Crane Load Chart

By Tord Martinsen, CEO · January 2026

A crane load chart specifies the maximum safe working load (SWL) a crane can lift at various boom lengths and angles. For offshore operations, the standard onshore load chart must be derated to account for dynamic loads caused by wave-induced vessel motion.

The dynamic load factor (ψ) represents how much additional force the payload experiences due to the relative motion between the crane hook and the payload. A dynamic factor of 1.0 means no additional dynamic load, while 1.5 means the load is 50% higher than its static weight.

Why Dynamic Load Factors Matter

During offshore lifting operations, waves cause the vessel to heave. When a payload is suspended from the crane, it experiences forces from:

Gravity — the static weight of the payload
Vessel heave — the crane hook moves up and down with the vessel
Payload inertia — the payload resists changes in motion
Snap loads — sudden tensioning of the wire when slack is taken up

The dynamic load factor captures these effects in a single number applied to the crane’s 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.

Maintaining Full Crane Capacity with Shock Absorbers

The key insight from crane load chart analysis: dynamic factors reduce your effective lifting capacity, sometimes dramatically. A crane rated for 10t SWL might only safely lift 5-6t in rough seas.

Shock absorbers solve this problem by limiting the dynamic factor to 1.3 or less, regardless of sea conditions. This means:

Full crane capacity maintained in higher sea states
Wider operational weather windows — fewer weather delays
Safer operations — controlled forces, no snap loads
Cost savings — smaller cranes can handle the same payloads

Norwegian Dynamics’ POLARIS crane shock absorber is specifically designed for this application. It achieves >90% efficiency in a single-cylinder design that is lighter, smaller, and cheaper than traditional multi-cylinder solutions.

Standards and Classification

Crane load charts for offshore operations are governed by DNV ST-0378, DNV RP-N202, API 2C, and EN 13852. Norwegian Dynamics products are designed and classed according to DNV ST-0378.