Safe Carriage of Ore Concentrates i n Double H u i l Bulkers
W a l t e r A b i c h t , T U Hamburg-Harburg^1. I n t r o d u c t i o n
I n January 1966, the M S "Kremsertor" capsized on her voyage t o Bremen i n the English Channel. T h e accident occurred eleven days after the vessel had l e f t the p o r t o f Novorossijsk, w i t h iron ore concentrate as cargo, j u s t after i t had passed the Bay of Biscay i n stormy weather.
T h e casualty was investigated by Prof. Wendel and his associate Arndt (1966). I t was f o u n d that, after a n upward m i g r a t i o n o f moisture, upper parts of the cargo liquefied, causing progressive cargo s h i f t i n g and, finally, capsizing o f the vessel.
T h e investigation received notice i n Germany as well as at I M O and evoked activities to improve the rules f o r the shipment of b u l k cargo which may liquefy. T h e f i r s t e d i t i o n of the "Code of safe practice for solid b u l k cargoes" ( B C Code) was pubhshed by I M O i n 1965. I n the following years, several amendments were made. Nevertheless, the present s i t u a t i o n is s t i l l unsatisfactory.
I n the current version o f the b u l k carrier code, the mean moisture of a concentrate cargo is taken as a criterion to indicate whether liquefaction is to be expected or not. Other i m p o r t a n t factors, as ship motions i n waves, vibrations, m i g r a t i o n o f moisture d u r i n g the voyage, d u r a t i o n of voyage etc. are ignored. Obviously, w i t h o u t observing these a d d i t i o n a l influences, a reliable prediction cannot be made. U n f o r t u n a t e l y , up to now, a better criterion does n o t exist. Because i t is d o u b t f u l whether such a criterion - suitable for practical use - can be established, ore concentrate cargo should be generally regarded as l i q u i d . I n m y opinion, i t is more p r a c t i c a l to develop an economically efficient bulker design w i t h sufficient s t a b i l i t y even i f the cargo has t o t a l l y or p a r t l y liquefied, t h a n to t r y to improve the liquefaction criterion.
T h e idea for the proposal presented here came d u r i n g the investigation o f the capsizing of the barge-pusher u n i t " F i n n - B a l t i c " . T h e vessel capsized after downward m i g r a t i o n of moisture and s h i f t i n g of the i r o n ore concentrate cargo i n December 1990 i n the B a l t i c Sea.
2. E x i s t i n g r u l e s f o r t h e s h i p m e n t o f o r e c o n c e n t r a t e s
T h e relevant rules i n the I M O B C code, intended to prevent capsizing due to liquefaction of cargo, are based o n the assumption t h a t liquefaction w i l l not occur i f the cargo moisture -measured prior t o loading - does not exceed the "transportable moisture l i m i t " ( T M L ) . T M L is defined as 90% o f t h e "fiow moisture p o i n t " ( F M P ) . F M P indicates the percentage of moisture at which the material starts to fiow i n a plastic manner when v i b r a t e d . T h e F M P is determined i n tests w i t h representative samples o f t h e material. T h e test procedures are described i n detail i n the B C Code.
Materials having a higher moisture content must be carried either i n cargo ships fitted w i t h specially designed portable divisions or i n specially constructed cargo ships w i t h permanent s t r u c t u r a l boundaries. I f T M L is not exceeded, no measures must be taken.
These rules are rather ineffective. T h e reasons for this judgement are:
- For the behavior of the cargo d u r i n g the voyage several factors are of importance. I t is impossible to predict the behavior i f only mean moisture content and flow moisture p o i n t are k n o w n .
- The moisture o f the m a t e r i a l is not constant, b u t d i s t r i b u t e d t h r o u g h o u t the consignment. D u r i n g the voyage, the d i s t r i b u t i o n varies w i t h t i m e , and accordingly, the occurrence o f liquefaction depends - among other factors - on the d u r a t i o n o f the voyage.
- T h e T M L criterion does not distinguish between upward and downward movement o f moisture. A t tlie t i m e the criterion was developed, mainly cases of capsizing after lique-faction at the surface of the cargo were k n o w n , Arndt (1966). However, capsizing after liquefaction at the b o t t o m of the cargo may also occur (Atkinson and Taylor 1994). Es-pecially i n the latter case, the moistm-e may rise u p to saturation at the b o t t o m , even i f the average moisture of the cargo is clearly below T M L . The cargo can then easily s h i f t along the steel deck.
T h e inadequacies of the T M L criterion call for an improvement of the existing rules. Becatise of the difficulties to predict prior t o shipment whether a cargo w i l l p a r t l y or t o t a l l y l i q u e f y d u r i n g the voyage, a new regulation should be based on the worst case scenario. T h e consequences for the design would be not too severe. As demonstrated i n the following, an arrangement of a double h u l l is the most convincing solution. The benefits attained by a double h u l l of sufficient w i d t h are: no capsizing i n case of liquefaction, h i g h p r o b a b i l i t y of s u r v i v i n g s m a l l damages occurring f r o m inside by the grab or f r o m outside by being rammed, no cumbersome portable divisions or permanent l o n g i t u d i n a l bulkheads w i t h i n the holds, smooth inner side walls, no troublesome procedures like determination of F M P and measurements of moisture contents.
M u c h can be done i n the design stage to keep the necessary w i d t h of the double h u l l small. A remaining loss o f cubic capacity of the holds can be avoided by reducing the height of the double b o t t o m , by rising the depth, a n d by enlarging the hatchways.
3 . R e s i d u a l s t a b i l i t y o f a b u l k c a r r i e r a f t e r l i q u e f a c t i o n o f c a r g o
I f a bulker carrying l i q u i d cargo is inclined, the cargo starts t o s h i f t creating a heeling moment which increases w i t h angle o f inclination. For obtaining the resulting r i g h t i n g a r m , this heeling moment has t o be subtracted f r o m the i n i t i a l r i g h t i n g moment o f the vessel and the remaining moment t o be d i v i d e d b y the weight of the ship. T h e residual arms must a t t a i n the m i n i m u m values required i n the existing stability rules. T h i s is to be observed i f the risk of capsizing shall not exceed the level which is n o r m a l for seagoing ships.
Fig. 1 shows the midship section o f a b u l k carrier of 48,000tdw, Heldt (1996). T h e i n i t i a l design is a single h u l l standard bulker w i t h alternating short and long cargo holds. T h e t o t a l number of holds is seven; the f o u r short holds are assumed to be loaded w i t h i r o n ore concentrate of 3 . 6 t / m ^ . T h e respective r i g h t i n g arms o f this bulker are p l o t t e d i n F i g . 2. I f the cargo has liquefied the vessel w i l l capsize because the residual r i g h t i n g a i m s are negative t h r o u g h o u t the t o t a l range o f angles of i n c l i n a t i o n up to 8 0 ° .
Heldt (1996) investigated t o what extent the stability after liquefaction can be improved by the arrangement of a double h u l l of 2 m i n w i d t h (case a), o f 4 m i n w i d t h (case b ) , and by the arrangement of a centreline division (case c ) . T h e centreline d i v i s i o n improves s t a b i l i t y most. F i g . 2. For practical reasons, however, a centreline division should be avoided because i t hampers the h a n d l i n g and stowage of cargo. F r o m this p o i n t of view, the double h u l l is better. T h o u g h the residual r i g h t i n g arms are smaller, the stability rules can be f u l f i l l e d i f the w i d t h of the w i n g space is broad enough. For the bulk carrier under consideration the w i d t h of the double h u l l should be 3m t o f u l f i l l I M O stability requirements.
T h e residual metacentric height w h i c h corresponds w i t h the d o t t e d r i g h t i n g a r m curve of Fig. 2, is GM' 1.20m (the p r i m e indicates t h a t the free surface effect of the cargo is included). T h i s result is not o n l y valid for the b u l k carrier presented i n F i g . 1, b u t f o r a l l b u l k carriers of n o r m a l shape. Generally, sufficient residual stability is t o be expected i f after liquefaction the metacentric height o f the vessel does n o t f a l l below GM 1.20m.
Lpp -B = D = A = CB = T 208.80m 28.00m 16.20m 60539t 0.85 11.87m
Fig. 1: V a r i a t i o n of tlie m i d s h i p section of a bulk carrier o f 48,000tdw, Heldt (1996): a) 2m wide double h u l l , b) 4 m wide double h u l l , c) single hull bulker w i t h centreline division.
60*
Angle o f r n c l l ' n a t i o n
Fig. 2: Residual r i g h t i n g arms after liquefaction o f concentrate cargeo for the variations a), b) and c) of the b u l k carrier i n F i g . 1. T h e d o t t e d Hne between curves a) and b ) indicates the l i m i t below which the residual lever ai-ms must not f a l l i f the intact stabihty rules o f the "Seeberufsgenossenschaft", SBG (1984) - w h i c h are equivalent to the I M O "Code o f I n t a c t Stability" (Res. A 749) - shall be satisfied. Broken lines f o r original b u l k carrier w i t h o u t l o n g i t u d i n a l bulkheads. Upper broken line for solid cargo, lower broken line f o r liquefied cargo.
4 . R e d u c t i o n o f t h e m e t a c e n t r i c h e i g h t o f a d o u b l e h u l l b u l k e r d u e t o l i q u e f a c t i o n o f c a r g o
T h e assumption t h a t the t o t a l cargo moves like a l i q u i d , is an extreme statement. I n reality, liquefaction does n o t start simultaneously i n a l l cargo holds. However, because i t cannot be excluded t h a t after some t i m e the t o t a l cargo has become easily movable, i t appears advisable to consider the worst case. A n a d d i t i o n a l advantage of this assumption is t h a t i t w i l l make a ship generally able t o carry l i q u i d cargoes w i t h o u t any stability problems.
T h e effect of a free l i q u i d on the metacentric height can be assessed by assuming t h a t the centre of g r a v i t y of the f l u i d is shifted f r o m the centre of the f l u i d to the metacentre o f the f l u i d i n the hold. F i g . 3 shows the cross section o f a double h u l l bulker.
boTTl = l-w^
12v ( 1 )
i is the transverse moment of i n e r t i a o f the free surface, v the volume o f the l i q u i d , I the l e n g t h of the h o l d p a r t l y f i l l e d w i t h l i q u i d cargo, and w the inside w i d t h of the hold.
F i g . 3: Fiee l i q u i d effect: the effective p o i n t of attack of the fluid weight moves f r o m the centre of gravity of the fluid bo to the metacentre of the fluid m
T h e s h i f t o f the centre of g r a v i t y o f the fluid f r o m bo to m shifts the centre o f g r a v i t y of the ship f r o m G t o G'. T h e distance GG is identical w i t h the loss of metacentric height AGM
caused by the l i q u i d . B y comparing the moments of mass, we get:
b ^ - p , - v = GG' - P ^ - C B - L - B - T ^ AGM = GG'= ^ • ^ ^ (2)
V is the volume o f the cargo l i q u i d , pc its mass density, pw the mass density of the sea water, CB the block coefficient, L the length, B the breadth, and T the d r a f t of the ship. Because o f the a i m to present the influence o f the double h u l l w i d t h b o n the loss of stability, the inside w i d t h of the cargo hold is expressed as:
Combining Eqs. (1) to (3), we get for the loss o f metacentric height:
B 12 pyj ÜB L 1 \ O J
T h i s f o r m u l a demonstrates t h a t the loss of s t a b i l i t y due to liquefaction decreases w i t h decreas-ing PCIPW, l/L, B/T, and w i t h increasdecreas-ing block coefficient CB and b/B.
5. M i n i m u m d o u b l e h u l l w i d t h f o r s u f f i c i e n t s t a b i l i t y a f t e r l i q u e f a c t i o n o f c a r g o
T h e GM of a bulker as shown i n F i g . 1 is relatively h i g h i f i t carries sofid concentrate cargo. For solid i r o n ore concentrate e.g., GM amounts to some metres even i f only every second cargo hold is loaded. T h e residual metacentric height after liquefaction of cargo, GM = GM-AGM should be about 1.20m. E q . (4) shows the parameters influencing AGM. I f i n t h e stage o f a double h u l l bulker design GM can be assessed and also the parameters pdPw, CB, l / L , and B/T are given, the necessary double h u l l w i d t h b/B can be calculated to get the required GM :
A _ i _ 3 / 12CB-iAGM/B) \
B - 2 y \ l { p J p ^ ) . { l / L ) . { B / T ) ) ^'
T h i s equation shows w h a t must be done to keep the double h u l l w i d t h small. T h e only param-eter which cannot be influenced by the designer, is the mass density r a t i o pc/pw Therefore, a m a x i m u m mass density f o r concentrate cargo should be specified f o r each ship. For the other parameters is to be aimed at: a h i g h CB, a small B/T, a s m a l l l / L by filling cargo only into some o f the holds, and - above a l l - a high AGM/B. For a h i g h AGM/B, i t is i m p o r t a n t to t r y t o get a h i g h GM w i t h o u t m a k i n g the vessel too broad and too stiff. U n f o r t u n a t e l y , some of these points have also opposite effects. E.g., i f l / L is kept small, the cargo must be stowed higher w i t h the result t h a t GM becomes smaller. Therefore, i n each i n d i v i d u a l case, i t must be well considered w h i c h solution is more effective.
A l l factors c o n t r i b u t i n g to this goal must be observed to get a s m a l l double h u l l w i d t h . O f course, the range o f v a r i a t i o n for the i n d i v i d u a l parameters is Umited. T h i s applies also t o GM. For the highest GM w h i c h may be tolerated, the shortest r o l l i n g periods o f existing standard b u l k carriers give some o r i e n t a t i o n . For iron ore concentrate cargo, the lower l i m i t may be set between 8 and 9 seconds, w h i c h is around one second and a half less t h a n the average r o l l i n g p e r i o d of a r o u n d 10 seconds. T h i s means t h a t e.g. f o r a bulker w i t h B = 2 4 m , the m a x i m a l acceptable GM is about 5.15m.
I n the design stage, i t w i l l be h e l p f u l to have a d i a g r a m w h i c h shows f o r each i n d i v i d u a l case how wide the double h u l l must be. Such a diagram is presented i n F i g . 4. A f a m i l y o f curves is p l o t t e d ; each curve is based o n a constant p r o d u c t o f CB and AGM/B . T h e other relevant parameters are t o be f o u n d as a p r o d u c t o f Pc/pw, B/T, a n d l/L on the v e r t i c a l axis. T h e m i n i m u m double h u l l w i d t h b/B needed f o r sufficient residual s t a b i l i t y is to be read f r o m
the abscissa. Example: A double h u l l bulker of 30,000tdw is subdivided as shown i n F i g . 5. T h e concentrate cargo is carried i n the four shorter of the seven holds. T h e stowage factor is assumed t o be 0.27m^/t, pc/py, =3.6, B = 2 4 m , T =10.50m, CB = 0.82, GM = 5.15m, A G M = 3.95m, and l/L = 0.34. T h i s yields CB • AGM/B = 0.135. T h e corresponding curve is p l o t t e d i n F i g . 4 (dashed line). For po/pn, • B/T • l/L = 3.6 • 2.286 • 0.34 = 2.80 we get b/B = 0.083. Hence, the m i n i m u m double h u l l w i d t h being necessary for sufficient residual stability is 6 = 0.083 • 24m = 2.00m. I t may be interesting t h a t this is j u s t the same double h u l l w i d t h required f o r an o i l tanker of the same size - not for stability reasons b u t for reasons of o i l p o l l u t i o n m i n i m i z a t i o n .
Fig. 4: Diagram for the d e t e r m i n a t i o n of the m i n i m u m double hull w i d t h . E x a m
-ple: CB • AGM/B = 0.135 and pc/pw •
B/T • l/L = 2.8. T h e double liuU w i d t h must be b/B = 0.083.
Fig. 5: Example p l o t t e d i n F i g . 4: dou-ble h u l l bulker o f 30,000tdw w i t h con-centrate cargo i n t h e four shorter holds; t o t a l length of holds filled w i t h cargo is l/L = 0.34. Oa-^-OAO 0.55 0.30 0.25 24.00 m 10.50 m 6M ft 6M 5.15 m 3 . 9 ï m 3 c / j w . 0.92 . 3.6
A smaller w i d t h t h a n 6=2.00m w i l l be obtained, i f the mass density of the cargo or the t o t a l length of the loaded holds are below the values given above. Also a m i n i m u m GM' < 1.20m reduces the w i d t h . Provided t h a t the other d a t a are left unchanged, we get e.g. b — 1.60m for pc/pw = 3.2. T h e necessary 6 w i l l become even smaller i f liquefaction is assumed t o occur i n only three of the f o u r holds filled w i t h car^o. We then get 6 = 0.99m for Pc/pw = 3.6 and b = 0.55m for pc/p^ = 3.2. I f instead of GM = 1.20m a value of GM' = 0.76m is accepted, we get b = 1.64m f o r pjpw = 3.6 and l/L = 0.34. However, the assumption o f a p a r t i a l liquefaction of cargo or the allowance of a smaller GM' reduces safety. Moreover, the a b i l i t y of the safe carriage o f cargoes which are already l i q u i d prior to loading, w i l l be lost.
6. C o n c l u d i n g r e m a r k s
Cargoes liable to Uquefy should be generally carried i n double h u l l bulkers. Such a regu-lation seems a l l the more acceptable, since the double h u l l w i d t h can be kept s m a l l and the loss of hold capacity can be compensated by o m i t t i n g the lower and upper w i n g tanks, b y a lower double b o t t o m , by larger hatchways etc. Tankers must be fitted w i t h a double h u l l t o minimize o i l p o l l u t i o n . Just the same, bulkers should be fitted w i t h a double h u l l t o avoid capsizing. Safety o f life at sea is an argument which is at least as serious as p r o t e c t i n g the marine environment!
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