Clasification Society Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, January 2016 - Part 10 SHIP UNITS - Chapter 1 General Requirements - Section 12 Corrosion additions
Section 12 Corrosion additions
12.1 General
12.1.1 The net scantling approach is described in Pt 10, Ch 1, 12.2 Net scantling approach. Corrosion additions are defined in Pt 10, Ch 1, 12.3 Corrosion additions and in-operation steel renewal criteria are defined inPt 10, Ch 1, 13 Steel renewal criteria.
12.1.2 The requirements for corrosion protection given in Pt 8 CORROSION CONTROL are to be complied with.
Figure 1.12.1 Net scantling approach
12.2 Net scantling approach
12.2.1 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 Pt 10, Ch 1, 12.1 General 12.1.2.
12.2.2 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.
12.3 Corrosion additions
12.3.1 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 Pt 10, Ch 1, 12.3 Corrosion additions 12.3.4. However, consideration will be given to alternative corrosion rates if these are contractually agreed between the Owner and Shipyard.
12.3.2 The total corrosion addition for a structural member is given by the following formula:
= 
mm, rounded up to the nearest 0,5 mm
where
| Nc | = | number of years of unit life where coating is not fully effective, see Pt 10, Ch 1, 12.4 Scantling compliance 12.4.6 Nc is not to be less than 10 years for new-builds and not less than 5 years for conversions and redeployments. Where cargo tanks remain uncoated, Nc is to be taken as equal to the unit design life |
| tc1, tc2 | = | corrosion rate for each side of the structural member, as given in Pt 10, Ch 1, 12.3 Corrosion additions 12.3.4 |
For example calculations of corrosion additions, see Pt 10, Ch 1, 12.4 Scantling compliance 12.4.4.
12.3.3 The corrosion rates for cargo and ballast water tanks given in Pt 10, Ch 1, 12.3 Corrosion additions 12.3.4 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 10, Ch 1, 12.3 Corrosion additions 12.3.4. The percentage time empty is not to be taken as less than 25 per cent.
12.3.4 The default coating life is to be taken as 15 years. Alternative corrosion additions may be derived using the general principles shown in Pt 10, Ch 1, 12.4 Scantling compliance 12.4.6 where an alternative coating life is specified.
Table 1.12.1 Corrosion rate for one side of structural member
| Compartment type | Structural member | Corrosion rate  (mm/year)
|
| Ballast water tank | Within 3 m below top of tank,see Note 1 | 0,15 |
| Elsewhere | 0,1 | |
| Cargo oil tank | Within 3 m below top of tank, see Note 1 | 0,125 |
| Bottom of single bottom tanks | 0,125 | |
| Elsewhere | 0,075 | |
| Exposed to atmosphere | Weather deck plating | 0,1 |
| Other members | 0,075 | |
| Exposed to sea-water | Shell plating | 0,075 |
| Fuel and lubricating oil tank see Note 3 | 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 |
| Chocks and supports for independent tanks located in a void space | ||
| 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 |
| NOTES | ||
| 1. This is only applicable to cargo tanks and ballast tanks with weather deck as the tank top. | ||
| 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. | ||
| 3. 0,07 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. | ||
12.3.5 To address the risk of pitting corrosion, the gross thickness of the bottom plating of tanks is not to be less than:
= 6 + 
where
|
= | number of years between surveys (not to be taken as less than 5 for new builds or 2,5 for conversions) |
and 
are defined in Pt 10, Ch 1, 12.3 Corrosion additions 12.3.2. 
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.
12.4 Scantling compliance
12.4.1 The minimum net thicknesses of structural items as required by Pt 10, Ch 3 Scantling Requirements 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 10, Ch 1, 12.3 Corrosion additions.
12.4.2 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.
12.4.3 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 10, Ch 1, 12.3 Corrosion additions.
- 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 10, Ch 1, 12.3 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 10, Ch 1, 12.3 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 10, Ch 1, 12.3 Corrosion additions;
Figure 1.12.2 Example calculations of 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 10, Ch 1, 12.3 Corrosion additions.
12.4.5 Any additional thickness specified by the Owner as Owner’s extra margin is not to be included when considering compliance with this Section.
12.4.6 For the subject analysis types, the corrosion applied to the gross scantlings prior to the compliance assessment is given in Table 1.12.2 Corrosion applied to the gross scantling for assessment.
Figure 1.12.3 Generic example unit life cycle
Table 1.12.2 Corrosion applied to the gross scantling for assessment
| Assessment | Stress calculations | Buckling capacity calculations | |
| Minimum thickness | Thickness |
|
N/A |
| Local strength (plates, stiffeners, and hold frames) | Thickness/sectional properties |
|
N/A |
| 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
|
|
| Local fine mesh |
|
||
| Fatigue | Global coarse mesh | 0,25
|
N/A |
| Local fine mesh | 0,5
|
||
| Sloshing | Sloshing |
|
N/A |
| Fracture | Global coarse mesh | 0,25
|
|
| Local extremely fine mesh | 0,5
|
N/A | |
| Ultimate strength | Ultimate strength | 0,5
|
0,5
|
| NOTES | |||
| For the assessment, the gross scantling used is not to include any Owner’s extra. | |||
| Copyright 2016 Clasification Society, International Maritime Organization, International Labour Organization or Maritime and Coastguard Agency. All rights reserved. Clasification Society, its affiliates and subsidiaries and their respective officers, employees or agents are, individually and collectively, referred to in this clause as 'Clasification Society'. Clasification Society Register assumes no responsibility and shall not be liable to any person for any loss, damage or expense caused by reliance on the information or advice in this document or howsoever provided, unless that person has signed a contract with the relevant Clasification Society entity for the provision of this information or advice and in that case any responsibility or liability is exclusively on the terms and conditions set out in that contract. |  |