Dynamic response of a hydroelastic model of a typical semi-submersible to waves and bergy-bit impact

El-Tahan, Hussein Wahba Mohamed (1985) Dynamic response of a hydroelastic model of a typical semi-submersible to waves and bergy-bit impact. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This study addresses the problems of semi-submersible platforms operating year-round under harsh environmental conditions. The main objective of this study is to develop techniques for modelling, construction, handling, and testing a model that is dynamically and structurally similar (as close as possible) to a typical semi-submersible and use it to study its motion and global structural response to wave forces and bergy bit impact. -- Extensive review of the available numerical, experimental and full-scale studies is presented. The review identified the need for developing techniques to overcome the problems associated with modelling, construction, handling and testing of a structurally and dynamically similar (hydroelastic) semi-submersible model. -- The review also identified the lack of data on the impact strength of iceberg ice and the need for experimental and analytical studies to investigate the problem of semi-submersible/bergy-bit impact. -- A hydroelastic model of a typical semi-submersible was developed. In addition to simulating hydrodynamic forces, a hydroelastic model achieves structural similarity (i.e. mode shapes, natural frequencies and stresses). The model, believed to be the first of its kind, was fabricated with 0.8 mm (0.03 in.) thick high impact polystyrene sheets. Cellulose acetate butyrate tubes, 1.58 mm thick, were used as bracings. The model was extensively instrumented with strain gauges to measure strains in critical sections of all the semi-submersible members. -- The response of the model to regular and irregular waves under operating/survival conditions and impacts of modelled bergy-bits is presented. -- The experimental response in waves is compared with the available numerical and full-scale values. The measured forces, stresses and motion due to simulated impact are presented and compared with numerical results of impact models. -- There is very good agreement between the experimental and analytical results of this study and other published results. -- The effect of varying local stiffness of the structure/ice on the impact force has been studied using a two-degrees-of-freedom model and introducing a local stiffness element. The impact strength and load-deformation characteristics of iceberg ice were obtained from a comprehensive ice testing program carried out as part of this study. -- The maximum bergy-bit mass and impact velocity conditions that a column can withstand, without local damage, have been estimated using the results of the impact tests on the semi-submersible models and blocks of iceberg ice. -- It was found that semi-submersibles designed for wave forces only may not be able to withstand impact of small bergy-bits without suffering local damage. -- The study demonstrated the viability and reliability of hydroelastic modelling, and the usefulness of hydro-elastic models in studying the response to impact. -- Based on the findings of the study, areas for further research have been identified and recommendations made for further work on the bergy-bit/semi-submersible impact.

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