Corrosion protection of crude oil cargo tanks new IMO regulations

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Corrosion protection of crude oil cargo tanks v2

© Lloyd's Register August 2012

Guidance note

Corrosion protection of crude oil

cargo tanks – new IMO regulations

Guidance for shipowners and operators, manufacturers and ship builders

1. Background to the new requirements

Following incidents resulting from structural failure in oil tankers, the IMO developed requirements aimed at inhibiting corrosion in cargo oil tanks by way of performance standards. These performance standards are now being made

mandatory by an amendment to SOLAS: regulation II-1/3-11, Corrosion Protection of Cargo Oil Tanks of Crude Oil Tankers, adopted by Resolution MSC.291(87).

The legislation applies to new crude oil tankers of 5,000 dwt or above engaged on international voyages as follows:

• Contract date: 1 January, 2013

• Keel laying date: 1 July, 2013

• Delivery date: 1 January, 2016

As the figure below illustrates, there are three options for any ship which is subject to the regulations. This guide explains the three options and suggests some factors to take into account when evaluating them.

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2. Corrosion in context

The last 20 years have seen rapid developments for shipping in terms of design, building technology and environmental requirements. However, these developments have not always been helpful in reducing corrosion attack on ship structures.

• Ships are larger and more structurally complex than ever, which leads to large surface areas requiring protection, and difficulties in access for the application, maintenance and inspection of coatings.

• High tensile steels have been used to reduce scantlings, and the resulting thickness reduction may reduce the life expectancy of ship structures.

• Double hull tankers have become the norm, and while their design improves the structural integrity for collisions and groundings, they present a significantly increased surface area requiring corrosion protection.

• Corrosion resistant alloys have been increasingly used, which provide better corrosion resistance than structural ship steels in marine environments or in corrosive cargoes, but also increase the complexity of material coupling consideration, and require special attention to maintenance and suitability for particular cargoes.

3. Corrosion protection options

The new corrosion protection regulations require corrosion protection in the cargo oil tanks, either by coating or other means, for the under deck cargo space (ullage space) and the cargo tank bottom. However, the corrosion mechanisms in each area are dissimilar and need to be treated accordingly.

The regulations provide three options to address the corrosion protection in these areas.

3.1 Option 1 – Coating

The first option requires the coating of cargo oil tanks during the construction of the ship in accordance with the

‘Performance Standard for Protective Coatings for Cargo Oil Tanks of Crude Oil Tankers’ (Resolution MSC.288(87)).

The coating system shall be tested in accordance with Resolution MSC.288(87) Annex 1, or its suitability shall be demonstrated by at least five years’ service experience with a final coating condition of “GOOD”. The coating’s

compatibility with any pre-fabrication primer shall be demonstrated and recorded on the system Type Approval Certificate.

The application of the coating system is subject to extensive controls detailed in Table 1 of the regulation. The compliance of the application with the regulation is to be ensured by a suitably qualified coating inspector. The yard is responsible for the preparation of the vessel’s Coating Technical File (CTF), which is to contain the specification of the coating system applied, a record of the shipyard's and shipowner's coating work, and detailed criteria for coating selection, job specifications, inspection, maintenance and repair.

3.2 Option 2 – Alternative means: Corrosion-resistant steel

The second option allows cargo oil tanks to be protected by what the regulation refers to as “alternative means”. One such example is the use of “corrosion-resistant” steels which have the goal of maintaining the required structural integrity for 25 years in accordance with Resolution MSC.289(87) ‘Performance Standard for Alternative Means of Corrosion Protection for Cargo Oil Tanks of Oil Tankers’. This resolution includes testing requirements for the material.

At the time of releasing this guidance, corrosion-resistant steel is the only recognised alternative to the application of a protective coating system.

Corrosion-resistant steels are being developed to improve corrosion resistance of ship structures when compared to that of normal shipbuilding steels. Different types are developed according to whether the application is intended for the bottom or top of the internal cargo oil tank. Both types are tested and approved to satisfy the requirements in MSC.289(87) in addition to other relevant requirements for ship material, structure strength and construction.

For other alternative means which may be developed in the future and recognised by the IMO, a specific performance standard including testing procedure(s) will be developed by the IMO by adding a new annex to the performance standard.

This will take into account experience gained through field tests for the novel prototype alternative conducted in accordance with SOLAS regulation II-1/3-11.4.

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3.2.1 Properties of corrosion-resistant steel

Corrosion-resistant steels are focused on the corrosion conditions found either in the under deck or the bottom of the cargo oil tank. These steels should not be confused with corrosion-resistant alloys; they are not stainless steels.

The steels are normal, weldable, higher tensile shipbuilding steel. They corrode in a similar way to that of normal ship steel in contact with seawater, and so would not bring benefits if used in the hull envelope or ballast tanks.

Corrosion-resistant steels find application in cargo oil tanks because of the effect of certain chemical elements added as micro-alloying additions. These increase the steel’s resistance to pitting corrosion in such environments. Pitting will still occur, but at a slower rate, which is considered manageable by the manufacturers. It is likely that two grades of steel will be developed: one to be used in the tank bottom to retard pitting; the other to combat the harsher conditions of the ullage space.

Consideration will also have to be given to the welding consumable used and it is likely that combinations of corrosion resistant steel and consumable will be approved. Where non-approved materials are used requirements for coating still apply.

3.2.2 Testing, maintenance and repair considerations

The corrosion approval tests for corrosion-resistant steels are stated in MSC.289(87). It should be noted that testing for the upper deck conditions takes 98 days while testing for the inner bottom conditions takes 168 hours for a welded joint. Testing is to be carried out at an approved test facility.

Corrosion resistant steels are similar to the ship steels already described by IACS in Unified Requirement (UR) W11, therefore the welding and weldability requirements are described in IACS UR W17, and the requirements for welding consumables are outlined in UR W28.

The full requirements for corrosion-resistant steels will be available in a new IACS Unified Requirement, which is expected to be published during 2012.

Global availability of corrosion-resistant steel for planned and unplanned repairs throughout the ship’s life will need to be considered. Corrosion-resistant steel of the appropriate type for the upper or bottom section will need to be available at the point of repair.

3.3 Option 3 – Exemption

The regulations allow a third option, in which a vessel’s flag state may choose to exempt a crude oil tanker from the requirements described in option 1 and option 2, if the vessel is built solely to carry cargoes or perform cargo handling operations with cargoes which do not cause corrosion.

At MSC 90 in May 2012 the IMO approved MSC.1/Circ.1421, “Guidelines on exemptions for crude oil tankers solely engaged in the carriage of cargoes and cargo handling operations not causing corrosion.” This includes criteria for identifying a crude oil which can be considered “benign”.

Any exemption, and the conditions for which it is granted, shall be recorded on an exemption certificate issued by the flag state.

4. Lloyd's Register class notation

Where the cargo oil tanks are coated or protected by the use of corrosion-resistant steels in accordance with the IMO regulations, Lloyd’s Register’s verification shall be indicated by the class notation ShipRight ACS(C), where ACS denotes anti-corrosion system and C denotes cargo oil tanks.

5. Next steps

5.1 Advice to shipyards

If either the coating or corrosion resistant steel option is chosen, the shipyard is to ensure during the pre-contract meeting that an appropriate approved coating system or approved corrosion-resistant steel has been specified in the contract documents. The attending Lloyd’s Register surveyor will be able to confirm the status of any specified materials.

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The application and installation of the compliant materials must be performed in accordance with IMO regulations. Any outstanding non-compliance related to the coating or corrosion-resistant steel shall be rectified before the vessel can be provided with a SOLAS certificate.

Corrosion-resistant steel is specifically manufactured for the upper deck and inner bottom of the cargo tanks, as separate grades of material and subjected to different corrosion tests. Therefore these materials should not be mixed or used for the incorrect area of the cargo oil tank.

5.2 Advice to shipowners and operators

The use of either coatings or corrosion resistant steel in the cargo oil tank offers clear advantages to inhibit corrosion, but the correct choice may be critical for your business depending on the initial costs and the through-life maintenance and repair requirements. Both methods will require a technical file to be maintained on board the ship detailing information such as the locations applied, the product applied, the thickness and the repair method.

If you have oil tankers contracted, in the build process or due for delivery on the dates identified as qualifying for the mandatory application of the new IMO legislation, a decision will have to be made to:

1) select and apply an approved coating; or

2) select and use an approved corrosion-resistant steel; or

3) specify that only non-corroding cargoes will be carried and have this accepted by the flag state.

If you intend to choose option 1 or 2 then Lloyd’s Register can provide the advice you need to ensure compliance with the new regulations in a timely fashion. Please contact your local office for more information.

5.3 Advice to steel manufacturers

All materials are to be manufactured at an approved works in accordance with Lloyd's Register requirements.

The steel is to be manufactured by the basic oxygen and electric furnace process unless specially approved by Lloyd's Register. Rolling practice is to be in accordance with IACS UR W11 procedures.

As mentioned in Section 3.2.2, the corrosion approval tests for corrosion-resistant steel as stated in MSC.289(87) require 98 days for testing and simulation of the upper deck conditions and 168 hours for testing and simulation of a welded joint in the inner bottom conditions. Testing is to be carried out at an approved test facility.

5.4 Advice to paint manufacturers and coating suppliers

Paint manufacturers should be aware that the testing of a candidate oil tank coating system, with a compatible pre- fabrication primer, will take a minimum of nine months to complete. This is also dependent on a test laboratory having the capacity to undertake the testing immediately. Systems selected for testing should therefore be submitted to the laboratory for testing as soon as possible, to best ensure their approval prior to the entry into force of the new regulations.

Systems intended for approval through the service experience route should have vessels selected for inspection that have documented service experience of carrying a variety of cargoes (for example, not those that are regarded as being non- corrosive or benign). In addition, full details of the application of the coating system must be provided, including surface preparation and the presence of primers, and any repairs or maintenance specified.

6. Your further questions answered

Q1. What are the regulatory source references?

A1. The regulatory amendment is:

SOLAS regulation II-1/3-11, Corrosion Protection of Cargo Oil Tanks of Crude Oil Tankers. Adopted by Resolution MSC.291(87)

The relevant performance standards are:

Performance Standard for Protective Coatings for Cargo Oil Tanks of Crude Oil Tankers: Res MSC.288(87); and Performance Standard for Alternative Means of Corrosion Protection for Cargo Oil Tanks of Crude Oil Tankers: Res MSC.289(87).

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The guidelines for maintenance and repair are:

Guidelines on Procedures for In-Service Maintenance and Repair of Coating Systems for Cargo Oil Tanks of Crude Oil Tankers: MSC.1/Circ.1399

The guidelines for administrations on issuing exemptions are:

Guidelines on exemptions for crude oil tankers solely engaged in the carriage of cargoes and cargo handling operations not causing corrosion: MSC.1/Circ.1421

Q2. What parts of the ship are required to be protected?

A2. The following areas are the minimum to be coated or otherwise protected according to the respective standards (Res MSC.288(87) and Res MSC.289(87)):

1) Deckhead with complete internal structure, including brackets connecting to longitudinal and transverse bulkheads. In tanks with ring frame girder construction the underdeck transverse framing to be protected down to level of the first tripping bracket below the upper faceplate.

2) Longitudinal and transverse bulkheads to be protected to the uppermost means of access level. The uppermost means of access and its supporting brackets to be fully protected.

3) On cargo tank bulkheads without an uppermost means of access the protection to extend to 10% of the tank’s height at centreline but need not extend more than 3 m down from the deck.

4) Flat inner bottom and all structure to height of 0.3 m above inner bottom to be protected.

Q3. What is IACS doing on this subject?

A3. Corrosion resistant steels are similar to the ship steels already described by IACS in UR W11, therefore the welding and weldability requirements are described in IACS UR W17, and the requirements for welding consumables are outlined in UR W28.

The full requirements for corrosion-resistant steel will be available in a new IACS Unified Requirement, which is expected to be published during 2012.

IACS has issued Recommendation 116, Performance Standard for Protective Coatings for Cargo Oil Tanks of Crude Oil Tankers: 5 years field exposure test in accordance with Res MSC.288(87). This provides an interim solution for the method of carrying out the 5 year field procedure.

Q4. Where can testing of coatings be performed?

A4. Lloyd’s Register has started auditing and approving test laboratories for the testing of cargo oil tank coatings in accordance with Res MSC.288(87). Approved laboratories are listed on CDLive and more will be added as the process continues.

Q5. Which coatings have been approved by Lloyd’s Register?

A5. All approved coatings will be listed on CDLive, indicating the approved locations of manufacture and any compatible primers.

Q6. How do I find out more?

A6. Please contact your local Lloyd’s Register office.