Offshore Crane Load Charts

Q: How is the allowable load calculated from the dynamic factor?

With a fixed factor, gross allowance = rating × ψ of the rating / ψ of the sea state; in this page’s example 10 t × 1.30 / 1.8 = 7.2 t. If the factor comes from the crane’s formula and depends on the mass, solve mass and factor together: the same 10 t rating then allows 6.50 t at ψ = 2.00.

Q: What is the difference between the dynamic factor and DAF?

The dynamic factor ψ is the crane rule’s factor that sizes the crane and its chart. The dynamic amplification factor (DAF) is the ratio of peak to static load for one particular lift, found by analysing that lift.

How to read a chart cell, where the sea-state derating comes from, why the dynamic factor can depend on the load, and how much capacity heave compensation and shock absorption can win back.

By Peter Wang, COO · January 2026 · Updated September 2026 · 10 min read

Blender illustration of a subsea module lifted off a barge alongside a vessel by the vessel's crane.

In brief

A crane load chart gives the safe working load (SWL) at each radius. An offshore chart adds columns for significant wave height ( $H_s$ ), each derated by a dynamic factor $\psi$ .
Read the cell at the worst radius of the lift and the first $H_s$ band at or above the forecast. The cell is the gross allowance: hook, rigging and other suspended tare come out of it.
Under DNV-ST-0378, an offshore crane’s $\psi$ is at least 1.3 for working loads up to 2,500 kN. It grows with the relative velocity between hook and load, and with the crane’s stiffness.
If $\psi$ is calculated from the suspended mass, it rises as the load falls, so load and factor must be solved together. Derating a 10 t rating ( $\psi = 1.30$ ) for a relative velocity of 1.25 m/s gives 6.50 t, not the 7.2 t a fixed factor gives.

A crane load chart specifies the maximum safe working load a crane can lift at each radius, boom length and angle. Offshore, the hook, the load, the deck and the sea surface all move relative to each other, so the chart is derated for the sea state: each wave-height column scales the rating by the rating’s dynamic factor over the larger factor for that sea state. Reading the right cell is the first check on any offshore lift.

How to read a chart cell

Example A is an illustrative offshore main-hook chart. Each wave-height column applies a fixed factor to the rating, here with $\psi_{\text{rating}} = 1.30$ :

\text{gross allowance} = \text{rating} \times \frac{\psi_{\text{rating}}}{\psi_{\text{sea state}}}

Radius	Ratingψ 1.30	H_s ≤ 2.0 mψ ≈ 1.5	H_s ≤ 2.5 mψ ≈ 1.7	H_s ≤ 3.0 mψ ≈ 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

Illustrative chart. A certified chart states its own rule set, factors and wave-height bands.

Find the 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 for the operation and round up to the first band at or above it. Between columns, the worse one applies.
Read the intersection. At 20 m and $H_s \le 2.5\ \text{m}$ the cell is 5.3 t. That is the gross allowable suspended load, not net cargo.
Check the basis. A certified chart’s $H_s$ cells are the approved allowances: use them as printed. If you derive an allowance yourself, from a rating and a $\psi$ calculated from the suspended mass, solve mass and factor together (below).

Where the derating comes from

The largest capacity reductions come from relative velocity and sudden changes in tension:

Lift-off from a moving deck. If the hook rises while the supply-vessel deck drops, the slings go from slack to fully loaded almost instantly, and the snap load can exceed the static load by a wide margin.
The splash zone. Buoyancy, drag, added mass and wave particle velocity change quickly as the load passes through the surface.
Crane and wire elasticity. The crane, wire, slings and load behave as a spring-mass system, so fast motion or poor timing amplifies the peak.
Short events. Lift-off, landing and snagging can set the maximum hook load even when the average sea state looks acceptable.

The chart’s dynamic factor $\psi$ captures these in one number for the crane. It is related to, but not the same as, the dynamic amplification factor (DAF) of a particular lift, which comes from analysing that lift. See dynamic amplification factor.

How the dynamic factor is calculated

For a lift from a supply vessel, the relative velocity between hook and load combines the hoisting speed with the vertical velocities of the crane tip and the deck:

Figure 1. Lift-off from a supply vessel. The hook rises at the hoisting speed while the crane tip and the deck move with the waves; the crane and wire act as a spring of stiffness

k

on the suspended mass

m

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

Here $v_L$ is the hoisting speed, $v_c$ the crane-tip velocity from the vessel’s motion and $v_d$ the deck velocity from the waves. They come from vessel response data, measured motion or time-domain simulation, or from conservative rule values when data is limited. The dynamic factor then follows from the relative velocity, the stiffness $k$ of the crane and wire, and the total suspended mass $m$ :

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

This is DNV-ST-0378’s $\psi = 1 + V_R\sqrt{C/(W g)}$ [4.1.3.2], rewritten with $W = m g$ and $C = k$ ; $v_c$ and $v_d$ are DNV’s $V_t$ and $V_{\text{in}}$ . The standard sets a floor: at least 1.3 for offshore cranes with working loads up to 2,500 kN. The details, including the minimum hoisting speed, are in DNV-ST-0378.

Fixed or mass-dependent ψ: two answers

Take the 12 m rating of 10 t at $\psi = 1.30$ , a relative velocity of 1.25 m/s and a stiffness of 400 kN/m. At 10 t the formula gives:

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

Example B shows what the rating then allows, depending on whether $\psi$ is fixed:

If ψ is	Calculation	Gross allowance
A fixed input (a rule table or an analysis result)	$10 \times 1.30 / 1.8$	7.2 t at $\psi = 1.8$
The crane’s own formula, $\psi(m)$	Solve $m\,\psi(m) = 13\ \text{t}$ , with $\psi(m) = 1 + 2.54842/\sqrt{m}$	6.50 t at $\psi \approx 2.00$

With the formula, a lighter load gets a higher factor, so the answer has to satisfy both at once:

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

That is about 35 % below the 10 t rating. Before using the simple ratio rule, confirm that the factor really is independent of the mass.

Figure 2. The solid curve is the rating’s budget:

\text{allowance} \times \psi = 13\ \text{t}

(10 t × 1.30), capped at 10 t. Fixed factors of 1.5 and 1.8 sit on it at 8.7 t and 7.2 t. The dashed curve is the crane’s own

\psi(m)

for this case (

v_r = 1.25\ \text{m/s}

k = 400\ \text{kN/m}

), plotted against the suspended mass: 1.81 at 10 t, rising as the load falls. It meets the budget at 6.50 t and

\psi = 2.00

. Worked example, not a certified chart.

Gross allowance is not net cargo

Every figure above is the gross allowable suspended load. The net payload allowance is what remains after the suspended tare:

\text{net payload} = \text{gross allowance} - \text{below-hook tare}

The tare is everything below the hook that is not cargo: the hook block, reeving, rigging, a spreader and any other suspended equipment. A compensator or shock absorber in the load path is part of that tare, so its weight comes off the allowance too.

Getting capacity back

Dynamic load is what takes capacity off the chart. Two things drive the peak, and a third case needs active control:

What sets the peak	What helps	How
One event: deck pick-up, snap or overload	POLARIS crane shock absorber	Takes the velocity mismatch into stroke and damping instead of into the hook load. It needs time to reset between events.
Repeated wave motion: splash zone, subsea lifts	Passive heave compensation: RIGEL or CYGNUS. Adaptive passive: ANTARES, with automatic damping control, where the load changes during the lift, such as air to water or a staged landing	Isolates the load from the crane tip’s heave, so the relative velocity at the load, and with it the lift’s DAF, stays low through every wave.
A residual-motion limit passive units can’t meet	Active heave compensation, such as VEGA (in development)	Measures the motion and drives the load against it; kept for the cases where that performance is needed.

Operational controls still set the starting conditions: controlled hoisting speed, a soft lift-off, no re-contact, avoiding resonant sea states and choosing the weather window. A compensated or shock-absorbed lift does not change the crane’s certified chart; it lowers the dynamic load the lift actually sees.

How much that can be worth, with this page’s numbers: the 10 t rating carries a budget of 13 t (10 t × 1.30). Uncompensated, the worked case allows 6.50 t. If the lift analysis shows that a compensator holds the peak to 1.3 times the static load, 13 t / 1.3 covers the full 10 t. The crane maker or class, and the marine warranty surveyor, must accept that basis. For a shock absorber, DNV-ST-0378 credits a hydraulic damper’s effect only if specially agreed, documented by calculations and tests that a surveyor witnesses [5.2.9.10]. For a first product check, use the selection guide.

Standards behind the chart

An offshore crane’s load chart follows the rules it is certified to: DNV-ST-0378 for DNV-certified offshore and platform cranes, EN 13852-1 in Europe, or API 2C for offshore pedestal cranes. A heave compensator in the load path is designed to DNV-ST-0378 and validated to DNV-RP-N202, and the lift as a whole falls under DNV-ST-N001, with DNV-RP-N103 for the dynamic analysis; see DNV standards for offshore lifting.

Frequently asked questions

What is a crane load chart?

A table of the maximum safe working load a crane can lift at each radius, boom length and angle. An offshore crane’s chart also has columns for significant wave height, each derated by a dynamic factor for the motion at sea.

How do you read an offshore crane load chart?

Find the row for the worst radius of the lift, take the column for the forecast significant wave height rounded up to the first band at or above it, and read the cell. That is the gross allowable suspended load: subtract the hook, rigging and other suspended equipment to get the net payload.

What dynamic factor applies to an offshore crane?

Under DNV-ST-0378 an offshore crane’s dynamic factor is at least 1.3 for working loads up to 2,500 kN. The actual value comes from the relative velocity between hook and load, the crane’s stiffness and the suspended mass; in the illustrative chart on this page it is about 1.5 at Hs 2.0 m, 1.7 at 2.5 m and 1.95 at 3.0 m.

How is the allowable load calculated from the dynamic factor?

With a fixed factor, gross allowance = rating $\times\,\psi_{\text{rating}}/\psi_{\text{sea state}}$ ; in this page’s example 10 t × 1.30 / 1.8 = 7.2 t. If the factor comes from the crane’s formula and depends on the mass, solve mass and factor together: the same 10 t rating then allows 6.50 t at $\psi = 2.00$ .

Does heave compensation increase crane capacity?

It does not change the crane’s certified chart. It lowers the dynamic load the lift actually sees by cutting the relative motion between hook and load. In this page’s worked case, if the lift analysis shows a peak of 1.3 times the static load, the 10 t rating’s 13 t budget covers the full 10 t, against 6.50 t uncompensated. The crane maker or class and the marine warranty surveyor must accept that basis.

What is the difference between the dynamic factor and DAF?

The dynamic factor $\psi$ is the crane rule’s factor that sizes the crane and its chart. The dynamic amplification factor (DAF) is the ratio of peak to static load for one particular lift, found by analysing that lift.

Basis and sources

DNV-ST-0378, edition July 2019 amended October 2021: the dynamic factor [4.1.3], the offshore-crane minimum values and relative velocity [8.2.2], load charts per wave height [8.2.4], and hydraulic dampers [5.2.9.10].
Examples A and B and the worked case are illustrative, with the stated inputs; the numbers are rounded as shown and recompute exactly from the formulas on this page.
EN 13852-1 Cranes, offshore cranes, general-purpose offshore cranes; API Spec 2C Offshore pedestal-mounted cranes.

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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.

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