Seakeeping (A008)

Seakeeping is the analysis method to help evaluate the seaworthiness and operability of a design in a seaway.

The program allows the user to look at wave induced ship motion. The approach is statistical using the frequency domain:

• The program will compute the unit wave induced ship response for 5 degrees of freedom DOF (Roll, Yaw, Pitch, Heave, and Sway). The ship unit wave response is different across the frequency range and forms a 'ship' characteristic known as the response amplitude operator (RAO). This may also be evaluated by physical measurement of model experiments.
  
  
• A 2D 'strip' approach is used in conjunction with a new multi-parameter conformal mapping technique to increase the accuracy with which the two-dimensional properties (TDP’s) are calculated. (Added Mass and Damping).
  
  
• Applying a statistical irregular sea (wave 'energy' spectrum) to obtain a 'statistical' ship motion response. The wave energy spectrum is a spectrum of the component wave heights at each frequency. Formulations for such spectra are available which relate to a characteristic wave height and periodicity. Sea state definitions can be defined by the specification of these variables.
  
  

The ship characteristic spectrum and the wave energy spectrum are combined to determine the 'response' spectrum. Summating the area and moments of area of the combined frequency domain 'response' spectrum across the range of frequencies will yield the 'statistical' ship response (RMS) for whatever DOF.

The statistical ship responses can be plotted or compared against threshold design requirement 'limiting' values. Such limits (known as criteria) are used to evaluate what wave height (sea state), ship heading (relative to wave direction) and ship speeds the vessel will be within the defined threshold limit. Many such limiting parameters can be assessed (either individually or in combination) including values based on estimated ship motion acceleration, which affects human behaviour, known as human factors. This type of assessment is sometimes called an operability assessment (inferring limits to the unimpeded operation of the ship due to the effects of wave induced ship motion).

Different ships may be compared by the above approach to see which one is less 'limited' by threshold values or design changes (to VCG, displacement etc) can be investigated to see if the effect on ship responses of such changes.

Simulated time domain ship motion responses for the 5 degrees of freedom can be computed for given ship speed, wave headings, and wave spectra. These have been used for real-time simulation.