Al-Hammadi, Mohammed Ridha Sadeq Abed; (2018) Scour and Scour Protection around Dynamically Active Marine Structures. Doctoral thesis (Ph.D), UCL (University College London).

Abstract

The fast growing and proven offshore wind energy industry provides the motivation for this study, in particular the prediction and control of scour around foundations. This thesis is concerned with the impact of the dynamic behaviour of marine structures (such as wind turbines) on scour and scour protection around foundations in non-cohesive and cohesive beds. Firstly, to investigate the potential of pile cyclic movement, a range of cases were tested in non-cohesive sediment. These concluded with a very long, multi stage test simulating the exposure of a monopile to a sequence of storm and calm weather conditions. The results have been verified at two scales and for two sand sizes. In cohesive beds, formation of a gap between the pile and the bed, and creation of a waterjet-like system was observed. For scour protection over non-cohesive beds, two rock sizes and one filter system were tested. A range of frequencies and amplitudes of cyclic movement of the pile were covered through the programme. For hydrodynamic conditions, unidirectional clear water currents were tested to isolate the impact of pile cyclic loading and minimize the effects of other factors. The study results in a number of significant findings. Firstly, in non-cohesive beds the cyclic movement of the pile alters the geometry of the standard scour hole, resulting in growth of the scour hole in both depth and extent. This growth is likely to continue progressively. In cohesive beds (clay-sand mixtures) the pile cyclic movement results in removal of clay particles and development of a scour hole under a flow velocity as low as the critical velocity of the sand particles used in the mixtures. For scour protection, a new failure mechanism has been detected under the effect of pile cyclic movement. The project has introduced post equilibrium scour as a significant and vital part in the scour process that, in addition to the pile cyclic movement, can be driven by other parameters. Prediction of post equilibrium scour will reduce the uncertainty in the current approaches to scour prediction and introduce a more efficient and sophisticated design methodology.

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