Analysis of the Three Methods for Computing Fatigue of Tanker Moorings Presented in API RP 2FPI
Authors: Shoup G.J. , Bhst S.U. 
Published: October 2008

In this study, the three API methods for computing fatigue damage are analyzed. The paper discusses the relative accuracy of the three methods, and also illustrates how the accuracy depends on mooring configuration and seastate intensity. The results will help mooring system engineers decide which API method is most appropriate for a particular case.

Introduction

The American Petroleum Institute Recommended Practice (2FP1) for Design, Analysis, and Maintenance of Mooring for Floating Production Systems presents three methods for determining the fatigue damage from environ. mental loading&gt; Each method, as explained in the API RP, assumes the designer is working with computer generated low frequency (long period) mooring tension spectrums and computer generated high frequency (wave period) mooring tension spectrums. Each method combines the low frequency and wave frequency tension spectrums in a different way. Fatigue analysis is usually done in the manner because computer simulators compute the low frequency motions and tensions separately from the wave frequency motions and tensions.

This paper presents the results of the three API fatigue analysis methods applied to mooring line tension histories generated by model testing. A spread moored tanker and a single point tanker mooring are analyzed. The three methods are also applied to synthetic time histories constructed by adding two sinusoids, one representing the low frequency tension component and the other representing the wave frequency tension component. The ratio of the tension amplitudes is varied to show the behavior of the three methods for all possible amplitude ratios.

API method 3, which is a rainflow cycle counting of the entire time history, is assumed to yield the most accurate fatigue damage estimate because it analyzes the entire time history directly. Methods 1 and 2 rely on tension spectrums to describe the load history, and are designed for use when tension time histories are not available. This is generally the case when computer simulations are used for the fatigue assessment. Commonly used mooring programs do not combine the low and high frequency mooring tension variations to form a true time history. For this reason, most designers must rely on model test results to use method 3. Because of the high cost of testing and lack of planning, the necessary time histories for implementing method 3 are usually not available.

The purpose of analyzing the model test time histories was to compare the three methods. Usually this type of comparison is not possible because time histories are unavailable. The comparison revealed that the fatigue life ratio of methods 1 and 2 to method 3 is a function of mooring configuration and seastate intensity.

The purpose of analyzing the synthetic tension histories was to determine the deviation of methods 1 and 2 relative to method 3. Synthetic results suggest method 1 can over estimate the fatigue life by a factor of 2, while method 2 can under estimate the the fatigue life by a factor of 2 (approximate values). Actual model test time history analysis show that method 1 can over estimate fatigue life by a factor of 3.5.