Springing Damping Of Tension Leg Platforms

Abstract

SUMMARY The present report describes an experimental investigation of the springing damping of a TLP column in calm water, current and waves. The tests were done as motion decay tests with a 1:40 scale model of a single column. In calm water without current the damping was linear at small amplitudes and represented a damping level of about 0.1 percent of critical damping. The most severe current and waves increased the damping by a factor of about 4 compared to calm water. Test results are also presented for the effect of varying radius of curvature at the lower edge of the column. Finally a series of numerical predictions have been done, based on numerical solution of the basic viscous flow equations. The calculations, limited to two-dimensional flow cases, correlate very well with the experiments. INTRODUCTION Tension leg platforms are normally designed to have their heave, roll, and pitch resonant frequencies well abovethe frequencies of the wave spectrum. This is in order to prevent first order wave forces from inducing excessive resonant motions in these directions, which could lead to serious fatigue stresses in the tethers. However, due to non-linear wave forces there will be some excitation forces occurring at the sum-frequencies of the components in the wave spectrum. Thus there is still a possibility of significant fatigue stresses in the tethers due to resonant heave, pitch, and roll. The phenomenon is often referred to as "springing" in the tethers. Present-day theories for second order excitation forces have limitations regarding accuracy. Even more serious is the lack of theory for prediction of the hydrodynamic damping of the very small, high-frequency motions in question. The investigations described in the present paper were done as part of a joint industry project in 1990 and have been confidential until now. SCOPE OF PRESENT INVESTIGATION The main objective of the investigations described in the present report is to provide experimental information on the hydrodynamic springing damping of a TLP column as influenced by the presence of current and waves, and by variation of radius of curvature at the lower edge of the column. A further objective is to show results of numerical calculations of the damping in calm water and compare them with the experiments. MODEL DESCRIPTION The experimental investigations were done as model tests with a 1:40 scale model of a TLP column of 25 m diameter and 37.5 m draught. Fig. 1 shows the model dimensions. The model was made of steel. Two versions of the model were tested, the two models having different radius of curvature at the lower edge. One version of the model had its lower edge machined to a sharp corner with radius of curvature less' than 0.1 The second version had a radius curvature of 20 mm, model scale. model was painted to obtain hydraulically smooth surface.