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The sputtering coefficient of polycrystalline gold bombarded by 10-40 keV Ar+ ions has been measured as a function of total ion dose and shown to exhibit oscillations in magnitude between 30 and 100%. Possible experimental errors which would give rise to such an oscillation have been considered, but it is apparent that these factors are unable to explain the measurements. It is proposed that a change in the Sublimation Energy associated with either bulk damage or formation of surface topographical features arising during ion bombardment may be responsible for the observed variations in sputtering coefficient.

This paper investigates modifications to the dynamics of a marine vehicle caused by a point of inflection in its hydrostatic righting moment curve. Such inflections are shown to yield multiple equilibrium angles due to static over turning moments and to the occurrence of a classical fold catastrophe. Both frequency domain analyses and numerical solutions of the equation of roll motion are used to evaluate the influence of such inflections on the vessel’s dynamics. The analyses demonstrate that inflectional righting moment curves can yield unexpected roll amplitude jump phenomena close to resonance. The paper concludes with a discussion of the effects of such behavior on vessel hull designs.

This paper presents comparisons between the results of hydrodynamic analysis and two sets of model test data for the wave-induced motion response of tensioned buoyant platforms. The comparisons are presented with emphasis on the measured and predicted behavior of the tether elements. The two sets of data used are from (i) tests performed jointly by Heriot-Watt University and University College London at the National Maritime Institute (NMI) Ltd., and (ii) published data from tests at the Norwegian Hydrodynamics Laboratory. The hydrodynamic analysis used in the comparisons is described, together with the assumptions underlying its formulation and the manner in which the lateral dynamics of the tethers are accounted for. The analysis and test data show good agreement for surge motions although discrepancies are observed for the tether tension amplitude response at certain wave frequencies. The paper also presents detailed tether tension time histories from tests in regular long-crested waves at NMI. These data demostrate the problem of high-frequency tension oscillations (often called ringing) in tethers. Although this feature is not modeled by the hydrodynamic analysis, time history data are presented to enable interpretation of the underlying physical mechanism of this phenomenon.

Conventional analysis of tension leg platform structures yields natural frequencies which are well separated from wave excitation frequencies. However, the results presented here show that the tethers can have lateral resonant frequencies in the wave frequency range if the platform is deployed in deep water. Analysis which ignores the lateral tether dynamics gives reasonable estimates of platform motions but can seriously underestimate tether displacements. It is usually assumed that tether tension does not vary with time. However, the vertical waves forces, reacted by the tethers, cause the tension to change with time and this can cause a Mathieu type of instability in the platform sway motion. This phenomenon is investigated using a simple energy balance technique and it is shown that square law fluid damping places an upper bound on oscillation amplitude; it is found that platform sway motions due to Mathieu excitation remain acceptable even in large waves.

This paper describes applications of an extension to a recently developed calculation method for vortex-induced response of risers and tethers. The vortex-induced vibration response for the excited modes is generated using a semi-empirical formulation which is applied within a time domain calculation. This makes it possible to take account of the variation of flow properties along the riser length, as well as in time. The calculation method has been verified by comparison with model test data for vortex-induced response to surface vessel motions in still water. This paper presents applications of the technique for vortex shedding due to the combined action of current, surface vessel motions and waves. The mathematics of the extended prediction technique is described and results are illustrated by presenting typical vortex-induced responses for single-tube risers. The results of the prediction method are used to highlight the relative magnitudes and nature of vortex-induced response excited by currents, surface vessel motions and waves.