Offshore Crane Types Compared
Main Types of Offshore Cranes
Offshore cranes fall into several categories based on their boom geometry and luffing mechanism:
- Knuckle boom cranes — the most common type on offshore platforms and supply vessels. The boom has an articulating joint (knuckle) that allows the boom to fold for stowage. Capacities from 5 to 300+ tonnes. Manufacturers include Liebherr, NOV (National Oilwell Varco), and Palfinger.
- Stiff boom (fixed boom) cranes — a straight boom without articulation. Typically higher capacity and longer reach than knuckle boom cranes. Used for heavy lifts and construction operations. Common on semi-submersible crane vessels (SSCVs).
- Ram luffing cranes — the boom angle is controlled by a hydraulic ram rather than a wire rope. Provides precise load positioning and is common on jack-up rigs and drilling platforms. Favell, SMST, and Huisman manufacture these.
- Pedestal cranes — a general category for cranes mounted on a fixed pedestal, which can use any boom type. The pedestal allows full 360° rotation.
How Crane Type Affects Heave Compensation
The crane type influences heave compensation requirements in several ways:
Boom stiffness: A stiffer boom transmits more of the vessel heave to the crane tip, increasing the dynamic amplification factor. Stiff boom cranes on floating vessels generally benefit more from heave compensation than knuckle boom cranes, which have some inherent compliance at the knuckle joint.
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 heave compensator shifts the natural period away from typical wave periods.
Integration: Active heave compensation (AHC) is typically integrated into the crane winch drum. Passive heave compensation (PHC) is installed in the wire rope path between the crane hook and the payload — this makes PHC systems crane-independent and suitable for any crane type.
A RIGEL or ANTARES compensator can be deployed on any crane because it sits below the hook, independent of the crane own systems.
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).
A heave compensator effectively increases the usable crane capacity for a given sea state by reducing the dynamic component of the load.
