A remotely operated well control station could offer commercial advantages over an umbilical laid from the main platform. These have been determined for CONOCO (UK) on behalf of the Hutton Partnership' by a conceptual design study of a tension moored single leg buoy. Based on a location typical of the northern North See with water depths in the range of 160–460 m, the control station would be unmanned with electrical power for the control equipment generated locally. The experience of Cameron Atkins Technology is presented In this paper not only to assist those interested In research but also as a note of caution to designers of single leg buoys. A tethered hull had been selected by Conoco as the basis for the concept to be developed. This concept offered the potential for good station-keeping control (low lateral and heave excursions) and a reduction in the roll/pitch response characteristics of a free floating catenary moored buoy. Although not In itself a novel concept, its application needed fundamental calculations of the hull/tether system dynamic response. Preliminary estimates of the dynamic response were based on linearised models. These results were considered to be conservative at wave excitation close to the system natural frequencies. A non-linear analysis was undertaken to confirm this hypothesis. The introduction of representative damping through viscous drag forces was expected to reduce the conservatism. The results from a non-linear time history analysis in regular waves identified a pronounced and unexpected resonant response which contradicted trends established by linear analysis. This was particularly important since the design was to be dynamically tuned to minimise hull response by adjusting the mass distribution and the location of the tether attachment to the hull. The resonant response phenomenon was investigated in some detail during the project.

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