As part of the OC7 Phase II project, I am analyzing a floating structure (VolturnUS-S design) as follows:
The incident waves travel along the x-direction. As part of the project, amongst other things, we are looking at the torque loads in the downstream pontoon 2. This is the torque along the pontoon longitudinal axis (z_p2)
I'm using 3 numerical models in OpenFAST:
NLR1: strip-theory model using rectangular elements,
NLR2: potential-flow model.
NLR3: strip-theory model using virtual cylinders as a work around when rectangular elements are not available.
The 3 models use the same nonlinear hydrostatics formulation.
Below you can see the torque observed in the pontoon 2 during a regular wave condition. The window of time corresponds to 3 wave periods approximately.
As observed, the NLR1 and NLR3 results are more or less consistent with other participants in the project using strip-theory models. However, the NLR2 model behaves different compared to other potential-flow models. A phase shift and a hump in the lower part of the torque signal can be observed.
The differences in terms of amplitudes between the strip-theory and the potential-flow models are due to the physics included in these models and will be covered in a publication.
In any case, something seems to be off with the NLR2 model. And such behavior was observed regadless of using the displaced or undisplaced platform position.
I performed a sensitity analysis to try to better understand what was going on:
The original results shown in the comparison against other participants corresponds to the vertical wave stretching with nonlinear hydrostatics (shown in this plot in blue). When not using the wave stretching capability (red solution) or when using wave stretching with linear hydostatics (black dashed solution), the results are quite different and aligned with other participants in the project.
It seems that the nonlinear hydrostatics may be incosistent with potential-flow hydrodynamic loads based on linear or second-order theory.
For future reference, I add the platform motion for the 3 different approaches considered. It's important to note that in this case the platform moves 1.95 m downwards for the heave equilibrium. This may challenge a bit the platfom pitch motion estimation when using linear hydrostatics.
As part of the OC7 Phase II project, I am analyzing a floating structure (VolturnUS-S design) as follows:
The incident waves travel along the x-direction. As part of the project, amongst other things, we are looking at the torque loads in the downstream pontoon 2. This is the torque along the pontoon longitudinal axis (z_p2)
I'm using 3 numerical models in OpenFAST:
NLR1: strip-theory model using rectangular elements,
NLR2: potential-flow model.
NLR3: strip-theory model using virtual cylinders as a work around when rectangular elements are not available.
The 3 models use the same nonlinear hydrostatics formulation.
Below you can see the torque observed in the pontoon 2 during a regular wave condition. The window of time corresponds to 3 wave periods approximately.
As observed, the NLR1 and NLR3 results are more or less consistent with other participants in the project using strip-theory models. However, the NLR2 model behaves different compared to other potential-flow models. A phase shift and a hump in the lower part of the torque signal can be observed.
The differences in terms of amplitudes between the strip-theory and the potential-flow models are due to the physics included in these models and will be covered in a publication.
In any case, something seems to be off with the NLR2 model. And such behavior was observed regadless of using the displaced or undisplaced platform position.
I performed a sensitity analysis to try to better understand what was going on:
The original results shown in the comparison against other participants corresponds to the vertical wave stretching with nonlinear hydrostatics (shown in this plot in blue). When not using the wave stretching capability (red solution) or when using wave stretching with linear hydostatics (black dashed solution), the results are quite different and aligned with other participants in the project.
It seems that the nonlinear hydrostatics may be incosistent with potential-flow hydrodynamic loads based on linear or second-order theory.
For future reference, I add the platform motion for the 3 different approaches considered. It's important to note that in this case the platform moves 1.95 m downwards for the heave equilibrium. This may challenge a bit the platfom pitch motion estimation when using linear hydrostatics.
