# SHEAR7

SHEAR7 is a frequency domain VIV prediction program developed by Prof. K Vandiver at MIT and distributed by AMOG. The link to SHEAR7 is based around OrcaFlex's ability to export SHEAR7 data files and SHEAR7 modes files.

## OrcaFlex SHEAR7 Interface

To use SHEAR7 for VIV analysis of a line, choose SHEAR7 for the line's statics VIV. You must also define further data specific to SHEAR7 on the line data form, as described below, and on the SHEAR7 data form.

When you perform the OrcaFlex static analysis, SHEAR7 will be run, and the drag coefficients calculated by SHEAR7 will be used by OrcaFlex instead of the user-specified drag coefficients from the line type form. The values of these SHEAR7 drag coefficients, and other results calculated by SHEAR7, are reported by OrcaFlex.

### SHEAR7 executable file location, SHEAR7 lift file location

In order for OrcaFlex to run SHEAR7, the location of the SHEAR7 executable file must be defined. In addition, SHEAR7 requires a file to specify lift coefficients, usually named common.cl. Typically the lift file will be the standard one supplied with SHEAR7.

Both of these file locations are given (either as relative or full paths) on the SHEAR7 data form. The location of the executable file must be given explicitly. The lift file location may be left blank, in which case OrcaFlex will look for a file named common.cl in the same folder as the executable file.

### SHEAR7 coupling method

Since the SHEAR7 analysis depends on the position of the line and, vice versa, the position of the line depends on the SHEAR7 analysis, OrcaFlex must perform a coupled, iterative statics calculation as follows:

1. set the line's drag coefficients to those specified on the OrcaFlex line type form
2. calculate the static position of the line using those drag coefficients
3. run SHEAR7 to analyse VIV for that position and update the line drag coefficients
4. recalculate the static position of the line for those new drag coefficients
5. repeat steps 3 and 4 until the static position has converged

Convergence is achieved when none of the nodes changes position (compared with the previous iteration) by more than $\textit{tol} \times l$, where $\textit{tol}$ is the value of convergence tolerance and $l$ the length of line represented by that node. Usually only 2 or 3 coupling iterations are required for convergence.

The choice of coupling method determines how the SHEAR7 modes file is handled.

• full coupling generates a new modes file at each coupling iteration (i.e. every time SHEAR7 is called)
• partial, automatic modes file coupling creates an modes file only on the first coupling iteration; this is used in all subsequent coupling iterations
• partial, user modes file coupling does not generate an modes file, but uses an existing one, specified on the data form, for all coupling iterations

Full coupling may well be more accurate than partial but, since generating an modes file can be time consuming, especially for more complex models, it can be significantly slower.

The partially-coupled approaches are more akin to running SHEAR7 separately. However, the ultimate static solution found by these coupled methods will still differ, because they include the effect of drag enhancement. When running SHEAR7 separately, the only way to account for drag enhancement is to explicitly modify drag coefficients in the OrcaFlex model, which is extremely impractical.

### First and last modes

For full and automatic-partial coupling methods, these values determine which modes are to be included in the modes file. They are not relevant to the partial, user modes file method.

A value of '~' for first mode is interpreted as the lowest-numbered transverse or inline mode; similarly, '~' for last mode means the highest-numbered such mode. OrcaFlex modal analysis classifies modes as either transverse and mostly transverse or inline and mostly inline. The choice of which set of modes, transverse or inline, to output to the automatically-generated modes file is determined by the selected response on the SHEAR7 data form. (This choice is not available for earlier SHEAR7 versions, for which transverse and mostly transverse modes are output.)

The SHEAR7MdsFile batch script command applies the same interpretation to the FirstMode and LastMode parameters.

### Include coupled objects

The automatically created modes file is generated by performing a modal analysis. This option determines which degrees of freedom are included in that modal analysis. When left unchecked, only the free degrees of freedom of the line are included; when checked, any coupled degrees of freedom are also included.

### SHEAR7 modes file

The location of the modes file to be passed to SHEAR7 (only relevant to the partial, user modes file coupling method). This may be given as either a relative or a full path. The modes file must be generated separately, which is normally done either from the modal analysis form or using batch script.

### Results output

When OrcaFlex calls SHEAR7 it captures the resulting output files and incorporates them in the OrcaFlex summary results for the line. These output files can also be exported, from the SHEAR7 data form or by using the SHEAR7OutputFile batch script command;

The output files are automatically exported if you run the static analysis in batch mode or from Distributed OrcaFlex. The files are named using a combination of the OrcaFlex data file name and the line name.

The enhanced drag coefficients can be obtained from a static state range graph of x drag coefficient (or indeed y drag coefficient which is identical).

## Technical details

The SHEAR7 interface operates by calling the SHEAR7 executable directly as follows:

1. a temporary folder is created to contain the SHEAR7 input and output files
2. a SHEAR7 .dat file is generated in the temporary folder
3. the SHEAR7 modes file is created (according to the coupling method) in the temporary folder and named common.mds
4. the specified lift file is copied to the temporary folder and renamed common.cl
5. if stick-slip hysteresis is modelled then a common.cat file and an .inhyst file are generated in the temporary folder
6. the SHEAR7 executable is run, and the SHEAR7 output files are read by OrcaFlex
7. the .plt file is parsed by OrcaFlex to find the drag enhancements factors $C\urm{f}$

This process depends heavily on the current implementation details of SHEAR7. We have developed and tested the SHEAR7 interface with the SHEAR7 versions listed on the SHEAR7 data form, but cannot guarantee that other versions of SHEAR7 will be compatible with the SHEAR7 interface.

 Notes: If the locations read from the .plt file do not correspond to the node locations of the line in OrcaFlex, OrcaFlex applies linear interpolation to obtain the corresponding $C\urm{f}$ values. This can occur if the discretisation of a user-supplied modes file differs from that of the OrcaFlex line. When the partial, user modes file coupling method is used, the interface to SHEAR7 will still name the copy of the specified modes file to common.mds. The associated .dat file and input echo in the .out will therefore reflect that the default modes file name has been used. For SHEAR7 4.11 and later, all file extensions are prefixed with s7. So .dat becomes .s7dat, .mds becomes .s7mds, etc.