Experimental Results on the Damping Ratio in Sample of a Real Umbilical

Abstract

The operation of offshore platforms depends on the functions performed by umbilical cables, among other structures. They are used to conduct hydraulic fluids and transmit electrical energy and control signals. These slender multi-component structures are designed to resist the movements and stresses imposed on them by the sea current, the relative displacement of the platform, and the movement of the internal fluid. Therefore, design interest lies on knowledge regarding the dynamic behaviour of these structures, in particular, their damping properties, in their portions (i) viscoelastic, (ii) because of internal accommodation between the components or (iii) external, hydrodynamic in nature. The present work therefore aims to experimentally characterise the damping factor and fundamental oscillation frequency of a sample of a real umbilical cable. These parameters are determined by decay tests in air under different conditions of initial displacement. The sample measured 6 metres in length and 129 mm in nominal external diameter. Displacement-time records were collected by means of wire displacement transducer for each test and three methods were applied to calculate the observed damping factor. Frequencies were evaluated by applying the Fast Fourier Transform (FFT) and observing the average period between peaks, troughs, and ascending zeros. Two different dissipation regimes were observed during the tests. The most intense one, characterised by accommodation between adjacent components of the structure, is preponderant in conditions of large amplitude of oscillation. When active, it induces substantial increases in the damping ratio and natural oscillation period. Results from cyclical bending test corroborate this claim, showing transition between two very distinct flexural rigidity regimes. Furthermore, careful assessement of the model applied to evaluate the damping ratio from the bilinear hysteretic cycle revealed an overestimation of this quantiity for amplitude regimes containing large post-slip displacements and a correction is proposed.