Aircushion Supported Mega-Floaters

Aircushion Supported Mega-Floaters

Van Kessel, J.L.F.

Huijsmans, R.H.M. (promotor)

Mechanical, Maritime and Materials Engineering

Offshore Engineering & Ship Hydromechanics

2010-02-01

The increase of the global population and expanding coastal mega-cities will necessitate an innovative pursuit of the utilization of the ocean space in which mega-floaters will play an important role in the future. These types of structures are very large floating artificial islands that can be used for various facilities and purposes similar to those on land. Compared to landfill methods mega-floaters generally have a smaller environmental impact than traditional land reclamation projects. They are indifferent to earthquakes and can be constructed at relatively low cost in a short period of time, independent of ocean depth and seabed conditions. Furthermore, the existing facilities can be easily expanded while they are functional and the space available inside the structure offers prospects for various activities and different use. This thesis describes a method to predict the dynamic behavior of aircushion supported mega-floaters in waves. These types of structures are supported by a large volume of air which is entrapped underneath the structure by vertical walls that extend sufficiently far underneath the water surface in a way that no air will escape when waves pass by. The method is based on a linear three-dimensional potential theory using modal expansions and a linear adiabatic law to describe the air pressure within the aircushion. It is the first method that is able to accurately predict the three-dimensional dynamic behavior and stresses of flexible aircushion supported structures of arbitrary shape in waves. The structure around the aircushion is modeled in the usual way by means of panels representing pulsating sources which are distributed over the mean wetted surface of the body. The free water surface underneath the structure is modeled by panels laying in the mean free surface of each aircushion. All panels associated with an aircushion represent a body without material mass, but having added mass, damping, hydrostatic restoring and aerostatic restoring characteristics. The results of this study indicate that the behavior of aircushion supported structures can be well predicted by means of a three-dimensional linear potential method. In case of rigid bodies, the numerical results were validated with model tests. Model tests with a conventional flexible barge served to validate the hydroelastic method. Unfortunately no experimental results are available for flexible aircushion supported structures. Therefore the numerical results of these structures are verified with analytical and FEM computations. Both the model tests and computations have shown that the application of aircushions can significantly influence the behavior of floating structures. The effect on the structural loads is significant and is particularly pronounced in the wave induced bending moments which are considerably reduced by the aircushions. A conventional mega-float structure has to be protected by breakwaters if it is located in open seas. These breakwaters will reduce the wave loads on the structure, but add to the total costs of the mega-float project. Another option is to support the structure by aircushions to reduce the wave induced bending moments and consequently the stresses. In general, the results of this study have shown that an aircushion supported structure will have significant advantages compared to conventional mega-floaters. In addition, the computational method as developed and proposed proved to be a suitable tool to optimize the cushion configuration for a particular application.

mega-float,
very large floating structure (VLFS)
aircushion support
compressibility
fluid-structure interaction
hydroelastic analysis
motions
dynamic behavior
wave forces
drift forces
wave field
shear forces
bending moments
stresses
frequency domain
floating runway

http://resolver.tudelft.nl/uuid:938839aa-01ab-4c64-bf6e-448ed519945e

2010-02-01

9789085704966

Institutional Repository

doctoral thesis

(c) 2010 Van Kessel, J.L.F.