A two-dimensional numerical and experimental study of resonant coupled ship and piston-mode motion

We consider resonant coupled ship and piston-mode motion in a two-dimensional setting. The setting is that of a ship section moored by a bottom mounted terminal which are exposed to regular waves. The ship section is allowed to oscillate in three degrees of freedom: Sway, heave and roll. We investigate in particular the damping caused by flow separation from the bilge keels. This has importance near resonance. The investigation is carried out by means of model tests as well as by a linear wavetank and a fully nonlinear numerical wavetank. Both are time-domain wavetanks based on a Boundary Element Method (BEM) and the mixed Eulerian–Lagrangian (MEL) formalism. Flow separation is modelled in the nonlinear numerical wavetank by means of an inviscid vortex tracking method. We focus on the, under resonant condition, large-amplitude massive piston-like fluid motion in between the ship and the terminal and the associated large ship motions. We do not focus on the sloshing problem associated with higher modes. In resonant condition, large piston-mode motion occurs when the sway and heave motions are downwards and towards the terminal simultaneously.An alternative formulation of the forces and moment is derived and implemented into the nonlinear numerical wavetank as a means to circumvent the problems related to the φtφt term in the Bernoulli equation in the presently adopted MEL approach.