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Hydrodynamic interference between risers

To take into account the hydrodynamic interference between different risers in a array the simple calculation procedure proposed by Huse has been implemented.

The hydrodynamic efforts on the lines are still given by the Morison equations but the incident fluid velocity on a riser is the addition of the current velocity, the wake velocity generated by the upstream risers and the dipole velocity generated by all the risers.

• Wake velocity : \(v_{wake} = U_0\exp[-0.639(y/b)^2]\)

  \(b = 0.25 \sqrt{C_{d}DX}\) : half-width of the wake,

  \(U_0 = v_{v_{current}} \sqrt{C_{d}D/X}\) : maximum wake velocity,

  \(X\) : the distance from the wake source,

  \(D\) : the riser diameter

  \(C_d\) : drag coefficient of the riser generating the wake

• Dipole velocity : \(\nu_{dipol} = \displaystyle \frac{VD(y^2-x^2)}{(y^2+x^2)^2}\) where \(V\) is the inflow velocity in x-direction and the riser center which is the source is at the origin.

Restrictions :

• Static analysis (permanent flow),

• Spacing between risers greater than a few diameters (at least 3 diameters),

• Reynold's number \(R_e : 3~10^5 \le R_e \le 3.6~10^6\).