Subsea resonance avoidance
RESONANCE: INCREASED INDUSTRY FOCUS
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.
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.
EXAMPLE OF RESONANCE
The figure below shows the system frequency, or period, (red line) for an object of 400 Te (added mass 1000 Te) lowered from the surface down to 2400m with a typical setting on crane and wire stiffness. The average stiffness of the Cranemaster is 200 kN/m.
As can be seen 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 a Cranemaster is included in the rigging the system frequency will be shifted upwards (blue line), and thereby avoiding the match with the wave induced frequencies.