Diffraction Analysis

Diffraction Analysis Master Skill

Overview

This skill provides guidance on hydrodynamic diffraction/radiation analysis using the available modules in digitalmodel. Three primary modules handle different aspects of the workflow:

Version

• Skill Version: 1.0.0
• Updated: 2026-01-27
• Category: Offshore Engineering

Module Comparison

When to Use Each Module

Feature Matrix

Unified Data Schema

All modules use the unified schema from diffraction.output_schemas:

from digitalmodel.modules.diffraction import (
    DiffractionResults,  # Complete analysis container
    RAOSet, RAOComponent, # RAO data structures
    AddedMassSet, DampingSet,  # Coefficient matrices
    HydrodynamicMatrix,  # 6×6 frequency-dependent matrix
    FrequencyData, HeadingData,  # Discretization
    DOF, Unit,  # Enumerations
)

DiffractionResults Structure

DiffractionResults
├── vessel_name: str
├── frequencies: FrequencyData
├── headings: HeadingData
├── raos: RAOSet
│   ├── surge: RAOComponent (magnitude, phase)
│   ├── sway: RAOComponent
│   ├── heave: RAOComponent
│   ├── roll: RAOComponent
│   ├── pitch: RAOComponent
│   └── yaw: RAOComponent
├── added_mass: AddedMassSet
│   └── matrices: List[HydrodynamicMatrix]  # 6×6 per frequency
├── damping: DampingSet
│   └── matrices: List[HydrodynamicMatrix]
└── source_file: str

Typical Workflows

Workflow 1: AQWA Analysis Only

from digitalmodel.modules.aqwa import AQWAAnalysis

# Direct AQWA analysis
analysis = AQWAAnalysis(folder="aqwa_results/")
analysis.run()

Workflow 2: AQWA → OrcaFlex Conversion

from digitalmodel.modules.bemrosetta import (
    AQWAParser, OrcaFlexConverter, validate_coefficients
)

# Parse AQWA
parser = AQWAParser()
results = parser.parse("analysis.LIS")

# Validate
report = validate_coefficients(results)
if report.is_valid:
    # Convert to OrcaFlex
    converter = OrcaFlexConverter(output_dir="./orcaflex")
    converter.convert(results)

Workflow 3: OrcaWave Analysis

from digitalmodel.modules.orcawave import OrcaWaveAnalysis

# Run OrcaWave (requires OrcFxAPI)
analysis = OrcaWaveAnalysis()
analysis.setup_model(vessel_file="vessel.yml")
analysis.run_diffraction()
results = analysis.get_results()

Workflow 4: AQWA vs OrcaWave Comparison

from digitalmodel.modules.diffraction import (
    DiffractionComparator,
    AQWAConverter,
    OrcaWaveConverter,
)

# Convert both sources to unified schema
aqwa_results = AQWAConverter("aqwa_folder/").convert_to_unified_schema()
orcawave_results = OrcaWaveConverter(model).convert_to_unified_schema()

# Compare
comparator = DiffractionComparator()
report = comparator.compare(aqwa_results, orcawave_results)
print(f"RAO match: {report.rao_match_percentage:.1f}%")

Workflow 5: Complete Pipeline with QTF

from digitalmodel.modules.bemrosetta import (
    AQWAParser, QTFParser, OrcaFlexConverter,
    CoefficientValidator, CausalityChecker,
)

# Parse main results
parser = AQWAParser()
results = parser.parse("analysis.LIS")

# Parse QTF
qtf_parser = QTFParser()
qtf_data = qtf_parser.parse("analysis.QTF")

# Validate
coef_validator = CoefficientValidator(check_symmetry=True)
coef_report = coef_validator.validate(results)

causality_checker = CausalityChecker()
kk_report = causality_checker.validate(results)

# Convert with QTF
converter = OrcaFlexConverter(output_dir="./output")
converter.set_qtf_data(qtf_data)
converter.convert(results)

CLI Commands

AQWA Module

# (Uses existing AQWA CLI if available)

BEMRosetta Module

bemrosetta convert analysis.LIS -o ./output
bemrosetta convert analysis.LIS --qtf analysis.QTF -o ./output
bemrosetta info analysis.LIS
bemrosetta validate analysis.LIS --strict --causality
bemrosetta convert-mesh hull.gdf -o hull.stl
bemrosetta status

Diffraction Module

# Batch processing
python -m digitalmodel.modules.diffraction.batch_processor config.yml

Output Formats

OrcaFlex Vessel Type YAML

VesselType:
  Name: FPSO
  Category: Vessel
  PrimaryMotion: Calculated (6 DOF)
  RAOOrigin: [0, 0, 0]
  RAOPhaseConvention: AQWA

Coefficient CSV

Frequency_rad/s,A11,A12,A13,A14,A15,A16,...
0.3,1.0e7,0.0,0.0,0.0,0.0,0.0,...
0.4,1.1e7,0.0,0.0,0.0,0.0,0.0,...

QTF CSV

Freq1_rad/s,Freq2_rad/s,Heading_deg,Surge_Re,Surge_Im,...
0.3,0.3,0.0,1.2e5,0.0,...

Validation Criteria

Coefficient Validation

• Symmetry: Added mass and damping matrices should be symmetric
• Positive definiteness: Diagonal elements non-negative
• Physical limits: No NaN/Inf values, reasonable magnitudes

Kramers-Kronig Causality

• Added mass A(ω) and damping B(ω) must satisfy K-K relations
• Tolerance: typically 10% relative error acceptable

RAO Validation

• Magnitude non-negative
• Phase in reasonable range (-360° to 360°)
• Physical trends (heave RAO → 1.0 at low frequency)

Best Practices

1. Always validate coefficients before using in OrcaFlex
2. Check K-K causality for added mass/damping consistency
3. Compare results when both AQWA and OrcaWave are available
4. Use unified schema for interoperability
5. Document water depth and frequency range assumptions

Related Skills

Module Locations

src/digitalmodel/modules/
├── aqwa/                    # AQWA analysis tools
├── orcawave/                # OrcaWave analysis
├── bemrosetta/              # Format conversion
│   ├── parsers/             # AQWA, QTF parsers
│   ├── converters/          # OrcaFlex export
│   ├── mesh/                # GDF, DAT, STL handlers
│   └── validators/          # Coefficient, causality
├── diffraction/             # Unified schemas
│   ├── output_schemas.py    # DiffractionResults
│   ├── aqwa_converter.py    # AQWA to unified
│   ├── orcawave_converter.py # OrcaWave to unified
│   ├── orcaflex_exporter.py # Export to OrcaFlex
│   └── comparison_framework.py # Compare results
└── hydrodynamics/           # Coefficient database

References

• OrcaFlex Documentation: https://www.orcina.com/webhelp/OrcaFlex/
• WAMIT Manual: https://www.wamit.com/manual.htm
• BEMRosetta: https://github.com/BEMRosetta/BEMRosetta