VIV analysis of a subsea power cable during installation in high current seas

Master thesis (2017)

Authors

D.F. Kallias Ntroumpis Mechanical Engineering

Contributors

A. Metrikine (supervisor 1)
Y. Qu (supervisor 1)
Connie Visser (supervisor 2)
Mohamed Hassan (supervisor 2)
Faculty
Mechanical Engineering
Copyright: © 2017 Dimitris Kallias Ntroumpis
Wake oscillator model Vortex induced vibrations VIV VIV on catenary shaped cables Cable integrity limits
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Published Date

25-09-2017

Language

English

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Faculty

Mechanical Engineering

Abstract

Tideway is a marine solutions provider for the offshore oil & gas and renewables and among other activities specialized in subsea power cable installation. To optimize the installation of power cables in high currents a better understanding of the dynamic behavior of the cables is needed. During installation of subsea power cables in high ocean currents, significant vibrations might occur. These vibrations are called vortex induced vibrations (VIV) and they are result of the vortex shedding behind the cables. Significant forces may arise due to VIV which can compromise the cable's integrity limits and further introduce problems to the installation procedure. The main objective of this research is to assess the effects of the VIV on the cables and provide solutions to improve workability.

In this thesis a coupled wake-structure model was created in order to study the interaction between the vortices and the cable. The model is based on the phenomenological model of the wake oscillator which can capture well the self-exciting, self-limiting behavior of VIV, therefore estimating accurately enough the lock-in region and vibration amplitude. A wake oscillator with acceleration coupling is used and different sets of tuning parameters tuned to 1DOF and 2DOF free vibration experiments are examined. The structure has been modeled in the finite element software OrcaFlex and the wake oscillator model was implemented as an external function in the programming language Python.

To confirm the validity of the above mentioned model two experiments were used. Firstly, the model was tested against the Delft flume experiment where the case of a vertical riser is examined and further the São Paulo experiment where the case of a steel catenary riser (SCR) is studied. The model was also compared with the Milan wake oscillator model from OrcaFlex's VIV toolbox. The model showed good agreement with the experimental results and found to be superior of OrcaFlex's VIV model.

Next, VIV analysis was performed for a case study. The analysis was done for different current speeds and directions and the effects of the VIV on the tension and the bending radius of the cable were determined. For lower current velocities, lock-in response and standing waves along the cable were observed, while for higher velocities multifrequency response, beating behavior and traveling waves were observed.

An important conclusion of this research is that the coupled wake-structure model used in this thesis is suitable for assessing VIV effects on catenary shaped cables. Furthermore, the analysis showed that catenary shaped cables with remaining length on the seabed exhibit smaller crossflow vibration amplitudes than straight cylinders.