Deck Beams and Supporting Deck Structures
Section 12 Tank Structures
A. General
Note
The arrangement and subdivision of fuel oil tanks has to be in compliance with MARPOL, Annex I, Reg. 12 A "Oil Fuel Tank Protection".
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 B or 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 B in the aft peak.
1.3 Peak tanks exceeding 0,06 L or 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 and sounding 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 the deck to the point where the water way have access 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 b e 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 tK for the common boundary is not less than 2,5 mm.
5.3 Fuel oil tanks adjacent to lubricating oil circulation tanks are not permitted.
5.4 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, Volume III, Section.10, B.5.is to be observed specifically.
6. Tanks for heated liquids
6.1 Where heated liquids are intended to be carried in tanks, a calculation of thermal stresses is required, if the carriage temperature of the liquid exceeds the following values :
T = 65
E
C in case of longitudinal framing,= 80
E
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 0EC and an air temperature of 5EC.
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 tmin need not be taken greater than 7,5 mm 7.3 For ballast tanks of dry cargo ships tmin need not be taken greater than 9,0 mm.
7.4 For oil tankers see Section 24, A.14.
1) For Indonesian flag ship, the cofferdams are also required between accommodation spaces and oil tanks.
8. Plating and stiffeners in the propeller area and in the engine room 8.1 General
From a vibration point of view shell and tank structures in the vicinity of the propeller(s) and the main engine should be designed such that the design criteria defined in 8.3 to 8.5 are fulfilled (see also Section 6, F.1 and Section 8, A.1.2.3).
8.2 Definitions
fplate2)= lowest natural frequency of isotropic plate field under consideration of additional outfitting and hydrodynamic masses [Hz]
fstiff2) = lowest natural frequency of stiffener under consideration 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 = ratio
" = 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]
= [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 3) fignition = main engine ignition frequency at ne
= [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 following frequency criteria:
for " $ 60o
for " < 60o
dr needs not to be taken less than 1,0.
8.4 Tank structures in propeller area
For vessel with a single propeller, plate fields and stiffeners of tank structures within dr = 5 should fulfil the following frequency criteria :
2) The natural frequencies of plate fields and stiffeners can be estimated by approved computer program
3) The number is valid for in-line engines. The ignition frequency for V-engines depends on the V-angle of the cylinder banks and can be obtained from the engine manufacturer.
for " $ 60o
for " < 60o
dr needs not to be taken less than 1,3.
8.5 Tank structures in main engine area
For vessels with a single propeller, plate fields and stiffeners of tanks located in the engine room should at all filling states fulfil the frequency criteria as summarized 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.
B. Scantlings
1. Definitions
k = material factor according to Section 2, B.2.
a = spacing of stiffeners or load width [m]
R = unsupported span [m] according to Section 3, C.
p = load p1 or pd [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 Tmin need not be larger than :
p = [kN/m2]
Tmin= smallest design ballast draught [m]
z = distance of structural member above base line [m]
p2 = load [kN/m2] according to Section 4, D.1.
tK = corrosion addition according to Section 3, K.
h = filling height of tank [m]
et = characteristic tank dimension Rtor bt [m]
Rt = tank length [m]
bt = tank breadth [m]
FpR = [N/mm2]
FL = membrane stress at the position considered [N/mm2] according to Section 5,D.1 JL = shear stress [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.
Table 12.1 - Frequency criteria
2.2 Above the requirements specified in 2.1 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]
2.3 Proof of plating 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 @ a @ R2 @ p @ k [cm3] W2 = 0,44 @ a @ R2 @ p2 @ k [cm3].
Where one or both ends are simply supported, the section moduli are to be increased by 50%.
The shear area of the girder webs is not to be less than:
Aw1 = 0.05 @ a @ R @ p @ k [cm2] Aw2 = 0,04 @ a @ R @ p2 @ k [cm2].
Aw2 is to be increased by 50 % at the position of constraint for a length of 0,1 R.
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 :
Fb = [N/mm2]
J = [N/mm2]
Fv = [N/mm2]
S if subjected to load p2 :
Fb = [N/mm2]
J = [N/mm2]
Fv = [N/mm2]
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.3.1 as for longitudinals. In this case for girders supporting transverse stiffeners the factors m = 1 and ma = 0 are to be used.
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 have to 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 considered when 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 formulae:
= [N/mm2]
τ
StPst = 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
τ
St is to be added to the shear stress JL due 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 . bSt above and below the stringer
Thereby the following requirement shall 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 p
tcrit = [mm]
S if subjected load p2
tcrit = [mm]
FD = compressive stress [N/mm2] b = breadth of face plate strip [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 be determined 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]
t2 = 0,45 @ a [mm]
for one side clad steel:
A = 0,25 S
for both side clad steel:
A = 0,25 S
t = plate thickness including cladding [mm]
tp = thickness of the cladding [mm]
r =
Rep = minimum nominal upper yield point of the cladding [N/mm2] at service temperature
ReH = minimum nominal upper yield point of the base material [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 however the reductions as per Rules for Ships Carrying Dangerous Chemicals in Bulk, Volume X, Section.4, 4 – 0.1.3 apply.
C. Tanks with Large Lengths or Breadths
1. General
Tanks with lengths Rt > 0,1 L or 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 resonance between 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 :
TR,b = [s]
f = hyperbolic function as follows:
= tanh
Period of wave excited maximum pitch motion :
Ts = [s]
= ahead speed of ship [kn] as defined in Section 1, H.5.
Period of roll motion :
Tr = [s]
cr = 0,78 in general
= 0,70 for tankers in ballast . 0,07 @ B in general
. 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. and B. 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 70 and 90% 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 % of the tank volume are to be provided. Where the tank is subdivided by at least one centre line bulkhead, 3% 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 waterline. 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 be damaged 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 formulae for t1 and 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 @ a @ R2 @ p @ k [cm3] 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.
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 corresponding piping 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 pd according to Section 4, D.2., but disregarding pv is 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 which the 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 coated 4) 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 coated 3) 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 BKI, the test may also be carried out after launching. Water testing may be carried out after application of a coating3),
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 BKI, the test may also be carried out after launching. Water testing may be carried out after application of a coating3),