Offshore Crane Types Compared
Separate boom geometry, luffing mechanism and mounting.
Three descriptors that can coexist.
Knuckle boom, ram luffing and pedestal mounting answer different questions about a crane.
Boom geometry
- Knuckle boom
- An articulating joint lets the boom fold.
- Stiff / straight boom
- A boom without an articulating knuckle joint.
Luffing mechanism
How the boom angle is changed.
- Ram luffing
- A hydraulic cylinder changes the boom angle.
- Wire-rope luffing
- A rope system changes the boom angle.
Mounting
- Pedestal-mounted
- Describes the support installation. It does not specify the boom geometry or luffing mechanism.
Check the actual mounting and slew limits in the equipment specification.
These descriptions can be combined. For example, Liebherr’s RL-K range combines ram luffing with a knuckle boom ↗.
One crane can carry several labels.
Knuckle boom
The articulating joint describes the boom geometry.
Ram luffing
The hydraulic ram describes how the main boom angle changes.
Pedestal mounting
The support arrangement is a separate descriptor.
Use the selected crane configuration, load chart, geometry and coupled lift model for the operation.
| Term | Describes | Check in the selected specification |
|---|---|---|
| Knuckle boom | Boom geometry | Articulation, working envelope and stowage arrangement |
| Stiff / straight boom | Boom geometry | Reach, configuration and the applicable load chart |
| Ram luffing | Luffing mechanism | Hydraulic luffing arrangement and operating limits |
| Pedestal-mounted | Mounting | Support, installation and slew limits |
How Crane Type Affects Heave Compensation
The crane type influences heave compensation requirements in several ways:
Boom stiffness: the crane structure is one spring in a chain that also contains the wire, rigging and payload — hook motion and the resulting amplification are properties of that coupled system, not of the boom type alone.
Wire rope length: Longer wire rope runs act as springs, changing the natural frequency of the lifting system. Resonance between wave period and the system natural period must be avoided — a compensator shifts the natural period, and the new period is verified against the project sea states.
Integration: the arrangement depends on the crane and compensation system. A below-hook passive unit can be independent of the crane controls, but the hook geometry, headroom, load range and certified lift basis still need checking. Active compensation may be crane-integrated or supplied in an inline system; confirm the proposed configuration and development status.
A RIGEL or ANTARES compensator sits below the hook, independent of the crane’s own control systems — deployment on a given crane is still checked against hook geometry, headroom, load range and the certified lift basis.
Crane Load Charts and Dynamic Loads
Every offshore crane has a load chart that specifies maximum safe working load (SWL) as a function of boom angle and radius. The load chart assumes static conditions — actual dynamic loads during a lift can exceed the chart values due to vessel motions.
DNV and NORSOK standards require that dynamic loads be accounted for in lift planning. The total hook load must remain below the crane SWL at the planned radius, including the dynamic amplification factor, rigging weight, and any subsea forces (drag and added mass).
Offshore crane types — frequently asked
What do offshore crane type labels describe?
What is the difference between knuckle boom and ram luffing?
How does crane type affect heave compensation?
Can a below-hook compensator be used with my crane?
Do crane load charts include dynamic loads?
Related on Norwegian Dynamics
- Crane Load Chart — Velocity formula, DAF derating, worked example.
- DNV Standards for Offshore Lifting — ST-0378, ST-N001, RP-N103 and how they apply to compensators.
- Heave Compensator Selection — Choose between PHC, APHC and AHC for your envelope.
Matching compensation to your crane type?
Below-hook compensation is one option. Share the crane configuration and load case so the interfaces, motion response and project scope can be assessed.
