Experimental investigations into the motions of vessels, less than twenty metres in length, stationed at single point moorings.

Abstract

The Royal National Lifeboat Institution (RNLI) is an independent charity whose purpose is to save lives at sea and end preventable loss of life. In order to achieve these goals 21 of their all-weather lifeboats are moored permanently to a single point mooring (SPM) and a further 19 lifeboat stations have a reserve secondary mooring. On the 23 rd March 2008 a Trent class lifeboat slipped her mooring and was damaged beyond economic repair and there are also numerous media reports of vessels breaking free from their coastal harbour SPMs resulting in damage and/or rescue crews being called out. The motivations for the experimental work presented in this thesis are the reported loss of human life and damage to vessels together with the lack of consistent SPM configurations. This project is aimed at an improved understanding of the motions experienced by a lifeboat and buoy in order to gain insight into the key factors which effect a SPM in order to provide guidance for full scale. Three changes in SPM configuration have been investigated namely: three changes in mooring line (hawser) length, two scales of mooring buoy and five shapes of mooring buoy. The novel contributions of this research include: (1) a detailed breakdown of the literature examining the motions of vessels at SPMs, (2) the creation and validation of a portable method of motion capture which can be used for small scale laboratory testing and in-situ full scale data recording, (3) experimental data on the effects of changes in buoy shape and size upon the motions experienced by a lifeboat the results of which suggest that the introduction of a twice scale buoy reduces the risk of mooring failure by reducing the motions of a lifeboat at a SPM, (4) investigations into the coupling of the 6 degrees of freedom motions of a model lifeboat moored in regular waves the results of which show that increasing the wavelength to longer than the length overall of the model leads to a breakdown in the coupling between surge-pitch, surge-heave and heave-pitch and (5) in-situ motion data, for both vessel and buoy, from a full scale lifeboat at a SPM the results of which indicate that it is passing harbour traffic that produces the peak excursions and therefore the RNLI’s SPMs should be positioned as far as is operationally practical from shipping routes. These experimental results and in situ measurements will contribute to improving the RNLI’s design and operation of its SPM. Furthermore these techniques are of use to Harbour Commissions monitoring the safe mooring of vessels e.g. the validated algorithm could be used to notify of extreme motions at a SPM via video surveillance.

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Identifiers

ORCID for Catherine Jayne Hollyhead: orcid.org/0000-0003-2934-2955

ORCID for Nicholas Townsend: orcid.org/0000-0001-6996-3532

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