GREEN SEA LOADS

National Maritime Research Institute, Japan

Yoshitaka OGAWA

Based on the experimental results in regular waves in a certain ship speed condition, it is found that

inflow of shipping water is approximately proportion to the square of the exceeded height of relative

water height at stem. Based on this relation, green sea load F can be approximated to the value that is

proportion to the square of exceeded height with the following expression as (Reference [1])

                                              (1)

where  is the coefficient, is a density of water, g is the acceleration of the gravity, B is a breadth of

ship,  is the amplitude of relative water height, f is the bow height at stem and  is exceeded

height of relative water height at stem. For example, as to 4knot ship speed condition of general cargo

ship, the coefficient  on No.1 hatch and No.2 hatch are approximated as 0.158 and 0.076 respectively

by the results of experiment in regular waves in same ship speed. As to 2knot ship speed condition of

general cargo ship, the coefficient  on No.1 hatch and No.2 hatch are approximated as 0.131 and

0.049 respectively. Though the coefficient may be changed by the ship type, ship speed and so forth,

the coefficient could be estimated by the use of estimation method of green sea load developed by the

authors (Reference [2]) without experiments.

The probability density function of green sea load is expressed with the following equation by the

probability density function of relative water height at stem height  as

                                                 (2)

When it is assumed that the spectrum of relative water height is narrow band, probability density

function of relative water height  is given by Rayleigh distribution as

                                               (3)

where standard deviation of relative water height. In this study,  is derived from measured one.

By combining Eq.1, Eq.2 and Eq.3, the probability density function of shipping water load is deriveed as

                               (4).

The exceedance of probability of green sea load P(F>F₀) can be expressed by the integral of Eq.4 as

                               (5).

Comparison between measured and estimated probability of exceedance of green sea load are shown

in Fig.1 and Fig.2 respectively. Values of them were converted to those of the real-ship scale. It is

found that the estimated results agree well with the experimental results. It is verified that the present

method is useful for the estimation of the probability of exceedance (the probability density function) of

green sea load accurately.

Fig.1 Probability of exceedance of green sea loads (General cargo ship, 4knot)

Fig.2 Probability of exceedance of green sea loads (General cargo ship, 2knot)

[1] Assessment of domestic rule on load lines from the viewpoint of deck wetness

Forth Osaka Colloquium on Seakeeping Performance of Ships, 2000

[2] Practical prediction methods for the shipping water load on the deck and its structure

Forth Osaka Colloquium on Seakeeping Performance of Ships, 2000

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