Hydrodynamic Diffraction Analysis in Ansys Aqwa

Explore how hydrodynamics challenges in offshore engineering are tackled using advanced tools like Ansys Aqwa. Learn about industry needs, engineering solutions, methods, and real-world examples to address wave-structure interactions effectively.

Challenges

Hydrodynamics is the study of how fluids (like water) move and how they interact with objects (like ships or platforms). For floating structures, the focus is on how waves, currents, and other forces affect the structure's stability, motion, and behavior. Hydrodynamics plays a critical role in the design and operation of offshore and marine structures. Companies in this sector face numerous challenges, including:

• Accurate Prediction of Wave-Induced Loads: Ensuring that structures like floating platforms, moored ships, or wave-energy devices can withstand extreme environmental conditions.
• Minimizing Motion Responses: Reducing motions like roll, pitch, and heave for operational safety and stability.
• Multi-Body Interactions: Accounting for hydrodynamic interactions in structures such as floating platforms, moored vessels, or offshore wind farms.
• Optimizing Mooring Systems: Designing systems that can resist drift forces, anchor loads, and low-frequency oscillations over time.
• Extreme Event Design: Preparing structures to withstand rogue waves, hurricanes, or storm surges.

The marine and offshore industries require advanced simulation techniques to meet safety, performance, and cost-efficiency standards.

Engineering Solutions

Engineering advancements have introduced sophisticated techniques to handle hydrodynamics in offshore structures, such as:

• Model Testing in Wave Tanks: Traditional but highly effective for understanding real-world behavior. However, it is costly and time-intensive.
• Computational Fluid Dynamics (CFD): Provides detailed, physics-based analysis but can be computationally expensive for large-scale models.
• Boundary Element Methods (BEM): Efficiently simulates linear wave-structure interactions using techniques like panel methods, which are ideal for early-stage designs.
• Coupled Simulations: Combining hydrodynamics with mooring dynamics or structural analysis to capture realistic behaviors.

Ansys Aqwa excels by integrating several of these approaches to provide efficient and accurate hydrodynamic solutions, from concept design to final verification.

Methods

Ansys Aqwa provides a powerful suite of tools for simulating wave-structure interactions and optimizing offshore systems. Its key methods include:

• Linear Radiation and Diffraction Analysis: Models how waves interact with floating or fixed structures.
• Multi-Body Hydrodynamic Interaction: Captures forces and motions in complex systems like platforms with moored support vessels.
• RAO Calculations: Provides response amplitude operators for all six degrees of freedom, helping engineers predict how a structure moves in various wave conditions.
• Second-Order Forces: Calculates drift forces and low-frequency oscillations, critical for mooring system design.
• Coupled Mooring and Station-Keeping Analysis: Integrates mooring dynamics and dynamic positioning to ensure station-keeping in real-world sea states.
• Frequency- and Time-Domain Simulations: Allows flexibility to analyze steady-state behaviors or transient responses.

By providing these capabilities, Ansys Aqwa enables engineers to design safer, more efficient offshore structures, meeting industry needs with reduced development time. The results are based on the six degrees of freedom (DOF):

• Surge: Forward and backward motion along the x-axis.
• Sway: Side-to-side motion along the y-axis.
• Heave: Vertical motion along the z-axis.
• Roll: Rotation about the x-axis.
• Pitch: Rotation about the y-axis.
• Yaw: Rotation about the z-axis.
  
  

Results

Ansys Aqwa has been used successfully to address various challenges in marine and offshore engineering. In the following example, a basic hydrodynamic analysis is performed for a cargo ship and its behavior under various wave conditions and directions is evaluated to ensure operational safety and structural integrity. The questions to address are: 1) How will the ship move in response to waves? (motions), 2) How much force will the waves exert on the ship? (wave forces), 3) Is the ship stable in rough seas?

Pressures and Motions. For a given wave frequency and direction, the pressure is calculated on the surfaces of the ship. It is possible to create animations and visualize results in tables as well. The user is asked about what wave components will be included (incident, diffracted and radiation waves, hydrostatic varying). The water elevation is another type of animation that can be generated (not shown). For the plots below, the ship motion is observed for ship's motion for wave at 45° and 0.31513 Hz (left-hand side), and for wave at 180° and 0.16552 Hz (righ-hand side).  

RAOs (Response Amplitude Operators). RAOs are at the heart of hydrodynamic motion analysis. They quantify how much a floating structure will move in response to waves of a specific frequency and direction. For the two cases shown, the 6 degrees of freedom are as follows. Notice that graphs include wave frequencies for each wave direction (45° and 135°). Aqwa also provides a 3D graph containing all wave frequencies and directions.

Natural Modes. Natural modes are the structure's fundamental motion patterns due to its interaction with waves, often considered in six degrees of freedom. These modes describe how a structure responds to hydrodynamic forces and moments generated by wave radiation and diffraction. They are fundamental in understanding how a structure interacts with waves and provide insights into potential resonance behaviors, and in the diffraction analysis, they are not tied directly to time- or frequency-domain simulations but are intrinsic to the structural and hydrodynamic properties of the system.

Added Mass. When a ship moves in water, it must push water aside. The water “resists” the motion, making the ship feel heavier. This extra resistance is called added mass. The added mass affects how the ship accelerates and decelerates in waves, influencing its motion. The SubType refers to the direction or mode of motion being considered and the Component refers to the response direction or the component of the added mass you want to observe. For instance,

• If you select SubType: Global X and Component: Global X, you are plotting the added mass resulting from surge motion in the surge direction.
• If you select SubType: Global X and Component: Global Z, you are plotting the added mass resulting from surge motion but looking at the response in the heave direction.

Additional Results. Ansys Aqwa provides more results relevant for further analyses. Some of them are described as follows:

• Quadratic Transfer Function (QTF): QTF is a key tool for understanding second-order wave forces. QTF calculates the second-order wave forces based on interactions between wave components of different frequencies. It helps analyze drift forces and slow oscillations caused by wave groups.
• Second-order wave forces: First-order forces are proportional to the wave height and dominate a ship's immediate response (e.g., heave, pitch, roll). Second-order forces are proportional to the square of the wave height and cause slower, cumulative effects like drift and mooring tension. These forces can cause slow drift or large motions, especially for moored ships or offshore structures.
• Multi-Body Hydrodynamics: When multiple floating bodies are near each other (e.g., two ships or a ship near a floating platform), their hydrodynamic interactions can’t be ignored; otherwise, they must be calculated.
• Hydrodynamic Interaction with Fixed Structures. While floating structures are the primary focus, fixed structures like breakwaters or gravity-based foundations also play a role. Diffraction analysis is used to study how waves are reflected, diffracted, and transmitted around the structure.

Ansys Solution Benefits

Ansys provides an integrated simulation platform that empowers engineers to tackle the most demanding engineering challenges. By combining advanced physics, robust geometry tools, and cutting-edge optimization, Ansys enables the development of innovative, efficient, and reliable products across industries. Whether designing offshore platforms, optimizing automotive designs, or creating groundbreaking renewable energy solutions, Ansys is the partner of choice for engineering excellence. Some tools are described below:

• Ansys SpaceClaim: A versatile CAD tool for 3D modeling, repair, and editing. Engineers can create complex geometries, simplify models, or optimize existing designs for simulation.
• Ansys Discovery: Combines CAD modeling with real-time simulation, enabling engineers to visualize performance changes interactively during design.
• Ansys Mechanical: It is a powerful tool for simulating structural behavior, offering capabilities for static, dynamic, and thermal analyses.
• Ansys Aqwa: It specializes in analyzing hydrodynamic behavior, making it an essential tool for marine and offshore industries.
• Parametric Studies: Tools like Ansys Workbench enable engineers to perform sensitivity analyses and evaluate the impact of design changes on system performance.
• Ansys OptiSLang: Automates design exploration and optimization, allowing engineers to evaluate multiple design iterations and uncover the most efficient configurations.

Ozen Engineering Expertise

Ozen Engineering Inc. leverages its extensive consulting expertise in CFD, FEA, optics, photonics, and electromagnetic simulations to achieve exceptional results across various engineering projects, addressing complex challenges like multiphase flows, erosion modeling, and channel flows using Ansys software.

We offer support, mentoring, and consulting services to enhance the performance and reliability of your systems. Trust our proven track record to accelerate projects, optimize performance, and deliver high-quality, cost-effective results for both new and existing water control systems. For more information, please visit https://ozeninc.com.

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