I»S.i3,C, CoiTimlttee "Design Loadfl" Contr i l . u t ion by F.F.van Gunsteren on 2.2.1.2. F l e x i b l e Body Loads
The 1976 r e p o r t addressed the problem o f t h e s t a t i s t i c a l t r e a t m e n t o f combined low~frequenoy r i g i d body mode s t r e s s e s and h i g h frequency v i b r a t o r y s t r e s s e s . The h i g h frequency wave-induced main h u l l v i b r a t i o n s - or SPRINGING- form, due
to the h i g h resonant frequency a separate l o a d caused by t h e v i b r a t o r y i n e r t i a f o r c e s . These f o r c e s are r e l e v a n t f o r
ship/wave resonant c o n d i t i o n s when the body i s f l e x i b l e . U n f o r t u n a t e l y no sound e n g i n e e r i n g method e x i s t f o r i n c o r p o -r a t i o n o f these loads i n the Qene-ral S t -r u c t u -r a l Design
Procedure.Such a mehod i s n o t a v a i l a b l e y e t . because a c c u r a t e c o r r e l a t i o n between t h e o r e t i c a l and f u l l s c a l e e x p e r i m e n t a l r e s u l t s i s not y e t possible,since»
- t h e r e i s a l a c k o f p r e c i s e i n f o r m a t i o n on waves -knowledge on damping i s s t i l l v e r y l i m i t e d
- u n c e r t a i n t i e s e x i s t i n the t h e o r y o f wave e x c i t a t i o n Let us consider these t h r e e areas w i t h r e s p e c t t o
r e c e n t f i n d i n g s and recommende f u t u r e work.
Precise i n f o r m a t i o n on wave c h a r a c t e r i s t i c s , e.g. t h e occurrence o f wave groups and t h e v a r i a t i o n o f wave energy w i t h t i m e , i s r e q u i r e d f o r t h e p r e d i c t i o n o f s p r i n g i n g o f
ships i n a seaway i n c l u d i n g the c o r r e c t i o n f o r d e v i a t i o n s from the frequency response method. T h i s c o r r e c t i o n f o r t h e l n f l u e n c ( od t r a n s i e n t s i n weakly damped systems i s c o n s i d e r a b l e accordln* t o Van G u n s t e r e n d ) f o r damping o f l e s s than 3 p e r c e n t o f t h e c r i t i c a l damping. See f i g u r e 3.2 o f r e f e r e n c e ( 1 ) .
The c o r r e c t i o n c o u l d e x p l a i n t h e increased d i f f e r e n c e between measured and c a l c u l a t e d s p r i n g i n g s t r e s s e s w i t h I n c r e a s i n g wave h e i g h t as r e p o r t e d i n r e f e r e n c e (2) and c o u l d a s w e l l e x p l a i n one c o n c l u s i o n o f the v a l u a b l e and e x t e n s i v e f u l l
s c a l e research on the -Esso Bonn-, t h a t t h e damping c o e f f i c i e n t e s t i m a t e from the response spectrum i s about 25 p e r c e n t l a r g e r than o b t a i n e d from t r a n s i e n t s . F o r an e s t i m a t e o f the c o r r e c t i o n more i n f o r m a t i o n on waves than alone the spectrum has t o b« i known.Also n o n - s t a t i o n a r y e f f e c t s should be known.I'^rther work
i n t h i s f i e l d , e . g . on wave groups and i t s r e l a t i o n w i t h spectrum shape, i s recommended.
The second area - damping - has been s t u d i e d by means o f ( 6 ) , ( 7 ) ( 8 ) , ( 9 ) , ( 1 0 ) , ( 1 1 ) . ( 1 2 ) . From these s t u d i e s t h e o r d e r of magnitude o f t h e s p r i n g i n g damping c o e f f i c i e n t can be established.See f i g u r e s 5.4 and 5.5 o f r e f e r e n c e ( 1 ) .
However f o r an a c c u r a t e p r e d i c t i o n nore i n f o r m a t i o n on v a r i o u s souces o f damping i s needed,In t h i s f i e l d t h e r e i s a need f o r model t e s t s , e . g . on t h e i n f l u e n c e ol' speed on t h e hydrodynamic damping a t h i g h f r e q u e n c i e s and the i n f l u e n c e o f mean s t r e s s and s t r e s s d i s t r i b u t i o n on s t r u e x t u r a l damping.
-2-The wave e x c i t a t i o n èf s p r i n g i n g i s t h e l a s t area o f i n t e r e s t . For t h e d e t e r m i n a t i o n o f t h e v i b r a t o r y wave e x c i t a t i o n t h e l o n g i t u d i n a l d i s t r i b u t i o n o f wave e x c i t i n g f o r c e s along t h e l e n g t h o f t h e s h i p should be known q u i t e a c c u r a t e l y .
Progress by means o f experiments has been made. Tests w i t h segmented models ( 1 3 ) , ( 1 4 ) . ( 1 5 ) and p r e s s u r e measurements oh model s c a l e ( 1 6 ) , ( 1 7 ) . ( 1 8 ) show, that»
a) S t r i p t h e o r y may g i v e o n l y good p r e d i c t i o n s o f wave e x c i t i n g f o r c e s i n waves longer than h a l f t h e s h i p l e n g t h and i n the lower speed range.
b) C o r r e l a t i o n between measurements and r e s u l t s , c a l c u t a t e d w i t h the s t r i p t h e o r y , i s very poor f o r wave l e n g t h s s m a l l e r than h a l f t h e s h i p l e n g t h and speed over Fn-0.15, which g e n e r a l l v cover s e r v i c e speeds.
c) The f i r s t order wave e x c i t a t i o n f o r s m a l l wave l e n g t h r a t i o s tJikes p l a c e p r i m a r i l y a t t h e bow,to a l i m i t e d e x t e n t a t t h e o t e r n and i s s m a l l between b o t h ends o f t h e s h i p .
d) The wave e x c i t i n g f o r c e s are l i n e a r l y dependent on wave h e i g h t i n t h e range o f moderate speeds and wave h e i g h t s .
e) 17onllnear wave e x c i t a t i o n might i n c r e a s e w i t h wave h e i g h t and
s h i p speed. ^ There i s a need f o r t h e development o f t h e o r e t i c a l methods I n t h i s
r i e l d i n order t o match t h e v a l u a b l e e x p e r i m e n t a l r e s u l t s (14) on the l o n g i t u d i n a l d i s t r i b u t i o n o f wave f o r c e s on t h e h u l l o f a
r e s t r a i n e d model i n r e g u l a r head waves o f s m a l l wave l e n g t h s .
References
1. Van Guns t e r en, P. F. " S p r i n g i n g of s h i p s i n ^^^y^m r^ir^^ T h e s i s ,
D e l f t , 1 9 7 8 .
2. stiansen,S,G.,A.Man8our and Y.N.Chen -Dynamic Response o f L a r g e Great Lakes Bulk C a r r i e r s t o Wave-Excited Loads ^ p^>,^«« « n . 4,Annual Meeting.SNAMB,November 1977, 3. Robinson,D.C.I-Damping c h a r a c t e r i s t i c s o f s h i p s i n v e r t i c a l f l e x u r #
and c o n s i d e r a t i o n s i n h u l l damping i n v e s t i a a t l o DTMB Report 1876,December,1964
4. Detts,c.v.,R.E.D.Bishop and W.G.Prlce«"A s u r v e y o f i n t e r n a l h u l l _ ^ damping",Trans.RINA,vol 118,1976.
5. Jensen,J.J.and N.Fl.Madseni-Areview o f s h i p h u l l v i b r a t i o n " ,
fi H-*.^- « M « X . .ï'*''!'?^''*-*- " * * i v e r s i t y o f Denmark,Report 107,1976
6. Hart,H.H. ' t j - H u l l v i b r a t i o n s o f the cargo l i n e r Kouderkerk" Netherlands Ship R e s e a r c h C e n t r e TOO,Report No 143 S,October 1970
7. H y l a r i d e s . s . and R.van de G r a s f i - V L C C deckhouse v i b r a t i o n o a l o u l a t l o i compared w i t h measurements-,Monograph M6,
Netherlands M l r i t i m e I n s t i t u t e , J u l y , 1 9 7 6 . S.Catley.D. and C . N o r r i s i - T h e o r e t i c a l p r e d i c t l o n o f t h e v e r t i c a l
dynamic r e s p o n s e o f s h i p s t r u c t u r e s u s i n g f i n i t e elementsand c o r r e l a t i o n w i t h s h i p
Q M ^ r ^ i ^ ^ V « m n i o b i l i t y measurements-,ONR symposium London 197( 9. McGoldrick.R.T.»-Compari8on m between t h e o r e t i c a l l y end e x p e r i m e n t a l
determined n a t u r a l f r e q u e n c i e s and modes o f v i b r a t i o n o f ships-.DTMBreport 906,1954 10. MGGoldrick,R.T.i-Shlp v i b r a t i o n - , D T M B r e p o r t 1451,1960
11. H y l a r l d e a . S . t - H u l l v i b r a t i o n s o f a t h i r d g e n e r n t i o n c o n t a i n e r s h i p , c a l c u l a t i o n s compared w i t h measurements"
12. Moeyes,G.."Measuremj,rent"oi*"^".T'''"* ^"-^^^«^^ Ponograph MX4^7
D e l f t Progress^^'^«•^ i« «^^^^t waves" D e l f t Progress R . p ^ r t , v o l 2,1976.ppft7-107
-3-13. Kureai,T.«"On t h e e x c i t i n g f o r c e and response o f s p r i n g i n g o f ships",Paper no 28, , I n t .Synip.on the Dynswiics o f Marine V e h i c l e s and S t r u c t u r e s i n Waves,Universi College,London,1974,
14. G e r r i t s m a , J . and W.Beukelmani"Analysis o f t h e m o d i f i e d s t r i p t h e o r y f o r t h e c a l c u l a t i o n o f s h i p motions artd wave bending moments",TNG r e p o r t no 9€ S,kl967. 15. Lee,C.M.i"Heaving f o r c e s and p i t c h i n g moments on a semi-submerg X and r e s t r a i n e d P r o l a t e s p h e r o i d proceeding i n
r e g u l a r w a v e s " , U n i v e r s i t y o f C a l i f o r n i a , R e s e a r c h I n s t i t u t e o f Engineering,Report NA-€4-2,19e4 16. Kagawa,K,,K.0htaka and M.0nouei"A study o f wave induced v i b r a t l
n s " , J o u r n a l o f t h e S o c i e t y o f Naval A r c h i t e c t s of Japan,Paper 1-17 (1975) and Paper 2-7 (1977 17. Yamamoto,Y and H.0htsuboi"Wave f o r c e s e x c i t i n g s p r i n g i n g o f
s h i p s " . J o u r n a l o f t h e S o c i e t y o f JJaval A r e h i t a o t of Japan.
