Failure of moored structures from accumulated fatigue damage in shackles, connecting links, chain and wire rope components is common. When systems will be deployed for long periods, it is especially important to determine at the design, inspection and maintenance stages the fatigue damage. Since slack moored structures behave in a highly nonlinear manner, commonly used fatigue analysis procedures are normally inadequate. This paper reviews present probabilistic fatigue analysis methods, and provides a means for incorporating nonlinear mooring behavior into analysis and design to predict accumulated damage and remaining service life.
The procedures presented are general, and they are also applicable to ship and buoy moorings, offshore terminals, and guyed and tension leg platforms.
Structural fatigue is a function of ocean wave environment, load history and dynamic response characteristics of the structure. The basic steps of a mooring system fatigue analysis are:
vessel motion analysis1
mooring line dynamic analysis1
cyclic stress range characterization1 and
fatigue damage prediction.
Presently, there are three distinct approaches deemed applicable. These are:
frequency domain analysis;
time domain analysis1 and
a hybrid frequency/time domain method.
Selection depends on the degree of nonlinearity that is introduced through any of the five steps above, and the ultimate accuracy required of the analysis. The three approaches are schematically outlined in Fig. 1. A brief description of each follows.
Frequency domain analysis is applicable if all force-displacement response and fluid resistance characteristics can be reasonably approximated as being linear. In such a case, vessel motion analysis would be carried out in the frequency domain to provide vessel response using linear waves. From this, the displacement spectrum of the upper end of the mooring is then determined. A nonlinear time history dynamic analysis of the mooring line would then be carried out to determine the tension range transfer function. The displacement response spectrum and tension range transfer function are then used to determine the tension range response spectrum. A probability distribution function would be used to characterize the number of loading cycles associated with each tension range. A fatigue damage model is then applied to compute the accumulated damage and fatigue life of the system. Once the tension range transfer function has been developed, then fatigue can be determined for all sea states for which the linear assumption holds.
Time domain analysis is the most powerful analytic method applicable to nonlinear dynamic systems. Simply stated, this is a step-by-step numerical solution or "simulation" method. When properly handled, this approach can accommodate arbitrary loads or driving conditions, including irregular seas. Linearization of vessel motions, and other restrictive assumptions associated with frequency domain analysis, are not necessary. Vessel motions resulting from an actual or synthesized irregular wave record are used to determine displacement time history for the upper end of the mooring line. This would then be used as input, for a nonlinear dynamic time history analysis, to obtain line tension time history.