Recently the automated vessel requires the system which could evaluate the navigation safety of a ship in the present position and the future position estimated from weather information as well as recommend the optimum course and speed for the ship's safety. From the viewpoint of safe operation, it is primarily important to estimate the seakeeping performance of a ship in a seaway. Among various factors for evaluating it, ship motion is the most fundamental one. According to recent studies, deck wetness, propeller racing, slamming, rolling, vertical and lateral acceleration are the factors to evaluate seakeeping performance.
The systemic combination of those evaluated factors has the form of serial combination. A theory of reliability engineering is applied to the theoretical method which makes it possible to evaluate the seakeeping performance. The occurrence probability by Rayleigh's joint probability density function of the factors has been adopted for evaluating seakeeping performance. If the probability of occurrence of just one factor exceeds the critical occurrence probability, then the overall seakeeping performance fails and the ship may be endangered.
The Seakeeping Performance Index (SPI) is defined by measuring only any one factor, i.e. vertical acceleration, through the medium of relative dangerousness of the other factors. If one factor exceeds each extreme value or some factors come close to the extreme value, SPI would exceed '1' which is calculated from the occurrence probability. Namely, it means that the ship will face dangerous circumstance.
In this study, we compute the response amplitude of ship motions with the weather data provided in real time, and then obtain the involuntary speed loss with added resistance caused by wave and ship motion. Finally, numerical calculation is executed by stochastic process of the seakeeping performance elements.
After calculation above process, the results have been achieved to develope the system which can evaluate the synthetic seakeeping performance of a ship using the measured data of hull vertical acceleration.
The system is composed of the apparatus for measuring the vertical acceleration generated by the ship's motions, computer for calculating the synthetic seakeeping performance index and monitor for displaying the evaluating diagram of navigational safety of ship.
In order to apply this system in actual operation ship, computer simulation is conducted, and the results are compared with the products based on real ship operation. A Lpp 93m passenger ship, which is equipped with a single screw and diesel engine, is used to test this system on the seagoing vessel.
Therefore the results of this study for developing the navigation evaluation system with hull acceleration could be summarized as follows.
In order to evaluate the ship navigation safety in rough seaway, the onboard navigation safety evaluation system has been developed. This apparatus is used for measuring the vertical acceleration generated by ship's motion. This system is able to synthetically evaluate the navigational safety of a ship with one element of the data of vertical acceleration which is measured by accelerometer regardless of vessel's type or loading condition.
The results obtained by this evaluation system have appeared on a monitor for these operator's convenience using real time weather data. These results will be useful for the ship operators who are concerned about the optimum course and speed for safety of ships.
The results of this study are useful for application to design an automated vessel and develop the next generation voyage data recorder of a ship.