Resonance Avoidance

The concept of resonance has got increased focus recently as weight and water depths are increasing. The problem may occur when the wave period of the surface waves, and thus the frequency of the heave, generate boom tip movements which correspond to the system frequency of the load and wire. The heave may then create resonance movements in the subsea load and wire with destructive energy.

Operational Challenges

  • When the wave period of the surface waves, and thus the frequency of the heave, generate boom tip movements which correspond to the system frequency of the load and wire.

  • The heave creates resonance movements in the subsea load and wire with destructive energy.

  • Crossing of structures with large variation between weight in water and air.

Cranemaster Benefits

  • Uncontrolled dynamic forces and escalating movements of load during deep water operations avoided.

  • With Cranemaster® included in the rigging, system resonance period will be increased.

Subsea resonance with and without Cranemaster

This Orcaflex simulation shows how resonance for deep water lowering can be avoided by use of Cranemaster.

System frequency

The system frequency is primarily dependent on the properties of the load and on the overall wire stiffness. Since the wire stiffness changes with length, and thus with water depth, the system frequency will also change with water depth. At deep water operation it is therefore high probability that at some point during the lowering operation the system frequency and wave period interfere to create resonance which may cause uncontrolled dynamic forces on the load and wire.

This example shows the system frequency/period (red line) for an object of 400 t (added mass 1000 t) lowered from the surface down to 2400 m with a typical setting on crane and wire stiffness. The average stiffness of the Cranemaster® is 200 kN/m. The system frequency without Cranemaster® scan periods that with high probability will match the wave induced movements during the lowering, and resonance with escalating movements and forces to load and crane may occur. When Cranemaster® is included in the rigging, the system frequency will be shifted upwards (blue line), avoiding the match with the wave induced frequencies.

Featured References – Resonance Avoidance

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