Resonance can greatly amplify the motions and forces involved in lifting.
Where does it happen
– Hoisting on floating cranes.
– Cranes moving with the load hoisted.
– Lifting with fast crane movements.
Because it happens
– The crane motion period approaches the natural period of the hoisting system, which is usually a single or double pendulum.
– An aggravating factor is that this system has very low damping, which allows absorption of greater energy.
Consequences
– Increased load speed.
– Increased forces involved.
– Loss of control, load collision.
Example for floating crane
– A large ship may have a natural play or roll period of 12 to 17 seconds for a wave falling on the side of the hull.
– The hoisting with a crane using a cable with a length between 25 and 40 meters has a natural period of 10 to 14 seconds.
– If the period of the wave (Tp) is in the range of 12 to 17 s, it will amplify the movement of the ship and, consequently, greatly amplify the movement of the cargo, even if the wave has a low height.
– If the period of the wave at the time of the operation is 8 seconds, there will be little amplification, as there is no resonance.
How to mitigate
– Appropriate determination of environmental window for lifting (range of threshold values for wave height, period, wind speed, etc.)
– Assessment of environmental conditions at the time of operation
– Skilled and experienced operator, who manages to compensate the balance of the load at the beginning (middle…) of the operation, avoiding progression to resonance.
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