Qi, Z; (2015) Flushing Ballast Tanks. Doctoral thesis , UCL (University College London).

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Abstract

The non-indigenous species (NIS) transported by ships’ ballast water lead to destructive failure of the main ecologies giving rise to economic implications of many countries dependent on aquatic organisms. The International Maritime Organisation currently requires that ballast tanks are flushed three times with far ocean water. New protocols for cleaning technologies are still in discussion internationally. Current lacking is the science to understand how ballast tanks geometry and ballast water composition affect the NIS removal rate. This thesis describes a major contribution to this effort and identifies key engineering principles that should be taken into account to improve flushing efficiency. A combined experimental and theoretical study of flushing from ballast tanks is described. A hierarchy of laboratory scale models are designed, built and tested to understand the effect of geometry and stratification, with complexity increasing from 1×7, 2×2, 3×3 to 5×4 configurations. The experimental study is based on an optical method of interrogating the fraction of each compartment and whole tank that is cleaned. By drawing on modelling approaches applied in related areas, notably building engineering, a number of new mathematical models are developed that have no free variables (when resistance of pathways is the same) or require the use of closures for pressure drop coefficients. For homogeneous flow where stratification is negligible, the agreement between predictions and experiments is within 1.2%. Likewise, when resistances are different, the model is accurate, except when the inhomogeneity is significant. Three models are developed to include the influence of stratification. For miscible fluids, the stratified mixing model is accurate within 5% at Richardson number between 30 and 1000. We apply the validated models to examine how to change practical ballast tanks and how cleaning efficiency affects the total NIS removal. To enhance flushing, a single outlet should be placed far from the inlet.

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