ABSTRACT of the Doctoral Thesis

Static and Dynamic Modelling of Lightweight Floating Platforms Supported by Flexible Air Chambers

by Florian Toth (2014)

A novel concept for a floating platform usable as a solar power plant was presented. The floating platform consists of a plate-like frame structure which is supported by cylindrical air chambers. The chambers are formed by the platform-plate, a flexible membrane, and the water surface. Ballast weights at the bottom end of the membrane ensure the air-tightness of the open air chambers even during conditions with high waves.

Computational homogenisation was used to reduce the plane-periodic frame structure to an equivalent plate structure in order to reduce the complexity of the mechanical model. Thick plate properties, were computed based on the unit cell of the periodic structure. The localisation procedure for computing local field variables in the unit cells based on the analysis results of the homogenised plate was explained. A procedure to directly evaluate a failure criterion defined on unit cell level based on global section forces was developed. This procedure offers a reduction of the computational cost by pre-computing the failure surface of the unit cell.

A mechanical model of the floating platform under wave forcing was developed. The model is computationally very efficient and has a minimal number of input parameters, yet incorporates all important effects, making it suitable for preliminary design. The finite element method was used to model the deformable platform-plate. The supporting air chambers, coupling water surface and plate, were idealised by linear springs taking the flexibility of the membrane and the compressibility of the air into account. The hydrodynamic reaction forces arising due to pressure oscillations in the air chambers were determined by an acoustic sub-model of the surrounding fluid with free surface. In this way the complex fluid-structure-interaction could be accounted for by the use of equivalent spring-mass-daspot elements. Excitation forces from surface gravity waves were obtained from linear wave theory. The natural modes and transfer functions computed by the developed model were experimentally validated by model tests. Tests were carried out in regular and irregular waves to obtain transfer functions. The Ibrahim time domain method was employed to determine free floating oscillation modes, natural frequencies and damping factors experimentally.

A single air chamber shows a static instability: It globally buckles if a critical internal pressure is exceeded, resulting in the loss of load carrying capacity. This instability phenomenon was investigated computationally and experimentally. The reason for the instability was determined to be the unsymmetrical pressure distribution arising at the waterline of slightly inclined air chambers. This creates a bending moment which leads to global buckling of the chamber.

Finally, a prototype design suitable for the Mediterranean Sea was presented. The behaviour of the platform in irregular seas was evaluated based on probabilistic theory, confirming the feasibility of the proposed design.

revised 140702 (hjb)