The floating harbour transhipper : well dock hydrodynamics of a novel transhipment concept

The floating harbour transhipper (FHT) concept has been developed as an alternative to traditional (side-by-side) transhipping. This concept employs a well dock at the aft end of a storage mothership in order to provide a sheltered environment for the feeder vessel, thus increasing the operational weather window. The inclusion of materials handling equipment onboard the mothership can reduce loading times to a fraction of that of traditional transhippers and the negative pressure enclosure contains dust and spillage of bulk goods. The FHT concept provides a number of advantages, particularly in terms of wider weather windows, faster materials transfer and reduced environmental impacts. These translate to increased financial viability for smaller and more remote exporters (and importers) and long term sustainability in applications previously deemed ill advised. The well dock introduces some complex confined water effects that must be understood to unlock the full capabilities of the FHT concept. The feeder vessels entering the well dock represents a significant blockage and displaces a very large proportion of the water within the well dock. As the feeder vessel enters this constricted space it is expected to generate significant flow velocities and strong confined water effects. It was proposed that including vents within the design of the well dock could mitigate such unfavourable effects. The adoption of well docks in commercial transhipment is previously unprecedented, particularly in cases where a high blockage coefficient combines with a finite channel length and an incident sea state. Understanding the hydrodynamic influences on the feeder vessel during transhipment operations is the focus of this research project. For this first serious investigation into the concept, an experimental campaign was undertaken using scale models to explore two scenarios that are crucial to the success of the FHT concept; the seakeeping performance of the feeder vessel when it is docked; and the performance of the feeder vessel during docking and departure manoeuvres. In order to confirm the viability of the FHT concept, the docked seakeeping performance needs to confirm that material transfer is possible in larger sea states than conventional side-by-side transhipment. For the docking and departure manoeuvres, the controllability is a key consideration and contact between the vessels is of concern across all operations. In addition to conventional resistance measurements to assess the varying loads experienced, two dimensional particle image velocimetry was also employed to analyse the flow field while the feeder vessel enters and departs the well dock. Significantly better seakeeping performance was observed when the feeder vessel is docked within the well dock than when the same vessel operates in an open seaway. The well dock also has a significant effect on the manoeuvring performance with increased longitudinal force on the feeder vessel being observed when operating within the confined well dock. Trapped fluid at the enclosed end of the well dock caused interesting confined water effects during docking and amplified the effect when departing. Vents at the enclosed end of the well dock were proposed to reduce the confined water effects. The vents successfully reduced the adverse well dock effects with longitudinal force reducing with increased vent area. When exposed to a seaway, increased vent opening area led to more wave energy in the well dock and increased feeder vessel motion. The relative motion between the feeder vessel keel and the well dock floor was less favourable as well dock vent size increased. The inclusion of vents is quantified for both the seakeeping behaviour and the docking/departure performance. The use of vents is very beneficial when a feeder vessel docks or departs the well dock, however adverse effects on feeder vessel motions when docked and exposed to a seaway highlight the need for compromise. The optimum solution that covers all operational conditions likely requires the inclusion of relatively small vents. This project revealed that the challenges posed by this novel concept can be overcome through careful well dock design, confirming the FHT concept to be a viable alternative to traditional transhipping methods