Vessel Motions

A floating vessel can move in six independent ways, known as degrees of freedom (DOF):

• Surge — fore-and-aft translation (along the vessel’s length)
• Sway — side-to-side translation (athwartships)
• Heave — vertical translation (up and down)
• Roll — rotation about the longitudinal axis
• Pitch — rotation about the transverse axis
• Yaw — rotation about the vertical axis

For lifting operations, heave is the most critical DOF because it directly causes vertical motion at the crane tip. However, roll and pitch also contribute to crane tip heave — particularly when the crane is located away from the vessel’s centre of flotation. This coupling means that the effective heave at the crane tip can be significantly larger than the vessel’s pure heave motion.

The relationship between wave excitation and vessel response is described by Response Amplitude Operators (RAOs). An RAO is a transfer function that gives the vessel’s motion amplitude per unit wave amplitude, as a function of wave frequency and heading.

RAOs depend on the vessel’s hull form, displacement, loading condition, and speed. They are typically determined through hydrodynamic analysis or model testing and are unique to each vessel. Naval architects provide RAO data as part of the vessel’s motion characteristics.

To predict crane tip motion in a given sea state, engineers combine the vessel RAOs with the wave spectrum. The resulting motion spectrum gives the statistical distribution of crane tip heave — which is the input for heave compensator design.

Different vessel types have very different motion characteristics:

• Semi-submersibles — Excellent motion characteristics due to small waterplane area. Low heave RAOs. Preferred for heavy lifts and demanding operations.
• Monohull construction vessels — Moderate motion characteristics. Heave, roll, and pitch can be significant. Most common vessel type for subsea installation work.
• Jack-up barges — Zero heave when jacked up, but significant motion during transit and positioning. Limited to shallow water.
• Barges — High motion amplitudes, particularly in roll. Often used with heavy-lift cranes but require calm weather windows.

The vessel’s motion characteristics directly determine the required compensator specification — stroke, capacity, and response speed.

The crane tip heave amplitude and period determine the key design parameters for a heave compensator:

• Stroke — Must exceed the maximum expected crane tip heave amplitude (peak-to-peak) with a safety margin.
• Velocity — The compensator must respond fast enough to follow the heave motion. Maximum piston velocity is determined by the heave amplitude and wave period.
• Natural period — The compensator’s natural period must be tuned to avoid resonance with the dominant wave periods.

For operations where vessel motions are large or unpredictable, an adaptive passive system like Norwegian Dynamics ANTARES provides the flexibility to maintain performance across a range of conditions. For the most demanding cases, Norwegian Dynamics active heave compensator delivers maximum compensation efficiency regardless of vessel motion characteristics.