Procedure for Bottom Survey
Similar to other inspections, a bottom survey is a kind of visual inspection.
Tools used during a bottom survey are a test hammer, light, measuring tape, and a stretched string for measuring the depth of dents. A large hole or indent in bottom shell can be detected easily by anybody, but it is very difficult to
find any small indent or cracks while walking around the dock. The bottom area of the world's largest tanker, "Yahre Viking," has an area of approximately 30,000 square meters, which is about three times the area of a football ground. The bottom survey is hard work; if we get tired, we are likely to overlook large dents. Procedures for detecting defects in the hull when the ship is docked are described below.
6.1 Inspection of Bottom Shell in Dry Condition
If we inspect the shell immediately after water is discharged from the dock and when the hull is still wet, we cannot see the water on the shell if there is a leakage of ballast water through fine cracks in shell plate or remaining water. In the same way a bottom inspection is extremely difficult during the rainy season and on damp days because a large amount of dew remains on the shell plate due to the difference between ambient and shell temperatures. If major damage due to stranding or contact is known beforehand, even if the shell plate is
still wet and uncleaned, we should inspect damage as soon as possible to decide repair method and extent. In this case the bottom should be inspected again after the shell plate is dry and cleaned.
6.2 Inspection before Painting
Shell plating is to be inspected before re- painting. Because in a wet painted condition we cannot detect fine cracks. If you refuse bottom inspection on holidays or weekends, and carry out the inspection on the following working day, there is a possibility that painting will have already commenced and small cracks will be covered with paint.
6.3 Symmetrical Examination
Notwithstanding the bottom survey, the hull structure is generally symmetrical about the centreline. With the exception of local damage, such as dents due to contact and cracks, if we find a crack on the starboard side of the hull, there is a high probability of finding a similar crack on the port side at the same location, although there are exceptions to this case. We need to check both port and starboard sides paying attention to both sides.
6.4 Dirty Spots
On both bottom and side shells, if we find spots that are dirtier than the surroundings, there is a possibility of a flaw in the vicinity. The area where the paint has peeled off due to abrasion must be carefully examined.
Generally, such an area will be badly corroded.
6.5 Wet Locations
If we find partly wet spots in a dry shell area, small cracks may be concealed. After cleaning the shell plate, sometimes algae or barnacles remain at wet areas. Such locations often develop cracks. From this point of view, the bottom inspection should be carried out when the shell is completely dry.
6.6 How to Detect Dents
Large local dents can be easily detected but it is easy to overlook a dent that extends over a wide area. In particular, dents in the curved shell plate in the bilge, fore and aft peak parts are difficult to identify. If the side shell plate is viewed directly from the dock side, dents can be overlooked easily
because of the effect of light rays striking the plate at this area. There was an instance of a large dent being detected in the side shell plate after all inspections and repairs were completed and the ship was just about to sail.
1) Side shell plate
If the side shell plate is inspected by looking at it from various angles or by looking up from the dock , dents can be detected easily. Locations that are suspect may be observed later by looking down and inspecting the shell plate from the deck. A dent in the plate can be overlooked if the shell plate is examined from position A. The plate must also be viewed from position B to check for dents.
Tools used during a bottom survey are a test hammer, light, measuring tape, and a stretched string for measuring the depth of dents. A large hole or indent in bottom shell can be detected easily by anybody, but it is very difficult to
find any small indent or cracks while walking around the dock. The bottom area of the world's largest tanker, "Yahre Viking," has an area of approximately 30,000 square meters, which is about three times the area of a football ground. The bottom survey is hard work; if we get tired, we are likely to overlook large dents. Procedures for detecting defects in the hull when the ship is docked are described below.
6.1 Inspection of Bottom Shell in Dry Condition
If we inspect the shell immediately after water is discharged from the dock and when the hull is still wet, we cannot see the water on the shell if there is a leakage of ballast water through fine cracks in shell plate or remaining water. In the same way a bottom inspection is extremely difficult during the rainy season and on damp days because a large amount of dew remains on the shell plate due to the difference between ambient and shell temperatures. If major damage due to stranding or contact is known beforehand, even if the shell plate is
still wet and uncleaned, we should inspect damage as soon as possible to decide repair method and extent. In this case the bottom should be inspected again after the shell plate is dry and cleaned.
6.2 Inspection before Painting
Shell plating is to be inspected before re- painting. Because in a wet painted condition we cannot detect fine cracks. If you refuse bottom inspection on holidays or weekends, and carry out the inspection on the following working day, there is a possibility that painting will have already commenced and small cracks will be covered with paint.
6.3 Symmetrical Examination
Notwithstanding the bottom survey, the hull structure is generally symmetrical about the centreline. With the exception of local damage, such as dents due to contact and cracks, if we find a crack on the starboard side of the hull, there is a high probability of finding a similar crack on the port side at the same location, although there are exceptions to this case. We need to check both port and starboard sides paying attention to both sides.
6.4 Dirty Spots
On both bottom and side shells, if we find spots that are dirtier than the surroundings, there is a possibility of a flaw in the vicinity. The area where the paint has peeled off due to abrasion must be carefully examined.
Generally, such an area will be badly corroded.
6.5 Wet Locations
If we find partly wet spots in a dry shell area, small cracks may be concealed. After cleaning the shell plate, sometimes algae or barnacles remain at wet areas. Such locations often develop cracks. From this point of view, the bottom inspection should be carried out when the shell is completely dry.
6.6 How to Detect Dents
Large local dents can be easily detected but it is easy to overlook a dent that extends over a wide area. In particular, dents in the curved shell plate in the bilge, fore and aft peak parts are difficult to identify. If the side shell plate is viewed directly from the dock side, dents can be overlooked easily
because of the effect of light rays striking the plate at this area. There was an instance of a large dent being detected in the side shell plate after all inspections and repairs were completed and the ship was just about to sail.
1) Side shell plate
If the side shell plate is inspected by looking at it from various angles or by looking up from the dock , dents can be detected easily. Locations that are suspect may be observed later by looking down and inspecting the shell plate from the deck. A dent in the plate can be overlooked if the shell plate is examined from position A. The plate must also be viewed from position B to check for dents.
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A dent in the plate can be overlooked if the shell plate is examined from position "A". The plate must also be viewed from position "B" .
Have a look the side shell plate in the vertical and fore-aft directions.
2) Bottom shell
Small dents in the bottom shell plate can be detected easily if we bend our waist and look backwards to view the bottom shell between legs, thereby lowering your line of vision. When the beam of light is projected parallel to the bottom, a dent, if present, can be detected as it will appear dark. However, in a ship of riveted construction, the lapped parts of the bottom shell appear shaded and are likely to be mistaken for dents.
If we find a clearance between the keel block and the keel, a dent is likely to exist in the keel.
When the big and widly repairs works to the bottom are carried out , in the next docking widly dent may appear in the same area.
In case the floating dock , the dock itself may deform as the same as bottom. In this case a clearance between keel blok and keel may not be appeared.
Fig.6.1 Examination of Bottom Shell (1)
Lower our line of vision, If we inspect the side shell by looking down from the deck,we can easily detect dents .
Fig. 6.2 Examination of Bottom Shell (2)
point the light parallel to the shell
6.7 Measuring and Recording of Dents
If the dent is minor and repair is not necessary at that time , it may be recorded in the survey report without outstanding recommendation. At the next docking survey the same atra should be re- measured. if the size of the dent is increased, repairs should be recommended.
6.7.1 How to Measure Dents
Measurement using a stretched string is easier. Use two magnets for securing both ends of the string to the bottom shell, and measure the dent using a scale. This measure-ment can be performed single-hand. The measurement using transit is also useful,
6.7.2 Precautions during Measurement
Because the measurement is to be carried out to determine whether a dent has increased in depth by measuring the same location again during the next docking, care should be taken to record the measurement points and the reference points for measurements correctly, so that the dent can be measured at the same location and compared to the previous measurement. As shown in the figure below, A and C are taken as
reference points, and the depth of the dent is recorded as PR. During the next measurement, if A and B are taken as reference points and the
depth of the dent is taken as PQ, it indicates that the dent has reduced. The fwd-aft position and position in the breadth direction of the reference points should be recorded for future reference, as shown in the figure below. The reference points should preferably be taken at bulkheads or other locations where movement is considered to be minimal. In the case of a double bottom, record whether the tank is empty or full during the measurement.
Fig.6.4 Deformation of bottom shell
Fig.6.5 Measurement of Dent (1)
Fig.6.6 Measurement of Dent (２)
6.8 Bend in Bilge Keel
Although no requirements for bilge keels are prescribed in classification rules, if bilge keels are fitted, they should be inspected. Bilge keels might bend because of contact with the sea bed or contact with floating objects.
If we view the bilge keel from end to end, we can easily detect a bend.
Fig. 6.7 Damage in bilge keel
6.9 Damage to Internal Members
Even a minor dent of bottom shell plating it may accompany a damage of internal members such as frame, bulkhead or floor. In this case not only
external inspection in the dock but also internal inspection in that tank or
hold should be requested.
If a minor dent is located in the fuel oil tank, the remaining fuel oil must be shifted to other tanks to carry out the internal inspection. In winter,
sometimes transferring fuel oil is very difficult because of its low viscosity, so we should negotiate with the shipownwers to carry out the internal inspection at the next docking depends on the condition of damage.
Photo. 6.1 Slight Dent
Photo. 6.2 Inside the Double Bottom Tank in Area fo a slight Dent
6.10 Shell Plate Terminology and Shell ExpansionPlan
Refer to the Shell Expansion Plan when we want to check the size, type and thickness of a shell plate in which damage has occurred. The Shell Expansion Plan shows the bottom shell and side shell on one sheet of drawing with a 1:2 scale for units in the longitudinal and transverse (vertical) directions. That is, if the scale in the length direction is 1:100, the scale in the width direction is 1:50. In addition to shell data, the positions of holds and tanks, frame spacing and dimensions of all frames are also shown in this drawing. Plates on the shell are named as follows:
K for keel plate; plates adjacent to the keel starting from the garboard strake are named sequentially as A, B, C, D, E, F, G, H, J, L (not K as "K" is used for the keel plate; not "I" as it is likely to be misunderstood for some other symbol). The topmost strake (sheer strake) is named S (using the initial letter of sheer strake). For the same strake (say H-strake), the plates are numbered 1, 2, 3, and so on, starting from aft and proceeding forward. The fore and aft parts of the hull are slender, and the plates are narrow; therefore, at the stern, the plates adjacent to the C strake and nearer to the centre are divided into two strakes, C and D, while at the bow, the C and D strakes are combined to form the C strake. The tank top and bulkheads that fall on the side shell are indicated by broken lines; the frames are indicated by single dot and dash lines. The number above
mark is the plate thickness to distinguish it from other values. Class D, Class E, etc. indicating the kinds of stee,such as D class steel etc. The mark (<) shows a joint in the breadth direction of the plate; a long S shows a joint in the length direction or a block joint.
Fig.6.8 Shell Expansion (1) Fig 6.9 Shell expansion （２）
Fig.6.10 Shell Expansion (3)
B-17 and C-17 plate, both 16mm in thickness and C-17plate is D grade steel.
6-11 Repair symbol mark
There are 5 international repair marks. The origin is unknown.
Have a look the side shell plate in the vertical and fore-aft directions.
2) Bottom shell
Small dents in the bottom shell plate can be detected easily if we bend our waist and look backwards to view the bottom shell between legs, thereby lowering your line of vision. When the beam of light is projected parallel to the bottom, a dent, if present, can be detected as it will appear dark. However, in a ship of riveted construction, the lapped parts of the bottom shell appear shaded and are likely to be mistaken for dents.
If we find a clearance between the keel block and the keel, a dent is likely to exist in the keel.
When the big and widly repairs works to the bottom are carried out , in the next docking widly dent may appear in the same area.
In case the floating dock , the dock itself may deform as the same as bottom. In this case a clearance between keel blok and keel may not be appeared.
Fig.6.1 Examination of Bottom Shell (1)
Lower our line of vision, If we inspect the side shell by looking down from the deck,we can easily detect dents .
Fig. 6.2 Examination of Bottom Shell (2)
point the light parallel to the shell
6.7 Measuring and Recording of Dents
If the dent is minor and repair is not necessary at that time , it may be recorded in the survey report without outstanding recommendation. At the next docking survey the same atra should be re- measured. if the size of the dent is increased, repairs should be recommended.
6.7.1 How to Measure Dents
Measurement using a stretched string is easier. Use two magnets for securing both ends of the string to the bottom shell, and measure the dent using a scale. This measure-ment can be performed single-hand. The measurement using transit is also useful,
6.7.2 Precautions during Measurement
Because the measurement is to be carried out to determine whether a dent has increased in depth by measuring the same location again during the next docking, care should be taken to record the measurement points and the reference points for measurements correctly, so that the dent can be measured at the same location and compared to the previous measurement. As shown in the figure below, A and C are taken as
reference points, and the depth of the dent is recorded as PR. During the next measurement, if A and B are taken as reference points and the
depth of the dent is taken as PQ, it indicates that the dent has reduced. The fwd-aft position and position in the breadth direction of the reference points should be recorded for future reference, as shown in the figure below. The reference points should preferably be taken at bulkheads or other locations where movement is considered to be minimal. In the case of a double bottom, record whether the tank is empty or full during the measurement.
Fig.6.4 Deformation of bottom shell
Fig.6.5 Measurement of Dent (1)
Fig.6.6 Measurement of Dent (２)
6.8 Bend in Bilge Keel
Although no requirements for bilge keels are prescribed in classification rules, if bilge keels are fitted, they should be inspected. Bilge keels might bend because of contact with the sea bed or contact with floating objects.
If we view the bilge keel from end to end, we can easily detect a bend.
Fig. 6.7 Damage in bilge keel
6.9 Damage to Internal Members
Even a minor dent of bottom shell plating it may accompany a damage of internal members such as frame, bulkhead or floor. In this case not only
external inspection in the dock but also internal inspection in that tank or
hold should be requested.
If a minor dent is located in the fuel oil tank, the remaining fuel oil must be shifted to other tanks to carry out the internal inspection. In winter,
sometimes transferring fuel oil is very difficult because of its low viscosity, so we should negotiate with the shipownwers to carry out the internal inspection at the next docking depends on the condition of damage.
Photo. 6.1 Slight Dent
Photo. 6.2 Inside the Double Bottom Tank in Area fo a slight Dent
6.10 Shell Plate Terminology and Shell ExpansionPlan
Refer to the Shell Expansion Plan when we want to check the size, type and thickness of a shell plate in which damage has occurred. The Shell Expansion Plan shows the bottom shell and side shell on one sheet of drawing with a 1:2 scale for units in the longitudinal and transverse (vertical) directions. That is, if the scale in the length direction is 1:100, the scale in the width direction is 1:50. In addition to shell data, the positions of holds and tanks, frame spacing and dimensions of all frames are also shown in this drawing. Plates on the shell are named as follows:
K for keel plate; plates adjacent to the keel starting from the garboard strake are named sequentially as A, B, C, D, E, F, G, H, J, L (not K as "K" is used for the keel plate; not "I" as it is likely to be misunderstood for some other symbol). The topmost strake (sheer strake) is named S (using the initial letter of sheer strake). For the same strake (say H-strake), the plates are numbered 1, 2, 3, and so on, starting from aft and proceeding forward. The fore and aft parts of the hull are slender, and the plates are narrow; therefore, at the stern, the plates adjacent to the C strake and nearer to the centre are divided into two strakes, C and D, while at the bow, the C and D strakes are combined to form the C strake. The tank top and bulkheads that fall on the side shell are indicated by broken lines; the frames are indicated by single dot and dash lines. The number above
mark is the plate thickness to distinguish it from other values. Class D, Class E, etc. indicating the kinds of stee,such as D class steel etc. The mark (<) shows a joint in the breadth direction of the plate; a long S shows a joint in the length direction or a block joint.Fig.6.8 Shell Expansion (1) Fig 6.9 Shell expansion （２）
Fig.6.10 Shell Expansion (3)
B-17 and C-17 plate, both 16mm in thickness and C-17plate is D grade steel.
6-11 Repair symbol mark
There are 5 international repair marks. The origin is unknown.
 | Remove , fair and refit : This work is for rivetted ship. |
 | Crop, fair and refit : this work is for rivetted ship |
 | Fair in place: to recover the dent by heating and water cooling |
 | Partly renew: Only deformed part |
 | Renew : all plate renew |
Fig.6-11 Repair symbol mark
Fig.6-12 Example of repair mark
S-15 Renew , S-16 Partly renew 、N-15 and N-16: Fair in place,
S-15 26 (E) means : 26 is the thickness of plate and (E) is the grade of
steel. (E-grade steel), (B) is B-grade steel
Fig.6-12 Example of repair mark
S-15 Renew , S-16 Partly renew 、N-15 and N-16: Fair in place,
S-15 26 (E) means : 26 is the thickness of plate and (E) is the grade of
steel. (E-grade steel), (B) is B-grade steel
6-12 How to Predict Defective Locations
If defects such as dents are found in the side shell plate or the bottom plate, the location of the part where the defect has occurred should be recorded. However, a hold or tank with a dent cannot be identified from the outside. This has to be judged later by observing the Shell Expansion Plan, but the methods described below may be used to confirm the approximate position of the defect.
Fig.6.12 Identification of hold from outside (mast or bottom plug)
I
(1) Judging the position of dent by the mast or crane post
If the dock is wide, move the dock side so that we can see the mast or crane post on the deck. These are usually installed on a bulkhead. For instance, you can judge whether the dent is between No. 2 Hold and the No. 3 Hold. However, if the width of the ship covers almost the entire width of the dock, this method cannot be applied.
(2) Judging the position of the dent looking for the bottom plugs In double bottom tanks, bottom plugs are installed at the aft end of the tanks near the centreline of the ship for draining residual bilge in the tanks when the ship is docked. These plugs are coveredwith cement; therefore, if you observe a raised part similar to a small dish, it is a bottom plug. From the bottom plugs, you can aee the approximate position of the aft end of each tank and estimate the position of the damage.
Fig. 6.13 Identification of double bottom tank
If defects such as dents are found in the side shell plate or the bottom plate, the location of the part where the defect has occurred should be recorded. However, a hold or tank with a dent cannot be identified from the outside. This has to be judged later by observing the Shell Expansion Plan, but the methods described below may be used to confirm the approximate position of the defect.
Fig.6.12 Identification of hold from outside (mast or bottom plug)
I
(1) Judging the position of dent by the mast or crane post
If the dock is wide, move the dock side so that we can see the mast or crane post on the deck. These are usually installed on a bulkhead. For instance, you can judge whether the dent is between No. 2 Hold and the No. 3 Hold. However, if the width of the ship covers almost the entire width of the dock, this method cannot be applied.
(2) Judging the position of the dent looking for the bottom plugs In double bottom tanks, bottom plugs are installed at the aft end of the tanks near the centreline of the ship for draining residual bilge in the tanks when the ship is docked. These plugs are coveredwith cement; therefore, if you observe a raised part similar to a small dish, it is a bottom plug. From the bottom plugs, you can aee the approximate position of the aft end of each tank and estimate the position of the damage.
Fig. 6.13 Identification of double bottom tank
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