OCTG Purchase Specification: What a Mill Needs to Quote Casing and Tubing

A purchase order that reads "9-5/8 L80 casing, best price" cannot be quoted accurately. The mill can price a 9-5/8 L80 casing, but not your 9-5/8 L80 casing — because that single line leaves the weight, the connection, the length range, the drift, the service condition, and the certification level unstated, and each of those changes the wall thickness, the tooling, the test scope, and the price. The gap between what a buyer writes and what a mill needs to quote is where most OCTG procurement friction lives: back-and-forth emails, quotes that can't be compared, and — worst — a compliant shipment that isn't what the well needed.

This guide is the checklist we wish every RFQ arrived with. ZC Steel Pipe manufactures API 5CT casing and tubing across the grade ladder from H40 through Q125 — including sour-service and 13Cr grades, with BTC and premium connections — for EPC and drilling buyers in Africa, the Middle East, South America, and Southeast Asia. We process these purchase orders daily, and the nine line items below are the ones that, when present, let us return a firm quote on the first pass, and when missing, force the assumptions that later become disputes.

Why an incomplete RFQ costs you time and money

An OCTG request with only grade and OD is not wrong — it is incomplete in a way that transfers decisions to the mill. When a specification is silent, a compliant mill fills the gap with the cheapest defensible option: the lightest common weight, the plainest connection, the base grade type, a 3.1 certificate, standard drift. Every one of those defaults is legitimate under API 5CT. None of them is necessarily what your string design assumed.

The cost shows up in three ways. First, time: an under-specified RFQ generates a clarification cycle, and on a multi-line casing program that cycle can run a week before you have a quote you can put in a bid. Second, comparability: if two mills each fill the gaps with different assumptions, their quotes are not for the same product, and the lower number may simply be the lighter wall. Third, and most expensive, a wrong-but-compliant shipment: the mill ships exactly what the PO said, the MTC matches the PO, and the mismatch with the well is discovered at the rig. The time to catch a specification error is on the RFQ, before manufacturing begins — not on the mill test certificate, and certainly not at the wellsite.

The nine line items every OCTG RFQ needs

A complete OCTG line item answers nine questions. The table is the summary; the sections that follow explain what each one changes and where buyers get it wrong.

Read the table as a dependency chain, not a menu: item 3 (weight) fixes the wall and therefore items 6 (drift) and part of the price; item 1 (grade and type) drives item 7 (service testing); item 5 (range) drives the joint count that turns item 9 (footage) into a shipment. Leaving any one blank does not just omit that field — it moves everything downstream onto a default.

1 — Grade and type

State the grade and its type, because several API 5CT grades are families, not single materials. L80 is the clearest example: the standard defines L80-1, L80-3Cr, L80-9Cr, and L80-13Cr. All share the same 80 ksi minimum and 95 ksi maximum yield, but they are metallurgically different products — L80-1 is a carbon-manganese steel, while L80-13Cr carries 13% chromium for CO2 corrosion resistance. A PO that says "L80" defaults to Type 1.

The distinction matters because the qualifications do not overlap the way buyers assume. L80 Type 1 is an approved sour-service grade under API 5CT — it meets the hardness control (23 HRC maximum) that governs sulfide stress cracking resistance. What it does not have is CO2 or chloride corrosion resistance; that is what the chromium in L80-13Cr adds. So "L80 for our sour well" is ambiguous: if the threat is H2S, Type 1 is qualified; if the threat is wet CO2, you need the 13Cr grade, and only the type designation tells the mill which.

The priority grades where the type or heat-treat condition must be explicit: N80 (N80-1 normalized, or normalized-and-tempered, vs N80Q quenched-and-tempered only — different toughness and consistency), L80 (the four types above), and the sour grades C90 / T95, which carry hardness ceilings (25.4 HRC) and mandatory testing that N80 and P110 do not. For the full grade ladder with yields and hardness limits, see the API 5CT specification tables →.

2 — Outside diameter

OD is the one field buyers almost never omit, because it is the size they think in. State it as the fractional-inch label the industry uses — 4-1/2, 5-1/2, 7, 9-5/8, 13-3/8 — not a decimal or a metric conversion, because the label is the specification and the decimal OD follows from it. The only subtlety is consistency across a multi-string program: name each string's OD explicitly rather than "the intermediate casing," so a mill quoting the whole program prices the right size at each depth.

3 — Weight per foot

Weight per foot is where a great deal of quiet money moves, because for a given OD the weight is the wall thickness. A heavier weight is a thicker wall, a smaller ID and drift, a higher collapse and burst rating, and more steel to pay for. Specify the weight; do not leave it to "standard," because most sizes have several common weights and the lightest one is the cheapest quote.

There is a second trap inside this field: which weight convention. API 5CT Table C.18 lists nominal weight with threads and couplings — the figure that includes the coupling steel. Plain-end weight, the pipe body alone, is lower and is used in engineering formulas. When you turn a footage requirement into order tonnage, use the nominal (threads-and-couplings) weight, because that is what the mill invoices; calculating tonnage from plain-end weight under-orders. The all-sizes table with wall, ID, and drift for every weight is on the casing dimensions and weight tables → page.

4 — Connection

The connection is a specification in its own right, and it splits into API and premium. For casing, the API options are STC (short round thread — shallow, surface strings), LTC (long round thread — the general-purpose intermediate default), and BTC (buttress — deep, high-tension production strings, near-full pipe-body tensile efficiency). For tubing, the API options are NUE (non-upset) and EUE (external upset — the stronger, more common choice). State one; if you leave it blank on casing the mill defaults to LTC, and on tubing to EUE.

Above the API connections sit the premium metal-to-metal seals, and here the RFQ language matters. API round-thread and buttress connections are not gas-tight — for HPHT, gas, deepwater, and high-frac horizontal wells, a premium is the norm. But premium connections are proprietary and license-restricted: a mill can only cut the premiums it is licensed for. Name the connection you need — VAM TOP, Hydril PH-6, and so on — and add "or equivalent" if you will accept a licensed alternative. Specifying a brand the mill does not license just forces it to substitute silently. The connection selection guide is in the API 5CT couplings and connections → article.

5 — Range (length)

API 5CT groups OCTG into three length ranges, and the range you specify drives the joint count, the handling, and — importantly for import buyers — the shipping mode. Range 1 is the shortest, Range 2 is the workhorse for most casing and tubing, and Range 3 is the longest. The joint count for a given footage falls as the range lengthens, which changes the number of connections to make up and the handling on the rig.

For buyers importing into Africa, the Middle East, and South America, the range also decides how the pipe ships: Range 2 casing loads into a standard 40 ft container, while full-length Range 3 exceeds the internal length of a container and moves on flat-rack or as breakbulk — a real cost and lead-time difference. Specify the range; if it is left blank, the mill assumes Range 2. The full definitions and the casing-versus-tubing length bands are in the casing length ranges R1 / R2 / R3 → guide.

6 — Drift

Drift is the guaranteed minimum bore the pipe will pass, verified by a mandrel run through every joint, and the specification treats casing and tubing differently. Casing has a standard drift diameter fixed in API 5CT Table C.18 for each size and weight — if you do not state otherwise, the mill drifts to that standard. Tubing does not have a fixed drift in the specification; under API 5B, tubing drift is a purchase-order item. So a tubing RFQ that is silent on drift gets the standard API 5B mandrel for the size and weight, which may be smaller than a specific completion tool assumes.

If your completion runs a tool, plunger, or wireline assembly with a real clearance requirement, state the drift — either "special drift" with the diameter, or "standard API 5B drift" if the standard is acceptable — so the mill drifts to the number your completion actually needs rather than to the default. This is one of the few fields where the correct answer for casing and tubing genuinely differs, and competitors frequently present tubing drift as if the standard fixes it. It does not.

7 — Service and metallurgical condition

The service condition tells the mill which testing and metallurgical controls apply, and it is separate from the grade. The main flags:

• Sour service (NACE MR0175 / ISO 15156): if the well produces H2S above the threshold, state "NACE MR0175 sour service." This governs hardness control and, for the sour grades, mandatory testing. Note the qualification is conditional on temperature, H2S partial pressure, and chloride level — the RFQ should reference the service, and the well data lets the mill or you confirm the grade fits.
• CO2 / CRA: wet CO2 drives the chrome grades (L80-13Cr, Super 13Cr) — link this back to the type designation in item 1.
• Low-temperature toughness: if the operating or test temperature demands Charpy impact values beyond the standard requirement, state the temperature and energy.

Leaving service blank defaults the order to standard (non-sour) processing. A sour requirement discovered after the heats are rolled is not a paperwork fix — it can mean re-manufacturing. To match a grade to well conditions before you write the RFQ, use the pipe grade selector →.

8 — Inspection and certification

The certification level sets the mill's inspection scope and cost, and it belongs on the RFQ because it changes scheduling, not just paperwork. The core choice is the EN 10204 certificate type: a 3.1 certificate is validated by the manufacturer's own quality department against the actual heats; a 3.2 adds independent witnessing by a third party — SGS, Bureau Veritas, Intertek, or the purchaser's agent. Specify 3.2 when the end operator or project requires third-party witnessing, and specify it up front, because a 3.2 witness has to be scheduled into the test plan.

Beyond the certificate type, state any additional inspection the project requires: full-length drift verification, specific NDT (ultrasonic or electromagnetic), hydrostatic test pressure if non-standard, and the marking and traceability requirements. If your specification requires API monogram pipe, say so explicitly — it is a specific requirement, not an assumption. The certificate types and what each verifies are covered in the mill test certificate (EN 10204) → guide.

9 — Quantity and delivery

Close the line item with quantity and delivery terms. State quantity as both footage and tonnage where you can — mills frequently quote and invoice OCTG by weight, so giving the tonnage removes a conversion step and a place for the numbers to diverge. Note whether the footage is delivered length or nominal, because range tolerance changes the joint count and therefore the exact weight.

For delivery, state the Incoterm and destination — CIF Lagos, FOB Tianjin, DAP the jobsite — because the term decides who arranges and pays for freight, insurance, and import clearance, and it changes the quoted number materially. The risk-transfer and insurance implications of each term for a pipe shipment are in the Incoterms 2020 for pipe buyers → guide. Add the required delivery date, so the mill can tell you whether stock or a new rolling meets it.

A complete RFQ line item — worked example

Put the nine items together and a single line carries everything a mill needs to quote:

2-7/8 inch OD, 6.5 lb/ft, L80 (Type 1), EUE, Range 2, drift per API 5B, NACE MR0175 sour service, EN 10204 3.1 MTC, 500 joints, CIF Lagos, delivery within 10 weeks.

That line resolves all nine questions: grade and type (L80-1), size (2-7/8 tubing), weight and therefore wall (6.5 lb/ft), connection and upset (EUE), length band (Range 2), drift basis (API 5B), metallurgical requirement (sour), certificate (3.1), quantity (500 joints), and delivery (CIF Lagos, 10 weeks). No follow-up email is needed to price it.

To turn the joint count into the tonnage a mill will invoice: 500 joints of Range 2 tubing average roughly 30 ft each (the 28–32 ft band), so about 15,000 ft; at 6.5 lb/ft that is 97,500 lb, or about 44 tonnes. Stating both the joint count and the resulting tonnage on the RFQ lets the mill confirm the order size against stock immediately. (Range 2 length varies within the band, so treat the tonnage as nominal and let the mill firm it against actual footage.)

Casing vs tubing — what changes in the RFQ

The nine items apply to both, but three fields behave differently and are worth calling out so a casing template is not blindly reused for tubing:

• Connection vocabulary: casing uses STC / LTC / BTC and premium; tubing uses NUE / EUE and premium. Do not write "BTC" on a tubing line.
• Drift: casing drift is fixed in API 5CT Table C.18; tubing drift is a PO item per API 5B (item 6). This is the field most often carried over incorrectly from a casing template.
• Weight designation: tubing weight is quoted for the upset condition (the EU or NU weight), so a tubing weight must be paired with the connection type to be unambiguous.

The procurement traps we catch before manufacturing

Three specification gaps account for most of the corrections we make before a heat is rolled:

1. The type trap — "L80" with no type, when the well needs L80-13Cr for CO2 resistance. The mill is compliant shipping Type 1; the RFQ must name the type.
2. The weight trap — tonnage calculated from plain-end weight instead of nominal (threads-and-couplings) weight, under-ordering the steel; or a missing weight that stops the quote entirely.
3. The drift trap — a tubing RFQ silent on drift, getting the standard API 5B mandrel when the completion assumed a specific clearance.

Each is a one-line fix on the RFQ and an expensive fix anywhere later.

Purchase order guidance — the field checklist

Before you release an OCTG RFQ, confirm each line item carries:

1. Grade and type — e.g. "L80 Type 13Cr," not "L80."
2. OD — fractional-inch label per string.
3. Weight per foot — the specific weight, using nominal (T&C) weight for tonnage.
4. Connection — STC/LTC/BTC or NUE/EUE; premium by name "or equivalent."
5. Range — R1, R2, or R3.
6. Drift — casing standard, or tubing "per API 5B" / special drift with the diameter.
7. Service — NACE MR0175 sour, CO2/CRA, low-temperature, or standard.
8. Certification — EN 10204 3.1 or 3.2; NDT and API monogram if required.
9. Quantity and delivery — footage and tonnage; Incoterm, destination, date.

When you receive the pipe, the mill test certificate should trace to the heat number and match every field above — grade and type, weight, connection, drift, service testing, and the EN 10204 level. Verify the MTC against the PO line, not against a generic expectation: a compliant certificate matches what was ordered, so the check that protects you is whether what was ordered matches what the well needs.

To match a grade to well conditions before writing the specification, use the pipe grade selector →; for the full grade ladder and dimensions, see the API 5CT specification tables →.