Ship vibrations

SHIP VIBRATIONS P r o f . S. H y l a r i d e s

H o o g l e r a a r S t e r k t e en T r i l l i n g e n

s h i p v i b r a t i o n s , some c o n s i d e r a t i o n s on f u t u r e d e v e l o p m e n t s . I n t r o d u c t i o n ,

Recent l i t e r a t u r e r e v i e w p e r f o r m e d f o r ISSC ^ shows t h a t d u r i n g t h e l e t say l a s t 7 y e a r s t h e r e has been r a t h e r l i t t l e p r o g r e s s w i t h r e s p e c t t o developments i n t h e f i e l d o f s h i p v i b r a t i o n s . As an example can be m e n t i o n e d t h a t a t t h e l a s t

ICMES 2 C o n f e r e n c e i n Malmo p r a c t i c a l l y no a t t e n t i o n was p a i d t o t h e v i b r a t i o n s o f t h e p r o p u l s i o n s h a f t i n g . M a i n a t t e n t i o n was f o c u s e d on g e n e r a l d e s i g n a s p e c t s , f u e l s a v i n g , r e l i a b i l i t y and m a i n t e n a n c e .

The m a i n r e a s o n c o u l d be t h a t t h e e s s e n t i a l developments have now been t a k e n p l a c e and t h a t t h e need o f more advanced and a c c u r a t e methods becomes l e s s u r g e n t , p a r t l y because nowadays o n l y few h i g h powered v e s s e l s a r e b e i n g c o n s t r u c t e d . B u t maybe a l s o because t h e g e n e r a l a v a i l a b l e methods s u i t t h e need.

T h e r e f o r e i t i s e x p e c t e d t h a t now i t has become t i m e t o r e c o n s i d e r and t o e v a l u a t e f u r t h e r t h e g e n e r a l a v a i l a b l e d e s i g n methGds/1,2,3,4/. Maybe a t e x t b o o k , o f a scope as s u g g e s t e d by r e f e r e n c e s /5,6/, may f o l l o w soon.

The a v a i l a b i l i t y o f programs f o r programmable c a l c u l a t o r s / 7 / t o d e r i v e t h e v i b r a t i o n s i g n a t u r e o f t h e s h a f t i n g f o r c o m p a r i s o n reasons i s an o t h e r i n d i c a t i o n f o r developments i n t h e near f u t u r e . D e s i g n a p p r o a c h . To c o n s t r u c t s h i p s w i t h an a c c e p t a b l e v i b r a t i o n l e v e l i t i s e s s e n t i a l t h a t f r o m t h e v e r y b e g i n n i n g i n t h e d e s i g n c a r e i s t a k e n t o a v o i d resonances and h i g h e x c i t a t i o n / 5 , 6 / . Thus,

ISSC s t a n d s f o r t h e I n t e r n a t i o n a l Ship and O f f s h o r e S t r u c t u r e Congress t h a t i s o r g a n i s e d i n t e r n a t i o n a l l y each 3 y e a r s . I t c o n s i s t o f a S t a n d i n g Committee t h a t a p p o i n t s t a s k s t o a number o f T e c h n i c a l Committees. The r e s u l t s o f t h e s e t a s k s a r e r e p o r t e d and p r e s e n t e d a t t h e 3 y e a r s c o n g r e s s e s .

2 ICMES s t a n d s f o r I n t e r n a t i o n a l C o - o p e r a t i o n on M a r i n e E n g i n e e r i n g Systems. T h i s a s s o c i a t i o n i s comparable w i t h ISSC, i t s a c t i v i t i e s c o n c e r n t h e m a r i n e e n g i n e e r i n g a s p e c t s . I t i s younger t h a n ISSC, i t was founded i n a b o u t 1975, and i t has l e s s i n t e r n a t i o n a l r e c o g n i t i o n .

Het laboratorium voor

Scheepsconstructies

o f t h e c o m p l e x i t y o f t h e h u i l g i r d e r t h a t b e a n a l y s e d b y m e a n s o f t h e b e a m m e t h o d / 8 /

Ship V i b r a t i o n s

' be aware o f resonance and h i g h e x c i t a t i o n '

i s t h e s l o g a n i n t h e c o n s t r u c t i o n o f l o w v i b r a t i o n s h i p s and w i l l r e m a i n t h e s l o g a n as i s shown b e l o w . Resonances. I n t h e e a r l y d e s i g n s t a g e , when a d e t a i l e d f i n i t e e l e m e n t model i s s t i l l i m p r o p e r t o be a p p l i e d , s i m p l i f i e d methods a r e needed, as w e l l as i n t h e cases i n w h i c h o n l y an i n t r o d u c t o r y a n a l y s i s i s needed. For t h a t r e a s o n r a t h e r s i m p l i f i e d p r e d i c t i o n methods have been d e v e l o p e d .

For t h e h u l l g i r d e r , f o r example, t h e s e methods a r e b a s e d on s i m p l e beam c a l c u l a t i o n s o r on s t a t i s t i c a l methods. I n f i g u r e 1 the c r o s s s e c t i o n o f a s h i p ' s h u l l c l e a r l y i n d i c a t e s how b o l d i t i s t o t h i n k t h a t t h e h u l l g i r d e r m i g h t be r e p r e s e n t e d b y a s l e n d e r beam / 8 / . On t h e o t h e r hand, i n s p i t e o f t h e c o a r s e a p p r o x i m a t i o n , d o i n g i t i n t h i s way e a s i l y and r a t h e r c h e a p l y good r e s u l t s a r e o b t a i n e d .

A l t h o u g h t h e beam method t h u s shows a r e a s o n a b l e a c c u r a c y , c e r t a i n l y s u f f i c i e n t f o r t h e e a r l y d e s i g n stage p r a c t i c e , t o meet the v i b r a t i o n l i m i t s imposed by t h e c o n t r a c t u a l r e q u i r e m e n t s a more d e t a i l e d d e s i g n p r o c e d u r e i s most t i m e s needed, e s p e c i a l l y f o r n o n c o n v e n t i o n a l s h i p tjrpes l i k e f e r r i e s , c o n t a i n e r s h i p s and so on.

The use o f r a t h e r s i m p l i f i e d models t h u s r e a s o n a b l y s u i t s . Based on t h e t h u s o b t a i n e d r e s u l t s d e c i s i o n s havé t o be t a k e n t o a p p l y more complex methods o r n o t . F u t u r e a t t e n t i o n w i l l be f o c u s e d on s t a n d a r d i s i n g such i t e r a t i v e p r o c e d u r e s .

E x c i t a t i o n s .

Slow speed d i e s e l main engines w i t h 5 o r 4 c y l i n d e r s a r e p r o b a b l y t h e m a i n c o n c e r n f o r v i b r a t i o n i n modern merchant s h i p s , w h i l e t h e h u l l s u r f a c e f o r c e s i n way o f t h e p r o p e l l o r a r e no

l o n g e r c o n s i d e r e d as a m a i n problem/1/.' The t o p i c o f p r o p e l l o r i n d u c e d v i b r a t i o n has w e l l s t u d i e d and r e p o r t e d t o t h e p o i n t t h a t we have now reaOhed a s t a t e where enough i s known t o a v o i d

p r o p e l l o r g e n e r a t e d p r o b l e m s i n s e r v i c e , p r o v i d e d t h e r e i s t i m e and a commitment t o t r e a t t h e s u b j e c t a t t h e d e s i g n s t a g e / I / .

I n t h e p r o p e l l o r d e s i g n i t i s needed t o keep t h e e x c i t a t i o n f o r c e s and moments low. T h i s can be done / 9 / by e x p l o i t i n g t h e l i f t i n g s u r f a c e t h e o r y o r c a r r y i n g o u t model t e s t s and f u l l s c a l e measurements. The g e n e r a l o p i n i o n i s t h a t d e s i g n p r e d i c t i o n methods have t h e r e q u i r e d a c c u r a c y . A more d i f f i c u l t a s p e c t t o be c o n s i d e r e d f o r t h e p r o p e l l o r i n d u c e d p r e s s u r e e x c i t a t i o n i s caused by t h e f l o w i n s t a b i l i t y a r o u n d t h e a f t e r b o d y / 1 0 / . By i n s t a l l i n g a s t e r n t u n n e l o r f i n s t h e p r o p e l l o r e x c i t a t i o n can be c o n t r o l l e d t o a g r e a t e x t e n t . For such p r o b l e m s o n l y model t e s t s o r f u l l s c a l e e x p e r i m e n t s s u i t .

Ship V i b r a t i o n s

To a v o i d e x c e s s i v e v i b r a t i o n s caused by t h e m a i n e n g i n e r a t h e r few methods a r e a v a i l a b l e t o keep t h e e x c i t a t i o n lov7. I n c r e a s i n g t h e number o f c y l i n d e r s i s c o n t r a d i c t i n g t h e r e q u e s t o f a h i g h e f f i c i e n c y . T h e r e f o r e , most i m p o r t a n t l y t h e resonances have t o be s h i f t e d t o a c c e p t a b l e ranges o f t h e s h a f t r a t e o f r e v o l u t i o n s . A d d i t i o n a l l y , m e c h a n i c a l s o l u t i o n s l i k e b a l a n c e r s and f r i c t i o n a l s t a y s have t o be c o n s i d e r e d . O u t l o o k .

The appearance o f methods t o e s t i m a t e t h e r e l i a b i l i t y o f t h e p r o p u l s i o n s h a f t i n g s y s t e m / 1 1 / can be c o n s i d e r e d as i n d i c a t i v e o f new developments i n t h e near f u t u r e i n t h e f i e l d o f v i b r a t i o n a n a l y s i s i n g e n e r a l . I n t h i s r e f e r e n c e / l l / a p r o b a b i l i s t i c

m o d e l l i n g o f t h e t o r s i o n a l e x c i t a t i o n , as g e n e r a t e d by t h e d i e s e l e n g i n e and t h e p r o p e l l o r , i s i n t r o d u c e d as w e l l as t h e

s t a t i s t i c a l p r o p e r t i e s o f the t o r s i o n a l v i b r a t o r y s t r e s s

a m p l i t u d e a t any p o i n t on the s h a f t . I t i s shown t h a t s i g n i f i c a n t c r i t i c a l s , t h a t can n o t be i d e n t i f i e d by t h e t r a d i t i o n a l

d e t e r m i n i s t i c a n a l y s i s , are r e v e a l e d when t h e p r o b a b i l i s t i c a n a l y s i s i s used. F u r t h e r m o r e i t i s c l a i m e d / l l / t h a t t h e

s h a f t i n g segments, t h a t a r e most l i k e l y t o f a i l , a r e d i f f e r e n t ^ f r o m t h e segments t h a t , as shown by t h e t r a d i t i o n a l d e t e r m i n i s t i c method, have a h i g h e r mean s t r e s s . However, as s t a t e d i n t h e d i s c u s s i o n t o / l l / , i t must a l s o be r e a l i s e d t h a t , up t i l l now f o r t h e t o r s i o n a l v i b r a t i o n s o f t h e p r o p u l s i o n - s h a f t i n g - s y s t e m , t h e d e t e r m i n i s t i c c a l c u l a t i o n method has l e d t o v e r y a c c u r a t e and u s e a b l e r e s u l t s .

The e s s e n t i a l new a s p e c t i n t h e above approach i s t h a t no l o n g e r t h e d e t e r m i n i s t i c t r e a t m e n t i s c o n s i d e r e d b u t a

s t a t i s t i c a l a p p r o a c h . T h i s opens new p e r s p e c t i v e s and a more sound way o f t h i n k i n g . I s n ' t i t t r u e t h a t f u n d a m e n t a l l y t h e r e i s n e v e r a d e t e r m i n i s t i c phenomenon i n t h e cause o f t h e v i b r a t i o n s , because i t i s d e p e n d i n g on: d r a f t , speed, sea s t a t e and may be o t h e r e n v i r o n m e n t a l c o n d i t i o n s ? A s t a t i s t i c a l approach w i l l o n l y a p p l y i n t h e d e t e r m i n a t i o n o f t h e f o r c e d r e s p o n s e . The d y n a m i c a l c h a r a c t e r i s t i q u e s w i l l t o a v e r y l a r g e e x t e n d be d e t e r m i n e d by t h e d e t e r m i n i s t i c a s p e c t s . T h e r e f o r e , t h e d e t e r m i n a t i o n o f t h e n a t u r a l f r e q u e n c i e s w i l l r e m a i n an i m p o r t a n t a s p e c t . To o b t a i n t h e s e n a t u r a l f r e q u e n c i e s or t h e " v i b r a t i o n s i g n a t u r e " / ? / o f t h e s t r u c t u r e i t i s needed t o d e r i v e a l a r g e number o f computer programs, t h a t can r u n on d i f f e r e n t t y p e s o f computers, r u n n i n g f r o m progammable c a l c u l a t o r s v i a PC's t o main frame. N e a r - f u t u r e developments i n t h e s h i p v i b r a t i o n r e s e a r c h may t h e r e f o r e be c o n c e n t r a t e d on t h e g e n e r a t i o n o f two d i f f e r e n t s e r i e s o f computer programs:

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S h i p V i b r a t i o n s - d e t e r m i n i s t i c programs t h a t a r e used t o d e r i v e t h e n a t u r a l f r e q u e n c i e s and n o r m a l modes o f v a r i o u s s t r u c t u r a l components w i t h t h e use o f a l a r g e v a r i e t y o f c o m p l e x i t y o f t h e c o m p u t e r s ; - programs t o d e t e r m i n e t h e f o r c e d response o f s t r u c t u r e s and s t r u c t u r a l components, based on s t a t i s t i c a l methods, w h i c h i s a more f u n d a m e n t a l a s p e c t and t h e r e f o r e , f o r t h e

t i m e b e i n g , r e s t r i c t e d t o l a r g e r computersystems;

- d e v e l o p s t a n d a r d i s e d methods t o use t h e s e programs i n a s e q u e n t i a l way.

R e f e r e n c e s .

1. Ward, G.: " V i b r a t i o n s o f s h i p s t r u c t u r e s w i t h p a r t i c u l a r r e f e r e n c e t o w a r s h i p s . "

Advances i n M a r i n e S t r u c t u r e s , 86-07, pg 63. 2. Noonan, E.F.: " S h i p b o a r d v i b r a t i o n can be c o n t r o l l e d . "

Proc. o f Chesapeake Mar.Eng.Symp..Washington D.C., j a n u a r i 1986.

3. H a l a k a , M.K.: " S o l u t i o n o f s h i p v i b r a t i o n problems - c u r r e n t s i t u a t i o n and f u t u r e p r o s p e c t s . "

Proc. o f VTT Symp. 68, Espoo, F i n l a n d , 1986.

4. Ward, G.: "The a p p l i c a t i o n o f c u r r e n t v i b r a t i o n t e c h n o l o g y t o r o u t i n e s h i p d e s i g n work." RINA, 1982. 5. " V i b r a t i o n c o n t r o l i n s h i p s . " Handbook b y V e r i t a s , O s l o , Norway, 1987. 6. " B u i l d i n g and o p e r a t i o n o f v i b r a t i o n f r e e p r o p u l s i o n p l a n t s and s h i p s . " Handbook b y Bureau V e r i t a s , P a r i s , 1087.

7. Morehead, J . C : "Some programmable c a l c u l a t o r programs u s e f u l i n s i g n a t u r e a n a l y s i s . "

M a c h l n a r y V i b r a t i o n M o n o t o r l n g and A n a l y s i s , 83-05/06. 8. Jensen, J . J . and P.T.Pederson: "Recent advances i n beam

models used f o r s h i p h u l l v i b r a t i o n a n a l y s i s . "

Proc. o f Int.Symp. on Ship V i b r a t i o n s , CETENA, Geneva, 1985.

9. Fabro, R., P . F r a n d o l i and o t h e r s : " C o r r e l a t i o n between p r e v i s i o n a l c a l c u l a t i o n s and e x c i t a t i o n s u r v e y s o f p r o p e l l e r s and v i b r a t i o n s f o r a 25000 DWT motor t a n k e r o f a c l e a n / d i r t y p r o d u c t c a r r i e r . "

CETENA publ.No.69, March 1986.

10. Nonaka, S. and H.Tomlnaga: "Animprovement i n t h e a f t e r b o d y v i b r a t i o n o f 7 - c i l l n d e r e n g i n e powerd l a r g e f u l l s h i p . " ( i n J a p a n e s e ) . Trans.West.Jap.Soc.Nav.Arch.,No.68, 1985. 11. N l k o l a i d e s , E . , Perakls,A.N. and P a r s o n s , M . G . : " S t r u c t u r a l r e l i a b i l i t y o f m a r i n e d i e s e l e n g i n e p r o p u l s i o n s h a f t i n g systems." SNAME t r a n s a c t i o n s 86-06, pg 1 8 9 . P r o f . d r . i r . J.D. v a n Manen

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