VIV Induced Fatigue of Pipe-in-Pipe Systems |
 
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VIV Induced Fatigue of Pipe-in-Pipe Systems |
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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, which are then post-processed by LifeTime in a manner similar to a random sea fatigue simulation. Flexcom can create the time histories automatically for you, based on a series of static analyses which deform the riser system statically into specific mode shapes. Alternatively, you can apply the deflected mode shapes using sinusoidal boundary conditions in a series of dynamic simulations - this approach is more accurate in the sense that riser inertia and hydrodynamic loading are taken into account. Further details are provided in the referenced paper but the main points are summarised here.
Perform a modal analysis of the system in order to determine the natural frequencies and associated mode shapes. Perform a single mode VIV analysis of the proposed riser system using Shear7 to determine the dominant mode of response and its associated frequency.
Assuming that each exciting mode acts independently, take the normalised mode outputted from Modes corresponding to the discrete mode excited in the Shear 7 analysis and scale the normalised mode by the maximum displacement amplitude predicted by Shear7 for a given current load.
The deflected mode shape is applied using constant boundary conditions along the riser and solved statically, so that static force databases are available with axial force, local y and z bending moment at riser hotspots. You must create these static offset files manually, applying the mode shapes via *BOUNDARY->TYPE=CONSTANT. Refer to A03 - Pipe-in-Pipe Production Riser for an illustration of this approach. Flexcom subsequently uses these static force databases to fabricate dynamic time histories.
The deflected mode shape is applied using sinusoidal boundary conditions along the riser and solved dynamically, so that dynamic force databases are available with axial force, local y and z bending moment at riser hotspots. The displacements should oscillate with a time period which corresponds to the modal frequency. This approach is more accurate in the sense that riser inertia and hydrodynamic loading is taken into account. You must create these dynamic offset files manually, applying the mode shapes via *BOUNDARY->TYPE=SINUSOIDAL. Each dynamic file should request storage of axial force and local bending moment via *DATABASE or *TIMETRACE->TYPE=FORCE.
Enable SOURCE=TIMETRACE in the *SOURCE TYPE keyword and refer to the static solution database files via the *SEASTATE FILES keyword, with the FREQUENCY=Modal frequency defining the frequency of the exciting mode. Ensure that PIP VIV AUTOGENERATE=YES in the *SEASTATE FILES keyword (this is the default option).
The LifeTime fatigue analysis proceeds in the usual manner but rather than reading time histories of axial force, Y bending moment and Z bending moment at each hot spot from dynamic database files, they will be constructed from the static database files using the following method.
Time histories of Y and Z bending moment are generated from the following formula:
where:
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is the Y or Z bending moment time history at time t.
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is the local Y or Z bending moment from the static analysis.
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is the modal frequency of the excited mode.
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is time.
By default, the constructed time histories are one period in duration and contain 40 equally spaced data points, but you can vary these parameters if you wish.
Axial force is considered to be constant over the time history.
Once all the time histories are generated by Flexcom, the LifeTime fatigue analysis will proceed in the normal manner.
For an illustration of this methodology in practice, refer to A03 - Pipe-in-Pipe Production Riser.
Refer to the dynamic solution database files via the *SEASTATE FILES keyword, with the FREQUENCY=Modal frequency defining the frequency of the exciting mode. Ensure that PIP VIV AUTOGENERATE=NO in the *SEASTATE FILES keyword (this informs Flexcom not to automatically generate the time histories). Choose the relevant option under the *SOURCE TYPE keyword depending on whether you are using database or timetrace storage output for the time histories. The LifeTime fatigue analysis proceeds in the usual manner, reading time histories of axial force, Y bending moment and Z bending moment at each hot spot from the dynamic storage files.
•*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 or dynamic simulations which represent the various mode shapes.
•*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 A03 - Pipe-in-Pipe Production Riser.