Research on procedures to estimate 3D response of Vortex Induced Motion (VIM) of multi-column floaters based on 2D CFD calculations

Research on procedures to estimate 3D response of Vortex Induced Motion (VIM) of multi-column floaters based on 2D CFD calculations

Liu, Y.

Huijsmans, R.H.M. (mentor)
Maximiano, A.S. (mentor)
Koop, A. (mentor)

Mechanical, Maritime and Materials Engineering

Offshore and Dredging Engineering

2016-08-23

It is well known to offshore industry that current can trigger significant in-line and cross-line motions on floating offshore structures due to resonance. For a small dimension structure, like risers, this will lead to Vortex Induced Vibration (VIV). A lot of research has been carried out on this topic. However, for a large dimension structure, like semi-submersible floater, the frequency of motion is relatively low, resulting in the so-called Vortex Induced Motion (VIM). In the last decade, the concept of VIM has gained increasing attention. Nowadays it is one of the most important design topic for deep draft semi-submersibles, especially within the scope of riser fatigue analysis. Currently, model test is the preferred way to predict the VIM behaviour of a known offshore structure. However, due to time and cost restrictions, it is not feasible to perform VIM model tests in the early design stage. Therefore, a need for a fast and reliable method to predict VIM arises. A good candidate is performing CFD calculations. With the development of numerical method and hardware, at the moment CFD can provide reasonable VIM predictions. Compared with model test, CFD can save a lot of time and is relatively cheap to perform sequences of tests with different parameters which will greatly benefit the early design stage. Several 3D 3DOF CFD VIM calculations for offshore structures have been performed in MARIN and the result is quite comparable to model test result. However, about 2 weeks per calculation are required. At an early design stage, a faster prediction of the VIM behaviour of an offshore structure with desired parameters through CFD calculation is desirable. This target could be achieved through 2D CFD calculation. Compared with 3D CFD calculation, CFD VIM in 2D can be executed and accomplished within 1 or 2 days. The columns and pontoon of the floater will be treated separately in CFD. CFD VIM calculations in 2D could also provide reasonable and comparable results. It is feasible to study the VIM behaviour of multi-column floaters through 2D CFD calculations. As a consequence, in this project, the research on VIM behaviour of multi-column floaters will be performed in ReFRESCO, a commercial CFD code of MARIN. Different structural parameters (mass ratio, external damping levels, geometry of the columns) and environmental conditions (the velocity of current) will be investigated in this project, in terms of their influence on VIM behavior. This can give some insight about VIM behaviour of multi-column floaters and how to reduce it. Besides, the numerical set-up will also be surveyed, like time step and grid size sensitivities, to ensure the accuracy of the calculation. Finally, results from 2D CFD calculations will be compared with those from 3D CFD calculations. The feasibility of the estimation of a semi-submersible’s VIM response in 3D from 2D CFD calculations would be checked.

http://resolver.tudelft.nl/uuid:932b7311-e542-4c75-be2d-c73b3adb5647

Student theses

master thesis

(c) 2016 Liu, Y.