Knowledge / Shipyards and production

Heavy lifting and temporary supports in shipyards: understanding the load path

Examine centre of gravity, sling angle, structural load paths and temporary supports in shipyard lifting operations.

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A ship block may be strong in its completed vessel arrangement yet vulnerable while being lifted, turned or temporarily supported. The loads enter through different points, some permanent connections do not yet exist, and the surrounding structure may provide little restraint. Heavy-lift engineering therefore begins by defining the temporary condition. Crane capacity, sling capacity, lifting-point strength and block stability are connected questions, but none is a substitute for the others. This guide explains the reasoning and does not authorise a lift or provide rigging instructions.

Define the complete operation

A lift is a sequence: preparation, attachment, taking load, leaving supports, movement, possible rotation, landing and releasing the lifting gear. Each stage can have a different governing condition. The load may clear an obstruction at one point and approach it at another; the crane radius may change; the receiving supports may engage unevenly. Evaluate the complete path and the foreseeable abnormal conditions, not just a photograph of the intended suspended position.

HSE's lifting-planning guidance emphasises risk-based planning, appropriate resources and supervision in its UK regulatory context. Its value here is the planning principle, not a claim that UK law governs every yard. The actual operation requires competent planning, approved arrangements, suitable equipment and applicable local requirements. Naming a lift “routine” does not remove the need to understand its specific load and surroundings.

Establish weight, centre of gravity and uncertainty

The lifted weight includes the object and everything attached to it, with the allocation of rigging weight handled correctly in the crane assessment. Installed outfit, residual liquids, temporary steel and forgotten loose items can change both weight and centre of gravity. A design-model total may not represent the as-built block. Identify the source of each value and how uncertain or incomplete information is treated in the engineering assessment.

Centre of gravity affects load sharing and orientation. A small shift can matter when lifting points are closely spaced or when the arrangement is sensitive to imbalance. Do not assume equal sharing merely because two hooks or four sling legs are present. Geometry, stiffness, tolerances and the actual rigging arrangement influence which elements carry load. A drawing should state its reference axes and the meaning of every angle used in calculation.

An original two-support example

Consider a fictional rigid beam carrying a downward load W = 100 kN. Two vertical supports are 4 m apart, and the centre of gravity is 1.5 m from the left support. Assume static equilibrium, no other loads and ideal point supports. Taking moments about the left support gives right reaction R_right = 100 × 1.5/4 = 37.5 kN. The left reaction is 62.5 kN.

The reactions are unequal even though there are two supports. If the centre of gravity moves to the midpoint, the ideal reactions become 50 kN each. This elementary model demonstrates the effect of load position; it does not model a flexible block, a multi-leg sling, a crane or dynamic motion. It supplies no equipment selection or allowable load. A real lifting arrangement needs its own structural and rigging analysis.

Uncertainty should be tested in the direction that matters. If the teaching centre of gravity could lie between 1.3 and 1.7 m from the left support, the right reaction ranges from 32.5 to 42.5 kN in this model. The corresponding left reaction ranges from 67.5 to 57.5 kN. A single nominal answer would hide that distribution. The range is an assumed bound, not a statistically established confidence interval.

Sling angle changes force components

For another deliberately idealised example, suppose two identical sling legs symmetrically support a 100 kN load, each at angle θ from vertical. Static vertical equilibrium gives 2T cos θ = W. At 30° from vertical, each leg tension is approximately 57.7 kN. At 60° from vertical it is 100 kN. The angle reference must be stated: 60° from vertical is 30° from horizontal.

This calculation is not a sling-rating table. It excludes unequal load sharing, bends, hitch effects, connection geometry, dynamics and equipment-specific restrictions. The horizontal force components also have to be carried through the lifted structure or lifting arrangement. OSHA's safe-sling-use guidance directs attention to sling type, configuration and rated capacity. Applicable manufacturer information and competent assessment govern selection; arithmetic alone does not establish suitability.

Trace the force beyond the lifting eye

A lifting point is part of a larger load path. The eye or lug transfers load into welds, attached plating, stiffeners and the wider block. Local bending, out-of-plane loading, buckling and incomplete structural connections can govern before the gross tensile strength of a component is reached. The intended load direction matters. A point designed for one orientation cannot be assumed adequate after the block is rotated.

Temporary reinforcing steel may be needed for a particular engineered operation, but adding it is itself a design and fabrication task. Its connections, installation, inspection and eventual removal require control. A lifting point's inspection record should correspond to the actual item and configuration. Similar appearance to a previously used point is not proof of equal capacity, particularly after repairs, corrosion, deformation or a change in surrounding structure.

Crane capacity depends on configuration

A crane's permissible load depends on the applicable load chart and actual configuration, including radius and other specified conditions. The site also needs to support the crane's reactions and maintain suitable clearances. A maximum capacity printed on the crane is not the permissible load at every radius. For more than one crane, interaction and load sharing require specific planning rather than dividing total weight by the number of machines.

Environmental and dynamic effects must be included in the approved assessment. Wind on a large block can affect control even when its weight is well within the nominal lifting capacity. Starting, stopping, swinging or unintended snagging can change forces. This guide does not propose a universal dynamic factor or wind limit. Such values depend on the operation, equipment instructions and relevant engineering requirements.

Temporary support is engineered work

The landing arrangement has to carry the load safely before the crane is released, and remain suitable during subsequent work. Support height, contact location, stiffness, bracing and the capacity of the underlying ground or structure all matter. A block can be stable under vertical self-weight yet vulnerable to an added horizontal force, an eccentric work load or the removal of a connecting member.

HSE's temporary-works guidance treats temporary support as an engineered solution requiring appropriate design and coordination. Its construction-sector context should not be confused with a ship-specific standard. The transferable lesson is that short duration does not mean low consequence. Temporary arrangements should have a defined design basis, installation check, permitted use and controlled release or alteration process.

Coordinate people, routes and communications

An engineering calculation cannot prevent someone entering a dangerous position unless the work arrangement makes the exclusion and communication rules effective. Identify the controlled area, movement route, signal responsibilities and the means of stopping the operation. Blind areas, noise, simultaneous work and changing visibility can undermine a plan that appears simple on paper. The receiving team also needs to understand when the load is supported and when gear may be released.

OSHA 29 CFR 1915.116 addresses use of gear in US shipyard employment, including hazardous positioning around moving loads. It is an example of a jurisdiction-specific requirement, not a complete global procedure. Actual work must use the yard's authorised system and competent supervision. Nobody should rely on this article to decide where to stand or how to control a suspended load.

Manage changes and preserve evidence

Stop and review when the operation no longer matches its approved assumptions. A changed attachment point, additional outfit, unexpected tilt, damaged gear or different landing support can invalidate earlier calculations. The appropriate response is not to improvise until the load looks level. Define beforehand who can assess changes, who can authorise resumption and what information they need.

A useful record links the as-built load, weight and centre-of-gravity basis, lifting arrangement, equipment configuration, structural checks, inspections, environmental limits and responsibilities. It also records deviations and the final supported condition. Common mistakes are equal-share assumptions, angle-reference confusion, using overall crane capacity at the wrong radius and treating temporary steel as unimportant. The central lesson is to follow the force all the way from the lifted object to the ground, through every stage of the operation.

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