This paper presents comparisons between the results of hydrodynamic analysis and two sets of large scale model test data for the wave induced motion response of tensioned buoyant platforms. The comparisons are presented with emphasis on the measured and predicted behaviour 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 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 demonstrate the problem of possible vortex induced ringing for tethers with a large pretension. Although this feature is not modelled by the hydrodynamic analysis, the time history data are used to interpret the physical mechanism behind this observed vortex induced tether ringing.
Much research has been carried out on the wave induced motion response of tensioned buoyant platforms (TBPs). Comparisons between hydrodynamic analysis and the results of wave tank testing have been performed by several groups in research establishments and within industry. Most of these tests are relatively small scale although reference (1) describes large scale open sea testing. However, deficiencies still remain in the prediction of the way in which the dynamics of the mooring tethers and their interaction with the surrounding fluid influences the platform response and internal forces within the tethers themselves. These predictions are to a large extent dependent on achieving a quantitative understanding of the forces induced by these fluid/tether interactions vortex shedding effects being of particular interest here. Increases in the magnitude of internal force in the tethers due to dynamic magnification effects induced by vortex shedding or other fluid/structure interactions are of fundamental importance to the safe operation of production TBPs, both from the point of view of tether failure, and the costs of tether change out due to inadequate fatigue lives.
This paper presents the results of two large scale tests on tensioned buoyant platform motions and tether tension variations. The test data are compared with a hydrodynamic analysis and discrepancies between measured and predicted tether tension at some wave frequencies are highlighted. Time histories of tether tension variations are presented together with physical interpretations of the features observed in the data. This work is part of a research programme at the London Centre for Marine Technology, University College London into the detailed dynamic response of tensioned buoyant platform tethers.
