Frigate seakeeping - A comparison between results ob-tained with two computer programs

FRIGATE SEAKEEPING

A COMPARISON BETWEEN RESULTS OBTAINED WITH TWO COMPUTER PROGRAMS

By

D. Zigelman and I. Ganoni

A PARTIAL INTERFACE

FROM THE 5-D TO THE SCORES 2 SEAKEEPING PROGRAM

By

A. Birbnescu-Biran

1. INTRODUCTION

The following results have been obtained within the framework of an undergraduate, optional project in Ship Engineering.

The calculations have been carried out for a frigate assumedly similar to the Italian Navy Vessel MAESTRALE. Thus, the lines are based on the reconstitution of KEHOE et al, described in [li.

Details that were not clear enough were completed using the C series developed at Hamburg [2] as examples. More specifically, the stern, and even part of the afterbody follow somehow the line of the Cl model, while the bulb was inspired from the C8 model.

Hydrostatic calculations performed on the reconstituted hull yielded results consistent with published data of the Maestrale class.

The object of the project was to carry out seakeeping calculations by means of two different computer programs and to compare the results

thus obtained.

The first computer run was carried out with the 5-D program developed at M.I.T. A short description of this program can be found in [3].

The second run was obtained by means of Kaplan's Scores 2 program. This program is described in [4).

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FRIGATE SEAKEEPING

A COMPARISON BETWEEN RESULTS OBTAINED WITH TWO COMPUTER PROGRAMS

By

l-2

Within the investigated range the two programs yielded practically close results. Discrepancies appear mostly in irregular sea results,

2.2-Wave data

Regular wave results have been required for circular frequencies ranging between 0.2 and 2.4 rad/s. Besides these, the 5-D program has

utomatica1ly yielded also results for 4.4817 rad/s.

Irregular wave results have been calculated for one sea state defined by a Bretschneider spectrum having the following _parameters:

2. PROGRAM INPUT

2.1-Main ship characteristics

Length between perpendiculars, L 114.00 m

Length overall, LOA _122.70

Breadth, molded, B 12.90 Draft, T _4.10 Depth, D _8.78 Block coefficient, CB _0.49 -Displacement, _{3040,00 t} LCG, forward of midship _{-2.337 m}

Roll radius of inertia _5.16

Longitudinal radius of inertia _28.50

Wetted surface _{1591.4 m2}

Vertical center of gravity, KG _{5.835 m}

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significant wave height, H1/3 2.0 m

peak circular frequency, w 0.7796 rad/s

mean period Tm 6.74 s

This data has been measured near Hadera, by the Israeli Institute of Coastal Engineering, Technion - Haifa, during one ship trial.

As shown in the manuals [3] and [4], the 5-D seakeeping program uses the peak circular frequency as one of the two Bretschneider parameter, while the Scores 2 program uses the mean period as one parameter.

The resulting spectra displayed by the computer programs are shown in Fig.1.

The 5-D program printed out 40 spectral ordinates distributed

between 0.4678 and 2.3388 rad/s. The Scores 2 program yielded 12 ordinates equally spaced between 0.2 and 2.4 rad/s.

3. RESULTS

For regular seas, the 5-D program printed results at 12 wave frequencies equally distributed between 0.2 and 2.4 rad/s, plus results for 4.481 radIs, a frequency automatically added by the program. The SCORES 2 program printed results only at the 12 frequencies asked for in the input, i.e. 0.2 to

2.4 rad/s.

For irregular sea, tile 5-D program automatically printed response spectral ordinates at 40 wave frequencies unequally spaced between 0.4678 and 2.3388 rad/s. The SCORES 2 program yielded response spectral ordinates at exactly the 12 frequencies asked for in the input cards.

The range 0.4678 to 2.3388 rad/s stems from spectrum truncation at 0.6wp and 3wp (see §2.5.2 in [5]).

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Figures 2, respectively 3 represent amplitude response operators for heave, respectively pitch motion at a ship speed equal to 7.75 rn/s

(15 knots)

Figures 4-12 represent amplitude response operators for absolute vertical accelerations, for several ship speeds, at the following 3 points

Point i lies in the center of the helideck, point 2 represents the bridge and point 3 coincides with the main piece of artillery.

For all practical purposes, R.A.0.-s calculated over 13 frequencies by the 5-D seakeeping program look identical to R.A.O.-s smoothed through

12 points obtained from the SCORES 2 program.

Two outputs of the phase operator for one vertical acceleration are compared in Figures 13 and 14. The phase shift points to different

definitions of the phase origin in the two programs.

Irregular wave results are described in Figures 15 to 22. Thus, Figures 15 to 19 are plots of the EMS values of heave, pitch and absolute vertical accelerations, against ship speed. _{Corresponding numerical values} are centralized in Table 1.

R.M.S. values yielded by the 5-D program are consistently above those calculated over 12 frequencies by the SCORES 2 program. Now, R.M.S. values are obtained by integrating response spectrum ordinates. These integrations were carried out in the SCORES 2 program by trapezoidal rule, oyer 12 ordinates. This should explain why the resulted values were

point no. X before , m Y from CL, m Z above BL, m

1 -47.000 0.000 9.033

2 22.000 0.000 16.125

smaller than those obtained by integration over 40 frequencies, as in the 5-D program.

Figures 20, 21 and 22 were drawn in order to check this hypothesis. It is immediately evident that for the frequency spacing chosen in the first SCORES 2 run (0.2; 0.4; . . .2.4 rad/s) , response spectrum peaks were

simply missed.

The SCORES 2 program was run a second time, asking for regular sea results at 41 wave frequencies placed at 0.50; 0.55; 0.60 . . .2.50 rad/s.

The irregular sea results thus obtained for 3 ship speeds were superimposed as small circles over Figures 15-19. Contrary to expecta-tions part of the results fell under the previously obtained SCORES 2 output, part over the 5-D values. This can be seen also in Table 2.

In order to find an explanation, values obtained for 41 frequencies were superimposed as small circules also over the R.A.O.-s represented in Figures 4-12. The finer mesh identified well pronounced secondary peaks. No such secondary peaks appear in the 5-D output for 13 wave frequencies and no attempt was made at this stage to run the 5-D program for more regular wave frequencies.

Are such secondary peaks natural in R.A.O. curves? Theoretically calculated curves published, for example, by COLOMBO and CHILÒ [6) display such peaks, though of much lesser amplitude than found here and also

closer to the tail of the spectrum.

An idea about the relative magnitude of output deviations can be obtained from Table 3. As the program 5-D was run only once, its results were considered as the basis. The percentages presented in Table 3 are relative deviations of the two SCORES 2 runs from this basis.

4. CONCLUSIONS

The two seakeeping computer programs 5-D and SCORES 2 yielded practically equivalent results for the case investigated in this report.

The number of wave frequencies chosen for calculations evidently affect the results. In this respect, the program SCORES 2 leaves the user complete liberty in chosing the frequencies and his choice will hold for both regular and irregular wave calculations.

The user of the program 5-D can chose only the regular wave

frequencies. The frequencies for irregular sea calculations are defined by the program itself and they correspond to spectrum truncation as justified by theory.

It thus seems that the program 5-D better helps the user. On the other hand, the output format of this program is rather awkward as the user has to work through many pages in order to pick up the numbers and order

them in meaningful tables or curves. This drawback led to the development in the TECHNION of the program NV5D that will be described in a report written by S. Lipiner.

The output of the program SCORES 2 is much better organized and more concise. On the other hand more care must be exercised by the user in preparing his input.

One conclusion of this report would be to recommend carrying out another study in which the two programs will be run for one ship for which model test data are available. Such a study could yield a better appreci-ation of the actual relevance of output discrepancies and of the influence of wave frequencies used in calculations.

ACKNOWLEDGEMENTS

Te text was edited by Adrian Birbanescu-Biran, who also guided the project. All the typing was carried out by Mrs. Denise Meiselles.

REFERENCES

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[1] KEHOE James, GRAHAM Clark, BROWER Kenneth, and MEIER Herbert, Comparative Naval Architecture analysis of NATO and Soviet frigates, in Naval Engineers Journal, Vol. 92, No. 5, Oct. 1980, pp. 87-99 and No. 6, Dec. 1980, pp. 84-93.

[2) Widerstand, Propulsion, Bewegung und Beanspruchung schneller Verdraengungsfahrzeuge in glattem Wasser und in regelmaessigem Seegang, Ifs-Bericht Nr. 167, Institut fuer Schiffbau der Universitaet Hamburg, July 1966.

E31 User's manual for the 5 degrees of freedom seakeeping program, Design Laboratory - Department of Ocean Engineering, Massachusetts

Institute of Technology.

Tjser's manual for Scores 2 program - Israeli Navy version,

Hydromechanics, Inc., Plainview N.Y., Report No. 82-48, July 1982.

LOUKAKIS, Theodore A., Computer aided prediction of seakeeping performance in ship design, M.I.T. Report No. 70-3, August 1970.

[6] COLOMBO, A. , CHILÒ, B., Rilievi sperimentali sulla tenuta al mare

della nave "Ammiraglio Magnaghi", CETENA Report No. 538, Genova, January 1977.

HEAD SEA

H3 = 2.00

in w

= 0.7796 rad/s

P

TABLE i

- IRREGULAR SEA

RESULTS

SHIP SPEED

s R.M.S.

R.M.S. AMPLITUDE OF ABSOLUTE VERTICAL ACCELERATION

HEAVE

PITCH

POINT NO.

i

POINT NO.

2

POINT NO.

5-D

SCORES 2

5-D

SCORES 2

5-D

SCORES 2

5-D

SCORES 2

5-D

SCORES 2

0

0.1213

0.108 0.4751 0.441

0.2068

0.202

0.1474

0.142

0.2332

0.225

2.583

0.1370

0.129

0.5146 0.476

0.3015

0.292

0.2629

0.255

0.3973

0.385

5.167

0.170

0.161 0.5427 0.5 0.3901 0.375

0.4206

0.406

0.6103

0.588 7.75

0.2058

0.192

0.5569

0.509

0.4636

0.442

0.6037

0.573

0.8599

0.814

10.333

0.2223

0.202

0.5259

0.474

0.5843

0.552

0.6949

0.647

0.9826

0.915

12.917

0.2275

0.197

0.4672

0.414

0.6794

0.635

0.6539

0.597

0.9333

0.854 15.5

0.2516

0.208 0.4243 0.370

0.7100

0.656

0.6402

0.569

0.9058

0.813

HEAD SEA

i H /

= 2.00 m

w

= 0.7796 radIs

p

TABLE 2 - IRREGULAR SEA RESULTS OBTAINED WITH THE

SCORES 2 PROGRAM

*FrequeflcieS

S H IP

_SPEED

rn/s

R.M.S.

R.M.S. AMPLITUDE OF ABSOLUTE VERTICAL ACCELERATION

HEAVE

PITCH

POINT NO.1

POINT NO.2

POINT NO.3

12 41 12 41 12 41 12 41 12 41

Freqs.*

Freqs.*

Freqs.*

Freqs.*

Freqs.*

Freqs.*

Fre9s.*

Freqs.*

Freqs.*

Frecjs.* 7.751

0.192

0.189 0.509 0.513 0.442 0.426 0.573 0.524 0.814

0.750

10.334

0.202

0.225 0.474 0.501 0.552 0.515 0.647 0.651

0.915

0.909 12.917

0.197

0.249 0.414 0.470 0.635 0.629 0.597 0.734 0.854 1.000

HEAD SEA

H1/3

= 2.00

¡Xi

= 0.7796 m

TABLE 3 - IRREGULAR SEA RESULTS.PERCENT

DEVIATÏONS OF SCORES 2

RESULTS FROM 5-D RESULTS

*Frequenc ies

SPEED

R.M.S.

R.M.S. AMPLITUDE OF ABSOLUTE

VERTICAL ACCELERATION

HEAVE

PITCH

POINT NO.1

POINT NO.2

POINT NO.3

12

Freqs.*

41

Freqs.*

12

Freqs.*

41

Freqs.*

Freqs.*

Freqs.*

Freqs.*

Freqs.*

Freqs.*

Freqs.*

7.751 6.71 8.16 8.60 7.88 4.66 8.11 5.09 13.20 5.34 12.78

10.334

9.13 -1.21 9.87 4.73 5.53 11.86 6.89 6.32 6.88 7.49 12.917 13.41

-9.45

11.39 -0.60 6.54 7.42 8.70

-12.25

8.50

-7.15

R. M. S.

IATIONS

17.56 12.54 17.35 9.21 9.75 16.17 12.21 19.09 12.17 16.45

-HT Q z

r

rj

iL

.1 -Fig. 1.

Bretschneider spectra

generated by the computer

programs.

H1/3

2.Om

Oip

0.7796 rad/S

Tm

6.74 s

1:

Program 5-D

O

Program Scores 2

2.5

IIiI

w [rad/s]

LO

o. a

0.6

0.44

0.2

Fig!2. Heave R.A.O.

Head sea

Ship speed

7.75 rn/s

Program SCORES 2

Program 5-D

o

Program SCORES 2

41 regular wave

frequencies

1.0

2.0

çQ

0.5

ir.

4

¡0

2.0

Wave circular frequency [rad/s]

2.5

Fig.3. Pitch R.?.O.

Head sea

Ship speed

7.75 rn/s

Program SCORES 2

Program 5-D

Program SCORES 2

41 regular wave

frequencies

q

I.5

4.0

-o

e

o

Fig.4. R.A.O. of absolute vertical

acceleration at point 1.

Head sea

Ship speed

7.75 rn/s

Program SCORES 2

Program 5-D

o

Program SCORES 2

41 regular wave

frequencies

po

2.5

-2.0

/

I

I,

/

/

yave

circular frequercy [rad/s

1.0

L .0

-1.0

1-16

z-o

Wave circular frequency [rad/s]

Fig.5. R.A.O. of absolute vertical

acceleration at point 2.

Head sea

Ship speed O rn/s

Program SCORES 2

2.5

Program 5-D

-2.0

-15

-1.0

-05

-2 5

-2. 0

-1. 5

-LO

-0.5

I. O z. o

Fig.6. R.A.O. of absolute vertical

acceleration at point 2.

Head sea

Ship speed 2.583 rn/s

Program SCORES 2

Program 5-D

Wave circular

_{frequency {rad/sJ}

-J .4 J L

4.0

1-18

Fig.7. R.A.O. of absolute vertical

acceleration at point 2.

Head sea

Ship speed 5.166 rn/s

Program SCORES 2

Program 5-D

Wave circular frequency

_[rad/s] - - .,.

2.0

2.5 2.0 1.5 1.0 0.5

o

Fig.8. R.A.O. of absolute vertical

accelerat.on at point 2.

Head sea

Ship speed 7.75 rn/s

Program SCORES 2

Program 5-D

o

Program SCORES 2

41 regular wave

fr?quencies

Wave circuIr frequency

[radis]

/ / 2.5 - 2.0 1.0

_0.5

1-2O

Fig.9. R.A.O. of absolute vertical

\acce1ration at point 2.

Head sea

Ship speed 10.33 rn/s

Program SCORES 2

Program 5-D

o

Program SCORES 2

41 regular wave

frequenìcies

/

4

Wave'circular frequency [rad/si

2.0 1.5 2.5 u

0.5

if

Q

4.0

2.0

C)

1-21

Fig.10. R.A.O. of absolute vertical

acceleration at point 2.

Head sea

Ship speed 12.915 rn/s

Program SCORES 2

Program 5-D

o

Program SCORES 2

41 regular wave

frequencies

Wave circular frequency [radis]

/

2.0

2.5 2.0

_1.0

0.5

3 3 j ) C J -i L L i- 2

Wavcirçular frequency [r&d/si

2.0

1.5

o5

Fig.11. R.A.O. of absolute vertical

acceleration at poir

2.

Head sea

Ship speed 15.5 rn/s _2.5

Program SCORES 2

-- Program 5-D

1.0 z.o

o

4o

o

Fig.12. R.A.O. of absolute vertical

accel?ration at point 3.

Head sea

Ship speed 7.75 rn/s

Program SCORES 2

Program 5-D

.0

2.5

_2.0

_1 .5 /

Wave

_{ircular frequency [rad/s]}

/

10

o

Program SCORES 2

41 regular wave

frequencies

i

Figl3. Response -phase operator

of ahsolute vertical

acceleration at point 1.

Head sea

Ship speed 10.33 rn/s

Program 5-D

/

i

-90. 1

\

II

/

I

i

I

I

_I

I

i

\

i j

\

-18e J t

i

I

/

I

I

I

/

t

I

/

_/

'I

/

j

/

-270. \_j / t

/

_/

/

I

/

_/

Wave circular frequency [rad/s]

1.0

4,0

2.0

Fig.14. Response phase operator

of absolute vertical

acceleration at point 2.

Head sea

Ship speed 10.33 rn/s

Program SCORES 2

Wave circular frequency [rad/s

5.0 10.0

Fig.15. R.M.S. of heave

response in irregular

sea. o

Head sea

-3-

I-5.0

40.0

Program SCORES 2

Program 5-D

Program SCORES 2

41 regular wave

f requenc ies

Ship speed [m/s]

/

/

1-26 .30 o / .25 .20 .15 i O k .

o

/

/

/

/

/

/

/

//

/

5 . o

10.0

-

---7-

'N

7-I,

z

/

//

/

_\

5.0

Fiq.16. RJ1.S. of pitch

response in irregular

sea. o

Head sea

dio. 0

Program SCORES 2

Program 5-D

Program SCORES 2

41 regular wave

frequencies

\

Ship speed [rn/si

.60 .55 .50 .45 .40 35

5.0

Fig.17 R.M.S. of absolute

vertical acceleration

at point 1.

I-lead sea

Proqram SCORES 2

Program 5-D

/

/

/

/

/

/

/

/

/

/

/

/

/

o

Program SCORES 2

_/

41 regular wave

frequencies

/

10.0

/

/ / / / / / /

/

/

/

1-28 70 .60 .50 .40 .30 Ship speed [rn/s] _{- .20} 5.0 10.0

5.Q 10.0

Fig.18 R.M.S. of absolut

vertical acceleration

at point 2.

Head sea

Program SCORES 2

Program 5-D

o

Program SCORES 2

41 regular wave

frequencies

5.0 o. o

N

o

Ship speed [m/s] .70 .60 .50 .40 .30 .20 1 0

Fig.19 P..U.S. of absolute

vertical acceleration

at point 3.

Head sea

Program SCORES 2

0

LOO

/

/

/

j,

/

/

/

Proqram 5-D

O

_{Program SCORES 2}

41 regular wave

frequencies

/

/

/ /

/

L

/

/

5.0

10.0

/

N

/

Ship speed [m/s) .80 .60 .40 .20 5.0 10.0

- 04

- 0.

1.0

2O

WAVE FREQUENCY

Fig.20 Heave amplitude

response spectrum

Head sea

Ship speed 15.498 rn/s

Program 5-D

- 0.2

o

1-32

Fiq.21 Pitch amplitude response

spectrum

Head sea

Ship speed 15.498 rn/s

Program 5-D

o

Program SCORES 2

2.0

WAVE FREQUENCY

ds

V;

-

0.5

Op

Ti r

1 O: 0.5

1.0 WAVE FREQUENCY

1.5 T

Fig.22 Response spectrum of

vertical acceleration

at point 2.

Head sea

Ship speed 15.498 rn/s

Program 5-D

o

Program SCORES 2

1-34

Fig.23

Body plan

t

/

i

/

I

," /

4

1 4

/

;

'1'1II1

11

II

/

I'

\\'

//

/

//