Questions and answers: Surge and dynamic stress analysis for an offshore wind farm using BOSfluids

How does BOSfluids ensure the accuracy of the surge analysis results?

As is the case for every software program the accuracy of the results depends on the accuracy of the input. However, BOSfluids does provide users with multiple features to help the user to prevent incorrect results. Every simulation case is checked for input errors and warnings are shown when any anomalies are found in the user input. The BOSfluids flow solvers also detect any anomalies warns the user if these occur during the flow simulation. By default, a convergence check is performed by BOSfluids to determine if the used time step is fine enough to obtain converged results. During the post-processing, the user also has tools to review the results using output reports and the graphical interface.
Each version of BOSfluids is put through a large number of validation cases to ensure the produced results comply with analytical/experimentally obtained results.

Is it sufficient to run a stress analysis using static loads and a DLF (Dynamic Load Factor) of 1.5?

Performing a so called quasi-static stress analysis using a dynamic load factor could be sufficient. However, an actual dynamic analysis using the actual time history of the forces from the surge analysis gives more accurate results. Using a DLF could mean that you are overly conservative by applying unnecessary large forces, but on the other hand, it could also mean that you are missing the fact that a dynamic load could induce structural resonance, greatly underestimating the dynamic stresses.
BOSfluids makes it easy to perform a time-history dynamic stress analysis, by giving the required dynamic input file, but in CAESAR you can also find the BOSfluids button to import the force spectrum (FRC-file) to perform a quasi-static stress analysis.

How are forces exported from BOSfluids to Ansys to perform an FEA analysis?

When performing a dynamic analysis in Ansys a similar approach can be followed as for CAESAR II. I.e. BOSfluids can create an Ansys input file and export the time history of the forces. But BOSfluids also supports a more direct approach for Ansys. In BOSfluids you can specify a structural analysis and couple this to a surge analysis. The results obtained from the surge analysis are used to perform a structural analysis in Ansys. Ansys is run in the background and the results (stresses and displacements) are loaded into the BOSfluids results tab. Note that this requires a valid license for Ansys Mechanical.

How are forces exported from BOSfluids to the CAESAR II dynamic module?

BOSfluids exports “time history force” files that are then imported into the CAESAR II dynamic module to perform dynamic analysis of the piping system. In the dynamic module of CAESAR II, the user specifies for each pipe section a specific time history file, the location, and the direction of the force. Specifying this manually for each pipe section would mean a lot of work and is prone to mistakes. Therefore BOSfluids also writes the input file for the CAESAR II dynamic module ensuring the complete time history of the forces on all sections is taken into account.

Is ISOtracer part of BOSfluids?

ISOtracer is a separate Dynaflow software program for which you can obtain a separate license. At this moment an active BOSfluids license or subscription comes with a free license for ISOtracer.

Surge and dynamic stress analysis for an offshore wind farm using BOSfluids