Analysis of a multiple buoy instrument platform; A non linear model

The dynamics of a taut-line mooring suspended by two series (in-line) buoys is studied. Both monochromatic and pseudo-random surface wave models are used as forcing functions. Nonlinearities in hydrodynamic drag and cable elasticity are treated and motions are restricted to two dimensions in a vertical plane. Numerical solutions, based on a previous work for single buoys, were performed on a digital computer to study the effects of mooring line elastic properties and surface wave models on buoy system response. Comparisons of motion and tension responses between a relatively stiff (steel) line, approximated as a linear elastic element, and a compliant (nylon) line, requiring a nonlinear elastic description, are made. The mooring line material comparison permits the selection of the more stable platform mooring while the comparison of responses to both monochromatic and pseudorandom forcing functions allows the evaluation of wave model efficiency as a design tool (i.e., model realism versus solution time). It has been found that, within the limitations of this analysis and for the particular system parameters studied, the use of a monochromatic input provides a conservative and more efficient model than a pseudo-random input for both nylon and steel-moored system response. Furthermore, the steel-moored system, and in particular its lower buoy, provides the most stable underwater instrument platform. In all cases dynamic components of mooring line tension are negligible.