Hydrodynamics and Wave Impact Analysis
Welcome to this fluid tech tip where we will explore hydrodynamic and wave loading simulation. Below are two examples:
- A simulation of a boat traveling through rough seas.
- An oil rig platform being impacted by a massive wave.
Before we begin, remember that ANSYS offers a comprehensive range of solutions for simulating offshore applications:
- For slender bodies, use ANSYS Mechanical.
- For global assessment of large body dynamics on the sea surface, use ANSYS Aqua.
- For detailed assessment of response to complex wave loading, use ANSYS Fluent or a combination of ANSYS Fluent and ANSYS Aqua.
Simulation Focus
In this tech tip, we will focus on a detailed assessment of the response to complex wave loading using ANSYS Fluent. We'll use the example of an offshore platform impacted by a wave to determine the wave load on the pillar. Specifically, we will:
- Analyze different columns and deck forces as a function of time.
- Investigate green water phenomena to predict if waves will hit the decks.
Simulation Setup
The platform's geometry includes detailed features such as the derrick, helipad, and tanks. A polyhedral mesh is selected to capture these details accurately.
For this simulation, the multi-phase Volume of Fluid (VOF) model is used to track the surface between water and air phases. The setup includes:
- Selecting Volume of Fluid with two phases: air and water.
- Using open channel flow and open channel wave boundary conditions.
- Inputting wave parameters using the fifth-order solitary wave theory.
The velocity inlet is configured to input the wave, specifying free surface and bottom level quantities, indicating a sea depth of 54 meters.
Wave Theory and Simulation
Wave theories can be complex, so a boundary condition check is performed to ensure the selected wave theory is applicable. The transient simulation requires selecting a time step, which can be fixed or variable depending on the numerics used.
Results and Analysis
Upon completion, results are viewed in CFD Post to create an animation of the simulation. The results show:
- The wave impacting the offshore structure.
- Green water phenomena, where water reaches the deck level.
In CFD Post, forces on the column, deck, etc., can be recorded as a function of time.
Conclusion
Thank you for listening to this tech tip. For more resources, visit the ANSYS Computational Fluid Dynamics webpage and look for technology tips under CFD technology leadership.
For further assistance, contact Ozen Engineering, Inc.
Hi, for this fluid tech tip we'll be looking at hydrodynamics and wave loading simulation. Here are two examples. On one you see the simulation of this boat actually traveling through rough seas. And another one where you can see this oil rig platform being impacted by a massive wave.
Before we start, I wanted to remind you that ANSYS offers a complete range of solutions for simulation of offshore applications. You may be dealing with slender bodies where we could be using ANSYS Mechanical.
If you're only looking at a global assessment of large body dynamics on the sea surface, for example, you can use ANSYS Aqua. Or if you need a detailed assessment of the response to complex wave loading, you can use ANSYS Fluent or a combination of ANSYS Fluent and ANSYS Aqua.
But in this fluid tech tip, we'll be looking at a detailed assessment of the response to complex wave loading using ANSYS Fluent. We'll be using the example of this offshore platform impacted by the wave. The question we have is what is the wave load on the pillar?
Really, what we're going to determine are the different forces of the wave on the columns and deck as a function of time. At the same time, we'll also be looking at green water phenomena. Basically, we want to predict whether the wave will actually hit the decks.
Here you see the geometry of the platform, and you can see we actually included all the details. You can see the derrick, the helipad, some of the tanks located on the platform, etc.
I selected a polyhedral mesh in this case, and as you can see, the surface mesh is a high-quality surface mesh that captures all the details of the geometry. You can see the derrick is captured, and even the steps or railing are nicely captured by this polyhedral mesh.
For this simulation, I will be using the multi-phase volume of fluid or VOF model, which is used anytime you want to track a surface between two phases. In this case, we'll track the surface between the water phase and the air phase. I selected volume of fluid, and I have obviously two phases.
I also selected open channel flow and open channel wave boundary condition. I will input a wave. Now it's time to actually select the two phases. One will be air, and the other will be water. The material properties are already predefined from the library of materials inside Fluent.
I really wanted to show the velocity inlet because, as I told you, we're going to input the wave. So what we're going to do is actually give the free surface level and bottom level quantity. That means we have 54 meters of sea depth, and we're going to input a shallow wave.
We'll have one wave, use the fifth order solitary wave theory, define its height, length, etc. Here is an option I really like. Those wave theories are complicated, so I actually go to the boundary condition section and do a quick check.
Fluent is looking at my input for the wave and actually checking everything. The check passed, and it's telling me that the selected wave theory is applicable for this application. This is a transient simulation, so next is to select my time step.
I can obviously use a fixed time step, but depending on the numerics, you could be using a variable time step. It's always a good idea to check if a variable time step is available.
By just determining and inputting a limited amount of input, you can actually have the time step being selected automatically. Once the simulation is completed, I go to view any variables on the label, and there's a separate side for every run fall.
I go to CFD post to create the animation of the simulation, and here we see the result. We really see the wave that is actually going and impacting the offshore structure, and we see that we have green water phenomena, which is that the water actually reaches the deck level.
Of course, what I can do also in CFD post is record the force as a function of time of the water on the column, the deck, etc. Thanks for listening to this tech tip, and of course, we have many more and many more materials available for you in our technology tip page.
Simply go to the ANSYS Computational Fluid Dynamics webpage and look for the technology tips under CFD technology leadership. Thank you.
