Hydrodynamic loading of catenary mooring lines

Catenary moored ship and offshore structures exhibit large amplitude horizontal oscillations caused by low frequency components of the environmental loading, due to waves, wind and current, at the system resonant frequency. As a result of these large oscillations the mooring lines can undergo significant motion inducing large hydrodynamic drag forces. In many situations this line loading provides the major damping mechanism and reduces the resonant response of the floating vessel. The ability to predict induced mooring line tensions with accuracy, and the contribution of mooring line damping to total system response, is therefore of relevance to the offshore industry. Although a number of numencal models exist for the prediction of line tension and damping, the excitation of the mooring line at the point of suspension creates a complicated non-linear line response that is presently not well understood and inherently difficult to model. Given the nature of the line response, validation of numerical results with experimental data is of importance. This validation has however not been possible to date due to the extremely limited quantity of experimental data available. This thesis presents an experimental investigation into the response of small and large scale, single chain catenary mooring line models subject to a comprehensive range of top end excitation parameters. Measurements from the experimental investigation are compared with results from established numerical methods and those from a time domain numerical model developed in the thesis. Results from small and large model scale experiments are shown to be in reasonable agreement, providing further validation of existing numerical models. The new numerical model also shows good agreement and is demonstrated to be efficient and robust in its application to a wide range of line oscillation parameters.