for methods that can give reliable estimate for fatigue damage under all realistic conditions. This paper investigates the measured VIV for a free spanning pipeline model. The model has L/D ratios from 72 to 350 and includes both single and multi-spans. It is exposed to uniform current and is free to vibrate in both cross-flow (CF) and in-line (IL) direction. Bending strains are measured in CF and IL direction at ten locations along the pipe. The observed cross-sectional trajectories are analyzed, with particular focus on the complex interaction between CF and IL VIV, and the significance of higher order response components. The purpose of this investigation is to understand the behaviour of the free spanning pipelines with respect to response amplitudes, frequency and modal composition, and also to identify characteristic cross section trajectories for further investigation. This work is a continuation of the earlier published results by Nielsen et al. [1] and Soreide et al. [2].