Section 7 Corrosion additions

7.1 General

7.1.1 The net scantling approach is described in Pt 4, Ch 3, 7.2 Net scantling approach. Corrosion additions are defined in Pt 4, Ch 3, 7.3 Corrosion additions and in-operation steel renewal criteria are defined in the LR document Thickness Measurement and Close-Up Survey Guidance.

7.1.2 The requirements for corrosion protection given in Pt 8 Corrosion Control are to be complied with.

7.2 Net scantling approach

7.2.1 The net thickness of a structural element is that required for structural strength compliance with the design basis. The corrosion addition for structural elements is derived independently of the net scantling requirements. This approach clearly separates the net thickness from the thickness added to address the corrosion that is likely to occur during the in-operation phase. This approach enables the status of the structure with respect to corrosion to be clearly ascertained throughout the life of the unit. See Figure 3.7.1 Example calculations of corrosion additions.

7.2.2 The net thickness approach distinguishes between local and global corrosion. Local corrosion is defined as uniform corrosion of local structural elements, such as a single plate or stiffener. Global corrosion is defined as the overall average corrosion of larger areas such as primary support members and the hull girder.

7.3 Corrosion additions

7.3.1 The corrosion additions specified in this sub-Section are applicable to each of the two sides of a structural member and are given as a corrosion rate. The corrosion rate for each of the two sides of a structural member is specified in Table 3.7.1 Corrosion rate for one side of structural member. However, consideration will be given to alternative corrosion rates if these are contractually agreed between the Owner and Shipyard.

7.3.2 The total corrosion addition for a structural member is given by the following formula:

t_c

N_c (t_c1 + t_c2) mm, rounded up to the nearest 0,5 mm

where

N_c	=	number of years of unit life where coating is not fully effective, see Figure 3.7.2 Generic example unit life cycle. N_c is not to be less than 10 years for new-builds and not less than 5 years for conversions and redeployments where coating is not renewed. Where cargo tanks remain uncoated, N_c is to be taken as equal to the unit design life
t_c1, t_c2	=	corrosion rate for each side of the structural member, as given in Table 3.7.1 Corrosion rate for one side of structural member

For example calculations of corrosion additions, see Table 3.7.1 Corrosion rate for one side of structural member.

7.3.3 The corrosion rates for cargo and ballast water tanks given in Table 3.7.1 Corrosion rate for one side of structural member assume the tanks will spend 50 per cent of the time empty and 50 per cent of the time full over the unit design life and that the ballast tank is fitted with effective anodes. Where alternative regimes for individual tanks are specified, the corrosion rate may be adjusted by [percentage time empty/50] x corrosion rate from Pt 4, Ch 3, 7.4 Scantling compliance 7.4.6. The percentage time empty is not to be taken as less than 25 per cent.

7.3.4 The default coating life is to be taken as 15 years. Alternative corrosion additions may be derived using the general principles shown in Figure 3.7.2 Generic example unit life cycle where an alternative coating life is specified.

7.3.5 To address the risk of pitting corrosion, the gross thickness of the bottom plating of tanks is not to be less than:

t_grs

6 + N_t (20t_c1+ t_c2)

where

N_t

number of years between surveys (not to be taken as less than 5 for new builds or 2,5 for conversions)

t_c1 and t_c2 are defined in Pt 4, Ch 3, 7.3 Corrosion additions 7.3.2. t_c1 is the value for the side of the structural member within the tank.

Explanatory note:
This requirement ensures that there is sufficient bottom plating thickness remaining at thickness measurement survey so that pitting corrosion should not lead to loss of barrier integrity between inspections.

7.4 Scantling compliance

7.4.1 The minimum net thicknesses of structural items as required by Pt 4, Ch 3 Structural Design are to be rounded to the nearest 0,5 mm prior to the addition of Owner’s extras or corrosion additions. The applicable corrosion additions are given in Pt 4, Ch 3, 7.3 Corrosion additions.

7.4.2 The net section modulus, moment of inertia and shear area properties of local support members are to be calculated using the net thicknesses of the attached plate, web and flange.

7.4.3 The net section properties, shear area and section modulus of primary support members are to be calculated using the net thicknesses of the attached plate, web and flange plus half of the applicable corrosion addition specified in Pt 4, Ch 3, 7.3 Corrosion additions.

7.4.4 The net scantlings described in this sub-Section are related to gross scantlings as follows:

for application of the minimum thickness requirements, the gross thickness is obtained from the applicable requirements by adding the full corrosion additions specified in Pt 4, Ch 3, 7.3 Corrosion additions;
for plating and local support members, the gross thickness and gross cross-sectional properties are obtained from the applicable requirements by adding the full corrosion additions specified in Pt 4, Ch 3, 7.3 Corrosion additions;
for primary support members, the gross shear area, gross section modulus and other gross cross-sectional properties are obtained from the applicable requirements by adding one half of the relevant full corrosion additions specified in Pt 4, Ch 3, 7.3 Corrosion additions;
for application of buckling requirements, the gross thickness and gross cross-sectional properties are obtained from the applicable requirements by adding the full corrosion additions specified in Pt 4, Ch 3, 7.3 Corrosion additions.

7.4.5 Any additional thickness specified by the Owner as Owner’s extra margin is not to be included when considering compliance with this Section.

7.4.6 The corrosion allowance to be deducted from the gross scantlings prior to the compliance assessment is given in Table 3.7.2 Corrosion allowance to be deducted from the gross scantlings prior to the compliance assessment.

Table 3.7.1 Corrosion rate for one side of structural member

Compartment type	Structural member	Corrosion rate t_c1, t_c2(mm/year)
Ballast water and preload tanks (see Note 6)	within 3m below top of tank, see Note 1	0,15
Ballast water and preload tanks (see Note 6)	Elsewhere	0,1
Cargo oil tank (see Note 3)	within 3m below top of tank, see Note 1	0,125
	Bottom of tanks	0,125
	Elsewhere	0,075
Exposed to atmosphere	Weather deck plating	0,1
Exposed to atmosphere	Other members	0,075
Exposed to sea water (see Notes 6 and 7)	Shell plating	0,075
Exposed to sea water (see Notes 6 and 7)	Legs of self-elevating units	0,075
Exposed to sea bed and seawater	Legs, footings, mats of self-elevating units	0,2 (see Note 8)
Fuel and lubricating oil tank see (see Note 4)		0,05
Fresh water tank		0,05
Slop tanks		0,15
Void spaces, see Note 2	Spaces not normally accessed, e.g. access only via bolted manhole openings, pipe tunnels, inner surface of stool space common with a dry bulk cargo hold, etc.	0,05
Dry spaces	Internals of machinery spaces, pump room, store rooms, steering gear space, etc	0,05
Hold space bounding membrane liquefied gas tanks	side of hull structure within hold space where there is environmental control such as inerting	0
Note 1 This is only applicable to cargo tanks and ballast tanks with weather deck as the tank top.
Note 2. The corrosion rate on the outer shell plating in way of a pipe tunnel is to be taken as for a water ballast tank.
Note 3. 0,05 mm/year is to be added to the plate surface exposed to ballast for the plate boundary between water ballast and heated cargo oil tanks. 0,03 mm/year is to be added to each surface of the web and face plate of a stiffener in a ballast tank and attached to the boundary between water ballast and heated cargo oil tanks. Heated cargo oil tanks are defined as tanks arranged with any form of heating capability.
Note 4. 0,07 mm/year is to be added to the plate surface exposed to ballast for the plate boundary between water ballast and heated fuel oil tanks or lube oil tanks.
Note 5. Where a tank is loaded with contents not listed in Table 3.7.1 Corrosion rate for one side of structural member, e.g. drilling mud, corrosion rates will be specially considered.
Note 6. The corrosion rates indicated assume effective anodes are fitted to the steel boundary.
Note 7. Additional corrosion allowance in the splash zone is recommended.
Note 8. Additional margins greater than those indicated in the table may be required where the members are subject to high corrosion/wear rates.

1 Longitudinal bulkhead plating

Corrosion addition (Table 3.7.1 Corrosion rate for one side of structural member) = 0,075 mm/year for each side of the plating
Unit design life = 25 years
Effective coating life = 15 years
Total corrosion addition = (25-15) x (0,075 + 0,075) = 10 x 0,15 = 1,5 mm

2 Side shell plating (within 3 m below top of tank

Corrosion addition (Table 3.7.1 Corrosion rate for one side of structural member) = 0,075 mm/year for each side shell plating
Corrosion addition (Table 3.7.1 Corrosion rate for one side of structural member) = 0,15 mm/year for ballast water tank
Unit design life = 25 years
Effective coating life = 15 years
Total corrosion addition = (25-15) x (0,075 + 0,15) = 10 x 0,225 = 2,25 mm = 2,5 mm rounded

3 Cargo tank bottom plating

Corrosion addition (Table 3.7.1 Corrosion rate for one side of structural member) = 0,075 mm/year for cargo tank plating
Corrosion addition (Table 3.7.1 Corrosion rate for one side of structural member) = 0,1 mm/year for ballast water tank
Unit design life = 25 years
Effective coating life = 15 years
Total corrosion addition = (25-15) x (0,075 + 0,1) = 10 x 0,175 = 1,75 mm = 2 mm rounded

4 Stiffeners in ballast water tank

Corrosion addition (Table 3.7.1 Corrosion rate for one side of structural member) = 0,1 mm/year for ballast water tank
Unit design life = 25 years
Effective coating life = 15 years
Total corrosion addition = (25-15) x (0,1 + 0,1) = 10 x 0,2 = 2 mm

Note This example assumes that the tanks will spent 50 per cent of the time empty and 50 per cent of the time full over the unit design life.

Figure 3.7.1 Example calculations of corrosion additions

Figure 3.7.2 Generic example unit life cycle

Table 3.7.2 Corrosion allowance to be deducted from the gross scantlings prior to the compliance assessment

Assessment		Stress calculations	Buckling capacity calculations
Minimum thickness	Thickness		N/A
Local strength (plates, stiffeners, and hold frames)	Thickness/sectional properties		N/A
Local strength (plates, stiffeners, and hold frames)	Stiffness/proportions
Primary support members	Thickness/sectional properties	0,5	N/A
(Prescriptive)	Stiffness/proportions of web and flange
Strength	Global coarse mesh	0,5
Strength	Local fine mesh
Fatigue	Global coarse mesh	0,25	N/A
Fatigue	Local fine mesh	0,5	N/A
Sloshing	Sloshing
Fracture	Global coarse mesh	0,25	N/A
Fracture	Local extremely fine mesh	0,5	N/A
Ultimate strength	Ultimate strength	0,5	0,5
Note For the assessment, the gross scantling used is not to include any Owner’s extra.