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In NACE MR0175, Why are tubing and subsurface equipment treated as two separate categories?

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Question:

I need to clarify a confusion about NACE MR0175/ISO 15156-3. Why are tubing and subsurface equipment in Tables A.19 and A.20, respectively, treated as two separate categories? Tubing itself is subsurface equipment so why is it treated separately? Moreover, K90941 as mentioned in Table A.20 is recommended for subsurface equipment under any H2S partial pressure but not for tubing, exposed to the same condition; why? L-80 type 13 Cr is more cracking-resistant material than K90941; still it is not recommended for subsurface equipment apart from tubing; why? We are in a process of developing a sour gas field and purchased a copy of this standard to be a guideline for material selection. We need answers to these questions so we can select the most appropriate material for downhole casing/tubing.

Answer:

NACE MR0175/ISO 15156-3 reflects the experience of the oil industry and its experts in the use of materials in sour service over many years.
The separation of materials into Tables A.19, A.20, and A.21 allowed convenient grouping of the data available.
In some cases the differences you identify reflect the availability of different product forms manufactured from the different materials.
As indicated in the title of Table A.19, ISO 11960 L80 type 13Cr is acceptable for other subsurface equipment (other than tubing) providing the material fully meets the applicable material requirements of ISO 11960 L80 type 13Cr. Additionally as indicated in the title and notes of Table A.21, 420 (modified) having the chemical composition of ISO 11960 L80 type 13Cr is acceptable for packers and subsurface equipment.
In all cases the data presented reflect successful laboratory testing of an alloy or successful field experience with the alloy used in the product form listed.
For martensitic alloys not listed in Tables A.19, A.20, and A.21 qualification of the alloy for use in accordance with ISO 15156-3 can be carried out in accordance Annex B.

This question is in relation to NACE MR0175/ISO 15156-3 A.6.2, Table A.19, A.20 and A.21

Reference: ISO 15156 Maintenance Panel Inquiry #2005-22

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