Shell Defect

Shell Defects

8. Shell Defects8. Shell PlatingAt the renewal of the cracked or weared out plates, it is necessary to see the "ShellExpansion Plan" . Because there are many kind of steel plate such as mild stee platel,hightensile 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 DentsMost damage to the bottom shell is due to contact with objects on the seabed. Bottomscratches are slight damages. Such damage happens when the ship operates in shallowwaters such as the Mississippi river, near the coastline, etc. In many instances, bottomdamage 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/orholes appear with scratches. In this case, as a matter of fact, fractured shell plate shouldbe 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 shouldrecommend repairs. However, there have been many instances where the shipowner hasbelieved that there were no abnormalities and dents were found during the bottominspection, which naturally gives the shipowner a headache. The shipowner usuallydecided sailing schedule beforehand, they hate prolonging docking period because ofmajor repairs, they persists in putting off major repairs, and try to carry out simple repairswithin the drydocking period. That is why sparks usually fly between the Surveyor and theshipowner's representative.Regardless of experience of bottom inspection, the surveyor always feels uneasy beforedocking survey.

Fig.8-1 Damage in bottom shell

(2) Dents due to pantingWhen the ship sails in rough weather without reducing speed, dents might occur in thekeel and A strake starting from the fore peak tank to the middle part of No.1 Water BallastTank , because of the relation between ship speed and curvature of the forward bottom. Inthe 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

1.Introduction

2.Docking Survey

3.Survey Item

4.Type of Dry Dock

5.Safety in Dock

6.Procedure of BottomInspection

7.Course in Dock

8.Shell Defect

9.Stern Frame

10.Rudder

11.Propeller

12.Anchr

13.Chain Cable

14.Sea Valves

Hull Surveywelcome to

shell defect http://docking.ship-doctor.com/8/8_1.html

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members.

(3) Corrosion of keel and adjusent A strakeRe-painting of the area on the keel block is impossible. So after undocking and also longboyage, these unpainted area is much more corrosive than painted area. the corrosion ofwelded beads is more heavy than the plate itself.At the bottom survey previous un-painted area which we can find easily because of muchrust should be carafully examined espacially welded beads. When the beads isexcessively corroded, rust should be removed and it is necessary to re- weld and afterwell painted.(4) Corrosion of beadsGrooved corrosion, which at first glance looks like cracking often occurs in heat affectedzones of seam and butt weld. It means the both side of beads. The corroded beads shouldbe gouged and re-welded.(5) Wrinkled corrosionSmall craft and ships adopting the transverse system of framing might develop wrinkles inthe transverse direction in the midship area of the bottom shell. These wrinkles areconsidered to occur because of buckling. If possible, it is better to fit the transversecarlings in the double bottom tank to prevent buckling.(6) Sea chestAfter 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 formsa discontinuity with the rest of the bottom shell; therefore, cracs might apears at filletwelds of girders and floor plates.

Fig.8-2 Cracks in Sea Chest


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Photo 8.1 Corrosion of Keel and A strake in way of forward bottom

Photo. 8.2 Ship with False KeelWhen the ship with a fales keelis seated on the ordinary keel blocks, fales keel and centergirder may be heavily damaged.

iPhoto. 8-3Big 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


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Photo 8-5 Slight DentA 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. Asshown in the macro etching, this corrosion appears in a heat-affected zone.

Photo 8.7 Corrosion of Beads (2) Macro Etching


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Photo 8-8 Reed Screen Bottom Plate CorrosionStress corrosion due to buckling in the midship region of the bottom plate in atransversely- framed construction; the dent itself is small. Reinforcement by fitting acarling inside the tank is recommended.

9.2 Bilge KeelIn large tankers with the midship coefficient approaching 1.0, the bilge keel may not befitted when the ship is newly built. If bilge keels have not been provided on both sidesduring the bottom inspection, check whether the ship had no bilge keels when the shipwas constructed or whether they have been ripped off during a marine casualty. If thereare traces of welded beads on the bilge strake, we may conclude that the bilge keels havebeen 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 keelThe 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 endsAged ships which constructed with rivet not weld. Rivets at the ends of the bilge keelsoften worked loose. In a welded ship, the welds at the both ends are provided with largeleg lengths, but sometimes cracks are found at the ends.

(3) Local joints of the bilge keelThe block butt in the hull becomes a local joint in the bilge keel. If welding at this locationis defective, cracks appear in the joint. If the crack progresses and reaches the bilgestrake, water penetrates into the hull. In case of tankers, this defect leads to marinepollution. During a bottom survey, check the side shell plates at the location of the bilgekeeljoin.

(4) Box-type bilge keelIn fine and high speed ships, box-type bilge keels are frequently used. The box structureis watertight, but water sometimes penetrates into the box through small cracks in thewelds of local joints. If the local joint is wet, there is a possibility that water has penetratedinto the box. In such cases, carry out the air test, find locations where leaks start andrecommend welding repairs.

(5) Corrosion of the bilge strakeAlthough not directly related to the bilge keel, when we inspect the bilge keel, check forcorrosion of the bilge strake above it. Bige strake above the bilge keelis corrosive than other strake.


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Photo 8-9 Drformationof 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 shellthrough a flat bar. If it is welded directly to the shell, there is a strong possibility that cracksmight 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)


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Fig. 8.5 Crack at Bilge Keel End and joint of bilge keelCracks 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) Corrosion2) Dents and fracture due to contact with quays or floating objects3) Cracks in the longitudinal or transverse direction as a result of development of cracks inthe internal members4) Wear to the shell due to internal corrosion in addition to the above defects5) Cracks migh occur in the side shell due to shearing forcecaused by inappropriatejumping loads. however, no cases of damage to a side shell due to shear caused by loadsin the longitudinal direction have been reported.

Fig. 8-7 Corrosion of the side shell

Fig. 8.8 Damage due to ContactNumber 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 iswhy corrosion of the side shell progresses in the region between the fore peak tank and


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the end of No. 1 hold, and between the aft peak tank and the engine room. In aged shipsthe fracture openings sppears in way of the engine room. When the shell plate has theexcessive rust, the plate thickness should be measured.

(2) Dents and fracture due to contact

1) Both forward and aft ends of flat partsDents and fractured openings are likely to appear in these parts when the ship comes intocontact with the quay when berthing. These defects are often observed in car carriersandships operating in narrow waterways, such as the St. Lawrence waterway are alsoobserved to have dents at these area.

2) Fore and aft body near the waterlineDents appear in these area when the ship is pushed by a powerful tug boat while berthing.

3) SternDeformation apperas at the stem when the ship hits floating objects. Sometimes it isdifficult to find such a derormation just looking frome side. It is better to examine from justin front of the ship.Indent of the shell is difficult to distinguish so sometimes it is better to examinfrom thedeck looking dowawards .

4) Near the bell-mouthDents and fractured openings are caused when the anchor fluke frequently hits the sideshell.

5) Range of abrasion by chainGrooved corrosion due to chafing by chain appears in this area. cf 8-(2)-2

6) Near the hatch openingsIn ships that load/unload cargo from/to barges, dents are often caused when the barge orthe cargo comes into contact with the ship.

7) Propeller and rudderThe propeller and rudder might hit floating objects, get fouled in fishing nets, propellerblades might be bent, guard ring might fall off, and the portable box for the ruddersometimes drops off.8) Bilge keelInstances have been reported of a part of or the entire bilge keel dropping off after a shiphit a floating object. The dmage of the bilge keel have been observed frequently. Carefullycheck the deformation of billge keel lookimg throughfrom fore end to aft end.

(3) CracksCracks in the shell plate mostly develop from cracks of the internal members and theyspread 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 bulkheadA large number of internal members, such as frames and stringers, are provided in arelatively small area of the fore peak tank. It means that the fore peak tank is a rigidconstructioncollision. While the hold adjacent to the fore peak tank is a large, broad spacewith a small numbers of internal members per unit volume. Moreover, this region is oftensubject to large wave impacts, which cause cracks in No. 1 hold.As the reinforcement against panting, classification rules request side stringers orbrackets in the region between the collision bulkhead and 0.15L from the bow. But at theend of these members crack appears and develop to the side shell. Therefore, the vicinityof the ends of these members should be carefully inspected . Initial cracks appear in thevertical direction and have a length of 50 mm to 100 mm.


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Fig.8-9 Extension of internal crack to shell plate (1)


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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 superstructuresCracks sometimes appear at the sheer strake at the fore and aft ends of thesuperstructure (Bridge) of "Three Islander" ship and long Poop or long F'cle ships becauseof hoging 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 tankIn ships with a long aft peak tank such as ocean tugs boats or some car carriers, aft peakabove the rudder is flat and wide. sometimes crack because of stern vibration appears inthe APT and may propagate to shell plate. Care should be taken.

(7) Bilge partCracks 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


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.(8) Vicinity of bulkheadsCracks may also occur in the vertical direction along the bulkhead due to the difference inrigidity of the frame and the bulkhead. these cracks propagete toshell plate. Thissituation is shown in the figures below. Shell plate In the vicinity of bulkhead should becarefully 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) CorrosionThe shell plate is generally painted when the ship is in drydock but in the hold it is not wellpainted. In the following area shell plate is thinner than other area. If necessary thetickness should be measured in aged ships. Locations to be checked carefully are shownin the figure below.

Fig.8-13 Corrosion from insideNumber in the circle corresponds th he folowings..

(1) Chain locker bottomVentilation 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 theshell plate, corrosion does not appear in the shell plate. However, in hte normal ships, theshell plate forms a part of the chain locker, side shell plate corresponding to chain lockerbottom shoul be carefully examined.

Fig. 8-14 Chain locker bottom

(2) Tank topCorrosion at the sides of the tank top plate proceeds faster than other area, but it is notfaster than in chain locker. If corrosion at the sides of the tank top plate in the aged shipsis neglected, it extends to the shell plate and sometimes it leads to corroded openingsappear in the shell plate. In ships where the sides are raised, such as bulk carriers, thisproblem does not occur.(3) Aft end of 'tween deckIn ships with a 'tween deck, bilge water generally accumulates at the aft end of the deckand causes corrosion that extends to the shell plate. This is not a major problem, except inaged ships.(4) Near the forecastle aft bulkheadBilge water in the forecastle accumulates at the aft end on both side. Therefore, forecastleend wall on both lowest corners to be carefully ezamined .


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(5) Corrosion below side scuttlesStarting with Special Survey No. 2, it is mandatory to measure the thickness of the shellplate below side scuttles. Rules prescribe inspections of the condition of the shell platebelow side scuttles after removing the lining during Special Survey No. 3. In practice, thespace below the side scuttle is narrow and is covered with lining boards. Sea water oftenenters through open side scuttles; the humidity is so high, and corroded fracture openingsgradually appear in the shell plate. Because this area is above the waterline, we need notbe 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 waterflooding the ship through side scuttles;therefore, when we enterin 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 surroundingshell plate. If rusting is excessive, enter the cabin after completion of the bottominspection and ask to remove the lining and examine the state of corrosion in thesuperstructure side plate.

Ref. The Classification Rules In the Special Survey N0.3 request as follows : "The lining inway of the side scuttles is to be removed as required by the surveyor, and the shell platingshould be examined."

Fig.8-15 Pay attention to the area below the side scuttles

Photo 9.11Attention 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 corrosionof the bilge strake. ,SectionNo.


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Shell Defect