The importance of properly representing line stiffness in mooring analysis

Simplified mooring analysis methods don´t model the true stiffness characteristics of mooring lines, don´t model the effects of mooring line tails, and/or don´t model the on-deck length of mooring lines. Such simplified analysis methods might either overestimate or underestimate mooring line loads and vessel excursions. In this paper, comparisons are made between static mooring analyses using correct nonlinear mooring line stiffness and linear approximations of that stiffness. Comparisons are made between analyses representing the entire mooring line length and representing only the outboard length. Comparisons are made between analyses using only wire lines and using wire lines with tails. The purpose of tails is to decrease mooring line stiffness and thus reduce peak wave-induced mooring loads. But this effect is not evident in ordinary static mooring analysis. Comparisons are made between mooring lines with and without tails, using a kinematic mooring analysis technique to represent fairlead motions caused by wave-induced vessel motions.