A versatile and efficient numerical analysis is developed to compute the responses of a moored floating system composed of multiple floating structures. Structures such as tankers, semisubmersibles, FPSOs, SPARs, TLPs, and SPMs connected by mooring lines, connectors or fenders may be analyzed individually or collectively including multiple interaction. The analysis is carried out in the time domain assuming rigid body motion for the structures, and the solution is generated by a forward integration scheme. The analysis includes the nonlinearities in the excitation, damping, and restoring terms encountered in a typical mooring system configuration. It also allows for instabilities in the tower oscillation as well as slack mooring lines. Certain simplifications in the analysis have been made, which are discussed. The exciting forces in the analysis are wind, current, and waves (including a steady and an oscillating drift force), which are not necessarily collinear. The waves can be single frequency or composed of multiple frequency components. For regular waves either linear, stretched linear or fifth order theory may be used. The irregular wave may be included as a given spectral model (e.g., PM or JONSWAP). The vessels are free to respond to the exciting forces in six degrees of freedom—surge, sway, heave, roll, pitch, and yaw. The tower, when present, is free to respond in two degrees of freedom—oscillation and precession. The loads in the mooring lines are determined from prescribed tension-strain tables for the lines. Rigid mooring arms can be analyzed by allowing for compression in the load-strain table. Fenders may be input similarly through load compression tables. In order to establish the stability and accuracy of the solution, comparison of the results with linearized frequency domain analysis was made. The analysis is verified by several different model test results for different structure configurations in regular and random seas. Some of the interesting aspects of nonlinear system are shown with a few examples.

Dimelow, D. J., Neilson, R. D., and O’Donoghue, T., 1996, “Non-Linear Tower Motions in Waves,” Proc. of Sixth Conf. on Offshore and Polar Engineering, Los Angeles, CA, pp. 240–246.
Gottlieb, O., 1996, “Bifurcations of a Nonlinear Small-Body Ocean Mooring System Excited by Finite Amplitude Waves,” Proc. of Int. Conf. Offshore Mechanics and Arctic Engineering, ASME.
Chakrabarti, S. K., 1990, Nonlinear Methods in Offshore Engineering, Elsevier, Amsterdam.
Chantrel, J., and Marol, P., 1987, “Subharmonic Response of Articulated Loading Platform,” Proc. of Sixth Conf. on Offshore Mechanics and Arctic Engineering, Houston, TX, pp. 35–43.
Chakrabarti, S. K., 1987, Hydrodynamics of Offshore Structures, Computational Mechanics Publications, Southampton, UK.
S. K.
, and
D. C.
, “
Motions of Articulated Towers and Moored Floating Structures
ASME J. Offshore Mech. Arct. Eng.
), pp.
J. N.
, “
The Drift Force and Moment on Ships in Waves
J. Ship Res.
, pp.
You do not currently have access to this content.