A novel reduced order model for vortex induced vibrations of long flexible cylinders

TL;DR

This paper introduces a new reduced order model for vortex induced vibrations of long flexible cylinders, combining a van der Pol oscillator and a linear state-space model, validated against experimental data.

Contribution

The paper develops a novel reduced order model that captures both in-line and cross-flow VIV for long flexible cylinders using system identification from high fidelity simulations.

Findings

Model accurately predicts VIV behavior in full-scale flexible cylinders.

Validated results show good agreement with experimental data.

Efficient approach for analyzing VIV in offshore flexible risers.

Abstract

In this manuscript the development of a reduced order model for the analysis of long flexible cylinders in an offshore environment is proposed. In particular the focus is on the modelling of the vortex induced vibrations (VIV) and the aim is the development of a model capable of capturing both the in-line and cross-flow oscillations. The reduced order model is identified starting from the results of a high fidelity solver developed coupling together a Finite Element Solver (FEM) with a Computational Fluid Dynamics (CFD) solver. The high fidelity analyses are conducted on a reduced domain size representing a small section of the long cylinder, which is nevertheless, already flexible. The section is forced using a motion which matches the expected motion in full scale, and the results are used for the system-parameter identification of the reduced order model. The reduced order model is identified by using a system and parameter identification approach. The final proposed model consists in the combination of a forced van der Pol oscillator, to model the cross-flow forces, and a linear state-space model, to model the in-line forces. The model is applied to study a full scale flexible model and the results are validated by using experiments conducted on a flexible riser inside a towing tank.