We have recently identified an issue with the L02 OC4 semi-sub example, where incorrect linear damping coefficients were used as input to the semi-submersible model.

To explain the problem, the original damping coefficients were referenced from a comparison study against tank test results, documented as part of the Offshore Code Collaboration study OC6 project – Phase I. The coefficients were then calibrated in OrcaFlex, through a series of free-decay tests. Those tests used damping coefficients with force units of newtons (N), however, the example models uses units of kilonewtons (kN). This difference in units was not accounted for as part of the model building process, which led to the transfer of incorrect coefficients to the ‘Other damping’ page of the OrcaFlex vessel type data form.

In light of this mistake, the supporting OrcaWave and OrcaFlex models have been corrected, and the latest version of the example is now available for download from the usual L Diffraction examples page of the Orcina website. For clarity, the correct coefficients are shown below:

This serves as an important reminder that the available published Orcina examples are intended for illustrative purposes, meaning they should be checked for accuracy and modified appropriately to suit the individual needs of an application or project.

We apologise to anyone who might have been inconvenienced by this oversight. Should you need any further clarification, then please do contact us for further details.