Framework to research and design wrinkle free very large flexible offshore solar platforms by adding permeability

Abstract

Offshore solar power installations can be increased in size and efficiency compared to land based installations due to the availability of space and the cooling effect of water. To increase survivability in large waves, very large floating solar platforms can be made flexible. However, a flexible structure is susceptible to wrinkling due to combinations of mooring loads, wave dynamics and the flexibility of the platform. Wrinkling is a limit state which is not commonly researched in maritime engineering, since generally structures are designed to be as stiff as possible. Wrinkling can lead to the global collapse of a very large flexible solar platform and should therefore be minimised. Very large floating platforms have an impact on the environment due to the shadow underneath. This problem can be diminished by adding holes to the platform. Furthermore, holes can be functional as manholes for maintenance of the bottom of the platform. Scientific research on the wrinkling behaviour of membranes has pointed out that wrinkling behaviour depends on the geometry of the membrane, which can for example be changed by adding holes to the membrane. The aim of this thesis is to set up a design and analysis framework for wrinkled membranes with permeability. In order to define this framework, (i) reliable computational indicators are developed; (ii) the influence of permeability on membrane wrinkling is investigated and; (iii) the framework is verified on a fictive case of an integrated stiff solar panel on a membrane. The first objective in this thesis is to find reliable computational indicators to assess the wrinkling behaviour of a membrane. Current concepts of offshore solar platforms are flexible membranes including mooring systems based on concentrated tension loads. To study the wrinkling behaviour of a (subregion) of such a flexible floating solar platform, in this study, it is modelled as a rectangular membrane subjected to uniaxial tension; following the available literature on the wrinkling problem. Numerical analysis of the wrinkling behaviour is first done by non-linear post-buckling calculations. However, it appeared that the results of this calculation are highly dependent on the initial deformation which is used. An alternative method which does not depend on user defined input was found in studying the second principal stress field: when negative second principal stress is present in the membrane, wrinkling can occur. When no negative second principal stress is present in the membrane, wrinkling will not occur. Furthermore, the distribution and the area of the negative second principal stresses region are indicators for the wrinkle distribution and amplitude. For a robust assessment of the susceptibility of a membrane to wrinkling, a study of the negative second principal stress field should thus be preferred over a post-buckling analysis. The second goal of this thesis is to research the influence of permeability on membrane wrinkling. The effect of holes on the wrinkling behaviour is researched by studying their influence on the second principal stress field. The aim is to find wrinkle free configurations and study the mechanisms which minimise wrinkling behaviour. Therefore, the size, location and shape of holes are varied systematically in membranes of different sizes. It was found that holes should be chosen so that when the membrane is strained, they deform in a way so that they bring tension in the regions of the membrane which are subjected to negative second principal stress in the non-permeable configuration. Furthermore, holes should be chosen such that the loadpaths in the membrane are not interrupted. The framework to research and design wrinkle free very large flexible solar platforms by adding permeability is verified and has proven to be effective by applying it on the configuration consisting of a stiff solarpanel mounted on a flexible membrane. In this configuration, the framework has led to holes by which the wrinkling region is localised. From the three parts of this thesis, it is concluded that a research framework for wrinkling of membranes with permeability was found in the analysis of the second principal stresses and the driving mechanisms were investigated and verified. To further develop this framework, research on the effect of stiffening the membrane and minimising wrinkling under load conditions other than tension are recommended.