The average wind loads are an important design parameter for mooring or for a Dynamic Positioning (DP) system of offshore structures. Traditionally, these loads have been obtained in wind tunnels. Nowadays, these loads can be obtained using CFD with good accuracy, i.e. within 5 to 10% compared to experiments, at low costs and in acceptable turn-over time. Furthermore, CFD can play an important role in obtaining insight into scale effects.
The steps to be taken in CFD to determine the wind loads are illustrated in the figure below:

1. Obtain correct geometry for topsides, see Figure 1
2. Construct computational mesh, see Figure 2
3. Calculate flow field around the vessel and determine the loads on the topsides, see Figure 3
4. Repeat the calculation for the angles under interest and determine the Cx, Cy and Cm curves, see Figure 4
Other important aspects for wind loads that can be investigated with CFD are:

• Scale effects
Within the OO2 JIP wind load measurements have been carried out for different types of vessels. In [2] the following types have been calculated with CFD and compared with the OO2 measurements:

• LNG carrier, membrane type, Figure 5
• LNG carrier, moss type, Figure 6
• Shuttle tanker, Figure 7 and Figure 8
• Schematic FPSO, Figure 9
Figure 1: Obtain correct geometry for topsides.
Figure 2: Construct computational mesh.

Figure 3: Calculate flow field around vessel and determine the wind loads. Presented is pressure distribution on LNG Membrane carrier.
Figure 4: Repeat calculations for all angles under interest to determine the load curves for Cx, Cy, and Cm.

Figure 5: Wind loads on Membrane type LNG carrier.

Figure 6: Wind loads on Moss type LNG carrier.

Figure 7: Wind loads on Shuttle tanker, shallow draft.

Figure 8: Wind loads on Shuttle tanker, deep draft.

Figure 9: Wind loads on schematic FPSO.

Figure 10: For wind loads the geometric details are important. In the figures above the solution is presented for a schematic FPSO without (left) and with (right) gaps present between the blocks. The difference in wind loads between the two cases is almost 20%.

Figure 11: CFD can be used to determine the wind velocity field around vessels and the wind loads on shielded vessels as presented above. Wind direction is from top to bottom. Note that due to the presence of the shuttle tanker in the wake of the FPSO the location of the wake changes.

References
[1] Koop, A., Klaij, C. and Vaz, G.; “Predicting Wind Loads for FPSO Tandem Offloading using CFD”, OMAE 2010-20284.
[2] Koop, A., Rossin, B. and Vaz, G.; “Predicting Wind Loads on Typical Offshore Vessels using CFD”, OMAE 2012-83449.