Modes Operation |
 
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Modes Operation |
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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.
•*RISER TYPE ($MODES section) is used to specify the riser type and other parameters relating to the Modes facility for generating output for subsequent input to SHEAR7.