OrcaFlex Manual - Orcina

Fatigue Analysis, Commands
436
w
For SHEAR7 fatigue analysis the load cases are specified by a set of SHEAR7 .plt output files. These are most easily
generated using the direct SHEAR7 interface, together with the standard OrcaFlex automation facilities – the .plt
files are automatically exported if you run the direct SHEAR7 interface in batch mode.
Each load case is assigned an exposure level. For regular load cases this is the total number of occurrences of waves
within the wave class. For the other methods the exposure level is specified as the total time exposed to waves
within the wave class.
Choice of fatigue analysis method
As described above OrcaFlex can perform three different types of fatigue analysis: regular, rainflow or spectral.
Rainflow fatigue is the most accurate of the methods, but also the most time consuming and demanding of disk
storage. The time and storage requirements can be somewhat alleviated by careful selection of load cases. The other
factor which can be adjusted is the duration of the irregular wave load case simulations. In our experience it is often
possible to achieve accurate damage predictions with simulations of 20 minutes duration.
Regular wave fatigue analysis is much faster and requires much less disk storage than rainflow fatigue. The wave
scatter conversion facility provides an efficient and productive way to generate a regular wave scatter table from a
random sea scatter table. Provided that the regular wave bin discretisation is performed well, the results from a
regular wave fatigue analysis will generally agree well with an equivalent rainflow analysis.
The spectral fatigue analysis method was originally included to provide a very quick alternative to the other
methods. The spectral fatigue method in OrcaFlex is much more difficult to use effectively than the other methods.
This is largely due to weaknesses and limitations in the response calculation approach used to generate response
RAOs. If you do perform a spectral fatigue analysis in OrcaFlex then it is very important that you check that the
spectral response RAOs are smooth. The response calculation method often results in very noisy RAOs which in turn
result in gross over-predictions of damage. It is our experience that use of the spectral fatigue method usually
results in poor and inaccurate results.
Recommendations
The advent of multi-core processors and the wave scatter conversion facility mean that regular wave fatigue
analysis is often just as fast as spectral fatigue analysis, as well as giving much more reliable and accurate answers.
Because of this we no longer recommend the use of spectral fatigue analysis in OrcaFlex.
This then reduces the choice of methods to regular and rainflow. Because of the calculation time and disk storage
advantages it is clearly desirable to use regular wave fatigue. Certainly during system design these advantages are
significant because they allow for greater coverage and exploration of the design space.
Another effective strategy is to use regular fatigue analysis for the bulk of the time and switch to rainflow analysis
for a final, more detailed check. If the regular wave fatigue analysis predicts a system life significantly in excess of
the design life then this final detailed check could be omitted.
8.2 COMMANDS
File Menu
New
Clears previously entered Fatigue Analysis data and resets data to default values.
Open
Opens a Fatigue Analysis file. These files can be either binary format (.ftg) or text files (.yml).
� The .ftg extension is used for binary format files. If the file does contain results then these will be available
without having to perform the time-consuming calculation again.
� The .yml extension is used for the text file format. These are YAML files and are intended to be used for
automation purposes.
Just as is the case for OrcaFlex data files, the binary file has strong version compatibility features. For example, when
OrcaFlex attempts to open a binary .ftg file written by a later version of the program it is able to report informative
compatibility warnings. The program is not able to be as helpful and informative when working with text data files
across program versions. Whilst we strive to achieve as much compatibility as possible for text data files across
program versions, we cannot achieve the same level of compatibility as that for binary data files.