Ship motions memo no. 2. Notes on the correlation of ship motion characteristics with weather and sea states

HIP MOTIONS MO NO,. 2

NOTES ON THE CORRELATION OP SHIP MOTION CHARACTERISTICS

WITH WEiTh AND SEA STATE

A Introduction

The first memo in this series outlines certain

elemen-taz'y techniques for sunmarizing the information contained in graphical ship motion records. This menio will show how

such motion information can be correlated with itself and

with weather and sea state information. Again the

discus-sion will be concerned with simple, qualitative techiliques, since, by and large, graphical reco±ds do not lend, them-selves to highly detailed analysis,

In the previous notes, the individual record was taken to be the basic unit of reference. It was shown that the

characteristics of a given motion record can be defined

in terms of a rather small number of parameters, as swning

that one is willing to forgo a complete power spectrusa

analysis. Furthermore, the weather and. sea state conditions prevailing when the record is taken can also be defined by

hale-dozen or so parameters.

The various parameters of notion, weather, etc., taken: together define a Space, and. each record is then a point in this parameter space. Eècause of the inherent variability

n the data these. points are scattered rather than being.

smoothly, arranged on analytic surf aces. To the -extent that

the pointstend to cluster near lines, surfaces, etc., we

say that the data are corrélated,.by the laws defining such surfaces. Our object here will be to first define a simple

.araeer space and then cosideth

clerflg

points within this pace,

BssenttaPFametez!3 of Zhjn Ot1OU and. Seatbe.

A reasonably complete definition of the record and its ambient conditions should nC1iie enough parameters

to eorwey the following infoi'mati.on: ship moflon as shown

in the record; wind condittou; sea state ani ähip

iüfor-iaation such as .coürs, speed, and loading. Date and.

loca-tion could be added to the list, but this informaloca-tion is

-auxiliary as far as the corz'elat ion of motion and weather is concerned.

1otion Paraeta's: In our simplified representation (see. Memo Noo 1), the most important parameters of the motion

are; = RMS instanteneous amplitude;. = median period.1; and ri = N0/N1. hei'e are several other parameters that give more form to the power spectxuin, but the above. three

will suffice for the purposes of this discusion.

VJind Parameters: The parameters most essential for charao-tertziüg wind conditions are wind direction and. wind speed.

Duration and fetch also play an important role in deter-mining the sea state resulting from a given wind velocity. However, these latter parameters wi]. not .usi1y be known to the ship-bome observer.

Sea State Parameters: In a reasonably coherent seaway, three parameters will usually suffice to define the sea

state: prevailing direction of the sea;

revaiiing wave-length; and wave height or sea state nujaber. Vihere confused

or superposed sea states occur it may be necessaiy to dis-tinguish between sea and swell, etc.9

but this tends to

get messy and will usually represent an undue refinement.

Ship Parameters: The most important ship parameters are course and. speed. In certain cases loading,

et al,

may

be of interest, There is

someo.veriap between these para-meters and. the wind and sea stat' parameters. As far as. ship motions are concerned, what is 'of primary interest

i the relative motion between ship. and wind or ship and sea. Hence ship course can be combined with wind direction

to provide a single relative -direction parameter, and ditto

for ship course and sea direction. In many cases, ship

speed may be so nearly constant as

to not be a

parameter

at all.

Suai: Foi' the purposes of this discuss ion, we will

confine ourselves to a inimurn number of parameters.

Specifically, consider a space defined by the following;

For motion

-(5 = RMS instantaneous amplitude

= median period1

= = zero crossings of amplitude/ditto of

derivative

For wind

V = wind speed

wind, direction, relative from bow For sea

S = sea state number

= sea direction, relative from bow L = dominant wve length

-4-Ship course is taken care of by the use, of relative angles;

ship speed Is assumed to.be constant. This gives a space. of eight parameters, which is by no ieans prohibitively

complex.

C. ypical Scatter Diagrams.

Each record occupies a more or less well defined. point

in the above parameter space. The collection of all such

points summarizes our biowledge about the given ship motion and. corresponding weather, - Because of the inherent

varia-bility in the data these points will be some'vthat scattered.. We can study the correlations between the various parameters

by constructing various sections of parameter space, or scatter diagrams.

Intemal Correlations of iot1on Data:

-The motion data canbe correlated against itself by

plotting a scatter diagram of 6 versus T1 and. ri versus T1. These plots should give a pretty good indication of the natural frequency of the vessel.

Motion Versus Wind:

Péxiaps the most useful plot here i 6 versus wind direction, o( , with wind, speed, V, as a paraeter..P1 and

n 'versus the same parameters would be of somewhat less

interest,

Motion Versus Sea:

Here a plot of 6 versus sea direction with sea state as a parameter ihould. give useful information. It would.

also be worb1.vIliIe to include wave3ength as a parazieter

in several plots, ain, P1 and n could a3so be plotted versus ibe saae parameters.

ese scatez ataraas

e also have the leug.tei

ainpit-ide atd period d Thtbuti<n 1nthe preceding memo

3. H. cIADWiOK/eti