A fluid impulse nonlinear theory of ship motions and sea loads

Abstract

A new nonlinear seakeeping theory is presented for the prediction of the large amplitude motions and wave induced loads on ships in severe seastates. The hydrodynamic forces on the ship are evaluated using a new fluid impulse theory which expresses the nonlinear Froude–Krylov, radiation and diffraction forces as time derivatives of integrals of velocity potentials over the instantaneous ship wetted surface circumventing the time consuming evaluation of the pressure from Bernoulli's equation. Analogous expressions are derived for the structural loads. The free surface problems for the ship wave disturbance are solved on vertical planes fixed relative to an earth fixed frame using a 2D + t slender body theory. The linear free surface condition is enforced on the ambient wave profile assumed locally horizontal, a nonlinear boundary condition is imposed on the ship hull and the resulting boundary value problem is solved using the two-dimensional time-domain wave source potential. Computations are presented illustrating the performance of the new theory. The application of the theory to the evaluation of the extreme statistics of the ship responses and structural loads in a stochastic seastate is also addressed.