The cumulative distribution function of the stress range can be expressed as In the simplified fatigue assessment method, the two-parameter Weibull distribution is used to model the long-term distribution of fatigue stresses. Weibull distribution most effectively define loading data on offshore structure. The loading history is represented in probabilistic terms by a probability distribution function. The sum total of wave occurrence fractions for all fatigue load cases should be 100. So, basically it is the contribution of that fatigue case towards the total fatigue damage. The вЂWave Occurrence FractionвЂ™ represents the percentage of time that the structure encounters a specific load case. Each environment case will be characterized by its significant wave height, the zero-crossing period and wave occurrence fraction. There may be multiple environments throughout the lifetime of the structure, and each will contribute to the assessment. Since waves are the primary cause of cyclical stress on a structure, the fatigue analysis needs to be performed for the environment loading history that the structure will experience during its lifetime. When carrying out fatigue assessment of an offshore structure, one vital input is the environment in which the structure will be operating. The following parameters are obtained from the S-N curves: The details of these classes can be found in APPENDIX 1 of the ABS Guide on Fatigue Assessment of Offshore Structures. There are eight such nominal classes: B, C, D, E, F, F2, G, and W. They look like the below:ДЄ consideration in these calculations is that linear cumulative damage (as in the Palmgren-Miner method) applies.ДЇurther, the S-N curves are presented according to the вЂclassвЂ™ of the geometrical details of the structure. In the approach used by ABS Guide for Fatigue Assessment, two-segment S-N curves are used. Usually both N and S are displayed on logarithmic scales. Here, A and m are вЂfatigue strength coefficientвЂ™ and вЂexponentвЂ™ respectively and are determined from tests. The general relationship between the Stress Range (S) and the number of cycles to failure (N) is given by: These curves are plotted based on experimental tests done on specimens. S-N curves are plots of stress range versus the number of cycles to failure for a material. This is the approach adopted by the S-N curves method. Since fatigue is a result of cyclical stress, it will be useful to know how many cycles of stress a material can withstand under a certain stress range before it fails. The best way to calculate Fatigue demand is through a thorough structural analysisДЄnother important concept is that of Stress Concentration Factor (SCF) which is a factor introduced to take care of the localized rise in stress in a structure due to local geometrical variations (like a rat hole in way of the connection) The fatigue demand is calculated in terms of stress ranges that the structure experiences. Since the load varies, the stress too varies about a mean value. If the fatigue strength is more than the fatigue demand, the structure is safe. The former is the amount of stress that the structure experiences due to the variable loads, while the latter is the capacity of the structure to withstand fatigue loads. The key concepts here are Fatigue Demand and Fatigue Strength. In this article, weвЂ™ll discuss a simplified method for performing fatigue analysis of offshore structures using American Bureau of Shipping (ABS) Guide for Fatigue Assessment of Offshore Structures (June 2020). Usually, it is the welds which develop the cracks first that then propagates to the rest of the structure. In some cases, the vibrational loads of the structure may also result in fatigue. It is estimated that over a lifetime of 20 years, an offshore structure will experience 10 8 cycles of stress. This phenomenon is called вЂfatigue crackingвЂ™. Over a short period of a few months, this cyclical variation of stress doesnвЂ™t affect the structure, but over the lifetime of the vessel, constantly experiencing cyclical stresses may lead to cracks in the structure. These cyclical loads result in cyclical stresses on the structure too. By their nature, the resulting wave loads on the structure are cyclical. A simplified method of performing fatigue analysis of offshore structuresДЄn offshore structure is subject to environmental loads of waves, wind and current.

Calculate fatigue strength