Fatigue analysis in the frequency domain

In summary, the conversation discusses the use of a TLP platform to examine and measure wave forces acting on tendons, with a focus on calculating fatigue and producing S-N curves. It is noted that the tendons will always be in tension due to buoyancy, with the exception of breaking waves or sudden encounters with the bottom of the structure. The question then asks for clarification on the distinction between "instant forces" and "cyclical" forces.
  • #1
Dimis414
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Assuming that a TLP platform is examined and the wave forces acting on the tendons of the mooring lines are presented in RAOs (as a function of wave frequency). Using Jonswap spectra for North sea (i.e peakness factor equal to 3.3) we can obtain Hs (significant wave height); Tp (peak period) and the acting forces on the tendons for the specific location. These forces are instant forces from waves but not cyclic loading. The question is how to calculate fatigue in the tendons from these RAOs values, and how to produce S-N curves from RAOs values.
 
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  • #2
If this is a TLP (Tension Leg Platform) retained by tendons there are two planes of wave loading, vertical due to buoyancy and horizontal due to fluid drag on the submerged portion of the floating structure and its anchoring tendons will always be in tension due to the structure buoyancy.
Only in the case of breaking waves would any instant horizontal forces as opposed to cyclical be expected; and, the vertical component due to structure buoyancy would be expected to be cyclical, except for the case where the wave crest would suddenly encounter the bottom of some significant horizontal face on the structure.

Please clarify your "instant forces but not cyclical" statement.
 

1. What is fatigue analysis in the frequency domain?

Fatigue analysis in the frequency domain is a method used to assess the potential for failure in a material or structure due to repeated loading or stress cycles. It involves analyzing the response of a structure to cyclic loading at different frequencies, rather than just a single frequency. This allows for a more comprehensive understanding of potential failure modes and can help determine the fatigue life of a structure.

2. How is fatigue analysis in the frequency domain different from other methods?

Fatigue analysis in the frequency domain differs from other methods, such as time domain analysis, by considering the frequency content of the loading. This is important because different frequencies of loading can have varying effects on the structure's fatigue life. Time domain analysis only considers the response of the structure at a single frequency, while frequency domain analysis looks at the response over a range of frequencies.

3. What are the benefits of using fatigue analysis in the frequency domain?

Using fatigue analysis in the frequency domain allows for a more accurate and comprehensive assessment of a structure's fatigue life. It can also help identify potential failure modes that may not be apparent in other methods. Additionally, frequency domain analysis can be more efficient and cost-effective, as it allows for the use of simpler models and requires less computational resources than time domain analysis.

4. What are some common applications of fatigue analysis in the frequency domain?

Fatigue analysis in the frequency domain is commonly used in the aerospace and automotive industries, where structures are subjected to repeated loading and must have a long fatigue life. It is also used in the design and analysis of bridges, wind turbines, and other large structures that are exposed to cyclic loading. It can also be applied to smaller components, such as engine parts, to ensure their durability and reliability.

5. What limitations should be considered when using fatigue analysis in the frequency domain?

While fatigue analysis in the frequency domain has many benefits, it also has some limitations. It may not accurately capture the effects of complex loading patterns or interactions between different frequencies. Additionally, it relies on assumptions and simplifications that may not always reflect real-world conditions. It is important to carefully consider these limitations and validate the results with physical testing before making design decisions based on frequency domain analysis.

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