Upper deck areas
The longitudinally continuous upper deck of a bulk carrier suffers hull girder stress. The longitudinal bending causes an axial
force on the upper deck that may cause cracking of the deck plate at the locations where the stress is concentrated.
Bulk carriers have cargo hatchways for the convenience of cargo handling facilities. These hatchways reduce the ship's torsional strength and invite concentrated stress at the hatchway corners which may be evident by cracking of the deck plates in these areas.
Cross deck strips come under stress by transverse bending. The transverse bulkheads provide transverse strength to a bulk carrier and the cross deck strips provide the strength to withstand the resultant axial forces in a transverse direction.
Check points on the upper deck
Deformation Buckling of cross deck strips 6.1.4
Generally, longitudinal beams are arranged under the longitudinally continuous upper deck outboard of the side lines of the
cargo hatchways. This is called the longitudinal system. When the deck beams for cross deck strips are also arranged in this manner, buckling
of the cross deck strips may take place due to insufficient strength against the axial forces acting on them in a transverse direction. The
transverse system is the preferred method of construction for cross deck members. Particular attention should be given to buckling of the main
deck on those ships where the cross deck strips are arranged in the longitudinal system.
: Comparison of stiffening systems for cross deck
There are various types of cracking in the upper deck. Those propagating from the cargo hatchways are generally considered serious to the ship's safety:
The large cargo hatchway openings reduce the torsional strength of the hull and invite stress concentration at their
corners on the upper deck. In this regard, upper deck plating at hatchway corners is one of the focal points for cracking. Particular attention should be paid to these areas during inspection.
: Cracking at hatchway
Upper deck plating at deck fittings
Various metal fittings are welded to the upper deck plating. These installations may cause stress concentrations at the welded joints or have defects in the welds. Deck platings in the vicinity of manholes, hatchside coaming end brackets, bulwark stays, crane post foundations and deck houses, etc. are to be carefully watched for cracking.
: Various cracking in upper deck plating
Hatch coamings are subjected to hull girder stress. Although they are not critical longitudinal strength members, they should be watched carefully to ensure that these cracks do not spread. Cracking may be initiated at defects in welded joints and metal fittings to the coamings that will invite stress concentration. Such cracking is considered serious to the ship's safety because it may be the initiation of a fracture on a large scale.
: Cracking in hatch coaming
Corrosion on deck 6.1.6
Thinner steel structures on deck, such as cross deck strips, hatch coamings, hatch covers, etc., are easily corroded and often holed.
The best way to deter corrosion is to keep the structure well coated and painted. The parts most liable to corrosion in the upper deck area are as follows:
Cross deck strips
The thickness of cross deck plating between hatchways is designed about a half of that of main strength deck plating because it is not a longitudinal strength member. However, cross deck strips provide an important part of the transverse strength of the ship, and corrosion and waste of the cross deck plating may be considered serious to the ship's soundness.
|L (m)||At main deck plating||At cross-deck strip|
|Handy bulk carrier||177.00||17.5 mm (32 H/T)||10.0 mm (MS)|
|Panamax bulk carrier||215.00||20.5 mm (36 H/T)||10.5 mm (MS)|
|Capesize bulk carrier||280.00||33.0 mm (40 H/T)||12.0 mm (MS)|
: Examples of comparison of thickness of main deck and cross deck
The thickness of hatch covers is approximately the same as that of cross decks. Holes in hatch covers caused by corrosion lead to water ingress in cargo holds, which may lead to shifting of cargo and/or problems with the stability of the ship.
When steam pipes are arranged beside hatchside coamings, the corrosion progress of the coaming is very rapid. Corrosion holes of the coaming plates lead to the same problems as those associated with hatch cover corrosion.
Weathertight doors, small hatches and wall ventilator covers
Not only covers, door plates and coamings but also hinges, gaskets and clips are to be always kept in good condition.
Standing pipes on deck
Vent and sounding pipes from water ballast or fuel oil tanks and ventilation pipes for closed spaces under the upper deck are liable to corrosion. If these pieces become holed, seawater comes directly into the tanks or cargo cause contamination of fuel oil, cargo damage, shifting of cargo, and/or stability problems.
Forecastle aft wall
The bilges in forecastle space may cause corrosion of the bulkhead where it meets the deck. In flush decked bulk carriers, the boatswain store aft wall may be corroded in the same manner. Large bulk carriers generally do not have forecastle and have their boatswain stores down below the upper deck in fore peak spaces. Bilges left in such spaces also cause corrosion of the aft end bulkheads which separates boatswain store and No.1 cargo hold. Such wastage may lead to water ingress, cargo damage, cargo shifting, and/or stability problems.
: Forecastle end bulkhead
Structural features 6.2.1
On typical bulk carriers, the topside and bilge hopper tanks compose a double hull surrounding the cargo space, which together with the double bottom provides hull strength and rigidity. Single hulled side shells provided with individual transverse frames are located between the topside and bilge hopper tanks. In recent designs, these hold frames and end brackets are thinner than the side shell and are not constructed with web frames and side stringers as is the case with general cargo carriers.
Below is a comparison of thickness of hull skin plates and hold frames
in cargo hold.
|Tanktop (mm)||Hold frame (mm)||Side shell (mm)|
|Handy BC||15.0 (MS)||9.0 (MS)||12.0 (MS)||10.0 (MS)||12.5 (MS)|
|Panamax (BC)||17.0 (MS)||10.0 (36HT)||12.5 (HT)||11.0 (36HT)||15.5 (32HT)|
|Capesize (BC)||18.5 (MS)||10.0 (36HT)||17.0 (HT)||12.0 (36HT)||17.5 (32HT)|
: Comparison of thickness of hull skin and hold frames
Corrosion and waste of hold frames 6.2.2
Corrosion generally attacks thinner steel structures and is accelerated in thinner plates. In the time a thicker steel plate loses half of its original thickness a thinner plate might corrode completely.
Among the various members composing cargo hold structures, the
hold frames are usually the thinnest structures, especially at the web plates. In addition, the hold frames also have more surface area exposed, in that both surfaces of the plate are susceptible.
This may mean accelerated corrosion in the hold frames, the
thinnest among all the members in cargo holds. If corrosion and waste become excessive, failure of hold frames invites additional loads to the
adjacent ones, which may lead to failure throughout the side shell structure.
: An example of a corroded hold frame
Transverse bulkheads may also be susceptible to accelerated
corrosion, particularly at the midheight and at the bottom. Particular care should be exercised when inspecting hold frames and transverse
bulkheads, in that these members may appear in deceptively good condition. Tanktop and side shell plating generally corrode from the
steel surface facing the cargo hold and corrosion from inside the double bottom is usually less than that from cargo hold side.
Regarding the corrosiveness of cargoes, coal is among the most
corrosive of cargoes carried on board bulk carriers. Thickness measurement surveys reveal that bulk carriers which have been employed
in carriage of coal suffer more serious corrosion to their cargo holds than those engaged in the carriage of any other cargoes.
Cargo hold frames should also be carefully inspected for mechanical damage, corrosion and waste, because many cargoes will damage hold frames through direct contact. This damage will invite corrosion from seawater brought on board in loading operations.
The most important aspects of cargo hold inspections are the
condition of side shell structures and their reinforcements. Special attention should be paid to the condition of hold frames and their
connection to the shell plating.
Transbulkheads and associated structures 6.2.9
Bulk carrier watertight transverse bulkheads at the ends of dry cargo holds are constructed in various ways, which in general can be categorized as either vertically corrugated with or without upper or lower stools, double plated with or without upper or lower stools, or plane bulkheads vertically stiffened.
It may be necessary that certain holds bounded by the foregoing categories of bulkheads are partially filled with water ballast in order to achieve a satisfactory air draught at the loading/discharge berths. The filling is restricted to correspond to the dry cargo hold scantlings. However, for deep tank corrugated bulkheads at the ends of cargo holds which are designed to be fully filled with water ballast, the scantlings are increased substantially from that for ordinary watertight transverse bulkheads.
The opportunity is taken to emphasize that for ordinary transverse watertight bulkheads, in addition to withstanding water pressure in an emergency situation, i.e., flooding, the bulkhead structures constitute the main structural strength elements in the
structural design of the intact vessel. Ensuring that acceptable strength is maintained for these structures is therefore of major importance.
The structure may sometimes appear to be in good condition
when it is in fact excessively corroded. In view of this, appropriate access arrangements should be provided to enable a proper close-up inspection and thickness assessment.*
It is imperative to realize that in the event of one hold flooding, the transverse watertight bulkheads prevent progressive flooding and therefore also prevent the ship from sinking.
What to look for 6.2.14
The following are examples of the more common damage/defects
that may occur:
"Fractures" at the boundaries of corrugations and bulkhead stools, particularly in way of shelf plates, shredder plates,
deck, inner bottom, etc.
"Buckling" of the plating/corrugations, leading to the failure and collapse of the bulkhead under water pressure in an emergency situation.
"Excessive wastage/corrosion", in particular at the midheight and bottom of bulkheads, which may look in deceptively good condition. This is created by the corrosive effect of cargo and environment, in particular when the structure is not coated. In this respect special attention should be given to the following areas:
bulkhead plating adjacent to the shell plating;
bulkhead trunks which form part of the venting, filling and discharging arrangements between the topside tanks and the hopper tanks;
bulkhead plating and weld connections to the lower/upper stool shelf plates;
weld connections of stool plating to the lower/upper stool shelf plates and inner bottom;
in way of weld connections to topside tanks and hopper tanks;
any areas where coatings have broken down and there is evidence of corrosion or wastage. It is recommended that random thickness determination be taken to establish the level of diminution; and
other structures, e.g., diaphragms inside the stools, particularly at their upper and lower weld connections.
Typical fracturing at the connection of transverse bulkhead structure
Damage caused by cargoes 6.2.15
In cargo holds, tanktop plating and side shell structures are
apt to be damaged by cargo handling operations.
At loading and unloading ports for coal or iron ore, large grab buckets, high capacity cargo loaders, bulldozers and pneumatic hammers may be employed for cargo handling operations.
Large grab buckets may cause considerable damage to tanktop
plating when being dropped to grab cargo. Use of bulldozers and pneumatic hammers may also be harmful to cargo hold structures and may
result in damage to tanktops, bilge hoppers, hold frames and end brackets.
Lumber cargoes may also cause damage to the cargo hold structures of smaller bulkers that are employed in the carriage of light bulk cargoes and lumbers.
Combination cargo/ballast hold
In bulk carriers having combination cargo/ballast holds, cracks may often be found at or near the connection of the stool of the transverse bulkhead and the tank top.
Cracking at the connection of bulkhead stool and tanktop
All capesize and panamax bulk carriers and some handy bulkers have
combination cargo/ballast hold(s) to keep the necessary draught. The bulkhead boundaries of the spaces are designed to comply with the
requirements for deep tank bulkheads. In these holds cracks may often be found at the connection between the transverse bulkhead and the tanktop. These cracks can be detected by visual inspection or by noting leakage from the double bottom tanks.
Side stringers and/or side shells in way of No.1 cargo hold along the collision bulkhead are often found cracked. This kind of damage is considered to be caused by insufficient continuity between fore peak construction and cargo hold structure.
On large bulk carriers such as capesize and panamax bulkers, bilge hopper plating around the knuckle line may be cracked along the bilge hopper transverse webs. This is considered to be caused by insufficient local reinforcement.
Cracking around the collision bulkhead
Cracking in bilge hopper
Corrosion and wastage of steel, especially in the upper part of the topside tanks, should be carefully watched.
Though the water ballast tanks of newer bulk carriers are well
protected against corrosion, the upper portion is susceptible to corrosion because the protective coating will easily deteriorate due to heat from the upper deck and the cyclic wet/dry effect of seawater.
Bilge hopper/double bottom tanks
When carrying out inspections of these tanks, particular attention should be paid to any cracking, deformation or deterioration of coating.
Cracking in bilge hopper and double bottom tanks 6.4.2
Cracks may be found at the intersections of longitudinals and
transverse members and at other locations as follows:
Intersections of longitudinals and solid floors
Cracks may be found in the side, bottom and/or tanktop longitudinals at intersections with solid floors or bilge hopper transverses. Cracks also may be found in the floors or transverses occurring at the corners of the slots cut for longitudinals.
Cracking in tanktop/bottom longitudinals
Cracking of longitudinals at areas of structural discontinuity
Longitudinals may be cracked at the ends of additional (partial) side girders provided in the double bottom below cargo hold bulkheads or at the side walls of bilge wells for cargo holds, due to additional stress concentration caused by the structural discontinuity at those connections.
Cracking at the end connection with side walls of bilge well and cracking at the end of an additional girder
Bilge hopper transverse
Cracks may be observed in transverse webs in bilge hoppers initiating from the slot openings for longitudinals and at the knuckled corners of the lower ends of the hoppers.
Check points in bilge hopper transverse
Corrosion must be carefully watched in the inspection of water ballast tanks, particularly in older bulk carriers over 10 years of age. In general, the condition of the steel and protective coatings will be in satisfactory condition much longer in the double bottoms than in the topside compartments. However, even double bottom tanks will deteriorate in time due to the continual ballasting of the ship.
Corrosion accelerated by heat
Since the late '70s, problems with heavy corrosion in double bottom water ballast tanks adjacent to fuel oil tanks have appeared. In some cases, the corrosion was worse in areas closer to the fuel oil tank boundaries. In those ships, fuel oil tanks were installed.
Areas where heavy corrosion due to heat effect may be found (hatched areas)
: Progress of corrosion in water ballast tanks adjacent to fuel oil tanks (FOT)
The fuel oil heating system was adopted following changes to the properties of fuel oil, mainly an increase in viscosity. For economic reasons, ship operators began to use low grade bunker oil which needs heating in order to decrease the viscosity. At the beginning of this
trend, the temperature required in the fuel oil tanks was not high enough to accelerate the corrosion of the steel in the adjacent spaces.
However, in recent years, the grade of bunker oil being used requires the temperature in the tank to be 80 degrees C or more. Such a temperature can accelerate corrosion of the steel in the tanks,
particularly in the vicinity of the boundaries of the fuel oil tanks.
Areas under suction bell mouths
Bottom plates are often eroded under the suction bell mouths in tanks. On drydocking of an older ship, the bell mouths should be dismantled for examination of the condition of the shell plates below the bell mouths.
Bottom ends of sounding pipes 6.5.1
A sounding pipe has a pad plate at its bottom end for protection of the tank bottom against the strike of the sounding scale's lead. During inspections the extent of diminution of the protection plate should be examined.
Connection trunk between topside and bilge hopper spaces 6.5.2
Connection trunks provided between topside and bilge hopper
spaces are to be carefully watched for signs of corrosion and waste of the steel works inside.
On some bulk carriers, bilge hopper tanks and topside tanks form one integral tank connected with trunk spaces. The inside surface of a connection trunk is liable to corrosion and should be examined arefully.
Connection trunk entrance in topside tank
* Refer to section 5.3 of annex A to resolution A.744(18), Guidelines on the enhanced programme of inspections during surveys of bulk carriers and oil tankers, as amended, and to the Guidelines on the means of access to structures for inspection and maintenance of oil tankers and bulk carriers (MSC/Circ.686).