The effect of type wave spectrum on bending moment estimates

ARC EF

THE EFFECT OF TY OF WAVE

SPECTRUM ON BENDING MOMENT ESTIMAIES

by N. Hogben arid J.A. Ewing

Report on a study instigated as a result of

_discuasion

between Dr.

_Warnainck

_{and Dr. Hogben. regarding the ISSC Committee}

I

programme, held at Feltham in Apri]. 1965. _{It is recognised that}

other members of the Committee may like to see results for other

THE EFFECT OF .nk

OF WAVE SPECTRUM

ON BENDING. M0M1T ESTIMATES

by.N. Hogben and J.A. Ewing

INTB.ODUCTION

This study has been undertaken to guide the drafting of recommendations for a

sta.ndard spectrum. The object is to iñdicate the degree

of

sensitivity of bending morent estimates to the type of spectrum assumed. For this poe-pose comparisons have

bèen made covering ₃ basic spectrum types at each of

₃

different mod.a]. periods. DETAflaS OP COMPARISON

Spectral Ipes

In each case the spectra have

been

normaliaed to a variance corresponding to a

significant

height of 8 metres. It is to be noted that type (3) has been chosen

as representative

of

an actual measured spectrum containing a "Swell bump" at the

low frequency end.

Response

Operators

Response operatore

as derived by Swaan Reference (ii.) for a Series 6o model at 2 values block coefficient CB=O.7O and 0.80 at a Fronde number of 02 have been

used. These operators were mea8ured in regular waves at 1700 to the model héading

(nearly head seas). Results

Results in the form of bending moment coefficients for each of 3 ship lengths, 6o m. 160m. and 28 w.,, are preaentéd in both tabular and graphical form. The

fóliowing

nomenclature has been used0

(i)

Darbyshire Reference (i)

(2) I.SSS.C. 196k Reference (2)

M

Vertical mid.ahip wave bezing

moment

amplitude

(Reference ¿f)

M113 Significant value of vertical wave bending moment double amplitude,

-2-calculated, from 4X /Varianoê of

bending moment

spectrum

p Specific danaity of water

g

Acceleration due to gravity

Ship beam

Ship Length

Block coefficient

Wave amplitude (Reference ¿4.)

TABLE 0F VALUES OP (

_4_)

IN METRES

pgLB

Fr Freude number 0.2 signifiò'ant wave height 8m. and wave di reotïon 1700.

Spectrum Modal Period

Seco rEis

L=60 in. L=160 m. L=280 in.

C1çO. 70

CB=O60 CBO.7O C=0. 80

Cr0.70 CB0.BO

r

RERERENC5

(i)

DARB7SHIBE, J.

_{"The One Dimensional Wave Spectrum in the Atlantic Ocean}

and in Coastal Waters".

Conference on Ocean Wave

Spectra, Baston Maryland, U.S.A.

Nat. Acad. Sci., Washington, May 1961.

Report of Environmental Conditions Committee r.s.S.C. I 964.

MOSKL)VIITZ, L., PThRSON, w.j.

"Wave Spectra Estimated from Wave Record8 obtained by thé

0.W.S. Weather

Explorer and 0.LS. Weather Reporter.

New York University College of

Engineering Report Vol. I

1962.

80

'n.

O

7

o o

w

tn

w

u'.,

>-20

w

z

J

/

DÀRBVSHIR

Íss:c

1964

Ñvu Jill

0.05

010

WAVE FREQUENCY

_{MODAL PERIOD}

0.15-

005

Q

DARBYSHIRE

4964.

NYÌJ JHI

I 'I 11 13 15 11 13 15

MODAL PERIOD SECONDS

ii

13

IS..

COMPARiSON 0F BENDING MOMENTS