SHEAR7 Interfacing Operation |
 
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SHEAR7 Interfacing Operation |
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The main purpose of this module is to quickly and efficiently run a full SHEAR7 analysis with very little effort from the Flexcom user. This section outlines the theory behind the modal operation for SHEAR7 input, current transformation, input file creation, and finally running a SHEAR7 analysis.
What actually happens when you request data for input to SHEAR7 is the following. The actual eigensolution proceeds in exactly the same way as when the SHEAR7 interface is not invoked – the generation of SHEAR7 input is purely a postprocessing operation. Once the eigensolution is completed, Modes highlights the mode shapes of interest for SHEAR7 analysis as pure Bending modes, and in the case of SCRs, these modes are further categorised into In-Plane Bending and Out of Plane Bending modes. Although not relevant to SHEAR7, the remaining modes are classified as pure Axial modes, pure Torsional modes, and Unknown (typically mixed) modes.
This is an important step in the process. The reasoning is this. Modes is naturally a full 3D eigensolver, and all possible types of modes including bending, axial and torsional motions, and combinations of these, can be predicted. However of these, only pure bending modes are of consequence for the SHEAR7 calculations, so Modes does what it can to identify these. The first step in this process (for TTR and SCR types only) is to immediately eliminate any mode for which the maximum displacement occurs in either DOF 1 (likely to be a pure axial mode) or DOF 4 (likely to be a pure torsional mode). Any such modes are classified as Axial and Torsional respectively. Thereafter the program applies different criteria depending on the riser classification.
For a TTR, pure bending modes are assumed to occur in identical or nearly identical pairs. So Modes searches for these pairs and categorises one of them as Bending and the other as Unknown. Any singly-occurring mode is likewise deemed Unknown and excluded from the SHEAR7 data. The application of this rule assumes your TTR model is symmetric (or almost symmetric) with respect to horizontal displacements; for unambiguous results you should take care to ensure this is the case in setting up your model.
Modes outputs the SHEAR7 data for a TTR to a single file entitled common.mds – this is the name SHEAR7 requires you to use for this input. The common.mds data is in exactly the required format for SHEAR7, and comprises details of modal distributions of displacement and curvature for each mode in the data. A “header block” at the top of the file lists the actual natural frequencies. For further details, refer to MDS File Format.
For an SCR, the division of Bending modes into In-Plane and Out of Plane modes is a relatively straightforward operation. For In-Plane modes, displacements occur only in the plane of the SCR. Note that this plane can be oblique to the global co-ordinate axes. There is no requirement to set up your model in either of the XY or XZ planes, although this would be usual. Likewise Out of Plane modes have displacements normal to the initial plane of the SCR only. Any mode combining motions in all three directions is deemed Unknown.
Modes routes SCR SHEAR7 data to two files, named common.inp (for in-plane modes) and common.out (for the out of plane modes). The reason is that the two sets of modes will be excited by different current distributions and so will typically be considered in separate SHEAR7 runs. When doing your SHEAR7 analysis, you will need to rename one or other common.mds as appropriate. You can also indicated to Modes in advance whether you are interested in in-plane or out of plane modes, in which case the relevant common file will be renamed automatically for you.
Earlier versions of the software did not possess the User riser type option. This capability was added subsequently to cater for models for which the SHEAR7 interface was not producing meaningful output (for example, 3D spool pieces). Specifically, for the TTR and SCR options, the modal displacements relate to a riser plane, which is computed internally as a vertical plane which includes the start and end points of the structure. Obviously this is valid for standard models which lie in a single plane, but is not very useful arbitrary structures which do not lie in any particular plane. If you invoke the User option, both global and local displacements are presented as part of the standard program output. This allows for extraction of the normalised local element displacements and curvatures for a given mode shape which can then be translated for use in a subsequent SHEAR7 analysis. Note that Modes does not make any assumptions about what constitutes a bending mode for a user-defined riser (the “rules” mentioned for TTR and SCR risers are irrelevant in this context), so the onus is on you to interpret the mode shapes manually, and to explicitly specify which modes you wish to include in the SHEAR7 output. Any modes which you decide to include are classified as Bending, and any ones you exclude are deemed Unknown.
Modes outputs the SHEAR7 data for a User riser to a single file entitled common.mds – this is the name SHEAR7 requires you to use for this input. The common.mds data is in exactly the required format for SHEAR7, and comprises details of modal distributions of displacement and curvature for each mode in the data. A “header block” at the top of the file lists the actual natural frequencies.
After the modal frequencies and shapes have been satisfactorily obtained from $MODES, the $SHEAR7 interfacing module allows you to convert the Flexcom model into SHEAR7 format, transform current profiles to x/L format, and run SHEAR7.
The model is constructed quickly by assuming the element set included in the common.mds file, or mds set, is the desired set to be converted to the SHEAR7 format. From this set, a number of properties/values are obtained from the modal analysis elements including the unit flag, drag diameters, outer diameters, inner diameters, unit masses, internal fluid masses, added masses, internal fluid mass, element lengths, number of segments, riser type and axial stiffness. Some of these values are used directly in the creation of the SHEAR7, while others are used in the determination of other SHEAR7 parameters. Young's modulus, for example, is back-calculated from the axial stiffness and area; other values such as diameters and added mass coefficients are used explicitly. All values used in the SHEAR7 analysis are presented in the Flexcom output file, SHEAR7 input file, and additionally in the SHEAR7 output file once the analysis has run. It it is recommended to check both outputs to ensure that the desired element set and properties have been used, as very occasional default values may not be desirable.
After the model has been converted, and the current transformed, SHEAR7 is employed. Flexcom will notify you if there is an issue with SHEAR7 licensing, or if you have specified an invalid path to the SHEAR7 executable. Unfortunately, due to time it takes for SHEAR7 to search for a dongle, generating *.dat files without a SHEAR7 dongle will take significantly longer. Currently, the onus is on the user to post-process SHEAR7 results from here, but this feature will likely be supplemented in a future version.
1.BLOCK 1 - Unit System
2.BLOCK 2 - Structural & Hydrodynamic Data
3.BLOCK 3 - Current Data
4.BLOCK 4 - S-N and SCF Data
5.BLOCK 5 - Computation/Output Options
6.BLOCK 6 - Supplementary Data
Once provided the SHEAR7 input file (*.dat), there are 4 calculation options supported by SHEAR7. Option 2 is used exclusively by Flexcom.
1.Option 0 – This option calculates the natural frequencies and modes shapes being computed. This option does not compute the VIV response or damage rate.
2.Option 1 – This option calculates the natural frequencies and mode shapes, then additionally perform a VIV analysis.
3.Option 2 – This is the sole option employed by Flexcom’s uncoupled mode. This option uses externally (Flexcom provided) structural natural frequencies and mode shapes located in a file called “common.mds”. The VIV response is calculated based on these predicted mode shapes.
4.Option 3 – This option is similar to option 2, except this option allows the user to specify the name of the *.mds file.
Since $SHEAR7 employs calculation Option 2, in which the mode shapes and frequencies are already available to SHEAR7 in the “common.mds” file, there is some information that is not required by SHEAR7 to perform the VIV analysis. This is as follows:
1.Flag for Structural Model
2.Effective Tension at Origin
3.Inertia
4.Submerged Weight/Length
In the writing of SHEAR7 input file, $SHEAR7 will use assign nominal values for these unnecessary parameters.
1.Flag for Structural Model: “999”. Any number could be used when calculation Options 2 and 3 are employed as the modal analysis has already been performed and the *.mds file generated. However for clarity that an external file is being used, a token value of 999 is written.
2.Effective Tension at Origin: “0.100000E+01”. It is beneficial to use a non-zero value as SHEAR7 will issue an warning if the value is set to zero. A value of 1.0 should potentially prompt users to review the Flexcom documentation, in which case they will discover this value is not necessary.
3.Inertia: Calculated. Although this value is unnecessary, a value is calculated anyway.
4.Submerged Weight/Length: "0.00000E+00”.
VIV is only considered to be caused by flow that is normal to the riser. This is not generally an issue for TTRs, because they can be seen as vertical or almost vertical, and there is no specific vertical plane flow. However, it has a significant impact for in-plane current flow which excites out-of-plane modes for SCR and SLWR configurations. The normal current used in this case is determining by the inclination of the element. Another limitation is that SHEAR7 only provides functionality whereby the user can specify the normal flow velocity not taking into account the directionality. This often poses a problem in the modelling of SLWRs as the flow is actually reversed in the hogbend. While is not completely accurate to consider the flow as being all in the same direction, this is the only way that SHEAR7 currently allows, and it is generally an acceptable solution as it is typically more conservative. If you wish to use your own methodology for the application of current, an option has been reserved for explicit definition in terms of x/L. Elevation is converted to SHEAR7 x/L format by using each nodal locations resulting in a largely detailed current profile. A minimum of two points are required to define a current profile. When the current is defined in terms of x/L, that explicit current value is used directly in the SHEAR7 input file.
A number of default values are automatically assumed which allows you to quickly generate the SHEAR7 input file, with very little mandatory effort. While for most cases, the default values will be sufficient, there are a number of supporting keywords should you decide to change them. Most of the default values correspond to bare riser parameter as recommended by SHEAR7. These default values are presented in the table below:
Default Parameters and Coefficients
Property |
Default Value |
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Added Mass |
1.0 |
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Strouhal Number |
0.18 |
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Bandwidth |
0.40 |
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CL Table |
1 |
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Bet Control Number |
4 |
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Damping Coefficients |
Reynolds Number Still Water (DampCoeff0) |
1.00 |
A/D Still Water (DampCoeff1) |
0.20 |
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Low VR Regions (DampCoeff2) |
0.18 |
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High VR Regions (DampCoeff3) |
0.20 |
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Axial Flow Regions (DampCoeff4) |
0.00 |
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Power Cutoff Ratio |
0.05 |
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Primary Zone Amplitude Limit |
0.3 |
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Lift Coefficient Reduction Factor |
1.0 |
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Power Ratio Exponent |
1 |
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Other values that are specific to the model and zones are extracted, evaluated, or calculated from the modal restart or database files. The unit flag is derived from the gravitational constant. Hydrodynamic diameter, outer diameter, inner diameter, added mass, are extracted from the modal database. Modulus of elasticity is calculated from axial stiffness, and area calculated from the outer diameter.
While it might appear that a lot of input data is required to build the model, if the default values are used, then realistically there are only a small number of keywords that need to be explicitly defined. These include *RESTART, *CURRENT, *FATIGUE DATA and *S-N CURVE.
SHEAR7 recommends not attempting to model too much detail in zones. The zone capability in SHEAR7 should not be used to include small structural details. On the other hand, Flexcom will seek to capture even slight changes in geometry when it builds the SHEAR7 model. This may be problematic in areas like tapered joints with changing geometry may cause the generation of multiple zones. In order to allow you to best capture these geometric changes as you see fit, *ZONES can be used to generate sections with equivalent properties.
In the generation of these equivalent sections, the maximum drag diameter is used while a weighted average of other parameters is used. It’s worth noting that when internal fluid is present, the total weight of the internal fluid will be calculated and smeared over the section. Additionally, the user should be careful as to which set is used as a zone; the set specified for zone generation is not required by Flexcom to be continuous.