A new paper entitled ‘Guidance on Stability of Lifts’ outlines key strategies for ensuring safe and stable heavy lift operations when moving loads onto, off or between ships in port or at sea.
After years of collaboration, a working group initiated by the DNV-hosted Heavy Lift Exchange Forum has issued Guidance on Stability of Lifts – a detailed operational guideline for stable and safe heavy lift operations, aimed at reducing the risk of loss of life, property and the environment during these hazardous undertakings.
The paper was developed with expertise from BBC Chartering, BigLift Shipping, DNV, Heerema Marine Contractors, Jumbo Maritime, SAL Engineering and United Heavy Lift.
The paper covers several methods for lift calculations, presenting them in a comparative table for ease of reference and providing detailed examples and explanations.
These five methods are the Virtual CoG (centre of gravity) concept; the Kaps method, which takes the CoG method further and is widely used in the maritime industry; the Nikitin method, which defines an envelope for the CoG in which static and tip-over stability gives a stable equilibrium; and standard and extensive versions of numerical computer simulation methods, which require specialist software.
Some methods may be more applicable than others, depending on the lift or the stage of the planning process. For example, Virtual CoG and Kaps allow users to evaluate the initial stability of the lift, Nikitin provides insight on stability against the overturning of two-chain suspension arrangements, while numerical computer simulations allow a much more detailed analysis that incorporates simulation of the effects of forces such as wind or the motion of a crane.
“The numerical simulations are useful to understand the behaviour of a lift at certain phases of a lifting operation,” said Sebastian Becker, team lead CAD design, SAL Engineering.
Before getting into the fundamentals of these methods, though, the paper presents a flow chart for assessing the lifting stability of a rigging arrangement. All of the calculations and considerations that the paper goes on to discuss – such as sliding and effective inclination angles, or disturbing factors like wind force, rigging length tolerance, steering line forces, crane movement, CoG shift, vessel motions and the degree of friction at the lifting points, if applicable – feed into this flow chart, which prompts the user to review the draft rigging arrangement at various points before finalising the plan.
Naturally, the chart is not exhaustive, but it is a useful starting point for designing safe rigging arrangements.
The paper is unique among the many guidance documents available for operations in the maritime sphere, said Karsten Behrens, managing director of SAL Engineering. “It enables readers to assess the stability of particular lifting arrangements by using the provided methods and data. This puts them in the position to achieve a ‘stable lift’, which is one that remains in a balanced condition, within a safe margin, when subjected to predefined disturbing factors,” he said.
BigLift added: “This guideline fills a critical gap by providing clear, step-by-step procedures and assessments, particularly for unique cargo shapes that previously lacked specific guidance.”
