catenary-riser

# Catenary Riser Analysis Skill

Analyze catenary and lazy wave riser configurations for static shape, forces, tension distribution, and OrcaFlex model generation.

## When to Use

- Catenary riser static analysis
- Lazy wave catenary design and optimization
- Riser static configuration calculation
- Catenary force and tension analysis
- Generating OrcaFlex models from catenary geometry
- Fatigue loading extraction from riser configurations
- Pipe property calculations (buoyancy, effective weight)

## Prerequisites

- Python environment with `digitalmodel` package installed
- Riser geometry and material properties
- Environmental data (water depth, current if applicable)

## Analysis Types

### 1. Simple Catenary Analysis

```yaml
catenary:
  simple_catenary:
    flag: true
    geometry:
      top_end:
        x: 0.0
        z: 100.0
      touchdown:
        x: 500.0
        z: 0.0
    properties:
      outer_diameter: 0.2032
      wall_thickness: 0.0127
      steel_density: 7850
      internal_fluid_density: 800
      seawater_density: 1025
    output:
      shape_file: "results/catenary_shape.csv"
      forces_file: "results/catenary_forces.csv"
      plot_file: "results/catenary_plot.html"
```

### 2. Lazy Wave Catenary

```yaml
catenary:
  lazy_wave:
    flag: true
    geometry:
      water_depth: 1500.0
      hang_off_angle: 8.0
      buoyancy_section:
        start_arc_length: 800.0
        end_arc_length: 1200.0
        buoyancy_factor: 1.5
      target_sag_bend_depth: 800.0
      target_hog_bend_depth: 600.0
    pipe_properties:
      outer_diameter: 0.273
      wall_thickness: 0.0159
    output:
      configuration_file: "results/lazy_wave_config.csv"
      plot_file: "results/lazy_wave_plot.html"
```

### 3. OrcaFlex Model Generation

```yaml
catenary:
  orcaflex_model:
    flag: true
    catenary_config: "results/lazy_wave_config.csv"
    orcaflex_settings:
      line_name: "Riser1"
      segment_length: 5.0
      include_buoyancy_modules: true
    output:
      yml_file: "models/riser_model.yml"
```

## Python API

### Simple Catenary Equation

```python
from digitalmodel.modules.catenary.catenary_equation import CatenaryEquation
import numpy as np

catenary = CatenaryEquation()
top_end = (0.0, 100.0)
touchdown = (500.0, 0.0)

horizontal_tension = catenary.solve(
    top_end=top_end,
    bottom_end=touchdown,
    unit_weight=500.0
)

arc_lengths = np.linspace(0, catenary.total_length, 100)
x, z = catenary.get_shape(arc_lengths)
tensions = catenary.get_tension(arc_lengths)
```

### Catenary Riser Analysis

```python
from digitalmodel.modules.catenary.catenary_riser import CatenaryRiser
from digitalmodel.modules.catenary.pipe_properties import PipeProperties

pipe = PipeProperties(
    outer_diameter=0.2032,
    wall_thickness=0.0127,
    steel_density=7850,
    internal_fluid_density=800
)

riser = CatenaryRiser(pipe_properties=pipe)
results = riser.analyze(
    water_depth=1000.0,
    hang_off_angle=10.0,
    top_tension=1500e3
)

print(f"TDP position: x={results['tdp_x']:.1f}m")
print(f"Total length: {results['total_length']:.1f} m")
```

### Lazy Wave Catenary

```python
from digitalmodel.modules.catenary.lazy_wave_catenary import LazyWaveCatenary

lazy_wave = LazyWaveCatenary()
config = {
    "water_depth": 1500.0,
    "hang_off_angle": 8.0,
    "buoyancy_section": {
        "start_length": 800.0,
        "end_length": 1200.0,
        "net_buoyancy": 300.0
    },
    "pipe_properties": pipe
}

results = lazy_wave.analyze(config)
sag_bend = results["sag_bend"]
hog_bend = results["hog_bend"]
```

### OrcaFlex Model Building

```python
from digitalmodel.modules.catenary.orcaflex_model import OrcaFlexModelBuilder

builder = OrcaFlexModelBuilder()
builder.set_line_from_catenary(
    catenary_results=results,
    line_name="Riser1",
    segment_length=5.0
)
builder.add_buoyancy_section(
    start_arc_length=800.0,
    end_arc_length=1200.0,
    buoyancy_od=0.6
)
builder.export_yml("models/riser.yml")
```

## Key Classes

| Class | Purpose |
|-------|---------|
| `CatenaryEquation` | Mathematical catenary solver |
| `CatenaryRiser` | Main analysis orchestrator |
| `LazyWaveCatenary` | Dynamic catenary with buoyancy |
| `PipeProperties` | Material and geometric properties |
| `OrcaFlexModelBuilder` | Model export to OrcaFlex |

## Output Formats

### Catenary Shape CSV

```csv
arc_length,x,z,tension,curvature,bend_moment
0.0,0.0,100.0,1523456.7,0.000123,1875.4
10.0,9.8,98.5,1518234.5,0.000145,2215.6
```

## Best Practices

1. **Consistent units** - Use SI units (meters, Newtons)
2. **Coordinate system** - Z positive upward, origin at seabed
3. **Segment resolution** - Use small segments near critical points
4. **Curvature check** - Verify minimum bend radius constraints

## Related Skills

- [orcaflex-modeling](../orcaflex-modeling/SKILL.md) - Run OrcaFlex simulations
- [fatigue-analysis](../fatigue-analysis/SKILL.md) - Fatigue at critical locations
- [structural-analysis](../structural-analysis/SKILL.md) - Stress verification

## References

- API RP 2RD: Design of Risers for Floating Production Systems
- DNV-OS-F201: Dynamic Risers