STATISTICS OF LOCAL MOTIONS ON A SHIP

The realization of the local motion history of an LNG ship and of its cargo tank structure is a complex process involving two separate developments. First, it is necessary to determine the statistical properties of the local motions history. The difficulty here is that a wave with a certain frequency observed at a fixed point in space is observed at a different frequency at a point which is translating. The frequency which is observed on a ship proceeding into a seaway is called the encounter frequency. It is demonstrated that there are as many as three different waves from one direction which can result in one encounter frequency. This non-unique mapping of the wave frequency into the encounter frequency causes some numerical problems which require careful evaluation. Second, it is necessary to generate realizations of the motion history long enough to obtain meaningful statistics from the nonlinear modes of tank sloshing. The standard techniques used to generate a realization are either computer intensive or inadequate for the case of long records. A new procedure involving a sum of sine terms, each with a randomly varying phase history, is developed. The paper concentrates on these two somewhat disjointed but necessary facets of the local motions problem.