SLENDER BODY MODELS OF TLP AND GBS 'RINGING'

This paper develops a theory which describes the phenomenon of 'ringing' which can affect Tension Leg Platform (TLP) and Gravity Base Structures (GBS). Experimental and full scale data illustrates a discussion of the physics of the response and slender body theory is invoked to explain the loading terms which are at least cubic in the wave height. To compare model test results with predictions, wave kinematics are calculated from the measured wave profiles using a modified form of Wheller stretching to determine free surface values which satisfy the nonlinear free surface conditions. These are the 'destretched' into the fluid vertically below the wave elevation sensor. To a good approximation, large waves travel without distortion over distances typical of TLP and GBS structural dimensions, so a zero dispersion assumption is used to determine the response of a multi- leg structure. Encouraging agreement between model test and theory is demonstrated. Similar agreement can be achieved by some analytical models of wave kinematics which might be used in the absence of model test data. A slender body approach could form the basis for an interim recommended practice for design to include ringing.