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8/10/2019 Vol2_Section12

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Section 12 - Tank Structures A 12 - 1

Section 12

Tank Structures

A. General

1. Subdivision of tanks

1.1 In tanks extending over the full breadth of the ship

intended to be used for partial filling, (e.g. oil fuel and fresh

water tanks), at least one longitudinal bulkhead is to be

fitted, which may be a swash bulkhead.

1.2 Where the forepeak is intended to be used as tank,

at least one complete or partial longitudinal swash bulkhead

is to be fitted, if the tank breadth exceeds 0,5 Bor 6 m,

whichever is the greater.

When the afterpeak is intended to be used as tank, at least

one complete or partial longitudinal swash bulkhead is to

be fitted. The largest breadth of the liquid surface should

not exceed 0,3 Bin the aft peak.

1.3 Peak tanks exceeding 0,06 Lor 6 m in length,

whichever is greater, shall be provided with a transverse

swash bulkhead.

2. Air, overflow and sounding pipes

Each tank is to be fitted with air pipes, overflow pipes andsounding pipes. The air pipes are to be led to above the

exposed deck. The arrangement is to be such as to allow

complete filling of the tanks. The height from upper surface

of deck to their openings is to be at least 760 mm on the

freeboard deck and 450 mm on a superstructure deck. See

also Section 21, E.

The sounding pipes are to be led to the bottom of the tanks

(see also Rules for Machinery Installations, Volume III,

Section 11).

3. Forepeak tank

Oil is not to be carried in a forepeak tank or a tank forward

of the collision bulkhead . See also SOLAS 74, Chapter

II-2, Reg. 15.6 and MARPOL 73/78, Annex I, Reg. 14.4.

4. Cross references

4.1 Where a tank bulkhead forms part of a watertight

bulkhead, its strength is not to be less than required by

Section 11.

4.2 For pumping and piping, see also Rules for

Machinery Installations, Volume III, Section 11. For Oil

fuel tanks see also Rules for Machinery Installations,

Volume III, Section 10. For tanks in the double bottom,

see Section 8, B.5.

4.3 For cargo oil tanks see Section 24.

4.4 For dry cargo holds which are also intended to be

used as ballast water tanks, see C.2.

4.5 For testing of tanks, see H.

4.6 Where tanks are provided with cross flooding

arrangements the increase of the pressure head is to be taken

into consideration (see also Section 29-I, J. and

Section 36, G.).

5. Separation of fuel oil tanks from tanks for other

liquids

5.1 Fuel oil tanks are to be separated from tanks for

lubricating oil, hydraulic oil, thermal oil, vegetable oil,

feedwater, condensate water and potable water by

cofferdams1)

.

5.2 Upon special approval on small ships the

arrangement of cofferdams between oil fuel and lubricating

oil tanks may be dispensed with provided that:

.1 the common boundary is continuous, i.e. it does not

abut at the adjacent tank boundaries, see Fig. 12.1

Where the common boundary cannot be constructed

continuously according to Fig. 12.1, the fillet welds

on both sides of the common boundary are to be

welded in two layers and the throat thickness is

not to be less than 0,5 t (t = plate thickness);

Fig. 12.1 Continuous common boundary

replacing a cofferdam

.2 stiffeners or pipes do not penetrate the common

boundary;

.3 the corrosion allowance tKfor the common boundary

is not less than 2,5 mm.

1)For Indonesian flag ship, the cofferdams are also

required between accommodation spaces and oil tanks.


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Section 12 - Tank Structures A12 - 2

5.3 Fuel oil tanks adjacent to lubricating oil circulation

tanks are subject to the provisions of Rules for Machinery

Installations, Volume III, Section 10, B.2.1.5 in addition

to the requirements stipulated in 5.2 above.

5.4 Fuel oil tanks and lubrication oil tanks adjacent to

refrigerated spaces to comply with Rules for the

Construction of Refrigerating Installation, Volume VIII,

M.2.8.

5.5 For fuel tanks which are heated up to a temperature

which is higher than the flash point 10 C of the relevant

fuel, Rules for Machinery Installations, Rules for Machinery

Installations, Volume III, Section.10, B.5. must be observed

specifically.

6. Tanks for heated liquids

6.1 Where heated liquids are intended to be carried intanks, a calculation of thermal stresses is required, if the

carriage temperature of the liquid exceeds the following

values:

T = 65 C in case of longitudinal framing,

= 80 C in case of transverse framing.

6.2 The calculations are to be carried out for both

temperatures, the actual carriage temperature and the limit

temperature T according to 6.1.

The calculations are to give the resultant stresses in the

hull structure based on a sea water temperature of 0 C

and an air temperature of 5 C.

Constructional measures and/or strengthenings will be

required on the basis of the results of the calculation for

both temperatures.

7. Minimum thickness

7.1 The thickness of all tank structures is not to be less

than the following minimum value:

tmin = 5,5 + 0,02 L [mm]

7.2 For fuel oil, lubrication oil and fresh water tanks

tminneed not be taken greater than 7,5 mm

7.3 For ballast tanks of dry cargo ships tminneed not

be taken greater than 9,0 mm.

7.4 For oil tankers see Section 24, A.13.

8. Plating and stiffeners in the propeller area

and in the engine room

8.1 General

From a vibration point of view shell and tankstructures in the vicinity of the propeller and the

mainengine should be designed such that the design

criteria defined in 8.3 to 8.5 are fulfilled.

8.2 Definitions

fplate2)

= lowest natural frequency of isotropic plate field

under consideration of additional outfitting and

hydrodynamic masses [Hz]

fstiff

2)

= lowest natural frequency of stiffener underconsideration of additional outfitting and

hydrodynamic masses [Hz]

dp = propeller diameter [m]

r = distance of plate field or stiffener to 12 o'clock

propeller blade tip position [m]

dr = ratior

dp

= flare angle of frame section in propeller plane

measured between a vertical line and the tangent

to the bottom shell plating

n = maximum propeller shaft revolution rate [1/min]

z = number of propeller blades

fblade = propeller blade passage excitation frequency

at n [Hz]

=1

60n z [Hz]

ne = maximum main engine revolution rate [1/min]

nc = number of cylinders of main engine

kstroke = number indicating the type of main engine

= 1,0 for 2-stroke (slow-running) main engines

= 0,5 for 4-stroke (medium speed) main

engines

fignition = main engine ignition frequency at ne

=1

60k

stroken z [Hz]

8.3 Shell structures in propeller area

Plate fields and stiffeners of shell structures in vicinity of

the propeller(s) within dr = 3 should fulfil the followingfrequency criteria:

for 60o

fplate

>4,6

dr

fblade

fstiff

>4,6

dr

fblade

for < 60o

fplate

>2,3

dr

fblade

drneeds not to be taken less than 1,0.

2)The natural frequencies of plate fields and stiffeners can be

estimated by approved computer program


3/7

Section 12 - Tank Structures A 12 - 3

8.4 Tank structures in propeller area

Plate fields and stiffeners of tank structures in vicinity of

the propeller(s) within dr= 5 should fulfil the following

frequency criteria:

for 60o fplate

> 6,3d

r

fblade

fstiff

>6,3

dr

fblade

for < 60o

fplate

>3,15

dr

fblade

drneeds not to be taken less than 1,3.

8.5 Tank structures in main engine area

Plate fields and stiffeners of tanks located in the engine

room should at all filling states fulfil the frequency criteriaas summarised in Table 12.1.

Generally, direct connections between transverse engine

top bracings and tank structures shall be avoided. Pipe

fittings at tank walls etc. shall be designed in such a way

that the same frequency criteria as given for plates are

fulfilled.

Table 12.1 Frequency criteria

Engine type Mounting type Application area Frequency criteria

Slow speed Rigid Tanks within engine room

fplate> 1,2 . Fignition

fstiff > 1,2 . Fignition

and

fplate< 1,8 . Fignitionor

fplate> 2,2 . Fignition

Medium speed

Rigid or semi-resilient Tanks within engine room

fplate

< 0,8 . Fignition

or

fplate > 1,2 . Fignition

and

fstiff< 0,8 . Fignitionor

fstiff> 1,2 . Fignition

Resilient

Tanks within engine

length up to next platform

deck above inner bottom

fplate< 0,9 . Fignitionor

fplate< 1,1 . Fignition


4/7

Section 12 - Tank Structures B12 - 4

B. Scantlings

1. Definitions

k = material factor according to Section 2, B.2.

a = spacing of stiffeners or load width in [m]

= unsupported span in [m] according to Section

3, C.

p = load p1or pdin [kN/m2] according to Section

4, D.; the greater load to be taken.

For tank structures of tanks adjacent to the shell the pressure

p below Tminneed not be larger than:

p = [kN/m2]p

1 10 (T

minz) p

0c

f1

z

Tmin

Tmin = smallest design ballast draught [m]

z = distance of structural member above baseline.[m]

p2 = load in [kN/m2] according to Section 4, D.1.

tK = corrosion allowance according to Section 3, K.1.

h = filling height of tank in [m]

et = characteristic tank dimension tor btin [m]

t = tank length in [m]

bt = tank breadth in [m]

p = [N/mm

2

]

235

k

2

S

3

2

L S

0,89

L

L = design hull girder bending stress at the position

considered in [N/mm2] according to Section

5,D.1

L = shear stress due to longitudinal hull girder

bending in [N/mm2] at the position considered

see also Section 5,D.1.

nf = 1,0 for transverse stiffening

= 0,83 for longitudinal stiffening

For the terms "constraint" and "simply supported" see

Section 3, D.

2. Plating

2.1 The plate thickness is not to be less than:

t1 = 1,1 a + tK [mm]p k

t2 = 0,9 a + tK [mm].p2 k

2.2 Above the requirements specified in 2.1 above the

thickness of tank boundaries (including deck and inner

bottom) carrying also normal and shear stresses due to

longitudinal hull girder bending is not to be less than:

t = [mm]16,8 nf ap

p

tK

2.3 Proof of buckling strength of longitudinal and

transverse bulkheads is to be carried out according to

Section 3, F. For longitudinal bulkheads the design stresses

according to Section 5, D.1. and the stresses due to local

loads are to be considered.

3. Stiffeners and girders

3.1 Stiffeners and girders, which are not considered

as longitudinal strength members

3.1.1 The section modulus of stiffeners and girders

constrained at their ends, is not to be less than:

W1 = 0,55 a2 p k [cm

3]

W2 = 0,44 a2 p2 k [cm

3].

Where one or both ends are simply supported, the section

moduli are to be increased by 50 percent.

The cross sectional area of the girder webs is not to be lessthan:

Aw1 = 0.05 a p k [cm2]

Aw2 = 0,04 a p2 k [cm2].

Aw2is to be increased by 50 percent at the position of

constraint for a length of 0,1 .

The buckling strength of the webs is to be checked

according to Section 3, F.

3.1.2 Where the scantlings of stiffeners and girders are

determined according to strength calculations, the following

permissible stress values apply:

S if subjected to load p:

b = [N/mm2]

150

k

= [N/mm2]

100

k

v = [N/mm2]b

2 32180

k

S if subjected to load p2:

b = [N/mm2]180k

= [N/mm2]

115

k

v = [N/mm2]b

2 32200

k

3.2 Stiffeners and girders, which are to be considered

as longitudinal strength members

3.2.1 The section moduli and shear areas of horizontal

stiffeners and girders are to be determined according to

Section 9, B. as for longitudinals. In this case for girderssupporting transverse stiffeners the factors m = 1 and ma.=.0

are to be used.


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Section 12 - Tank Structures B 12 - 5

3.2.2 Regarding buckling strength of girders the

requirements of 2.3 are to be observed.

3.3 The scantlings of beams and girders of tank decks

are also to comply with the requirements of Section 10.

3.4 For frames in tanks, see Section 9, A.2.2.

3.5 The stiffeners of tank bulkheads are to be attached

at their ends by brackets according to Section 3, D.2. The

scantlings of the brackets are to be determined according

to the section modulus of the stiffeners. Brackets must be

fitted where the length of the stiffeners exceeds 2 m.

The brackets of stiffeners are to extend to the next beam,

the next floor, the next frame, or are to be otherwise

supported at their ends.

3.6 Where stringers of transverse bulkheads are

supported at longitudinal bulkheads or at the side shell,

the supporting forces of these stringers are to be consideredwhen determining the shear stress in the longitudinal

bulkheads. Likewise, where vertical girders of transverse

bulkheads are supported at deck or inner bottom, the

supporting forces of these vertical girders are to be

considered when determining the shear stresses in the deck

or inner bottom respectively.

The shear stress introduced by the stringer into the

longitudinal bulkhead or side shell may be determined by

the following formula:

= [N/mm2]StP

St

2 bSt t

Pst = supporting force of stringer or vertical girder

[kN]

bSt = breadth of stringer or depth of vertical girder

including end bracket (if any) [m] at the

supporting point

t = see 2.2

The additional shear stress is to be added to the shearSt

stressLdue to longitudinal bending according to Section

5, D.1. in the following area:

0,5 m on both sides of the stringer in the ship's

longitudinal direction

0,25 .bStabove and below the stringer

Thereby the following requirement must be satisfied.

4. Corrugated bulkheads

4.1 The plate thicknesses of corrugated bulkheads as

well as the required section moduli of corrugated bulkhead

elements are to be determined according to 2. and 3.,

proceeding analogously to Section 11, B.4.

The plate thickness is not to be less than tmin, according

to A.7, or

S if subjected to load p1

tcrit = [mm]b

905D tK

S if subjected load p2

tcrit = [mm]b960

D tK

D = compressive stress [N/mm2]

b = breadth of face plate strip in [mm]

4.2 For the end attachment Section 3, D.4. is to be

observed.

5. Thickness of clad plating

5.1 Where the yield point of the cladding is not less than

that of the base material the plate thickness is to bedetermined according to 2.1.

5.2 Where the yield point of the cladding is less than

that of the base material the plate thickness is not to be less

than:

t1 = 0,55 a [mm]pk

AtK

t2 = 0,45 a [mm]p2k

AtK

for one side clad steel:

A = 0,25 Stp

2 t1 S r S

tp

2 t1 S r2

for both side clad steel:

A = 0,25 Stp

t1 S

tp

t1 S r

t = plate thickness including cladding in [mm]

tp = thickness of the cladding in [mm]

r =Rep

ReH

Rep = minimum nominal upper yield point of the

cladding in [N/mm2] at service temperature

ReH = minimum nominal upper yield point of the base

material in [N/mm2] according to Section 2, B.2.

5.3 The plate thicknesses determined in accordance with

5.1 and 5.2 respectively may be reduced by 0,5 mm. For

chemical tankers the reductions as per Rules for ShipsCarrying Dangerous Chemicals in Bulk, Volume X,

Section.4, 4 0.1.3 apply.


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Section 12 - Tank Structures E12 - 6

C. Tanks with Large Lengths or Breadths

1. General

Tanks with lengths t> 0,1 Lor breadths bt> 0,6 B(e.g.

hold spaces for ballast water) which are intended to be

partially filled, are to be investigated to avoid resonancebetween the liquid motion and the pitch or roll motion of

the ship. If necessary, critical tank filling ratios are to be

avoided. The ship's periods of pitch and roll motion as well

as the natural periods of the liquid in the tank may be

determined by the following formulae:

Natural period of liquid in tank :

T ,b = [s]1,132et

f

f = hyperbolic function as follows:

= tanh h

et

Period of wave excited maximum pitch motion :

Ts = [s]L

1,17 L 0,15 vo

= ahead speed of ship [kn] as defined inv0

Section 1, H.5.

Period of roll motion :

Tr = [s]

cr B

GM

cr = 0,78 in general

= 0,70 for tankers in ballast

0,07 B in generalGM

0,12 B for tankers and bulk carriers.

2. Hold spaces for ballast water

In addition to the requirements specified under 1. above

for hold spaces of dry cargo ships and bulk carriers, which

are intended to be filled with ballast water, the following

is to be observed:

.1 For hold spaces only permitted to be completely

filled, a relevant notice will be entered into the

Certificate.

.2 Adequate venting of the hold spaces and of the

hatchway trunks is to be provided.

.3 For frames also Section 9, A.2.2 is to be observed.

D. Vegetable Oil Tanks

1. Further to the regulations stipulated under A. andB. for vegetable oil tanks, the following requirements are

to be observed.

2. Tanks carrying vegetable oil or similar liquids, the

scantlings of which are determined according to B., are

to be either fully loaded or empty. A corresponding note

will be entered into the Certificate.

These tanks may be partially filled provided they are

subdivided according to A.1.2. Filling ratios between 70and 90 percent should be avoided.

3. In tanks carrying vegetable oil or similar liquids

sufficient air pipes are to be fitted for pressure equalizing.

Expansion trunks of about 1 per cent of the tank volume

are to be provided. Where the tank is subdivided by at least

one centre line bulkhead, 3 per cent of the tank may remain

empty and be used as expansion space.

E. Detached Tanks

1. General

1.1 Detached tanks are to be adequately secured against

forces due to the ship's motions.

1.2 Detached tanks in hold spaces are also to be provided

with anti floatation devices. It is to be assumed that the

hold spaces are flooded to the load water line. The stresses

in the anti floatation devices caused by the floatation forces

are not to exceed the material's yield stress.

1.3 Detached oil fuel tanks should not be installed in

cargo holds. Where such an arrangement cannot be avoided,

provision is to be made to ensure that the cargo cannot bedamaged by leakage oil.

1.4 Fittings and pipings on detached tanks are to be

protected by battens, and gutterways are to be fitted on the

outside of tanks for draining any leakage oil.

2. Scantlings

2.1 The thickness of plating of detached tanks is to be

determined according to B.2.1 using the formula for t1and

the pressure p as defined in 2.2.

2.2 The section modulus of stiffeners of detached tanks

is not to be less than:

W = c a2 p k [cm

3]

c = 0,36 if stiffeners are constrained at both ends

= 0,54 if one or both ends are simply supported

p = 9,81 h [kN/m2]

h = head measured from the load centre of plate

panel or stiffener respectively to the top of

overflow; the height of overflow is not to be

taken less than 2,5 m.

2.3 For minimum thickness the requirements of A.7

apply in general.


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Section 12 - Tank Structures H 12 - 7

F. Potable Water Tanks

1. Potable water tanks shall be separated from tanks

containing liquids other than potable water, ballast water,

distillate or feed water.

2. In no case sanitary arrangement or correspondingpiping are to be fitted directly above the potable water tanks.

3. Manholes arranged in the tank top are to have sills.

4. If pipes carrying liquids other than potable water

are to be led through potable water tanks, they are to be

fitted in a pipe tunnel.

5. Air and overflow pipes of potable water tanks are

to be separated from pipes of other tanks.

G. Swash Bulkheads

1. The total area of perforation shall not be less than

5% and should not exceed 10% of the total bulkhead area.

2. The plate thickness shall, in general, be equal to

the minimum thickness according to A.7. Strengthenings

may be required for load bearing structural parts.

The free lower edge of a wash bulkhead is to be adequately

stiffened.

3. The section modulus of the stiffeners and girders

is not to be less than W1

as per B.3., however, in lieu of

p the load pdaccording to Section 4, D.2., but disregarding

pvis to be taken.

4. For swash bulkheads in oil tankers see also

Section 24, D.

H. Testing for Tightness

1. Testing of oil fuel, ballast, trimming, feed water,

fresh water and anti-rolling tanks is to be effected by a

combination of a leak test by means of air pressure and

an operational test by means of water or the liquid for whichthe tank is intended to be used. The air pressure is not to

exceed 0,2 bar gauge. The increased risk of accident while

the tanks are subjected to the air pressure is to be observed.

Butt welds made by approved automatic or semiautomatic

processes on erection welds need not be tested, provided

that these welds are carefully visually examined and are

free of repairs. The results of the non-destructive

examinations made at random to the satisfaction of the

Surveyor shall not reveal significant defects. If there is

evidence from inspection results that the quality of these

welds has been downgraded significantly, the extent of

the leak testing may be increased to the Surveyor's

discretion.

2. Where one tank boundary is formed by the ship's

shell, the leak test is to be carried out before launching.

For all other tanks leak testing may be carried out after

launching. Erection welds as well as welds of assembly

openings are to be coated3)

after the leak test is carried

out. This applies also to manual weld connections of

bulkheads with the other tanks boundaries and of collaring

arrangements at intersections of tank boundaries and e.g.frames, beams, girders, pipes etc. If it is ensured that in

adjacent tanks the same type of liquid is carried, e.g. in

adjacent ballast tanks, the above mentioned weld

connections may be coated3)

prior to the leak test.

All other welded connections in tank boundaries may be

coated prior to the leak test if it is ensured by suitable means

(e.g. by visual examination of the welded connections) that

the connections are completely welded and the surfaces

of the welds do not exhibit cracks or pores.

3. Where the tanks are not subjected to-the leak test

as per 2. but are leak tested with water the bulkheads area

in general, to be tested from one side. The testing should

be carried out prior to launching or in the dock. Subject

to approval by the Society, the test may also be carried out

after launching. Water testing may be carried out after

application of a coating3)

, provided that during the visual

inspection as per 2. above deficiencies are not noted. The

test head must correspond to a head of water of 2,5 m above

the top of tank or to the top of overflow or air pipe,

whichever is the greater.

4. The operational test may be carried out when the

ship is afloat or during the trial trip. For all tanks the proper

functioning of filling and suction lines and of the valvesas well as functioning and tightness of the vent, sounding

and overflow pipes is to be tested.

5. For testing of cargo tanks see Section 24, A.15.

3)Shop primers are not regarded as a coating within the

scope of these requirements.

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