Fuel Savings Up To 5% Thanks To Ship Trim Optimization With Omnis Marine

Author: Maurits Van Den Boogaard, CFD Product Engineer at Cadence 

It may not be the first angle you'd normally think about when talking about curbing emissions in the shipping industry, but ship trim optimization has recently gained enormous momentum as indeed it can significantly reduce fuel consumption. Depending on trim angle, the drag on a ship changes, even if speed and draft remain the same. As such, imposing an optimized trim angle during the loading process in the harbor ensures minimal average resistance during the vessel’s journey.
 

The Financial Benefits of Trim Optimization

Besides the positive impact on the environment, there is of course, the financial benefit that is of big interest for shipping companies. Because of the potential for reduced fuel costs, it becomes financially interesting to create a database of optimum trim angles based on a complete trim optimization study for various cruise conditions. Trim optimization can be performed regardless of vessel type and age, though of course: the bigger the vessel, the bigger the benefit. For some vessels, such as cruise ships, the trim angle cannot be adjusted with the same flexibility, since passenger comfort and facilities impose relatively strict constraints. The benefit is the largest for ships that often need to sail in part-load conditions, such as ro-ro carriers and smaller containerships. In those cases we are talking about fuel savings up to 5%. That is a serious amount: for a container feeder of about 2000 TEU, sailing at 22kn, this would correspond to a savings of about 35 barrels of oil per day [source]! 

"For ships that often need to sail in part-load conditions, such as ro-ro carriers and smaller containerships, we are talking about fuel savings up to 5%. For a container feeder of about 2000 TEU, sailing at 22kn, this would correspond to a savings of about 35 barrels of oil per day."

The Importance of Dynamic Trim

In the past, ship designs were optimized for one single speed and draft, even though most ships will experience a wide range of different conditions, requiring various changes to these parameters. Today, thanks to Computational Fluid Dynamics (CFD), a game-changing tool, matrixes of optimal trim and draft conditions can be generated quickly and with great accuracy.

Hundreds of simulations, one for each combination of initial trim, draft and speed, need to be run to create an optimal trim database, without the need for any model testing. Computations are performed at full scale, because using reduced models would not result in accurate prediction of turbulence, which affects optimum trim angles considerably. Local recirculation and flow detachment can lead to different force predictions between model- and full scale, as the Reynolds number, which governs these phenomena, cannot be preserved in the geometrical scaling. Compared to a potential code that doesn’t include the effects of turbulence, the resistance-increasing effects of wall-roughness (fouling) can be also included in the CFD analysis. As such, CFD can provide a much more realistic result of ship resistance during its entire lifetime.

Given the accuracy and consistency of the results, CFD also delivers detailed information about complex hydrodynamic problems to a 3D space. The workflow can be almost completely automated, as all required actions from the software can be scripted. All a naval architect or marine engineer needs to do is enter their specific ship conditions, and the software sets up and runs all simulations, rapidly and autonomously, on a single workstation or HPC cluster. 

"The workflow can be almost completely automated. All a naval architect or marine engineer needs to do is enter their specific ship conditions; the software sets up and runs all simulations, rapidly and autonomously, on a single workstation or HPC cluster. "

Our Approach

In practice, Omnis Marine’s C-Wizard matrix mode creates n x m x p computations: one for each {draft (n),trim (m)}-couple for a provided list of speeds (p). The displacement is kept the same for all draft-trim combinations. The Z-coordinate of the free surface also remains the same for all computations, as the ship is translated/ rotated to ensure iso-displacement conditions. Moreover the user has the option to use open-water data for real propeller performances through an actuator disk, which further increases the accuracy and realism of the results, while keeping CPU cost low.

Example of open water performance data that can be read by Omnis Marine during the simulation through an actuator disk

The drag forces, moments and dynamic trim and sinkage computed by the flow solver, are written out for each combination of (draft, trim, velocity) in the post-processing step, which also yields the displacement at each draft. As an example, here is an optimized trim table for a particular vessel that can be prepared based on the CFD results obtained from Omnis Marine.

Example of an optimized trim table

It is interesting to note that the whole project for all the hundreds of computations, will be performed with one single mesh! This capability significantly reduces total computation time needed, due to the fact that the geometry and domain need to be meshed only once, and it also ensures highest accuracy. The numerical uncertainty inherent to the creation of different meshes is also removed simultaneously. This is achieved via Omnis Marine’s unique Adaptive Grid Refinement (AGR), which takes care of all the necessary refinements for the free-surface during the simulation in an anisotropic, automatic and dynamic way.

Adaptive grid refinement in action to capture the free surface

Conclusion

Trim optimization is a relatively easy method for ship owners to reduce operational expenses. While in the past it may not have been possible to obtain accurate resistance predictions for a large matrix of initial drafts, trim angles and velocities, CFD is now the tool of choice to obtain a database of optimal trim angles quickly and with high accuracy. The unique features of Omnis Marine, such as the single-mesh approach with adaptive grid refinement and the usage of real open-water data for propulsion, position the software as the ultimate CFD toolbox for matrix-based resistance applications.

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