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TRANSV. SUBDIVISION PLAN SHOWING POSITION

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R o R o P A S S E N G E R F E R R I E S

Recent accidents w i t h Ro-Ro passenger ships show rapid capsizing;

Current methods to assess damage stability are inadequate;

In this paper possible improvements on existing ferries and new designs are described t o obtain a higher standard of safety.

Q U E S T I O N A B L E D A M A G E S T A B I L I T Y : E U R O P E A N G A T E U A Y ; T R A N S I E N T A S S Y M E T R I C F L O O D I N G (Spouge). T Y P E O F SUBDIVISION P E R M E A B I L I T Y I N C R E A S I N G B U O Y A N C Y IN SHIP'S WINGS, SIDE WALLS (D.K. Brown, Aston and Rydill). HOW TO M E E T F U T U R E S T A N D A R D S P R O P O S E D B Y IMO S U B - C O M . S E P T . '87 LONDON. CJR-HANNAH)

IMPROVING EXISTING F E R R I E S - NEW' D E S I G N H U L L I N T E G R I T Y , WATER ON C A R D E C K , FINS R A P I D D I S E M B A R K A T I O N : 'ME5' S T A B I L I T Y C H E C K , C A R G O P R O B L E M S , P R O C E D U R E S S T E E R I N G - MAN0EL:VRING AT PORT A R R I V A L IN STRONG WINDS. H E E L I N G IN SHARP T U R N AT F U L L S P E E D B Y A SUDDEN 'HARD O V E R ' R U D D E R

Questionable Damage Stability Range

Ro-Ro passenger damage s t a b i l i t y requirements for pa'senger ships according to the SOLAS 197't Convention, including the amendments, should be revised.

The British Department of Transport (1980) and The Netherlands Shipping Inspection (1983) improved SOLAS by implying e x t r a requirements: Final Stability A r m > 0.05 ' m-Range 7° m i n i m u m . F u r t h e r m o r e , during any stage of flooding the margin line should not be immersed.

This s t a b i l i t y standard is applicable to passenger vessels and Ro-Ro passenger ships without making any d i f f e r e n c e .

However, it should be realised that both types are behaving quite d i f f e r e n t l y when struck by a flared forebody w i t h an extending bulb underneath. On the conventional passenger ship, when l i s t i n g , the bulkhead deck o f t e n comes quite near to the waterline and when rolling in waves, flooding over the bulkhead deck can easily occur. However, the flow of water entering is l i m i t e d by f i r e bulkheads and partitions between cabins.

On the Ro-Ro vessel there is no c o m p a r t m e n t a t i o n in the Ro-Ro space above the bulkhead deck, (which normally extends over the f u l l breadth), and in a few minutes a large mass of water can move freely and wildly over a large surface of this vehicle deck. The moment of inertia of the vessel's waterline is enormously reduced and rapid capsizing is likely w i t h i n a few minutes. For this reason Ro-Ro vessels should comply t o a higher damage s t a b i l i t y requirement.

In view of the increasing t r a f f i c density, and the quite realistic probability of being hit at a p a r t i t i o n bulkhead, a 2 compartment standard for all Ro-Ro vessels should apply.

Moreover an alarming phenomenon is indicated b)' Mr. Spouge in his 'Investigation of the Sinking of the Ro-Ro Ferry EUROPEAN G A T E W A Y ' (The RINA Apr. '85, The Naval A r c h i t e c t , March, '86), the so-called 'Transient A s y m m e t r i c Flooding.'

A f t e r being hit at her side by the bulb of SEASPEED V A N G U A R D , a mass of water entered via the bulb-hole which represented a wave front moving into the engine room. Equalising of the surface went on quite slowly. The dynamic character of this calamitous insult on the ship's s t a b i l i t y has been underestimated.

The sloped surface (10-13°) caused a larger angle of heel than would follow f r o m the assumed standard ' s t a t i c ' flooding c a l c u l a t i o n . This complication caused the side of the bulkhead deck to dip well below the waterline diminishing the moment of i n e r t i a .

The flared bow of the SEASPEED V A N G U A R D had holed the topside of the EUROPEAN GATEWAY allowing water t o enter f r e e l y .

Personally 1 greatly appreciate this thorough investigation; for the f i r s t t i m e a t t e n t i o n has been paid to the 'Dynamics of Transient A s y m m e t r i c Flooding' which caused the unexpected immersion of the bulkhead deck, followed by flooding of the Ro-Ro space.

Further research on this subject is urgently needed and should be carried out on a large scale. Scale errors on model testing are to be a\oided.

Consequences on Type of Stfcdivision

Transverse Subdivision

Should flooding calculations be carried out in the f u t u r e by application of 10° sloped masses of water entering during the f i r s t minutes? ( M N I F L O O D C A L C . )

be accessible for inspection.

Considering the fire-hazard, the polythene drums are to be kept at a safe distance f r o m shell plating and bulkheads, where outside repair welding is likely to

occur. ' Drums could be lashed either v e r t i c a l l y or

hori-zontally, depending on location.

Polythene balls bundled in nets might also be applied in narrow compartments, (collapse-safety because of natural f o r m ) .

Longitudinal Subdivision

In a vessel which is subdivided by continuous longitudinal bulkheads (at B/5 f r o m the ship sides) the rate of overflow of the mass of water f r o m the port wing tank to the SB wing tank, via the cross-over duct, plays a very important role in the heeliiig of the vessel; ducts should be made as large as possible (2-3 f r a m e spacings at least). The Netherland Shipping Inspection requires a maximum overflow t i m e lapse 'A 1 minute in which case this asymmetric e f f e c t ;nay be disregarded. Research on the rate of overflow should be carried out on a large scale imitating' all discontinuities and sharp edges of the vessel's cross-over duct, assuming several sizes of the hole in t;:e damaged shell.

Permeability

Current permeability figures appear to be unrealistic: 8 5 % for engine rooms is applied, however a more likely figure for engine compartments is 90-95 9,1 In order to minimise immersion and heeling of a vessel by masses of water entering, it is logical to reduce the permeability of the flooded compart-n j - compart-n t s , especially icompart-n the w a t e r l i compart-n e .

Permanent buoyancy could be applied in the void wing spaces by stowage of empty drums; this idea, according t o Mr. D.K. Brown, was carried out on merchant vessels sailing in convoys during World War I I , by stowing empty oil drums in the sides of the tween deck spaces at waterline level. One of these ships HECTOR w s i hit by 6 torpedoes and s t i l l took several hours t o sink. Moreover the ship sank in upright condition.

Permanent buoyancy should be developed and might improve Ro-Ro Passenger vessels which are b u i l t under the 1965 Rules and presently do not comply w i t h the l a t e r 1980 Standards of the British Department of Transport.

Polythene drums or balls seem to be suitable in the void wing spaces because they cannot corrode and can be easily removed. (Many alternatives are mentioned in the table of

Steel drums are heavy, might rust and will collapse at 6 metre water pressure. However they might be suitable in the wings of engine compartments because of f i r e - h a z a r d .

By proper stowage a permeability of 50-60% could be achieved, and it is a challenge to all of us to find a p r a c t i c a l method of f i l l i n g the void spaces during f i t t i n g out and local removal in case of repair. Loss of deadweight and costs of permanent buoyancy seem to be reasonable.

A much greater increase of damage stability range can be achieved by surrounding the Ro-Ro space by a double hull: so-called 'Side Walls'. (Aston and RvHill. The Naval A r c h i t e c t , A p r i l , '87).

In case of a wall width of 0.13 B, the moment of inertia of the waterline is doubled and the vessel could survive a completely flooded cardeck.

In case of application of side walls, having about 8 f t . ' c o n t a i n e r - w i d t h ' and f i l l i n g these 'subdivided' spaces by polythene drums, the vessel most probably w i l l survive a collision at her side.

The depth of impact w i l l be reduced by the more resistant steel structure in the ship's side. (On top of the side walls a marine 'escape slide and r a f t ' can be accommodated t o allow for a rapid disembarkation).

The Ro-Ro design of maximum hull safetv is characterised by continuous:

Longitudinal bulkheads at B/5 below the bulkhead deck (without w a t e r t i g h t doors). 2- Void Wing tanks f i l l e d up w i t h drums. 3. Side walls around the Ro-Ro space. Engineers could sail w i t h w a t e r t i g h t doors in transverse E.R. bulkheads 'open' in order to be capable of immediate action in case of f i r e , short c i r c u i t or leakage.

Closing these w a t e r t i g h t doors is no longer a must ' w i t h i n one minute' because engine compartments are protected by B/5 void wing tanks on both sides.

T I I E L I F E B E L T i v i r . D . K . B r o w n o f t h e B r i t i s h M i n i s c e r y o f D e f e n s e g a v e m e a s t r o n g s u p p o r t t o t h e o r i g i n a l f o o t b a l l -p r o -p o s a l o n b u o y a n c y d u r i n g R I N A , J u n e 1 9 8 6 , o n T h e S ' - ^ f e s h i p P r o j e c t : S h i p S t a b i l i t y a n d S a f e t y . H e t o l d a b o u t l e s s o n s f r o m t h e p a s t l e a r n e d i n W o r l d W a r I I , w h e n e m p t y o i l d r u m s a t s h i p s s i d e s i n t h e t w e e n d e c k s a v e d t h e s h i p s f r o m r a p i d c a p s i z i n g . S i n c e 1 9 8 6 . a f e w H o R o v e s s e l s c a p s i z e d r a p i d l y w i t h l o s s o f l i f e . - A f a s t c a p s i z e s h o u l d b e I I . I P O S S I B L E w h e n c o n s i d e r i n g p a s s e n g e r v e s s e l s o n d s h i p s - c a r r y i n g d a n g e r o u s c a r g o . - A p r a c t i c a l L I F E B E L T a r o u n d t h e v e s s e l , w h e t h e r i n -s i d e o r o u t -s i d e ( -s p o n -s o n -s ) o f t h e s h e l l s h o u l d l e a v e t i m e t o g e t 2 0 C 0 l i v e s o f f t h e v e s s e l . - I t i s r i g h t t o d i s t i n g u i s h b e t w e e n : p r e s e r v a t i o n o f w a t e r p l a n e i n e r t i a a n d p r e s e r v a t i o n o f b u o y a n c y i n t h e h u l l . - H o w e v e r , w e m u s t c o n s i d e r t h e i n i t i a l h e a v y l i s t i n t h e f i r s t m i n u t e s a f t e r t h e c o l l i s i o n , i f e x t r a p e r -m a n e n t b u o y a n c y i s o n l y i n s t a l l e d i n t h e w a t e r l i n e a r e a - D r u m s d e e p e r d o w n , t h a n , h a l f o f t h e d r a f t w i l l s u p p o r t t h e v e s s e l a l r e a d y a t a s m a l l e r l i s t . - S q u a r e - o r s h a p e d b l o c k b u i l d i n g s o f p o l y s t y r e n e f o a m , w e l l l a s h e d , a p p e a r t o b e t h e c h e a p e s t s o l u t i o n f o r b u o y a n c y i n s i d e v o i o w i n g s p a c e s . L o w s p e c i f i c w e i g h t m a k e s i t a t t r a c t i v e : 15 K g / m ^ . R o w ? v e r , i t s h o u l d b e o f f i r e r e t a r d i n g q u a l i t y , a n d c o v e r e o b y f i r e p r o o f m a t e r i a l t o p r e v e n t a f i r e w h i c h m i g h t b e c a u s e d b y f l a m e - c u t t i n g a n d w e l d i n g d u r i n g r e p a i r s o n c a r o e c k a n d s h e l l . F o r t h e m o m e n t , s t e e l - o r a l u m i n i u m d r u m s s e e ; r i t o b o a p r a c t i c a l s o l u t i o n f o r p e r m a n e n t b u o y a n c y i n t h e w i n - ^ s o f t i T i n s v o r s e e n g i n e s p a c e s b u t t h e s e s p a c e s w i l l g i v e m a n y p r o b l e m s b e c a u s e t h e y a r e n o r m a l l y o c r ^ u p i e d b y m a c h i n e r y .

- W e s h o u l d e n v i s a g e b h e s i d e b o x o r d o u b l e - h u l l -p r i n c i p l e w h i c h i s a p p l i e d t o c e l l u l a r c o n t a i n e r s h i p s , c o n t a i n e r - b u l k c a r r i e r s , ' . n d c h e r ; i c a l t a n k e r s , - T h e f l o a t i n g C r y d o c k h a s t o f l o a t o n h e r s i d e b o x e s a n d i n c a s e t h e w i d t h o f t h e s e b o x e s i s 0,1$ b , t h e w a t e r p l a n e i n e r t i a o f t h e b o x e s ' i s i d e n t i c a l t o t h e w a t e r p l a n e i n e r t i a a t b r e a d t h b b e t w e e n t h e b o x e s . - W e s h o u l d d e s i g n a E o E o v e s s e l t o b e c a p a b l e t o f l o a t o n h e r s i d e b o x e s w i t h c e n t r e h o l d ( s ) f l o o d e d . - T o o b t a i n t h i s h o w e v e r , t h e s i d e b o x e s a r e t o b e f i l l e d w i t h p e r m a n e n t b u o y a n c y a n d d a m a g e i s s u p p o s e d t o b e n o l o n T c r t h a n 0,03 L + 5 m L e i n g t h e l e n g t h o f a a i i - a g e , d e f i n e d b y I M O . - " ^ ' r e e b o a r d o f t h e s i d e b o x e s t o h e a d e q u a t e i n o r d e r t o c o m p e n s a t e t h e l o s t b u o y a n c y . - T h e C a t a m a r a n c a n n o t b e a p p l i e d w i t h c r o s s f l o o d i n ^ d u c t s a n a p e r m a n e n t b u o y a n c y i s s t r o n g l y r e c o m m e n c e d i n t o t h e s i d e h u l l s t o p r e v e n t a o s p s i z e a f t e i - h u l l -Q a m a g e . - A s ? i n m i e t : i c b u o y a n c y i n a v e s s e l t o b e a v o i d e d a s s t a t e d b y M r B r o w n a s a w a r n i n g t o d e s i g n e r s . - M a n y s o c a l l e d n o n w a t e r t i g h t c o m p a r t m e n t s h o l d b a c k w a t e r f o r a l o n g w h i l e , p a r t i c u l a r l , y r e . f r i g e r a t e a s p a c e s a n d s t i ' o n g r o o m s ; q u i t e l a r g e a s y m m e t r i c m o m e n t s c o u ] . d d e v e l o p . R u l e s t o a p p l y r e a l a c t u a l p r e m e a b i l i t i e s i n v i e w t o b e a b l e t o c a l c u l a t e v ; i t h p e r m i a n e n t b u o y a n c y a r e n e e d e d .

I E ( i ' ^ F L A I i K D S P O N S O N S - I n p r i n c i p l e I a g r e e o n s l i g h t l y f l a r e d , s i d e s i r i o r d e r t o p r o v i d e r e s e r v e b u o y a n c y n e e d e d f o r s i t u a t i o n s o f v e h i c l e d e c k f l o o a i n g , w h i l e m i n i m i z i n g t h e i n c r e a s e o f b e a m o f t h e s h i p i n t h e w a t e r l i n e a n d t h e a s s o c i a t e d i n c r e a s e s i n r o l l i n g s t i f f n e s s a n d s h i p r e s i s t a n c e . - I n f i r s t i n s t a n c e I t r i e d t o i m p r o v e t h e s a f e t y o f e x i s t i n g R o R o - s h i p s w i t h o u t t h e a p p l i c a t i o n o f s p o n s o n s . - S p o n s o n s a r e v e r y e f f e c t i v e i n d e e d , h o w e v e r v e r y e x p e n s i v e f o r t h e o w n e r , b e c a u s e o f t h e f a c t t h a t c o n v e r s i o n d o e s n o t a p p l y t o t h e s h i p o n l y b u t t e r m i n a l s , r a m i p c o n n e c t i o n s m o s t p r o b a b l y a r e t o b e a d a p t e d a n d i n s o m . e c a s e s a n y i n c r e a s e o f b e a m i s e v e n i m p o s s i b l e ( l o c k s - H u l l ) . - S p o n s o n s a r e a d d i n g a b o u t 6 ( ' 0 t o n t o t h e l i g h t w e i g h t o f a c o m m o n f e r r y a n d w i t h o u t e x t r a d i s p l a c e n e n t t h e d r a f t m i g h t i n c r e a s e 0 , 2 m . - A d d i n g d r a f t t o a f e r r y i s s t r i c t l y p r o h i b i t e d ; a n d m a n y f e r r i e s a l r e a d y a r e s u f f e r i n g f r o m t o o m i u c h d r a f t - a f t , d u e t o a s t e r n b e i n g t o o s l i m . - T h e r e f o r e I s c e t c h e d t h e s p o n s o n s w i t h v e r t i c a l s i a e s a n d g a v e t h e m i a v e r y s o f t b i l g e r a d i u s i n o r d e r t o m i n i m i z e e x t r a r e s i s t a n c e . - I V I y s p o n s o n s m i g h t b e t o o b u l k y b u t g i v e a n y e x t i ' a b u o y a n c y t o c o m p e n s a t e f o r t h e e x t r a w e i g h t . - I ' v i o r e o e v e r , f l a r e d s i d e s a r e a n u i s a n c e t o t h e s h i p s s t a f f w h e n m a n o e u v r i n g a l o n . g s i d e q u a y s o r i n t o a l o c k a t h i g h - t i d e a n d l o v / - t i d e . - M a n y b e l t i n g s a r e n e c e s s a r y a t s e v e r a l l e v e l s t o a v o i d d a m a g e t o t h e s h e l l p l a t i n g a n d t h e s e a f a r e r s a r e a s k i n g u s t o p r o v i d e v e i t i c a l s h i p s s i d e s a s f a r a s p o s s i b l e a n d f i t t h e v e s s e l w i t h v e r t i c a l c y l i n d r i c c o r n e r s a f t .

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E X T R A C T :

Traditional damaged stability calcula-tions, which were used at the design stage lo check that Ihe ship complied wilh Ihe rules, indicate that sinking would initially have been on an even heel (i.e. no listj, since the compart-ments which flooded were symmetri-cal about the centreline. The free-surfa-ce loss due to water spreading over the four compartmentó below the

bulk-head deck would eventually have been sufficient to give the ship a negative metacentric height (GM), which would have caused a sudden capsize, or at least a lurch to an angle of loll. This does not agree with the available evi-dence (figuur 13), which strongly Indi-cates that the ship began to heel imme-diately after the collision, and that this heel steadily increased, at least until the ship grounded, A more detailed consideration of the flooding is impos-sible using this approach of its omis-sion of lime dependency. The traditio-nal approach is therefore unsuited to , explain this rapid sinking.

f>/1aximum penetration of the bulbous bowwas2.0mfrom the side shell (2.2m from the line of moulded beam), al-though the 0.5m deep frames caused damage up tot 2.5 m inside the shell.

Maximum penetration of* the upper deck was 3.5m from Ihe side shell (4.0m from the line of moulded beamj.

The technical investigation

of the flooding

The object of the investigation of the flooding was to determine firstly what had caused the unusually rapid heeling and sinking, and secondly what, if any-, thing, could have been done to prevent it.

Traditional d a m a s e d stability calcu-lations

The EUROPEAN GATEWAY complied, with the current UK statutory require-ments for subdivision of passenger vessels, which implement SOLAS 1960. These effectively require a one-compartment standard, where the ship will survive with any one compartment below the bulkhead deck flooded. The bulkhead deck for the EUROPEAN GA-TEWAY was the main vehicle deck, and Ihe spaces above Ihis deck are neglec-ted In the calculations since ihey are not required lo be watertight, although they would have provided some buoyancy, especially in a rapid sinking.

Probabilistic damaged stability cal-culations

Although the probabilistic approach is only a sophisticated application of a; large number of traditional damaged stability calculations, and therefore contains the same faults, it does indica-te the general level of safety of the ves-sel in terms of survival following floo-ding.

The EUROPEAN GATEWAY would not-comply with the new probabilistic da-maged Slabiiity regulations adopted in IMO Resolution A 2 6 5 (viii) as an alter-native to SOLAS 1960 for passenger ships. This requires a subdivision index (based on the ship's length, passenger/ crew numbers and lifeboat capacity) of 0.583, while the ship's achieved index (based on simplified point probabilities of compartment damage and ship sur-vival) was only 0.437. Limited experien-ce with these subdivision indiexperien-ces indi-cates that the EUROPEAN GATEWAY was safer than most Ro-Ro vessels, due to its substantial subdivision below the main vehicle deck; but was considera-bly less safe than required under the probabilistic regulations, largely due to its lack of freeboard to the main vehicle deck.

C a u s e s of the heeling

Various possible causes of the obser-ved heeling were considered at the In-vestigation, concentrating on those which may have started the process off, since once a list to starboard was achieved, water collecting on that side of the ship would help lo continue the heeling. It would certainly have requi-; red a considerable moment to achieve this initial list, since the EUROPEAN GATEWAY'S metacentric height before' the collision wat 2.87 m, with free-sur-face losses of 0.74 m. This would requi-re a moment of 12.0MNm, to cause a 5* list, and 24.2MNm to immerse the main vehicle deck (i.e. the lip of the hole abo-ve the waterline).

Transient asymmetric flooding

The generator room, which flooded first, is a shallow " U " shape, containing the generator and numerous pumps, pipes, fioorplates and pillars. The da-mage hole was on the starboard side, extending for half the height of the compartment, while the only signifi-cant exits were the comparatively small doors to the engine room (on the centreline aft) and to the stabiliser room (on the forward pon side). It is li-kely that the many obstructions to the flow of water across this compartment prevented the water surface becoming level, which Is Ihe implicit assumption in traditional damaged stability calcu-lations.

In the initial stages, the wave was ob-served to move across the compart-ment like a wall. Subsequently, as wa-ter poured in at about 20 tonnes/sec on the starboard side, a considerable gra-;dient would probably have remained 'on the water surface, albeit badly dis-'toned by the turbulent flow and the'

sloshing response to the ship's motion,' This effect could have caused heeling to starboard, decaying from the initial large heeling moment to a negligible moment as the compartment filled up.

A mean slope of 10°. for instance, oh !the water surface In the oenerato/

room would cause a heeling moment of 12,4 MNm,'

Such asymmetric flooding of symme-trical compartments has not been pro-, posed before, to the author's knowled-ge, but both NMI Ltd and the German consultants Schiffko (on behalf of Townsend Thoresen) were indepen-dently driven to conclude that this ef-fect must have been present, since the other possible causes described above seem inadequate to explain the obser-; ved heeling. Subsequently, the Court reached the same conclusion.

Some lessons from the

accident

Improvementsto damage control procedures

The sinking of the EUROPEAN GATE-WAY following the collision occurred mainly because it was impossible to close the watertight doors sufficiently quickly. Until this accident, It was com-rirton for UK ferries to operate with wa-tertight doors in the machinery spaces open, except in fog, The relevant sec-tion of the Merchant Shipping Regula-' tions, 1980, states that every watertight door "shall be kept closed at sea except when it is required to be opened for the working of the ship". The practice is therefore justified to some extent by' the need for the small complement of engineers to have immediate access to all the machinery compartments in ca-se of breakdowns or fires. The Court considered that it was reasonable for the EUROPEAN GATEWAY to have open the two doors furthest aft, but that It was not necessary for the working of the ship to leave the door between the generator room end the stabiliser room open.

The NMIFLOOD simulations suggested that even with this door closed, the EU-ROPEAN GATEWAY would probably have sunk in the weather conditions at the time of the accident, ellthough she might have survived in calm water. (Data from Ref. 3 was used to evaluate the likelihood of capsi2ing due to wa-ves.) With all doors initially open, the simulations demonstrated that only power-operated doors, closed within 50 seconds of the collision, could have saved the ship. The Court accordingly recommended that all ferries be fitted with power-operated doors (Indeed, this had largely become UK practice following the accident).

Improvements to the subdivision of Ro-Ro ships

Although the EUROPEAN GATEWAY satisfied the current UK requirements for passenger vessels, it proved to be vulnerable to this type of accident in the particular circumstances where the hull was breached below the waterline and also just above the bulkhead deck. This deck (the main vehicle deck) beca-' me immersed at only 10' heel, allowing water to flood the entire length and width of the ship. The NMIFLOOD si-mulations showed that once 12' heel had been reached, water entered this space at such a rate,that even with all watertight doors closed the ship would' eventually have capsized or grounded. Probabilistic methods of calculating damage survivability may well provide a more reliable basis than the current damaged stability and load line rules, both for assessing possible improve-ments and for regulating the subdivi-sion of these ships.

Implications of transient asymmetric flooding

Only the lack of a betier explanation for the sinking of the EUROPEAN GATE-WAY points towards the existence of asymmetric flooding in symmetrical compartments.

Furthermore, it is likely that such a phe-nonrfenon would only occur in certain crowded compartments or with certain sizes of damage holes. Nevertheless, the concept has considerable impor-tance for the assessment of the dama-ged stability of ships. In panicular, It suggests that rapid local flooding may be accompanied by rapid heeling, and that in such cases the statutory free-board requirements may be inadequa-te to prevent exinadequa-tensive further floo-ding. More research into this pheno-menon is underway at NMI Ltd, Finally, a model reproduction of the sinking, in-cluding consideration of scale effects, would confirm or modify the NMI-FLOOD simulation, and could be exten-ded to show the probability of this type of sinking recurring in the future.

2 7

Conclusions ^

EURO-PEAN G A T E W A Y following a collision, occured surprisingly rapidly. The tech¬ ' nical investigation, although relying on

somewhat uncertain evidence, was 'able to interpret the collision

accepta-bly, but was driven to postulate a new phenomenon - transient asymmetric flooding - to account for the rapid heel-ing of the EUROPEAN GATEWAY. The sinking occured because the ship had its three watertight doors ih the machinery spaces open at the time of the collision, and could not close them sufficiently quickly. A simulation of the ;flooding revealed that the doors would have had to have been closed within 50 seconds of the collision to have saved the ship. Furthermore, Ihe extensive vehicle deck, exposed, by the damage, and the low freeboard of this type of ship, made the EUROPEAN GATEWAY certain lo sink once it had reached 12° heel in its damaged condition.

The investigation demonstrated that power-operated watertight doors are essential for Ro-Ro ferries, but that even with these, such ships are extre-mely vulnerable to rapid flooding. As the Report of the Formal lnvestiga-| tion warns, "it cannot be satisfactory to proceed upon the basis that no passen-' ger vessel will ever again suffer a fate similarto that of the EUROPEAN GATE-WAY". It is to be hoped that the lessons from the sinking of this ship will be hee-ded in time to prevent the catastrophic loss of life, which must surely occur. If a fully-loaded Ro-Ro ferry is ever the vic-tim of such a collision.

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In reply lo Mr Hannah, the estimated vertical centre of gravity (KG) in the casualty condition was based on an inclining

experiment which was carried out on this ship in November 1980. The centres of gravity of the trailers on board were assumed to Jse 2 m above the relevant deck, as advised in the stability booklet. The KG estimate was therefore as good as is usual wilh such estimates, but Mr Hannah's concem about its accuracy is cenainly justified, since the implicit suggestion, a varialion of which is made by M r Heather, is that a higher KG and hence a lower metacentric height (GM) could have allowed the rapid heeling to be interpreted in a traditional way as a simple loss of sialic stability, withoutMving recourse to transient asymmetric flooding.

The author would agree that the GM could have been slightly lower than estimated, and consequently that the magnitude ol the transient asymmetric flooding could have been somewhal less, but is convinced that this does not allow the transient effects to be dispensed with altogether. Il should be noted that, although the GM is commonly over-estimated, there is no actual evidence that ii was so in this case.

Fig. 14 gives the simulation results requested by Dr Morrall for the maximum damage case defined in the current damaged stability regulations, which for this ship consists of damage 6 ' 9 m wide, extending from the baseline upwards without limit and 4 m inboard from the ship's side. With the watenicht doors closed, and using the same transient effects as in the simulation of the actual damage condition, the vessel heels over rapidly, but fails to Immerse the main deck sufficiently, before the Generator room iills and thc ship rights itself wilh n fin.il freeboard of 0'93m. Water which reached the main deck during the transient heeling causes a residual heel of 3 -4°. With the watcrtiuhi doors open, the ship heels over and capsizes within 40 sec of thc collision, l l must be noted, however, thai the transient effects for this condition are even less certain than for the actual damage case.

O P E N W A T E R T I G H T D O O R S

TO B E C L O S E D W I T H I N 3 0 - 5 0 S E C .

Mr Heather's forthright comments on watertight doors are appreciated. Mr Meek and Mr Adams, Mr Brown, Mr Hobson, Mr Cleary and Lt Fiebrandt, and Professor Kundu all raise the same issue. Mr Brown makes an apt reference lo the sinking of HMS VICTORLV, which was holed by the ram bow of another warship. It was calculated that the vessel would have survived If all watertight doors and gun ports had been closed; and although an order to this effecl was given one minute before the collision, it would have taken three minutes to carry out, and by then the flooding was out of control. It is not difficult to conclude that waieniahl doors which are slow to close are extremely dangerous, and it is tragic that merchant ship designers had I'orgotien this by the ume the EUROPEAN GATEWAY was buill. Mr Brown and M r Heather further judge that watertight doors should not bc filled below the waterline at all. However, this deceptively simple conclusion ignores the good reasons why the doors were pui there in the first place.

Doors between machinery spaces are used for many

watchkeeping and maintenance tasks, as well as lor escape routes in some vessels, and their elimination would make these tasks much more difficult. This may be acceptable on well-manned warships, but the small increase in complement, which Mr Brown allows would be necessary, may be economically unrealistic on merchant ships. The use of remote machinery monitoring and fire delecuon systems (which the EUROPEAN GATEWAY did not have) make doors less vital, but any restriction in access to the -source of an accident such as a fire may allow it lo gel out ol control. Watertight doors therefore, while decreasinp the vessel's safety inlhe event of a collision, which is a remote risk but has extremely serious consequences, also iricrease its safety in the event ol' machinery lires and othe'r similar accidents, which are rather more likely though less serious. The question ot whether or not to ill waierilghi doors and, 11 lilted, whether to leave them open or closed, depends on balancing these two risks. The Inquiry into the loss ofthe EUROPEAN GATEWAY decided ihal with the dala at present available any conclusion could only be subjective. (Their interpretation ofthe legal position is given in reply lo Mr Cleary and L l Fiebrandt). The author's subjective conclusion is that power-operated watertight doors are necessary, and that they should be lelt open only where the trequency ol engineers passing through them is high and the risk ot collision is low. Collision data cenainly support their closure in fog, and may also give support, as Mr Heather recommends, for their closure , when coming into and out of harbour.

3.R. SPOUGE

1 > 0 0 R S

TO B E CLOSED

F E R R Y E U R O P E . A N G A T E W A Y + S P O t s i S O N S

" SIDE. W A L L S "

T Z

S I D E - W A L L S

F U T U R E S T A B I L I T Y S T A N D A R D S ! j ° ^ _ i ° _ ' ! I l i i l _ l ! d i y . ! l i _ 5 . i - n . Ë ? . I l Ë i _ E L 2 E 2 . i l Ë _ Ë Y . = i ' ^ i . _ k £ D . Ë i Q 1 9 7 9 ?_ R e . : D a m a g e s t a b i l i t y o f p a s s e n g e r s h i p s . : D r a f t a m e n d m e n t s t o r e g u l a t i o n 1 1 - 1 / 8 o f S O L A S 1 9 7 4 i s s u e d b y t h e IMO S u b c o m m i t t e e o n s t a b i l i t y 3 2 e s e s s i o n . R e q u i r e d S t a b . A r m C u r v e i n f i n a l d a m a g e d c o n d i t i o n : C Z m i n = 0 , 1 0 , r a n g e m i n = 1 5 ° , a r e a m i n 0 , 0 1 5 m r a d . I n i n t e r m e d i a t e s t a g e s : C Z m i n = 0 , 0 5 m r a n g e m i n = 7 ° M a x i m u m a n g l e o f h e e l 1 5 ° b e f o r e e q u a l i z a t i o n 7 ° a f t e r f l o o d i n g o f 1 c o m p . 1 2 ° a f t e r f l o o d i n g o f m o r e c o m p I n f i n a l d a m a g e d c o n d i t i o n , h e e l i n g m o m e n t b y p a s s . , o r b o a t s . o r w i n d , H m o m e n t ( C Z - 0 , b 5 ) x d i s p l a c e m e n t s h o u I d b e m e t . - C Z m i n = 0 , 1 0 m , i s I o g i c a I t o o u r o p i n i o n . - a r e a m i n = 0 , 0 1 5 m , u p t o t h e c r i t i c a l a n g l e , i s ^ e f i n T h g t h e ' a m o u n t o f e n e r g y w h T c h c o u l d b e a b s o r b e d b y t h e d a m a g e d v e s s e l a n d i s i n l i n e w i t h t h e f a m o u s s t a b i 1 i t y - c r i t e r i j ? _ o _ f _ R a h o I a . - r a n g e m i n = ' 1 5 s e e m s " " t ó " " B ë o f n o s e n s e , b e c a u s e o f t h e f a c t t h a t t h e c r i t i c a l a n g l e h a s b e e n p a s s e d a I r e a d y . . o n a r o r o v e s s e l t h e c r i t i c a l a n g l e i s a t i m m e r s i o n o f t h e c a r d e c k ( - 8 ° ) ^ ' ® ^ ^ ° . o n a p a s s . v e s s e l t h e c r i t i c a l a n g l e i s a t i m m e r s i o n o f t h e c o r r i d o r ( - 1 4 ° ) w h i c h i s r u n n i n g a l o n g t h e w i n g c a b i n s , o n t o p o f t h e b u I k h e a d d e c k . P Z\ T h i s p r o p o s e d a r e a 0 , 0 1 5 m r a d a n d 1 5 ° r a n g e a r e d i f f i c u l t t o i m p l e m e n t o n a c u r r e n t t y p e o f r o r o p a s s e n g e r v e s s e I , w h i c h i s c h a r a c t e r i z e d b y a l o w - l e v e l b u l k h e a d d e c k , w i t h t h e r o r o s p a c e e x t e n d i n g o v e r t h e f u l l b r e a d t h a b o v e t h i s d e c k . - W i t h t w o e n g i n e c o m p a r t m e n t s b e i n g f l o o d e d , t h e f r e e s u r f a c e r e d u c t i o n i s l a r g e a n d p r e s e n t s g e n e r a l l y t h e m o s t c r i t i c a l d a m a g e d c o n d i t i o n . B e c a u s e o l t h e l o w f r e e b o a r d , t h e c h a r a c t e r o f t h e C Z -c u r v e i s \ i n s t e a d o f n o r m a l l y — a n d t o m e e t t h e r e q u i r e m e n t o f C Z m i n = 0 , 1 0 - r a n g e m i n = 1 5 ° , t h e o n l i e s t w a y i s t o d e s i g n a v e r y b e a m y ' s t i f f v e s s e l . R e a s o n f o r t h i s i s t h e s m a l l h e e l i n g a n g l e ( 7 ° - 8 ° ) a t w h i c h t h e b u l k h e a d d e c k i s r e a c h i n g t h e w a t e r l i n e a n d w h e r e t h e C Z s t a b i l i t y a r m i s a t i t s m a x i m u m . P - R e c e n t c a l c u l a t i o n s a r e i n d i c a t i n g a v a l u e o f m i n . i n t a c t C M r e q u i r e d o f a b o u t 5 m ! w h i c h w i l l r e s u l t i n w i l d l y r o c k i n g m o t i o n s o f t h e s h i p w h e n s a i l i n g i n b e a m s e a s - f i n s t a b i l i z e r s c a n h a r d l y c o p e w i t h t h i s .