Fatigue analysis in Flexcom is typically based on a series of random sea dynamic analyses, representing the loading experienced by an offshore structure over the course of its lifetime. Random variations in axial force and bending moment are translated into stress cycles which are then used to estimate fatigue damage. This approach is adopted in the vast majority of cases to estimate design life of offshore structures.
An alternative modelling approach is adopted to estimate fatigue induced by vortex induced vibration (VIV), where a structure is excited at a natural frequency by incident current loading. Fatigue of the riser is normally estimated by Shear7 following a modal analysis, and this is not related to LifeTime in any way. A more specialised case of VIV induced fatigue is that of pipe-in-pipe systems, where the VIV of the outer pipe forces the inner pipe to move accordingly. Given that the displacement and stresses in the inner pipe are effectively governed by the VIV response of the outer pipe, it is not possible to estimate fatigue damage in the usual manner via Shear7. Instead Flexcom adopts a novel approach based on a solution methodology proposed by Williams & Kenny (2017). The computational procedure involves the construction of regular/periodic time histories of bending moment, derived from the results of a static analysis of a riser system deformed into a specific mode shape, which are then post-processed by LifeTime in a manner similar to a random sea fatigue simulation. Refer to VIV Induced Fatigue of Pipe-in-Pipe Systems for further details.
There are three basic steps in performing a fatigue analysis with LifeTime, as outlined in the following sections.
If you are performing a standard fatigue analysis based on random sea loading experienced by an offshore structure over the course of its lifetime:
•Perform a Flexcom random sea analysis for each fatigue seastate. In setting up each analysis, you must store time histories of axial force, Y bending moment and Z bending moment at each location (hot spot) of interest. Earlier versions of Flexcom required these parameters be stored via timetrace output, whereas more recent versions also allow you to store this data using the more widely used database output. You can also optionally store any other time history of interest.
If you are performing a specialised fatigue analysis estimating fatigue damage of an inner pipe in a pipe-in-pipe configuration:
•Once the fatigue simulation commences, Flexcom automatically constructs the time histories of axial force, Y bending moment and Z bending moment at each hot spot for you. In preparation, you must have performed a series of static simulations in advance, each of which deforms the structure into a specific mode shape. Refer to VIV Induced Fatigue of Pipe-in-Pipe Systems for further details.
Specify the fatigue analysis input data. This would typically include the following information:
•The names of the Flexcom simulations and their corresponding percentage occurrences. For a standard fatigue analysis, these will be the names of the random sea dynamic analysis. For the specialised case of VIV fatigue of an inner pipe of a pipe-in-pipe configuration, these will be the names of the static simulations which represent the various mode shapes.
•Stress concentration factor
•S-N curve data
•Probability density function, whether Rayleigh or Dirlik
•Whether to use bending stresses only or combined bending and axial stresses
•Whether to include thickness effects, and if so, what threshold thickness to employ
•A list of the fatigue hot spots of interest
•Run LifeTime. When running, the program loops over all Flexcom simulations and all of the hot spots, and calculates fatigue damage using three different methods, each of which is described in Fatigue Analysis Methods. Further details are provided in Program Outputs.
•Numerous keywords are contained in the $LOAD CASE section, including data such as environmental parameters (e.g. current and waves).
o*DATABASE is used to request the storage of database output.
o*DATABASE CONTENT is used to customise the contents of the database output files.
o*TIMETRACE is used to request the storage of timetrace output (this option is not relevant for VIV Induced Fatigue of Pipe-in-Pipe Systems as you must use static database output in this case).
•Numerous keywords are contained in the $LIFETIME FATIGUE section, which corresponds to LifeTime Mode 1.
o*FATIGUE DATA is used to assign fatigue data to hot spot sets.
o*HOT SPOT SETS is used to define the fatigue analysis hot spots - these are the locations on the structure for which fatigue life estimates are required.
o*PDF is used to specify the probability density function to be used in calculating fatigue life estimates from stress spectra (this option is not relevant for VIV Induced Fatigue of Pipe-in-Pipe Systems as fatigue computations are based on regular/periodic time histories).
o*PROPERTIES is used to assign effective structural properties to hot spot sets for use in calculating stresses.
o*SEASTATE FILES is used to specify the names of the Flexcom simulations and their corresponding percentage occurrences. For a standard fatigue analysis, these will be the names of the random sea dynamic analysis. For the specialised case of VIV Induced Fatigue of Pipe-in-Pipe Systems, these will be the names of the static simulations which represent the various mode shapes.
o*S-N CURVE is used to define fatigue analysis S-N curves.
o*SOURCE TYPE is used to indicate the type of data storage file you wish to use as input to the fatigue analysis.
If you would like to see an example of how these keywords are used in practice, refer to A01 - Deepwater Drilling Riser.