Sour Service

Sour service means a wellstream or process stream containing hydrogen sulphide. Hydrogen sulphide promotes hydrogen embrittlement and sulphide stress cracking in high-strength steel, so the material is not chosen by strength but by hardness: the steel must be soft enough and clean enough that it will not crack in the presence of H2S and water. The governing document is NACE MR0175 / ISO 15156.

Why ordinary high-strength pipe is the wrong answer

Cracking in sour service is driven by hardness and by the presence of water with the H2S. As yield strength rises, so does hardness, and above a threshold the steel becomes susceptible to sulphide stress cracking. The practical consequence is counter-intuitive to anyone used to buying on strength: for a sour well, a higher grade can be the wrong grade, and a maximum hardness of 22 HRC is what actually governs the material selection.

The grades that are used

For casing and tubing to API 5CT, the sour-service grades are L80 type 1, C90, T95 and C110, each defined with a specific heat treatment and a hardness limit. Ordinary N80 is not a sour-service grade: it can meet the hardness requirement in some product forms, but the standard does not qualify it, which is why enquiries for sour wells are quoted against L80 or C90 rather than N80.

For line pipe, the answer is API 5L PSL2 with additional requirements. A sour line pipe specification normally adds a hardness limitation, a restricted carbon equivalent, limits on sulphur and phosphorus, and one or both of two tests: NACE TM0284 for hydrogen induced cracking, which is measured as crack length and crack thickness ratios, and NACE TM0177 for sulphide stress cracking under load.

What is normally called out

What has to be on the enquiry

A sour service enquiry is meaningless without the partial pressure of H2S, because ISO 15156 divides service into regions by H2S partial pressure, total pressure, pH and chloride content, and the permitted material follows from which region the well falls in. Supply the H2S partial pressure and the temperature at minimum, and the grade and hardness limits follow.

• Give the H2S partial pressure. Everything else is secondary, and a specification without it cannot be qualified.
• State the hardness limit required. 22 HRC maximum is the usual figure and has to be tested, not assumed.
• Ask for the test, not the claim. HIC to NACE TM0284 and SSC to NACE TM0177 are test reports, and they belong in the documentation list.
• Watch the welds as well as the pipe body. Seam and weld hardness limits are separate from the parent metal limits and are often missed.
• Think about the temperature. Sour service limits are given for a temperature range, and a material qualified at 20 °C may not be qualified at 120 °C.
• Documentation matters more here. Material traceability to EN 10204 3.1 or 3.2 and the test reports travel with the pipe.

The threshold that makes a service sour

Sour service is defined by the presence of hydrogen sulphide in a water-bearing environment, not by a single concentration number. For material qualification, ISO 15156 / NACE MR0175 works from the H2S partial pressure together with pH, chloride content, temperature and total pressure.

A commonly used screening value is an H2S partial pressure of about 0.3 kPa (0.05 psi); above this the service is normally treated as sour. Below it, the service may still require qualification under the full ISO 15156 rules, so the screening value is a flag, not a substitute for the standard.

The numbers that decide the material

Two sets of numbers come up on every sour enquiry: the hardness ceiling the material has to meet, and the acceptance ratios for the hydrogen induced cracking test. Both are worth having on the specification rather than discovering them at the inspection stage.

Hardness limits by material class

Hardness is the number that governs selection, not yield strength: a higher-strength grade that exceeds the ceiling is the wrong material for a sour well, however strong it is. The 22 HRC figure is the general ceiling for carbon and low-alloy steel; the standard lists a small set of exceptions, and anything outside those has to be qualified against the specific service conditions.

HIC acceptance ratios

Hydrogen induced cracking is tested to NACE TM0284: a coupon is held in a sour solution, then sectioned and measured. The result is reported as three ratios, and the purchase specification sets the maximum allowed for each.

NACE TM0284 is the test method; it does not fix the pass mark. The 15 / 5 / 2 values are the limits most project specifications write in, and your specification can be tighter. SSC to NACE TM0177 is the companion test under load, and the same point applies: the method is standard, the acceptance criterion is yours.

Material selection in five steps

1. Establish the H2S partial pressure, temperature, pH and chloride content.
2. Decide whether the service is sour under ISO 15156 / NACE MR0175.
3. Choose the material class: carbon or low-alloy steel with a hardness cap, martensitic stainless (13Cr class), duplex stainless or austenitic stainless.
4. Add the required tests: hardness, SSC to NACE TM0177 and HIC to NACE TM0284 where they apply.
5. Write the hardness limit, test methods and acceptance values into the purchase order and the mill certificate.

The most expensive mistake is to select a grade first and check the sour rules afterwards; the order is the other way round.

API 5L Annex H: sour service for line pipe

Annex H is the part of API 5L that applies when PSL2 line pipe is ordered for sour service – that is, where the produced or transported fluid contains hydrogen sulphide in the presence of water. It is a different route from the one used for casing and tubing: line pipe is ordered to API 5L with the annex called out, while casing and tubing are ordered to API 5CT and qualified against NACE MR0175 / ISO 15156 as described above.

What calling Annex H changes in the order

• Chemistry and hardness. Annex H adds limits on composition and hardness on top of the normal PSL2 requirements, and those limits apply to the pipe body and to the weld. The values follow the edition named on the order, so the edition is fixed before the heat is rolled.
• Cracking tests. Hydrogen induced cracking and sulphide stress cracking testing is specified against the annex rather than left to the mill’s routine programme, and the acceptance ratios belong on the purchase order rather than being settled at the inspection stage.
• Welding. A sour line pipe order carries requirements on the weld and on the heat affected zone, which is why the pipe is normally ordered with the weld procedure and the inspection scope agreed in advance.
• Documents. The certificate has to state the annex, the edition and the test results; if a third party inspects, the scope has to be agreed before production rather than before shipment.

Where a project cites Annex H without naming an edition, ask before the order is placed. The requirements are set by the edition, and a certificate cannot be re-written to a different edition after the pipe has been tested and marked.

Frequently asked questions

Where to go next

• Steel pipe weight calculator – work out the weight of any size and wall thickness
• Pipe schedule chart – wall thickness for every schedule, carbon and stainless
• Request a quotation – send the specification, quantity and destination port
• API 5CT L80 tubing – a sour-service grade in detail
• API 5CT N80Q casing – the standard grade, and why it is not the sour answer
• Casing and tubing (OCTG) – API 5CT well casing and tubing, J55 through P110 and Q125
• ASTM A790 Duplex Pipe – duplex stainless pipe where sour service meets chlorides