VIV Analysis Skill

Assess vortex-induced vibration (VIV) susceptibility for risers and tubular members with natural frequency calculations and safety factor evaluation.

When to Use

• VIV analysis for risers and pipelines
• Natural frequency calculation for tubular members
• Vortex shedding frequency analysis
• VIV fatigue damage assessment
• Tubular member VIV screening
• Safety factor evaluation against VIV criteria

Prerequisites

• Python environment with digitalmodel package installed
• Member geometry and material properties
• Current velocity profiles
• For risers: tension distribution along length

Analysis Types

1. Natural Frequency Analysis

Calculate natural frequencies for tubular members.

viv_analysis:
  natural_frequency:
    flag: true
    member:
      length: 50.0
      outer_diameter: 0.5
      wall_thickness: 0.025
      material:
        youngs_modulus: 207e9
        density: 7850
      boundary_conditions: "pinned-pinned"  # or fixed-fixed, fixed-pinned
    added_mass:
      coefficient: 1.0
      fluid_density: 1025
    output:
      frequencies_file: "results/natural_frequencies.json"
      mode_shapes_file: "results/mode_shapes.csv"

2. Vortex Shedding Assessment

Evaluate vortex shedding frequencies against natural frequencies.

viv_analysis:
  vortex_shedding:
    flag: true
    member:
      outer_diameter: 0.5
      length: 50.0
    current_profile:
      type: "power_law"  # or uniform, linear, custom
      surface_velocity: 1.5
      power_exponent: 0.143
    strouhal_number: 0.2
    output:
      shedding_frequencies: "results/vortex_shedding.csv"
      lock_in_check: "results/lock_in_analysis.json"

3. VIV Susceptibility Screening

Quick screening for VIV susceptibility.

viv_analysis:
  screening:
    flag: true
    members:
      - name: "Riser1"
        outer_diameter: 0.273
        length: 1500.0
        natural_frequency: 0.15
      - name: "Jumper1"
        outer_diameter: 0.2032
        length: 30.0
        natural_frequency: 2.5
    current_velocity: 1.2
    criteria:
      reduced_velocity_min: 4.0
      reduced_velocity_max: 8.0
    output:
      screening_report: "results/viv_screening.json"

4. Tubular Member VIV Analysis

Complete VIV analysis for tubular members per design codes.

viv_analysis:
  tubular_members:
    flag: true
    members:
      - name: "Brace1"
        geometry:
          outer_diameter: 0.324
          wall_thickness: 0.0127
          length: 12.0
        end_conditions: "fixed-fixed"
        effective_length_factor: 0.7
    environment:
      current_velocity: 1.0
      water_depth: 100.0
    design_code: "DNV-RP-C205"
    output:
      analysis_report: "results/tubular_viv_analysis.json"
      safety_factors: "results/viv_safety_factors.csv"

Python API

Natural Frequency Calculation

from digitalmodel.modules.viv_analysis.viv_analysis import VIVAnalysis
from digitalmodel.modules.viv_analysis.viv_tubular_members import VIVTubularMembers

# Initialize analysis
viv = VIVAnalysis()

# Define member properties
member = {
    "length": 50.0,
    "outer_diameter": 0.5,
    "wall_thickness": 0.025,
    "youngs_modulus": 207e9,
    "density": 7850,
    "boundary_conditions": "pinned-pinned"
}

# Calculate natural frequencies
frequencies = viv.calculate_natural_frequencies(
    member,
    n_modes=5,
    added_mass_coefficient=1.0,
    fluid_density=1025
)

for i, freq in enumerate(frequencies):
    print(f"Mode {i+1}: {freq:.3f} Hz, Period: {1/freq:.3f} s")

Vortex Shedding Analysis

# Vortex shedding frequency
diameter = 0.5  # meters
current_velocity = 1.5  # m/s
strouhal = 0.2

shedding_freq = viv.vortex_shedding_frequency(
    diameter=diameter,
    velocity=current_velocity,
    strouhal_number=strouhal
)
print(f"Shedding frequency: {shedding_freq:.3f} Hz")

# Reduced velocity
natural_freq = 0.15  # Hz
reduced_velocity = viv.reduced_velocity(
    velocity=current_velocity,
    frequency=natural_freq,
    diameter=diameter
)
print(f"Reduced velocity: {reduced_velocity:.2f}")

# Lock-in check
is_lock_in = viv.check_lock_in(
    reduced_velocity=reduced_velocity,
    vr_min=4.0,
    vr_max=8.0
)
print(f"Lock-in condition: {is_lock_in}")

Tubular Member Analysis

from digitalmodel.modules.viv_analysis.viv_tubular_members import VIVTubularMembers

# Initialize tubular member analysis
tubular = VIVTubularMembers()

# Define member
member_props = {
    "name": "Brace1",
    "outer_diameter": 0.324,
    "wall_thickness": 0.0127,
    "length": 12.0,
    "end_conditions": "fixed-fixed",
    "effective_length_factor": 0.7,
    "youngs_modulus": 207e9,
    "steel_density": 7850
}

# Run analysis
results = tubular.analyze(
    member=member_props,
    current_velocity=1.0,
    water_depth=100.0,
    design_code="DNV-RP-C205"
)

print(f"Natural frequency: {results['natural_frequency']:.3f} Hz")
print(f"Shedding frequency: {results['shedding_frequency']:.3f} Hz")
print(f"Reduced velocity: {results['reduced_velocity']:.2f}")
print(f"VIV susceptible: {results['is_susceptible']}")
print(f"Safety factor: {results['safety_factor']:.2f}")

VIV Fatigue Assessment

from digitalmodel.modules.viv_analysis.viv_fatigue import VIVFatigue

# Initialize VIV fatigue analysis
viv_fatigue = VIVFatigue()

# Calculate VIV-induced stress range
stress_range = viv_fatigue.calculate_stress_range(
    amplitude=0.5,  # VIV amplitude in diameters
    diameter=0.324,
    wall_thickness=0.0127,
    mode_shape="first_mode"
)

# Calculate fatigue damage from VIV
damage = viv_fatigue.calculate_damage(
    stress_range=stress_range,
    frequency=0.5,  # Hz
    duration=3600,  # seconds
    sn_curve="DNV-D"
)
print(f"VIV fatigue damage: {damage:.6f}")

Key Classes

VIV Parameters

Strouhal Number

Reduced Velocity Ranges

Added Mass Coefficients

Configuration Examples

Complete VIV Screening Workflow

basename: viv_screening

viv_analysis:
  # Step 1: Natural frequencies for all members
  natural_frequencies:
    flag: true
    members:
      - name: "Riser"
        length: 1500.0
        outer_diameter: 0.273
        wall_thickness: 0.0159
        boundary: "tension_controlled"
        top_tension: 500e3
      - name: "Jumper"
        length: 25.0
        outer_diameter: 0.2032
        wall_thickness: 0.0127
        boundary: "fixed-fixed"

  # Step 2: Current profile analysis
  current_analysis:
    flag: true
    profiles:
      - name: "1-year"
        surface_velocity: 0.8
      - name: "100-year"
        surface_velocity: 1.5

  # Step 3: VIV susceptibility
  susceptibility:
    flag: true
    criteria: "DNV-RP-C205"
    output:
      report: "results/viv_susceptibility.html"
      summary: "results/viv_summary.json"

Output Formats

Natural Frequencies JSON

{
  "member_name": "Riser1",
  "n_modes": 5,
  "frequencies": [0.15, 0.42, 0.78, 1.21, 1.72],
  "periods": [6.67, 2.38, 1.28, 0.83, 0.58],
  "boundary_conditions": "tension_controlled",
  "effective_tension": 500000.0
}

VIV Screening Report

{
  "member": "Brace1",
  "natural_frequency_hz": 2.5,
  "shedding_frequency_hz": 0.62,
  "reduced_velocity": 4.94,
  "is_susceptible": true,
  "lock_in_margin": 0.94,
  "safety_factor": 1.02,
  "recommendation": "VIV suppression required"
}

Best Practices

1. Include added mass - Always account for added mass in frequency calculations
2. Conservative Strouhal - Use appropriate Strouhal for surface roughness
3. Multiple modes - Check several natural frequency modes
4. Current profiles - Use depth-varying current profiles
5. Safety margins - Apply appropriate safety factors per design code

Design Code References

Related Skills

• catenary-riser - Riser configuration
• fatigue-analysis - VIV fatigue damage
• structural-analysis - Stress verification

References

• DNV-RP-C205: Environmental Conditions and Environmental Loads
• DNV-RP-F105: Free Spanning Pipelines
• Blevins, R.D.: Flow-Induced Vibration