General Mechanical

General Mechanical

ANSYS Aqwa – Mud Layer Stiffness

    • FCNQP
      Subscriber

      In AQWA Theory Manual the mud layer springs don't seem to provide a continuous force-displacement law. For simplicity let's considere the viscous spring force. In this case, the damping force is such that (k -> constant; v -> node velocity):



      • 0, if the node is above mud line

      • k x v, if the node is below the seabed

      • k x v x sin(theta)^2, in between


      Now, the theta variable is defined from the water depth (d), the mud layer thickness (z) and the node position (zj):



      So, let's assume d=200 [m] and an unitary mud layer. In this case, for the point near the seabed and near the top of the mud layer, respectively:



      • zj=-200 -> theta=0 -> sin(theta)^2=0 -> F=0

      • zj=-d+z -> theta=pi/2 -> sin(theta)^2=1 -> F= k x v


      So it seems the law provided is not connecting to the under seabed and above mud line regions in the proper way.


       


      Is this correct?


      Thanks in advance

    • pjschofi
      Ansys Employee

      Your observations are correct, but the theory manual is not correctly defining what the program is computing. The equation for theta is incorrect, as are the equations used in determining the reaction force of the sea bed on the mooring line. The correct equations are as follows:



      The theory manual will be updated at the next release.

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