Knowledge / Maritime cargo

Cargo securing at sea: motions, load paths and the securing plan

An engineering guide to cargo securing through forces, geometry, contact conditions and inspection.

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Cargo securing is an engineering problem involving much more than counting chains. Cargo mass and centre of gravity, its location on the vessel, motion-induced accelerations, contact conditions and the strength of every attachment act together. The objective is a continuous load path from the cargo into the ship structure, with evidence that this path remains effective throughout the voyage. This article explains general principles. It does not replace the approved Cargo Securing Manual, a cargo-specific engineering assessment or competent operational judgement.

Establish the governing documents

For relevant cargoes within SOLAS scope, stowage and securing follow the ship's Administration-approved Cargo Securing Manual. Applicability and exemptions depend on the vessel, cargo and voyage. The UK Maritime and Coastguard Agency's explanation of SOLAS chapter VI is an official national example of this relationship, rather than a statement that every UK implementation detail applies to every flag.

The IMO CSS Code provides an international framework for safe stowage and securing. Recommendations can acquire binding effect through national requirements or approved ship documentation. The IMO/ILO/UNECE CTU Code, addressing packing within cargo transport units, is non-mandatory in itself. Securing a container to a vessel and securing a machine inside that container are separate tasks with different load paths.

Translate motion into force

The basic effect of linear acceleration is mass multiplied by acceleration. Cargo on a ship, however, does not experience just one constant acceleration in one direction. Roll and pitch create effects that depend on distance from the vessel's motion reference points. Moving an unchanged machine higher or farther along the ship can change the securing demand. Inclination also introduces a component of gravity parallel to the supporting deck.

Vertical motion changes the normal force available to generate friction. A friction resistance observed on a dry quay therefore cannot be treated as a permanent contribution throughout a voyage. Water, oil, ice, paint condition, crushed timber and surface damage introduce further uncertainty. An assessment must identify its acceleration combination and friction assumptions. Values from unrelated calculation methods should not be combined merely because each looks conservative in isolation.

Geometry and the weakest connection

A chain's entire tensile capacity does not resist horizontal movement. Its inclination divides tension into vertical and horizontal components; its direction in plan then determines the useful transverse or longitudinal component. A steep lashing can develop substantial tension while providing little horizontal restraint. Lashings on opposite sides of a cargo item may also not contribute equally against movement in one particular direction.

The cargo lug, shackle, chain, tensioner, deck fitting, weld and supporting structure belong to the same force-transfer system. A certificate for one component does not establish the capacity of that system. Breaking strength, safe working load and maximum securing load are different terms and must not be interchanged. Equal force sharing between lashings of different stiffness and length also requires justification: a short, stiff member can attract high load before a longer member becomes effective.

Stops and chocks can transmit forces through compression, but their contact areas and foundations must resist the resulting pressure. A strong steel stop placed against a weak pallet does not create a strong restraint. Packing compression or cargo settlement can reduce pretension. Consequently, a securing assessment must consider the mechanical behaviour of packaging and support materials, as well as the metal hardware visible in a photograph.

Direct restraint and frictional restraint differ

In direct securing, a member transfers force opposing cargo movement through its attachment points. In a top-over arrangement, pretension may instead increase contact force to increase friction. The same chain or strap can serve different mechanical purposes, so the calculation must distinguish them. A contribution based on pretension falls if packaging compresses or the tensioning device loosens. Direct restraint also needs suitable geometry and attachments to provide the expected resistance.

Increasing pretension indefinitely is not a solution. The cargo casing, upper edges or supports may not withstand the resulting local pressure. Sensitive machinery can suffer internal damage while its exterior still looks sound. Sharp edges can require protection, but the protective material's resistance to sliding, crushing and cutting must be considered. A high component rating is not reliable engineering evidence when the force path remains unclear.

If departure and later inspections differ, simply recording a loose strap is insufficient. Consider whether the cargo moved, supports compressed or attachment points deformed. The same visible symptom can have different causes. Records of repeated events can inform improvements to support materials or inspection access for the next shipment, with suitable technical reassessment of the change.

An illustrative calculation and its limits

Consider a wholly hypothetical machine of mass 12,000 kg. Assume a horizontal acceleration of 0.40 g and, only for this simplified calculation, a normal force equal to m g. With g = 9.81 m/s², the horizontal inertial force is approximately 47.1 kN. An assumed friction coefficient of 0.20 provides approximately 23.5 kN, leaving approximately 23.5 kN to be resisted by other means. These are teaching assumptions, not recommended design values for any voyage.

If an ideal tension member acts at 45 degrees to the horizontal in the effective plane, balancing that residual force requires approximately 33.3 kN of tension. This does not specify an acceptable lashing or number of lashings. The example excludes pretension effects on normal force, vertical acceleration, motion combinations, load sharing, safety factors, movement before engagement and attachment capacities. It demonstrates only why geometry changes the required tension.

If the same machine has its centre of gravity 1.8 m above its base, the horizontal force also creates an overturning moment. Assessing this requires the base width, the possible pivot edge and the moment arms of restraints. A layout adequate against sliding can still be inadequate for a tall, narrow item. Sliding and tipping must therefore be treated as distinct limit states; satisfying one does not automatically satisfy the other.

Make the plan executable

A useful securing plan identifies the cargo and its orientation, centre of gravity, bearing areas, attachment points, hardware and acceptance criteria. The installer should not have to infer where each component belongs. An arrangement that requires reaching through an inaccessible gap or tensioning gear without safe working space is not practically complete. Loading sequence matters because adjacent cargo can obstruct attachment points that were accessible earlier.

Photographs provide useful evidence of identity, arrangement and visible condition, but do not replace material certificates, capacity verification or examination of concealed welds. Inspection records should connect the installed arrangement to identifiable equipment. Damaged, heavily worn or unmarked components cannot simply be credited with the capacity of apparently similar sound equipment. Where a condition is uncertain, its effect on the accepted arrangement must be resolved before relying on it.

Field modifications require the same discipline. Moving an attachment, removing a chock or rotating the cargo can alter the geometry assumed in the calculation. Changes should return to the person responsible for acceptance and to the current version of the plan. This prevents the next watch from confusing an earlier drawing with the arrangement actually installed on board.

Environmental loads and enforceable limits

Deck cargo may require assessment of wind and shipped-water effects in addition to forces derived from vessel accelerations. A light object with a large enclosed surface behaves differently from a compact, dense machine. Covers, temporary protection and protrusions can also attract loads. Their detachment can endanger people or neighbouring cargo even if the main item stays in place. Voyage suitability therefore concerns more than the principal cargo mass.

Voyage or weather restrictions assumed in an assessment must be operationally achievable. If avoiding certain wave conditions is part of the basis, there must be a realistic way to monitor conditions, make decisions and implement alternatives. Writing a restriction in a report does not itself control the hazard. A complete plan identifies who checks the restriction, using what information and at which decision point.

Inspect without creating another hazard

Settlement, temperature changes and vibration can alter the securing condition. Inspection intervals follow vessel documentation and operational circumstances. Sending personnel between moving cargo items in severe weather is not a defensible way to verify safety. Observation, access and intervention need safe conditions. Unusual noise, visible movement or a loose restraint should prompt investigation of the cause, rather than an automatic instruction to apply more tension.

Common errors include crediting every lashing in the same direction, assuming optimistic friction, overlooking the cargo's own attachment strength, and judging deck capacity solely from total tonnes. Adding more chains does not repair an incorrect load path. Changes of speed or course may reduce motions, but they do not retrospectively validate a deficient securing assessment. Reliable securing combines documented cargo data, ship-specific engineering, correct installation and safe monitoring.

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