California Sea Grant College Program
Structural Organization of Large-Scale Floating Runways Using a Floating-Mat Hydrodynamic Model
- Author(s): Ma, Jian
- et al.
The optimization of the strength distribution of the floating runway under wave loads was explored and the techniques needed to make this optimization possible were developed.
The "Hydrodynamic Influence Matrix" was defined to capture the hydrodynamic properties of the floating structure and to incorporate these into the motion equations of the floating structure. Great effort was made to improve the accuracy of the Hydrodynamic Influence Matrices for short waves.
A low-order panel method was developed first to compute the Hydrodynamic Influence Matrices, which were then incorporated into the finite-difference form of the motion equations for the floating-mat model. The spectra of the responses of the floating runway were computed by discretizing the incident wave spectrum into elemental waves and solving the finite-difference equations for each elemental wave. The low-order method required a huge number of variables to treat a realistic floating runway and the resulting numerical accuracy of the computation was insufficient.
A high-order method was then developed to improve the accuracy in the hydrodynamics computation for short waves. Double fifth order interpolation functions were used to represent the source distribution on a high-order panel. Analytically exact formulations and approximate formulations using asymptotic series were developed and combined to compute the singular integrations involving the Rankine part of the Green function and its gradient. Gaussian quadrature was used to compute the integrations involving the remnant part of the Green function. The high-order Hydrodynamic Influence Matrices were then computed with acceptable accuracy for waves with wavelength close to the panel dimension and were incorporated into the high-order motion equations for the floating-mat model. These were solved using a Galerkin method to get the responses of the floating runway in a wave system.
The optimization for the strength distribution of the floating runway was carried out based on consideration of the structural reliability using gradient projection method. Preliminary optimization results for various model structures and the full-size floating runway have been achieved. For the small model structures local optima satisfying the Kuhn-Tucker condition were found. For the large model structure and the full size floating runway preliminary optimized results with reduced total weight were presented as a reference for future work.