In maritime cargo operations, bending moment (BM) and shearing force (SF) are routinely monitored during cargo planning and loading. On bulk carriers, these two parameters are usually sufficient to ensure the vessel’s structural integrity under most operating conditions. However, torsional moment — the twisting moment acting along the ship’s longitudinal axis — is frequently overlooked, especially by deck officers accustomed to bulk operations.
This oversight, though common, may pose serious risks on container ships, where the cargo distribution and structural configuration make torsional stress a critical factor.
Structural and Operational Distinctions: Bulk Carriers vs Container Ships
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Parameter
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Bulk Carrier
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Container Vessel
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Cargo Distribution
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Homogeneous, symmetric across holds
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Highly segmented, dependent on discharge sequence
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Structural Design
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Enclosed, rigid structure
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Open deck with longitudinal flexibility
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Loading Software Focus
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Bending Moment & Shearing Force
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BM, SF, Torsional Moment, Stability, Trim
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Port Rotation
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1–3 ports per voyage
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Often 5–10 ports with frequent re-stow operations
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Bulk carriers typically load cargo such as coal, grain, or ore evenly into centerline holds. The cargo hold plan and loading manual focus on longitudinal bending and shear forces. As such, loading instruments for bulk carriers do not normally account for torsional moments — a justifiable simplification given their stable and symmetric cargo layout.
Why Torsional Moment Must Be Monitored on Container Vessels
Container vessels, on the other hand, are designed for multi-port voyages, with containers often stowed based on discharge ports. This results in transverse asymmetry, especially when containers are stacked more heavily on one side (port or starboard) within a given container bay or row.
Torsional moment arises from this athwartships imbalance, and though bending moment, shearing force, trim, and GM (metacentric height) may be within permissible limits, excessive torsion can lead to:
- Longitudinal hull twisting
- Deformation of hatch coamings and lashing bridges
- Loss of watertight integrity at hatch covers
- Accelerated fatigue stress on the ship’s structure
Role of Modern Loading Computers
Leading loading computer systems such as MACS3, CASP, and SM88 provide real-time calculations for:
- Torsional Moment curve along the vessel's longitudinal axis
- Maximum permissible torsional stress at critical frame locations
- 3D structural integrity monitoring, including cross-sectional torsion effects
These systems are mandatory on most container vessels and are routinely audited during PSC inspections and Classification surveys.
Operational Recommendations for Deck Officers
- Ensure transverse balance in each bay during stowage planning.
- Cross-check the torsion moment diagram before finalizing the cargo plan.
- Communicate clearly with stevedores during loading/discharging to maintain lateral balance.
- Include torsion stress awareness in cargo operation briefings and voyage planning meetings.
- Follow Classification or Flag Administration guidelines regarding maximum allowable torsion moment.
Conclusion: Torsional Moment is Not Optional — It’s Critical
Ignoring torsional moment can lead to structural deformation, cargo damage, or even loss of vessel integrity in severe cases. While bulk carriers may operate safely without accounting for torsion, container ships — with their complex loading patterns and multiple port rotations — cannot afford such an oversight.
As vessel designs evolve and cargo operations grow more dynamic, torsional stress has become a frontline concern for naval architects, ship operators, and deck officers alike. Incorporating torsional moment into every cargo planning process is not just about compliance — it is a commitment to safety, efficiency, and structural longevity.
Haiphong, 24/Jun/2023
BBC – SMR Marine Training department
Capt. Dang Quang Dan
