Mooring Line Calculator

1. Wind drag
   F_wind = 0.5 · ρ_air · C_d · A_air · V_wind²
2. Current drag
   F_cur = 0.5 · ρ_water · C_d · A_water · V_cur²
3. Total horizontal load
   F_total = F_wind + F_cur + F_wave
4. Line distribution with angles and sharing
   η = cos(θ_H) · cos(θ_V) · S
   H_line = F_total / (N · η)
5. Design tension and MBL
   T_design = (H_line · DAF) + T_pre
   MBL_req = T_design · SF
6. Elastic stretch (optional)
   ΔL = (T · L) / (E · A)

1. Collect design wind, current, and wave drift assumptions for the task.
2. Estimate projected areas above and below water for the unit.
3. Choose drag coefficients consistent with your geometry and guidance.
4. Enter the number of lines that resist the governing load direction.
5. Enter realistic horizontal and vertical lead angles at the fairlead.
6. Set load sharing, dynamic factor, pretension, and safety factor.
7. Run the calculation, then download CSV or PDF for documentation.

1) Why mooring loads matter on construction sites

Mooring forces control station keeping for barges, jack-up support units, floating platforms, and temporary berths. A small change in wind speed can noticeably raise demand because drag grows with velocity squared. This calculator reports wind, current, and wave-drift components separately, so planners can see which driver governs and document assumptions in exportable reports.

2) Wind loading inputs and typical ranges

The tool uses air density 1.225 kg/m³ and computes F = 0.5·ρ·Cd·A·V². For bluff marine shapes, a wind drag coefficient of 1.0–1.5 is common for quick screening, while air projected areas of 150–600 m² often represent work barges and deck cargo outlines. Always use project metocean values for design wind.

3) Current loading and submerged geometry

Water density is taken as 1025 kg/m³, which makes current loads significant even at 0.5–2.0 m/s. Underwater projected area should reflect hull draft, pontoons, and any large appendages normal to flow. Current drag coefficients of 0.8–1.2 are frequently used for preliminary checks, but detailed models should follow verified guidance.

4) Wave drift force as a direct input

Wave drift is entered directly in kN because it is often obtained from separate analyses or vendor data. For sheltered works, drift may be near zero; for exposed operations, hundreds of kN can occur depending on sea state and footprint. Recording the drift value alongside wind and current helps explain conservative or optimized mooring decisions.

5) Line count, angles, and efficiency

Not every installed line resists the governing direction. Enter the effective number of lines and the lead angles at the fairlead. The calculator applies cos(θH)·cos(θV) to reflect geometric efficiency; for example, 20° horizontal and 10° vertical gives about 0.925 efficiency before sharing. Large vertical angles reduce horizontal holding quickly.

6) Load sharing, dynamics, and pretension

Mooring systems rarely share perfectly, so a load-sharing factor of 0.7–0.9 is often used for screening where stiffness and geometry differ. Dynamic amplification factors commonly range 1.1–1.6 to represent gusts, vessel motion, and snatch effects. Pretension can stabilize the system but increases reported design tension.

7) Sizing with MBL and safety factors

The calculator computes design line tension and multiplies by the selected safety factor to produce required minimum breaking load (MBL). Safety factors of 2.0–3.5 are frequently seen in practice depending on consequences, redundancy, and operational control. Use certified line MBL, termination efficiency, and inspection limits for final acceptance.

8) Materials, stiffness, and stretch awareness

Material choice affects handling and elongation. Polyester typically offers moderate stretch with good durability; nylon stretches more and can reduce peak loads but may increase offsets; HMPE provides low stretch and high strength with careful attention to bending and abrasion. The optional stretch estimate uses elastic modulus (GPa) and line length to show order-of-magnitude elongation.

1) What does “required MBL” represent?

Required MBL is the minimum breaking load the line should meet after applying your safety factor to the calculated design tension. It supports early selection and documentation, not final certification.

2) Should I enter gust wind speed or sustained wind?

Use the wind basis your project specifies. If you use a sustained value, consider a higher dynamic factor. If you use gust values directly, keep the dynamic factor consistent with that assumption.

3) How do I choose the number of resisting lines?

Count only the lines that meaningfully oppose the governing load direction. Lines far off-angle contribute less due to cosine losses, so use an effective count that reflects the layout and expected load direction.

4) Why can the same line pass in one layout and fail in another?

Angles and sharing control how much of the total load turns into tension in each line. Higher vertical lead angles, fewer resisting lines, or poorer sharing increases per-line demand quickly.

5) Can I verify a vendor line instead of sizing a new one?

Yes. Select “Capacity check” and enter either the certified MBL or diameter. The tool reports utilization as design tension divided by capacity to indicate pass or fail under the chosen factors.

6) Is the diameter sizing accurate for procurement?

Diameter sizing is indicative and uses a simplified MBL ≈ k·d² relationship. Procurement should use manufacturer certificates, termination efficiency, inspection criteria, and any project-specific reduction factors.

7) What if I have wave loads from a separate analysis?

Enter the wave drift force directly in kN and keep the same wind and current basis. This keeps the calculator aligned with your external study while still reporting per-line tensions and exportable summaries.