Methods Used and Principal Functions Provided

From a numerical definition of the ship's underwater hull form, expressed as offsets at a number of transverse sections, the draught and the trim, the seakeeping module computes the hydrodynamic coefficients (added mass and damping for heave, sway, roll, and sway-roll) for each section over a range of wave frequencies. Two hydrodynamic theories are available in this module, the Ursell-Tasai multiple potential theory, References 41, 42, 43 and 44 and the Frank close-fit theory, References 36. In the first, the ship section is represented by a two-parameter Lewis section and in the second by a number of source singularities. The theory most appropriate to a particular hull form can be selected.

The regular responses module interpolates the sectional hydrodynamic coefficients at the appropriate wave encounter frequencies (which depend on ship speed and heading) so that the total ship hydrodynamic characteristics can be obtained by summation, References 45.. Using the ship's speed, weight distribution parameters (LCG, KG and radii of gyration) and appendage data, the simultaneous linear equations for ship motions are then solved to give the Transfer functions, consisting of the motion amplitudes and phases with respect to a wave crest amidships.

Non-linear roll damping is introduced in the roll equation in the form of a quasi-linear term which is added to the linear coefficient. Since the computation of this term requires the value of roll amplitude, the equation for lateral motions have to be solved by an iterative procedure. The method of Ikeda, References 38. is used to calculate roll damping contributions due to appendage, friction, eddy and lift effects. These results represent the motions of the ship in long-crested sinusoidal waves for a range of wave frequencies and ship headings.

Both of the added resistance and dynamic loads modules use previously calculated sectional hydrodynamic coefficients and regular responses. The first module computes added resistance coefficients over a range of wave frequencies and ship headings, References 40.. The second module computes regular shear forces, bending moments and torsional moments at any location along the ship.

The ship's behavior in regular waves - regular responses, added resistance and dynamic loads - are calculated from the regular response data.

Using sea-state data the irregular responses module combines the regular ship motions, loads and added resistance coefficients with the Wave Spectrum and Spreading function to produce ship motions, total motions at specified locations, loads and mean added resistance in a short-term irregular sea state. The relative motions and relative velocities computed by this module are also used to evaluate the incidence of slamming, deck wetness and propeller emergence.

The long-term performance assessment module combines series of short-term responses with wave statistics and seakeeping criteria to evaluate the long-term seakeeping performance of the vessel, References 39..