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!

Mooring Chain Bending Fatigue (following Bureau Veritas Guidance Note NI 604 DT R00 E)

Mooring line fatigue damage normally considers only tension load cycles. However, sometimes the mooring chain immediately adjacent to the fairlead (the ‘Top Chain’) can experience significant in-plane and out-of-plane bending which causes additional fatigue damage.

BV guidance note Fatigue of Top Chain of Mooring Lines due to In-plane and Out-of-plane Bendings, [NI 604 DT R00 E, Aug-14], (‘BV note’) describes a Top Chain fatigue damage assessment procedure which includes both tension and bending. Here we describe how to derive the necessary OrcaFlex results to use with the procedure outlined in the BV note.

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OrcaFlex Development plans for 10.2 and beyond

In the past we have not been very expansive about our development plans for OrcaFlex – usually nothing more than a short bullet list on our website. We’d like to engage more with our users about upcoming OrcaFlex developments. This post is intended to kick-start us in that direction.

But before we get into the details, here are some points that might be useful background:

  • For roughly the last 10 years OrcaFlex has been released annually, usually around September or October.
  • The latest release of OrcaFlex is 10.1c. Version 10.1a was released in October 2016.
  • The version number denotes the major release – 10.1 is the current major release, 10.2 will be the next major release.
  • Each version number has a minor revision letter with a marking the first minor revision, b indicating the second minor revision, etc.
  • Minor revisions are intended to fix bugs rather than introduce major new functionality.
  • Our website details OrcaFlex releases since version 9.5 in 2011.
  • We also occasionally remove functionality – usually when we believe this is not being used.

In terms of individual functionality changes (both additions and removals), our software team plan to blog more about these on other occasions. This post summarises our planning process and states where we are currently with our plans for version 10.2 and beyond.

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Less Common Applications for OrcaFlex

OrcaFlex is primarily used in the offshore oil and gas industry, typically for installation analysis and in-place design of riser and mooring systems, pipelay analysis, field construction, etc., as well as being widely used in the oceanographic community. However, OrcaFlex is an extremely versatile package, and over the years we have seen it used for the global dynamic analysis of a wide variety of applications. Starting what will be an occasional series of blog postings about less common applications for OrcaFlex, we kick-off with a summary of these.

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DNV OMAE paper, “Impact of Bathymetry on the Mooring Design of an Offshore Floating Unit”

In a recent paper (Impact of Bathymetry on the Mooring Design of an Offshore Floating Unit, OMAE2016-54965), DNV GL authors Jaiswal, Vishnubholta, Cole, Gordon and Sharma examine the effect of using an accurate seabed bathymetry on typical mooring results. Their motivation was to understand potential limitations in the assumption made by most mooring software that slope of the seabed is constant along the direction of each mooring line.

To explore this they consider the case of a hypothetical 8-legged all-chain-moored semi-submersible, located over a simple escarpment (with a constant profile). Consequently the water depth at the anchors varied between c150m and c250m. They considered two different seabed representations: (1) the accurate bathymetry, and (2) an approach equivalent to the constant slope assumption in most mooring software.

OrcaFlex is used for static (including wind, wave & current) and coupled dynamic time domain analysis. For 1st order dynamics waves are included. For 2nd order dynamics, wind and waves are included.

Results for line pre-tensions and the min and max values of vessel offset and line tensions are compared for each seabed representation. They conclude that there are “significant differences” in vessel offset and maximum mooring line tension when using different seabed representations. The authors point out that these results can’t be generalised, but further conclude that where there is uneven seabed bathymetry and marginal safety factors, it may be advisable to use a more accurate bathymetry.

As an aside to the paper summary, please note that we are actively enhancing OrcaFlex functionality for traditional mooring analysis. Wave Frequency Frequency Domain was added in v10.0 (Oct-15), and we’re currently working on a Low Frequency Frequency Domain solver – this should be forthcoming in OrcaFlex v10.1 (cOct-16), unless any last minute hitches postpone it to 10.2.

You can download the paper from the ASME paper here.

The next OMAE conference is to be held in Trondheim, Norway, in June 2017. Orcina are planning to attend and we look forward to seeing you there.