Helmholtz resonance (moonpool)

Helmholtz (piston-mode) resonance in a moonpool is the near-rigid vertical oscillation of the entrapped water column in a vessel’s vertical opening at its natural frequency, analogous to a Helmholtz acoustic resonator. The piston-mode natural frequency is approximately the square root of g over (draft plus an added-length correction), omega = square root of g over (h + delta_h), where h is the column length and delta_h accounts for added mass at the opening; sloshing modes occur at higher frequencies. When the wave encounter frequency nears this value the column heaves with large amplitude, raising drag, green water, and downtime for drillships, spar and offshore-construction vessels. Damping plates, recesses, and shaped openings detune or suppress the response.