Dynamic Amplification Factor (DAF)
What is Dynamic Amplification Factor?
The Dynamic Amplification Factor (DAF) is the ratio of the maximum dynamic load to the static load during a lifting operation. A DAF of 1.0 means no dynamic effect — the crane sees only the static weight of the payload. A DAF of 2.0 means the peak dynamic load is twice the static weight.
In offshore lifting, waves cause the crane tip to move up and down while the payload hangs below. This relative motion creates dynamic forces in the lifting system — tension spikes when the crane tip accelerates upward, and slack when it drops. The DAF captures how much worse these dynamic forces are compared to a calm-water lift.
Typical DAF values for offshore lifts without compensation range from 1.2 to 2.5, depending on sea state, crane stiffness, and sling length. With a heave compensator installed, the DAF can be reduced to 1.05–1.3, which means smaller cranes can lift the same payload or the same crane can operate in rougher seas.
How to Calculate DAF
The simplest DAF estimate comes from DNV-OS-H205 (now DNV-ST-N001), which gives:
DAF = 1 + aheave / g
where aheave is the maximum vertical acceleration at the crane tip and g is gravitational acceleration (9.81 m/s²). For a sinusoidal heave motion with amplitude ζ and period T:
aheave = ζ × (2π/T)²
For example, with 1.5 m heave amplitude and 8 s period: aheave = 1.5 × (2π/8)² = 0.93 m/s², giving DAF = 1 + 0.93/9.81 = 1.09. This is the crane-tip DAF only — the hook DAF is higher because the sling and payload form a spring-mass system that amplifies the crane tip motion, especially near resonance.
More accurate DAF calculations require modelling the full dynamic system: crane boom stiffness, wire rope elasticity, sheave friction, sling arrangement, and payload hydrodynamic properties. Tools like OrcaFlex are commonly used for this.
DAF and Heave Compensator Selection
The primary engineering benefit of a heave compensator is DAF reduction. By decoupling the payload from the crane tip motion, the compensator absorbs the dynamic forces that would otherwise be transmitted through the lifting system.
A well-tuned passive heave compensator typically reduces DAF by 60–90%, meaning a lift that would see DAF 2.0 without compensation drops to DAF 1.1–1.4 with compensation. This has direct cost implications:
- Smaller crane capacity needed for the same payload
- Wider weather windows — lift in Hs 2.5 m instead of Hs 1.0 m
- Reduced risk of sling overload and dropped objects
- Lower dynamic loads on subsea structures during landing
For heavy subsea lifts where DAF control is critical, an adaptive passive compensator like ANTARES offers the best balance of DAF reduction and operational simplicity.
