Author Arjen Koop, Jaap de Wilde, André Luís Condino Fujarra, Oriol Rijken, Samuel Linder, Johan Lennblad, Nora Haug, Amal Phadke
Title Investigation on reasons for possible difference between VIM response in the field and in model test
Conference/Journal OMAE ASME 35th International Conference on Ocean, Offshore and Arctic Engineering, Busan, South Korea, 2016
Paper no. 54746
Month June
Year 2016

Abstract
Floating offshore structures, such as production semisubmersibles
and spars, can exhibit significant in-line and
transverse oscillatory motions under current conditions. When
caused by vortex shedding from the floater, such motions are
generally called Vortex-Induced Motions (VIM). For semisubmersibles
these motions could have a strong impact on the
fatigue life of mooring and riser systems. Some field
development studies indicate that the VIM induced fatigue
damage for larger diameter Steel Catenary Risers (SCRs) can
have a magnitude equal to or larger than the wave-induced
fatigue damage.

The VIM phenomenon for multi-column floaters is
characterized by complex interactions between the flow and the
motions of the floater. Presently, model tests are the preferred
method to predict the VIM response of a multi-column floater.
However, several studies indicate that the observed VIM
response in the field is less than what is observed in model test
campaigns: typical model test results are very conservative.
Using such test results in the development of mooring and riser
design can easily result in very conservative designs which can
have a significant impact on mooring and riser cost, or even
affect SCR selection and/or feasibility.

The primary objective of the VIM JIP was to increase the
physical insight into the VIM phenomenon. This knowledge is
then used to address possible areas that could explain the
differences between the results from model tests and field
observations. To address these objectives, the JIP focused on
model testing and CFD studies. A key segment of the JIP was
the use of identical semi-submersible hull geometries for the
numerical and experimental studies thereby facilitating the
interpretation of the various response comparisons.

The JIP identified that a CFD model, at model-scale
Reynolds number, can reasonably well reproduce the VIM
response observed in model tests. However, to have confidence
in the CFD results extensive numerical verification studies
have to be carried out. The effect of external damping was
investigated in model tests and in CFD calculations. Both the
numerical and experimental results show that external damping
significantly reduces the VIM response. Comparisons between
CFD results at model- and full-scale Reynolds number indicate
that Froude scaling is applicable, with minor scale effects
identified on the amplitudes of the VIM motions. Changing the
mass ratio of the floater has a small influence on the VIM
response. Experimentally it was found that VIM response under
inline or transverse waves is slightly smaller than without the
presence of waves and is wave heading and wave height
dependent. The presence of waves does not explain the
observed differences between model test results and field
observations. The effect of unsteady current on the VIM
response is minimal.

Based on the results from the JIP it is concluded that
increased external damping reduces the VIM response. The
questions that remain are if the increased external damping is
actually present in full-scale conditions and if the mooring and
riser systems provide the required damping to reduce the VIM
amplitudes.

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