Where chart capacity goes when the sea moves — and how to keep the peak load near the floor.
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.
Worked example: from velocities to allowed payload
The two formulas above chain into one derating calculation. Illustrative deck lift: hoisting at vL = 0.5 m/s, crane-tip vertical velocity vc = 0.6 m/s, supply-vessel deck velocity vd = 0.8 m/s, payload 10 t, effective crane-and-wire stiffness 400 kN/m (illustrative), chart cell 10 t, minimum factor 1.30.
- Relative velocity. vr = ½ vL + √(vc² + vd²) = 0.25 + √(0.36 + 0.64) = 1.25 m/s
- Dynamic factor. √(k/m) = √(400 000 / 10 000) ≈ 6.3 s−1, so ψ = 1 + (1.25 / 9.81) × 6.3 ≈ 1.8
- Allowed payload. ψ = 1.8 exceeds the 1.30 floor, so the chart cell derates: 10 × 1.30 / 1.8 = 7.2 t — a 28% capacity loss from relative motion alone.
Cut the relative velocity — a shock absorber for the single snap at pick-up, heave compensation for repeated wave-cycle motion — and ψ falls back toward the floor, recovering the same chart cell toward its full 10 t. Values are illustrative; the design factor for a real lift comes from analysis.
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 POLARIS crane 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 a single velocity mismatch — deck pick-up, snap, overload — is absorbed before it becomes peak hook load. It needs time to reset between events; where slack–snap cycles repeat through the wave zone, heave compensation is the right tool.
- 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. In practice it is reserved for the narrow band of cases where the performance justifies the cost and complexity; for most load-chart cases an adaptive-passive compensator such as ANTARES closes much of the gap without external power.
- 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.
Crane load charts — frequently asked
What is a crane load chart?
Why are offshore crane load charts derated?
What dynamic factor should I use?
How is allowed payload calculated?
How do I get capacity back?
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.