The Effect of Cable Dynamics on the Station-Keeping Response of a Moored Offshore Vessel

An anchoring system for an offshore structure must meet certain prescribed requirements controlled by factors such as the site environment, operational constraints and the vessel employed. Its adequacy, survival and ability to stay on site must, therefore, be checked out with proper methods of analysis. The inclusion of cable dynamics is an important consideration in the dynamic analysis of a moored vessel. In this paper, mooring line equations of motion are derived for a multi-component, n-segment model using Lagrange’s modified equation, permitting anchor motion, and then numerically solved to yield time histories of cable displacements and cable tensions for the various cable configurations that can occur. Initial conditions can be provided through the mooring line static catenary equations. The nonlinear restoring force terms in the vessel equations of motion are generated through the dynamic tension- displacement characteristics of individual lines. The equations of motion of the moored vessel subjected to an open ocean environment are then numerically solved to yield time histories of vessel motions and cable tensions. An example involving a moored production barge is examined and results are compared with those of previous work in which a quasi-static cable configuration is employed.