EFFICIENT RELIABILITY-BASED DESIGN OF MOORING SYSTEMS
Keel Larsen, MARINTEK (Norwegian Marine Technology Research Institute), Trondheim, NORWAY
ABSTRACT
Uncertainties both in the environmentally induced load effects and in the strength of mooring line components make a rational design of mooring systems a complex task. The methods of structural reliability, taking these uncertainties into account, have been applied in an efficient probablistic analysis procedure for the tension overload limit state for mooring lines. This paper outlines the philosophy and methodology for this procedure, followed by numerical examples of a turret moored ship. Both "base case" annual failure probabilities and results from a number of sensitivity analyses are presented.
It is demonstrated that the reliability-based design procedure can be effectively utilized to quantify the safety against failure due to tension overload of moorings. The results of the case studies indicate that the largest uncertainties are associated with the distribution parameters of the chain link and steel wire rope segment tension capacity, and the modelling of the environment. The modelling of spreading angles between waves, wind and current vs. colinearity, and double-peaked vs. single-peaked wave spectrum models are key parameters in the reliability assessment.
SURGE AND PITCH COUPLED NONLINEAR RESPONSES OF A SINGLE POINT MOORING SYSTEM
Rujian Ma, Department of Mechanical Engineering, University of Petroleum, China, Dongying, Shandong, P. R. China
Guixi Li, Institute of Technology, Dezhou Petroleum Machinery Plant, Dezhou, Shandong, P. R. China
ABSTRACT
The nonlinear dynamic analysis of the single point mooring systems under the action of random sea waves was carried out by means of nonlinear spectral analysis. The study indicates that it is possible to solve nonlinear vibration problems by using spectral analysis directly. It is not necessary to linearize the nonlinear terms in this method so that the errors introduced by linearization can be eliminated. Therefore, this method will provide a convenient and accurate tool for solving nonlinear random vibrations.
