```html Seastead Simulation Software Guide

Seastead Hydrodynamic Simulation Guide

Analysis & Software Recommendations for Tension-Leg Platform Design

Design Analysis Summary

Your design represents a Tension-Leg Platform (TLP) variant with diagonal buoyant columns. Key hydrodynamic challenges include:

Displacement: ~16.3 tonnes (36,000 lbs)
Draft: 12 ft per column
Waterplane Area: ~25 ft² (4 small columns)
Leg Spread: 50×74 ft at base
Pretension: Cable system counters buoyancy uplift
Stability: Pendulum stability from low CG vs buoyancy centers
Reynolds Number: ~10⁶ (drag dominated, potential flow invalid)
KC Number: Varies by sea state (viscous drag critical)
Thrust: 4×2.5m props (low disc loading)

🏆 Primary Recommendations

OrcaFlex

Commercial Industry Standard Best for Cables

The gold standard for your specific application. OrcaFlex is specifically designed for offshore marine systems with complex mooring configurations.

Advantages:
  • Native handling of slack/taut cables with bend stiffness
  • Multi-body dynamics with 6-DOF coupling
  • Buoyant column objects with Morison equation drag
  • Thruster models with flow interaction
  • Time-domain simulation with irregular waves (JONSWAP, PM spectra)
  • Can model the 45° angle legs as "Lines" with buoyancy
Considerations:
  • Expensive (~$15-25k license)
  • Windows only
  • Steep learning curve for custom dynamics
  • Diffraction analysis requires WAFA license or external coupling

Modeling Strategy: Represent the 4 legs as Buoyant Lines with 45° orientation, platform as 6D Buoy, cables as Lines with seabody connections. Use Vessel Type data for drag coefficients of non-standard hull.

ANSYS AQWA

Commercial Hydrodynamic Diffraction

Best for calculating wave forces and RAOs (Response Amplitude Operators) on the submerged portions, though limited cable dynamics.

Advantages:
  • 3D diffraction/radiation for the angled columns
  • Can couple with ANSYS Mechanical for structural analysis
  • Handles multiple interacting bodies
  • Good for pressure vessel stress analysis (FSI)
Limitations:
  • Cable modeling requires Nauticus or manual coupling
  • Frequency domain primarily (time domain available but limited)
  • Difficult to model the exact cable-pretension system

🛠️ Specialized & Open Source Options

OpenFOAM (waves2Foam)

Open Source CFD

Full Navier-Stokes simulation for accurate drag prediction on your non-standard geometry.

Best for: Validating drag coefficients at 0.5-1 MPH, visualizing flow around the angled legs and platform underbelly.

  • Free and open source
  • Handles viscous effects (critical for your drag estimation)
  • Can model propeller interaction using actuator disk or MRF
  • Python automation available
  • Extremely steep learning curve (3-6 months)
  • Meshing complex geometry difficult
  • Time-consuming setup for moving bodies
  • Requires HPC for meaningful results

MoorDyn + MATLAB/Python

Open Source Mooring Dynamics

Specifically designed for mooring line dynamics. Can couple with your own equations of motion for the platform.

Use case: Model the cable system accuracy, couple with simplified 6-DOF platform model.

ProteusDS

Commercial Flexible Body

Canadian software excellent for unconventional marine structures. Better at flexible bodies and cables than most competitors.

Advantage: Native handling of pressurized vessels and cable dynamics. Good for prototyping your concept.

📐 CAD-Integrated Preliminary Design

Rhino + Grasshopper + Orca3D

Commercial Plugin

For preliminary hydrostatics and stability analysis before full simulation.

⚠️ Critical Modeling Considerations for Your Design

  1. Morison vs. Diffraction: At 4ft diameter and expected wave periods (3-8s), your columns are borderline between Morison equation (drag dominated) and diffraction regime. You may need both: diffraction for heave/pitch, Morison for surge/sway drag.
  2. Cable Stiffness: The 45° angle creates a geometric stiffness matrix. When modeling, ensure cables are pre-tensioned correctly to match the 50×74 ft footprint vs the 40×16 ft deck.
  3. Added Mass: The angled orientation means added mass varies with direction. Ensure your software allows directional added mass coefficients or uses 3D diffraction.
  4. Thruster Interaction: With 2.5m props close to the 4ft diameter legs, you may have thruster-leg interaction. OrcaFlex can model this with wake models; OpenFOAM can solve it directly.
  5. Pressure Vessel Effects: The 10 PSI internal pressure increases hoop stress but also slightly increases stiffness. For dynamic analysis, ensure your structural damping accounts for the steel shell vibration modes.

Recommended Workflow

1
Preliminary Design: Use Rhino/Orca3D to verify hydrostatics and initial stability. Check that 36,000 lbs matches the buoyancy of 4×(half of 24ft submerged)×π×(2ft)²×64 lb/ft³ ≈ 38,000 lbs (close to your weight, good).
2
Hydrodynamic Coefficients: Run ANSYS AQWA or WAMIT (open source alternative) to get added mass and damping matrices for the angled column configuration.
3
Time Domain Simulation: Import results into OrcaFlex (or use native if AQWA-NAUT). Model the cables with actual stiffness properties (E for stainless steel, ~28e6 psi). Run irregular wave simulations (Bretschneider spectrum, Hs=3-6ft for testing).
4
Validation: If budget allows, use OpenFOAM to validate drag coefficients at low speed (0.5-1 MPH) since this is non-standard hull form.
5
Maneuvering: In OrcaFlex, add the thruster models with 2.5m propellers, verify station-keeping capability against current (expected tidal currents vs. thrust available).

🎓 Academic/Free Alternatives

💡 Pro Tip: Simplified Verification Model

If software access is limited, create a scaled physical model (1:25 scale in a wave tank) and validate against MoorDyn or a simplified MATLAB/Simulink model using Cummins' equation:

(M + A∞)ξ¨ + ∫K(t-τ)ξ˙(τ)dτ + Cξ + F_cables(ξ) = F_wave + F_thrusters

Where K is the retardation kernel from AQWA/WAMIT, and F_cables is the nonlinear cable restoring force.

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