﻿ Dynamic analysis: Quasi-dynamic analysis

# Dynamic analysis: Quasi-dynamic analysis

For many applications, such as modelling mooring lines attached to a vessel, the accuracy of the full finite element representation of the line, whereby it is discretised into individual nodes that each carry degrees of freedom, is not required. Often, the interest lies only in the vessel and the finer details of the forces acting within the mooring lines are irrelevant: all that matters is to obtain a realistic approximation of the forces applied by the mooring lines to the vessel.

This form of analysis is known generally as quasi-dynamic analysis. It is a long-established method for mooring analysis, and is described, for instance, in Bureau Veritas NR 493. The method is implemented by a number of codes, including ARIANE and MIMOSA.

For a typical quasi-dynamic analysis the only calculated degrees of freedom in the system are those of the vessel. It is this reduction in the amount of calculation which leads to the significant reductions in analysis time which can be obtained from quasi-dynamic analysis compared to a full finite element solution. In addition to the obvious simplification, it is often possible to use the explicit time domain solver with a relatively large time step, while still retaining sufficient accuracy, for such models, leading to further performance gains.

To carry out quasi-dynamic analysis in OrcaFlex, you make use of the analytic catenary representation for lines to avoid the full finite element calculation. The mooring line loads are calculated from classical analytic catenary equations. Clearly, some simplification is required in order for us to be able to solve for the lines without invoking the full finite element method, and that simplification is that the analytic catenary does not account for inertia or bend stiffness. For mooring lines, this may be a reasonable approximation. Also, analytic catenary lines do not allow for any damping: if this is not a reasonable assumption, an additional source of damping can be introduced through vessel other damping data.