Title
Used to label all output of the fatigue analysis.
Damage calculation
A variety of methods are available for calculating damage:
- Homogeneous pipe stress, which assumes a pipe made of a straight, uniform, homogeneous, linear material. The damage calculation is based on ZZ stress. This method is appropriate for metal risers.
- A stress factor-based approach, in which the stress is assumed to comprise a tensile contribution (proportional to either wall tension or effective tension) and a bending contribution (proportional to curvature). This method is commonly used for umbilicals, and could also be used for unbonded flexibles.
- The mooring fatigue method calculates damage from effective tension ranges using T-N curves.
- The histograms method does not involve damage at all, but instead performs rainflow cycle counting and collates these cycles into histograms.
- SHEAR7 has the damage calculated externally to OrcaFlex, by SHEAR7, allowing you to collate fatigue damage from a number of SHEAR7 load cases.
- Externally calculated stress follows the stress factor approach, but with the stress calculated by means of an externally-specified result. A demonstration of this method is in the external function examples.
Analysis type
Three types of fatigue analysis are available:
- Regular analysis must be based on a number of regular wave simulations that represent the various load cases that will occur. For each of these load cases, a single-occurrence damage value is calculated based on the last wave cycle in the simulation. This damage value is then scaled up by the specified number of cycles expected to occur during the structure's life to give the total load case damage value. Finally these total damage values for each load case are summed, over all load cases, to give the overall total damage.
- Rainflow analysis is normally based on a collection of random wave simulation load cases. A cycle-counting technique is used to break down each random wave load case into a series of half-cycles, and the damage from each half-cycle is summed according to the Palmgren-Miner rule (for details, see the book by Maddox and the paper by Rychlik). This gives the damage value for that load case, which is then scaled to the specified exposure time. Finally, these total load case damage values are summed over all load cases, to give the overall total damage.
- Spectral analysis calculates damage in the frequency domain using statistical methods. The method requires a power spectral density function (PSD) for a particular load variable (stress or tension). The PSD is used to calculate damage using either Dirlik's formula or the Rayleigh distribution, according to the choice of cycle range distribution. The damage is scaled to the specified total exposure time for the load case. Finally these total load case damage values are then summed for each load case to give the overall total damage. Two different forms of spectral analysis are available. The frequency domain form obtains the PSD from a frequency domain simulation, while the response RAO form combines response RAOs for the load variables of interest, e.g. stress, tension etc, with the load case wave spectrum to give PSDs for load. The response RAOs are generated either by a time domain response calculation simulation, or a frequency domain simulation.
Units
The units to be used for the fatigue analysis, for both the fatigue analysis data and for its results. The units are specified in the same way as elsewhere in OrcaFlex.
These units do not have to be the same as the units in the various load case simulation files. If they differ, then the results from that simulation file will automatically be converted to the units specified for the fatigue analysis. This can be useful, since it allows the fatigue analysis to be done using m and MN as the length and force units (giving stresses in MN/m2 = MPa), for example, even if the simulation load cases use m and kN (which corresponds to stresses in kN/m2 = kPa). Similarly, in US units, the fatigue analysis can use inches (giving stresses in ksi) even if the simulation files use feet as the length unit.
If you change units, then all existing fatigue analysis data are automatically changed to match the new units. As elsewhere in OrcaFlex, this is helpful if you want to enter data in some other set of units: simply change the units to those of the new data, enter the new data, and then change the units back to their original values.
Cycle range distribution (spectral analysis only)
The spectral fatigue calculation assumes that cycle ranges follow this statistical distribution: either Dirlik's formula or the Rayleigh distribution. The Rayleigh distribution is appropriate if the variation of the response is a narrow banded random Gaussian process, whereas Dirlik's formula is applicable even if the variation of the response is not narrow banded. For this reason we recommend using Dirlik's formula.
