Orcina news

Here you will find the latest news on the development of OrcaFlex. Alongside our LinkedIn page, it is a valuable source of information about what we are up to!

OrcaFlex 9.4 released

A whole year has now passed since we released OrcaFlex 9.3 which means that it is time for the release of OrcaFlex 9.4! As always it takes time to produce and despatch installation CDs but clients with up-to-date MUS contracts can expect to receive the software some time around the end of August.

The major enhancements in version 9.4 fall into the following categories:

  • Improved performance of the OrcaFlex post-processing spreadsheet.
  • Enhancements to modal analysis: whole system analysis, better performance.
  • Enhancements to fatigue analysis: mean stress, mooring fatigue using T-N curves, SHEAR7 fatigue.
  • Line contents modelling: free-flooding, slug flow, spatial and temporal variation of contents.
  • Vessel data import enhancements for generic text files.
  • Better handling of vessel RAO data which omits zero period response.
  • Improved modelling of solid contact at edges and corners.
  • Line floats wizard now supports Reynolds number dependent drag.

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.

OrcaFlex spreadsheet

The Excel spreadsheet, supplied with OrcaFlex, that is used for pre-processing and post-processing has been completely re-written for version 9.4.

Previous versions used Excel VBA code to interface with OrcaFlex. Excel VBA has the significant limitation that it cannot be used to create multi-threaded code and so cannot take advantage of modern multi-core and multi-processor hardware. The version of the spreadsheet supplied with version 9.4 has been re-implemented as a COM add-in.

From a user’s perspective this change is largely immaterial – the operation of the spreadsheet remains essentially unchanged. The principal benefit is that the code is now multi-threaded and can process multiple load cases in parallel. Naturally, this can result in significant performance gains.

There are a number of other more minor enhancements to the spreadsheet, the most notable being:

  • The spreadsheet can now take advantage of Excel 2007 and later supporting very large worksheets. Older versions of Excel were limited to 65536 rows and 256 columns, but the more recent versions of Excel have effectively removed these limits.
  • A new facility has been added to automate generation of text data files. This is very similar to the existing facility for generating batch script files.

Modal analysis

The modal analysis facility has also received significant attention, although the basic operation is largely the same as in previous versions.

Possibly the most important development is the implementation of a new eigen-solver, using an iterative Lanczos algorithm, which has significant performance benefits. This new solver is limited in that it cannot extract all modes of the system. However, provided that you require no more than a few hundred modes, the new solver will be used and will solve very quickly. The benefit is felt for large systems and the new solver will typically solve in a matter of seconds large systems that require minutes or even hours for the old solver.

The other major enhancement is the implementation of modal analysis for the whole system. Previous versions of the program could only perform modal analysis for a single line within the model. The new capability fits naturally into the existing modal analysis user interface. We have no doubt that a great many users of the program will benefit from this development. Indeed, we have made extensive use of it to help deal with client support queries.

Together with a number of other minor improvements, these changes to the modal analysis capability greatly improve its utility.

Fatigue analysis

OrcaFlex’s fatigue analysis feature has seen regular development over recent releases, and version 9.4 is no exception.

The S-N curve data specification now allows for mean stress effects. Three different models have been implemented to describe these effects, namely Goodman, Soderberg and Gerber. Mean stress effects can be included for all types of fatigue analysis that make use of S-N curves.

Two new damage calculation options have been added: Mooring and SHEAR7.

Mooring fatigue is based on T-N curves. These are similar to S-N curves, but instead relate number of cycles to failure (N) to tension range (T).

SHEAR7 fatigue is somewhat different from the other damage calculation options. The difference is that the damage is actually calculated by SHEAR7. The OrcaFlex code merely collates the output from a series of SHEAR7 analyses. OrcaFlex reads the SHEAR7 output files, sums the damage and presents the results in either graphical or tabular form. Combined with the direct SHEAR7 interface and the OrcaFlex batch form, this allows you to automate an entire SHEAR7 fatigue analysis.

Line contents modelling

All previous versions of OrcaFlex included a very basic and simple model for line contents. Contents density could be specified, but it was assumed to be constant for the entire length of the line. OrcaFlex 9.4 introduces a host of new features to make contents modelling much more flexible.

Line contents can now be specified to be free-flooding. This results in the line being filled with seawater up to the instantaneous water level. Another new option has been added which allows axial inertia due to contents to be omitted from the analysis.

The other new contents modelling option is named slug flow. Although intended primarily for modelling of slugs, this option allows for both spatial and temporal variation of contents. We envisage the feature also being used to model static density variation, flow-lay, etc.

The slug flow option takes as input a specification of the density variation along the line. In addition a velocity is defined which determines how the contents flow. This velocity can vary over the course of the simulation. We have taken care to ensure that contents progress smoothly between line elements to minimise noise due to transient loading. The code accounts for the resulting variations of mass, weight and centrifugal and Coriolis forces.

Whilst we have not attempted to model complex multi-phase flow conditions, we do feel that the new facilities are in keeping with the level of detail typically modelled using OrcaFlex.

Vessel data import

The process of importing hydrodynamic data has been improved in a number of ways.

Generic text data may now be combined in one file and imported in a single operation. Previously, displacement RAOs, load RAOs, QTFs, and added mass and damping all had to be in separate files.

Generic text files may now specify the conventions and units of their data. If the conventions or units of the data being imported differ from those in the OrcaFlex model, then the data will be converted to match them at the time of import.

Extrapolation of RAO data towards the zero period response

Extrapolation of RAO data towards zero period response is now handled differently. Previous versions did issue warnings if any extrapolation was required. However, the extrapolation typically resulted in a non-zero response at zero period which is clearly non-physical.

Although we would always encourage users to read and take notice of the warnings, it is possible to make the program handle such input data in a more useful way. So, from version 9.4, if extrapolation is required for short period waves, the program will now always extrapolate towards a zero response for zero period.

Modelling of solid contact at edges and corners

Elastic solid contact is now handled slightly differently for objects with non-zero contact diameter, e.g. line-solid contact. The algorithm used in previous versions had some weaknesses. The essential problem was that the reaction force always acted normal to the solid face closest to the node. When a node is near an edge or a corner then this results in a discontinuity which in turn can lead to poor convergence and noisy responses.

In addition, the previous algorithm had the unfortunate property that contact could, under some circumstances, occur for nodes outside the shape. This problem most commonly afflicted nodes just beyond the end of a curved plate, e.g. a bellmouth.

Version 9.4 solves these problems by adopting a minor change to the contact algorithm. The reaction force now acts in the direction of the vector pops where po is the position of the node and ps the position of the closest point on the surface of the solid to po. When ps is on one of the faces of the shape then the new and old algorithms are identical. However, when ps is on an edge or at a corner then the reaction force is no longer normal to one of the faces.

Line floats wizard

The line floats wizard is a widely used and very useful feature. However, it has not been updated for some time and consequently did not make full use of new features added in recent versions of OrcaFlex.

The most significant change is that Reynolds number dependent drag coefficients for the base line are now supported. The program still assumes that the drag acting on the floats is independent of Reynolds number.  However, this is a much more reasonable assumption given the typical size of the floats used for offshore buoyancy.