Lifting heavy and oversized cargo is a critical part of project freight operations. Ensuring stability during these lifts is paramount to safety and success. The Guidance on Stability of Lifts by the Heavy Lift Exchange Forum provides valuable insights into assessing and managing lifting stability. Here’s a simplified yet professional breakdown for PFN members.
What is Lifting Stability?
A stable lift means the cargo remains balanced within safe limits when subjected to disturbances like wind, vessel motions, or rigging tolerances. Key factors include:
- Center of Gravity (CoG): The higher the CoG relative to lifting points, the more unstable the lift.
- Rigging Geometry: The length and angle of slings (primary and secondary suspensions) impact stability.
- Friction at Lift Points: Friction-dependent lift points (e.g., belly slings) are less stable than fixed points (e.g., pad-eyes).
Key Steps to Assess Lifting Stability
The guidance outlines a structured approach:
1. Draft a Rigging Plan
- Define cargo geometry, weight, CoG, and lifting points.
- Ensure equilibrium: The combined CoG of cargo and rigging should align vertically with the crane hook.
- Design Tips:
- Longer primary slings = more stable.
- Secondary slings tilted outwards improve stability; inwards reduce it.
- Prefer fixed lift points over friction-dependent ones.
2. Check Friction-Dependent Lift Points
If slings rely on friction (e.g., wrapped around cargo):
- Calculate the sliding angle—the tilt at which cargo starts slipping.
- Assess friction factors (material, surface condition, lubrication).
- Mitigate sliding risks with friction-enhancing measures (e.g., anti-slip pads).
3. Evaluate CoG Position
- CoG below lifting points? Generally stable.
- CoG above lifting points? Potentially unstable—requires stability assessment.
- Methods to Assess Stability:
- Virtual CoG Method: Quick but simplistic; checks if CoG is within a “stability triangle.”
- Kaps Method: Analytical, accounts for sling angles and weights.
- Nikitin Method: Evaluates overturning stability for complex rigging.
- Numerical Simulations (e.g., Orcaflex): High accuracy but time-consuming.
4. Define Operational Limits
Once stability is confirmed, set operational boundaries:
- Max wind speed
- Tugger line forces
- Wave height and crane motions
- CoG tolerance
Common Challenges & Solutions
Wind Forces
- Wind creates lateral forces, tilting the cargo.
- Calculate wind load using:Fw=0.5⋅ρ⋅v2⋅A⋅cFw=0.5⋅ρ⋅v2⋅A⋅c(where ρρ = air density, vv = wind speed, AA = projected area, cc = drag coefficient).
Sling Tolerances & Stiffness
- Length variations in slings can tilt cargo.
- Use pre-measured or matched slings to minimize imbalance.
Dynamic Effects (Vessel & Crane Motions)
- Pitching, rolling, and crane movements introduce horizontal forces.
- Dynamic analysis software helps simulate worst-case scenarios.
Practical Takeaways
- Prefer fixed lift points where possible.
- Optimize rigging geometry—long primary slings, outward-tilted secondary slings.
- Always assess friction for belly slings and conical/cylindrical cargo.
- Use multiple stability methods—Virtual CoG for quick checks, Kaps/Nikitin for detailed analysis.
- Set clear operational limits and monitor conditions during lifts.
Final Thoughts
Stability in heavy lifting isn’t just about strength—it’s about balance, geometry, and anticipating real-world variables. By following structured assessments and leveraging industry methods, PFN members can execute lifts safely and efficiently.