A new methodology has been developed in the form of a Fortran computer program, NICDAF, to perform Non-linear Integrated Coupled Dynamic Analysis of Floaters. The program computes six degrees of freedom rigid body motions, in time domain, at a fraction of time required by other methods. It captures all coupling effects of the connected risers and mooring lines, which are important for accurate motions of a platform in deep waters. The method computes respective restoring, inertia, and drag forces, using their quasi-static configurations at each time step instead of actual dynamic equilibrium based configurations. This results in substantial savings in computation time. In addition, the program is capable of including frequency dependency of added mass and radiation damping more accurately, taking due account of the system non-linearity. Comparison of case study results of dynamic motions from NICDAF with results obtained from rigorous coupled dynamic analysis, using ABAQUS, showed very good agreements. This means coupled dynamic analysis of platforms connected with risers and mooring lines can be performed even for routine and repetitive design and analysis work, using NICDAF.
Designs of mooring systems and risers, connected to platforms in deep waters, are dominated by the motions of the platforms. An over prediction of motion would require costly risers and moorings, whilst an under prediction of motion can lead to catastrophic consequences. Accurate computation of motions of floaters, operating in deep waters, is very important for the safety, reliability, and associated costs of the risers and moorings. These are resonant responses due to slowly varying forces from wind and waves, at natural frequencies of the platform. Therefore, in addition to accurate computation of wind and wave forces, actual damping values of the system of platform and connected risers and mooring lines are required for a realistic dynamic motion.
