Roll computer

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For prèsentati'n at the 16th Meeting of the American Towing

Tank Conference, Sao Paulo, Brazil, 9 August 1971.

-Lab.

_{y. Scheepsbouwbnd}

Hogeschool

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t) e. r_{111 i} J ROLL COMPUTER by Joseph E. Russ

Naval Ship Research and Development Center

29 MEl 1980

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CONTENTS INTRODUCTION

PHASE A. FEASIBILITY STUDY OF TANK MOMENT MEASUREMENT _USING PRESSURE GAUGES

PHASE B. HULL HYDRODYNAMIC CHARACTERIZATION PHASE C. EVALUATION OF ROLL-COMPUTING DEVICE FUTURE WORK

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ROLL COMPUTER

I. Introduction

The United States Navy is interested in developing _a

ship-board device which computes the unstabilized roll of a ship,

given its tank-stabilized roll motion, hull hydrodynamic

char-acteristics, and the moment generated by the stabilizing _passive

antiroll tank. The idea is to compare in _{some manner the}

com-puted, unstabilized roll to the measured, stabilized _{roll in}

order to evaluate the effectiveness of, and perhaps _{tune, the}

passive tank. Cargo safety considerations and time* may

pre-clude making the comparison by simply running the _{tank dry and}

then partially filled.

Some early developmental requirements imposed on the device were that it

1) _{be simple enough to be operated by members of} _{the ship's}

crew .

offer the user the option of determining under _what

conditions the tank would effect minimum roll, _i.e..

full, empty, or partially, filled. _{The device can be} used to determine an optimum tank condition. for existing

environmental conditions, in addition to a simpler

eval-uation of the tank. Time and cargo safety considerations

may limit the device's use to evaluation of a relatively fixed tank condition.

provide a comparison of instantaneous values of

sta-bilized and estimated unstasta-bilized roll, and also a

comparison of various specifie _{time averages of these}

quantities. _{Tentative displays include dual beata}

oscilloscope traces of computed and stabilized roll,

and digital displys of one or .two weighted _averages

of these quantities.

*The time to bring fluid to operating ievels in some tanks

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predict unstabilized roll to within a maximum of ±20 per cent of its actual value.

5) be easily adaptable to many different operating

conditions of the same ship, on the basis of simple dockside measurements made of the metacentric height, natural roll period, displacement, and underway

measurement of speed.

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Phase A. Fèasibility Study ofTank Moment Measurement Using Pressure Gauges

Besides hull hydrodynamic parameters and the stabilized

roll measuremént, the tank moment acting _{on the ship is an}

additional qUantIty necessary to compute unstabilized roil

thotions, using an assumed linear model of ship dynamics. _As

an inItial step in the development of the roll computer, a

feasibility _{tudy was made tö determine if the required tank}

mothent could be well approxiniated on the basis of a practical

number of pressure measurements made at selected points in

the tank.

The shIp for which the device is to be initially

devel-oped is the ARIS-3 _{(Advanced Range Instrumentation Ship)}

Maritime

Administration

_{Type C4-S-A3 troop ship.} _{The ship is}

selected because a model of convenient size already exits.

The associated passive roll tank is shown in Figure 1.

The tank tnödel was mounted on a device equipped to

oscil-late the tank in a roll mode about a fixed axis. _{Three pres}

sure gauge configuratidns of those judged most likely to enable accurate computation of tank moment were selected for test

purposes. These gauge configurations are shown s insets of Figure 2. The test objectives were o establish which, if any, of the arrangements (6 wäs arbitrarily chosen as the macinu

numbér of gauges toibe used in conjunction with the roll

corn-puter) provided pressure measurements enabling _{the total tank}

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I M - M1

t dt

Io

Tldt

i.e. _{a percentage error computed as the ratio of the average}

value of the difference magnitude

- MT?I to the average

value of the true moment magnitude

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within IO per cent of its true value according _{to a specified}

error criterion, over the frequency range of interest for this particular ship.

Additional objectives of the test were to determine the

necessity of gauges on the tank eiìd walls, and the relative importance of the varioùs gauge locations.

Tests were conducted for each of the three _{gauge config-} L.

urations for sinusoidal ñotions of frequencies rànging _from

.1 to .7 Hz, for roll amplitudes of 1, _{3, and 5 degrees, the}

maximum angle possible on the oscillator. _{For each run,}

var-iables required for computation of

_the

_{actual tank moment were}

measured and processed on-site by a small analog computer to

yield the time history of the actual tank moment. A similar

arrangement was used to cotfipute the approximate tank moment from pressure measurements made within the tank.

Boundaries of areas enclosing the gauges _{were chösen}

such that each gauge lay at the centroid of its associated

area. The moment contribution of each gauge is then the. measured pressure multiplied by the area and an appropriate moment arm. The difference between the. tank moment _NT

_and

its approximation M _{was also corputed by the analog computer}

for use in a performance index for each configuration. _The

criterion chosen to evaluate the appröximate moment relative to the true moment was ....

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Results. Results of the study ara presented in Figure 2. This fiure.indicates that, of the three arrangements tested, the one designated "B" best meets the imposed error criterion of 10 per cent, for the amplitudes and frequencies at which the tests were run.

III. Phase B. Hull Hydrodvnamic Characterization

In order to provide reliable input data for the feas

i-bility study, ARIS-3 hull hydrodynamic characteristics in the nature of a roll/tank moment transfer function were deter-mined by frequency response techniques, rather than the simpler and probably less accurate dockside technique envisioned for

use with the prototype roil computer. This roll/tank moment

transfer function is the heart of the computing device, and is essentially a calm-water characteristic independent, within linear assumptions, of the particular seaway in which the

ship is operating.

A roll axis passing through the CG and fixed in space,

was used for the tests. Frequency response tests were

con-ducted by oscillating the model, at speed, about this roll axis, using a push rod driven by an electric motor with

con-tinuously variable speed. The resulting roll records

_and

the record of push rod force provide frequency response amp-litude and phase information relating the roll to the applied

momen t.

Figure 3 presents an experimentally derived roll/moment frequency response obtained for the ARIS-3 model at a speed of 2.27 knots. Superposed on the experimental data are mag-nitude and phase characteristics obtained by a digital

com-puter algorithm designed to fit the experimental data in a

mean square sense. The transfer function associated with

these frequency response characteristics is used in synthe-sizing the roll computer.

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IV. PhaseC. Eva1uatIon of Roli-Coimuting Device

An expérimental study was next made to determine whether theroll computer met the specified performance requirements. A mathematical model of the dev'ie was programmed on an analog computer which was used iñ conjunction with model, tests of

the ARIS-3 in regular and irregular waves. _{RUns were first}

made with the uns tabil.ized iodel operatIng' in bow and quarter-ing waves over the significant frequency range of its roll. This provided roll/wave slope transfer functions upon which were superposed ±20 per cent lines to define a band within

which estimates of the roll computer would be deemed acceptable. Runs were then made in beam and bow quartering seas with the stabilizing tank operating, and tank moment and stabilized roll were introduced as input to the ánalôg simulation of the roll computer. Its output, the unstabilized roll 'estimate,

was then sed to determine the unstabilized roll/wave slope

transfer function basedon the compute.d roll.

Figure 4 shows a plot òf ±20 per cent bounds of the "trué" unstabilized roll t.ransfe.r function and points of the estimated transfer function based on the computed roll in

a beam se.ao ' The data is preliminary. añd is a result of

cur-sory on-site analysis of strip chart records. The figure shows'

that the estimates are close to being within the ±2Q per cent specification for frequencies on each side of resonance., but that in the immediate neighborhood of resonance the estimated motion is decidedly smaller than the actual, motion and falls

within error bounds of ±30 per cent. It is hoped that a more.

carefulanalysis of the:data., including a study of the effect of hull damping factor in the sensitive resonant area, will improve the acòuracy of the computer over the entire frequency

range. Data for the. quartering sea case, run t determine coupling effects on thecomputerTs performance, is of similar

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Future Work

Future work will first be directed toward a thorough analysis of the data obtained from the seaway experiment, and if necessary an improvement tqill, be attempted to resolve

the inaccuracies just mentionedo This will be followed by

a brief study of various averaging devices whose output is mt-ended to be displayed on the roll computer.

A prototype roll computer will be built, of integrated circuits and evaluated in full scale trials, in the latter part of 1971.

Preliminary Conclusions

The roll computing device, based on a linear model,

.g'ive surprisingly good estimates 'if uns tbilized roll,

except in the area of resonance. It is hoped this

discrep-ancy will be resolved by accurate data analysis.

For ARIS-3, the device appears valid for waveslopes up

to 0l8 at resonant frequency, corresponding to a full scale

sinusoidal wave amplitude of about 8 feet.

The assumption of a fixed roll axis appears to be jus-tified for this application.

J.E. Russ

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PHASE

MAGNI'IUDE

'ARIS-3 HULL MODEL

SPEED 2.27 KNOTS

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MAGNITUDE (ExPErIMrìwL)

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PHASE (EXPERIMENTAL)

COMPUTER FIT OF NAGNITUDE AD PHASE .\ \.;..

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FIGURE 3 - Typical Rolì/Mc.:!cnt Frequency Response

of AlUS-3 Hull

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.9 FRF.CtJENCY (Hz) FIGURE 1i

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TRANSFER FUNCTION BASED CN ROLL COMPUTER ESTIMATE

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FUNCTION 3O PERCENT OF TR TEAN3F