Abstract. 1 Introduction

Transcription

1 Buoyancy and strength of existing bulk carriers in flooded conditions J. Jankowski, M. Bogdaniuk, T. Dobrosielski Polski Rejestr Statkow, Gdansk, Poland Abstract Bulk carriers have been designed with a small margin of safety. The extensive corrosion and very difficult operational conditions cause that some old bulk carriers have insufficient strength to withstand the heaviest storms. In particular, the insufficient strength concerns the side structures and hatch covers and, after flooding, the corrugated bulkheads, double bottom structure and the general strength of the ship. The paper presents analyses of the buoyancy, stability and strength of bulk carriers in flooded conditions. The analyses are performed for different loading conditions: alternate loading (cargo in every second hold) and uniform (cargo in each hold) distribution of heavy cargo. Introduction Heavy losses of bulk carriers in the period since 990 [] (over 00 ships and over 600 lives lost) have caused international organizations to start explaining the reasons of these catastrophes. The majority of them are damages of hull structure in heavy weather conditions []. The analyses of bulk carriers losses enabled to determine typical scenarios of their sinkage (for example [2]): - appearance of cracks of side plating or even break in the side due to excessive corrosion of the end connections of side frames, - flooding of the hold with sea water, - collapse of the corrugated bulkhead loaded at one side (alternate loading condition) by heavy cargo and water, or/and - significant change of the ship's trim causing deck submergence, - wave attack on hatch covers of undamaged holds in extremely heavyweather conditions and damage of hatch covers (hatch covers are designed to withstand the load of.7 m height of water),

2 Marine Technology II - flooding of the neighbouring holds and sinkage of the ship due to lack of buoyancy. Bulk carriers have been designed with a small margin of safety. Higher tensile steels were used widely. Accelerations acting on the cargo of large density overload the corroded structure. This causes damage to structure members - most frequently the frames side brackets in thefirstholds and, as a result, the cracks of side plating or even collapse of the side. After flooding the hold, the corroded corrugated bulkheads are normally not able to sustain the load of heavy cargo and water acting at one side as it happens in alternate loading conditions. The collapse of a bulkhead creates a very dangerous situation for the ship, leading usually to its sinking. It should be stressed that the corrosion of high strength steel, often used for bulk carriers construction, causes more intensive decrease of structure strength than it is in the case of a ship built of normal steel. 2 Buoyancy, Stability and Strength of Bulk Carriers in Damaged Conditions The damage stability of bulk carriers was analysed in [] and the analysis showed that:. The loss of buoyancy due to flooding can occur in the following cases (see Table ): a) on small bulk carriers with two cargo holds in any loading condition and at flooding at least one hold or engine room, b) on bulk carriers with five cargo holds at flooding two first holds or the aft hold and engine room, especially in heavy cargo condition, c) on bulk carriers with seven cargo holds only in one case, at heavy cargo condition and with the engine room and aft hold flooded simultaneously. 2. When thefirstholds (or the aft hold and the engine room) are flooded, the forward part of the ship (the aft part of the ship) submerges and water can get through hatch covers and ventilation heads to the undamaged holds. This normally results in the loss of the ship. 3. The flooding of a single compartment (a hold or the engine room) of bulk carrier with more than four holds does not cause losss of its buoyancy.. The damaged but having positive buoyancy bulk carriers are stable. 5. The trim and heeling angles of damaged bulk carriers in an equilibrium position are rather small and the cargo cannot shift at these angles. Figure : Division of the bulk carrier into watertight compartments

3 Marine Technology II 5 Table. Numbers of the compartments flooded after which the ship loses positive buoyancy (for example, 2 & 3 means that compartment No 2 and 3 have been flooded, see Fig. ) - PRS program applied. Loading 2 holds condition Ship's length 5 holds Ship's length [m] 7 holds Ship's length [m] Ballast Heavy cargo Homo geneous cargo M 88 3& 3& &5 2 3 &2 3& & &6 5& & In Table, 2 & 3 means that the engine room and the aft hold have been flooded, and 6 & 7 means that hold No. & 2 on ship withfiveholds or holds No & 3 on ship with seven holds (see Fig. ) have beenflooded,and so on. Corrosion reduces the thickness of the structure members during operation of the ship. This results in decreased strength of particular part of hull structures and the whole ships, especially in ships built of higher tensile steels. Therefore, the flooding of one hold is very dangerous for existing bulk carriers, as it can lead, according to the scenario, to progressive flooding. To stop such flooding, sufficient strength of corrugated bulkheads (as a second line of defence) and a whole ship is required. However, the corrugated bulkheads of existing bulk carriers are not able to withstand loads resulting from the simultaneous loading of the bulkhead by cargo and water from flooded hold, (see Fig. 2) and therefore, I ACS (International Association of Classification Societies) proposed a new standard of safety [3] for such bulkheads, which is based on ultimate strength concept.

4 6 Marine Technology II beam model of corrugation Figure 2: Load of the bulkhead in flooded condition Bending of the bulkhead corrugations is considered applying simple beam model shown in Fig. 2. Collapse of the beam is assumed in form of two plastic hinges located at the lower end and in the middle of the beam. Bulkhead corrugations are considered as strong enough to withstand bending by the transverse pressures, if they fulfil the following condition [3]: <\ () F\^Z, + Z. <2 where, F is the total transverse force acting on the corrugation, / is the height of the corrugation, cr/r is the yield stress of material, and Z/, Z* are the elastic section moduli of the corrugation cross-section at the lower end and the middle of the corrugation span. Elastic section moduli applied in () instead of plastic ones, which formally should be applied for consistency of the ultimate strength concept, have been chosen purposely. Such a solution gives the bulkheads a factor of safety as the elastic moduli have smaller value than the plastic one. Another component of safety factor results from the assumption that the beam is simply supported at the upper end (see Fig. 3). Additionally, the section moduli Z/, and Z* are to be calculated applying effective width concept. Bending capacities of corrugated bulkheads between holds No. and No. 2 have been examined according to the standard () for six bulk carriers at least 0 years old. Alternate and homogenous loading conditions were considered. Hold No. was additionally flooded. The required increase of corrugation thickness Af above the actual thickness &, have been calculated. It was assumed that ta are, due to corrosion diminution, 2 mm smaller than values of thickness applied on the new ships. Corrosion additions equal to mm were included into values of Af. Such additions are required by classification societies for 5 years of ships operation, (to next special survey). Results of required increase of thickness (in non-dimensional values) are shown in Table 2. The results show clearly that all the considered bulkheads do not satisfy condition () in

5 Marine Technology II 7 alternate loading conditions, when holds No. are additionally flooded. Similar situation occurs in homogenous loading conditions, but, in this case Af is considerably smaller. Table 2. Required increase of corrugation thickness. Ship LBP M Number of holds ta [mm] alternate loading LBP in Table 2 means length between perpendiculars. C+Af ^homogenous loading IACS proposed also a new standard of double bottom safety for existing bulk carriers in flooded conditions [3]. In the standard total vertical force acting on flat part of inner bottom in a hold, between hopper tanks and lower stools of the bulkheads, is compared to shear capacity of bottom floors and girder webs at the edges of this part of the bottom. The vertical force is the algebraical sum of weight of cargo, weight of flooded water and the buoyancy force. To take into account dynamic components of the total force, the value of the buoyancy force is appropriately underestimated. Double bottoms of the six ships reported in Table 2 were examind according to the proposed standard. All the double bottoms appeared to be sufficiently strong. However, the smallest ratio of the shear capacity to the resultant vertical force (for midship hold of ship No. in Table 2) is only.03. The biggest value of the ratio, equal to 2.06, was found for the first hold of ship No 2. These values suggest that some existing bulk carriers may have too weak double bottoms to withstand flooding of the holds. The general strength of existing bulk carriers in flooded conditions depends on length of the ship. This can be seen in Table 3. Flooding of a midship hold of large bulk carrier causes the sagging type of deflection especially in homogeneous loading (see also Fig. 5). In the sagging condition, the deck is compressed and this can become dangerous for large bulk carriers since, after flooding a midship hold, the increase of the deck compression can lead to buckling of the deck structure. The numbers in the table mean: - the "numerator" is the maximum stress, caused by maximum bending moment at still water with one hold flooded and by 0.8 of the design wave bending moment, in relation to the permissible stresses equal to 75k, MPa; k is the material factor for higher tensile steel applied, - the "dominator" is the number of hold flooded at which the maximum bending stress occurs.

6 8 Marine Technology II Table 3. Maximum stresses due to bending in relation to permissible stresses (numerator) and No. of hold flooded (dominator) - PRS program applied. Ballast Load condition Heavy Cargo Homogenous Cargo Ship's Lenght [m] Ships with 5 holds Ships with 7 holds j i 3 Influence of Loading Condition on Bulk Carrier Safety Influence of the loading condition on the ship strength is significant. The homogeneous loading of bulk carriers by heavy cargo causes much smaller bending moments than the distribution of the cargo in every second hold (see Fig.5). The change of the ship loading from alternate into homogeneous causes also the change of the hull deflection from hogging into sagging. This is dangerous, as mentioned above, when midship hold is flooded but it is advantageous when the first holds are flooded, which normally happens. Resignation from the alternate loading condition significantly decreases the loads of bulkheads what results in smaller values A/ required to increase the corrugation thickness (see Table 2). However, majority of the bulkheads still will not satisfy condition (). The change of the loading condition most significantly influences the loading of the bottom structure. Bending stresses in the principal members of the bottom structure are much smaller (even in the flooded condition) in the homogeneous loading than in the alternate loading. This can be seen comparing the results of FEM calculations presented in Fig. 6. In alternate loading condition bending stresses in side frames, in empty holds, are much greater than in homogenous loading. The external sea pressures acting on the bottom of an empty hold cause rotation of the hopper tanks around the bilge. This type of deformation increases stresses in the frames above the values caused by local bending of frames by sea pressure acting on ship sides. In addition, the stresses in the side frames take their maximum values at lower parts of the frames where excessive corrosion wastage usually is observed in existing bulk carriers. Deformations and stresses in side structure of a panamax bulk carrier in alternate and homogenous loading conditions were calculated for values of the design loads required by the Rules for Classification and Construction of Segoing Ships of Polski Rejestr Statkow. Results of the calculations are shown in Fig. 3. As the first step of calculations, a coarse FEM model of three cargo

7 Marine Technology II 9 holds was considered. Then a fine FEM shown in Fig. 3 was applied. model of a part of side structure, DISPLACEMENTS: LOW) CASE : DISPLACEMENTS; LOAD CASE 2 : a) alternate loading b) homogenous loading Figure 3: Deformation and maximum stresses in side structure at alternate and homogenous conditions (VAST program applied). Bending stresses in the side frames, in the alternate loading, appeared to be about two times bigger than in the case homogenous loading (see Fig. 3). The increase of the trim after flooding thefirstor the last holds is dangerous for hatch covers, as normally the flooding of the first or the last holds causes the submergence of the deck and the hatch covers of existing bulk carriers are not able to withstand the wave loads. The increase of the trim is smaller in the homogeneous loading than in the case of alternate loading (Fig. ) and the first case is more safe from the point view of hatch covers strength. Figure : The trim of bulk carrier with seven holds, loaded homogeneously and alternately, after flooding thefirsttwo holds (PRS - program applied)

8 50 Marine Technology II Alternate Loading Condition «) ALTERNATE LOADING Homogeneous Loading Condition *o*v»» A Loading condition : o = «ir?o c Homogeneous Loading Condition and Hold Flooded Figure 5: Still water bending moments and and shear forces in bulk carrier of Z,=85 m (seven holds) (PRS program applied). a - the stresses resulting from bending of the bottom primary members /- deflection Figure 6: Results of the zone strength analysis of bulk carrier ( =25 m) (MAESTRO program applied).

9 Marine Technology II 5 Heavy cargo is usually loaded in every second hold of a bulk carrier to raise the centre of mass of the ship because uniformly distributed cargo in all holds makes the ship too "stiff " and great transverse accelerations can occur, which is inconvenient for the crew. To investigate the influence of the change of the loading condition on transverse accelerations (UT\ these accelerations have been calculated at the bridge of a bulk carrier of =25 m (seven holds), in different sea states and different angles between the ship and waves. The sea state is determined by significant wave height #, and by characteristic period T and relative angle (3 ((3=80 determines the head waves). Results of the calculations are presented in Table. Numbers in the table determine the average from /0 of the heighest transverse accelerations in given sea states. It can be seen from the table that:. the change of loading condition from alternate to homogeneous increases the transverse accelerations at bridge in average by 25%, 2. the change of angle for example from to (3=20 to 50 reduces a% by 60%. The calculations show that transverse accelerations can be controlled (reduced) by changing the course of the ship in relation to waves. Table. Transverse acceleration at the bridge in different loading condition, in m/s^ (#,,n (2.5,.) (3.5,5.3) (.5, 6.) (5.5, 6.9) (6.5, 7.8) (7.5, 8.8) (8.5, 9.8) (9.5, 0.7) (,.7) (3,2.8) (5, 3) (7, 6.) Conclusions alternate P homogenous P Heavy losses of bulk carriers during the last five years resulted in discussion on the minimum safety standards, which should take into account damage conditions (one hold flooded). The strength of existing bulk carriers is not sufficient to withstand the loads resulting from the damage condition and it is necessary:

10 52 Marine Technology II - to strengthen their structure, or - to decrease their loading. Owners usually choose the second solution as the strengthening of the ship is expensive. The decrease of bulk carrier loading can be obtained by: - resigning from the loading conditions which generate the greatest bending moments or shear forces, changing the alternate loading of the ship by heavy cargo (cargo in every second hold) into the uniform distribution of the cargo in all holds. Uniformly distributed cargo in all holds decreases the loads acting on the bottom, on the bulkheads and on the side structure. However, such loading condition causes: - the increase of transverse accelerations, which can be decreased at sea by changing the relative angle between the ship's course and the wave direction, - the sagging type deflection of the ship which can lead, after flooding the midship hold on large bulk carriers, to buckling of the deck and collapse of the whole ship but such deflection is adventagous when the first holds are damaged and flooded, which normally happens. The damaged and flooded forward part of bulk carrier increases the trim which normally results in deck submergence. Subsequently, the waves attack and damage the hatch covers of unflooded holds. The increase of trim after flooding the first holds of ship loaded homogeneously is smaller than in the case of alternate loading. The flooding of one compartment (hold) of a bulk carrier with more than four holds does not cause the loss of its buoyancy and the damaged bulk carriers with positive buoyancy are stable. The above conclusions can be summed in the statement that the changing of the alternate loading condition into homogeneous loading significantly increases the safety of bulk carriers subjected to damages of foremost holds. References. IMO, MSC 65/INF., 5, "Bulk Carrier Safety, Report of the correspondence group on the safety of ships caring solid bulk cargoes". 5, March Williams A., Faulkner D., "MV DERBYSHIRE, The Search, Assessment and Survey". Colloquium, The Royal Institution of Naval Architects, London, March Report MSC67//3, date , "Bulk Carriers Safety Submitted by ACT. Jankowski I, Purowski H : "Information Concerning the Stability of Bulk Carriers in Damage Conditions". PRS, Technical Report No. 5, 995