KNOWLEDGE HUB / OFFSHORE LIFTING

Crane Load Chart

Read the chart. Then check the load basis.

Find the right chart cell, identify the dynamic-factor assumption, and separate gross suspended load from net payload.

By Peter Wang, COO · January 2026 · Reviewed August 2026 · 11 min read

Read a chart cell, step by step.

EXAMPLE A · Illustrative offshore main-hook chart. The marked intersection is 20 m radius and the Hs ≤ 2.5 m band.

Illustrative figures from the published article. Each column embeds a fixed factor: gross allowance = rating × 1.30 / ψ. A certified chart states its own rule set, factors and H_s bands.
Lifting radius	Rating — ψ 1.30 floor	H_s ≤ 2.0 m — ψ ≈ 1.5	H_s ≤ 2.5 m — ψ ≈ 1.7	H_s ≤ 3.0 m — ψ ≈ 1.95
12 m	10.0 t	8.7 t	7.6 t	6.7 t
16 m	8.4 t	7.3 t	6.4 t	5.6 t
20 m	6.9 t	6.0 t	5.3 t	4.6 t
24 m	5.6 t	4.9 t	4.3 t	3.7 t

Find your radius row. Use the working radius at the worst point of the lift path — radius grows as the boom luffs out, and capacity falls with it.
Pick the sea-state column. Take the forecast significant wave height H_s for the operation and round up to the next chart band — between columns, the worse one applies.
Read the intersection. That cell is the gross allowable suspended load for the case, not net cargo. Subtract hook, reeving, rigging, spreader, below-hook equipment and other suspended tare to obtain the net payload allowance.
Check the basis. The step between columns is a fixed dynamic factor ψ embedded in this illustrative chart. If ψ is calculated from total effective suspended mass, solve the factor and that lumped mass together as shown below. The DAF article covers where the factor comes from — and how compensation brings it down.

02 / CHECK THE ASSUMPTION

Does the factor change with the mass?

EXAMPLE B · A separate worked case: a 10 t reference chart cell with a 1.30 minimum factor. These branches use different assumptions; this is not a with/without-compensation comparison.

FIXED EXTERNAL FACTOR

Use the ratio rule.

The factor is supplied independently and does not change with suspended mass.

10 × 1.30 / 1.8

7.2 t gross allowance at fixed ψ = 1.8

Read the fixed-factor basis ↗

MASS-DEPENDENT FACTOR

Solve mass and factor together.

In the velocity formula, the factor rises as the effective suspended mass falls.

m · ψ(m) = 13 t

6.50 t gross allowance at ψ ≈ 2.00

Published worked inputs: v_r = 1.25 m/s and k = 400 kN/m.

Read the coupled calculation ↗

Confirm the approved chart and project basis before applying either branch. These are the article’s worked examples, not certified load-chart values for a project.

03 / ALLOW FOR SUSPENDED TARE

Gross allowance is not net cargo.

Subtract hook, reeving, rigging, spreader, below-hook equipment and other suspended tare from the gross allowance.

GROSS ALLOWANCE− Suspended tare= Net payload allowance

What is driving the peak load?

Use the existing equipment guide to distinguish a single shock event from repeated wave-cycle motion.

Compare the operating cases →

THE ENGINEERING DETAIL

Equations, assumptions and supporting guidance.

The full technical explanation follows, with its original figures, references and worked calculations.

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 the governing rule applies to the total static suspended load at the chart reference when checking against the chart. The two are related but not interchangeable: a DAF is a measured or computed response ratio for a stated load channel, while ψ follows the chart’s own rule set. A factor of 1.5 means the crane and rigging must withstand 50% more load than the stated static suspended-load reference.

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 gross allowable suspended load?

For a conventional crane and rigging system, a dynamic factor can be estimated from relative velocity, stiffness and total effective suspended lumped mass at the chart reference:

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

Here $v_r$ is relative velocity, $g$ is gravitational acceleration, $k$ is effective crane-and-wire stiffness, and $m$ is the total effective suspended lumped mass at the chart reference. Many offshore checks also apply a minimum factor set by the rule and lift category.

Take the worked case used below: $v_r=1.25\,\mathrm{m/s}$ , $k=400\,\mathrm{kN/m}$ , a 10 t chart cell and a 1.30 minimum factor. At an initial total effective suspended lumped mass of 10 t:

\psi(10)=1+\frac{1.25}{9.81}\sqrt{\frac{400\,000}{10\,000}}=1.8059

There are two different calculation branches:

External, mass-independent factor. If ψ = 1.8 is a fixed rule-table or analysis input, the ratio rule gives $10(1.30/1.8)=7.2\,\mathrm{t}$ . The comparison card and Figure 1 below illustrate this fixed-factor branch.
Mass-dependent velocity formula. If ψ came from the equation above, it rises as the total effective suspended mass falls. With m expressed in tonnes, $\psi(m)=1+2.54842/\sqrt{m}$ , and the allowable dynamic load is $10(1.30)=13\,\mathrm{t}$ . Therefore:

m\,\psi(m)=m+2.54842\sqrt{m}=13 \quad \Rightarrow \quad m=6.5019\,\mathrm{t},\quad \psi=1.9994

Rounded for engineering communication, the coupled gross allowable suspended load is 6.50 t at ψ ≈ 2.00 — about 35% below the 10 t chart cell. This is not net cargo: subtract hook, reeving, rigging, spreader, below-hook equipment and other suspended tare. Before applying the simple ratio rule, confirm whether the factor is mass-independent.

Reducing snap loads with shock absorption, or hook-to-payload relative motion with heave compensation, can bring the lift closer to the minimum factor and reduce the derating.

Fixed-factor comparison (mass-independent)

The card and curve below show the separate ratio-rule branch, where ψ is a fixed external value that does not change with payload. They are a simplified comparison, not a certified POLARIS load chart and not the coupled ψ(m) result above.

Example chart capacity10 tat one selected radius

Minimum dynamic factor1.30used as the reference floor

Fixed ψ ≤ 1.3 10.0 t

Fixed ψ = 1.5 8.7 t

Fixed ψ = 1.8 7.2 t

Mass-independent ratio rule: gross allowable suspended load = chart capacity × 1.30 / fixed ψ. 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.

Figure 1 — Mass-independent comparison only: gross allowable suspended load = 10 t × 1.30/ψ once a fixed ψ exceeds the floor. If ψ comes from the velocity formula and depends on m, solve the coupled equation instead; the worked inputs give a gross allowable suspended load of 6.50 t at ψ ≈ 2.00. Simplified example, not a certified load chart.

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.

Which fix applies

Two ways a chart cell gets derated

The dynamic factor ψ is what takes capacity off the chart. Two quite different things produce it, and they take different equipment.

Single shock event

Deck pick-up, snap and overload

One velocity mismatch sets the peak — the load comes off a moving deck, or the wire goes slack and re-tensions. A crane shock absorber takes that mismatch into stroke instead of into the hook load, so the chart cell is sized for the lift rather than for its worst instant. It needs time to reset between events.

Drivers: relative velocity at engagement · rigging stiffness · deck motion

Cap the peak with — a crane shock absorber

See pile-run protection →

Repeated wave-cycle motion

Splash-zone crossing and subsea lowering

Here the hook and the payload cycle against each other for the whole lift, not once. A passive heave compensator keeps the sling in tension through every cycle, so ψ stays near its floor for the duration — and the same crane keeps more of its chart in a rougher sea.

Drivers: crane-tip heave · wave period · water entry

Hold the factor near its floor with — passive heave compensation

See splash-zone crossing →

Most real lifts contain both. The design-basis ψ comes from an analysis of the actual sequence, not from the chart — that is what a CONSTELLATION study produces.

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?

The table of maximum safe working load (SWL) a crane can lift at each radius, boom length and angle. Offshore, the chart value is the start of the check, not the end — dynamic loading must be applied on top.

Why are offshore crane load charts derated?

Because hook, payload, deck and sea surface move relative to each other. Snap loads at lift-off, splash-zone force changes and spring-mass amplification in the crane and wire all raise the peak hook load above the static weight, and the chart must absorb that through a dynamic factor.

What dynamic factor applies to an offshore lift?

The applicable factor or analysis method comes from the approved crane chart and contract-nominated project basis. In this illustrative calculation, m is the total effective suspended lumped mass at the chart reference represented by the selected stiffness; it is not automatically net cargo mass.

How is the allowable suspended load calculated from the load chart?

If ψ is an external, mass-independent value, use gross allowable suspended load = chart capacity × floor / ψ; fixed ψ = 1.8 gives 7.2 t from a 10 t cell with a 1.3 floor. If ψ comes from the velocity formula on this page, it changes with total effective suspended mass and must be solved together with m. The worked inputs give a gross allowable suspended load of 6.50 t at ψ ≈ 2.00. Net cargo is lower after subtracting hook, reeving, rigging, spreader and other suspended tare.

How can load-chart derating be reduced?

A POLARIS crane shock absorber can reduce a discrete impact response within its sized force–stroke envelope; passive heave compensation can reduce repeated hook-to-payload response. Quantify either benefit with matched cases and apply the certified chart and project criteria.

Related resources

Reading the load chart for a compensated lift?

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.

ND SolutionPOLARIS →Crane shock absorber – reduce snap loads and stay closer to chart capacity.

Dynamic Amplification FactorHow DAF turns relative motion into higher hook load.

Crane Shock AbsorptionShock absorbers limit snap loads during deck lifts and overload events.

Heave Compensator SelectionChoose PHC, APHC or AHC for splash-zone and subsea motion cases.

Free download · 8 pages

Offshore Lift Load Cases — ND benchmark figures

What a lift actually measures, with and without passive compensation, from Norwegian Dynamics' own time-domain simulations. It reproduces no standard — the value is the figures, which no standard publishes.

Get the benchmark figures →