| Scenario | Wind (m/s) | Current (m/s) | Hs (m) | Lines | Angles (plan/vert) | MBL (kN) | Typical outcome |
|---|---|---|---|---|---|---|---|
| Harbor standby | 12 | 0.6 | 0.8 | 6 | 15° / 8° | 1200 | Low utilization; good reserve margin. |
| Moderate exposure | 20 | 1.0 | 1.5 | 6 | 20° / 10° | 1200 | Mid utilization; check DAF and sharing. |
| High exposure | 28 | 1.5 | 2.2 | 4 | 30° / 12° | 1200 | High utilization; add lines or reduce angles. |
- Wind force: Fw = 0.5 · ρair · Cd,w · Aw · Vw2
- Current force: Fc = 0.5 · ρwater · Cd,c · Au · Vc2
-
Wave drift force (approx.):
Fv ≈ Cwave · Hs2 · B
You can also input wave drift force directly if known.
- Applied horizontal load: Fapp = (Fw + Fc + Fv) · G where G is the gust factor.
- Geometry effectiveness: η = cos(θplan) · cos(θvert)
- Shared line tension (per line): Tshare = Fapp / (N · η) · U where U is the uneven load factor.
- Design line tension: Tdesign = Tshare · DAF · SF + Tpre
- Enter wind and current data for your design case.
- Choose wave method: compute from Hs or input direct drift force.
- Set the number of effective lines and their angles.
- Add pretension and an uneven load factor if sharing is imperfect.
- Provide line MBL and allowable utilization for WLL checks.
- Adjust gust, DAF, and safety factors to match your method.
- Press calculate to view results and download CSV or PDF.
Design Inputs and Load Sources
This calculator estimates mooring demand by combining wind, current, and wave drift actions commonly assessed during temporary works, berthing, and marine lift operations. Wind and current loads use drag relationships based on density, projected area, velocity, and a selectable coefficient. Wave drift can be entered directly from studies or approximated using significant wave height and a beam term to reflect broadside exposure. If you have measured metocean data, enter site speeds and areas representative of the worst credible orientation case.
Load Combination and Adjustment Factors
After the individual actions are calculated, the tool sums them into a total horizontal environmental load and applies a gust factor to represent variability and short‑term peaks. A dynamic amplification factor then increases shared line tension to reflect surge, snatch, and vessel motions. Finally, a safety factor provides an overall margin for modelling uncertainty, equipment condition, and operational variability.
Mooring Geometry and Load Sharing
Mooring lines rarely align perfectly with the dominant load. Plan and vertical angles reduce the effective horizontal resistance, so the calculator applies cosine components to capture this loss of efficiency. The number of effective lines should reflect only those contributing in the load direction. An uneven load factor can be applied when line lengths, winch settings, or fairlead positions cause imperfect sharing.
Capacity Checks and Utilization
The calculated design tension per line is compared with the line’s minimum breaking load and an allowable utilization that represents a working limit. The utilization percentages provide a quick screening for feasibility and highlight when additional lines, improved angles, or reduced environmental exposure are needed. Where axial stiffness is available, a simple elastic stretch estimate is included to support clearance and winch travel checks.
Field Reporting and Scenario Comparison
For site teams, the results panel summarizes key forces, geometry efficiency, and the governing utilization in a format suitable for briefings. CSV export supports record keeping and scenario tables, while the PDF report provides a consistent snapshot for permits and method statements. Use the example scenarios as a starting point, then tune coefficients and factors to match your project standards and observed conditions.
1) What does “effective cosine factor” mean?
It represents how plan and vertical angles reduce the horizontal component of line capacity. Smaller cosine values increase tension for the same environmental load.
2) When should I use direct wave force input?
Use it when you already have wave drift or surge forces from a mooring study, model test, or specification. It avoids tuning the Hs coefficient.
3) How do I choose the uneven load factor?
Start with 1.10–1.30 when line lengths, winches, or fairleads differ. Use higher values only with justification, because it directly increases per‑line tension.
4) Is pretension multiplied by dynamic and safety factors?
No. Pretension is added after the factored tension calculation to represent the set line tension maintained by winches or stoppers.
5) Why might my utilization be low but motions still large?
Low utilization checks strength, not serviceability. Long, elastic lines can allow large excursions even at modest loads. Review stretch, clearance, fender limits, and operational constraints.
6) Can this replace a full mooring analysis?
No. It is a screening tool. Detailed design may require catenary geometry, seabed interaction, stiffness curves, and time‑domain dynamics per project standards.