bhattacharyya, jacket launch barge test

7/27/2019 Bhattacharyya, Jacket Launch Barge Test

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Ocean Engng, Vo l . 12 , No . 2 , pp . 161-17 6, 1985. 0029--8018/85 $3 .00 + .00P r i n t e d i n G r e a t B r i t a i n . ( ~) 1 98 5 P e r g a m o n P r e s s L t d .

A N E X P E R I M E N T A L A P P R O A C H T O E V A L U A T E T H ES T R U C T U R A L P E R F O R M A N C E O F A J A C K E T L A U N C HB A R G E *V . G . I D IC H A N D Y , '~ N IT I N D R A R . J O G L E K A R ~ a n d S U B R OT O K U M A R BH A T TA C H A R YY A

O c e a n E n g i n e e r i n g C e n t r e , I n d i a n I n s t i t u te o f T e c h n o l o g y , M a d r a s 6 0 0 0 3 6 , I n d iaA b s t r a e i - - T h e s t r u c t u r a l p e r f o r m a n c e o f a l a u n c h b a rg e i n l o a d - o u t a n d l a u n c h i n g o f a n o f f s h o r es t e e l j a c k e t i s t h e c r u c i a l c r i t e r io n o f i t s d e s i g n . T h e o v e r a l l b e n d i n g o f t h e h u l l g i r d e r s u b j e c t e d t ot h e d e s i g n l o a d f o r m t h e m a i n t h e m e o f t h e s t ud y . I n t h is p a p e r , a p p r o a c h o f p h y s i ca l m o d e l l i n gh a s b e e n a d o p t e d t o s t u d y t h e p r o b l e m i n t h e c o n t e x t o f a c a se s t u d y w h i c h is o r i e n t e d t oe s t a b l i s h , i n g e n e r a l t e r m s , t h e m o d e l d e s i g n p r i n c i p l e s , t h e r o l e o f s im i l i t u d e a n d t h e s i m u l a t i o no f t h e v a r i o u s l o a d i n g c a s es . W i t h c o n s i d e r a b l e s o p h i s t i ca t i o n o f t h e t e c h n i q u e s o f e x p e r i m e n t a ls t r e s s a n a l y s i s , l a r g e s c a l e p r o b l e m s ( o f w h i c h t h e p r e s e n t s t u d y i s a n e x a m p l e i n t h e a r e a o fo f f s h o r e s t r u c t u r e s ) c a n b e e f f i c i e n tl y t a c k l e d . I t i s s e e n t h a t s u c h a n e x p e r i m e n t a l a p p r o a c h c a ne f f i c i e n t l y s e r v e a s a n a d j u n c t t o c o m p u t a t i o n a l m o d e l , i n a d d i t i o n t o t h e v i s u a l q u a l i t i e s o f t h i sa p p r o a c h w h i c h is o f m u c h i n t e r e s t t o t h e p r a c t i s i n g e n g i n e e r .

1 . I N T R O D U C T I O NT H E LA UN CH b a r g e p l a y s a c r u c i a l r o l e i n l o a d - o u t a n d l a u n c h i n g o f a n o f f s h o r e s t e e lj a c k e t , w h i c h a r e t w o m a j o r s e q u e n t i a l o p e r a t i o n s i n t h e j a c k e t i n s t a ll a ti o n p r o c e s s . T h es t r u c tu r a l p e r f o r m a n c e o f t h e l a u n c h b a r g e , a s a l o a d e r i n t h e l o a d - o u t a n d a l a u n c h e r i nt h e l a u n c h i n g o p e r a t i o n s , i s o f c r u c ia l i m p o r t a n c e i n it s d e s i g n s i n c e it e s s e n t i a l l y i n v o l v e sl o n g i t u d i n a l b e n d i n g o f t h e h u l l g i r d e r s u b j e c t e d t o v e r y l a rg e d e c k l o a d s . E s p e c i a l l y inl a u n c h in g , w i t h t h e r o t a t io n o f th e r o c k e r a r m s , t h e e n t i r e j a c k e t l o a d is c o n c e n t r a t e d o nt h e r o c k e r a r m h i n g e s , c o n s t i t u t i n g a s i t u a t io n o f s i n g u l a r i n t e r e s t t o t h e d e s i g n e r .T h o u g h , c o m p u t a t i o n a l m e t h o d s o f t h e h u l l g ir d e r s t r e n g t h a n a l ys is n o r m a l l y s u ff ic e s i ns u c h p r o b l e m s , t h e u s e o f p h y s i c a l m o d e l s a s a p o w e r f u l a n d r e l i a b l e ( d u e t o s i g n if i c a n ts o p h i s t i c a t i o n o f t h e p r e s e n t d a y m e a s u r e m e n t t e c h n i q u e s ) t o o l o f s t r u c t u r a l d e s i g n i sw e l l a c c e p t e d i n m o d e r n t i m e s ( H o s s d o r f , 1 9 7 4 ) a n d a s s u c h r e s o r t e d t o i n t h e p r e s e n ti n v e s t i g a t io n . I n g e n e r a l , f o r t h e p r o b l e m s i n v o l v i n g l a r g e s c a l e s t r u c t u r e s , i t is a l w a y s o fi n t e r e s t , d u e t o i n h e r e n t c o m p l e x i ty o f t h e p r o b l e m w h i c h f o r c e s m a n y s i m p l i fy i n ga s s u m p t i o n s t o b e m a d e i n a c o m p u t a t i o n a l m o d e l , t o s e e k a n a l t e r n a t e " o p i n i o n " f r o ma n e x p e r i m e n t a l a p p r o a c h u s i n g s c a l e d m o d e l , w h i c h a g a i n i n v o l v e s a s s u m p t i o n s a n da p p r o x i m a t i o n s a l b e i t o f d i f f e r e n t n a t u r e s ( H o s s d o r f , 1 97 4) . F o r s h i p - li k e s t r u c t u r e s , ap i o n e e r i n g i n v e s t i g a t i o n ( C h a p m a n a n d T a y l o r , 1 97 0) is h i g h l y i n s t r u c t i v e i n o r d e r t ob r in g o u t t h e c o m p l e x i t y o f s u ch a s t u d y . H o w e v e r , i n th e p r e s e n t c o n t e x t , t h e l a u n c hb a r g e b e i n g o n e o f t h e " s i m p l e s t " s h i p f o r m i n t e r m s o f s tr u c t u r a l c o n f ig u r a t i o n a n d w i t hv a s tl y i m p r o v e d i n s t r u m e n t a t i o n s y s t e m s b e i n g a v a il a b le , t h e w o r k r e p o r t e d i n th is p a p e ris m a i n l y o f e n g i n e e r i n g in t e r e s t ( r a t h e r t h a n m e t h o d o l o g i c a l ) in t h e d e f i n e d c o n t e x t .* T h e w o r k r e p o r t e d h e r e w a s c a r r ie d o u t i n th e O c e a n E n g i n e e r i n g C e n t r e , l i T M a d r a s , I n d i a .~, C o r r e s p o n d e n c e t o : V . G . I d i c h a n d y . S r S c ie n t if i c O f f i c e r, O c e a n E n g i n e e r i n g C e n t r e , I T T , M a d r a s 6 0 0 0 3 6 ,I n d i a .-~ P r e s e n t ly G r a d u a t e S t u d e n t , M e m o r i a l U n i v e r s i t y , S t J o h n ' s , C a n a d a .

161


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162 V . G . IDICHANDYe t a l .2 . O B J E C T I V E S

T h e p r e s e n t s t u d y s e e k s a c o n f i r m a t i o n o f t h e s t r u c t u ra l d e s ig n o f a l a u n c h b a r g e u n d e ro p e r a t i o n a l c o n d i t i o n s , c o n c e n t r a t i n g o n o v e r a l l b e n d i n g o f th e h u l l g i rd e r b y t h em e t h o d s o f e x p e r i m e n t a l s t r e s s a n a l y s i s , u s i n g a s c a l e d m o d e l . I t i s a n e x e r c i s e i n t h ed o m a i n o f li n e a r e la s t ic t h e o r y a n d d e m a n d s i d e n ti f ic a t io n o f th e s e c t i o n s o f g e n e r a le l a s t i c t h e o r y w h i c h a r e a p p l i c a b l e t o t h e p r o b l e m a t h a n d , b o t h f o r t h e d e s i g n o f t h em o d e l a n d f o r th e i n t e r p r e t a t io n o f th e t e s t r e s u lt s . L i s t e d b e l o w a r e t h e s p e c if ico b j e c t i v e s o f t h e s t u d y .( i) T o o b t a i n t h e p a t t e r n o f t h e l o n g i tu d i n a l b e n d i n g s t r e s s e s o f th e b a r g e h u l l g i rd e rs u b j e c t e d t o ( a ) c o n c e n t r a t e d l o a d o n t h e r o c k e r a r m a t v a r i o u s s t a g e s o f t h e d e s i g nj a c k e t l o a d o f 6 0 0 0 t o n s , a l l a t e v e n k e e l ; ( b ) d e s i g n j a c k e t l o a d o f 6 0 0 0 t o n s a t t y p i c a lp o s i t i o n s , w i t h r e s t r i c t e d o r u n r e s t r i c t e d t r i m s .( ii ) To o b t a i n t h e v a r i a t i o n o f s tr e s s e s , i f a n y , d u r i n g t h e l a u n c h i n g o f a 1 5 0 0 - to nj a c k e t ,

( iii) T o s t u d y t h e b e h a v i o u r o f t h e b a r g e m o d e l s u b j e c t e d t o i d e a l is e d h o g g i n g a n ds a g g i n g m o m e n t s i n o r d e r t o c o m p a r e t h e s c a l e d u p s t r e s s v a l u e s f o r t h e p r o t o t y p e w i t ht h o s e o b t a i n e d f r o m t h e t h e o r e t i c a l c a l c u l a t i o n s .I n a l l t h e a b o v e t e s t s , s t i l l w a t e r e n v i r o n m e n t i s a s s u m e d .3. C H O I C E O F M O D E L M A T E R I A L

T h e d e c i d i n g c r i te r i a f o r t h e c h o i c e o f m a t e r i a l f o r s t r u c t u r a l m o d e l l i n g a r eh o m o g e n e i t y , i s o t r o p y , g o o d r a n g e o f el a s ti c l in e a r it y , s u i t a b l e m o d u l u s o f e la s t ic i t y a n dP o i s s o n ' s r a t io . I n a d d i t io n , e a s e o f m e c h a n i c a l w o r k a b i l i t y , f a v o u r a b l e t h e r m a lp r o p e r t i e s a n d a p p r o p r i a t e e x p e r i e n c e w i t h t h e m a t e r i a l a r e t h e o t h e r f a c t o r s w h i c hg o v e r n t h e c h o i c e ( H o s s d o r f , 1 9 7 4) . B a s e d o n t h e s e c r i te r i a , a n a c r y l ic r es i n ,c o m m e r c i a l l y k n o w n a s p e r s p e x o r p l e x i g l a s s h a s b e e n c h o s e n , w h i c h i n c i d e n t a l l y i s aw i d e l y u s e d m a t e r ia l f o r s tr u c t u r a l m o d e l li n g .4. S I M I L IT U D E R E L A T I O N S H I P S

T h e b e n d i n g o f th e h u l l g i rd e r , a s is u su a l , is t r e a t e d a s a f r e e - f r e e b e a m w i t ha s s o c i a t e d N a v i e r ' s a s s u m p t i o n . T h e l o a d s a r e w e l l d i s t r i b u t e d a n d a c t o v e r a f i n i t el e n g t h . T h e r e f o r e , o n l y t h e l o n g i t u d i n a l o v e r a l l b e n d i n g n e e d t o b e e v a l u a t e d a n d t h et r a n s v e r s e s t r e s s e s o f th e b i a x i a l s t r e s s f i el d c a n b e n e g l e c t e d f o r b e i n g s m a l l. T h e e f f e c to f t h e d i f f e r e n c e o f P o i s s o n ' s r a t io (V p a n d Vm, p a n d m d e n o t i n g t h e p r o t o t y p e a n d t h em o d e l r e s p e c t i v e l y ) o n t h e p r e d i c t i o n q u a n t i t i e s i s i g n o r e d . W i t h t h e s e s i m p l i f y i n ga s s u m p t i o n s , t h e s c a l i n g l a w s f o l l o w .

Xw = X~ = XpX3 (1)XM = XwXt (2)X , = X M X J X e X i = X,X~/XeXs (3)

w h e r e k r d e n o t e s t h e r a t io o f t h e p r o t o t y p e p r o p e r t y t o th a t o f th e m o d e l , K b e in g t h es a id p r o p e r t y ; B , W a r e t h e b a r g e b u o y a n c y a n d w e i g h t r e s p e c t iv e l y , M i s t h e b e n d i n gm o m e n t , E i s t h e m o d u l u s o f e l a s t ic i t y , l i s t h e l i n e a r c h a r a c t e r i s t i c l e n g t h , i i s t h e


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Performance of a launch barge 163moment of inertia of the barge cross section, c is the distance of the fibre from thesectional neutral axis, s ( = i / c ) is the section modulus at the fibre, p is the density of thefloating medium and is the unit elongation (strain). Finally

c r p = E p ~ . p /( 1 - V p ) 2 . (4)5. DESIGN OF THE BARGE MODEL

Design of the model is a crucial phase of any model study. For a ship-like structure, thedesign effort is compounded by the various type of interconnections between a very largenumber of structural members, all of which are not amenable to modelling. It is only theabstracted concept of the prototype structural arrangement that actually goes into themodel. As in any design, constraints of material availability and ease in fabrication needto be kept in view. The suitability of the model for the appropriate instrumenta tion andthe load application must be taken account of, since the model must support thelaunching operation (see Section 2), it must be fitted with all functional arrangements,e.g. winch assembly, launch beam, rocker arm etc. The simulation of launching is doneby a scaled model of a typical jacket structure and therefore the model scale is decided bythe requirements of the jacket* an d the same is adopted for the barge (hi = 50.98). Thestructural lay out of the barge model is shown in Fig. 1.The deck, bottom, side shell and the longitudinal bulkheads are longitudinallystiffened. The transverse bulkheads are vertically stiffened. The aft region (below thetwo rocker arm hinges) is stiffened heavily with closely spaced longitudinal diaphragms.The deck and the bottom transverses form a closed ring with the side frames at allframes. Due to obvious impracticability of modelling the longitudinals in the model scale(which results in plate thickness < 1 mm) a "smeared" or orthotropic modelling of thestiffened panels is resorted to. It is easy to find an equivalent thickness maintain ing thelongitudinal stiffness (flexural rigidity in the stiffened direction) of a prototype panel(:T-imoshenko, 1959), which can be scaled down by k, (t being the thickness), thusresulting in a reasonable plate thickness in the model scale (however, ht < hi). Now, inorder to maintain the ratio of the proto type longitudinal and transverse flexural rigiditiesin the model, a suitable transverse beam configuration (equispaced in the panel, be tweenthe bulkheads) need to be found. This is done by trial and error with a chosen flat barconfiguration so as to arrive at a reasonable moment of inertia to stiffener spacing ratio.The considera tion for the search is that the depth of the stiffener should not be too large(or it will occupy more tank space) nor too small (or it will act as a doubler). Thisprocedure is carried out for every panel of the deck, bo ttom, side shell, longitudinal andtransverse bulkheads and leads to (a) the transverse spacings are not uniform over thewhole model, (b) the transverses are not located at the geometrically similar positions.The scaling ratios of the model are hr = 25, hE = 66.67, hp = 1.025. The deck structuresincluding rocker arms, launch beams etc. are not modelled structurally as these areessentially functional items. However, their lengths and locations are geometricallysimilar.

* The barge had been used to support the load-out as well as the launchingof the 1500-ton jacket model. Thejacket model was tested for upendabilityalso, which, in fact. decided the model scale. This investigation srecently reported (Bhattacharyya t a l . , 1984).


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Performance of a launch barge 165In longitudinal bending of the hull, the transverse bulkheads transmit deck loads

predominantly in the inplane mode and not in the bending mode. Bending of thetransverse bulkheads occur in case of differential hydrostatic pressure. In the presentstudy, these bulkheads are designed using equivalent orthotropic modelling, buthowever, no attempt was made to measure and correlate the static response of thesebulkheads as it is not of interest in this study.

A necessary fall out of resorting to d istor ted modell ing ~.t < ~.1 is that the volume of thefree spaces of the tanks are less than that the factor ~3 would suggest (8% difference isfound in this model). However, this does not impair the results of the structuralresponse, though the scaled up ballast data will have (if certain tanks are full) some errorwhich may be acceptable and can be accounted for.

One of the major design requirements of the barge model is that it must maintain lightdraft similarity. This makes the design process extremely sensitive to the choice of ~,,which has to be selected such that kw = kB in Equation (1) is mainta ined. In addit ion,accurate estimate of the weight of the major deck fitting (with significant weightcontribution) must be known in advance in order to work out an overall estimate ofweight distribution before carrying out the actual design exercise. In the present case, theself weight matching with design trim similarity is achieved with 97% accuracy. A view ofthe completed model is presented in Fig. 2.

6. A NOTE ON FABRICATIONThough the launch barge is of extremely simple configuration involving no hullcurvature, yet its fabrication is highly demanding on workmanship. The dimensional

tolerance of the individual members are kept within +0.05 mm and the overalldimensions are kept exact. The support arrangements for the winch assembly (driven bya motor), pulley for the jacket movement, bollard and mooring eyes are designed fromthe point of view of ease in fabrication. The entire process of fabrication, it is estimated,involves a joining length of over 130 m consisting of over 300 individual pieces. Thewater tightness of all the tanks are tested under appropriate hydrostatic load beforemounting the deck panels.

7. SELECTION OF MEASURING POINTS, LOADING SCHEME ANDINSTRUMENTATIONAt selected measuring points, electrical resistance strain gauges are used all of whichare water proofed by silicon rubber. The under water gauges have compensating gauges

kept under similar conditions. A data logger (SOLATRON, UK) is used to record thestrains from the gauges. In order to get a good picture of the stress distribution over theentire length of the barge hull, most measuring points are located on the deck and thebottom in the longitudinal plane of symmetry of the hull. At two sections, gauges arefixed on the side shells (P & S) and off centreline in orde r to check on the stress variation.The gauge locations are shown in Fig. 3.

A simulated jacket load of 6000 tons is fabricated of mild steel plates using certainsimplifying assumptions on the load distribution pattern in both the longitudinal and thetransverse directions with its weight and the location of the CG scaled down.The jacket model used for the launching is made of perspex based on its hydrostaticand geometric characteristics. A detailed discussion on similitude relations applicable tothe design of a jacket model may be found in the work by Bhattacharyya (1984).


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FIG. 2 . T he B arge M odel .

FIG . 5 . Idea l i sed hogg ing tes t in p rog ress .


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FIG. 10. Positioning of the simulated load m progress.

FI6. 12. The Barge and Jacket model.


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Performan ce of a launch barge 1698 . A N O T E O N T E S T S

B e f o r e c a r r y i n g o u t t h e t e s t s w i t h th e f l o a t i n g b a r g e m o d e l , i t i s n e c e s s a r y t o s u b j e c tt h e m o d e l t o s o m e s i m p l e l o a d in g c as e s i n o r d e r t o c o m p a r e t h e r e s u lt s w i th t h et h e o r e t i c a l c a l c u l a t io n s . A l s o i t is i m p o r t a n t t o c h e c k t h e e f f i c ie n c y o f t h e j o i n t s ( o fp e r s p e x p l a t e s ) i n t r a n s m i t t i n g t h e l o a d s . T h i s m a y b e d o n e b Y r e c o r d i n g s t r a i n s a t t h et o p f i b r e o f t h e d e c k a n d a t t h e l o n g i t u d i n a l b u l k h e a d , b o t h c l o s e t o t h e j o i n t s a s isp h y s i c a l l y p o s s i b l e. T h e s e v a l u e s , f o r a s i m p l e l o a d i n g c o n d i t i o n , s h o u l d c o m p a r e w e l lw i t h t h o s e c a l c u l a t e d a t t h e j o i n t i n q u e s t i o n i f t h e j o i n t i s g o o d .

9 . T E S T S A N D R E S U L T Sldealised hogging and sagging conditions

T h e s e t u p d e t a i l s a r e s h o w n i n F ig . 4 a n d a t y p ic a l v i e w in F ig . 5 . T h e m e a s u r e m e n t s .a r e t a k e n a t 5 c h o s e n s e c t i o n s . T h e r e s u l t s a r e p r e s e n t e d i n F i g s 6 a n d 7 f o r S e c t i o n 7 .T h e r e s u l t s fo r t h e e f f i c ie n c y o f jo i n t s a r e p r e s e n t e d i n T a b l e 1 .Concentrated loads on rocker arms

C o n c e n t r a t e d l o a d s a r e p l a c ed o n a n a r r o w w o o d e n b a s e k e p t a c r o s s d i r e c tl y o v e r t h er o c k e r a r m h i n g e s . T h e f l o a t in g m o d e l is b a l l a s t e d t o e v e n k e e l in e a c h c a s e b y s t a r t in gf r o m t h e f o r w a r d m o s t t a n k s . A t y p i c a l r e s u l t i s s h o w n i n F i g . 8 .

-1 .7( .1 .11 . 1 3

( 1 . 1 9 )

- 1 . 5 6

1 . ? * 1 .7 *" 1 . 64( . 1 . 6 6 ) ( . 1 . 6 6 ) ( - 1 . 6 6 )

- 1 . 5 6( - 1 . 6 6 )

- 1 . 7 - I . 5 9~ . 6 6 ) (-,.66~NNI NS t r a s i n k g l c m 2Colculo tec l va lues ~ in b r o c k e t

FIG. 6. M od elstresses for idealised hogging at Section 7,


10/16

170 V. G. [DICHANDYet al.

8 4 7 . 1( S O S )5 6 3( 8 8 0 )

8 3 8 . 6( - 8 5 6 )

847.1 8 4 7 . I( 8 0 6 ) ( ~ 6 ) 847.2( 8 0 6 )

.6( - 8 5 6 )

9 0 0 .3 8 4 2 . 1 1,( 8 5 6 ) ( 8 5 6 )

c _ _Stress in kg lcm 2C a l c u l a t ~ v a l u e s g i v e n i n br Q ck ct

FIG. 7. Prototype stresses for idealised Hogging at Section 7.

+

1 2m m=,1 m2

T A B L E 1 .

3[ ] .~ D e c k p l a t i n gL o n g i t u d i n a lI b u l k h e a d{

Type of moment Strain Stress in the deck Stresses in the bulkh eadgauge (kg cm -2) (kg cm--')number Measured Calculated Measured Calculated

Hogging

Sagging

1 1.7 1.66 1.58 1.482 1.7 1.66 1.63 1.483 1.6 1.66 1.51 1.481 1 . 5 9 1 . 6 6 1 . 5 8 1 . 4 82 1.55 1.66 1,59 1.483 1.55 1.66 1.47 1.48


11/16

P e r f o r m a n c e o f a l a u n c h b a r g e 1 71

3 , 3 8 3 . 3 8 4 . 0 3 . 5 62 . 3 1 2 . 3 4 0 ~ ) ( 1 6 8 4 , 3 ) ( 1 9 ~ 0 ) ( 1 7 7 4 . 0 ) ~1i7~11_. 2 ) 3 t ' ~ O ~ 3 z n ~ e ~~ J a ~

o ~ s 7 . 3 ) o ~ o " ~ o ) 0 s, . . . ~ . o ~

; ' l ; ~ ' ~ ; - ~ 2 " 3 2 ; i . & ( B . 8 )~ ' - . ~ : ( - , ~ 0 ~ ~ - , ~ ~,,,~,~ ~ ' ~ F ~ ( ' ~ ) ' ~ 0 ~ ? )0 .1( 5 3 . 0 ) S t r e s s : k g / c m 2

S t e S S P a t t e r n

0 . 7 ~ I ~ 1 6 2 I 6 . 6 2 '( , o 2 ) _ , , ( 6 s 5 ) : (~ ) ' , ; :. . - , - - ~ . . . . . : . . . . . . - : . . . . . . . . . . . , . . . . . . . . . . . . ~ - _ _' 4 . 2 5 6 . ~ i g ,,' ,i ( S I T ) ' ( ~ 4 ) f ' ; :. . . . I . . _ _ _ I I

" - - i - - ~ - i - 4 . 2 5 II 5 . 8 .5 - - - r - i . . . . . . . . . . . . . . . . . . 't . . . v - , . . . . . d _ _ . l ,i i' ' ( S t 7 ) ~ ( 7 9 4 ) , , '

0 . 7 5 i 4 . 8 2 i 6 . 6 2 o J I ,( 1 0 2 ) ) ( 6 5 5 ) I ( 0 0 9 ) o ' I ,

I I I : I IB a l l a s tM o O e l : k g B a l l a s t D e t a i l sP r o t o t y p e : t o n

A l l P r o t o t y p ev a l u e s i n b r a c k e t

R o c k e r a r m l o a d 3 1 . 8 k g ( 4 3 1 9 t )D r a f t = 1 0 . 4 m ( 6 . 3 m )

F nG . 8 . L o a d i n g a n d h u l l s t r e s s p a t t e r n .

1 0 3 1 0 2 1 .3 2 0 . g 6 1 . 2 2 G 9 60 2 9 ( 5 3 3 . 2 ) ( . 5 ~ 3 ) ( 6 ~ Z - 8 ) ( 4 ? $ .4 ) (~ 0 .7 ,: ) ( t. 4 S .? ) 0 . 2 6 0 . 1 8 0 0 5I ) ( ; ; :

I I I- - . 9 )- 0 ] - 01 1 - 0 . 6 - 0 .7 7 - 0 9 4 - 0 9 6 - 0 . 7 6 I - 0 . 2 6 ( -1 6 1 .& )( - 5 3 ) ( - 5 3 ) ( - 3 1 7 . 8 ) ( o 4 0 7 . 8 ) ( -4 9 9 . 8 ) ( - 5 0 6 / , ) ( - & 0 2 5 ) ( - 1 3 6 . 9 )S t r e S S : k g l c m 2S t r e s s P a t t e r n

!!275 i st, . z, 94 i I i(3731) i (733 1) (6711) i I I- - ~ - - - t - . . . . . t - . . . . . . . . . . . . . . . . . 4 . . . . . - I . . . . . . . " F - - - - - "

I I I I |i i 4 7 8 I I_ ~ ~ _ ( ~ , 9 , ) ( ~ , , ' . " ' :

I' l I / . 7 8 / . . 4 I t I |] _ ( 6 ~ , 9 t ) ( 5 9 8 1 )_ _ . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IL ,. . . . .~ . . . . . . . .i I

2 75 1 5 4 z , ~ tt ,! Ic373~) I (7~31> (671o 0 I ] ul6 0 0 0 1 S i m u l a t e d l o a d ( f u l lB a l l a s t B o l l a s t D e t a i l s D r a f t 1 1 c m ( 5 . 6 m )M o d e l : k gP r o t o t y p ~ : t o n

F I G . 9 . L o a d i n g a n d h u l l s t r e s s p a t t e r n .

A l l I : ~ t o t y p ev a l u e s i n b r a c k e t6 0 0 0 TS ; m u l o t e d l o a d


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172 V . G . IDICHANDYet al.

Simulated load at fina l positionThe 6000-ton simulated load is positioned with the aft end of the load (total length =

115 cm) at the rocker arm hinge by lowering it on to the launch girder of the model in itsexact location by a crane, ensuring proper contact. Ballasting is done simultaneously toachieve an even keel configuration. Several ballast condi tions are tried. A typical result isshown in Fig. 9. A view of the loading sequence is shown in Fig. 10.

TABLE 2. DA TA FOR VARIOUS STAGES OF LAUNCHINGCondit ion Jacket load Ballast in kg

Wt in tons CG from aft (m) Tanks A Tanks B Tanks C Tanks D Tanks Ea 6000 33.3 -- -- 19.544 22.788 4.864

(2654.4) (3094.9) (660.7)b 6000 23.0 -- -- 19.544 22.788 4.864

(2654.4) (3094.9) (660.7)c 6000 13.0 -- -- 19.544 22.788 4.864

(2654.4) (3094.9) (660.7)d 6000 3.0 -- -- 19.544 22.788 4.864

(2654.4) (3094.9) (660.7)e 6000 concen trated -- -- 19.544 22.788 4.864

load at (2654.4) (3094.9) (660.7)hingesValues in brackets represent prototype ballast in tons.

Simulated design load at various launching stagesA few stages of the forward movement of the jacket in launching, details of which are

presented in Table 2, are simulated by positioning the load appropriately on the bargemodel and then measuring the strains. The results are presented.in Fig. l l. However ,due to practical difficulties to simulate a dynamic phenomena by an equivalent static onethe condition with the load CG on rocker arm hinges is not tried, a condition whichcauses excessive trim. Instead an equivalent concen trated load is applied on the hinges.La unch i ng o f j a cke tA simulated launching operation with a 1500-ton jacket model (see Fig. 12) isperformed and dynamic recording of strains at some chosen locations are obtained. Atypical record is shown in Fig. 13. The pulses on the left side corners define the locationsof the jacket during launching.

10. DISCUSSION OF RESULTSIn the tests with idealised hogging/sagging moments (see Figs 6 and 7)_it is seen that the

experiments and the calculated values agree well within 90-95%. This can be consideredto be extremely satisfactory and it validates the assumptions made in formulating theproblem. The joints can also be considered (see Table 1) to be satisfactory.


13/16

P e r f o r m a n c e o f a l a u n c h b a r g e 1 73P O S l l l0 N O F C GO F T H E L O A D( - I S ? . 0 ) ( - 3& 5 . 3 ) ( - 10 6 3 ) ( - 2 f~ , S ) ( 20 3 . 1 ) ( 4 3 3 3 ) L . . ( 3 1(~ S )0 - 0 . 3 2 - 0 ~ - 1 -/ ,2 - 0 . 5 & . 0 4 1 OJbS aam + 0 , 7 61 o a . ~ . . o . ? s ( -s e .s ) ( o ) ( o )

o ~ ~ ~ , 3 o o

, I I ' o- - , )" - - " - ' = ' 1 ~ . , . . ~ # ~ . ~ - 0 .1 S - 0 .1 3 i - 0 . S l ' - 0 , 2 S ( 0 )* 0 - 3 2 . 0 - 3 2 - ~ ( - 10 0 .1 ) ( -$ 1D . ? ) ( - 3 M = ) ( - 10 15 )( 16 5 .8 ) ( I T . S ) o l , 0 1" * 1 .0 7" * 0 - 6

( S ~ ) ( Sb 't ,1 ) ( 31 1 .0 ) t r k m : 0 . 3 6 7 " P O S I T I O N F C G( O ) O F T H [ L O A D o( 4 ~ 9 ) ( ? 22,0) 21.9)*0 .SS .1.45 i I . ~ S ( N r ~ t )( - t 0 9 3 ) ( - , ) e ~ . ) ( - l s 6 . s ) _ . ~ 1 9 ( 41 .s )

- , i iI I ' ~ ~ ~ i ' A lI 0' 0 CE I ~ ' 'l I - ~ . o ( - 1 3 s . I )j ~ A ~ - O & 9 - I . 4 2 - 1 - 2* 0 . 16 * 0 , 2S * 0 3 ? * O S * 0 , 20 ( - 3 ~ / . 0 ) ( - ? S 0 , 6 ( - 6 3 0 , 5 ) ( - 4 , 0 ~0 )( 13 4 ) 0 3 3 . 5 ) ( 3 0 0 , 3 ) ( * 21 i6 . S ) ( . 1S 0 . 1 ) P O S lT I O H O F C Gt r i m : 1 . 5 8 " O F T HE L 0 ~

( b ) ( 4 ~ s ) ( ? s ~ ) ( m s ~ ) 1 3 m - . - j m i 4 ~ )* 0 - 0 8 * 1 . 5 1 , ~ 4 ( S 2 2 . 1 ), 1 194 1. 1) ( 14 1. 3) ( 3 1 3 - 9 ) ~ - - I ~ 1 |~ 1 . 0 , n . 0 , 3 - " x o

I % " I 0: )l

- 0 . 0 ~ - O N - 0 . 7 3 - 1 . 2 6 - 3 . 0 S - 1 . S 3 ( - 6 ~ - 1 )( - 3 3 & ) ( - 20 0 , 2 ) ( - 3 S 3 .? ) ( - 6 6 % 3 ) ( - 10 8 & ~) ( - $ 6 2 . 3)( C ) t r i m = 3 . 3 7 " P O S IT IO N O F C O(1126.1) (1773.9) OF THE LOAD* 3 4 8 0 * 3 .S ~0 ( 14 12. ? ) .

( 1 2 2 & . 3 ) - ~- * 2-83S ( . 9 8 0 . 2 ) ~ ' , - 3 m( s ~ . s ) . 2. 4sv~ ., .. .'~ ~ 3 . , I ( , , , ) l( ) . ? ) ( 1 1 0 . 1 ) ( & N . I I ) , . ZI S " ;1 .1 f; O* 0 . 2 , 0 . 2 2 1 * 0 - 0

( 3 4 . 7 ) (47.1)

i ' (~ " I

*O-~ 0S 0 . 1 ( 0 )

. I0 - 0 ,126( - 6 6 , 1 )

0 ( & 9 . S )* 0 - 1iE 'II0 - 0 . 4( - 19 9 . 3 )

I (- 11S1~( - S 1 4 . 2 )- 0 -S O A - 1 .? 0 1 " 1 . 9 3 S - 3 - 0 5 6 - 3 , 3 71 - 2 - S 3 2 - 1 . N B( - ~6 - 9 ) ( - 9 0 0 . 9 ) ( - 10 3 1 .3 ) ( - t 6 15 .5 ) ( - 178 3 -3 ) ( - 13 & ~2) ( - ? r e . S )t r i m = 8 . 3 9 *( d ) CO.Ce.TOA;S.( t 6 7 9 . ~ ) ( , , 2 . s ) LOAd OF~ . l e k g. 3 . 3 7 1 *3.&D1 ( 1 3 9 ? . 2 )t 5 6 S ~ ( 10 9 3. 7 ) ~ ~ ( 9 21. ? )

D S I A ' 4, I 7 ( - 2 . . , )i I I I- 0 . 6 3 - 1 . 292 ~ 3 . 23 7 - 3 . 0 5 6 - 3 , 235 - 3 ~ - 2JI G 7( - 3 3 3 . ? ) ( - 6 k . 2 ) ( - 1 1 6 4 . ? ) ( - 1 6 1 8 . 5 ) ( - 1 7 2 3 - 9 ) ( - 1 6 ~ . 7 ) ( - I T S & . ? )

S T R ES S k g l c m 2( e ) t r i m : 8 . 3 9 " P R O T O T Y ' P E 1d .U E S I N I R A C K E T S

F IG . 1 1 . S t r e s s v a r i a t i o n s f o r d i f f e r e n t f r o z e n s t a g e s o f l a unc h ing .


14/16

174 V. G. IDICHANDYe t a l .

oI

C O o th i n g ec

1

3

I I I I I j m / 1 T 1m /mI I 1~ j m/m

T o p

4 . o i i o i i~Jm / m m l m

C O a ~~ . . ~ i n g e 3 5

o1 I

o iI ~ m l m

6

ogQ .g

B o t t o r nS t r a i n G a u g e s a t s e c t i o n s 3 , 5 & 6

T r i m : 2

FiG. 13. Stra in recording during launching.

T h e c o n c e n t r a t e d r o c k er a r m l o a d s , w h e n c o u n t e r b a l a n c e d b y b a ll a st i n o r d e r t or e st r ic t a n y r i g id b o d y r o t a t i o n c a n p r o d u c e v e r y h i g h s t r e s s es a s is e v i d e n c e d i n F i g . 8 ,w h e r e a 7 2 % o f t h e d e s i g n l o a d r e s u l t s i n a s t re s s o f 1 9 9 3 k g c m - 2 . C l e a r l y , a n y l a r g e l o a da t t h e a f t ( a s i n a s i t u a t i o n o f l a u n c h i n g ) m u s t b e a c c o m p a n i e d b y r i g i d b o d y r o t a t i o n( w h i c h d o e s h a p p e n d u r i n g l a u n c h i n g ) f o r t h e s y s t e m t o b e s a fe . I n o t h e r w o r d s , r i g idb o d y r o t a t i o n ( t r i m ) m u s t n o t b e r e s t r i ct e d i n s u c h a c a s e.


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Performance of a launch barge 175The case with the design load with its aft end on the rocker arm hinges can be

considered to be a realistic one during load-out. From Fig. 9, it is clear that the stressesare within acceptable limits (maximum being 657.8 kg cm -2) considering the still waterenvironment o f the system.

The results for the cases at various frozen stages of the launch ope ration (see Fig. 11)show substantial increase in stresses (439.5-1773.9 kg cm -2) in the forward region a longwith major variations of the stress profile. However, it may be noted that during the lasttwo stages (see Figs ll c and d) the partial submergence of the jacket tip is expected, thusreducing the ne t bending moment and trim, resulting in lower stress level than indicated.This, in fact, is amply clear from the results of the dynamic strain records (see Fig. 13)where at location 5 (closest to the maximum stress zone) an increase in stress of the orderof 200 kg cm -2 is obtained for a 1500-ton model jacket . The maximum prelaunch stressat forward can be reasonably assumed to be of the order of l/4th (due to linearity ofload-deflection) of that shown in Fig. ll a, which is 110 kg cm -2. Thus, the increase instress is nearly 200% of the prelaunch value in an appropriately modelled launchingoperation in contrast to an increase of 300% (see Figs lla and d) obtained with thesimulated load. This brings out the fundamental importance of the jacket submergenceduring the course of the launch in reducing the otherwise large stresses in the forwardregion of the launch barge.The assumption that the transverse stresses at the deck and the bottom are negligible isclear from the results presented in Fig. 14.

0 8 ~ 3 0 91 - o 2 - 0 1

24

--'15- 2 8 - 1

. i s _ 0.4,; "I0 5

R o c k e r o.rm loocl 44.18 kg (6000 t )S t r e s s : k g / c m 2FIG. 14. Transversestresses in the model.

Based on the above results, it can be concluded that for the realistic loading casesconsidered, the structural performance of the launch barge is satisfactory and the designadequate.

11. CONCLUDING REMARKSPrincipal results of a model testing program dealing with structural performance of ajacket launch barge have been reported. Though the study pertains to a typical case, itmay be looked upon as a general treatment of the defined engineering problem based onthe laws of similitude. Design, fabrication and testing of the model as well as some resultstranscends the typicality of the example chosen. Though the design and the fabrication ofthe model can be quite time consuming, requiring much effort, once it is accomplishedthere is virtually no limitation on the load cases that can be considered. With increasingtendency to use scaled models to solve practical design problems, this study hopefullybrings out the merit of such an approach in the context of an important structuralproblem concerned with the offshore structures. It may be noted that an experimental


16/16

176 v . G . IDICHANDYet al.a p p r o a c h , d u e t o i ts v i s u a l q u a l i t i e s c a n b e o f m u c h i n t e r e s t t o th e p r a c t i s i n g e n g i n e e r i na d d i t i o n t o i ts i n t r i n s i c a d v a n t a g e s t o t a c k l e a p r o b l e m i n a n i n d e p e n d e n t m a n n e r .Acknowledgements--The a u t h o r s a r e g r a t e f u l t o M r N . K . S a w h n e y o f M a z a g o n D o c k L t d , B o m b a y , f o r t h es p o n s o rs h i p o f a c o m p r e h e n s i v e r e s e a r c h p r o j e c t o f w h i c h t h is f o r m e d a p a r t . T h e y t h a n k M r N . S . M o h a n r a mo f M a z a g o n D o c k f o r t a k i n g d e e p i n t e r e s t i n t h i s w o r k a n d f o r p r o v i d i n g s u f f i c i e n t d a t a f o r t h e s a m e . T h ea u t h o r s a c k n o w l e d g e t h e c o n s t a n t e n c o u r a g e m e n t p r o v i d e d t o t h e m b y P r o f . V . S . R a j u o f t h e O c e a nE n g i n e e r i n g C e n t r e a n d v a l u a b l e s u g g e s t i o n s p r o v i d e d t o t h e m b y P r o f . C . G a n a p a t h y o f t h e O c e a nE n g i n e e r i n g C e n t r e i n f o r m u l a t i n g t h e e x p e r i m e n t a l p r o g r a m .

R E F E R E N C E SBHAIqACHARYYA,S. K. 1984 . On app l ica t ion o f s imi l i tude to in s ta l la t ion ope ra t ion o f Of f sho re S tee l Jacke ts .Appl. Ocean Res. 6 (4), 221-226.BHA'I'rACHARYYA,S. K. , JOGLEKAR, N . R . and IDICHANOY, V . G . 1984 . On expe r ime n ta l inves t iga t ion o fl o a d - o u t , l a u n c h i n g a n d u p - e n d i n g o f o f f s h o r e s t e e l j a c ke t s . Appl. Ocean Res. ( in press) .CrlAPMAN, J . C . and T AYLOR, P . F . 1970 . Inves t iga t io n on a s t ruc tu ra l mo de l o f the S .S . Ocean Vulcan. Q.

Trans. RINA 112(3), 257-277.HOSSDORF, H . 1974 . Model Analysis of Structures. V a n N o s t r a n d R e i n h o l d , N e w Y o r k .TIMOSHENKO, S. P. an d WOtr~OWSKV-KmEt3EX,S. 1959. Theory of Plates and Shells. M c G r a w - H i l l , N e w Y o r k .

bhattacharyya, jacket launch barge test

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