Numerical investigations on transient behaviours of two 3-D freely floating structures by using a fully nonlinear method

Abstract

Two floating structures in close proximity are very commonly seen in offshore engineering. They are often subjected to steep waves and, therefore, the transient effects on their hydrodynamic features are of great concern. This paper uses the quasi arbitrary Lagrangian-Eulerian finite element method (QALE-FEM), based on the fully nonlinear potential theory (FNPT), to numerically investigate the interaction between two 3-D floating structures, which undergo motions with 6 degrees of freedom (DOFs), and are subjected to waves with different incident angles. The transient behaviours of floating structures, the effect of the accompanied structures, and the nonlinearity on the motion of and the wave loads on the structures are the main focuses of the study. The investigation reveals an important transient effects causing considerably larger structure motion than that in steady state. The results also indicate that the accompanied structure in close proximity enhances the interaction between different motion modes and results in stronger nonlinearity causing 2^nd-order component to be of similar significance to the fundamental one.

Keywords

fully nonlinear interaction / two floating structures / transient behaviours / potential flow / QALE-FEM method

References

[1] Dean R.G., Dalrymple R.A. Water wave mechanics for engineers and scientists, 1991, Singapore: World Scientific Publishing Co., 170-187

[2] Hong S.Y., Kim J.H., Cho S.K., Choi Y.R., Kim Y.S. Numerical and experimental study on hydrodynamic interaction of side-by-side moored multiple vessels. Ocean Engineering, 2005, 32(7): 783-801

[3] Kashiwagi M., Endo K., Yamaguchi H. Wave drift forces and moments on two ships arranged side by side in waves. Ocean Engineering, 2005, 32(5–6): 529-555

[4] Koo W.C., Kim M.H. Fully nonlinear wave-body interactions with surface-piercing bodies. Ocean Engineering, 2007, 34(7): 1000-1012

[5] Kristiansen T., Faltinsen O.M. Application of a vortex tracking method to the piston-like behaviour in a semi-entrained vertical gap. Applied Ocean Research, 2008, 30(1): 1-16

[6] Ma Q.W., Wu G.X., Eatock Taylor R. Finite element simulation of fully non-linear interaction between vertical cylinders and steep waves. Part 1: Methodology and numerical procedure. International Journal for Numerical Methods in Fluids, 2001, 36: 265-285

[7] Ma Q.W., Yan S. Quasi ALE finite element method for nonlinear water waves. Journal of Computational Physics, 2006, 212(1): 52-72

[8] Ma Q.W., Yan S. QALE-FEM for numerical modelling of nonlinear interaction between 3D moored floating bodies and steep waves. Int J Numer Meth Engng, 2009, 78: 713-756

[9] Maiti S., Sen D. Nonlinear heave radiation forces on two-dimensional single and twin hulls. Ocean Engineering, 2001, 28: 1031-1052

[10] Smith BG (2007). Extreme waves and ship design. 10th International Symposium on Practical Design of Ships and Other Floating Structures, Houston, USA.

[11] Sun L., Eatock Taylor R., Taylor P.H. First and second order analysis of resonant waves between adjacent Barges. Journal of Fluids and Structures, 2010, 26(6): 954-978

[12] Wang C.Z., Wu G.X., Khoo B.C. Fully nonlinear simulation of resonant motion of liquid confined between floating structures. Computers and Fluids, 2011, 44: 89-101

[13] Yan S., Ma Q.W. Numerical simulation of fully nonlinear interaction between steep waves and 2D floating bodies using the QALE-FEM method. Journal of Computational Physics, 2007, 221(2): 666-692

[14] Yan S, Ma QW (2008). Numerical investigations on responses of two moored 3D floating structures to steep waves. Proc Int Offshore and Polar Engng Conf (ISOPE2008), Vancouver, Canada, 322–310.

[15] Yan S., Ma Q.W. QALE-FEM for modelling 3D overturning waves. International Journal for Numerical Methods in Fluids, 2010, 63: 743-768

[16] Yan S., Ma Q.W., Cheng X. Fully nonlinear hydrodynamic interaction between two 3D floating structures in close proximity. Journal of Offshore and Polar Engineering, 2011, 21(3): 178-185

[17] Yan S, Ma QW, Cheng X (2011b). Fully nonlinear simulation of two floating structures in close proximity subjected to oblique waves. Proc Int Offshore and Polar Engng Conf (ISOPE2011), Hawaii, USA, 104–112.