twu 1986 aquacultural-engineering

7/28/2019 Twu 1986 Aquacultural-Engineering

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Aquacultural E ngineering 5 (1986) 271-286

O n a W a v e D i s s ip a t i o n M e t h o d fo r O f f sh o r eA q u a c u l t u r e A r ea s

S . W . T w u , R . C . K a o a n d H . H . H w u n gHyd raulics and Ocean Engineering Depar tment and Graduate School, Cheng-KungUniversity, Tainan, Taiwan. ROC

A B S T R A C TThis pap er d escribes a propo sed f lexible f loat ing s tructure to provide amethod o f wave d i s s ipa t ion for an aquacu l ture area . 7he s t ruc ture i sm a i n l y m a d e o f net-tubes which are f i l led wi th f loat ing balls. An u m b er o f wa ys o f p l a cem en t f o r t h e f le x i b l e s tr u ct u re h a ve b eeninvest igated in hydraul ic m od el s tudies a nd the results are presen ted inthis paper . From these s tudies i t i s con chM ed that i ra cer tain nu m be r o frows o f these f loat ing s tructures are co nne cted wi th pro per spacings,then the co m ple x f loat ing s tructure wi l l serve as a go od wave shel ter ingsystem fo r an of fshore aquac ul ture area.

INTRODUCTIONW i t h t h e c o n t i n u e d g r o w t h o f i n d u s t r y a n d t h e s t e a d y i n c r e a s e o fp o p u l a t i o n i n t h e p a s t fe w d e c a d e s , T a i w a n h a s a s t r o n g f e e l in g o fd e f i c i e n c y o f r e s o u r c e s o f a l l k i n d s . I t i s d e s i r a b l e f o r t h e R e p u b l i c o fC h i n a ( R O C ) g o v e r n m e n t to t a k e m e a s u r e s t o o b t a in r e so u r c e s tos a t is f y th e i r n e e d s . O n e w a y i s t o e x t e n d f i s h e r ie s to d i s t a n t o c e a n s .B u t t h is g o a l h a s b e e n l i m i t e d b y t h e p o l i c ie s o f e c o n o m i c s e a a r e a s o ff o r e ig n c o u n t r i e s, T h u s f i s h e r y e x t e n s i o n p r o je c ts h a v e b e e n c o n f i n e dt o l o c a l w a t e r s , a n d o t h e r w a y s o f f i s h e r y d e v e l o p m e n t h a v e t o b es o u g h t. D e v e l o p m e n t o f n e a r - s h o r e a q u a c u l t u r e a r e as i n t h e v i ci n it y o fT a i w a n t h e r e f o r e t a k e s p l a c e . T h i s i s a fa i rl y g o o d i d e a , b u t a n o t h e rp r o b l e m a r is es . W a v e c o n d i t i o n s i n n e a r b y s e a a r e a s o f f T a i w a n a r eu n f a v o r a b l e f o r a q u a c u l t u r e e x p a n s i o n .

271Aquacultural Engineering 0144-8609/86/S03.50-- Elsevier Applied SciencePublishers Ltd, England, 1986. Printed in Great Britain


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2 7 2 S. W. Twu, R. C Kao, H. H. HwungThe Tropic of Cancer lies across the middle of Taiwan. The climateought to be subtropical. However, in winter the weather is colder incomparison with other localities of the same latitude. The reason isthat Mongolian anticylones occur in the vicinity of Lake Baikal everyfive or six days in the winter and move southeast. High pressure andsteep pressure gradients cause monsoons which blow over the EastChina Sea and Taiwan Strait and send cold air to Taiwan fromOctober to April. The duration of winds exceeding 5 m s- t has beenrecorded to be more than 50 days.On the other hand this island cannot escape tropical disasters.

Typhoons attack or approach Taiwan along various routes, as shownin Fig. 1, from early summer to October or even November. Calmwater can only be expected in April, May and June.

F ig . I . A p p r o a c h r o u te s o f T y p h o o n s ( 1 8 9 7 - 1 9 8 3 ) .


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O n a w ave d i s s ipa t i on m e t h od f o r o f f shore aqua cu l t u re a reas 2 7 3W i t h t h e e x c e p t i o n o f th e s o u t h e r n c o a s t, e v e r y c o a s t o f T a i w a n

s u ff e rs f r o m m o n s o o n w a v e s c o n t i n u o u s l y in th e w i n t e r. I n th is s e a s o ns i g n if ic a n t w a v e h e i g h t s a r e e s t i m a t e d t o b e 3 m o n a v e r a g e , w i t hp e r i o d s o f a r o u n d 1 0 s. I n s u m m e r , a l m o s t a ll t h e co a s ts o f T a i w a n a r es u b j e ct to t h e m e n a c e o f a t ta c k b y t y p h o o n - g e n e r a t e d w a v e s . D u r i n gt he t y p h o o n p e r i o d , i m m e n s e w a v e s e x c ee d i n g 1 0 m h a v e o c c u r r e dq u i t e o f t e n . T h e r e f o r e , t h e a q u a c u l t u r e a r e a s h a v e s u f f e r e d s e r i o u sw a v e a t ta c k p r o b l e m s . I f t h e a q u a c u l t u r e a r e a s a r e t o b e d e v e l o p e d ,s o m e w a v e d is s ip a t i o n m e a s u r e s m u s t b e t ak e n . F o r e c o n o m i c r e as o n sa n d b e c a u s e o f d e f i c i e n t p o r o s i t y , t h e tr a d i t i o n a l w a v e s h e l t e r i n g s t r u c -t u r e s s u c h a s b r e a k w a t e r s a r e n o t s u i t a b l e f o r w a v e r e d u c t i o n f o ra q u a c u l t u r e a r e a s. T h e r e f o r e , a n a r r a y o f n e t - t u b e s a n d b u o y a n t b a ll sh a s b e e n p r e s e n t e d f o r t h i s p u r p o s e . I n c o m p a r i s o n w i t h b r e a k w a t e r s ,t h e a r ra y o f n e t - tu b e s a n d b u o y a n t b a lls d o e s n o t h a v e a s g o o d a w a v er e d u c i n g c a p a b il it y b e c a u s e o f h i g h p o r o s it y . H o w e v e r , s o m e m e t h o d sf o r im p r o v i n g w a v e d i s si p a ti o n c a n b e p r o v i d e d . A c o u p l e o f t h e s em e t h o d s w i ll b e d i s c u s s e d i n t hi s p a p e r .

T H E O R E T I C A L C O N S I D E R A T I O N ST h e o r e t i c a ll y , s o m e p r o p e r t i e s o f th e a r r a y o f n e t - tu b e s a n d b u o y a n tb a ll s, s u c h a s d r a u g h t a n d t h e n a t u r a l p e r i o d o f o s c i l la t i o n , a r e c o n -s i d e r e d a s h a v i n g v i s i b l e i n f l u e n c e o n w a v e d i s s i p a t i o n a b i l i t y . F o re x a m p l e , a c c o r d i n g t o A i r y ' s w a v e t h e o r y , 8 0 % o f t h e w a v e e n e r g y i sd i s t ri b u t e d w i t h in o n e f o u r t h o f t h e d e p t h f r o m t h e s u r fa c e . S o s t ru c -t u r es i n th e u p p e r p a r t o f th e o c e a n w i ll d i s si p a te l a r g e r a m o u n t s o fe n e r g y t h a n t h o s e i n th e l o w e r p a r t. T h e r e f o r e , t h e d r a u g h t d e p t h o f as t r u c t u r e i n w a t e r i s c o n s i d e r e d t o b e a n i m p o r t a n t f a c t o r i n i t s w a v ed i s s i pa t i on a b il it y .

W a v e s b e h i n d t h e m o v a b l e p e r f o r a t e d s t r u c t u r e a r e g e n e r a t e d b yt w o s o u r c e s : o n e is t h e d i r e c t t r a n s m i s s i o n o f w a v e s a n d a n o t h e r is t h ew a v e s m a d e b y o s c i l l a t io n o f t h e s t r u c t u r e . S o i f t h e r e is a p h a s e l a g o fj u st ~ d e g r e e s b e t w e e n i n c i d e n t w a v e s a n d t h e o s c il la t io n o f th e s tr u c -t u re , t h e w a v e c r e s t o f th e f o r m e r w ill m e e t w i t h t h e w a v e t r o u g h o f th el a t t e r . A s m a l l w a v e o r e v e n c a l m w a t e r s u r f a c e , d e p e n d i n g o n t h er e l a t i v e w a v e h e i g h t , w i l l b e p r e d i c t e d . G e n e r a l l y s p e a k i n g , t h e r e i sa l w a y s a t i m e l a g b e t w e e n t h e i n c i d e n t w a v e s a n d t h e o s c i l l a ti o n o f t h e


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2 7 4 S . W . T w u , R . C K a o , H . H . H w u n gs tr u c tu r e . T h e a m o u n t o f t im e l a g d e p e n d s m a i n l y o n t h e r a ti o o f t h en a t u r a l p e r i o d o f th e s t r u c tu r e (T n) t o t h e p e r i o d o f i n c i d e n t w a v e ( T )a n d a l so o n t h e r e s is t a n t f o rc e t o m o v e m e n t o f t h e s tr u c tu r e . T h e r e la -t i o n s h i p o f p h a s e l a g t o T,/T and r e s i s t an t f o r ce i s s how n i n F i g . 2 .H o w e v e r , t h e n a t u r a l p e r i o d o f t h e s t r u c t u r e h a s a c l o s e r e l a t io n t o i tsm a s s a n d r i g i d i t y . A c c o r d i n g t o t h e o ~ ' , t o i n c r e a s e m a s s w i t h o t h e rf a c t o r s r e m a i n i n g c o n s t a n t w i l l i n c r e a s e t h e n a t u r a l p e r i o d o f t h es t ru c t u r e , a n d t h e n t h e p h a s e a n g l e in c r e a se s . H o w e v e r , w h e n t h e m a s so f ba l l s i s i nc r eas ed , i t s r e t u r n i ng f o r ce , i . e . buoyan t f o r ce mi nus i t sw e i g h t , i s d e c r e a s e d . T h i s m e a n s i t s r e s i s t a n t f o r c e i s d e c r e a s e d t o o ,a n d t h e d e c r e a s e d r e s i s t a n t f o r c e w i l l r e s u l t i n a d e c r e a s i n g o f t h ep h a s e a n g l e w h e n t h e p h a s e a n g l e is s m a l l e r t h a n 7 r/2. F u r t h e r m o r e ,t h e p h a s e a n g l e w i l l h a v e a d i r e c t e f f e c t o n t h e d a m p i n g e f f i c i e n c y .T h e r e f o r e , i f t h e s p e c i f i c g r a v i ty o f t h e b u o y a n t b a ll s is r e d u c e d , it isn o t c l e a r w h e t h e r t h e w a v e d a m p i n g e f fi ci e n cy w i ll b e i n c r e a s e d o rd e c r e a s e d . T h i s is a d e b a t a b l e p r o b l e m a n d w i ll b e e x a m i n e d i n t h ee x p e r i m e n t a l s t u d i e s l a te r.

I f a s t r uc t u r e i n f l u i d i s t o be acce l e r a t ed , a f o r ce mus t be g i ven t ot h e s t r u c t u r e a n d t h e f l u i d a r o u n d it. T h i s c a n b e p r e s e n t e d a s

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O n a w ave d i s s ipa t i on m e t h od f o r o f f shore aqua cu l t u re a reas 2 7 5in w h i c h F i s t h e a c t i n g fo r c e , a t h e a c c e l e r a t i o n o f th e s t r u c t u r e , m t h em a s s o f t h e s t r u c t u r e a n d r n' t h e a d d e d m a s s . m ' h a s a c lo s e r e l a ti o n tot h e s h a p e o f t h e s t ru c t u r e . F o r e x a m p l e , a s p h e r i c a l b o d y h a s a n a d d e dm a s s o f o n e h a l f t h e m a s s o f f lu i d d i s p l a c e d b y t h e s p h e r e , a n d b o d i e so f o t h e r s h a p e s w i ll h a v e a l a r g e r a d d e d m a s s t h a n t h e s p h e r i c a l s h a p e .A c c o r d i n g t o t h e p r e v i o u s e q u a t i o n , it" t h e s a m e w a v e f o r c e is ac t in g ,t h e b o d y' w i t h l a r g e r a d d e d m a s s w i ll s h o w a n i n c r e a s e o f p h a s e a n g l ea n d t h u s i n c r e a s e t h e w a v e d i s s i p a t i o n e f f ic ie n c y .

T h e o r e t i c a l l y t w o o r t h r e e r o w s o f a rr a y s o f n e t t u b e s a n d f lo a t in gb a ll s s h o u l d h a v e a b e t t e r d i s s i p a t i o n a b i li ty t h a n a s i n g l e ro w . I f t h et w o o r t h r e e r o w s o f d e v i c e s a r e c o n n e c t e d b y w i r e s a n d t h e s p a c i n gb e t w e e n t h e m is k e p t a s o n e h a l f o f th e w a v e l e n g t h , t h e b e s t d i ss i p a -t i o n e f fi c ie n c y s h o u l d b e o b t a i n e d . T h e r e a s o n is t h a t t h e w a t e r p a r -t ic l es at t h e w a v e c r e s t s e c t i o n a r e p r o g r e s s i n g i n t h e d i r e c t i o n o f t h ew a v e ' s a d v a n c e , b u t t h e w a t e r p a r t ic l e s a t t h e w a v e t r o u g h s e c t i o n a r ep r o g r e s s i n g i n th e r e v e r s e d i r e c t i o n . S o i f t w o r o w s o f d e v ic e s a r ev e r ti c a ll y f lo a t e d in w a t e r a n d s p a c e d w i t h o n e h a l f o f t h e w a v e l e n g th ,w i t h w i r e c o n n e c t i n g t h e m , t h e n w h i l e o n e r o w i s a c t i n g w i t h f o r w a r df o r c e , t h e o t h e r i s a c t i n g w i t h b a c k w a r d f o r c e , a n d s o t h e y o f f s e t e a c ho t h e r . I n t h is s i t u a t i o n t h e s e t w o r o w s o f f l o a t i n g s t r u c t u r e s f u n c t i o nj u s t l i ke a fL xed b o d y a n d t h e d i s s i p a t i o n a b i li t y is i n c r e a s e d .

I f t h e c o m b i n a t i o n o f t w o r o w s o f th e s e f lo a t in g s t ru c t u r e s w e r eu s e d t o p r o t e c t t h e a q u a c u l t u r e a r e a f r o m l a r g e w a v e a t t a c k , a n dt h e s e t w o r o w s w e r e c o n n e c t e d b y w i r e s t o k e e p a c e r t a i n d i s t a n c eb e t w e e n t h e m , t h e n t hi s j o i n e d s t r u c t u r e w o u l d h a v e a b e tt e r w a v ed i s s i p a t i o n e ff e c t f o r th e w a v e s o f a c e r t a in p e r i o d , c o r r e s p o n d i n g t ot h e s p a c i n g b e t w e e n t h e t w o r o w s o f a r ra y , t h a n t o t h e w a v e s o f o t h e rp e r i o d s . H o w e v e r , t h e r e a r e w a v e s c o v e r i n g a w i d e r a n g e o f p e r i o d s int h e o p e n s e a a n d t h e s p a c i n g b e t w e e n t h e r o w s o f t h e s tr u c t u r e s c o u l dn o t b e a d j u s t e d a s t h e w a v e p e r i o d s c h a n g e . T o s o l v e t h i s p r o b l e m , as e r i e s o f a c e r t a i n n u m b e r o f r o w s o f t h e f le x i bl e s t r u c t u r e s , s a y fi v e o rs ix , c a n b e p l a c e d , w i t h s p a c i n g s s e l e c t e d a c c o r d i n g t o t h e p r o p e r w a v ep e r i o d o c c u r r i n g in th e a q u a c u l t u r e a r e a . T h e w a v e l e n g t h c a n b ec a l c u l a t e d b y

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276 S. la' i Twu, R. C. Kao . H. H. H w u n go r b y f ir st c a lc u l a ti n g t h e w a v e l e n g t h in d e e p w a t e r ( L , ) b y

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t h e n f i n d i n g t h e d /L v a l u e i n t h e t a b l e f r o m t h e k n o w n v a l u e o f d/L~,w h e r e T is t h e w a v e p e r i o d a n d d a n d L t h e w a t e r d e p t h a n d w a v el e n g t h r e s p e c t i v e l y , i n t h e a r e a . S o w h e n t h e N p e r i o d s , s a y T ~ ,T 2 .. .. TN , a r e s e l e c t e d a s th e d e s i g n w a v e p e r i o d s , t h e n t h e c o r r e -s p o n d i n g w a v e le n g th s , s a y L l, L : , . . . , L x , c a n b e c a l c u la t e d . I n o r d e rt o g ai n a h ig h d a m p i n g e f f ic i en c y , N + I r o w s o f t h e f lo a t i n g s t r u c t u r e s ,w i t h s p a c i n g L ~ / 2 , L 2 / 2 . . . . , L x ~2 b e t w e e n e a c h t w o r o w s , c a n b ep l a c e d . A l l t h e s e p r e v i o u s t h e o r e t i c a l c o n s i d e r a t i o n s h a v e b e e ns t u d i e d i n t h e m o d e l t es t a n d a r e p r e s e n t e d i n t h e fo l l o w i n g s e c t io n .

E X P E R I M E N T A L S T U D I E ST h e e x p e r i m e n t a l s t u d ie s o n t h e w a v e d a m p i n g e f fi c ie n c y o f th e a r ra yo f n e t - t u b e s a n d f l o a t i n g b a l l s h a v e b e e n c a r r i e d o u t i n t w o f l u m e s .T h e f ir st fl u m e is m a d e e n t ir e ly o f c o n c r e t e , w i th d i m e n s i o n s o f 5 0 ml e n g t h , 1.0 m w i d t h a n d 1 .0 m d e p t h . T h e s e c o n d f l u m e is m a d e o fc o n c r e t e w i t h d i m e n s i o n s o f 7 5 m l e n g th , 1"0 m w i d t h a n d 1"2 md e p t h . O n e s i d e o f it w a s c a s t w i t h a s h e e t o f g l as s 3 6 m l o n g f o ro b s e r v i n g t h e e x p e r i m e n t s . B o t h t h e f l u m e s h a v e a w a v e g e n e r a t o rw i t h a n A C m o t o r o f 7 .5 H P , b y w h i c h a w a v e p a d d l e is o s c i l la t e dt h r o u g h a l in k a g e , c o n t r o l l e d b y a c o n t i n u o u s s p e e d c h a n g e r . T h ew a v e h e i g h t s a n d p e r i o d s c a n b e o b t a i n e d b y a d j u s t m e n t . T h e w a v ep e r i o d s g e n e r a t e d r a n g e f r o m 0"6 s t o 4 .0 s .

A w a v e r e c o r d e r c o n t a in s a 6 - c h a n n e l p e n o s c i l l o g r a p h w i th am p l i -f i e r w h i c h c a n c o n t i n u o u s l y r e c o r d w a t e r s u r f a c e o s c i l l a t i o n d e t e c t e db y p a r a l l e l w i r e , r e s i s t a n c e t y p e w a v e g a u g e s . T h e s e w a v e g a u g e s c a nt r a n s f e r t h e v a r i a t io n o f w a v e h e i g h ts i n t o t h e v a r i a t i o n o f e l e c t ri cc u r r e n t , t h e n r e c o r d i t b y w a v e r e c o r d e r a f t e r i t h a s b e e n m a g n i f i e da n d f i l t e r e d . T h r o u g h t h e s i g n a l v a r i a t i o n r e c o r d e d b y t h e w a v er e c o r d e r , t h e w a v e a m p l i t u d e c a n b e e s t i m a t e d b y r a ti n g cu r v e .T h e w a v e f lu m e a n d t h e m o d e l a r e s h o w n in F ig . 3.


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q = 0 . 4~ = 0 . 3

s I E I I I - d / L0 I 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6

R e l a t i o n s h i p b e t w e e n w a v e t r a n s m i s s i o n c o e f f ic i e n t a n d r e la t iv e d e p t h .

F i g . 7 .

K1 . 0 x x ,,

0 . 9 x " , x

0 . 8 ' : "

0 . 7 q = 0 . 5) " b = 0 . 3

0.~ 01 I r I L L d /L I 0 . 2 0 . 3 0 , 4 0 . 5 0 . 6R e l a t i o n s h i p b e t w e e n w a v e t r a n s m i s s i o n c o e f f i ci e n t a n d r e la t iv e d e p t h .

Fig. 8.

K

1 . 0

0 . 9

0 . 8

0 . 7

X X XX X ~k. x

q = 0 . 6; , ' b = 0 . 3

0 . 6 , a I ~ i I d / LO . I 0 . 2 0 . 3 0 . 4 0 , 5 0 . 6

R e l a t i onsh i p bep, veen ,,,,ave t r ansm i s s i on coe f f i c i en t an d r e l a ti ve de p t h .


10/16

2 8 0 S. 144. Tw u, R. C Kao, H. H. HwungT h e n e x t p r o b l e m t o b e s t u d i e d i n t h e h y d r a u l i c m o d e l w a s t h e

e f f e c t o f t h e s p e c i f i c g ra v i ty o f t h e f l o a t in g b a ll s o n t h e w a v e d a m p i n gabi l i ty . F o ur ca ses of sp ec if ic grav i ty , Yb = 0-9, Yb = 0.7, ?'b = 0-5 an dYb = 0 -3 h a v e b e e n t e s t e d a n d t h e r e s u lt s a r e s h o w n b y s t ra i g h t l in e s inF i g s 9 , 1 0 a n d 1 1 . T h e s e f i g u r e s s h o w t h a t t h e r e l a t i o n s k i p o f w a v ed a m p i n g a b i l it y t o s p e c i f ic g r a v it y o f t h e f l o a t in g s t r u c t u r e s is n o t a sg o o d a s e x p e c t e d . H o w e v e r , t h e y s t i l l s h o w a r o u g h t r e n d t h a t t h ef l oa t i ng s t r uc t u r e s , hav i ng l ow s pec i f i c g r av i t y , s ha r e a s ma l l w avet r a n s m i s s i o n c o e f f i c i e n t i n s h a l l o w w a t e r .T h e t h i r d p r o b l e m t o b e d i s c u s s e d is th e r e l a t i o n s h i p o f t h e tr a n s-m i s s i o n c o e f f i c i e n t w i t h w a v e s t e e p n e s s , w h i c h is t h e r a ti o o f w a v e

Fig , 9 .

K1.00 .90 . 8

0 .70 .6

q ,= 0. 9~ b= 0 .9

Yb= 0.3T b = 0 .7Yb= 0.5

J i t i i ' - d /L0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6

R e l a t i o n s h i p o f s p e c i fi c g r a v i t y o f f le x ib le s t r u c t u r e t o w a v e t r a n s m i s s i o ncoe f f i c i en t .

K1.00 .90 .80 .70 .6

q = 0 . 5 Yb = 0 - 5

O . I 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6

Fi g . 10 . R e l a t i onsh i p o f spec i f i c g rav i t y o f f lex i b le s t ruc t u re t o w a ve t r ansm i s s i oncoe f f i c i en t .


11/16

O n a w a v e d i s si p a t io n r n e t h o d f o r o f f s h o r e a q u a c u l t u r e a r e a s 28 1

F i g . 1 1 .

K

0.90.80.70.60.5

~ ; b 0"6 70.3",0.5

I i I i I I ~ d/L0 . I 0 . 2 0 . . 3 0 . 4 0 . 5 0 . 6

Rela t ionship of spec if ic gravi ty of f lexible s t ruc ture to wave t ransmiss ioncoefficient.

h e i g h t t o w a v e l e n g t h H / L . T h e e x p e r i m e n t a l d a t a h a v e b e e n p l o t t e da s F i g s 1 2 , 1 3 a n d 1 4 . T h e s e f i g u re s s h o w t h a t th e w a v e t r a n s m i s s i o nc o e f f i c i e n t d e c r e a s e s w i t h t h e i n c r e a s e o f w a v e s t e e p n e s s . T h i s m e a n st h a t t h e f lo a t i n g s t r u c t u r e s h a v e a b e t te r w a v e d a m p i n g a b i l i ty f o r t h el a rg e r w a v e s , a n d t h i s p h e n o m e n o n w i l l b e n e f i t t h e a q u a c u l t u r e a r e a s.B e c a u s e i t is f l ex i b le , th e f l o a t i n g n e t - t u b e s t r u c t u r e is a l w a y s w a v e da s t h e a t t a c k i n g w a v e s p a s s . T h e s t r u c t u r e ' s o s c i l la t io n s m a y d i m i n i s hi t s w a v e d i s s i p a t i o n e f f i c ie n c y . A n i n v e s t ig a t i o n o f f u n c t i o n s o f th ew i r e c o n n e c t o r s b e t w e e n t w o r o w s o f th is f l o a t in g s t r u c t u r e h a s b e e n

K1.00.9.0.80.70.60.5

q=o.5 yb= 0.5

2 4 6 x I0 2Fig. 12. Re la t ionsh ip be tw een wave t ransm iss ion coeffic ient and wave s teepness .


12/16

2 8 2 S. W. Twu, R. C. Kao, H. H. Hwung

F i g . 1 3 .

Kl. O

0 . 9

0 . 8

0 . 7

0 . 6

o

q - 0 . 47b= 0.5

' ; ' , ~ ' ; ' l , ' ' , o x , o , * " ' / "R e l a t i o n s h i p b e t w e e n w a v e tr a n s m i s s i o n c o e f f i c i e n t a n d w a v e s t ee p n e s s .

F i g . 1 4 .

K1 .0

0 . 9

0 .8

0 . 7

0 . 6

0 . 5

8 , o ' D ~ o

q - 0.6 ; b " 0 . 5

i i i0. 4 I ~ I 4 ; ~ ' I " H i / LI O x l O zR e l a t i o n s h i p b e t w e e n w a v e t r a n s m i s s i o n c o e f f i c ie n t a n d w a v e s t e ep n e s s .

p e r f o r m e d . I n t h is st u d y t w o r o w s o f f l o a t in g s t r u c t u r e s h a v e b e e np l a c e d i n t h e w a v e f l u m e w i th s p a c i n g o f o n e h a l f o f t h e w a v e l e n gt h .T w o c a s e s h a v e b e e n t e s t e d . T h e y a r e w i t h w i r e s a n d w i t h o u t w i r e sc o n n e c t i n g t h e t w o r o w s o f s t r u c t u r e , r e s p e c t iv e l y , a s i n F ig s 4 (c ) a n d(d ). T h e r e s u lt s w i th w i r e s h a v e b e e n s h o w n i n F i g. 1 5 a n d t h o s e w i t h -o u t w i r e s in F ig . 1 6. T h e c o m p a r i s o n h a s b e e n m a d e i n F i g . 1 7. It h a sb e e n p r o v e d t h a t t h e c o n n e c t i n g w ir e s i n c r e a s e th e w a v e d a m p i n gab i l it y o f t he f l oa t i ng s t r uc t u r e .T h e s e t w o r o w s o f f l o a t in g s t r u c t u r e s, w i t h a c e r t a i n s p a c i n g a n dw i re s c o n n e c t i n g th e m , w i ll h a v e b e t t e r w a v e d a m p i n g a b il it y o n l y f o r


13/16

O n a w a v e d B s i p a t io n m e t h o d f o r o f fs h o r e a q u a c u i m r e a r ea s 2 8 3

F i g . 1 5 .

K1.00 .90 .80 .70 .6

B = 2S / L = I / 2

q = 0 . 6

O ' ' ' i ' ~ ' " d /L. I 0 , 2 0 3 0 . 4 5 0 . 6W a v e t r a n s m i s s i o n c o e f fi c i en t t h r o u g h t w o r o w s o f f l o a ti n g s t r u c t u r e s w i t h

w i r e s c o n n e c t e d .

F i g . 1 6 .

KI . O

Io . s i

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I " B=2

e ~ q = 0 . 6_ . . . . .L ~ - . o . ~-

_ ! . Jo . , 0 . 2 o i ~ o i ~ o . ~ o 1 6 = ( : I L LW a v e t r a n s m i s s i o n c o e f f i c i e n t t h r o u g h t w o r o w s o f f l o a t i n g s t r u c t u r e s

without wires c o n n e c t e d .

F i g . 1 7 .

K1.00.9 B = 2

~.~/( ,b)~'-a) SIL = I120 . 8 ~ q = 0 . 6--% b= o.30.7

0 . 6 0 1 ~ , A , ~ f I ~ d / L. i 0 . 2 0 . 3 o . 4 0 . 5 o 6C o m p a r i s o n o f w a v e t r a n s m i s s i o n c o e f f ic i e n t b e t w e e n t h e c a se s ~a) w i t h o u t

w i r e s a n d ( b ) w it h w i r e s .


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2 8 4 S. Vvl Twu, R . C. K a o . H . H . H ~ n gw a v e s o f a c e r t a i n p e r i o d . F o r w a v e s o f o t h e r p e r i o d s , t h e c o n n e c t i n gw i r e s d o n o t c o n t r i b u t e t o t h e d a m p i n g e f f i ci e n c y . A c t u a l l y , i n t h eo p e n s ea , t h e a q u a c u l t u r e a r e a w i ll e n c o u n t e r w a v e s o f a w i d e ra n g e o fp e r i o d s a n d n o t a s p e c i f i c p e r i o d . T o f a c e t h i s p r o b l e m , a c e r t a i nn u m b e r o f p e r io d s , s a y T~, T , , . . . , T .,,. m a y b e c h o s e n , a c c o r d i n g tot h e d i s tr i b u t io n c o n d i t i o n o f w a v e p e r i o d s a t t h e a r ea . T h e n t h e c o r r e -s p o n d i n g w a v e l e n g t h s, s ay L ~, L , _ , . . . , L x , a r e c a l c u l a te d . T a k e h a l fo f t h e s e w a v e l e n g t h s , L [ / 2 , L , _/2 . . . . L ; . /2 , a s t he s pa c i ngs o f t heN + I r o w s o f fl o a ti n g st r u c tu r e s . T h e n th is c o m p l e x s e t o f f lo a ti n gs t r u c t u r e s i s c o n s i d e r e d t o f u n c t i o n w e l l f o r a l l p e r i o d s , i n a c e r t a i nr a n g e , o f w a v e s . I n t h is p a p e r , f o u r w a v e p e r i o d s , 0 -8 , 1 - 2 , 1 .6 a n d1 .8 s, w e r e c h o s e n a s t h e d e s i g n w a v e p e r i o d s . S o f iv e r o w s o f th ef lo a t in g s tr u c t u r e s w e r e p l a c e d i n t h e w a v e f l u m e a n d w e r e c o n n e c t e dw i t h w i r e s , w i t h s p a c i n g s L ~ / 2 , L 2 / 2 , L 3 / 2 a n d L 4 / 2 b e t w e e n t h e m .A f t e r a s e r ie s o f m o d e l t e st s o f w a v e p e r i o d s r a n N n g f r o m 0 "8 t o 2"0 s,t h e r e s u l t s w e r e p l o t t e d i n F i g . 1 8 a s r e p r e s e n t e d b y t h e s o l i d l i n ec u r v e . I n t h i s f i g u r e a n o t h e r c u r v e , i n a b r o k e n l i n e , r e p r e s e n t s t h er e s u lt s o f th e s a m e f l o a t in g s t r u c t u r e s b u t w i t h o u t t h e c o n n e c t i n g w i r e sb e t w e e n t h e m .

0.9K 0 . 8

0 . 70 60 . 50 .40 . 30 . 20 . t

F i g . 1 8 .

0 .3 1 2 0 . 2 t 9 0 . 1 6 7 0 .1 3 9 0 . 11 9 0 . 1 0 4 0 . 0 9 2 , . . . . . . - d / L

0... O~

0 0 o0

0

o 0oo

0o0 o

o

x

0 . . . . w i t h o u t w i r e sx ~ w i t h ~ l r e s

[ l [ , L L , [ 1 L I I I I ' = m0 .8 I .O 1 .2 1 .4 1 .6 1 .8 2 .0

W a v e t r a n s m i s s i o n t h r o u g h f iv e r o w s o f f lo a t i n g s t ru c t u re s .


15/16

O n a w a v e d is s ip a t io n m e t h o d f o r o f f s ho r e a q u a c u l t u re a r ea s 285D I S C U S S I O N A N D C O N C L U S I O N S

( 1 ) I f t h e f lo a t in g s t r u c t u r e p r e s e n t e d i n t hi s p a p e r is to b e u s e d i n a na q u a c u l t u r e a r e a t o d a m p i n v a s i v e w a v e s , i t is b e t t e r t o p l a c e th e mv e r t ic a l ly t h a n h o r i z o n t a l ly .

( 2) F r o m F i g s 5 , 6 . 7 a n d 8 , w e r e a l i z e t h at , f o r v e r t ic a l p l a c i n g o f t h es t r u c t u r e , t h e d e e p e r t h e s i t e w h e r e t h e s t r u c t u r e i s p l a c e d , t h el o w e r t h e w a v e t r a n s m i s s i o n c o e f f i c i e n t o b t a i n e d , if h / d is k e p tc o n s t a n t. B u t th is d o e s n o t m e a n t h a t it is m o r e e c o n o m i c to p l a cet h e m i n d e e p e r w a t e r t h a n i n s h a l l o w w a t e r , b e c a u s e t h e d r a u g h th , w h i c h i n d i c a t e s t h e a m o u n t o f m a t e r i a l , w i ll b e i n c r e a s e d a s t h ew a t e r d e p t h is i n c r e a s e d w h e n h /d is k e p t c o n s t a n t .

( 3 ) T h e s p e c i f i c g r a v i t y o f t h e f l o a t i n g s t r u c t u r e p r e s e n t e d i n t h i sp a p e r is n o t a d o m i n a n t f a c t o r in t h e w a v e d a m p i n g a b il it y o f th es t r u c t u r e .

(4 ) T h e w a v e t r a n s m i s s i o n t h r o u g h t hi s f lo a ti n g s t r u c t u r e d e c r e a s e s ast h e w a v e s t e e p n e s s , H / L , i n c r e a s e s . T h i s m e a n s t h a t t h e f l o a t i n gn e t - t u b e s h a v e a b e t t e r w a v e d a m p i n g a b i li ty f o r t h e l a r g e r w a v e s ,a nd i t is e a s y t o s e e t h a t t he s e c ha r a c t e r i s t i c s w i ll do w e l l i n t hea q u a c u l t u r e a r e a s.( 5) F o r a c e r t a i n w a v e l en g t h , L , t h e t w o r o w s o f f l o a ti n g s t r u c t u r e sw i t h s p a c i n g L / 2 b e t w e e n t h e m a n d w i re s c o n n e c t in g th e t w or o w s w ill h a v e a b e t t e r w a v e d a m p i n g e f fi c ie n c y t h a n t h o s e w i th -o u t t h e c o n n e c t i n g w i r es .

( 6) A c t u a l l y in a p p l i c a ti o n t o a n a q u a c u l t u r e a r e a , w e c a n c h o o s e ap r o p e r n u m b e r o f w a v e l e n g t h s , s a y L ~, L_~ . . . , L , ~ , w h i c h a r eb a s e d o n t h e w a v e c o n d i t io n s a t t h a t a r ea . T h e n N + I r o w s o f t hef l o a ti n g s t r u c t u r e s a s p r e s e n t e d i n th i s p a p e r , w i t h s p a c i n g s o fL I / 2 , L 2 / 2 . . . . . L ,~ /2 b e t w e e n e a c h t w o a n d c o n n e c t i o n s w i th w ir esa s i n F i g . 4 ( e ) , c a n b e p l a c e d b e f o r e t h e s i t e o f t h e a q u a c u l t u r ea r e a . T h i s c o m p l e x s t r u c t u r e w i ll s e r v e a s a g o o d w a v e s h e l t e r in gs t r u c t u r e f o r t h a t a r e a .

A C K N O W L E D G M E N TT h i s i n v e s ti g a ti o n w a s s u p p o r t e d b y th e C o u n c i l o f A g r i c u l tu r e R O C .A p p r e c i a t i o n is e x p r e s s e d t o M r H . C . T s a o , e n g i n e e r o f t h e B u r e a u o f


16/16

2 8 6 S. w. T wu, R. C. Kao. H. H. HwungF i s h e r i e s , T a i w a n , f o r s e v e r a l i n f o r m a t i v e c o n v e r s a t i o n s o n f l o a t in gs t r u c t u r e s .

B I B L I O G R A P H YG i le s , M . L . (1 9 7 9 ) . D e t e r m i n a t i o n o f m o o r i n g l o a d s a n d w a v e t r a n sm i s si o nf o r a f l o a t i n g t i r e b r e a k w a t e r . Special i ty Co nf on Coa stal S tructures,A l e x a n d r i a , V i rg i n ia , 2 , 1 0 6 9 - 8 6 .H a r r i s , A . J . & W e b b e r , N . B . ( 1 9 6 8 ) . A f l o a t i n g b r e a k w a t e r . Proc. l l th Conf .on Coastal Engineering, 2 , 1 0 4 9 - 5 4 .I p p e n , A . T . ( 1 9 6 6 ) , Estuary and Coas t l ine Hydrodynarn ics , M c G r a w - H i l l ,N e w Y o r k .K a t o , J., H a g i n o , S . & U e k c t a , Y . ( 1 9 6 6 ) . D a m p i n g e f f e c t o f fl o a t i n g b r e a k -w a t e r t o w h i c h a n t i -r o l l in g s y s t e m is a p p l i e d . Proc. lOth Conf. on CoastalEngineering, 2 , 1 0 6 8 - 7 8 .K e n n e d y , R . J. & M a r s a l e k , J . (1 9 6 8 ) . F l e x i b l e p o r o u s f l o a ti n g b r e a k - w a t e r s .Proc. l l th Conf. on Co astal Engineering , 2 , 1 0 9 5 - 1 0 3 .M a r k s , W . ( 1 9 6 6 ) . A p e r f o r a t e d m o b i l e b r e a k w a t e r f o r fi x e d a n d f l o a t in ga p p l i c a t i o n s . Proc. lOth Conf. on Coastal Engineering, 2 , 1 0 7 9 - 1 2 9 .T e r r e t t , F. L . ( 1 9 6 6 ) . M o d e l s t u d i e s o f a p e r f o r a t e d b r e a k w a t e r . Proc. l l thC on f on C oas ta l Eng ineer ing , 2 , 1 1 0 4 - 2 0 .W i e g e l , R . L . ( 1 9 6 0 ) . T r a n s m i s s i o n o f w a v e p a s t a r i g i d v e r t i c a l th i n b a r r i e r . J.Waterways and H arbors D ivis ion, 86 , I - 12 .W i e g e l , R . L . & C h e n , K . ( 1 9 7 0 ) . F l o a t i n g b r e a k w a t e r f o r r e s e r v o i r m a r i n a s .Proc. 12th Conf. on Coastal Engineering, 3 , 1 6 4 7 - 6 6 .

twu 1986 aquacultural-engineering

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