CFD has become an accurate, efficient and cost-effective design tool to predict the current loads on typical offshore constructions. Arjen Koop & Frederick Jaouen, a.koop@marin.nl


CFD provides valuable insight into current loads on offshore constructions


A


n accurate assessment of the current loads is important for Dynamic Positioning and mooring


analysis. Traditionally, current loads were obtained by means of wind tunnel tests or tow tests in water. Although this has been the working method for many years in the industry, the accuracy of the results might be hampered due to blockage effects, scale effects and floor boundary layer effects in the wind tunnel.


Nowadays, CFD is being used as an efficient design tool to predict the current loads on offshore constructions. Within the Current Affairs JIP [5], tools and guidelines have been developed to assist engineers in the assessment of current loads in different design stages. CFD and model-scale experi- ments were compared and there was good comparison between the two. To extrapo- late the results to prototype configurations, scale effects can be investigated with CFD. The quality of the results must be ensured by following an extensive verification procedure for both model and full-scale as shown in [3] and [4].


Shallow water In shallow water, the clearance under the vessel becomes smaller, resulting in an increase of the current loads due to the blockage of the flow under the vessel. The Oil Companies International Marine Forum’s (OCIMF) experiments for tank- ers often serve as the reference to obtain


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Figure 1: Results for CY coefficient for a shuttle tanker positioned side-by-side to an LNG carrier comparing CFD results with experimental results, to be published in [2]


the increase of the current loads for different water-depth-to-draught ratios. However, for different vessels this ratio might change requiring new model tests or CFD calcula- tions. In [1] and in Figure 3, the CFD results are shown to accurately match the experi- mental results. Blockage effects and scale effects are known issues when carrying out model tests for shallow water but these ef- fects can be efficiently quantified with CFD.


Side-by-side current loads During a side-by-side offloading operation two ves- sels are positioned close to each other and significant shielding or interaction effects can occur. When a vessel is in the wake of another vessel the force coefficient can even change sign, meaning that the vessel is sucked towards the upstream vessel. With CFD, the side-by-side current loads can be calculated and the results agree well with


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