PetE322 Casing Design

7/31/2019 PetE322 Casing Design

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PetE 322 Drilling Engineering II, Chapter 6 Spring 2004

Evren zbayolu, PhD, Tel: 210 5663, e-mail: [email protected] Page 1 of 39

PetE 322 Drilling Engineering II

Chapter 6 Casing Design

In the course of drilling are laid bare rocks differing in their lithological composition,physico-mechanical properties, in the extent of their saturation with and the type of theimpregnating fluid, in the formation pressure abnormality factors and the solidity of the rocks.Alongside stable, for instance, carbonaceous rocks, in which the well bore can remainuncased for a long time, one comes across insecure and incoherent ones. Such rocks readilycrumble, cave in, or are susceptible to plastic deformations and bulge up inward the well boresoon after having been drilled off. Particularly often unstable rocks are encountered atcomparatively shallow depths and also within zones of diastrophism. Among insecure rocksare many chemogenic ones susceptible to plastic flow under the effect of rock pressure,especially at elevated temperatures. To prevent any derangement in the stability of the bo-rehole walls the well bore has to becased off. For such purposes, steel pipes are used, called

casings.

Introduction

Choosing the correct size, type, and amount of casing that is used in well construction is ofutmost importance to the success of the well. The casing must be of sufficient size andstrength to allow the target formations to be reached and produced.

The design of the integrated casing, cementing, mud, and blowout prevention control programmust take into account. The depths at which freshwater, hydrocarbon, salt, coal seams andother mineral-bearing formations are expected to be penetrated, the formation fracture

gradients and pressures expected to be encountered, and other pertinent geologic andengineering data and information about the area.

The main functions of the casing in any well are:! Maintain hole integrity! Isolate abnormally pressured zones! Protect shallow weak formations from heavier mud weights required in the deeper

portions of the holeo Prevent release of fluids from any stratum through the wellbore (directly or

indirectly) into the waterso Prevent communication between separate hydrocarbon-bearing strata (except

such strata approved for commingling) and between hydrocarbon and water-bearing strata

o Prevent contamination of freshwater-bearing strata! Support unconsolidated sediments! Provide a means of controlling formation pressures and fluids

Casing being installed should conform to American Petroleum Institute (API) standards andcement should meet API standards and when mixed with water of adequate quality does notdegrade the setting properties.

The diameter, weight and strength must be calculated with respect to realistic load conditions

during the lifetime of the well. If an exploration or appraisal well is planned, the casing design


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must also take into account temperature expansion causing an increased collapse force on theproduction casing.

When preparing the casing design, information from offset wells and well drilled within thearea along with the geological and pressure related conditions in the well must be taken intoaccount and the casing strings set at such depth that full control of the well can be maintainedat all times.

Installation should be in such a way that they anchored and can withstand the pressures andloads from fluids and gases that can be expected in the well. Casings must also be of suchquality that it can withstand particularly corrosive media in the well (H2S, CO2 etc.), ifexposed to such formations.

Types of Casings

There are primary four types of casings that are commonly used in well construction.

Conductor Pipe

Other names for this casing are drive pipe or stove pipe.

Surface formations are loose and consolidated. They consist mainly of sand and stones. Dueto this, the surface hole will need to be cased off before any drilling can take place. Largeheavy walled pipe is often driven to a point of refusal or a safe depth.

The conductor pipe is the first casing to be put in place, and is generally installed before therig arrives on location. On land, the hole for this shallow casing is often dug with an auger

drill mounted on a truck, or driven using a diesel or steam hammer. Such casing can be drivento 250 feet. Conductor casing measuring between 16 to 24" outside diameter is used onshore,and between 24 to 48" for offshore. However the size of this casing will depend on the depthof the hole, the deeper the hole, the larger the casing.

On land, a cellar is dug. This will house the well if completed. The cellar will normally houseonly one well. There are three ways of putting this casing into the ground:! Driven (driving with a diesel hammer)! Drilled (the hole drilled beforehand the casing is then run and cemented in to place)! Drill and drive (The pipe will be driven to refusal. A bit will be run and a short section of

hole drilled. The pipe will then he hammer in until refusal, the same processes repeateduntil the pipe is at the required depth)


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There are many reasons for such pipe:! To return drilling fluid and cutting back to surface and clean the hole.! Protect fresh water sands.! Stop washouts under the drilling rig.! To give a base and support for the next string of casing! Protect the following casing string

As the hole is drilled, cuttings must be removed. To remove the cuttings, a drilling fluid ispumped and circulated around the hole. This fluid, often referred to as spud mud, is pumpeddown the drill string, out from the bit and back up between the drill string and drive pipe andreturned it a cleaning system. For this to be effective, a riser is attached to the drive pipe.

Surface Casing

Surface casing is set to

! Protect, water sands! Case unconsolidated formations! Provide primary well control! Support other casing! Case off lost circulation zones

Such a string would be run and cemented back to surface. It is normally the first casing tosupport some for of secondary well control equipment. Equipment such as a diverter or 211/4BOP.

Major characteristics! Casing size range from 7 5/8 on shallow holes to 20 inch on deep holes.! Hole may have severe hole erosion! Shallow string may be pumped out easily! Drilling fluid often viscous with little water loss control or drilled with water! Casing may stick easily in unconsolidated formations! Lost circulation or water flows could be a problem! Most area require that it be cemented to surface! Guide shoe and float collar are commonly used! Packing off can cause high pressure and can burst the casing! Often run thought the bent section of the hole! Often run to depths of 3000 feet plus! Often need top cement job to complete cementing to surface


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Drilling this hole section is normally fast and little time is wasted. If spud mud is used, itwould only be at the start. Formation clays would be use to continue with minimum chemicaladded.

In some area, a pilot hole may be drilled, then opened up to a larger size. Drilling tools are bigand cumbersome and the fewer BHA changes the better.

It is often in this section of the hole the start of directional work begins. Running into otherwells could also become a problem. On multi-well platform, there can be as many as 36 holesall going in different directions, guiding the bit past some of the other wells can be a problem.

Shallow gas can also be a problem. For such area, a diverter should be used. In such a case thewell can never be closed in. Diverters of today have been vastly improved, and if rigged up

properly is an asset. Such a rig up would consist of the diverter package with 12 in. outlet, butjust as important are the dump lines a dump line dropping 90 feet to the water line on a jackup has a siphoning effect and will suck both gas and water away from the rig floor.

Intermediate Casing

Of all the casing run, this one normally takes the worst beating. Prolonged drilling can andoften will damage it. Corrosion is common as such a string will often cover salt zones.

The justification for this string is to cover many of the problem zones that are encountered inthe top sections of the hole such as lost circulation and water flows.

Such zone need to be isolated as the drilling fluid weight may have to be raise in the deepersection of the hole.

It will also be used to support the completion and any other string that may be ran later.The casing point "where the shoe is to be set" must be in a firm and solid formation as thesecondary well control equipment will be installed on top. A leak off or integrity test will becarried out to test the shoe.

Should there be any leak, the shoe must be re-cemented by squeezing it off as the casing mustwithstand any drilling fluid raise in weight or kick from the formation.

Often run to depths of 5000 or 6000 ft, and would normally cemented back to surface. A shoe,float collar and sometimes a cement diverter will be used.


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This string will often be the largest cement job and could be done in two stages. The cementdiverter" is a ported tool the same size of the casing that is opened and closed with plugs anddarts.

The first section will be cemented. The cement is pumped down the casing and out of the shoeinto the annulus. As this is often a two stage job a water spacer will be pumped ahead of thecement and should end up just around the outside of the cement diverter once the tail cementis put in place with the wiper plug.

An opening dart will then be dropped and landed on the sleeve of the cement diverter.Pressure will be applied to push the sleeve into the open position. The well will then becirculated from the ports and the hole conditioned ready for the next stage. The second stagewill then be pumped and a closing plug will be pumped and bumped.

Production Casing

The fourth but not necessarily the final string of pipe run in the hole is the production casing.The production casing is used to control the hydrocarbon bearing zones that will be produced.This string of pipe adds structural integrity to the well-bore in the producing zones.

It is necessary to conduct the hydrocarbons to the surface. Production casing should be setbefore completing the well for production. It should be cemented in a manner necessary tocover or isolate all zones which contain hydrocarbons.

A calculated volume of cement sufficient to fill the annular space at least five hundred (500)feet above the top of the uppermost hydrocarbon zone should be used.

Casings must also be of such quality that they can withstand particularly corrosive media in

the well (H2S, CO2 etc.), if expected to be exposed to such formations.

This string would normally be the longest string run and may often be cemented in stages soas not to brake down the lower formation. It must also be of sufficient strength that should the

production strings leak, it will contain the formation pressure that will migrate to surface andshould be design to cover the expected life span of the well.

If there are indications of inadequate primary cementing (such as lost returns, cementchanneling, or mechanical failure of equipment) in the surface, intermediate, or productioncasing strings, the casings should be evaluated, by pressure testing the casing shoe, running acement bond log or a cement evaluation tool log, running a temperature survey, or a

combination before continuing operations. If the evaluation indicates inadequate cementing,the casing should re-cement and if necessary perforated and squeezed with cement.


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When a liner is used as production casing, the testing of the seal between the liner top andnext larger string must be conducted as in the case of intermediate liners.

Selection of Casing Size

The controlling factor in the selection of casing size is the outer diameter of the productionstring. Considerations in the determination of this factor outlined as follows:! Drilling Cost: As hole diameter increases, so will the cost of drilling and completion. The

cost of large diameter holes should be balanced against expected economic advantages.! Methods of Production: A well may flow naturally in its early history, but later may

require pumping. The hole should be large enough to accommodate necessary productionequipment.

! Possibilities of Multizone Completion: Hole sizes should be large enough to handleequipment for multizone completion, if this is a reasonable possibility.

! Number of Intermediate Strings: If the expected drilling conditions necessitate one or

more intermediate strings, the maximum size of the production string will be limited.! Nature of the Fluids Produced: This factor is important primarily because it affects thechoice of production equipment, and in turn, the downhole equipment and accessorieslimit the minimum hole size.

! Rig Limitations: Normally, the selection of a rig depends on size and depth of the hole tobe drilled. There are cases, however, when rig selection is limited in a given area. In thesecases, sizes of hole and casing are determined by rig capabilities.

! Workovers: If experience indicates that remedial work is commonly needed, hole sizeshould be large enough to accommodate the necessary equipment.

! Availability of Casing: Shortage of casing has, in many instances, been determined factorin establishing production string size.

! Common Practice: Even after careful consideration of the above factors, the experiencesof others in given areas and situations should be studied before final determination ofcasing size.

! Type of Well: In an exploratory well, the prime purpose of drilling is to prove up theexistence of commercial zones. Frequently, it is necessary to set casing, and in the interestof economy, slim-hole drilling and completion may be used to extend the exploratory

budget. This type of drilling and completion, however, should be weighed againstpossible production problems at some later date.

When the outside diameter of the production string has been established, bit sizes and sizes ofintermediate and surface casings can be determined. In selecting the hole size for a given

casing OD, it is necessary to consider the coupling OD for the casing and to provide sufficientclearance to allow for mud cake and also casing appliances such as centralizers and scratchersand hole conditions, such as caving formations and crookedness. The following table givesminimum bit sizes for all API casing sizes.


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Table 1 Minimum bit sizes with corresponding casingsCasing OD Coupling OD Recommended Clearance Minimum Hole Size Minimum Bit Size

(in) (in) (in) (in) (in)

4 1/2 5.000 1.000 6.000 6

5 5.563 1.250 6.813 7

5 1/2 6050.000 1.250 7.300 7 3/86 6.625 1.750 8.375 8 3/8

6 5/8 7.390 1.750 9.140 9 1/2

7 7.656 2.000 9.656 9 3/4

7 5/8 8.500 2.500 11.000 11

8 5/8 9.625 3.000 12.625 13 3/4

9 5/8 10.625 3.250 13.875 14 3/4

10 3/4 11.750 3.250 15.000 15

11 3/4 12.750 3.500 16.250 17

13 3/8 14.375 3.500 17.875 18

16 17.000 3.500 20.500 20 3/4

20 21.000 3.500 24.500 25 1/2

Selection of Casing Setting Depths

The selection of the number of casing strings and their respective setting depths are generallybased on a consideration of the pore-pressure gradients and fracture gradients of theformations to be penetrated. Usually, the pore pressure and fracture pressure are expressed asequivalent circulating density, and are plotted vs depth. Usually, the mud densities are chosento provide an acceptable trip margin above the anticipated formation pore pressures to allowfor reductions in effective mud weight caused by upward pipe movement during trippingoperations. A commonly used trip margin is 0.5 ppg or that will provide 200 to 300 psi pfexcess bottomhole pressure over the formation pressure.

Following figure illustrates the relation between the casing setting depth and these gradients.


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Casings setting depth identification

To reach the depth objective, the effective drilling fluid density shown at point-a is chosen toprevent the flow of formation fluid into the well. However, to carry this drilling fluid densitywithout exceeding the fracture gradient of the weakest formation exposed within the wellbore,

the protective intermediate casing must extend at least to the depth at point-b, where thefracture gradient is equal to the mud density needed to drill to point-a. Similarly, to drill topoint-b and to set intermediate casing, the drilling fluid density shown at point-c will beneeded and will require surface casing to be set at least to the depth at point-d. When possible,a kick margin is subtracted from the true fracture gradient line to obtain a design fracturegradient line. If no kick margin is provided, it is impossible to take a kick at the casing settingdepth without causing hydrofracture and a possible underground blowout.

Other factors, such as protection of freshwater aquifers, the presence of vugular lost-circulation zones, depleted low-pressure zones that tend to cause stuck pipe, salt beds thattend to flow plastically and to close the borehole, and government regulations, also can affect

casing setting depth requirements.

Grades of Casings

Casing is graded on the basis of its minimum yield strength. API recognizes five grades andthe corresponding minimum yield strengths. The yield strength for these purposes is definedas the tensile stress required producing a total elongation of 0.5 % of the length (except P-110,where elongation is 0.6 %).


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Table 2 Casing grades, and minimum yield strengthsCasing

Grade

Minimum Yield Strength

(psi)

F-25 25,000H-40 40,000J-55 55,000

N-80 80,000P-110 110,000

Casing Dimensions

Casing is designated also by length range into which it falls. API standards (5A) establishthree length ranges with limits and tolerances as shown in the following table.

Table 3 API casing lengths

Range Length Range(ft)

Minimum Length(ft)

Maximum Length Variation(ft)

1 16 25 18 62 25 34 28 53 Over 34 36 6

Casing is designated also by i) outside diameter, and ii) wall thickness or nominal weight. Thefollowing table is taken from API Spec.5A, lists 74 sizes of casing, varying diameter from 4.5in to 20 in, in wall thickness from 0.205 to 0.595 in, and in weight from 9.50 to 94.00 lb/ft.


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Table 4 API recognized casing list according to their sizesOD Wall Thickness Nominal Weight Available Available OD Wall Thickness Nominal Weight Available Available

(in) (in) (lb/ft) Grades Threads (in) (in) (lb/ft) Grades Threads

0.205 9.50 F,H,J S 0.250 20.00 F S

0.250 11.60 J S,L 0.300 24.00 H S

0.250 11.60 N,P L 0.328 26.40 J S,L

0.290 13.50 N,P L 0.328 26.40 N L

0.337 15.10 P L 0.375 29.70 N,P L0.220 11.50 F,J S 0.430 33.70 N,P L

0.253 13.00 J S,L 0.500 39.00 N,P L

0.296 15.00 J S,L 0.264 24.00 F,J S

0.296 15.00 N,P L 0.304 28.00 H S

0.362 18.00 N,P L 0.352 32.00 H S

0.228 13.00 F S 0.352 32.00 J S,L

0.224 14.00 H,J S 0.400 36.00 J S,L

0.275 15.50 J S,L 0.400 36.00 N L

0.304 17.00 J S,L 0.450 40.00 N,P L

0.304 17.00 N,P L 0.500 44.00 N,P L

0.361 20.00 N,P L 0.557 49.00 N,P L

0.415 23.00 N,P L 0.281 29.30 F S

0.238 15.00 F S 0.312 32.30 H S

0.288 18.00 H S 0.352 36.00 H S

0.288 18.00 J S,L 0.352 36.00 J S,L

0.288 18.00 N L 0.395 40.00 J S,L

0.324 20.00 N L 0.395 40.00 N L

0.380 23.00 N,P L 0.435 43.50 N,P L

0.434 26.00 P L 0.472 47.00 N,P L

0.245 17.00 F S 0.545 53.50 N,P L

0.288 20.00 H S 0.279 32.75 F,H S

0.288 20.00 J S,L 0.350 40.50 H,J S

0.352 24.00 J S,L 0.400 45.50 J S

0.352 24.00 N,P L 0.450 51.00 J,N,P S

0.417 28.00 N,P L 0.495 55.50 N,P S

0.475 32.00 N,P L 0.545 60.70 P S

0.231 17.00 F,H S 0.595 65.70 P S

0.272 20.00 H,J S 0.300 38.00 F S

0.317 23.00 J S,L 0.333 42.00 H S

0.317 23.00 N L 0.375 47.00 J S

0.362 26.00 J S,L 0.435 54.00 J S

0.362 26.00 N,P L 0.489 60.00 J,N S

0.408 29.00 N,P L 0.330 48.00 F,H S0.453 32.00 N,P L 0.380 54.50 J S

0.498 35.00 N,P L 0.430 61.00 J S

0.540 38.00 N,P L 0.480 68.00 J S

0.514 72.00 N S

0.312 55.00 F S

0.375 65.00 H S

0.438 75.00 J S

0.495 84.00 J S

20 0.438 94.00 F,H S

8 5/8

7 5/8

9 5/8

6 5/8

7

16

13 5/8

11 3/4

10 3/4

4 1/2

5

5 1/2

6

Casing Threads and Couplings

Individual casings are usually joined by means of threaded couplings. Couplings are graded in

the same manner as casing, and the physical properties of a coupling must be at least equal tothose of the casing section joints. Couplings are classified also according to the outsidediameter and the wall thickness of the casing with which they are to be used. Finally,couplings are classified as either long or short in accordance with the lengths of the threads ofthe casing with which they are to be used. Table above also shows the couplings available foreach size and grade of casing. API Spec.5A gives the dimensions of long and short threadsand couplings as shown in the following figure.


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Thread and coupling schematic representation

In this figure, de is the outside diameter of the casing, dc is the outside diameter of thecouplings, d1 is the pitch diameter at distanceL1 from vanish point, d2 is the pitch diameter athand-tight plane, J is the distance from end of casing to center of coupling for power-tightmake-up,Lc is the length of coupling,Lt is the distance from end of casing to vanish point ofthreads,L1 is the distance from vanish point of casing threads to plane ofd1,Mis the distancefrom face of coupling to hand-tight plane, dr is the diameter of coupling recess, and t is the

casing wall thickness. Numerical dimensions are given in the following tables.

Table 5a For short threads and couplingsde t d1 d2 J L c L t L 1 M dr dc

4 1/2 All 4.43175 4.40337 0.500 5 2.000 0.625 0.704 4 19/32 5.000

5 0.220 4.93175 4.90337 0.750 6 1/2 2.500 0.625 0.704 5 3/32 5.563

5 X 4.93175 4.90337 0.500 6 1/2 2.750 0.625 0.704 5 3/32 5.563

5 1/2 0.228 5.43175 5.40337 0.750 6 3/4 2.625 0.625 0.704 5 19/32 6.050

5 1/2 X 5.43175 5.40337 0.500 6 3/4 2.875 0.625 0.704 5 19/32 6.050

6 All 5.93175 5.90337 0.500 7 3.000 0.625 0.704 6 3/32 6.625

6 5/8 All 6.55675 6.52837 0.500 7 1/4 3.125 0.625 0.704 6 23/32 7.390

7 0.231 6.93175 6.90337 1.250 7 1/4 2.375 0.625 0.704 7 3/32 7.656

7 X 6.93175 6.90337 0.500 7 1/4 3.125 0.625 0.704 7 3/32 7.656

7 5/8 0.250 7.55675 7.52418 0.875 7 1/2 2.875 0.625 0.709 7 23/32 8.500

7 5/8 X 7.55675 7.52418 0.500 7 1/2 3.250 0.625 0.709 7 23/32 8.500

8 5/8 0.264 8.55675 8.52418 0.875 7 3/4 3.000 0.625 0.709 8 23/32 9.625

8 5/8 X 8.55675 8.52418 0.500 7 3/4 3.375 0.625 0.709 8 23/32 9.625

9 5/8 0.281 9.55675 9.52418 0.625 7 3/4 3.250 0.625 0.709 9 23/32 10.625

9 5/8 X 9.55675 9.52418 0.500 7 3/4 3.375 0.625 0.709 9 23/32 10.625

10 3/4 0.279 10.68175 10.64918 1.250 8 2.750 0.625 0.709 10 27/32 11.750

10 3/4 X 10.68175 10.64918 0.500 8 3.500 0.625 0.709 10 27/32 11.750

11 3/4 0.300 11.68175 11.64918 0.750 8 3.250 0.625 0.709 11 27/32 12.750

11 3/4 X 11.68175 11.64918 0.500 8 3.500 0.625 0.709 11 27/32 12.750

13 3/8 All 13.30675 13.27418 0.500 8 3.500 0.625 0.709 13 15/32 14.375

16 All 15.93175 15.89918 0.500 9 4.000 0.625 0.709 16 3/32 17.000

20 All 19.93175 19.89918 0.500 9 4.000 0.625 0.709 20 3/32 21.000

Table 5b For long threads and couplingsde t d1 d2 J L c L t L 1 M dr dc

4 1/2 All 4.43175 4.40337 0.500 7 3.000 0.625 0.704 4 19/32 5.000

5 All 4.93175 4.90337 0.500 7 3/4 3.395 0.625 0.704 5 3/32 5.563

5 1/2 All 5.43175 5.40337 0.500 8 3.500 0.625 0.704 5 19/32 6.050

6 All 5.93175 5.90337 0.500 8 1/2 3.750 0.625 0.704 6 3/32 6.625

6 5/8 All 6.55675 6.52837 0.500 8 3/4 3.875 0.625 0.704 6 23/32 7.390

7 All 6.93175 6.90337 0.500 9 4.000 0.625 0.704 7 3/32 7.656

7 5/8 All 7.55675 7.52418 0.500 9 1/4 4.125 0.625 0.709 7 23/32 8.500

8 5/8 All 8.55675 8.52418 0.500 10 4.500 0.625 0.709 8 23/32 9.625

9 5/8 All 9.55675 9.52418 0.500 10 1/2 4.750 0.625 0.709 9 23/32 10.625


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When casing is furnished with threads and couplings, the length is taken to be the overalllength pf pipe plus attached coupling, i.e., the length as measured from the uncoupled end of

pipe to the outer face of the coupling at opposite end, with the coupling made up power-tight.When two or more lengths are made up to form a section or a string of casing, the overalllength of string is less than the sum of the individual lengths of casing because of the make-uploss at the couplings. The make-up loss per unit joint for a string made up power-tight is

1

2i cL L J=

whereJis a dimension defined above.

Example

Calculate the make-up loss per joint for 7 in 26 lb/ft casing with short threads and couplings.

Solution

From table-4, it has been seen that for 7 in 26 lb/ft casing, wall thickness is 0.362 in. Thus,from table-5a,Lc is found to be 7.25 in, andJis determined as 0.500 in. Therefore,

( ) ( )1

0.5 7.25 0.5 3.1252

i cL L J= = = in/joint

When casing is run, the couplings forming the joints in the upper sections are in tensionbecause of the weight of the casing suspended below them. These joints must, of course,

possess sufficient strength to resist rupture or deformation under the axial stresses to whichthey will be subjected. Additionally, they must be leak-resistant in tension if the casing stringis to perform its functions properly. From the previously conducted studies, it has beenconcluded that; i) Although several factors influence the leak resistance of a joint made up inthe conventional manner, yield strength is apparently the controlling factor, and, ii) Forstandard API round threads, a joint in tension is leak-resistant up to the yield strength of the

joint (and usually beyond), and hence the leak resistance in tension is greater than thatrequired in service.

The axial tension load which can be supported at a casing joint is called the joint strength.

Joint strength depends on grade, size and weight of the casing, and on the effective length ofthe threads. The following empirical equations are recommended by API for calculating theminimum joint strengths for standard API round-thread casings:

For short thread casings

( )( )( )33.71 0.071 24.45 0.742js e eF C d d t t =

and, for long thread casings

( )( )( )1.647 25.58 0.071 24.45 0.742jl e eF C d d t t =


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where Fjs and Fjlare minimum joint strengths for short and long thread casings, respectively(lb), Cis a constant depending on the casing grading, given in the table below, de is the outerdiameter of the casing (in), and, tis the wall thickness of the casing (in).

Table 6 Constant CJoint Grade C

F-25 134H-40 182J-55 243

N-80 282P-110 369

Joint strengths calculated from the equations given above are presented in the followingtables. All Joint strengths given in these tables in 1,000 lbs.


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Table 7a Joint strengths (in 1,000 lbs) of API casingsOD Nominal Weight F-25 H-40 J-55 J-55 N-80 N-80 P-110 P-110

(in) (lb/ft) S S S L S L S L

9.50 71 96 128

11.60 159 189 220 288

13.50 258 33715.10 394

11.50 84 152

13.00 178 210

15.00 210 247 288 377

18.00 354 463

13.00 95

14.00 139 186

15.50 211 247

17.00 234 275 320 418

20.00 382 500

23.00 440 576

15.00 108

18.00 179 239 278 323

20.00 366

23.00 432 565

26.00 646

17.00 121

20.00 195 259 299

24.00 320 370 430 562

28.00 511 669

32.00 582 762

17.00 118 160

20.00 191 254

23.00 300 344 400

26.00 345 395 460 602

29.00 520 681

32.00 578 756

35.00 635 831

38.00 688 900

20.00 138

24.00 227

26.40 333 378 439

29.70 505 661

33.70 581 760

39.00 676 885

6 5/8

7

7 5/8

4 1/2

5

5 1/2

6


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Table 7b Joint strengths (in 1,000 lbs) of API casingsOD Nominal Weight F-25 H-40 J-55 J-55 N-80 N-80 P-110 P-110

(in) (lb/ft) S S S L S L S L

24.00 161 288

28.00 252

32.00 295 393 43736.00 448 499 581

40.00 655 858

44.00 729 954

49.00 812 1062

29.30 185

32.30 279

36.00 318 442 462

40.00 477 521 606

43.50 670 875

47.00 727 952

53.50 841 1100

32.75 196 26540.50 338 450

45.50 518

51.00 585 680 890

55.50 750 981

60.70 1081

65.70 1180

38.00 222

42.00 236

47.00 507

54.00 593

60.00 668 778

48.00 260 35254.50 545

61.00 613

68.00 695

72.00 868

55.00 258

65.00 423

75.00 662

84.00 753

20 94.00 359 487

16

9 5/8

10 3/4

11 3/4

13 3/8

8 5/8

Weights of Casing

In discussing casing weights, it is necessary to differentiate between plain-end weight,average weight with threads and couplings, and nominal weight of casing. The plain-endweight of casing is the weight without threads and couplings. The average weight (per foot) ofa length of casing is the weight with threads at both ends and a coupling attached power-tightat one end. It is determined from the plain-end weight, the weight of metal removed in cuttingthreads at each end, and the weight of the coupling. Nominal weights are used foridentification purposes and in cases where it is not necessary to specify the exact averageweight per foot. Conventionally, neither the length ranges of the casing nor the dimensions of

the threads and couplings are considered in calculations involving the weight of a section of a


16/39



casing string, since satisfactory accuracy can be obtained by use of nominal weightcorresponding to the outer diameter and wall thickness of the casing.

The following tables give plain-end weights, coupling weights, and weights with threads andcouplings for all API sizes of casings. The weights with threads and couplings are based on 20ft lengths. These weights must be adjusted for lengths longer or shorter than 20 ft in.

Table 8a Weights of API casings

OD Nominal Weight Plain-end Weight Short Long Short Long

(in) (lb/ft) (lb/ft) (lb) (lb) (lb) (lb)

9.50 9.40 6.05 9.55

11.60 11.35 6.05 9.07 11.47 11.54

13.50 13.04 9.07 13.20

15.10 14.98 9.07 15.12

11.50 11.23 10.18 11.50

13.00 12.83 10.18 12.56 13.07 13.12

15.00 14.87 10.18 12.56 15.08 15.13

18.00 17.93 12.56 18.14

13.00 12.84 11.44 13.12

14.00 13.70 11.44 13.97

15.50 15.35 11.44 14.03 15.59 15.64

17.00 16.87 11.44 14.03 17.09 17.14

20.00 19.81 11.44 14.03 20.03

23.00 22.54 14.03 22.70

15.00 14.65 14.53 15.03

18.00 17.57 14.53 18.29 17.91 17.99

20.00 19.64 18.29 20.02

23.00 22.81 18.29 23.13

26.00 25.80 18.29 26.06

17.00 16.69 19.97 17.29

20.00 19.49 19.97 24.82 20.04 20.17

24.00 23.58 19.97 24.82 24.06 24.1828.00 27.65 24.82 28.16

32.00 31.20 24.82 31.64

17.00 16.70 18.34 17.20

20.00 19.54 18.34 20.01

23.00 22.63 18.34 23.67 23.03 23.15

26.00 25.66 18.34 23.67 26.02 26.13

29.00 27.82 23.67 29.12

32.00 31.68 23.67 32.01

35.00 34.58 23.67 34.86

38.00 37.26 23.67 37.48

20.00 19.69 26.93 20.55

24.00 23.47 26.93 24.26

26.40 25.56 26.93 34.23 36.32 26.5129.70 29.04 34.23 29.91

33.70 33.04 34.23 33.83

39.00 38.05 34.23 83.73

6 5/8

7

7 5/8

4 1/2

5

5 1/2

6

Coupling Weight Weight, Threaded and Coupled


17/39



Table 8b Weights of API casings

OD Nominal Weight Plain-end Weight Short Long Short Long(in) (lb/ft) (lb/ft) (lb) (lb) (lb) (lb)

24.00 23.57 35.58 24.75

28.00 27.02 35.58 28.13

32.00 31.10 35.58 47.48 32.14 32.48

36.00 35.14 35.58 47.48 36.11 36.42

40.00 39.29 47.48 40.48

44.00 43.39 47.48 44.48

49.00 48.00 47.48 48.98

29.30 28.04 39.51 29.32

32.30 31.03 39.51 32.25

36.00 34.86 39.51 55.77 36.01 36.46

40.00 38.94 39.51 55.77 40.01 40.44

43.50 42.70 55.77 44.11

47.00 46.14 55.77 47.47

53.50 52.85 55.77 54.02

32.75 31.20 45.53 32.65

40.50 38.88 45.53 40.2045.50 44.22 45.53 45.44

51.00 49.50 45.53 50.63

55.50 54.21 45.53 55.25

60.70 59.40 45.53 60.34

65.70 64.53 45.53 65.37

38.00 36.69 49.61 38.22

42.00 40.60 49.61 42.08

47.00 45.56 49.61 46.94

54.00 52.57 49.61 53.82

60.00 58.81 49.61 59.94

48.00 45.98 56.23 47.64

54.50 52.74 56.23 54.28

61.00 59.45 56.23 60.87

68.00 66.11 56.23 67.40

72.00 70.60 56.23 71.81

55.00 52.36 78.98 54.70

65.00 62.58 78.98 64.71

75.00 72.72 78.98 74.63

84.00 81.97 78.98 83.68

20 94.00 91.41 98.25 93.86

16

9 5/8

10 3/4

11 3/4

13 3/8

8 5/8

Coupling Weight Weight, Threaded and Coupled

Liners

A liner is an abbreviated oil string extending from the bottom of the hole upward to a pointapproximately 100 ft above the lower end of the protection string, where it is suspended froma liner hanger and sealed off. Its function is similar to that of an oil string. Its obviousadvantage over a conventional string (which would extend from a bottom of the hole to thesurface) is economy, since fewer pipes is needed for a liner. This distinct advantage is notrealized without certain disadvantages, among them, the possibility of leakage at the top

point, at which the liner is suspended.

API liners are manufactured only of grade J-55 steel, however, any grade, weight, and type ofcasing may be used as a liner. Sizes of liners are designated in the same manner as casingsizes, i.e., by outside diameter and wall thickness. The length limits and tolerances for liners

are the same as for casing in ranges 2 and 3. The tentative API liner list, with plain end weightand inside diameter for each size, is given in the following table.


18/39



Table 9 API linersOutside Diameter

(in)

Wall Thickness

(in)

Plain-end Weight

(lb/ft)

Inside Diameter

(in)

31/2 0.289 9.91 2.9224 0.286 11.34 3.428

41/2 0.290 13.04 3.9205 0.362 17.93 4.276

51/2 0.361 19.81 4.77865/8 0.417 27.65 5.791

Effects of External Pressure

If casing is lowered into a hole, the pressure outside the casing may be greater than pressureinside the casing because of fluid pressure in the formation opposite the casing or because of

the column of fluid standing between casing and hole. When the excess of external pressureover internal pressure is of sufficient magnitude, there is a tendency for the casing to collapse.If collapse is preceded by permanent deformation, the casing is said to have experienced

plastic failure. If, on the other hand, collapse occurs under elastic deformation, failure is saidto be elastic. The ability of casing to withstand external pressure without experiencing either

plastic or elastic failure is called collapse resistance.

The subject to collapse resistance of casing has been investigated thoroughly from both thetheoretical and experimental standpoints. As a result of these investigations, it has becomeapparent that collapse resistance is determined by! The ratio of pipe diameter to wall thickness

! The characteristics of the material of construction! The axial tension or axial compression to which the casing is subjected

In late 1800s, research on collapse resistance, Pc, leaded theoretical expressions forexplaining this phenomenon.

22

2 1

11

c

e

EP

d

t

=

whereEis the modulus of elasticity, is the Poissons ration, and de/tis the ratio of the outerdiameter and wall thickness of the casing. The values of average yield strength, Ya, for thevarious grades of casings are given in the following table.

Table 10 Average yield strengthCasing

grade

Maximumde/t for

plastic collapse

Average yield strength

(psi)

H-40 40.02 50,000J-55 30.73 65,000

N-80 23.91 85,000

P-110 18.57 123,000


19/39



From the stand point of the fact that, the engineers must make their designs on the basis ofminimum properties of the material, the following equations are widely used in drillingindustry, and recommended by API.

For elastic failure,

6

2

x46.95 10

1

c

e e

Pd d

t t

=

For plastic failure with 14ed

t< ,

2

1

1.50

e

c a

e

d

tP Y

d

t

=

For plastic failure with 14ed

t ,

1.887 0.0345c ae

P Yd

t

=

The Steward equation for 43.5ed

t<

65,0001040c

e

Pd

t

=

The Steward equation for 43.5ed

t

6

3

x37.66 10c

e

Pd

t

=

The Steward equations are used to calculate the collapse pressures for grade F-25 casing only.

Also, F-25 has no elastic failure case. The rest of the equations are used to predict collapsepressures for casing of grades H-40, J-55, N-80 and P-110.


20/39



Example

Calculate the collapse resistance for a 51/2 in17 lb/ft J-55 casing.

Solution

Except for grade F-25 casing, it is necessary to determine which of the equations given aboveis applicable. Since collapse resistance is considered to have been exceeded if either plastic orelastic failure occurs, the lower of the two values predicted by the plastic or elastic equationsmust be used. The key to the selection of the proper equation is the ed t ratio. From table-10,

limiting value for the elastic failure of grade J-55 can be found. From table-4, tfor J-55 51/2in, 17 lb/ft casing is 0.304. Therefore,

5.518.09

0.304ed

t= =

From table-10, it is seen that, since 30.73ed t< , the failure will be plastic, and, since

14ed t> , collapse pressure will be determined from

( )

1.887 1.8870.0345 65000 0.0345 4500

18.09c a

e

P Yd

t

= = =

psi

The following tables give collapse resistance for casing of all API sizes and grades. With afew exceptions, the collapse resistances are calculated using the equations presented above.


21/39



Table 11a Collapse resistance of API casings

OD Nominal Weight

(in) (lb/ft) F-25 H-40 J-55 N-80 P-110

9.5 1,920 2,550 3,320

11.6 4,540 5,930 8,59013.5 7,350 10,640

15.1 12,780

11.5 1,820 3,130

13 3,930

15 4,980 6,520 9,430

18 8,550 12,390

13 1,660

14 2,440 3,170

15.5 3,860

17 4,500 5,890 8,520

20 7,580 10,910

23 8,900 12,870

15 1,540

18 2,780 3,620 4,740

20 5,690

23 7,180 10,380

26 12,380

17 1,370

20 2,360 3,060

24 4,250 5,550 7,850

28 7,110 10,290

32 8,490 12,280

17 1,100 1,370

20 1,920 2,500

23 3,290 4,300

26 4,060 5,320 7,220

29 6,370 9,220

32 7,400 10,700

35 8,420 12,180

38 9,080 13,130

20 1,100

24 1,970

26.4 3,010 3,930

29.7 4,910 6,180

33.7 6,070 8,780

39 7,530 10,900

6

6 5/8

7

7 5/8

Collapse Resistance (psi)

4 1/2

5

5 1/2


22/39



Table 11b Collapse resistance of API casingsOD Nominal Weight

(in) (lb/ft) F-25 H-40 J-55 N-80 P-110

24 950 1,430

28 1,580

32 2,110 2,74036 3,420 4,470

40 5,390 7,420

44 6,320 9,140

49 7,370 10,660

29.3 860

32.3 1,320

36 1,710 2,220

40 2,770 3,530

43.5 4,280 4,760

47 4,900 6,120

53.5 6,110 8,830

32.75 650 83040.5 1,340 1,730

45.5 2,300

51 2,870 3,750 3,750

55.5 4,420 5,040

60.7 6,790

65.7 8,540

38 620

42 940

47 1,630

54 2,270

60 2,840 3,680

48 560 74054.5 1,140

61 1,670

68 2,140

72 2,880

55 290

65 640

75 1,010

84 1,480

20 94 410 520

13 3/8

16

Collapse Resistance (psi)

8 5/8

9 5/8

10 3/4

11 3/4

Effects of Internal Pressure

During the entry of formation fluid into casing, as well as in such operations as squeezing andfracturing, casing is often subjected to high internal pressures. In the lower portions of acasing string, external pressure is normally greater than internal pressure. In the upper

portions of the string, however, external pressure is negligible, since at the surface, there isneither formation pressure nor significant fluid column pressure opposite the casing.Therefore, any appreciable internal pressure (whether resulting from entry of fluid or fromsurface pump pressure) would cause an excess of internal pressure over external pressure inthe upper portions, with a resulting tendency of the casing to fail by longitudinal splitting. The

excess internal pressure at which this type pf failure takes place is called bursting pressure.


23/39



Conventionally, bursting pressure, Pb, for steel pipes (pressure required to rupture the pipe) iscalculated from Barlows formula

2b

e

S tP

d=

where Sis the tensile strength of steel, tand de are explained in the previous sections. For oilwell casing, failure is considered to have occurred when applied stress exceeds yield strength,resulting in permanent deformation of the pipe. This type of failure takes place at stressesconsiderably below those which cause actual rupture. In the view of this, the effect of internal

pressure on casing is best analyzed in terms of internal yield pressure of the casing bysubstituting minimum yield strength, Ym, for tensile strength in Barlows equation. A secondconsideration in the application of Barlows formula is the fact that casing wall thickness mayvary, within API tolerances, by 12.5 % from nominal values. This means that, if nominalvalues for wall thicknesses are used, bursting or internal yield pressures may be as much as12.5 % lower than values predicted by Barlows equation. Making allowance for this, the

expression for minimum internal yield pressure, Pi, becomes

( )2 1 0.125 1.75m e mi

e

Y d YP

dt

t

= =

Following tables give internal yield pressures for casing of all API sizes and grades, ascalculated from the equation above.

Example

Calculate the minimum internal yield pressure for N-80, 7 in, 38 lb/ft.

Solution

For this casing, ed t value can be calculated as

712.96

0.540ed

t= =

From table-2, Ym for N-80 is determined as 80000 lb. Thus, internal yield pressure for N-80, 7in, 38 lb/ft is calculated as

( )1.75 800001.7510800

12.96m

i

e

YP

d

t

= = =

psi


24/39



Table 12a Internal yield pressures of API casingsOD Nominal Weight

(in) (lb/ft) F-25 H-40 J-55 N-80 P-110

9.50 1,990 3,190 4,380

11.60 5,350 7,780 10,690

13.50 9,020 12,41015.10 14,420

11.50 1,930 4,240

13.00 4,870

15.00 5,700 8,290 11,390

18.00 10,140 13,940

13.00 1,810

14.00 3,110 4,270

15.50 4,810

17.00 5,320 7,740 10,640

20.00 9,190 12,640

23.00 10,560 14,520

15.00 1,74018.00 3,360 4,620 6,720

20.00 7,560

23.00 8,870 12,190

26.00 13,920

17.00 1,620

20.00 3,040 4,180

24.00 5,110 7,440 10,230

28.00 8,810 12,120

32.00 10,040 1,380

17.00 1,440 2,310

20.00 2,720 3,740

23.00 4,360 6,340

26.00 4,980 7,240 9,960

29.00 8,160 11,220

32.00 9,060 12,460

35.00 9,960 13,690

38.00 10,800 14,850

20.00 1,430

24.00 2,750

26.40 4,140 6,020

29.70 6,890 9,470

33.70 7,890 10,860

39.00 9,180 12,630

Internal Yield (psi)

4 1/2

5

5 1/2

6

6 5/8

7

7 5/8


25/39



Table 12b Internal yield pressures of API casingsOD Nominal Weight

(in) (lb/ft) F-25 H-40 J-55 N-80 P-110

24 1,340 2,950

28 2,470

32 2,860 3,93036 4,460 6,490

40 7,300 10,040

44 8,120 11,160

49 9,040 12,430

29.3 1,280

32.3 2,270

36 2,560 3,520

40 3,950 5,750

43.5 6,330 8,700

47 6,870 9,440

53.5 7,930 10,900

32.75 1,140 1,82040.5 2,280 3,130

45.5 3,580

51 4,030 5,860 8,060

55.5 6,450 8,860

60.7 9,760

65.7 10,660

38 1,120

42 1,980

47 3,070

54 3,560

60 4,010 5,830

48 1,080 1,73054.5 2,730

61 3,090

68 3,450

72 5,380

55 850

65 1,640

75 2,630

84 2,980

20 94 960 1,530

13 3/8

16

8 5/8

9 5/8

10 3/4

11 3/4

Internal Yield (psi)

Effects of Axial Loading

The effect of axial tension is twofold. First, it tends to cause the casing to fail by longitudinaldeformation or yielding, and second, it lowers the resistance of casing to collapse.Considering the first effect, the stress at which permanent deformation takes place is theminimum yield strength, Ym. If the axial load is sufficiently large to cause deformationanywhere along the pipe, the deformation will occur at the root of the last perfect thread, sincethe cross-sectional area of the pipe is least at that point. The axial load causing longitudinalyielding is, therefore,

a m jF Y A=


26/39



where Aj is the area at the root of the thread, which can be determined from the followingtables, including for all sizes and weights of casings.

Table 13a Cross-sectional dimensions of API casings

Casing OD Nominal weight Drift diameter Plain-end area Root thread area(in) (lb/ft) (in) (sq in) (sq in)

9.5 21.951 3.965 2.766 1.775

11.6 18 3.875 3.338 2.347

13.5 15.517 3.795 3.836 2.844

15.1 13.353 3.701 4.407 3.416

11.5 22.727 4.435 3.304 2.2

13 19.762 4.369 3.773 2.67

15 16.891 4.283 4.374 3.271

18 13.812 4.151 5.275 4.171

13 24.122 4.919 3.776 2.561

14 22.541 4.887 4.029 2.81

15.5 20 4.825 4.514 3.29917 18.092 4.767 4.962 3.747

20 15.235 4.653 5.828 4.613

23 13.253 4.545 6.63 5.414

15 25.21 5.399 4.308 2.981

18 20.833 5.299 5.168 3.841

20 18.518 5.227 5.777 4.45

23 15.789 5.115 6.709 5.382

26 13.825 5.007 7.589 6.262

17 27.04 6.01 4.911 3.444

20 23.003 5.924 5.734 4.267

24 18.821 5.796 6.937 5.47

28 15.887 5.666 8.122 6.66632 13.947 5.55 9.177 7.71

17 30.303 6.413 4.912 3.361

20 25.735 6.331 5.749 4.198

23 22.082 6.241 6.656 5.105

26 19.337 6.151 7.549 5.998

29 17.157 6.059 8.449 6.899

32 15.453 5.969 9.317 7.766

35 14.056 5.879 10.173 8.622

38 12.962 5.795 10.959 9.408

de /t

6 5/8

7

4 1/2

5

5 1/2

6


27/39



Table 13b Cross-sectional dimensions of API casings

Casing OD Nominal weight Drift diameter Plain-end area Root thread area

(in) (lb/ft) (in) (sq in) (sq in)

20 30.5 7 5.792 4.115

24 25.417 6.9 6.904 5.213

26.4 23.246 6.844 7.519 5.828

29.7 20.333 6.75 8.541 6.85

33.7 17.732 6.64 9.72 8.029

39 15.25 6.5 11.192 9.501

24 32.67 7.972 6.934 5.02

28 28.371 7.892 7.947 6.032

32 24.502 7.796 9.149 7.234

36 21.562 7.7 10.336 8.421

40 19.166 7.6 11.557 9.642

44 17.25 7.5 12.763 10.848

49 15.485 7.386 14.118 12.203

29.3 34.252 8.907 8.249 6.11

32.3 30.849 8.845 9.128 6.99

36 27.343 8.765 10.254 8.116

40 24.367 8.679 11.454 9.315

43.5 22.126 8.599 12.559 10.421

47 20.391 8.525 13.572 11.434

53.5 17.66 8.379 15.547 13.408

32.75 38.53 10.036 9.178 6.788

40.5 30.714 9.894 11.435 9.045

45.5 26.875 9.794 13.006 10.616

51 23.888 9.694 14.561 12.171

55.5 21.717 9.604 15.947 13.557

60.7 19.725 9.504 17.473 15.082

65.7 18.067 9.404 18.982 16.592

38 29.166 10.994 10.791 8.178

42 35.285 10.928 11.944 9.331

47 31.333 10.844 13.402 10.788

54 27.011 10724 15.463 12.849

60 24.028 10.616 17.301 14.687

48 40.53 12.559 13.525 10.546

54.5 35.197 12.459 15.513 12.535

61 31.104 12.359 17.486 14.508

68 27.864 12.259 19.447 16.468

72 26.021 12.191 20.769 17.791

55 51.282 15.188 15.378 11.81265 42.666 15.062 18.407 14.841

75 36.529 14.938 21.413 17.847

84 32.323 14.822 24.112 20.546

20 94 45.662 18.936 26.917 22.456

16

de/t

9 5/8

10 3/4

11 3/4

13 3/8

7 5/8

8 5/ 8

In analyzing the effect of axial loading on collapse, it should be noted that the collapseresistance given in the tables (also calculated using the equations given in the effects ofexternal pressure section), apply to casing subjected to no axial stress. The presence of axialstress has no effect on the resistance of a tube to elastic collapse, however, it can

substantionally reduce resistance to plastic failure. If the (plastic) collapse pressure for casing


28/39



under no axial load is Pc, and the collapse pressure corrected for the effect of an axial load Wis Pcc, then,

cc t

c o

P S

P S=

where So is the yield stress in pure tension, and St is the apparent yield stress in the tangentialor peripheral direction in the presence of axial stress, Sz, which is defined by

z

WS

A=

where A is the cross-sectional wall area of the tube. Defining K, which is a constantdetermined by the dimensions and the material of construction of the tube,

2 oK AS=

API recommended the corrected collapse resistance as

( )2 23cccP

P K W W K

=

where W is the weight of the casings suspended below the point of concern. K can bedetermined using the tables given below.


29/39



Table 14a K values (1,000 lbs) for API casings

Casing OD Nominal weight

(in) (lb/ft) H-40 J-55 N-80 P-110

9.50 278 360

11.60 434 567 821

13.50 652 944

15.10 1084

11.50 430

13.00 490

15.00 569 744 1076

18.00 897 1298

14.00 403 524

15.50 587

17.00 645 844 1221

20.00 991 1434

23.00 1127 1631

18.00 517 672 879

20.00 982

23.00 1141 1650

26.00 1867

20.00 573 745

24.00 902 1179 1707

28.00 1383 2001

32.00 1560 2258

17.00 491

20.00 575 747

23.00 865 1132

26.00 981 1283 1857

29.00 1436 2078

32.00 1584 2292

35.00 1729 2503

38.00 1863 2696

24.00 690

26.40 977 1278

29.70 1452 2101

33.70 1652 2391

39.00 1903 2753

7 5/8

K (1,000 of pounds)

4 1/2

5

5 1/2

6

6 5/8

7


30/39



Table 14b K values (1,000 lbs) for API casingsCasing OD Nominal weight

(in) (lb/ft) H-40 J-55 N-80 P-110

24.00 901

28.00 795

32.00 915 118936.00 1344 1757

40.00 1965 2843

44.00 2170 3140

49.00 2040 3473

32.30 913

36.00 1025 1333

40.00 1489 1947

43.50 2135 3090

47.00 2307 3339

53.50 2643 3825

32.75 918

40.50 1144 148745.50 1691

51.00 1893 2475 3582

55.50 2711 3923

60.70 4298

65.70 4670

42.00 1194

47.00 1742

54.00 2010

60.00 2509 2941

48.00 1352

54.50 2017

61.00 227368.00 2528

72.00 3531

65.00 1841

75.00 2784

84.00 3135

20 94.00 2692

11 3/4

13 3/8

16

8 5/8

9 5/8

10 3/4

K (1,000 of pounds)

Example

Calculate the collapse resistance of 7 in, 26 lb/ft N-80 casing if a 2000 ft section of 7 in, 29lb/ft N-80 is suspended below it.

Solution

For N-80 7 in 26 lb/ft casing, from table-13a, A is determined as 7.549 in2. Thus, from tabl-14a, K for this casing is found as 1283000 lb. Collapse resistance of N-80, 7 in, 26 lb/ft casingis determined from table-11a as 5320 psi.

Weight of 2000 ft of N-80 7 in 29 lb/ft is

( )( )2000 29 58000W= = lb


31/39



Since, from table-13a, ed t is equal to 19.337, which indicates that the failure will be plastic.

Thus, using

( ) ( )

2 2 12 12 6x x x

53203 1.646 10 0.010 10 0.058 10 5060

1283000

ccc

PP K W W

K

= = = psi

The following table is a list of all API casing sizes which fail elastically at zero axial stress.Also, shown are minimum loads at which the equations for which plastic collapse apply. Theeffect on H-40 casing is not considered, since this casing is not normally used in combinationstrings, i.e., a section of it is not normally subjected to axial stress by the suspension ofanother section below it.

Table 15 - API casing sizes which fail elastically at zero axial stressGrade OD Nominal weight

(in) (lb/ft) pounds psi

13 3/8 48.00

16 65.00

20 94.00

8 5/8 24.00 36,800 5,300

11 3/4 47.00 22,500 1,680

13 3/8 54.50 123,500 7,960

13 3/8 61.00 9,000 520

16 75.00 201,200 9,400

16 84.00 104,800 4,340

9 5/8 40.00 43,500 3,800

11 3/4 60.00 20,800 1,200

13 3/8 72.00 309,200 14,8906 5/8 24.00 35,000 5,040

7 26.00 106,000 14,040

7 5/8 29.70 235,200 27,540

8 5/8 40.00 127,500 11,030

9 5/8 43.50 558,500 44,470

9 5/8 47.00 386,200 28,460

10 3/4 51.00 816,200 56,060

10 3/4 55.50 658,500 41,290

10 3/4 60.70 348,500 19,940

P-110

N-80

Minimum axial load at which plastic collapse equations apply

J-55

H-40

Design Factors

As discussed in the previous sections, joint strength, Fj, is a measure of the resistance ofcasing to failure in tension at the joints; tensile yield load, Fa, is a measure of the resistance ofcasing to failure in tension based on the minimum yield strength and the cross-sectional areaof the material of construction; collapse pressure, Pcc, is a measure of the resistance of casingto failure by collapse under external pressure; internal yield pressure, Pi, is a measure of theresistance of the casing to failure by yielding or bursting from internal pressure. All thesequantities indicate maximum allowable stresses based on minimum physical properties towhich casing can be subjected without failure.

It is seldom desirable to subject any material to its maximum allowable stress. This isparticularly true for the material of which casing is constructed, since minimum physical


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properties are determined statistically, and the properties of any individual length of casingmay deviate considerably from the statistical average. Accordingly, a concept called safetyfactor is introduced, which is the ratio of maximum allowable stress to actual working stress.If safety factors are designated by N with appropriate subscripts), the working loads and

pressures to be sustained by casing are

Joint Load j

j

F

N=

Axial Load a

a

F

N=

External Pressure cc

c

P

N=

Internal Pressure i

i

P

N=

API suggested the following design (safety) factors according to the field experiences.

ForPcc, Nc : 1.00 1.50ForFj and a, Nj and a : 1.50 2.00ForPi Ni : 1.00 1.75

Design of a Combination String

A combination string, i.e., a casing string consisting of more than one section, is used in orderto obtain a string which will satisfy the desired design factors with the least investment. Thus,the starting point for a design is a statement of the weights and grades of casing available,together with the designed factors to be employed. In connection with the latter, it should benoted that the physical properties almost universally considered are joint strength, collapse

pressure, and internal yield.

Once the available casing and the design factors to be used have been determined, all gradesand weights of casing which will not meet the requirements for internal yield are eliminated.The worst possible conditions are used in determining loading data. In line with this, the

internal pressure (for design purposes) is assumed to be full reservoir pressure, Pres, and theexternal pressure is assumed to be zero. Thus, minimum allowable internal yield strength forthe casing tube used in the string is

i res iP P N=

For casing which will meet the requirements for internal pressure, the controlling factor in thelower portions of the string is collapse pressure, and the controlling factor in the upper

portions of the string is joint strength (or, possibly, longitudinal yielding).

For purposes of investigating the setting depth limitations imposed by collapse resistance, it isassumed that the external pressure is that due to the external fluid column, and that the


33/39



internal pressure is zero. Accordingly, the lowest section of the casing string will becomposed of the least expensive weight and grade which will satisfy the equation

0.052c c sP N L=

whereLs is the setting depth for the casing (ft), and is the density of the external fluid (ppg).In determining setting depths for sections other than the lowest, the effect on collapse pressureof longitudinal tension must be considered. This normally involves the use of either trial-and-error or graphical solutions.

At some point up the hole, collapse resistance ceases to be the controlling factor in casingstring design. From this point to the top of the string, the primary consideration is jointstrength and longitudinal yielding. In this region, the casing must be designed to satisfy theequations

j jF W N=

and

m j aY A W N =

where Wis the weight of casing suspended below the casing under consideration.

Example

Design a 7 in, 8000 ft combination casing string for a well where the mud weight is 12 ppg

and the formation pressure gradient is 0.5 psi/ft, suing the worst possible loading assumptions.All weight of API casing in grades J-55 and N-80 are available. The design factors to besatisfied are 1.125 for collapse, 2.00 for joint strength, 1.25 for yield strength, and 1.00 forinternal yield.

Solution

The available casings along with pertinent physical properties are listed as follows.

Grade WeightPi

(tab.12)Pc

(tab.11)K

(tab.14)Fjl

(tab.7)Fjs

(tab.7)Ym

(tab.2)Aj

(tab.13)

J 20 3740 2500 747,000 254,000 55,000 4.198J 23 4360 3290 865,000 344,000 300,000 55,000 5.105J 26 4980 4060 981,000 395,000 345,000 55,000 5.998

N 23 6340 4300 1,132,000 400,000 80,000 5.105N 26 7240 5320 1,283,000 460,000 80,000 5.998N 29 8160 6370 1,436,000 520,000 80,000 6.899N 32 9060 7400 1,584,000 578,000 80,000 7.766N 35 9960 8420 1,729,000 635,000 80,000 8.622N 38 10800 9080 1,863,000 688,000 80,000 9.408

The reservoir pressure can be estimated using the given information above. If no informationis available, a reasonable gradient can be assumed.


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( )8000 0.5 4000resP = = psi

The minimum internal yield for any section of the string must be

( )4000 1.00 4000i res iP P N= = = psi

This requirement excludes J-55, 20 lb/ft casing (which has an internal yield pressure less than4000 psi) at any point in the string. Since all other weights and grades have internal yieldstrength greater than 4000 psi, they are retained for further consideration. The lowest sectionof the string will consist of the least expensive casing available for which the collapse

pressure is at least

( )( )( )0.052 0.052 1.125 12 8000 5620c c sP N L= = = psi

Therefore, the lowest section (which will be designated as section-1) will consist of N-80, 29lb/ft casing with long threads and couplings. The length of section-1 is limited physically only

by the axial load which can be sustained at the top joint of the section. Considering jointstrength,

max

520000260000

2.00j

j

FW

N= = = lbs

and considering yield strength,

( )( )max

80000 6.899 4420001.25

m j

a

Y AWN

= = = lbs

The maximum length of section-1 is, therefore,

maxsec 1

2600008970

29L

= = ft

which is greater than the setting depth. This means that, collapse pressure is the controllingfactor for this portion of the string.

The next lowest section (will be called section-2), will consist of the next lighter casing,namely, N-80, 26 lb/ft with long threads and couplings. Neglecting the effect of axial tension(due to the weight of section-1, since the length of section-1 is not exactly known belowsection-2), and knowing from the tables that Pc for N-80, 26 lb/ft is 5320 psi, the setting depthof section-2 is

( ) ( )

53207580

0.052 0.052 1.125 12c

s

c

PL

N= = = ft.

This is the first assumed setting depth of section-2. Assuming that this setting depth is correct,the weight of section-1 (below section-2) is


35/39



( )( )8000 7580 29 12180W= = lb

For this axial load, the collapse pressure of section-2 should be corrected as

( ) ( )2 2 12 9x x53203 1.646 10 0.445 10 12810 5270

1283000c

ccPP K W W K

= = = psi

So, setting depth of section-2 is updated to

( ) ( )

52707510

0.052 0.052 1.125 12cc

s

c

PL

N= = = ft

This is the second assumed setting depth of section-2. Under this assumption, the weight ofsection-1 becomes

( )( )8000 7510 29 14210W= = lb

and, hence, the corrected collapse pressure becomes

( )12 9x x5320

1.646 10 0.606 10 14210 52601283000

ccP = = psi

for section-2. The third assumption for the setting depth of section-2 is

( ) ( )5260 7490

0.052 1.125 12sL = = ft

and the weight of section-1 below section-2 becomes

( )( )8000 7490 29 14790W= = lb

Since the iteration results are close to each other, the collapse pressure for section-2 is 5260psi, and the weight of section-1 is 14790 lb. So, maximum length for section-2 has to bedetermined. The maximum joint load is

max

460000230000

2.00j

j

FW

N= = = lb

and the maximum yield load is

( ) ( )max

80000 5.998384000

1.25m j

a

Y AW

N= = = lb

Since the weight of casing suspended below section-2 is 14790 lb, the maximum length ofsection-2 is


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( )maxsec 2

230000 147908280

26L

= = ft

which is greater than the setting depth. Hence, collapse pressure is still the controlling factorin this portion of the string. The next available lighter casing will be investigated. Section-3

will consist of N-80, 23 lb/ft casing with long threads and couplings, which has anuncorrected collapse pressure of 4300 psi. Again, neglecting the effect of axial tension due tothe weights of sections 1 and 2, the first assumed setting depth for section-3 is

( ) ( )

43006130

0.052 1.125 12sL = = ft

On this basis, the weight of section-2 below section-3 is

( )( )7490 6130 26 35400W= = lb

Total axial load below section-3 (due to section 2 and 1) is

14790 35400 50200W = + = lb

The corrected collapse pressure for section-3 is

( )12 12x x4300

1.281 10 0.008 10 50200 40901132000

ccP = = psi

from which, the second assumed setting depth for section-3 is

( ) ( )

40905830

0.052 1.125 12sL = = ft

By continuing the trial-and-error procedure, the setting depth for section-3 is calculated to be5780 ft. For this setting depth, the total weights of section-1 and section-2 is 59200 lb, and thecollapse pressure of section-3 is 4060 psi. The maximum length of section-3 can bedetermined by checking the maximum allowable joint load and maximum yield load,

max

400000

2000002.00W = = lb

and

( ) ( )max

80000 5.105327000

1.25W = = lb

respectively. The maximum length of section-3 is

( )maxsec 3

200000 59200

612023L

= = ft


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which is again greater than the setting depth. Thus, collapse pressure continues to be thecontrolling factor, and will determine the setting depth of section-4. The least expensive of theremaining grades and weights is J-55, 26 lb/ft casing with short threads and couplings. Thesetting depth of section-4 is found by trial-and-error to be 5310 ft, the total weight of sections1, 2 and 3is 71400 lb, and the collapse pressure of section-4 is 3730. The maximum allowable

joint load and yield loads for section-4 are, respectively,

max

345000172500

2.00W = = lb

and

( )( )max

55000 5.998264000

1.25W = = lb

The maximum length of section-4 is

( )maxsec 4

172500 714003890

26L

= = ft

Since 3890 ft is less than the setting depth of section-4 (which is 5310 ft), the setting depth forsection-5 is not governed by collapse pressure, but by joint strength. Section-5 is composed ofJ-55, 26 lb/ft with long threads and couplings (stronger than J-55, 26 lb/ft with short threadsand couplings), has a setting depth given by

sec 5 5310 3890 1420L = = ft

For section-5, maximum allowable joint and yield loads are, respectively,

max

395000197500

2.00W = = lb

and

( )( )max

55000 5.998264000

1.25W = = lb

The weight of all casings below section-5 is

( )x71400 26 3890 172500+ = lb


197500 172500960

26

= ft

The setting depth of section-6 is


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sec 6 1420 960 460L = = ft

It is obvious that, section-6 must consist of casing with a joint strength greater than that ofsection-5 (i.e., greater than 395000 lb). No weight of J-55 casing will satisfy this requirement,and, therefore, N-80, 23 lb/ft casing with long threads and couplings must be used. For

section-6, allowable joint and yield loads are, respectively,

max

400000200000

2.00W = = lb

and

( ) ( )max

80000 5.998327000

1.25W = = lb

The weight of all casings below section-6 is

( )x172500 26 960 197500+ = lb


200000 197500110

23

= ft

The setting depth of section-7 is

sec 7 460 110 350L = = ft

Section-7 must consist of casing with a joint strength greater than 400000 lb. The obviouschoice is N-80, 26 lb/ft casing with long threads and couplings. For this casing, fromcalculations previously made, the maximum joint load and maximum yield load are, 230000lb and 384000 lb, respectively. The total weight of the previous sections is 200000 lb. Themaximum length for section-7 is, therefore,

230000 2000001150

26

= ft.

Since this is greater than the allowable setting depth of section-7, this section can continue tothe top of the hole. Summarizing,


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SectionInterval

(ft)Length

(ft)Grade

Weight(lb)

Coupling

1 7490-8000 510 N-80 29 L2 5780-7490 1710 N-80 26 L3 5310-5780 470 N-80 23 L4 1420-5310 3890 J-55 26 S5 460-1420 960 J-55 26 L6 350-460 110 N-80 23 L7 0-350 350 N-80 26 L

The calculations involving design factors are tabulated below;

Section Ni Nc Nj Na1 2.04 1.277 35.2 37.32 1.81 1.125 7.77 8.11

3 1.58 1.125 5.60 5.714 1.24 1.125 2.00 1.915 1.24 3.54 2.00 1.676 1.58 11.55 2.00 2.047 1.81 19.50 2.20 2.30

The design problem discussed above was worked out in a great deal more detail than isnormally necessary. It is obvious that, once the joint strength becomes the controlling factor,it is unnecessary to check for collapse resistance for each section. It is also apparent that, inmost instances, the design factor for axial yielding will be satisfied if the design factor for

joint strength is satisfied.

Date post:	05-Apr-2018
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PetE322 Casing Design

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