Resonance Avoidance

Every spring-mass system has a natural frequency — the frequency at which it tends to oscillate when disturbed. In a passive heave compensator, the mass is the suspended load and the spring is the compressed gas in the accumulator.

When the wave-driven crane tip motion oscillates at or near this natural frequency, the system responds with amplified motion — the load moves more than the crane tip rather than less. This amplification can be dramatic: an undamped system at resonance would theoretically have infinite response.

In practice, damping limits the peak response, but even a well-damped system can see amplification factors of 2–5× at resonance. This is clearly unacceptable for a device designed to reduce motion.

The natural period of a passive heave compensator depends on the load mass and the gas spring stiffness. For typical offshore lifting systems, natural periods fall in the range of 10–30 seconds.

The dominant wave periods in most operational areas range from 5–15 seconds (T_p). This means that for well-designed systems, the natural period is longer than the wave period, and the system operates in the sub-resonance region where compensation is effective.

However, resonance risk increases when:

• The gas volume is too small (making the spring too stiff and reducing the natural period).
• The load is lighter than the design case (reducing the mass and thus the natural period).
• Long-period swell is present (increasing the excitation period towards the natural period).

Engineers use several strategies to ensure heave compensators operate safely away from resonance:

• Sufficient gas volume — Larger gas volumes produce softer springs with longer natural periods, pushing the resonance well above the wave period range.
• Adequate damping — Hydraulic damping limits the amplification at resonance to acceptable levels, providing a safety margin even if conditions push the system closer to its natural frequency.
• Load range analysis — The compensator must be checked across the full range of expected loads, not just the design point. The lightest load typically gives the shortest natural period and therefore the highest resonance risk.
• Adaptive tuning — Systems like Norwegian Dynamics ANTARES automatically adjust their gas spring characteristics to maintain the optimal natural period as load conditions change, inherently avoiding resonance across a wider operating envelope.

Resonance avoidance is not just about the compensator — it applies to the entire lifting system. The crane wire, sheaves, and any subsea rigging all have their own stiffness and mass, creating a coupled system with multiple potential resonance modes.

At greater water depths, the wire’s elasticity becomes significant, and the coupled wire-compensator system can have resonance frequencies different from either component alone. This is why deepwater operations require careful coupled dynamic analysis that models the entire system from vessel to seabed.

For critical operations, time-domain simulations using site-specific wave data and actual vessel RAOs are used to verify that resonance is avoided across all foreseeable conditions. Norwegian Dynamics provides engineering support for these analyses as part of compensator selection and sizing — see our compensator selection guide for more information.