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Questions from reader:

Dear Raputra,

I appreciate your help to verify on whether the below material can be used as substitute for API 5L Gr B materials for sea water carrying pipeline.

a) A516 Gr. 70 , 12mm x 6' x 20' Plate

The current pipeline material is API 5L Gr B:

Pipe, 78”, STD, 5L-B, GMAW, BE

Carbon Steel,

Material: API 5L GR.B

Manufacturing Process: GMAW

Thickness: 0.375 in

Length: 6metres.

GN

My Answers:

Dear GN,

A516 Gr 70 is suitable substitute for API 5L Gr B in this application. A516 Gr 70 has higher tensile strength compare to API5L Gr B but its yield properties is about the same. The chemical composition is about similar.

So, the main comparison in this kind of situation (since both are carbon-manganese steel) is usually based on tensile strength, yield stress and elongation. Since the pipeline is designed to a certain design code, in terms of calculation of the required thickness, the minimum strength based on the design code is required. So, if you substitute with material with lesser grade material say A516 Gr 60, then you may need to recalculate the required thickness, which may result in thicker pipeline thickness required.

However, my other concern is on the material thickness supplied. Since you’ll be welding A516 with 12mm thickness to API5L with 9.5mm thickness, there is potential to have hi-low during welding. Furthermore, the current API 5L piping has been in service for sometimes and therefore corrosion might have taken place making the current pipeline thickness lesser. This can be ok if they fit-up properly i.e. internal edge trimming etc OR ensuring the ID matches the current pipeline ID during plate rolling.

For corrosion protection, I think the internal wall of the pipe shall be protected from sea water corrosion. My recommendation is by applying Epoxy coating. Since the pipeline diameter is relatively big, installation of sacrificial anodes (using zinc) can also be considered. I have seen pipeline of similar size using internal sacrifical anodes in addition to epoxy-coated wall. Included in the design were manholes for interval internal visual inspection. For more than 25 years, it seems that the pipeline condition was still excellent.

Hope this info helps.

regards,

Raputra

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