Crane Load Chart
By Tord Martinsen, CEO · January 2026
Practical application: For practical application of this topic, see POLARIS crane shock absorber and engineering studies and analysis.
A crane load chart specifies the maximum safe working load (SWL) a crane can lift at different radii, boom lengths and angles. Offshore, the available chart capacity must be checked against dynamic loading because the hook, payload, deck and sea surface can all move relative to each other.
The dynamic load factor (ψ), often discussed together with DAF, is the multiplier applied to the static payload or crane chart capacity. A factor of 1.5 means the crane and rigging must withstand 50% more load than the static weight alone.
Where the extra load comes from
The largest capacity reductions usually come from relative velocity and sudden tension changes:
- Lift-off from a supply vessel or barge – the crane hook and the cargo deck can move in opposite directions. If the hook rises while the deck or container drops, the sling can go from slack to fully loaded almost instantly. That snap load can exceed the static payload by a wide margin.
- Splash-zone crossings – buoyancy, drag, added mass and wave particle velocity change quickly as the payload passes through the free surface. The result is a varying hook load and a higher dynamic factor.
- Crane and wire elasticity – the crane, wire, slings and payload behave like a spring-mass system, so fast motion or poor timing can amplify peak tension.
- Lift-off, landing and snagging – short events can dominate the maximum hook load even when the average sea state looks acceptable.
The dynamic load factor captures these effects in a single number applied to the crane load chart.
How to calculate relative velocity?
For deck lifts, classification rules commonly estimate relative velocity 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 crane lifting velocity, v_c is the crane-tip vertical velocity from vessel motion, and v_d is the deck or payload vertical velocity from wave motion.
We estimate v_c and v_d from vessel response data, measured motion, metocean data or time-domain simulation. When data is limited, conservative rule-based values can be used.
How to calculate dynamic factor and allowed payload?
For a conventional crane and rigging system, a dynamic factor can be estimated from relative velocity, stiffness and payload mass:
\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 effective crane and wire stiffness, and m is the payload mass.
\psi is then used to derate the crane chart. Many offshore checks also apply a minimum dynamic factor, commonly 1.3 depending on rule set and lift category, so a calculated value below the minimum does not increase the chart capacity.
As an example, assume a crane has 10 t chart capacity for a deck lift and the applicable minimum dynamic factor is 1.3. What is the allowed overboard payload if the calculated dynamic factor is 1.2 or 1.8?
For 1.2, the minimum factor still controls, so the allowed payload remains 10 t. For 1.8, the allowed payload becomes:
m = 10 \cdot \frac{1.3}{1.8} = 7.2\ \text{t}
By reducing snap loads with shock absorption, or hook-to-payload relative motion with heave compensation, the lift can often be brought closer to the minimum dynamic factor instead of forcing a large chart derating.
Illustrative load-chart effect
The example below shows the same crane chart capacity checked with different dynamic factors. It is a simplified calculation example, not a certified POLARIS load chart.
Calculation: allowed payload = chart capacity x 1.30 / dynamic factor. If a shock absorber keeps the peak load near the minimum factor, the same chart cell can remain much closer to full capacity.
How to reduce load-chart derating
The practical question is not only what the dynamic factor is, but what causes it. Different load cases need different equipment.
- Snap loads and deck pick-up: use a POLARIS crane shock absorber. The absorber adds controlled stroke and damping between crane and payload, so sudden velocity mismatch is absorbed before it becomes peak hook load.
- Splash-zone crossings and subsea lifts: use passive heave compensation to reduce hook-to-payload relative motion. RIGEL and CYGNUS cover simpler passive cases; ANTARES is used for complicated or multi-step subsea lifts with changing buoyancy.
- Topside active heave compensation: use active heave compensation where residual motion must be minimized. VEGA is the usual starting point for topside motion compensation, with subsea active compensation considered only where the performance justifies the cost and complexity.
- Operational controls: use controlled hoisting speed, plan soft lift-off, avoid re-contact, avoid resonant sea states and use suitable weather windows. Equipment reduces the peak load, but the lift procedure still sets the starting conditions.
For a first-pass product check, use the heave compensator selection guide. For a full review, send the crane radius, SWL, lift speed, payload, sea state, wave period and lift sequence.
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 where applicable.
Related resources
Working on a lift that needs this?
If load-chart derating is limiting the lift, send the crane and load case. We can separate snap-load, transfer-lift and splash-zone cases and suggest the practical next step.