Vertical motion analysis

Amarcon vertical motion calculation method

When starting a calculation the user has to select the calculation settings. If the calculation setting is based on the criterion "bottom touch probability" (this means that PROTIDE should use a probabilistic calculation method), then the Amarcon vertical motion calculation method will be used. Actually the application OCTOPUS by Amarcon is used. This application can analyze how a particular ship will move considering given conditions like waves, current, course, ship type, ship dimensions, load and stability parameters. PROTIDE presents a variety of these conditions and asks OCTOPUS how the ship will move. OCTUPUS answers with the 0th to the 4th spectral moments of the ship movements per motion point. PROTIDE uses them to calculate the most probable extreme per motion point and selects the point that is the most critical for later calculations.

Motion scenarios

It is not known which exact conditions a ship will encounter during a channel voyage, so PROTIDE will present OCTOPUS with a set of equally possible scenarios and store all the motion periods, significant vertical motions and sinkages (squat) for a given course, water level and expected swell. Later in the calculation, when a simulation is done over possible conditions, these scenario's will randomly be selected to calculate the bottom touch probability.

Input

1. Ship and load: type, length, width, draft (front and back), dead weight, water displacement, metacentric height, free fluids correction and roll period (all set by the user when requesting a tidal window)
2. Water depths (based on depth / draft ratios 1.1, 1.2 and 1.4)
3. Speed and course (speed is set by the user and course is determined by the channel
4. The expected swell and wave spectra (in steps of 0.1 meter between the minimum and maximum expected swell, possible wave spectra are selected)
5. Wave direction, standard deviation and spread (set by the provider, mostly around NNW)

Calculation method

OCTOPUS is presented with a set of ship and mass parameters. From a list of possible hulls, the one that suits the given parameters best is selected and scaled to match the length, width and depth. With this modeled ship the Response amplitude operators (RAO's) are calculated. Next the responses are calculated for a set of wave spectra. These responses are calculated for all wave directions around the ship (360 degrees in steps (what size?) and a set of speeds). PROTIDE then varies in wave direction, the needed headings to keep course in the channel and speed to get a retrieve possible motion scenario's.

Output

All analyzed motion patterns are stored and labeled with depth / draft ratio, heading and wave height. At the moment a vertical motion analysis is needed during the tidal window calculation, one vertical motion is randomly selected from all the calculated responses with the correct depth / draught ratio, heading and wave height.