IACS rules1 define the double side skin on bulk carriers as a configuration (figure 7.1) where each ship side is constructed by the side shell and a longitudinal bulkhead connecting the double bottom and the deck. Hopper side tanks and topside tanks may, where fitted, be integral parts of the double side skin configuration. The minimum double side width,W , is suggested not to be less than 1 m measured ds perpendicular to the side shell.
According to ABS2, double sides on bulk carriers enhance protection of the primary structural members against cargo related corrosion and mechanical damage, as well as provide a barrier against extensive flooding due to low-impact side shell damage. The exposure of damage-prone transverse frames of conventional bulk carriers is eliminated on double side bulkers, whereas the creation of stiffer side structure eliminates the flexing or fatigue of conventional side frame structures. From an operational point of view, the damage per ton of cargo discharged can be six times lower than the conventional bulk carriers and the time required for cargo discharge is decreased, due to the smooth hold sides. The advantages and disadvantages of the double side skin (DSS) in comparison with the single side skin (SSS) bulk carriers have been summarized by ABS in table 7.1, which examines various aspects of the design, such as corrosion, flooding, mechanical damage, maintenance and steel weight.
Figure 7.1
Modern3 double side bulk carrier [3]
Table 7.1
Comparison of DSS bulk carriers to SSS bulkers [2]
Table 7.1 (continued)
Comparison of DSS bulk carriers to SSS bulkers [2]
Spyrou et al.4 present the modifications proposed on the midship configuration of a double side Panamax by Oshima shipyard to compensate for the reduced cargo hold space. Instead of modifying the principal hull dimensions, larger inner bottom area was considered and as a result, smaller topside and hopper tanks were used, as shown in figure 7.2.
Figure 7.2
Layout of a typical cargo hold of a Panamax for DSS and SSS construction [4]
The typical structure adopted for the double sides of large bulk carrier ships consists of longitudinal stiffening with transverse webs (alternative 1, figure 7.3). Alternative designs were proposed and assessed by Fricke et al.5 Alternative 2 included arrangement of transverse webs in the sides at each frame location. In this way the width of the double side can be reduced because the space is not affected by longitudinals. The transverse web structure is heavier than that with longitudinals, because additional plates are arranged there and because increased plate thicknesses are required in the upper part of the side shell and longitudinal bulkhead to achieve satisfactory buckling strength.
Alternative 3 is a mixed design with longitudinals on the longitudinal bulkhead and transverse frames at the side shell as possible. Their support is provided by transverse webs and by stringers which are arranged at a distance of three frame spacings. The advantage compared with alternative 1 is more space in the double hull, which allows a reduced width to be realized. Contrary, the transverse webs require an increased thickness in the lower part, due to high shear forces and the necessary openings at this area.
Finally, alternative 4 provides a curved shell for the inner skin. By forming the inner skin from an unstiffened curved shell, its increased buckling strength is utilized. The whole curved part is only stiffened by transverse webs and a few stringers. This design provides buckling strength to global hull girder stresses as well as to local pressure forces and bending of the transverses.
Figure 7.3
Double side design alternatives [5]
Hsu et al6 studied the strength aspects of double side skin bulk carriers. They state that DSS in a bulk carrier is initially designed to support the shearing force, especially for bulk carrier with alternate hold loading. Additionally, compared to SSS design, DSS proved also very good transverse performance. Hsu evaluated the strength performance of two identical ships, the one with single side skin and the other provided with double side skin. Considering shear strength, the shear stress levels of shell were found quite different. For a 1000 KN vertical shear force, the maximum shear stress of the SSS ship is 11.75N/ mm2, but only 6.78 N/ mm2for the DSS ship. This means that the shell plate thickness of the SSS ship is dominated by shear stress, while the shear strength of outer shell for the DSS ship has safety margin of up to 40%. The study on the transverse strength revealed that the transverse webs in DSS design with the highest stress level are prevented by longitudinal bulkhead and inner bottom from exposing to a high corrosive cargo environment. However, the stress intensive areas of side frames in SSS design are totally exposed to the cargoes. Taking under consideration the operational aspects of the design, the total steel weight of DSS ship is increased 3.55% compared to SSS, whereas the available cargo hold volume is reduced 3.54%.
Soares et al7 assessed the reliability levels of a conventional single hull bulk carrier compared to a double hull bulk carrier. It was found that double hull design has a higher level of reliability, whereas it maintains the same safety level for both sagging and hogging conditions. Contrary, failure has a higher probability in sagging than in hogging in single hull ships.
The analysis of the stress distribution near collapse concluded that the behavior of the two bulk carriers is very similar for both sagging and hogging bending moment. Under sagging collapse bending moment, most of the deck has already collapsed as well as the intersection of the side shell with the bottom of the wing tank. For hogging, the collapse bending moment is achieved with buckling of the bottom plating. However the inner bottom longitudinals and the bottom girders have already collapsed.
Additionally, the corrosion progress taken under consideration, sagging case was found always to be the dominant mode of failure for the single hull. However, for the double hull bulk carrier, the hogging case become the dominant one at 5,10 and 15 years. Figure 7.4 shows the time dependent probability of failure normalized by the initial value (for as built thickness), for the two designs assessed. BDH stands for the double hull bulk carrier and BSH is the single hull bulk carrier.
Figure 7.4
Time dependent probability of failure [7]
Ozguc et al8 studied the collision resistance and residual strength of single side skin and double side skin bulk carriers subject to collision damage. The main results of the study considering various collision cares are given below:
• The ship structural design has very significant influence on the collision resistance. The collision energy absorption capability depends on the thickness of outer shell, inner shell, side stringers, transverse webs, width of the side ballast tank and width of lower and upper wing tanks.
• Energy absorption when rupture of the outer shell of DSS occurs is approximately 10% less than the energy absorbed by the outer shell of SSS. However, the maximum energy absorbed, i.e. the energy absorbed when the skin of cargo hold (inner shell for DSS, outer shell for SSS) ruptures, in 2.2 times more for DSS than for the SSS.
• For all cases, DSS has higher rupture energy than SSS.
• DSS bulk carriers have higher safety index than the SSS bulk carriers in hogging and in sagging conditions under similar collision damage
scenarios, and this index value is greater in the hogging case compared to that in the sagging case.
• Ultimate sagging moments of resistance in intact and damaged hulls are considerably less than ultimate hogging moment.