Methods for data analysis at ship model testing in waves

D. RADEV (*) - R. KISHEV (*)

Methods for Data Analysis

at Ship Model Testing in Waves

(1 Bulgarian Ship Hydrodynamics Centre, 9003 Varna, Bulgaria.

TECIINISCHE UNIVER_SITEIT

Laboratarium voor

Schco;:hydromeohanioa

Archief

Md:c....1viog 2, 2:728 CD Delft Tel.: 015- 7..3373_{- Fr-x: 015 - 761036}

METHODS FOR DATA ANALYSIS AT SHIP MODEL TESTING IN WAVES

SUMMARY

The proper evaluation and the

possibility

of

controlling

the ship behaviour in real operational

conditions

represents

one of the most important stages

of the

technology

for the

development of new types of ships and floating

structures

or

the improvement of existing ones. The

investigations

can be

realized at different

levels:

theoretical,

experimental

or

combined approaches can be used.

In the

theoretical

approach

to

determine

the

seakeeping

characteristics,

the

analytical

methods

are

not

always

applicable,

mainly

due to the

hull-wave

interaction

and

related effects. In these cases, the application

of

numerical

methods is necessary.

On the other

hand,

the

experimental

approach

requires

an

accurate methodology, an hardware

configuration

and

applied

software. In this paper,

the

problems

connected

with

this

approach are examined in detail. The object is the development

of methods which

should

guarantee

reliable

ship

behaviour

prediction results.

METODI PER L'ANALISI DEI RISULTATI OTTENUTI

ATTRAVERSO

PROVE

SU MODELLI IN PRESENZA DI ONDE

SOMMARIO

La corretta valutazione ed ii

controllo

del

comportamento

della nave nelle reali condizioni operative

rappresentano

uno

degli elementi piu' importanti per lo sviluppo

di

nuovi

tipi

di

nave

e

strutture

galleggianti,

come

pure

per

miglioramento delle caratteristiche tecniche

od

operative

di

quelle esistenti. Gli studi relativi possono essere effettuati

per via teorica, sperimentale o con

approcci combinati.

Nella determinazione delle caratteristiche

di tenuta

al

mare

per via teorica, l'approccio

analitico non e' sempre possibile

a causa della complessita'

del problema, principalmente dovuta

all'interazione tra carena ed onda ed agli

effetti

connessi.

Bisogna allora procedere attraverso

l'applicazione di tecniche

di calcolo numerico.

D'altra parte, lo studio del

fenomeno

per

via

sperimentale

richiede

una

metodologia

accurata

e

necessita

di una

configurazione hardware e di software applicativo.

In

questa

memoria Si analizzano i problemi connessi con questo approccio

ed i metodi che garantiscono l'ottenimento di risultati validi

nella previsione del comportamento in mare della nave.

INTRODUCTION

The proper evaluation and control of ship behaviour in real operational

conditions represent one of the most important stages of the technology for

development of new types of ships and floating structures, as well as

of the process of improvement of their technical or exploitational

charac-teristics. The investigations are realized on different levels, theoretical,

experimental or combined approaches being used (1),(2).

In the theoretical way of determining seakeeping characteristics, the analytical methods are not always applicable, due to the complexity of the

problem, mostly in respect to hull-wave interaction and related effects.

Thus the application of approximate approaches used is connected with the schematization of the ship hull and the flow round it, the effective

adaptation of definite numerical methods for solution and their computational realization.

The experimental investigation in a model test tank includes accurate

methodology, hardware and applied software, which should guarantee reliable ship behaviour prediction results.

METHODOLOGICAL BASIS OF EXPERIMENTAL RESEARCH

In the BSHC experimental tanks, tests with both free-running and towed models are carried out (3),(4). The free-running tests are conducted with

models of 3-4 m length in the manoeuvring and seakeeping basin (MSB). The

apparatus complex mounted on the model includes systems for control, drive and ultrasonic trajectory indication (USS), as well as a number of measuring devices for registration of model oscillations along and about its inertial axes. Output from different transducers is transmitted to shore via a

multichannel telemetric system (TMS).

The tests with towed (or self-propelled) models are carried out in

the deep water towing tank, (DWT). The transducer's outputs are connected

directly to the analog inputs of the computer system or stored on

magnetic tape. In the case of forced oscillations harmonical excitation is imposed on the captured model by means of mechanical devices.

The tests described, involve operations for measurement of a large number of physical values, characterized by different process frequencies; their transmittance to shore; acquisition and registration of experimental

data enormous in volume in real time; processing and analYsis of the

SHIP PROJECT .--, (:,

( THEORETICAL

' 1---.

k 'EVALUMON

-OVERALL :BEHAVIOUR ESTIMATION, -FINAL REPORT

V

nil

fk":66-;"117ali . ;;]147_1

Forced

Amplitude

_Oscillations

.1111Wiric

7~11

Towing

,Model Tests'

in Waves

Fig, lGeneraJ paths pf informatiUm

flows--.

1111MS

11

I

The generalized scheme of information flows in hardware sense is shown in Fig. 1.

The ship model can be most generally treated as a dynamics system with 6 degrees of freedom under random (wave) excitation, as shown in Fig. 2.

Every mode of motion, as well as the other responses, are considered as linear output reactions of the dynamical system, which can be universally

characterized by their transfer function Hu(iw)=1Hu(w)l.e4k6), and it is

proved (5), that pure quadratic responses, such as added resistance, thrust, torque etc. can be treated ih an analogrus way, by introducing so called

"quadratic" (bi-frequency) transfer function Hu(wl,w2). The evaluation of these system response-amplitude operators (RAO) appear to be the main goal of the seakeeping tests, as they are invariant in respect to the

excitation characteristics. Thus, if the input excitation is,most generally, characterized by its energy spectrum, S (w), the respective spectrum of

reaction will be

Su(w) =

1Hu(w)12.Sc(w)

Every ordinate of the RAO can be considered as system reaction to a

single harmonic excitation. This gives a possibility for ship model testing in successive series of regular waves with given frequency and amplitude, which simplifies the experimental procedure and data processing, but leads

to losses in testing time. On the contrary, generating in the basin irregular waves with prescribed spectral characteristics, the RAO can be evaluated

by mathematical statistics and spectral analysis, which decreases several times the duration of experiments.

At ship model motion in severe waves, impulse loadings occur, such as slamming pressures along the bottom and sides, which has to be subjected to

peak analysis.

The experimental data handling

is a key moment,

determining the duration of

the experiment and the loading of facilities.

To ensure proper implementation of all different kind of dynamical tests at high quality and scientific level, complex automation of all experimental

stages based on contemporary measuring and computer technique, the use of efficient mathematical methods for planning of the investigations, data processing and representation, is needed. To meet this demand,a close loop automated system for seakeeping investigations was developed at BSHC (8),(9),(10),(11),(12), on the basis of minicomputer PDP 11/34. General

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structure of the computerized approach is realized. _{According to Fig. 1,}

it includes: preliminary theoretical estimations, based on simplified

methods; automated implementation of the tests, data acquisition and

initial processing; visualization and graphic presentation; overall

analysis and prediction of general ship behaviour _{in waves.}

GENERAL DESCRIPTION OF THE COMPUTERIZED SYSTEM SOFTWARE FOR SEAKEEPING INVESTIGATIONS

The software of the computer system for seakeeping investigation is built

out of separate subsystems, according to their destination, on a module principle (Fig. 3). In the course of the system's design and development an essential place occupy the data base common for the different models, as well as the connections between them. An approach is chosen during

which the software is divided into two parts, according to its functions:

universal software

and

specialized software;

program interface is created between the two parts. The first part

includes the registration subsystem, as well as the subsystem for visualiza-tion and graphic presentavisualiza-tion. The specialized software includes the

subsystem for theoretical prediction of seakeeping characteristics and

the subsystem for processing the results from model _{tests in waves or}

forced oscillations tests.

Common data base reduce the problems that arise when data processing

is carried out. Redundancy of data generation _{is reduced because growth is}

simplified since each alternation of data base need be done just once,

instead of once for each program.

A program module is developed, which arranges the output information files in accordance with the standards recommended by the XVI ITTC

(7).

Universal Software

The data registration subsystem consists of three main parts (Fig. 4):

data base initialization; real time software (15);

software for preliminary processing.

-OD

Software for Data Base Initialization

DATA REGISTRATION SOFTWARE

Software for Prelimi- rary

Processing

Express Evaluaticns

SOFTWARE FOR THEORETICAL PREDICTION Real Time Software

Detailed Theoretical Predictions

COMPUTER SYSTEM FOR SEAKEEPING INVESTIGATIONS COMMON DATA BASE Fig.

3. General structure of the

automated system for seakeeping

investigations

SOFTWARE FOR VISUALI- ZATION

AND

GRAPHICAL

REPRESENTATION

SOFTWARE FOR SEAKEEPING TESTS Software for Model Tests

in Waves

System initializa-tion Determination of carrying frequencies Test for stationarity Filtration

COMMON DATA BASE

Fig. 4. Universal software for initial data processing

Time series Data selection Data acquisition and regi-stration

340 TECNICA ITALIANAINL 4 - 197

This system's modules are Started successively, in view. of their functional ensurance,

The software for data base, initialization'

includes

entering the necessary

initial test parameters according to its type, as well as :information regarding registrating devices. Effective dialogue is provided, all the

parameters are easily controlled, the changes are immediately registered.

Carrying out seakeeping tests necessitates software back-up for the following functions, in real time:

RI

- experimental data acquisition and registration. from TMS and USS for

free-running model tests and from analog-digital converter(ADC);

ship model trajectory and speed! determination and their graphical presentation. at seakeeping tests with free-running models,

- control of model course stabilization at free model tests.

Using the programs included

in

the software for preliminary processing,

conversion' of the measured values to physical, their evaluation with

amplification and, calibration coefficients is done. Also, the values are

sorted up according to channels

in

a common file

from

which! data, for a

given channel can! be derived if desired.

The selected time series can be visualized and controlled' on! a display, and stationary test it made (Fig. 4). The basic frequencies of the process, are evaluated by spectral analysis and, if necessary, digital filtration can be

done by Remetz method, thus removing the noise frequencies and the trend.

-The software for visualization and graphical presentation (14)is based

on two universal programs for visualization and graphical presentation of time series' and discrete functicns allowing definition, of Window,

incorporated of several graphics, diagram! scaling, etc.

This software is universal both for the program modules ensuring the

theoretical prediction and for those ensuring' the experimental inmestigation,

, Specialized Software

For express evaluation of test conditions and expected reactions (ampli-tudes of motion, added, resistance and related speed loss etc.), some

simplified methods are used, based on statistics or approximate relations.

-Formation of general data base is under way in BSHC, and experience

gained helps much in this respect. More precised and detailed predictions of seakeeping characteristics are effected through linearized methods based on strip theory assumptions and conformal mapping or close-fit technique for inertia and damping evaluations. The corresponding

program modules are integrated in a complex computerized procedure for overal estimation of ship behaviour in real environmental conditions. The software for forved oscillation model tests (10) comes in sequence after the programs ensuring data acquisition, which are two types: acquisition and registration of experimental data as time series via ADC, and acquisition and registration of integral values from special microprocessor.

Direct connection with the software for visualization and graphical

output is realized, which serves for effective control of the experiment, as well as for drawing up of output documentation.

The basic functions of this software are:

determination of hydrodynamic forces' sine and cosine components

by Fourier analysis,(digital or electromechanical); and

identification of hydrodynamic coefficients in motion differential equations.

The major part of the specialized software consists of routines for

data processing at experiments in waves. The program modules are realized on FORTRAN or ASSEMBLER languages. In view of voluminous calculations connected with statistical analysis of time series, special attention is

paid to algorithms for fast processing . The faster action of all algorithms

is ensured by complete use of integers, and utilizing of standard ASSEMBLER routines (9). DEC-LSP program package is used mostly (8). The basic functions of this developed set are as follows (Fig. 5):

statistical analysis;

histograms and distributions, allowing processing of up to 20000 points per second;

regular wave data processing, including amplitude and phase analysis; auto-correlation function;

cross-correlation function;

spectral analysis of experimental data, comparison of the results with standard spectra, computation of spectral moments.

.

-Hito-

grams and distri- butions

A

GRAPHICAL AND TABULAR OUTPUT

SHIP BEHAVIOR ESTIMATION

FINAL

REPORT

Fig. 5. Basic functions of the specialized

seakeeping software

Amplitude spectrum

1111

Tables

Official documents printing

441

YD R CD YN Atl I C COEFFICIENTS Phase spectrum

COMMON

DATA

B A SE

Auto- correla-Ampli -Power spectrum Generated theore-Response

FORCED OSCILLATIONS EXPERIMENTS

tion tude and spec-ical amplitude func - tion spectrum tral mo-spec - trum operator ments

The fast spectral processing is effected by standard ASSEMBLER module for fast Fourier transform and using built-in tables of the sinus-function. The transformation of 800 points per second is approximate

processing speed. Rough spectra are smoothed in frequency domain by

Nanning or Haan spectral windows. cross-spectral analysis;

calculation of response amplitude operators;

- bi-spectral analysis, realized by an original and economical algorithm, in which only single fast Fourier transform is needed of generalized time series (5)

peak analysis;

final processing, including spline approximation and/or multiple linear regression analysis.

CONCLUSIONS

United approach of computerized investigation of ship seakeeping

qualities leads to a sharp decrease in the time consumed and increases the reliability of the results obtained. The system affords a possibility for repeated and quick access to the necessary data, which creates

prerequisites for their systematization and use in the development of new methods for investigation and prediction of ship behaviour.With the

minimum of exertions and expenses, without radical modifications, automation of new test types can be realized upon necessity.

On the grounds of the harmoniously built computerized data processing

system and making use of a common data base, the data systematization

is easily realized.

All prerequisites are present for developing further the closed system for off-line simulations and a real time simulation systems for training

of ship masters and off-shore operators to handle properly the vehicle in real exploitation conditions.

REFERENCES

1. Bogdanov P., Dynamic Development of the Bulgarian Ship Hydrodynamics Centre in the Period 1971-1981 and Main Future Tasks and Activities, Jubilee Scientific Session of the BSHC, 1981, vol.l.

-Bogdanoy P., Andonov K., Stoyanov S., Lyutov N., Automation of Engineering Labour Experiment and Manufacturing Process at BSHC -Application of Progressive Methods, Results and Trends for Further Development, SMSSH'84.

Galchook V.Y., Solovyov A.P., Scientific Experiments Technique, Leningrad, Sudostroenie, 1982.

Kishev R., Instruction Mannual: Implementing Ship Model Tests in Waves, BSHC, 1980.

Kishev ft., Radev D., Wave Energy Convertor Efficiency in Irregular Waves, SMSSH'83.

Kishev R., Ivanov N., Pomeranetz L., On the Practical Evaluation of Hydrodynamic Coefficients of Semisubmersible Platforms, Based on Systematic Model Tests, 4th BOSS Conference, Delft, 1985.

Proposed Standard Format for Exchange of Seakeeping Data on Computer-Compatible Media, 16th ITTC Proceedings, 1981, vol.1, p.494-503.

Radev D., Stephanov A., Program System for Statistical Processing

of Time Series by Optimization Algorithms for Minicomputer, Rousse,

1981.

Radev D., Kishev R., Stephanov A., Stanev Tz.,Sea-Keeping Qualities Experimental Investigation of Ship by Automated System, S.I., Varna,

1982.

Radev D., Milanov E., Stephanov A., Spassov S., On the Software Development of Planar Motion Mechanism Tests at BSHC, SMSSH'82.

Radev D., Stephanov A., Software for Automated Experimental Investigation

of Seakeeping and Manoeuvring Qualities of Ship and Off-shore Structure Models, SMSSH'83.

Radev D., Stephanov A., Development and Realization of a Method for

Real Time Data Acquisition of Slamming Pressure Signals, SMSSH'86.

Stanev Tz., Radev D., Instruction Mannual: Implementing of Seakeeping

Tests with Free Running Models, BSHC, 1981.

Stephanov A., Kostova T., Basic Software for Graphic Display, Rousse,

1981

Stephanov A., Martinov P., Real Time Software for Experiments with

Free-Running Ship Models, XVII Scientific Session Dedicated to Radio

Day, Sofia, May 1982.

Zlatev Z., An Algorithm and Computer Program for Multiple Linear Regression Analysis, SMSSH'81.

344 TECNICA ITALIANA N. 4 - 1987