Crane Load Chart

How to calculate relative velocity?

According to classification society rules the relative velocity is typically given as:

v_r =\frac{1}{2}v_L + \sqrt{v_c^2+v_d^2}

Where $v_r$ is the relative velocity, $v_L$ is the cranes lifting velocity, $v_c$ is the vertical velocity of the crane due to wave motion and $v_d$ is the deck velocity due to wave motion.

We can help estimating $v_c$ and $v_d$ using simulation tools or conservative approaches can be used.

How to calculate dynamic factor and allowed payload?

Normal cranes have the dynamic factor calculated as:

\psi =1 +  \frac{v_r}{g} \sqrt{\frac{k}{m}}

Where $v_r$ is the relative velocity, $g$ is acceleration of gravity, $k$ is the crane stiffness (which varies with crane angle etc.) and $m$ is the payload.

$\psi$ is typically used to derate crane lifting capacity during offshore lifts, however it is normally never allowed to operate with a value less than 1.3.

As an example let us say we have a crane with SWL 10t lift capacity for deck lifts and has a dynamic design factor of 1.3. What will the overboard lifting capacity be if the calculated dynamic factor is 1.2 and 1.8?

In the first case it will be 10t as it doesn’t matter if the dynamic factor is below 1.3. For the second case the allowed lifting capacity will be:
$m = 10 \cdot \frac{1.3}{1.8} = 7.2\ \text{t}$

By using shock absorbers it is possible to maintain full capacity as the dynamic factor can usually be kept to 1.3 or less.