Shielding effect for quartering seas

We are doing a DAF study for a lift analysis (in the time domain) considering a side-by-side vessel configuration (as shown in the figure below).

For the DAF study, the shielding effect is activated using the -obstacle command in MOSES, particularly for waves originating from 225 deg, 270 deg, and 315 deg headings.

 The table below shows the comparison of DAF values for the above three headings considering with and without the shielding effect.

As per the results, there is a reduction in DAF for beam sea in the shielded condition (as expected). However, for 225 deg and 315 deg, the DAF values are increasing in the shielded condition.

It is not clear why MOSES is reporting higher DAF for quartering seas seeing that there is shielding from the cargo barge for almost the entire length of the lift vessel.

We have also tried some sensitivity with smaller mesh distance and time steps. However, it is the same outcome.

Any other suggestions?

 

Here below some additional information on the MOSES model:

  • “3DDif” diffraction type is considered for both lift vessel and cargo barge.
  • The two-vessel bodies are in the 'box' shape model for effective mesh evaluation.
  • The configuration is side-by-side with the lifting system at the vessel port-stern.
  • The hydrodynamic database of the lift vessel considers the cargo barge with -obstacle.
  • The hydrodynamic database of the cargo barge considers the lift vessel with -obstacle.
  • Hi Jamhari,

    Thank you for your question.  It might be that the green vessel is "too" near the size of the pink vessel.

    Below are some pictures and plots.  The first picture is the set up.  The plot titles include the value of obstacle, either true or false. The plots are of the mooring line tensions.  I noticed the mooing lines on the port side (mooring lines that would run under the obstacle) are lower.  The mooring lines tension change on the starboard side is not as prominent.  Also for quartering seas the lines at the the bow show a small difference.

    We can continue this conversation via a service request.  Do you know how to initiate a service request?

    I hope this helps.

    georgina