Forced oscillation techniques in use at the Delft Shipbuilding Laboratory for analysing ship motions

R e p o r t No. 233.

LABORATORIUM VOOR

SCHEEPSBOUWKUNDE

TECHNISCHE HOGESCHOOL DELFT

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FORCED OSCILLATION TECHNIQUES I N USE AT THE DELFT

SHIPBUILDING LABORATORY FOR ANALYSING SHIP MOTIONS.

by

P r o f . i r . J . G e r r i t s m a .

L J

Prepared f o r t h e P l a n a r M o t i o n Mechanism p o s t g r a d u a t e cour AEW, H a s l a r .

Forced o s c i l l a t i o n t e c h n i q u e s i n use a t t h e D e l f t S h i p b u i l d i n g L a b o r a t o r y f o r a n a l y s i n g s h i p m o t i o n s .

by

P r o f . i r . J . G e r r i t s m a .

As f a r as known t o t h e A u t h o r , t h e f i r s t f o r c e d o s c i l l a t i o n t e s t w i t h a s h i p model was c a r r i e d o u t i n 1 9 ^ 6 by M.D. Haskind and I.S. Rieman i n t h e C e n t r a l Aero-Hydrodynamic I n s t i t u t e "H.E. J o u l t o v s k y " m R u s s i a L l ] . I t concerned t h e f o r c e d h e a v i n g m o t i o n o f a s i m p l e m a t h e m a t i c a l s h i p f o r m a t zero speed o f advance.

The t e s t r e s u l t s showed f o r t h e f i r s t t i m e t h e f r e q u e n c y dependent damping and hydrodynamic mass a s s o c i a t e d w i t h t h e harmonic h e a v i n g m o t i o n o f a s h i p model i n a f r e e s u r f a c e . F i g u r e 1 g i v e s a schematic d r a w i n g o f t h e e x p e r i m e n t a l s e t up. A s o f t s p r i n g i s used as a

dynamometer; t h e compression o f t h e s p r i n g i s p r o p o r t i o n a l t o t h e e x c i t i n g h e a v i n g f o r c e .

I n a d d i t i o n t o t h e phase d i f f e r e n c e between upper and l o w e r s i d e o f the s p r i n g , s u f f i c i e n t i n f o r m a t i o n i s a v a i l a b l e t o d e t e r m i n e t h e t o t a l mass and t h e damping o f t h e system. The e x p e r i m e n t can be c a r r i e d o u t f o r a range o f f r e q u e n c i e s w h i c h i s o f i n t e r e s t f o r s h i p m o t i o n s i n waves.

Since 1950 s i m i l a r t e s t t e c h n i q u e s were d e v e l o p e d a t t h e D e l f t S h i p -b u i l d i n g L a -b o r a t o r y , t o s t u d y f u n d a m e n t a l a s p e c t s o f s h i p m o t i o n s i n

waves and, i n a l a t e r s t a g e , t h e s t e e r i n g and manoeuvring c h a r a c t e r i s t i c s o f s h i p s .

F i r s t o f a l l t h e d e s c r i b e d "Haskind and Rieman" system was extended t o i n c l u d e a c o u p l e heave- and p i t c h e x c i t a t i o n f o r 2,5 meter s u r f a c e s h i p m o d e l s , see F i g u r e 2. E x p e r i m e n t s w i t h S e r i e s S i x t y ship-models showed t h e i m p o r t a n c e o f t h e hydrodynamic c o u p l i n g e f f e c t s between heave and p i t c h and o f t h e damping c o e f f i c i e n t s . C 2 J .

A d e t a i l e d comparison w i t h s t r i p - t h e o r y c a l c u l a t i o n s was p o s s i b l e f o r each o f t h e c o e f f i c i e n t s o f t h e e q u a t i o n s o f m o t i o n . I n t h i s way a more r e f i n e d a n a l y s i s c o u l d be made o f t h e m e r i t s o f t h e s t r i p - t h e o r y t h a n a comparison o f c a l c u l a t e d and measured m o t i o n s .

S o f t s p r i n g o s c i l l a t o r s have t h e g r e a t advantage t h a t t h e dynamometer i s a l m o s t i n s e n s i t i v e f o r h i g h f r e q u e n c y v i b r a t i o n s o f t h e t o w i n g

-2-2 c a r r i a g e . However a d i s a d v a n t a g e i s t h e f r e q u e n c y dependent m o t i o n a m p l i t u d e o f t h e s h i p model a t c o n s t a n t o s c i l l a t o r a m p l i t u d e because o f t h e s m a l l s p r i n g c o n s t a n t . T h i s hampers more or l e s s t h e i n v e s t i g a t i o n o f n o n - l i n e a r i t i e s . A f u r t h e r d i s a d v a n t a g e i s t h e r a t h e r poor d e t e r m i n a t i o n o f p h a s e - d i f f e r e n c e s f r o m t h e g r a p h i c r e c o r d i n g s . Quadrature components o f hydrodynamic f o r c e s can be v e r y s m a l l a t low and h i g h f r e q u e n c i e s and c o n s e q u e n t l y an a c c u r a t e phase d e t e r m i n a t i o n i s r e q u i r e d . As an example t h e f r e q u e n c y dependent m o t i o n a m p l i t u d e i n t h e case o f a f o r c e d l a t e r a l m o t i o n i s shown i n F i g u r e 3 .

T h i s p a r t i c u l a r t e s t was c a r r i e d o u t t o a n a l y s e t h e b e h a v i o u r o f an

anchored s h i p i n waves, which was meant t o be used as a d r i l l i n g p l a t f o r m .

Forced o s c i l l a t i o n t e s t s w i t h p r a c t i c a l l y f r e q u e n c y independent a m p l i t u d e s are p o s s i b l e when s t i f f e l e c t r o n i c dynamometers a r e used. To a v o i d c o m p l i c a t e d d a t a r e d u c t i o n t h e n a t u r a l f r e q u e n c y o f t h e model-dynaiïïometer system s h o u l d be l a r g e r by a f a c t o r o f 5 t o 10 t h a n t h e

h i g h e s t f o r c e d f r e q u e n c y w h i c h i s o f i n t e r e s t . However t h i s may i n t r o d u c e a h i g h n o i s e l e v e l i n t h e m e a s u r i n g system due t o r e s o n a n t v i b r a t i o n s i n d u c e d by t h e t o w i n g c a r r i a g e .

The p r e s e n t e l e c t r o n i c system a t D e l f t S h i p b u i l d i n g L a b o r a t o r y employs a m u l t i p l i c a t i o n o f t h e measm-ed s i g n a l by s i n u t and coscot and an

i n t e g r a t i o n t o s e p a r a t e t h e a u p l i t u d e s o f t h e in-phase and t h e q u a d r a t u r e components o f t h e hydrodynajnic f o r c e s [ 3 ] .

T h i s F o u r i e r a n a l y s i s e l i m i n a t e s v e r y e f f e c t i v e l y t h e n o i s e f r o m t h e measurements b u t c r i t i c a l e x a m i n a t i o n i s necessary t o a v o i d c o n d i t i o n s where t h e n o i s e - s i g n a l r a t i o i s t o o l a r g e .

The e l e c t r o n i c d a t a r e d u c t i o n system can be used i n c o n n e c t i o n w i t h a v a r i e t y o f m e c h a n i c a l o s c i l l a t o r s .

A v a i l a b l e a r e two v e r t i c a l o s c i l l a t o r s f o r s u r f a c e s h i p s and submarines and one h o r i z o n t a l o s c i l l a t o r . A l s o a combined r o l l , sway and heave mechanism i s a v a i l a b l e t o t e s t a n t i - r o l l i n g t a n k s (see F i g u r e s k and 5 ) ,

A v a r i e t y o f t e s t s has been c a r r i e d o u t w i t h t h e d e s c r i b e d a p p a r a t u s . They i n c l u d e t h e e x p e r i m e n t a l d e t e r m i n a t i o n o f t h e c o e f f i c i e n t s o f t h e e q u a t i o n s o f m o t i o n f o r p i t c h , heave, sway, yaw, surge and r o l l f o r s u r f a c e s h i p s , d r i l l i n g p l a t f o r m s , h o v e r c r a f t s and submarines.

D e t a i l e d e x p e r i m e n t s were made and a r e s t i l l i n p r o g r e s s t o v e r i f y t h e o -r e t i c a l methods f o -r t h e c a l c u l a t i o n o f s h i p m o t i o n s i n waves. E x t e n s i v e s y s t e m a t i c t e s t i n g on p a s s i v e a n t i r o l l i n g t a n k s was p u b l i s h e d t o enable the p r a c t i c a l d e s i g n o f such d e v i c e s [h\

An example o f a c o m p l i c a t e d o s c i l l a t i o n t e s t concerns t h e e x p e r i m e n t a l

-3

d e t e r m i n a t i o n o f t h e d i s t r i b u t i o n o f damping and hydrodynamic mass a l o n g t h e l e n g t h o f an o s c i l l a t i n g shipmodel [ 5 j •

I n t h i s case a segmented model was used as shown i n F i g u r e 6,

Each o f t h e seven segments was connected t o a s t r o n g box g i r d e r by means o f a dynamometer. By n e g l e c t i n g t h e s m a l l i n t e r f e r e n c e e f f e c t s , between t h e segments, a mean v a l u e f o r damping and mass f o r each segment was found and a r e a s o n a b l e e s t i m a t e o f t h e d i s t r i b u t i o n o f t h e s e hydrodynamic q u a n t i t i e s o v e r t h e l e n g t h o f t h e s h i p model c o u l d be made (see f i g u r e

T)-A comparison o f a measured and c a l c u l a t e d d i s t r i b u t i o n o f a heave damping c o e f f i c i e n t and a heave damping c r o s s c o u p l i n g c o e f f i c i e n t i s shorn i n F i g u r e 8.

S i m i l a r work w i t h a segmented model i s now i n p r o g r e s s f o r l a t e r a l m o t i o n s . These t e s t s w i l l complete a s e r i e s o f e x p e r i m e n t s w i t h o s c i l l a t i n g c y l i n d e r s i n heave, r o l l and sway ^ 6 ] .

For a n a l y s i n g t h e r o l l i n g m o t i o n o f s h i p models i n t e r n a l o s c i l l a t o r s may be used. I n an e a r l y s t a g e o f t h e work use was made o f r o t a t i n g w e i g h t s t o e x c i t e r o l l i n g moments. Q u i t e c o m p l i c a t e d systems are needed t o a v o i d p a r a s i t i c moments and i t seems d i f f i c u l t t o e l i m i n a t e v a r y i n g mass moments o f i n e r t i a .

An e l e g a n t method f o r t h e g e n e r a t i o n o f a r o l l i n g moment i s t h e use o f the g y r o s c o p i c moment o f a r o t a t i n g g y r o s c o p e .

The yawing moment w h i c h w o u l d be p r e s e n t when one g y r o i s u s e d , can be e l i m i n a t e d by r o t a t i n g two gyroscopes (see Figiure 9 )

-A d i s a d v a n t a g e o f f r e e r u n n i n g models i s t h e more c o m p l i c a t e d measuring system o f t h e s h i p m o t i o n s . R o l l i n g has a s t r o n g c o u p l i n g w i t h swaying and c o n s e q u e n t l y swaying m o t i o n s have t o be measured and i n c l u d e d i n the a n a l y s i s o f t h e f o r c e d r o l l i n g e x p e r i m e n t .

T h i s s h o r t and i n c o m p l e t e s u r v e y o f t h e f o r c e d o s c i l l a t i o n work c a r r i e d out a t D e l f t shows t h a t a wide v a r i e t y o f e x p e r i m e n t s i s p o s s i b l e . I n p a r t i c u l a r f u n d a m e n t a l e x p e r i m e n t s can be c a r r i e d o u t t o check and s t i m u l a t e t h e o r e t i c a l methods f o r t h e c a l c u l a t i o n o f s h i p b e h a v i o u r w h i c h can bemused i n s t e a d o f more e m p i r i c a l approaches.

-k-k

R e f e r e n c e s .

[ l ] M.D. Haskind and I . S . Rieman,

" B u l l e t i n de l'Académie des Sciences des U.S.S.R.". Classe des Sciences t e c h n i q u e s - 19^6 no. 10

[2] J . G e r r i t sma, "An e x p e r i m e n t a l a n a l y s i s o f s h i p m o t i o n s i n l o n g i t u d i n a l r e g u l a r waves'.' I n t e r n a t i o n a l S h i p b u i l d i n g P r o g r e s s 1950. [ 3 ] H. J . Z u n d e r d o r p , M. B u i t e n h e k , " O s c i l l a t o r t e c h n i q u e s a t t h e S h i p b u i l d i n g L a b o r a t o r y " , R e p o r t no. I l l , S h i p b u i l d i n g L a b o r a t o r y , T e c h n o l o g i c a l U n i v e r s i t y D e l f t 1 9 6 3 . [h] J . J . v . d . Bosch, J.H. V u g t s ,

"On R o l l Damping by F r e e - S u r f a c e Tanks".

T r a n s a c t i o n s R o y a l I n s t i t u t e o f Naval A r c h i t e c t s I 9 6 6 .

[ 5 ] J . G e r r i t s m a , W. Beukelman,

" D i s t r i b u t i o n o f damping and added mass a l o n g t h e l e n g t h o f a s h i p model".

I n t e r n a t i o n a l S h i p b u i l d i n g P r o g r e s s 1 9 ^ 3 .

[ 6 ] J.H. V u g t s ,

"The hydrodynamic c o e f f i c i e n t s f o r s w a y i n g , h e a v i n g and r o l l i n g c y l i n d e r s i n a f r e e s u r f a c e " .

L i s t o f f i g u r e s :

F i g u r e 1: O s c i l l a t o r used by H a s k i n d and Rieman.

F i g u r e 2: S o f t S p r i n g O s c i l l a t o r

F i g u r e 3: Frequency response measured w i t h a s o f t s p r i n g o s c i l l a t o r .

F i g u r e k: Segmented model.

F i g u r e 5: A n t i - r o l l i n g t a n k o s c i l l a t o r .

F i g u r e 6: Arrangement o f o s c i l l a t i o n t e s t s w i t h segmented model.

F i g u r e 7: D i s t r i b u t i o n o f s e c t i o n a l damping c o e f f i c i e n t s a l o n g t h e l e n g t h o f a s h i p m o d e l .

F i g u r e 8: Comparison o f t h e c a l c u l a t e d d i s t r i b u t i o n o f mass and damping and c o r r e s p o n d i n g e x p e r i m e n t a l v a l u e s f o r a s h i p m o d e l .

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D i s t r i b u t i o n o f s e c t i o n a l d a m p i n g c o e f f i c i e n t s a l o n g t h e l e n g t h o f a s h i p m o d e l .

F I G U R E 8.

C o m p a r i s o n o f t h e c a l c u l a t e d d i s t r i b u t i o n o f mass a n d d a m p i and c o r r e s p o n d i n g e x p e r i m e n t a l v a l u e s f o r a s h i p m o d e l .