MARIN investigates VIM response of semi-submersible floaters
Conference/JournalMARIN Report 138
Date20 Dec 2023
MARIN develops a novel co-simulation approach to assess the effect of mooring and risers on the Vortex Induced Motion (VIM) response of a semi-submersible platform.
The VIM response of semi-submersibles is a resonance phenomenon which occurs when the vortex shedding frequency of the flow around the columns of the platform coincides with the natural frequency of the moored platform. These motions of the platform can have a significant impact on the predicted fatigue life of mooring and risers. Over the last decades physical model testing using Froude-scaled floater geometries has been the main method to estimate the VIM response.
However, studies based on available field measurements seem to indicate that the VIM response of the installed platform is significantly lower than what is predicted in the model tests. More specifically, VIM in the field seems to occur within a narrower range of flow velocities and with up to 50% lower amplitudes of the VIM motions, compared to model test results.
The VIM response of semi-submersibles is a resonance phenomenon which occurs when the vortex shedding frequency of the flow around the columns of the platform coincides with the natural frequency of the moored platform. These motions of the platform can have a significant impact on the predicted fatigue life of mooring and risers. Over the last decades physical model testing using Froude-scaled floater geometries has been the main method to estimate the VIM response.
However, studies based on available field measurements seem to indicate that the VIM response of the installed platform is significantly lower than what is predicted in the model tests. More specifically, VIM in the field seems to occur within a narrower range of flow velocities and with up to 50% lower amplitudes of the VIM motions, compared to model test results.
Contact
Arjen Koop
Senior Researcher/Teamleader
Joep van der Zanden
Senior Project Manager
Robert Heerink
Senior Project Manager
Jaap de Wilde
Senior Project Manager/Teamleider • Offshore
Impact on costs
Therefore, as a consequence of relying on standard model test information, overly conservative design guidance for moorings and risers may have been followed, resulting in a significant impact on costs, the feasibility of Steel Catenary Risers (SCRs) and changes to the hull design or riser system configuration.
Model test setup for floating offshore semi-submersible platform
Illustration of the flow field around a semi-submersible with a representative deep-water catenary mooring and riser system consisting of 14 anchor lines and two risers. The presented results are obtained using the coupled simulation approach of CFD (ReFRESCO) with a time-domain mooring code (aNySIM-XMF).
Illustration of the flow field around the semi-submersible during VIM with the forces on the columns and pontoons illustrated by the black and red arrows, where the black arrow is for the force on an individual column or pontoon and the red arrow is for the columns combined and the pontoons combined. Also, the total lift and drag forces on the semi-submersible and the motions of the semi-submersible are shown in time traces with the red dot illustrating the instant of time shown on the left.
Tags
floating wind turbinefloating windcfd developmentcfdcfd/simulation/desk studies