As now seems to be traditional, here at Orcina we have spent the summer months working very hard to complete a major upgrade to OrcaFlex. The result is OrcaFlex 9.3 which was formally released early in August. We have just despatched the final upgrade CD and so all customers with up-to-date MUS contracts will have the software very soon, if not already.
I should also point out that we have released a minor upgrade (version 9.3b) which fixes some bugs that have come to light in version 9.3a (the version included on the upgrade CD). As usual a patch to upgrade from 9.3a to 9.3b is available for download.
The major enhancements in version 9.3 fall into the following categories:
- Extreme value statistics post-processing of results variables.
- Text data file (in addition to existing binary data file).
- Enhanced interface to SHEAR7.
- Static state simulation files.
- Non-linear material properties for elastomeric bend stiffeners.
- API 2RD stress check.
- Coatings and linings for homogeneous pipe.
- Roll damping for vessels.
- Modal analysis improvements.
- Performance improvements.
As usual there are a large number of improvements to the program, far too many to discuss here. Full details can be found in the
OrcaFlex help file.
What follows are brief introductions to the new features that we consider to be most significant.
Extreme value statistics
Extreme Value Statistics results are available for all time history variables. Return values are estimated for a user-specified time duration, based on the time history of a selected variable.
Rayleigh, Weibull or Generalised Pareto distributions can be fitted. For the latter two, confidence limits are estimated for the return level and diagnostic graphs are presented indicating the goodness-of-fit of the selected model.
Text data file
OrcaFlex models can be saved to text data files in addition to the traditional binary .dat data file. Text data files can be edited in any standard text editor and are readable, well structured and self-documenting. The text data file offers benefits for QA and automation in particular and is intended to complement, rather than replace, the binary data file.
A simple example is shown below:
General: StageDuration: - 10.0 - 50.0 Lines: - Name: Line1 Length, TargetSegmentLength: - [60.0, 5.0] - [40.0, 2.0] - [120.0, 10.0]
This example first defines a 10s build-up stage followed by stage 1 with 50s duration. Then an OrcaFlex Line is created and named “Line1”. Finally the section data is specified: three sections are created with varying section lengths and segment lengths. Default values are used for all data which are not specified.
Enhanced interface to SHEAR7
Previous versions of OrcaFlex interfaced with SHEAR7 by exporting SHEAR7 input files (.dat structure file and .mds mode shapes file). This approach left the onus on the user to then run SHEAR7.
The enhanced interface calls SHEAR7 directly from OrcaFlex. One immediate benefit of the direct SHEAR7 interface is much simplified automation and file management.
The other benefit is that coupled VIV analyses can now be performed. The direct SHEAR7 interface achieves this by parsing the SHEAR7 output file to obtain the drag enhancement factors (Cf) predicted by SHEAR7. OrcaFlex then uses the enhanced drag coefficients and repeats the static analysis until a coupled solution is obtained.
Static state simulation files
One of the quirkier aspects of previous versions of OrcaFlex is that static state results could not be saved to file. The standard work-around to this limitation is to run a very short dynamic simulation with no dynamic excitation.
I’m pleased to say that this work-around is no longer needed. Static state results can now be saved to simulation files in much the same way as dynamic results can be – you simply save the model when it is in static state.
The batch form has been extended to allow specification of the analysis to be performed. If you specify dynamic analysis then the program processes the data file in the traditional way by running a dynamic simulation and then saving a dynamic simulation file. However, if you specify static analysis then the program performs the static calculation and then saves a static state simulation file.
This natural enhancement to the program is clearly useful when static analysis takes a significant amount of time. For example, the ability to save static state simulation files is particularly beneficial when using the direct SHEAR7 interface.
Non-linear material properties
The previous major upgrade to OrcaFlex (version 9.2) introduced built-in modelling support for bend stiffeners. A limitation of the built-in bend stiffener was that the material properties had to be linear. This limitation has now been removed and the material properties can be specified by a non-linear relationship between stress and strain.
This improvement has been made principally to allow for better and simpler modelling of elastomeric bend stiffeners but could also be valuable when modelling plastic deformation of steel pipes during installation.
The relationship between stress and strain can be specified either by supplying a table of values or through the Ramberg-Osgood formula. The former option is typically used for elastomeric bend stiffeners whilst the latter is applicable to steel pipes.
API 2RD stress check
A relatively simple (but hopefully valuable) addition to the program has been the inclusion of the stress check from the API 2RD code. This is a very commonly used strength criterion for the design of steel risers.
Implementation of the API 2RD stress check is a relatively simple exercise in post-processing of standard OrcaFlex output. However, the code as published contains a number of well-known errors. A recently published errata corrects some, but not all, of these errors.
OrcaFlex implements corrected forms of the code and so removes any doubts concerning the unfortunate errors in the published code documents. In addition, having the stress check built-in to the program significantly increases productivity.
In future releases of OrcaFlex we intend to implement additional code checks from other commonly used industry codes.
Coatings and linings for homogeneous pipe
The homogeneous pipe category of Line Type was introduced in OrcaFlex 9.2. This enables you to specify a homogeneous pipe line type by giving the basic structural properties such as material density, Young’s modulus, diameters etc., and OrcaFlex will calculate for you the derived structural values (e.g. mass per unit length, axial & bend stiffnesses, etc.).
This feature has been extended to cater for coatings and linings. Coatings and linings are typically used with steel pipes to model the additional mass and displacement of concrete coatings, plastic linings etc. They contribute mass, weight and displacement and also modify the pipe’s inner and outer diameters. However, they contribute no additional structural strength and are assumed not to be load bearing. Stress results are calculated based on stress diameters equal to the underlying pipe diameters.
Roll damping for vessels
Calculated vessels can now model the effects of roll damping. These effects are included in a simple model offering both linear and quadratic damping terms.
Modal analysis improvements
A number of improvements have been made to the modal analysis facility included in OrcaFlex:
- Modal analysis can be performed for lines which include torsion.
- Mode shape views can be animated which greatly helps visualisation of more complex mode shapes.
- Modelling of seabed friction in modal analysis calculations is now more robust. Previous versions of the program were somewhat prone to modal analysis failures related to seabed interaction and this change has been implemented to avoid such failures.
Previous releases of the program have suffered from performance problems when modelling certain drag-dominated systems. Most commonly this affected towed-array systems with high tow speeds. Such systems could suffer from poor convergence in both statics and implicit dynamics. Dynamic simulations could require very short time steps and often were faster performed with the explicit solver. These performance problems have been comprehensively addressed by this release of the program.
In addition, we have made performance improvements for models which include line clashing when using the implicit solver. Such systems can often now be modelled with longer time-steps than was possible in previous versions.
However, you still should use a short enough time-step to achieve your desired level of accuracy. Line clashing commonly leads to significantly non-linear responses. Hence, shorter time steps are required for accurate simulations than is the case for systems with less non-linearity.