  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.
