ABSTRACT

The dynamics of. single, taut, surface moorings in current and wave fields in the deep ocean are analyzed. Results of computer simulation by means of a numerical model of liberalized equations in the frequency domain are described. These are compared with actual data from a deep-ocean mooring obtained with densitometers and accelerometers. It is shown that metallic and synthetic portions of mooring lines respond differently to excitation. Interpretation of the results in terms of energy trans fer through mooring lines is presented.

INTRODUCTION

Through the discontinuity in density. the upper surface of the ocean offers a means to fix structures at sea when a reserve of" buoyancy at the surface is connected to a heavy anchor at the bottom. When water depth is large and rigid structures are not feasible, such a mooring becomes an attractive alternative. In the offshore industry this alternative is extensively used with single buoy moorings. The 'tension leg platform' proposed for deep-water operations and reported by Tubb(26) is another example of the increasing interest in this kind of structure by the offshore industry. But so far, deep-ocean moorings (Fig. 1) are mostly used for supporting current meters for Aurelian measurements of ocean currents and for suspending other sensors to monitor the water column.

The presence of surface waves poses the major problem for surface moorings. The dynamic loading induced by surface wave excitation may cause both structural and stability problems. The cyclic loading can lead to failure of mooring cables by excessive stress and fatigue. It is worth noting that of all the types of moorings, the moorings with surface expression have the least structural reliability. In their study of moorings set by the Woods Hole Oceanographic Institution, Walden and Panicked (27,28) found that surface moorings had a reliability of only 54% as compared to about 90% for subsurface moorings. Often, however, the problem of stability is of greater concern than structural reliability. The motion sensitivity of moored platforms and single buoy moorings is an important design consideration. When mooring cable are used to support oceanographic sensors the data quality may be affected by the energy transmitted down the line from surface waves. The wave excitation at '!:he buoy may cause high frequency responses of mooring cables and sensors attached to them. High frequency axial. excitation of Savories rotor current meters is found to cause amplification of the current records; see, for example, Gaul (7). In fact, Gould and Sawbuck (10) found horizontal kinetic energy densities obtained from current reg- o ords at depth on surface moorings to be consistently greater than on subsurface moorings at the same location. The information available on the effect of mooring motion on the quality of data frOIn.ocefl1lographic moorings (Pollard (22)) and the effect of stress cycling on the life of moored structures indicates that an understanding of the dynamics of surface moorings has a great practical value. The objective of the present paper is to report the results of numerical simulation of surface mooring dynamics and compare them with values actually measured in the deep ocean.

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