Shell Plating
At the renewal of the cracked or wearied out plates, it is necessary to see the "Shell Expansion Plan" . Because there are many kind of steel plate such as mild steel plate, high-tensile steel plate etc. Among them there are many kind of steel such as A-grade, B-grade, C-grade etc. When renew the shell plate, the same steel should be used.
Typical defects found in the bottom and the side shell are as follows.
8.1 Bottom Shell
(1) Bottom scratches and Dents
Most damage to the bottom shell is due to contact with objects on the seabed. Bottom scratches are slight damages. Such damage happens when the ship operates in shallow waters such as the Mississippi river, near the coastline, etc. In many instances, bottom damage due to contact starts from the forward part and disappears around midship.
Because of scratches, the bottom paint has peeled off and corrosion occurs. In this case, remaining paint and rust should be cleaned by shot blasting and properly re-painted . When the bottom has had contact with a coral reef or rocky seabed, large dents and/or holes appear with scratches. In this case, as a matter of fact, fractured shell plate should be cropped and renewed with damaged internal members. If the dents are relatively small, they may be left as they are with some effective internal reinforcement.
If we find a large fractured opening during the bottom inspection, naturally we should recommend repairs. However, there have been many instances where the shipowner has believed that there were no abnormalities and dents were found during the bottom inspection, which naturally gives the shipowner a headache. The shipowner usually decided sailing schedule beforehand, they hate prolonging docking period because of major repairs, they persists in putting off major repairs, and try to carry out simple repairs within the drydocking period. That is why sparks usually fly between the Surveyor and the shipowner's representative.
Regardless of experience of bottom inspection, the surveyor always feels uneasy before docking survey.
Fig.8-1 Damage in bottom shell
(2) Dents due to panting
When the ship sails in rough weather without reducing speed, dents might occur in the keel and A strake starting from the fore peak tank to the middle part of No.1 Water Ballast Tank , because of the relation between ship speed and curvature of the forward bottom. In the worst case, the floors in the tank and bottom stiffeners buckled.
Naturally, the Surveyor should recommend repairs to the shell, as well as the internal members.
(3) Corrosion of keel and adjusent A strake
Re-painting of the area on the keel block is impossible. So after undocking and also long voyage, these unpainted area is much more corrosive than painted area. the corrosion of welded beads is more heavy than the plate itself.
At the bottom survey previous un-painted area which we can find easily because of much rust should be carefully examined especially welded beads. When the beads is excessively corroded, rust should be removed and it is necessary to re- weld and after well painted.
(4) Corrosion of beads
Grooved corrosion, which at first glance looks like cracking often occurs in heat affected zones of seam and butt weld. It means the both side of beads. The corroded beads should be gouged and re-welded.
(5) Wrinkled corrosion
Small craft and ships adopting the transverse system of framing might develop wrinkles in the transverse direction in the mid-ship area of the bottom shell. These wrinkles are considered to occur because of buckling. If possible, it is better to fit the transverse carlings in the double bottom tank to prevent buckling.
(6) Sea chest
After removing the grating of the sea chest in way of the engine room or the pump room, climb onto the staging and inspect the internal parts of the sea chest. The sea chest forms a discontinuity with the rest of the bottom shell; therefore, cracks might appears at fillet welds of girders and floor plates.
Fig.8-2 Cracks in Sea Chest
Photo 8.1 Corrosion of Keel and A strake in way of forward bottom
Photo. 8.2 Ship with False Keel
When the ship with a fales keelis seated on the ordinary keel blocks, fales keel and center girder may be heavily damaged.
i
Photo. 8-3 Big damage in bottom (1)
Ballast water comes out from the double bottom tank.
Photo 8-4 Big damage in bottom (2)
Big damage can be detected by anybody
Photo 8-5 Slight Dent
A slight dent in a curved plate is difficult to detect
Photo 8-6 Corrosion of welded Bead (1)
Corrosion in a welded bead of a bottom shell plate looks like a crack, but is not a crack. As shown in the macro etching, this corrosion appears in a heat-affected zone.
Photo 8.7 Corrosion of Beads (2) Macro Etching
Photo 8-8 Reed Screen Bottom Plate Corrosion
Stress corrosion due to buckling in the midship region of the bottom plate in a transversely- framed construction; the dent itself is small. Reinforcement by fitting a carling inside the tank is recommended.
9.2 Bilge Keel
In large tankers with the midship coefficient approaching 1.0, the bilge keel may not be fitted when the ship is newly built. If bilge keels have not been provided on both sides during the bottom inspection, check whether the ship had no bilge keels when the ship was constructed or whether they have been ripped off during a marine casualty. If there are traces of welded beads on the bilge strake, we may conclude that the bilge keels have been ripped off during a marine casualty.
Damage to the bilge keel is as follows;
(1) Bilge keel dropped off, a part of bilge keel ripped off, kinks in bilge keel
The bilge keel is a member that does not need to conform to classification society rules. However, if it is damaged, it is normal to repair the bilge keel, usually under insurance.
(2) Cracks at ends
Aged ships which constructed with rivet not weld. Rivets at the ends of the bilge keels often worked loose. In a welded ship, the welds at the both ends are provided with large leg lengths, but sometimes cracks are found at the ends.
(3) Local joints of the bilge keel
The block butt in the hull becomes a local joint in the bilge keel. If welding at this location is defective, cracks appear in the joint. If the crack progresses and reaches the bilge strake, water penetrates into the hull. In case of tankers, this defect leads to marine pollution. During a bottom survey, check the side shell plates at the location of the bilge keel join.
(4) Box-type bilge keel
In fine and high speed ships, box-type bilge keels are frequently used. The box structure is watertight, but water sometimes penetrates into the box through small cracks in the welds of local joints. If the local joint is wet, there is a possibility that water has penetrated into the box. In such cases, carry out the air test, find locations where leaks start and recommend welding repairs.
(5) Corrosion of the bilge strake
Although not directly related to the bilge keel, when we inspect the bilge keel, check for corrosion of the bilge strake above it. Bige strake above the bilge keel
is corrosive than other strake.
Photo 8-9 Deformation of bilge keel
Photo 8.10 Corrosion of the Bilge Strake above the bilge keel
The bilge keel is not directly welded to the shell; it is generally connected to the shell through a flat bar. If it is welded directly to the shell, there is a strong possibility that cracks might develop in the bilge strake when the bilge keel suffers damage.
Fig. 8-3 Example of Bilge Keel
Fig. 8-4 Crack at Bilge Keel End (1)
Fig. 8.5 Crack at Bilge Keel End and joint of bilge keel
Cracks tat the joints of the bilge keel might develop, causing cracks in the bilge strake.
Fig. 8-6 Example of Box-type Bilge Keel
8.3 Side shellThe followings are the example of the defects in the side shell.
1) Corrosion
2) Dents and fracture due to contact with quays or floating objects
3) Cracks in the longitudinal or transverse direction as a result of development of cracks in the internal members
4) Wear to the shell due to internal corrosion in addition to the above defects
5) Cracks might occur in the side shell due to shearing force caused by inappropriate jumping loads. however, no cases of damage to a side shell due to shear caused by loads in the longitudinal direction have been reported.
Fig. 8-7 Corrosion of the side shell
Fig. 8.8 Damage due to Contact
Number in the circle corresponds to the followings
The side shell at the bow and the stern is thinner than the shell in midship part. That is why corrosion of the side shell progresses in the region between the fore peak tank and the end of No. 1 hold, and between the aft peak tank and the engine room. In aged ships the fracture openings spears in way of the engine room. When the shell plate has the excessive rust, the plate thickness should be measured.
(2) Dents and fracture due to contact
1) Both forward and aft ends of flat parts
Dents and fractured openings are likely to appear in these parts when the ship comes into contact with the quay when berthing. These defects are often observed in car carriersand ships operating in narrow waterways, such as the St. Lawrence waterway are also observed to have dents at these area.
2) Fore and aft body near the waterline
Dents appear in these area when the ship is pushed by a powerful tug boat while berthing.
3) Stern
Deformation appears at the stem when the ship hits floating objects. Sometimes it is difficult to find such a deformation just looking from side. It is better to examine from just in front of the ship.
Indent of the shell is difficult to distinguish so sometimes it is better to examine from the deck looking downwards .
4) Near the bell-mouth
Dents and fractured openings are caused when the anchor fluke frequently hits the side shell.
5) Range of abrasion by chain
Grooved corrosion due to chafing by chain appears in this area. cf 8-(2)-2
6) Near the hatch openings
In ships that load/unload cargo from/to barges, dents are often caused when the barge or the cargo comes into contact with the ship.
7) Propeller and rudder
The propeller and rudder might hit floating objects, get fouled in fishing nets, propeller blades might be bent, guard ring might fall off, and the portable box for the rudder sometimes drops off.
8) Bilge keel
Instances have been reported of a part of or the entire bilge keel dropping off after a ship hit a floating object. The damage of the bilge keel have been observed frequently. Carefully check the deformation of bilge keel looking through from fore end to aft end.
(3) Cracks
Cracks in the shell plate mostly develop from cracks of the internal members and they spread to the shell plate. Cracks might occur in the transverse or longitudinal direction; some cracks appears in the shape of a star.
1) Aft end of collision bulkhead
A large number of internal members, such as frames and stringers, are provided in a relatively small area of the fore peak tank. It means that the fore peak tank is a rigid construction collision. While the hold adjacent to the fore peak tank is a large, broad space with a small numbers of internal members per unit volume. Moreover, this region is often subject to large wave impacts, which cause cracks in No. 1 hold.
As the reinforcement against panting, classification rules request side stringers or brackets in the region between the collision bulkhead and 0.15L from the bow. But at the end of these members crack appears and develop to the side shell. Therefore, the vicinity of the ends of these members should be carefully inspected . Initial cracks appear in the vertical direction and have a length of 50 mm to 100 mm.
Fig.8-9 Extension of internal crack to shell plate (1)
Fig. 8-10 Extension of internal cracks to shell plate (2)
Fig.8-11 Crack near the bulkhead
3) Bilge keel See Section 9.2.
4) ,(5) Cracks in the sheer strake at the ends of superstructures
Cracks sometimes appear at the sheer strake at the fore and aft ends of the superstructure (Bridge) of "Three Islander" ship and long Poop or long F'cle ships because of hogging and sagging.
Sheer strake at the break of the superstructure should be carefully examined
Fig.8.12 Crack at the sheer strake.
(6) Shell plate in way of the the aft peak tank
In ships with a long aft peak tank such as ocean tugs boats or some car carriers, aft peak above the rudder is flat and wide. sometimes crack because of stern vibration appears in the APT and may propagate to shell plate. Care should be taken.
(7) Bilge part
Cracks at the lower ends of frames in cargo holds sometimes extend to the shell plate. Cracks in the shell plate appear in the horizontal direction
.
(8) Vicinity of bulkheads
Cracks may also occur in the vertical direction along the bulkhead due to the difference in rigidity of the frame and the bulkhead. these cracks propagete to shell plate. This situation is shown in the figures below. Shell plate In the vicinity of bulkhead should be carefully examined not only outside but in the hold side.
especially in aged ships
(9) Crack in rudder plate ; Please refer to the Rudder section.
4) Corrosion
The shell plate is generally painted when the ship is in drydock but in the hold it is not well painted. In the following area shell plate is thinner than other area. If necessary the thickness should be measured in aged ships. Locations to be checked carefully are shown in the figure below.
Fig.8-13 Corrosion from inside
Number in the circle corresponds the followings.
.
(1) Chain locker bottom
Ventilation of the chain locker is not enough and bilge water accumulates in the bottom, resulting in the rapid onset of corrosion. In large ships, the chain locker is isolated from the shell plate, corrosion does not appear in the shell plate. However, in hte normal ships, the shell plate forms a part of the chain locker, side shell plate corresponding to chain locker bottom soul be carefully examined.
Fig. 8-14 Chain locker bottom
(2) Tank top
Corrosion at the sides of the tank top plate proceeds faster than other area, but it is not faster than in chain locker. If corrosion at the sides of the tank top plate in the aged ships is neglected, it extends to the shell plate and sometimes it leads to corroded openings appear in the shell plate. In ships where the sides are raised, such as bulk carriers, this problem does not occur.
(3) Aft end of 'tween deck
In ships with a 'tween deck, bilge water generally accumulates at the aft end of the deck and causes corrosion that extends to the shell plate. This is not a major problem, except in aged ships.
(4) Near the forecastle aft bulkhead
Bilge water in the forecastle accumulates at the aft end on both side. Therefore, forecastle end wall on both lowest corners to be carefully examined .
(5) Corrosion below side scuttles
Starting with Special Survey No. 2, it is mandatory to measure the thickness of the shell plate below side scuttles. Rules prescribe inspections of the condition of the shell plate below side scuttles after removing the lining during Special Survey No. 3. In practice, the space below the side scuttle is narrow and is covered with lining boards. Sea water often enters through open side scuttles; the humidity is so high, and corroded fracture openings gradually appear in the shell plate. Because this area is above the waterline, we need not be excessively concerned about the danger of water flooding into the ship immediately. However, in the past, the "Umegaka Maru" owned by NK capsized because of water flooding the ship through side scuttles;
therefore, when we entering the dock, check the plate below the scuttles on the super-structure from the dock floor or dock sides and compare the condition with the surrounding shell plate. If rusting is excessive, enter the cabin after completion of the bottom inspection and ask to remove the lining and examine the state of corrosion in the superstructure side plate.
Ref. The Classification Rules In the Special Survey N0.3 request as follows : "The lining in way of the side scuttles is to be removed as required by the surveyor, and the shell plating should be examined."
Fig.8-15 Pay attention to the area below the side scuttles
Photo 9.11
Attention to the lower area of side scuttle
(6) Forward and aft of bulkheads See (8)
(7) Upper edge of bilge strakeSee 5) of Section 9.2 for external corrosion
of the bilge strake
Typical defects found in the bottom and the side shell are as follows.
8.1 Bottom Shell
(1) Bottom scratches and Dents
Most damage to the bottom shell is due to contact with objects on the seabed. Bottom scratches are slight damages. Such damage happens when the ship operates in shallow waters such as the Mississippi river, near the coastline, etc. In many instances, bottom damage due to contact starts from the forward part and disappears around midship.
Because of scratches, the bottom paint has peeled off and corrosion occurs. In this case, remaining paint and rust should be cleaned by shot blasting and properly re-painted . When the bottom has had contact with a coral reef or rocky seabed, large dents and/or holes appear with scratches. In this case, as a matter of fact, fractured shell plate should be cropped and renewed with damaged internal members. If the dents are relatively small, they may be left as they are with some effective internal reinforcement.
If we find a large fractured opening during the bottom inspection, naturally we should recommend repairs. However, there have been many instances where the shipowner has believed that there were no abnormalities and dents were found during the bottom inspection, which naturally gives the shipowner a headache. The shipowner usually decided sailing schedule beforehand, they hate prolonging docking period because of major repairs, they persists in putting off major repairs, and try to carry out simple repairs within the drydocking period. That is why sparks usually fly between the Surveyor and the shipowner's representative.
Regardless of experience of bottom inspection, the surveyor always feels uneasy before docking survey.
Fig.8-1 Damage in bottom shell
(2) Dents due to panting
When the ship sails in rough weather without reducing speed, dents might occur in the keel and A strake starting from the fore peak tank to the middle part of No.1 Water Ballast Tank , because of the relation between ship speed and curvature of the forward bottom. In the worst case, the floors in the tank and bottom stiffeners buckled.
Naturally, the Surveyor should recommend repairs to the shell, as well as the internal members.
(3) Corrosion of keel and adjusent A strake
Re-painting of the area on the keel block is impossible. So after undocking and also long voyage, these unpainted area is much more corrosive than painted area. the corrosion of welded beads is more heavy than the plate itself.
At the bottom survey previous un-painted area which we can find easily because of much rust should be carefully examined especially welded beads. When the beads is excessively corroded, rust should be removed and it is necessary to re- weld and after well painted.
(4) Corrosion of beads
Grooved corrosion, which at first glance looks like cracking often occurs in heat affected zones of seam and butt weld. It means the both side of beads. The corroded beads should be gouged and re-welded.
(5) Wrinkled corrosion
Small craft and ships adopting the transverse system of framing might develop wrinkles in the transverse direction in the mid-ship area of the bottom shell. These wrinkles are considered to occur because of buckling. If possible, it is better to fit the transverse carlings in the double bottom tank to prevent buckling.
(6) Sea chest
After removing the grating of the sea chest in way of the engine room or the pump room, climb onto the staging and inspect the internal parts of the sea chest. The sea chest forms a discontinuity with the rest of the bottom shell; therefore, cracks might appears at fillet welds of girders and floor plates.
Fig.8-2 Cracks in Sea Chest
Photo 8.1 Corrosion of Keel and A strake in way of forward bottom
Photo. 8.2 Ship with False Keel
When the ship with a fales keelis seated on the ordinary keel blocks, fales keel and center girder may be heavily damaged.
i
Photo. 8-3 Big damage in bottom (1)
Ballast water comes out from the double bottom tank.
Photo 8-4 Big damage in bottom (2)
Big damage can be detected by anybody
Photo 8-5 Slight Dent
A slight dent in a curved plate is difficult to detect
Photo 8-6 Corrosion of welded Bead (1)
Corrosion in a welded bead of a bottom shell plate looks like a crack, but is not a crack. As shown in the macro etching, this corrosion appears in a heat-affected zone.
Photo 8.7 Corrosion of Beads (2) Macro Etching
Photo 8-8 Reed Screen Bottom Plate Corrosion
Stress corrosion due to buckling in the midship region of the bottom plate in a transversely- framed construction; the dent itself is small. Reinforcement by fitting a carling inside the tank is recommended.
9.2 Bilge Keel
In large tankers with the midship coefficient approaching 1.0, the bilge keel may not be fitted when the ship is newly built. If bilge keels have not been provided on both sides during the bottom inspection, check whether the ship had no bilge keels when the ship was constructed or whether they have been ripped off during a marine casualty. If there are traces of welded beads on the bilge strake, we may conclude that the bilge keels have been ripped off during a marine casualty.
Damage to the bilge keel is as follows;
(1) Bilge keel dropped off, a part of bilge keel ripped off, kinks in bilge keel
The bilge keel is a member that does not need to conform to classification society rules. However, if it is damaged, it is normal to repair the bilge keel, usually under insurance.
(2) Cracks at ends
Aged ships which constructed with rivet not weld. Rivets at the ends of the bilge keels often worked loose. In a welded ship, the welds at the both ends are provided with large leg lengths, but sometimes cracks are found at the ends.
(3) Local joints of the bilge keel
The block butt in the hull becomes a local joint in the bilge keel. If welding at this location is defective, cracks appear in the joint. If the crack progresses and reaches the bilge strake, water penetrates into the hull. In case of tankers, this defect leads to marine pollution. During a bottom survey, check the side shell plates at the location of the bilge keel join.
(4) Box-type bilge keel
In fine and high speed ships, box-type bilge keels are frequently used. The box structure is watertight, but water sometimes penetrates into the box through small cracks in the welds of local joints. If the local joint is wet, there is a possibility that water has penetrated into the box. In such cases, carry out the air test, find locations where leaks start and recommend welding repairs.
(5) Corrosion of the bilge strake
Although not directly related to the bilge keel, when we inspect the bilge keel, check for corrosion of the bilge strake above it. Bige strake above the bilge keel
is corrosive than other strake.
Photo 8-9 Deformation of bilge keel
Photo 8.10 Corrosion of the Bilge Strake above the bilge keel
The bilge keel is not directly welded to the shell; it is generally connected to the shell through a flat bar. If it is welded directly to the shell, there is a strong possibility that cracks might develop in the bilge strake when the bilge keel suffers damage.
Fig. 8-3 Example of Bilge Keel
Fig. 8-4 Crack at Bilge Keel End (1)
Fig. 8.5 Crack at Bilge Keel End and joint of bilge keel
Cracks tat the joints of the bilge keel might develop, causing cracks in the bilge strake.
Fig. 8-6 Example of Box-type Bilge Keel
8.3 Side shellThe followings are the example of the defects in the side shell.
1) Corrosion
2) Dents and fracture due to contact with quays or floating objects
3) Cracks in the longitudinal or transverse direction as a result of development of cracks in the internal members
4) Wear to the shell due to internal corrosion in addition to the above defects
5) Cracks might occur in the side shell due to shearing force caused by inappropriate jumping loads. however, no cases of damage to a side shell due to shear caused by loads in the longitudinal direction have been reported.
Fig. 8-7 Corrosion of the side shell
Fig. 8.8 Damage due to Contact
Number in the circle corresponds to the followings
The side shell at the bow and the stern is thinner than the shell in midship part. That is why corrosion of the side shell progresses in the region between the fore peak tank and the end of No. 1 hold, and between the aft peak tank and the engine room. In aged ships the fracture openings spears in way of the engine room. When the shell plate has the excessive rust, the plate thickness should be measured.
(2) Dents and fracture due to contact
1) Both forward and aft ends of flat parts
Dents and fractured openings are likely to appear in these parts when the ship comes into contact with the quay when berthing. These defects are often observed in car carriersand ships operating in narrow waterways, such as the St. Lawrence waterway are also observed to have dents at these area.
2) Fore and aft body near the waterline
Dents appear in these area when the ship is pushed by a powerful tug boat while berthing.
3) Stern
Deformation appears at the stem when the ship hits floating objects. Sometimes it is difficult to find such a deformation just looking from side. It is better to examine from just in front of the ship.
Indent of the shell is difficult to distinguish so sometimes it is better to examine from the deck looking downwards .
4) Near the bell-mouth
Dents and fractured openings are caused when the anchor fluke frequently hits the side shell.
5) Range of abrasion by chain
Grooved corrosion due to chafing by chain appears in this area. cf 8-(2)-2
6) Near the hatch openings
In ships that load/unload cargo from/to barges, dents are often caused when the barge or the cargo comes into contact with the ship.
7) Propeller and rudder
The propeller and rudder might hit floating objects, get fouled in fishing nets, propeller blades might be bent, guard ring might fall off, and the portable box for the rudder sometimes drops off.
8) Bilge keel
Instances have been reported of a part of or the entire bilge keel dropping off after a ship hit a floating object. The damage of the bilge keel have been observed frequently. Carefully check the deformation of bilge keel looking through from fore end to aft end.
(3) Cracks
Cracks in the shell plate mostly develop from cracks of the internal members and they spread to the shell plate. Cracks might occur in the transverse or longitudinal direction; some cracks appears in the shape of a star.
1) Aft end of collision bulkhead
A large number of internal members, such as frames and stringers, are provided in a relatively small area of the fore peak tank. It means that the fore peak tank is a rigid construction collision. While the hold adjacent to the fore peak tank is a large, broad space with a small numbers of internal members per unit volume. Moreover, this region is often subject to large wave impacts, which cause cracks in No. 1 hold.
As the reinforcement against panting, classification rules request side stringers or brackets in the region between the collision bulkhead and 0.15L from the bow. But at the end of these members crack appears and develop to the side shell. Therefore, the vicinity of the ends of these members should be carefully inspected . Initial cracks appear in the vertical direction and have a length of 50 mm to 100 mm.
Fig.8-9 Extension of internal crack to shell plate (1)
Fig. 8-10 Extension of internal cracks to shell plate (2)
Fig.8-11 Crack near the bulkhead
3) Bilge keel See Section 9.2.
4) ,(5) Cracks in the sheer strake at the ends of superstructures
Cracks sometimes appear at the sheer strake at the fore and aft ends of the superstructure (Bridge) of "Three Islander" ship and long Poop or long F'cle ships because of hogging and sagging.
Sheer strake at the break of the superstructure should be carefully examined
Fig.8.12 Crack at the sheer strake.
(6) Shell plate in way of the the aft peak tank
In ships with a long aft peak tank such as ocean tugs boats or some car carriers, aft peak above the rudder is flat and wide. sometimes crack because of stern vibration appears in the APT and may propagate to shell plate. Care should be taken.
(7) Bilge part
Cracks at the lower ends of frames in cargo holds sometimes extend to the shell plate. Cracks in the shell plate appear in the horizontal direction
.
(8) Vicinity of bulkheads
Cracks may also occur in the vertical direction along the bulkhead due to the difference in rigidity of the frame and the bulkhead. these cracks propagete to shell plate. This situation is shown in the figures below. Shell plate In the vicinity of bulkhead should be carefully examined not only outside but in the hold side.
especially in aged ships
(9) Crack in rudder plate ; Please refer to the Rudder section.
4) Corrosion
The shell plate is generally painted when the ship is in drydock but in the hold it is not well painted. In the following area shell plate is thinner than other area. If necessary the thickness should be measured in aged ships. Locations to be checked carefully are shown in the figure below.
Fig.8-13 Corrosion from inside
Number in the circle corresponds the followings.
.
(1) Chain locker bottom
Ventilation of the chain locker is not enough and bilge water accumulates in the bottom, resulting in the rapid onset of corrosion. In large ships, the chain locker is isolated from the shell plate, corrosion does not appear in the shell plate. However, in hte normal ships, the shell plate forms a part of the chain locker, side shell plate corresponding to chain locker bottom soul be carefully examined.
Fig. 8-14 Chain locker bottom
(2) Tank top
Corrosion at the sides of the tank top plate proceeds faster than other area, but it is not faster than in chain locker. If corrosion at the sides of the tank top plate in the aged ships is neglected, it extends to the shell plate and sometimes it leads to corroded openings appear in the shell plate. In ships where the sides are raised, such as bulk carriers, this problem does not occur.
(3) Aft end of 'tween deck
In ships with a 'tween deck, bilge water generally accumulates at the aft end of the deck and causes corrosion that extends to the shell plate. This is not a major problem, except in aged ships.
(4) Near the forecastle aft bulkhead
Bilge water in the forecastle accumulates at the aft end on both side. Therefore, forecastle end wall on both lowest corners to be carefully examined .
(5) Corrosion below side scuttles
Starting with Special Survey No. 2, it is mandatory to measure the thickness of the shell plate below side scuttles. Rules prescribe inspections of the condition of the shell plate below side scuttles after removing the lining during Special Survey No. 3. In practice, the space below the side scuttle is narrow and is covered with lining boards. Sea water often enters through open side scuttles; the humidity is so high, and corroded fracture openings gradually appear in the shell plate. Because this area is above the waterline, we need not be excessively concerned about the danger of water flooding into the ship immediately. However, in the past, the "Umegaka Maru" owned by NK capsized because of water flooding the ship through side scuttles;
therefore, when we entering the dock, check the plate below the scuttles on the super-structure from the dock floor or dock sides and compare the condition with the surrounding shell plate. If rusting is excessive, enter the cabin after completion of the bottom inspection and ask to remove the lining and examine the state of corrosion in the superstructure side plate.
Ref. The Classification Rules In the Special Survey N0.3 request as follows : "The lining in way of the side scuttles is to be removed as required by the surveyor, and the shell plating should be examined."
Fig.8-15 Pay attention to the area below the side scuttles
Photo 9.11
Attention to the lower area of side scuttle
(6) Forward and aft of bulkheads See (8)
(7) Upper edge of bilge strakeSee 5) of Section 9.2 for external corrosion
of the bilge strake
No comments:
Post a Comment