Casing: purpose, types, properties and standards

Quick refresher

Strings are named by their purpose. The conductor holds back loose rocks near the surface. The surface casing shuts off fresh water and carries the blowout preventers. Intermediate strings isolate problem intervals. The production casing is the channel for production. A liner is hung inside the previous string.
We describe a pipe by its outside diameter, its wall thickness (English-language catalogues give the weight per foot in lb/ft instead), its grade and its connection type. An example is 177.8 × 10.36 mm, N-80, buttress thread (a trapezoidal thread).
The inside diameter is $D - 2t$ , where $D$ is the outside diameter and $t$ the wall thickness. We check whether a bit or another tool will pass by the drift diameter, which is a little smaller than the inside diameter.
The grade sets, first of all, the minimum yield strength of the steel, the stress at which it starts to stretch permanently. For N-80 it is 80,000 psi (552 MPa). For environments with hydrogen sulfide (a toxic gas), the crew chooses steels of limited hardness.
The rated characteristics of a pipe are the internal yield pressure (it grows in proportion to $t/D$ ), the collapse pressure (it falls sharply as $D/t$ grows), and the tensile strength of the pipe body and of the connection. Tension lowers the collapse resistance.
We choose the setting depths of the strings from the bottom up, by the pore pressure (the pressure of the fluid in the rock) and the fracture pressure (the pressure at which the rock breaks). We choose the sizes from the diameter needed at the bottom of the well.

Key relationships

d = D - 2t

inside diameter from the outside diameter and the wall thickness

m = 0.02466\,(D - t)\,t

mass of a metre of plain pipe, kg/m ( $D$ and $t$ in mm)

p_{\text{iy}} = 0.875\,\dfrac{2\,Y_p\,t}{D}

internal yield pressure (burst) from the Barlow formula

F_y = Y_p\,\dfrac{\pi}{4}\left(D^2 - d^2\right)

the tensile load at which the pipe body reaches its yield strength

Common pitfalls

Confusing the nominal outside diameter with the inside diameter. The inside diameter of a 177.8 mm string is about 150 to 164 mm, depending on the wall thickness.
Checking whether a bit will pass by the inside diameter, not by the drift diameter.
Assuming that a pipe is equally strong against internal and external pressure.
Assuming that a connection is as strong and as tight as the pipe body.
Confusing the English “conductor” (the Russian napravlenie) with the Russian konduktor, which is surface casing in English.

After this lesson you can

explain why a well is lined with casing strings, and name the purpose of the conductor, the surface, intermediate and production casing, and the liner
describe the parts of a casing string, from the shoe (the bottom end) to the casing head (the fitting at the top), and their role when the casing is run and cemented
from the size of a pipe, find its inside diameter, drift diameter (the size of the opening that is guaranteed to be clear), mass per metre and capacity, and check whether a bit will pass
explain what the steel grade and the rated characteristics of a pipe mean (how much pressure from inside, how much pressure from outside and how much pull the pipe can take: internal yield pressure, collapse pressure and tensile strength), and what they depend on
name the main types of threaded connections and the standards for casing, and explain how the pressures along the well set the setting depths of the strings

Why a well is lined with casing

The crew drills a well in intervals. Before drilling deeper, the crew lines each finished interval with a casing string and cements it (how a well is built). A string does several jobs at once:

near the surface, it shuts off fresh groundwater and is the base for the casing head and the blowout preventers
together with the cement, it forms the second barrier of the well: without a string, the well cannot be closed on a kick, an unwanted inflow of formation fluid (Well control: primary barrier and kick indicators)
it separates intervals with different pressures: formations that need heavy mud and formations that heavy mud would fracture must not be in the same open hole
it shuts off problem rocks: rocks that cave in or wash out, flowing salts (salts that creep slowly into the hole), loss zones
it is the channel for production or injection, and it keeps the productive formations apart for the whole life of the well

Casing pipe is one of the largest costs of a well, and it is hard, sometimes impossible, to repair a poor string. So designers try to keep the number of strings to the necessary minimum. But in exploration wells (wells drilled to find new accumulations of oil or gas), where the rocks are poorly known, they keep a spare option: to run one more string or a liner if the pressures turn out higher than expected.

Strings by purpose

Strings are classified by their purpose, not by their diameter. The same 244.5 mm pipe is an intermediate string in one well and the production string in another.

Casing strings in the order they are run

String: Conductor
Purpose: holds back the loose rocks near the surface and guides the mud into the flowline (the trough that carries it to the shale shakers, the screens that clean the mud)
Where it ends: tens of metres; often driven into the ground, cemented to the surface

String: Surface casing
Purpose: shuts off fresh water and unstable upper rocks, and carries the casing head, the blowout preventers and the following strings
Where it ends: hundreds of metres, in rocks that will hold the pressure of a closed well; cemented to the surface

String: Intermediate (technical) casing
Purpose: isolates problem intervals: unstable shales (clay-rich rocks), salts, loss zones, zones of abnormally high formation pressure (overpressure)
Where it ends: set by the pressures and the problems; there may be several such strings, or none

String: Production casing
Purpose: the channel for production or injection; separates the formations
Where it ends: at the top of the productive formation, or below its bottom

String	Purpose	Where it ends
Conductor	holds back the loose rocks near the surface and guides the mud into the flowline (the trough that carries it to the shale shakers, the screens that clean the mud)	tens of metres; often driven into the ground, cemented to the surface
Surface casing	shuts off fresh water and unstable upper rocks, and carries the casing head, the blowout preventers and the following strings	hundreds of metres, in rocks that will hold the pressure of a closed well; cemented to the surface
Intermediate (technical) casing	isolates problem intervals: unstable shales (clay-rich rocks), salts, loss zones, zones of abnormally high formation pressure (overpressure)	set by the pressures and the problems; there may be several such strings, or none
Production casing	the channel for production or injection; separates the formations	at the top of the productive formation, or below its bottom

The English conductor is the Russian napravlenie, and the Russian konduktor is the English surface casing. This pair often causes confusion in English-language drilling programmes.

The surface casing is the first string on which the crew can close the well with a preventer. So the crew sets its shoe, the bottom end of the string, in rocks that will hold the pressure of a closed well if a kick happens while the next interval is drilled. Otherwise the formation under a shallow shoe fractures. Then fluid can come to the surface along cracks around the string, and a crater forms at the wellhead.

A liner does not reach the surface. The crew hangs its top inside the previous string on a liner hanger: slips (toothed wedges) that press against the wall of the string. The overlap with the previous string is usually tens of metres, and the top of the liner is sealed with a packer (a rubber seal) and cement. The wide previous string stays above the liner, so it is easier to fit the tubing (the pipe that carries the oil or gas up to the surface) and the pump in it. A liner is also shorter than a full string, and the crew runs it on drill pipe. During cementing the crew can rotate it and move it up and down, which helps the cement to push the mud out. The weak points are the tightness of the overlap, and the fact that the upper string must withstand the production pressures. If necessary, the crew extends the liner to the surface with a tie-back string.

A drilling liner covers a problem interval so that the crew can drill on.
A production liner is run into the productive formation.
A stub liner (a patch) covers a damaged or worn section of a string.

The crew decides where to set the shoe of each string from the pore pressure (the pressure of the fluid in the rock) and the fracture pressure (the pressure at which the rock breaks). Both are written as equivalent densities, the density of mud that would give the same pressure. The crew works from the bottom up (choosing shoe depths). The crew also takes the problems into account: unstable shales, salts, loss zones, fresh water. The sizes are also chosen from the bottom up: from the diameter needed at the bottom for completion (making the well ready to produce) or testing, up to the strings above (how pressures set the casing programme). The bit for the next interval must pass inside the string. The string itself must go down a hole that leaves a gap for the cement.

Typical combinations of strings and bits (inch series)

String: 508 mm (20″)
Hole for the string: 660.4 mm (26″)
Bit inside the string: 444.5 mm (17½″)

String: 339.7 mm (13⅜″)
Hole for the string: 444.5 mm (17½″)
Bit inside the string: 311.1 mm (12¼″)

String: 244.5 mm (9⅝″)
Hole for the string: 311.1 mm (12¼″)
Bit inside the string: 215.9 mm (8½″)

String: 177.8 mm (7″)
Hole for the string: 215.9 mm (8½″)
Bit inside the string: 152.4 mm (6″)

String	Hole for the string	Bit inside the string
508 mm (20″)	660.4 mm (26″)	444.5 mm (17½″)
339.7 mm (13⅜″)	444.5 mm (17½″)	311.1 mm (12¼″)
244.5 mm (9⅝″)	311.1 mm (12¼″)	215.9 mm (8½″)
177.8 mm (7″)	215.9 mm (8½″)	152.4 mm (6″)

These combinations are typical, but they are not the only ones: the passage of the bit depends on the wall thickness (see the section on the drift). In the post-Soviet countries another series is also common, with strings of 426, 324, 245 and 168 mm (an example of a casing programme).

What a string is made of

A string is made up of separate pipes, each about 10–12 m long. Most casing pipes have an external thread cut at both ends, and couplings (short sleeves with an internal thread) join the pipes. One coupling is screwed onto each pipe at the factory. Pipe lengths are standardised in ranges.

Length ranges of casing pipes

Range: 1
Length, m: 4.88–7.62
Length, ft: 16–25

Range: 2
Length, m: 7.62–10.36
Length, ft: 25–34

Range: 3
Length, m: 10.36–14.63
Length, ft: 34–48

Range	Length, m	Length, ft
1	4.88–7.62	16–25
2	7.62–10.36	25–34
3	10.36–14.63	34–48

These ranges come from the standard API 5CT (ISO 11960). On the rig the crew measures every pipe and enters it in the tally of the string (the list of pipe lengths). The length is counted without the part of the thread that goes into the coupling. To get the string to the exact length, the crew uses short pipes called pup joints.

Parts are added to the string without which it cannot be run and cemented. From the bottom up:

Casing shoe (guide shoe): A short thick-walled piece with a rounded nose at the bottom end of the string. The bit can drill the nose out later. The shoe guides the string past ledges in the hole. It often has a check valve built in.
Shoe track: One to three pipes between the shoe and the float collar. The cement that mud contaminated during the displacement (the pushing of the cement down the string) stays in them, so clean cement is left at the shoe.
Float collar (check valve and landing ring): The check valve (also called the float valve) does not let the heavy cement slurry return from the annulus into the string. The top plug (the plug that follows the cement down the string) lands on the ring. The rise in pressure at that moment (the “bump”) shows that the displacement is over.
Centralisers: Spring or rigid cages on the body of the pipe. They hold the string in the middle of the hole, so that the cement surrounds it on all sides. Scratchers (wire brushes on the pipe) scrape the mudcake (the layer of mud solids on the wall of the hole) off the wall.
Stage collar: A collar with side ports (openings). The crew installs it if the string is cemented in two or more stages (Primary cementing).
Casing hanger and casing head (wellhead): At the top, the string hangs in the casing head on slips or on a threaded (mandrel) hanger. The head seals the space between the strings (Wellheads and running casing).

SchematicParts of a casing string and a liner. At the top the string hangs in the casing head. The liner hangs in the string above its shoe, with an overlap. The liner carries couplings and centralisers, and at the bottom it has a float collar, a shoe track and a shoe. Cement fills the annulus of the liner, the overlap and the lower part of the annulus of the string. Not to scale.

Size, wall thickness and drift

The nominal size of a casing pipe (the size by which it is named) is its outside diameter (OD). In the main standard it runs from 114.3 mm (4½″) to 508 mm (20″). Conductor pipes can be larger: 762 mm (30″) and 914 mm (36″). Pipes with a diameter smaller than 114.3 mm count as tubing.

For each diameter, pipes are made with different wall thicknesses. Metric catalogues give the wall thickness directly (177.8 × 10.36 mm). English-language catalogues give it through the nominal weight, the weight of a foot of pipe with couplings in pounds (7″, 29 lb/ft). A thicker wall makes a stronger pipe, a heavier string and a smaller inside diameter.

Inside diameter and mass per metre of a casing pipe

d = D - 2t, \qquad m = \rho_{\text{steel}}\,\pi\,(D - t)\,t

where:

$d$ inside diameter (nominal), m
$D$ outside diameter, m
$t$ wall thickness (nominal), m
$m$ mass of a metre of plain pipe, without couplings, kg/m
$\rho_{\text{steel}}$ density of steel, about 7850 kg/m³, kg/m³

Units: SI

Holds when

The cross-section area of the wall, $\frac{\pi}{4}(D^2 - d^2)$ , equals $\pi(D - t)t$ .
This is the mass of a plain pipe. The nominal weight in a catalogue is a little greater, because it includes the couplings or the upset (thickened) ends.
These are nominal sizes. Real sizes differ within the tolerances of the standard.

We need the inside diameter to calculate volumes (the capacity of the string is the area of its inside cross-section) and to check the passage of tools. We need the mass for the weight of the string and the load on the hoisting system (the cable and blocks that lift the string).

In other unit systems (2)

Metric units

m\,[\text{kg/m}] = 0.02466\,(D - t)\,t\;[\text{mm}]

$D$ and $t$ in mm.

$0.02466 = 7850\cdot\pi\cdot 10^{-6}$ (the density of steel, and the conversion of mm² to m²).

Oilfield units

m\,[\text{lb/ft}] = 10.69\,(D - t)\,t\;[\text{in}]

$D$ and $t$ in inches; 1 lb/ft = 1.488 kg/m.

$10.69 = 490\cdot\pi/144$ : the density of steel is 490 lb/ft³, and there are 144 square inches in a square foot.

The inside diameter of each pipe varies within the tolerances (the differences from the nominal size that the standard allows). To guarantee the passage of tools, the pipes are checked with a drift mandrel: a cylinder of a set diameter and length that is pulled through the whole pipe. The drift diameter is the guaranteed passage. The bit, the packer or another tool must be smaller than it.

Standard drift for casing pipes

Outside diameter of the string: smaller than 244.5 mm (9⅝″)
Drift diameter: $d$ − 3.18 mm (⅛″)
Drift length: 152 mm (6″)

Outside diameter of the string: from 244.5 to 339.7 mm
Drift diameter: $d$ − 3.97 mm (5/32″)
Drift length: 305 mm (12″)

Outside diameter of the string: larger than 339.7 mm (13⅜″)
Drift diameter: $d$ − 4.76 mm (3/16″)
Drift length: 305 mm (12″)

Outside diameter of the string	Drift diameter	Drift length
smaller than 244.5 mm (9⅝″)	$d$ − 3.18 mm (⅛″)	152 mm (6″)
from 244.5 to 339.7 mm	$d$ − 3.97 mm (5/32″)	305 mm (12″)
larger than 339.7 mm (13⅜″)	$d$ − 4.76 mm (3/16″)	305 mm (12″)

This is the standard drift of API 5CT (ISO 11960); $d$ is the nominal inside diameter. For some pipes the manufacturers offer a larger drift (alternative drift), so that a bit of the standard size can pass.

Worked exampleWhich bit will pass through the production casing

Given (assumed data)

String: 177.8 mm (7″); pipes with a wall of 9.19, 10.36 and 11.51 mm
Bits for drilling below the string: 155.6 mm (6⅛″) and 152.4 mm (6″)

Solution

Wall 9.19 mm: $d = 177.8 - 2\cdot 9.19 = 159.42$ mm, drift $159.42 - 3.18 = 156.24$ mm; mass of a metre $0.02466\cdot(177.8 - 9.19)\cdot 9.19 = 38.2$ kg/m; capacity $\frac{\pi}{4}\cdot 159.42^2\cdot 10^{-3} = 19.96$ L/m.
Wall 10.36 mm: $d = 157.08$ mm, drift 153.90 mm, mass 42.8 kg/m, capacity 19.38 L/m.
Wall 11.51 mm: $d = 154.78$ mm, drift 151.60 mm, mass 47.2 kg/m, capacity 18.82 L/m.
The 155.6 mm bit passes only the drift of 156.24 mm, with a clearance of 0.6 mm. The 152.4 mm bit passes the drifts of 156.24 and 153.90 mm (clearance 1.5 mm), but not 151.60 mm.

Answer: The 155.6 mm bit passes only into pipes with a wall of 9.19 mm. The 152.4 mm bit passes into pipes with a wall of 9.19 and 10.36 mm. Neither bit passes into pipes with a wall of 11.51 mm.

What it means. A wall thicker by 2.3 mm makes the pipe noticeably stronger and 9.0 kg/m heavier. But it narrows the passage, and the programme for drilling below the string must take this into account. If a string needs pipes with different walls, the thickest wall sets the passage. The nominal sizes follow the standard series; the rest of the data are assumed for illustration.

Steel: grades

The steel sets the strength of a pipe. The standard groups steels into grades. The letter is only a label for the group, and the number is the minimum yield strength in thousands of psi (ksi). For N-80 it is not less than 80,000 psi (552 MPa), and for P-110 it is 110,000 psi (758 MPa). The yield strength is the stress at which steel starts to stretch permanently. For some grades a maximum yield strength is also set. For steels meant for environments with hydrogen sulfide (a toxic gas that attacks steel) there is also a hardness limit.

Main casing grades

Grade: J-55, K-55
Yield strength, MPa: 379–552
Ultimate tensile strength, MPa, minimum: 517 (J-55), 655 (K-55)
Notes: conductors, surface casing, shallow wells

Grade: N-80
Yield strength, MPa: 552–758
Ultimate tensile strength, MPa, minimum: 689
Notes: a common grade for strings of medium depth

Grade: L-80
Yield strength, MPa: 552–655
Ultimate tensile strength, MPa, minimum: 655
Notes: limited hardness: suitable for environments with hydrogen sulfide

Grade: C-90, T-95
Yield strength, MPa: 621–724, 655–758
Ultimate tensile strength, MPa, minimum: 689, 724
Notes: environments with hydrogen sulfide at high loads

Grade: P-110
Yield strength, MPa: 758–965
Ultimate tensile strength, MPa, minimum: 862
Notes: deep wells and high pressures; in environments with hydrogen sulfide, only at high temperature

Grade: Q-125
Yield strength, MPa: 862–1034
Ultimate tensile strength, MPa, minimum: 931
Notes: ultra-deep wells, very high pressures

Grade	Yield strength, MPa	Ultimate tensile strength, MPa, minimum	Notes
J-55, K-55	379–552	517 (J-55), 655 (K-55)	conductors, surface casing, shallow wells
N-80	552–758	689	a common grade for strings of medium depth
L-80	552–655	655	limited hardness: suitable for environments with hydrogen sulfide
C-90, T-95	621–724, 655–758	689, 724	environments with hydrogen sulfide at high loads
P-110	758–965	862	deep wells and high pressures; in environments with hydrogen sulfide, only at high temperature
Q-125	862–1034	931	ultra-deep wells, very high pressures

The values come from the standard API 5CT (ISO 11960) and are rounded; 1 ksi = 6.895 MPa. In the CIS countries (the Commonwealth of Independent States, which includes Kazakhstan) the interstate standards are also used. One is GOST 31446, which is built on ISO 11960. The other is the older GOST 632-80, in which the grades are marked with Russian letters, in the order of increasing strength (transliterated: D, K, E, L, M). Grade E matches N-80 by yield strength.

Stronger is not always better. The stronger and harder the steel, the more sensitive it is to sulfide stress cracking. Atomic hydrogen forms when the steel corrodes in the presence of hydrogen sulfide. It enters the metal and makes it brittle, and a loaded pipe can break without warning. So for wells with hydrogen sulfide, and there are many of them in the fields of the Pre-Caspian Basin, the crew chooses steels of limited hardness. The crew uses high-strength grades only where there is no hydrogen sulfide, or where the temperature is high enough. Against carbon dioxide corrosion, chromium steels are used (for example, with 13% chromium).

Pipes are made seamless (without a weld) or electric-welded. Seamless pipes are used more often for the most important strings. As the temperature rises, the yield strength of steel falls a little. In deep hot wells the designer takes this into account when designing the string (Casing design).

Threaded connections

A connection must carry the weight of the string, withstand pressure from inside and outside, and not let fluids through. Most casing pipes are threaded and coupled: a short coupling with an internal thread joins the pipes. Integral connections are less common. The thread is cut directly on the thickened ends of the pipes, so the outside diameter of the connection is smaller. This matters when the gap between the string and the hole, or the previous string, is small.

Main types of casing connections

Connection: Round thread, short and long (API STC, LTC)
Design and seal: a triangular thread with rounded crests, 8 threads per inch; the seal comes from the thread compound (grease), which fills the gaps along the spiral of the thread
Where used: shallow strings, liquids, moderate pressures

Connection: Buttress thread (API buttress, BTC; OTTM in GOST 632)
Design and seal: a trapezoidal thread, 5 threads per inch, the loaded face is almost perpendicular to the axis, which gives high tensile strength; the seal comes from the compound
Where used: most strings of medium and large diameter

Connection: Premium (gas-tight) connections; OTTG in GOST 632
Design and seal: a metal-to-metal seal (metal presses on metal) and a torque shoulder (a stop face where the pipe ends meet); gas-tight
Where used: gas wells, high pressures and temperatures

Connection: Integral connections; TBO in GOST 632
Design and seal: the thread is cut on the thickened ends of the pipes, and the connection hardly projects
Where used: small gaps, liners

Connection	Design and seal	Where used
Round thread, short and long (API STC, LTC)	a triangular thread with rounded crests, 8 threads per inch; the seal comes from the thread compound (grease), which fills the gaps along the spiral of the thread	shallow strings, liquids, moderate pressures
Buttress thread (API buttress, BTC; OTTM in GOST 632)	a trapezoidal thread, 5 threads per inch, the loaded face is almost perpendicular to the axis, which gives high tensile strength; the seal comes from the compound	most strings of medium and large diameter
Premium (gas-tight) connections; OTTG in GOST 632	a metal-to-metal seal (metal presses on metal) and a torque shoulder (a stop face where the pipe ends meet); gas-tight	gas wells, high pressures and temperatures
Integral connections; TBO in GOST 632	the thread is cut on the thickened ends of the pipes, and the connection hardly projects	small gaps, liners

Manufacturers have their own names for premium connections. Their properties are confirmed by tests to the standard for connection testing. The names OTTM, OTTG and TBO come from GOST 632 and are transliterated from the Russian letters.

The tensile strength of a connection (the joint strength) can be smaller or larger than the strength of the pipe body. In Russian-language practice it is called the pull-out load: the load at which the thread starts to pull out of the coupling. For a round thread it is usually smaller. For a buttress thread and for premium connections it is close to the strength of the body, or higher. The designer calculates the string from its weakest element.

Most leaks in strings come not from the pipe body but from the connections. Most often they come from the way the connections were screwed together: the pipe was started at a slant, the thread was not cleaned or was damaged, the wrong compound was used, or the specified torque was not applied. So on the rig the crew cleans and inspects the threads and greases them. The crew screws the connections together with torque control (for a buttress thread, also by the position of the end of the coupling relative to the mark on the pipe). Before drilling the next interval, the crew pressure-tests the string (fills it with fluid and holds a set pressure to see that it does not leak).

Strength ratings of a pipe

A catalogue gives each pipe its rated characteristics. These are guaranteed values, calculated with the formulas of the standard (ISO 10400, API TR 5C3) for the minimum yield strength, allowing for the tolerances on the pipe dimensions. To design a string, the designer compares the expected loads, multiplied by design factors (safety margins), with these ratings (Casing design).

Internal yield pressure (the Barlow formula)

p_{\text{iy}} = 0.875\,\frac{2\,Y_p\,t}{D}

where:

$p_{\text{iy}}$ internal yield pressure (burst): the internal gauge pressure at which the hoop stress in the wall (the stress around the circumference of the pipe) reaches the yield strength, MPa
$Y_p$ minimum yield strength of the steel, MPa
$t,\ D$ nominal wall thickness and outside diameter, mm
$0.875$ allowance for the tolerance: the wall may be 12.5% thinner than nominal, —

Units: any consistent units: the pressure and the yield strength in the same units (MPa or psi), $t$ and $D$ in the same units

Holds when

The pipe is thin-walled, and the stress is taken as uniform across the wall (the derivation is in the advanced section).
The pressure outside is zero. In a well, the pipe feels the difference between the internal and the external pressure.
This is a rating of the pipe body. The pressure that the connection holds may be lower.

The pressure grows with the yield strength and with the ratio $t/D$ : the thicker the wall, or the smaller the diameter, the higher the allowable internal pressure.

Tensile strength of the pipe body

F_y = Y_p\,\frac{\pi}{4}\left(D^2 - d^2\right)

where:

$F_y$ the tensile load at which the stress in the pipe body reaches the yield strength (pipe body yield strength), N
$Y_p$ minimum yield strength, Pa
$D,\ d$ outside and inside diameters, m

Units: SI; it is convenient to take $Y_p$ in MPa and the diameters in mm: then $F_y$ comes out in newtons

Holds when

The load is axial, with no bending or pressure.
The strength of the connection is checked separately.

The weight of the string in mud, plus the forces when the crew moves the string up and down or sets it down, plus the bending in a curved hole, must stay below this load with a margin.

Collapse pressure is the external pressure at which a pipe loses its shape and flattens. Thick-walled pipes collapse when the steel on the inside surface reaches its yield strength. Thin-walled pipes collapse earlier, before the steel yields: the wall loses stability, like an empty tin can that someone steps on. Between these extremes there are transition ranges. So the collapse pressure falls sharply as the ratio $D/t$ grows, and the standard calculates it by different formulas, depending on $D/t$ and the grade. Ovality (a pipe that is not exactly round) and uneven wall thickness lower the collapse resistance. Axial tension (a pull along the pipe) also lowers it, and internal pressure raises it.

The external pressure on a string comes from the mud and the cement behind it. In intervals of flowing salts it comes from the salt itself, whose pressure approaches the overburden pressure (the pressure from the weight of all the rock above). It is most dangerous when the string is emptied inside: during lost circulation (mud escaping into the rock), when the well is being brought in (started up), and while producing gas. So the lower part of the string is often made thick-walled, and the upper part is designed for tension and internal pressure.

Worked exampleWhat the rated characteristics of a pipe mean

Given (assumed data)

Pipe: 177.8 × 10.36 mm (7″, 29 lb/ft), grade N-80: yield strength not less than 552 MPa
Collapse pressure from the tables of the standard: 48.4 MPa (7020 psi)
String: 3000 m, nominal weight with couplings 43.2 kg/m
Mud behind the string: 1.25 g/cm³

Solution

Ratio $D/t = 177.8/10.36 = 17.16$ .
Internal yield pressure: $0.875\cdot\dfrac{2\cdot 552\cdot 10.36}{177.8} = 56.3$ MPa, about 8160 psi.
Tensile strength of the body: the wall area is $\frac{\pi}{4}(177.8^2 - 157.08^2) = 5450$ mm², and $F_y = 552\cdot 5450 = 3.01$ MN, or about 307 tonnes-force.
Weight of the string in air: $3000\cdot 43.2\cdot 9.81 = 1.27$ MN, which is 42% of $F_y$ . In mud we multiply it by the buoyancy factor (the mud lifts the string, so it weighs less) $1 - 1.25/7.85 = 0.84$ : 1.07 MN.
If the string is emptied completely, the external pressure of the mud, $0.00981\cdot 1.25\cdot z$ , equals the collapse pressure at the depth $z = 48.4/(0.00981\cdot 1.25) = 3947$ m.

Answer: The pipe withstands 56.3 MPa from the inside but only 48.4 MPa from the outside. The pipe body starts to yield in tension at 3.01 MN. An emptied string in mud of 1.25 g/cm³ would collapse below 3947 m (with no design factors).

What it means. The collapse pressure of this pipe is lower than its internal yield pressure, and for thinner-walled pipes the difference is much larger. At 3000 m an emptied string carries an external pressure of 36.8 MPa. The collapse pressure is 1.32 times higher. Whether such a margin is enough is decided by the string design to the accepted norms, with the tension that lowers the collapse resistance taken into account. The sizes and ratings are from the standard tables; the string and the mud are assumed for illustration.

Standards for casing

The sizes, grades, threads, tests and rating calculations for casing are standardised. In world practice the main documents are issued by the American Petroleum Institute (API), and many of them have been adopted as international ISO standards too.

Main documents

Document: API Spec 5CT, ISO 11960
What it covers: the specification for casing and tubing: sizes, grades, chemical composition, tests, marking

Document: API Spec 5B
What it covers: threads of casing and tubing, their gauges and inspection

Document: API RP 5C1
What it covers: handling of pipe: transport, storage, screwing together, running

Document: API TR 5C3, ISO 10400
What it covers: formulas for the rated characteristics: collapse, internal pressure, tension

Document: API RP 5C5, ISO 13679
What it covers: tests of threaded connections for tightness and strength

Document: GOST 31446, GOST 632-80
What it covers: interstate standards for casing, used in the CIS countries

Document	What it covers
API Spec 5CT, ISO 11960	the specification for casing and tubing: sizes, grades, chemical composition, tests, marking
API Spec 5B	threads of casing and tubing, their gauges and inspection
API RP 5C1	handling of pipe: transport, storage, screwing together, running
API TR 5C3, ISO 10400	formulas for the rated characteristics: collapse, internal pressure, tension
API RP 5C5, ISO 13679	tests of threaded connections for tightness and strength
GOST 31446, GOST 632-80	interstate standards for casing, used in the CIS countries

The designations are as of the middle of the 2020s. Standards are revised from time to time, so a project names the particular edition.

Terms and symbols

Conductor, surface, intermediate and production casing: Casing strings in the order they are run. In Russian-language practice the conductor is called napravlenie and the surface casing konduktor.
Liner: A string that is hung in the previous string and does not reach the surface.
Casing shoe: The lower end of the string, with a guide nose. The depth at which the bottom of the string stands is also called the “shoe”.
Check valve and landing ring: They stop the cement from returning into the string, and they stop the top plug.
Centraliser: A device on the body of a pipe that holds the string in the middle of the hole.
Drift: The guaranteed passage of a pipe: a tool must be smaller than the drift diameter.
Nominal weight: The mass of a unit length of pipe with couplings. English-language catalogues give it in lb/ft, and it sets the wall thickness.
Casing grade: The class of the steel of the pipe. The number in the designation is the minimum yield strength in ksi.
Internal yield pressure: The internal pressure at which the wall reaches yield (burst rating).
Collapse pressure: The external pressure at which the pipe loses its shape (collapse rating).
Pull-out load: The tensile load at which the thread of a connection starts to pull out of the coupling.

Common mistakes and misconceptions

Wrong

“A 177.8 mm string means a 155.6 mm bit will pass inside.”

In fact

The passage depends on the wall thickness. For pipes with a wall of 10.36 mm the drift is 153.9 mm, and such a bit will not pass. Check by the drift diameter of the pipe in the string that has the thickest wall.

Wrong

“The higher the grade, the better.”

In fact

High-strength steels cost more and are more sensitive to sulfide stress cracking in the presence of hydrogen sulfide. We choose the grade for the loads and the environment, not “with a margin”.

Wrong

“If a pipe holds 56 MPa from the inside, it will hold the same from the outside.”

In fact

The collapse pressure is usually lower than the internal yield pressure, and it falls sharply as $D/t$ grows. For thin-walled pipes it is several times lower.

Wrong

“The strength of a string is set by the pipe body.”

In fact

A connection can be weaker than the body, both in tension and in tightness. Most leaks come from the threaded connections and from how they were screwed together.

Wrong

“A liner is an unfinished string.”

In fact

It is a full solution: it saves pipe and time, and it leaves a wide string above. Its weak points, the overlap and the load on the upper string, are taken into account in the design. If needed, the liner is extended to the surface.

In depth: advanced detail

Where the Barlow formula comes from

Imagine cutting a pipe of length $L$ in half along its axis. The internal pressure $p$ pushes the halves apart with the force $p\,d\,L$ : the pressure times the flat area $d\,L$ of the cut. The two walls, each with the cross-section $t\,L$ , hold them together, and a hoop stress $\sigma$ acts in them: $2\sigma t L = p\,d\,L$ . Barlow takes the outside diameter instead of the inside diameter. The force and the stress come out a little larger, that is, on the safe side, and $\sigma = p D/(2t)$ . Yield starts at $\sigma = Y_p$ , that is, at $p = 2Y_p t/D$ .

The factor 0.875 appeared because the standard allows the wall to be 12.5% thinner than nominal, and the rating must be guaranteed. The modern standard also gives formulas for the rupture of a pipe, allowing for the strain hardening of the steel (the steel gets stronger as it deforms) and for wall defects. The rupture pressure is higher than the yield pressure, but in string design the yield pressure is still the one most often used.

Why thin-walled pipes collapse earlier

Two limiting cases are described by the formulas of the theory of elasticity. A thick-walled pipe collapses when the hoop stress on the inner surface reaches the yield strength (the Lamé solution). A thin-walled pipe collapses when the ring of the wall loses stability and buckles. The stresses in the steel are then still below the yield strength.

Collapse pressure: yield and elastic loss of stability

p_Y = 2\,Y_p\,\frac{D/t - 1}{(D/t)^2}, \qquad p_E = \frac{2E}{1 - \nu^2}\cdot\frac{1}{(D/t)\,(D/t - 1)^2}

where:

$p_Y$ external pressure at which the inner surface of the wall reaches yield, MPa
$p_E$ external pressure of the elastic loss of stability of a perfectly round pipe, MPa
$E,\ \nu$ Young's modulus (for steel about 207,000 MPa) and Poisson's ratio (about 0.3), MPa, —
$Y_p,\ D/t$ yield strength and the ratio of the outside diameter to the wall thickness, MPa, —

Units: any consistent units of pressure and modulus

Holds when

A long, perfectly round pipe with a uniform wall, zero pressure inside and no axial load.
The standard reduces the theoretical $p_E$ by about 29% to allow for ovality and uneven wall thickness. Between the two limits it applies empirical formulas for “plastic” and “transition” collapse. The boundaries between these ranges depend on the grade.

$p_Y$ falls with growing $D/t$ roughly as $1/(D/t)$ , and $p_E$ falls as $1/(D/t)^3$ . So for thick-walled pipes the strength of the steel decides, and for thin-walled pipes the stability decides, and a stronger steel hardly helps them.

Worked exampleA thick-walled and a thin-walled pipe made of the same steel

Given (assumed data)

Steel: yield strength 552 MPa; $E$ = 207,000 MPa, $\nu$ = 0.3
Pipes: $D/t$ = 17.16 (the pipe from the example of the lesson) and $D/t$ = 40

Solution

$D/t$ = 17.16: $p_Y = 2\cdot 552\cdot 16.16/17.16^2 = 60.6$ MPa; $p_E = 454{,}945/(17.16\cdot 16.16^2) = 102$ MPa, where $2E/(1 - \nu^2) = 454{,}945$ MPa.
$D/t$ = 40: $p_Y = 2\cdot 552\cdot 39/40^2 = 26.9$ MPa; $p_E = 454{,}945/(40\cdot 39^2) = 7.5$ MPa, and with the reduction of 29% about 5.3 MPa.

Answer: For the thick-walled pipe both limits are high (60.6 and 102 MPa), and the standard gives 48.4 MPa: the pipe is in the intermediate region, where the empirical formula for plastic collapse applies. For the thin-walled pipe the elastic loss of stability (7.5 MPa, about 5.3 MPa by the standard) comes much earlier than yield (26.9 MPa).

What it means. A rise in $D/t$ by a factor of 2.3 lowered the yield limit a little more than twofold, and the stability limit 14 times. So thin-walled pipes of large diameter, the conductors and the surface casing, are especially vulnerable to external pressure. An example is cementing, when heavy cement slurry stands behind the string. The data in this example are assumed for illustration.

Connections to other topics

How a well is drilled: the process and the people involved — the sequence of well construction and an example of a casing programme.
Fracture gradient and leak-off testing — how the pore pressure and the fracture pressure set the shoe depths.
Pore pressure and overburden pressure — the pressures along the well and the window of mud densities.
Well control: primary barrier and kick indicators — the string with cement as part of the second barrier of the well.
Casing design — collapse, burst and tension loads, design factors and the design of the string.
Wellheads and running casing — casing heads, hangers and running the strings.
Primary cementing — how the string is cemented, and why the shoe, the valve and the centralisers are needed.
Well completion: opening the reservoir and the tubing string — the production casing and the liner in well completion.

Check yourself

1
Which string shuts off the fresh-water formations and is the base for the blowout preventers?
1. A.Conductor
2. B.Production casing
3. C.Surface casing
4. D.Liner
Show the answer
Correct answer: C. Surface casing
The surface casing (in Russian, konduktor) shuts off fresh water and unstable upper rocks. It is cemented to the surface, and the casing head and the blowout preventers are installed on it. The conductor (in Russian, napravlenie) is shorter and only holds back the loose rocks near the surface.
2
A 339.7 mm string has a wall of 13.06 mm. Will a 311.1 mm bit pass inside it?
Show the answer
The inside diameter is $339.7 - 2\cdot 13.06 = 313.58$ mm. For strings from 244.5 to 339.7 mm the drift is 3.97 mm smaller: $313.58 - 3.97 \approx 309.6$ mm. The 311.1 mm bit is larger than the drift, so it does not pass by the standard drift. The crew needs a pipe with a thinner wall or with a larger drift, or a smaller bit.
3
What does the number 110 mean in the grade designation P-110?
1. A.An internal yield pressure of 110 MPa
2. B.A minimum yield strength of the steel of 110,000 psi (758 MPa)
3. C.A nominal weight of 110 lb/ft
4. D.A maximum temperature of 110 °C
Show the answer
Correct answer: B. A minimum yield strength of the steel of 110,000 psi (758 MPa)
The number is the minimum yield strength in thousands of psi. The pressures that a pipe withstands also depend on its dimensions: the internal yield pressure depends on $t/D$ , and the collapse pressure on $D/t$ .
4
Find the internal yield pressure of a 244.5 × 11.05 mm pipe of grade P-110 (yield strength 758 MPa).
Show the answer
$p_{\text{iy}} = 0.875\cdot 2\cdot 758\cdot 11.05/244.5 \approx 60.0$ MPa, about 8700 psi. The factor 0.875 allows for the wall being up to 12.5% thinner than nominal.
5
How is a liner better than a full string run to the surface?
1. A.It does not need to be cemented
2. B.It is always tighter
3. C.It does not need a hanger
4. D.It is shorter: fewer pipes and less time to run, the crew can rotate it during cementing, and a wide string stays above
Show the answer
Correct answer: D. It is shorter: fewer pipes and less time to run, the crew can rotate it during cementing, and a wide string stays above
A liner saves pipe and time, is run on drill pipe, and leaves a wide string above. But an overlap appears, which must be tight, and the upper string must withstand the production pressures.
6
Why is the string in an interval of flowing salt made thick-walled and of a high grade?
Show the answer
Salt under load flows slowly and over time puts a pressure on the string that is close to the overburden pressure, so the external load is very large. The collapse resistance rises as $D/t$ falls and as the yield strength rises. So the crew puts thick-walled pipes against the salt, and sometimes two strings.

Updated: 19 October 2026