Chapter 7 - Casing Design

 

  hapter

asing Design

R e purpose of this chapter is to present I ) the primary functions of oilwell casing 2 ) the various types of casing strings used and 3 ) the procedures used in the design of casing strings. Introduction

Casing serves several important functions in drilling ahd completing a well. It prevents collapse of the borehole during drilling and hydraulically isolates the wellbore fluids from the subsurface formations and formation fluids. It minimizes da mag e of both the subsurfa ce environment by the drilling proc ess and the well by a hostile subsurface environ men t. It provides a high-strength flow flow conduit for the drilling fluid fluid to the su rface and, w ith the blowout preventers (BOP), permits the safe control of formation pressure. S elective perforation perforation of properly cem ented casing also permits isolated communication with a given formation of interest. As the search for commercial hydrocarbon deposits reaches greater depths , the num ber and sizes of the casing strings required to drill and to complete a well successfully also increases. Casing has become one of the most expensive parts of a drilli drilling ng p rogram : studies have shown that the average cost of tubulars is about 187 of the average cost of a completed well. Th us, an important responsibilityy of the drilling sponsibilit drilling enginee r is to design the least expensive casing program that will allow the well to be drilled and operated safely throughout its life. The savings that can be achieved through an optimal design, as well as the risk risk of failure fro m an impro per de sign, jjusti usti-fy a considerable engineering effort on this phase of the drilling program. Fig. 7 .1 show s typi typicak cak casing progra ms for deep wells in several different sedimentary basins. A well that will not encounter abnormal formation pore pressure gradients, lost circulati circulation on z ones, or sal saltt sections may require only conductor casing and surface casing to drill to the depth objective for the w ell. The c onduc tor casing is need-

ed to circulate the drilling fluid to the shale shaker w ithout eroding the surface sediments below below the rig and rig foundations when drilling is initiated. The conductor casing also protects the subsequent casing strings from corrosion and may be used to suppo rt structurall structurallyy som e of the wellhead wellh ead loa d. A dive rter system can be installed installed on the conductor casing to divert flow fr om rig personnel personnel and equipment in case of an unexpected influx of formation fluids during drilling to surface casing depth. The surface casing prevents cave-in of unconsolidated, weaker, near-surface sediments and protects the shallow, freshwater sands from contamination. Surface casing also also supports and protects from corrosion any subsequent casing strings run in the well. In the event of a kick, surface casing generally allow s tthe he flow to be contained by closing the BOP's. Th e BOP'S should not be closed unless the sing to which the BOP's are attached has been placed dee p enough into the earth to preven t a pressure-induced pressure-induced form ation fracture initiated initiated below the cas ing seat from reaching the surface. Subsequ ent flow flow through such fractures eventueventuall allyy can erode a large crater, up to several hundred hundred feet in diameter, which could completely engulf the rig. Surface-casing-setting depths are usually from 300 to 5 000 t into the the s edime nts. Because o f the possibilit possibilityy of contamination of shallow-water-supply aquifers, surfacecasing-setting casing-sett ing depth s and cem enting practices are subject to government regulations. Deeper wells that penetrate abnormally pressured Iormations, lost circulation zones, unstable shale sections, or salt sections sections generally generally will require one o r more strings o f intermediate casing between the surface-casing depth and the final well depth (Fig. 7.1 b). W hen ab norm al formation pore pressures are present in the deeper portions of a well, interme diate casing is needed to protect form ations tions below the su rface casing from the pressures created by the required high drilling-flui drilling-fluidd d ensity. Similarly , when normal pore pressures are found below sections having having abnormal pore pressure, an additional intermediate casing

 

CASING DESIGN

2 0 n.

13 - 3/8 in.

9

9 - 5 / 8 in . t e r n e d t eo

s

n

7 5 / 8 in . Drilling Liner

I

31000

. 7- 5/8in. Drilling Liner

5 in. P r o d u c titi o n L i n e r

O.OOOft

b 1 OFFSHORE

LOUISIANA MIO CE NE TREN D

Fig

7.1 7.1-Exa -Example mple

permits lowering permits lowering th e mud density to drill dee per formations tions economically. Interm ediate casing may also be required after a troublesome lost-circulation zone or an unstable unsta ble shale or salt section is penetrated , to prevent well problems while drilling below these zones. Liners are casing strings that do not extend to the surfacee but are suspended from the bottom o f the next largfac er casi casing ng stri string ng Fig. 7 . 1 ~ ) . Several hundred feet of overlap between the liner top and the casing seat are provid pro vided ed to promote a good cement seal. The principal advantage advant age of a liner is its lower cost. How ever, problems sometimes arise from hanger seal and cement leakage. Also, using a liner exposes the casing string above it to additional addit ional wear during subsequent drilling. drilling. A drilling-liner is similar to intermediate casing in that it serves to isolate troublesome zones that tend to cause well problems during drilling operations. roduction casing is casing casing set through the productive interval. This casin g string provide s protection fo r the enirironment in the event of a failure of the tubing string duringg production opera tions and perm its the production durin tubing tubi ng to be replaced o r repaired la ter in the life of a well. production liner is a liner set through the productive interval inte rval of the well. Production liners generally a re connected to the surface wellhead using a tieSback tieSbackcasing casing string when the well is comp leted. Tie-back casing is connected to the top of the liner with a specially designed connector. connect or. Production liners with tie-back casing strings are most advantageous when exploratory drilling below the pioductive interval is planned. Casing w ear resulting resulting from drilling operations is limited to the deeper portion of the well, and the productive interval is not exposed to Potential Potent ial dam age by the drilling fluid for n extended period. Use of production line rs with with tie-back casing strings also results in lower hanging weights in the upper part of the well and thus often permits a more economical design.

 

Cosing

7,OOOft.

5 in. P r oduc tion Cas ing

a 1 MISSISSIPPI S M AC A C K OV OV E R T R E N D

p

5/ 8 in n..

1

TEXAS DELAWARE BASIN ELLENBURGER TREND

casing progra ms.

7 1 Manufacture of Casing The three basic processes used in the manufacture of casing include 1) the seamless proce ss, 2) electricresistance welding, and 3 ) electric-f electric-flash lash welding. In the seamless process, a billet is first pierced by a mandrel in a rotary piercing mill. The heated billet is introduced into the mill, whe re it is gripped by two obliquely orien ted rolls that rotate and advance the billet into a central piercing plug Fig. 7.2a). T he pierced billet billet is processed through plug mills, where the wall thickness of the tube is reduced reduced by c entral plugs with two single-groove rolls Fig. 7.2b). Reeler s similar in design to the piercing mills are then used to burnish the pipe surfaces and to form a more uniform uniform wall thic thickness kness Fig. 7. 2 ~ ) . inally, sizing mills similar in design to the plug mills produce the finall uniform pipe dimensions and roundness Fig. 7.2d). fina

I ? ) ROTARY

l

PIERCING MILLS

PLUG MILLS

Fig

7.2-Manufact 7.2Manufacture ure

IcI

EELERS

I d ) SIZING

MILLS

of sea mle ss cas ing .

APPLIED DRILLING ENGINEERING

In the electr ic welding processe s, flat sheet stock is cut and formed, and the two edges are welded together, without the addition of extraneous metal, to form the desired tube. The electric-resistance process continuously makes casing from coiled sheet stock that is fed into the machine, f orm ed, and welded by an electric electric arc. The pipe leaving the machine is then cut to the desired lengths. The electric-flash electric-flash welding technique p rocesses a sheet by cutting cutti ng it to the desired dimension s, simultaneously forming the entire length to a tube, and flashing and press pressing ing the two edges together together to ma ke the weld. Some welded welded pipe is passed through dies that deform the steel sufficiently to exceed its elastic limit. This process raises the elastic elast ic limit directions. in the direction s tressed and reduces it in perpendicular The nominal size of casing is its OD . The strength of a given size casin g is controlled by the yield yield s trength and wall thickness of the steel. Steel used in casing is relativelyy mild (0.3 carbo n) an d can be normalized with small tivel amounts of manganese to increase its strength. Strength can also be increased by a quenching and tempering ( Q T) process, which is favored by most manufacturers because of its lower cost.

Yield Strength API Grade

Minimum Ultimate Tensile Strength

Minimum' Elongation Yo) 29.5 24.0 19.5 19.5 19.5 18.5 18.5 18.0 15.0

-40 J-55 K-55 C-75 L-80 N-80 C-90 C-95 P-110 'Test specimen with area greater than 0.75 sq. In.

The Am erican Petroleum Inst. (API) has developed stanstandards for casing and o ther tubular goo ds that have been been accepted internationally by the petroleum-producing industry. C asing is defined defined as tubular pipe w it ithh a range of OD's of 4.5 to 20 in. Among the properties included in the API standards2 for both pipe and couplings are

vide a unique designation designation for each g rade of casing adopted in the standards. The number designates the minimum yield strength of the steel in thousands of psi. The yield strength is defined by API as the tensile stress required to produce a total elongation per unit length length of 0.005 on a standard test specimen. This strain is slightly beyond the elastic limit. Since there are significant variations in the yield strengths measured on manufactured pipe, a minimum yield yield strength strength criterion, rather than an average yield stress, was a dopted. Based on considerable test data, the minimum yield yield strength should should be computed as 80 of the average yield strength observed. In addition to specifying the minimum acceptable yield yield strength of each

strength, physical dimensions, and quality-control test procedures. In addition to to these standards, A PI provides bulletins on the recommended minimum-performance properties3 and formulas4 for the computation of minimum-performance properties. The minimum-performance properties must be used in the design of casing strings to minimize the possibility of casing failure. API has adopted a casing gr de designation to define the strength characteristics of the pipe. Th e grade code consistss of a letter followed consist followed by a n umbe r. T he letter desi desiggnation in the API grade was selected arbitrarily to pro-

grade of casing, API specifies the maximum yield strength, the minimum ultimate tensile tensile strength, and the minimum elongation per unit length at failure (Table 7.1). It also stipulates that the amount of phos phorus in the steel steel must not exceed 0.04 and that the amount of sulfur must not exceed exceed 0.0 6 . In addition to the API grades, there are many proprietary steel grades that d o not conform to all API specifications but are widely used in the petroleum-producing industry. Streng th properties of commonly used non-API grades are given in Table 7 2 These steel grades are used

7 2 Standardization of Casing

TABLE 7.2-C 7.2-COMMONL OMMONLY Y

Non-API Grade

Mod. N-80 C-90* SS-95

USED NON-API GRADES OF CASING

Manufacturer one Star Steel Mannesmann Tube Co. Mannesmann Tube Co. Lone Star Steel Mannesmann Tube Tube C o. Lone Star Steel Mannesmann Tube Co. Mannesmann Tube Co. U.S. Steel Mannesmann Tube Co.

Yield Strength

Minimum Ultimate

Minimum'

(psi) Minimum Maximum

Tensile Strength (psi)

Elongation )

75,000 55,000~ 80,000 90,000 95,000. 75,000~ 95,000 95,000 92,000 125,000 140,000 150,000 155,000

-Test specimen with area greater than 0.75 sq in. ':Circumferential. Longitudinal. Maxim um ultimate tensile strength o off 120 ,00 0 psi.

 

TABLE 7.1-GRADES 7.1-GRADES OF CASING RECOGNIZED BY THE API

75,000 95,000 105,000

110,000 110,000

150,000 150,000 165.000 180,000 180.000

20.0

CASING DESIGN

for special applications that require very high tensile strength, special collapse resistance, or high-strength steels that are more resistant to hydrogen sulfide. The API Standards recognize three length ranges for casing. Range 1 (R-I) includes joint lengths in the range of 16 to 25 ft. Range 2 (R-2) is the 25- to 34-ft range, and Range Range 3 (R-3) is 34 3 4 ft and longer. It is also specified that when when casing is ordered ord ered from the mill in amounts greater than one carload, 95% of the pipe must have lengths greater than 18 t for R-1, 28 t for R-2, and 36 ft for R-3. In addition, 95 % of the shipment must have a m maxiaximum length variation no greater than 6 ft for R-1, 5 ft for R-2, and 6 ft for R-3. Casing is run most often in R-3 lengths to reduce the th e number of connections connect ions in the string.

1. Short round threads and couplings (CSG). 2. Long round threads and couplings (LCSG). 3. Buttress threads and couplings (BCSG). 4. Extremeline threads (XCSG). Before development of of API threads, most manufacturers manuf acturers used a sharp V-shaped thread that proved unsatisfactory with increases in well depth. Schematics of each of the API connectors are shown in Fig. 7.3. The CSG and LCSG connectors have the same basic thread design. Threads have a rounded shape and are spaced to give eight threads per inch. Because o off this, they are sometimes referred to as API &Round threads. The threads are cut with a taper of ?h in./ft on diameter for all pipe sizes. A longer thread run-out and coupling

Since casing is made up in single s ingle joints, R-3 lengths can can be handled easily by most rigs. To meet API specifications, the OD of casing must be he held ld within within a tolerance of of +0 .7 5% . However, casing manufacturers generally will try to prevent the pipe from being bei ng undersized to ensure adequate adeq uate thread run-out when machining a connection. Casing usually is found to be within the API tolerance but slightly oversized. The minimum permissible pipe-wall thickness is 87.5% of the nominal wall thickness. The maximum ID is controlled by the combined tolerances for OD and minimum wall thickness. The minimum ID is controlled by a specified rift diameterdiameter-the the minimum mandrel diameter diam eter that must pass unobstructed through the pipe. The length of a casing drift mandrel is 6 in. for casing sizes in the range of 4.5 to 8.625 in. For larger l arger casing sizes, size s, a ]?-in. drift manmandrel must be used. The drift mandrel is not long enough to ensure a straight pipe, but it will ensure the passage

of the LCSG provide a greater strength when needed. These are very commonly used connectors because of their proven reliability, rel iability, ease ea se of manufacture, and low low cost. As can be seen in Fig. 7.3a, the API Round Thread is cut with a 60 included angle angle and has rounded peaks and roots. When the coupling is formed, small voids exist at the roots and crests of each thread. Thread compound must be used to fill these voids to obtain a seal. This connection is not designed to be a dependable, highpressure seal for gases and solid-free, low-viscosity liquids. If the seal is ineffective, internal pressure acts to separate the threaded surfaces further. Because the threads ar e cut on a taper, stress rapidly increases as the threads are made up. The proper prope r amount of make-up is best determined determi ned by by monitoring mon itoring both the torque and the number of turns. A loose connection can leak and will have reduced strength. An over-tight connection can leak because of galling of the threads or a

of a bit a size less than the drift diameter. In some instances, it is desirable to run casing with a drift diameter slightly greater than the API drift diameter for that casing size. In these instances, casing that has passed an oversized drift mandrel can be specially ordered. Some of the more commonly available oversized drift diameters are given in Table 7.3. 7. 3. When non-API non-API drift requirements are specified, they should be made known to the the mill, the distributor, distrib utor, and the threading company before the pipe manufacture. Casing dimensions can be specified by casing size (OD) and nominal wall thickness. However, it is conventional to specify casing dimensions dimensio ns by size and weight per foot. In discussing casing weights, one should differentiate differ entiate between nominal weight, plain-end weight, and average weight wei ght for threads thread s and couplings. coupli ngs. The nominal weight per foot foot is not a true weight per foot but is useful for identification purposes as an approximate average weight per

cracked coupling. It can 'also have reduced strength and can produce a reduced drift diameter as a s a result of excessive yielding of the threaded casing end. Special thread compounds containing containing powdered metals are used to reduce frictional forces during connection make-up and to provide filler material for assisting in

TABLE 7.3-SPECIAL DRIFT DIAMETERS Courtesy of Lone Star Steel) OD Size in.)

Weight T C

Ibflft)

Wall Thickness in

7

23.00 32.00 46.10 32.00 40.00

0.317 0.317 0.453 0.595 0.352 0.450

49.70 40.00 43.50 47.00 58.40 59.20 62.80 45.50 55.5.0 65.7,O 60.00 65.00 71.80 72.00 86.00 81.40 88.20

0.557 0.395 0.435 0.472 0.595 0.595 0.625 0.400 0.495 0.595 0.489 0.534 0.582 0.514 0.625 0.580 0.625

.

foot. The plain-end weight per foot is the weight per foot of the pipe body, excluding the threaded portion and coupling weight. The average weight per foot is the total weight weig ht of an average joint of threaded pipe, with with a couco upling plin g attached power-tight at one on e end , divided by by the total length length of the average joint. In practice, the average weight weig ht per foot sometimes is calculated to obtain the best possible estimate of the total weight of a casing string. However, the variation between nominal weight per foot and and average weight per foot is generally gene rally small, and most design desi gn calculations calculatio ns are performed p erformed with the nominal nominal weight per foot. API provides specifications specifications for the following four types of casing connectors.

Drift Diameter in API

S~ecial

 

APPLIED DRILLING ENGINEERING O P W 18

112L I

TLPLn I l r n P l L COT O * D . I

a) API Round Thread Connector

b) API Buttress Thread Connector Fig. 7.3-API

c) API Extreme-Line Connector

connectors.

plugging any plugging any remaining small voids aro und the ro ots and crests in the threads. The compound used is critical to prevent gall galling ing an andd to obtain a leak-proof. pro perl y m adeup connecti connection. on. Care must bbee exercised to ens ure that a proper thread compound for the given connector is used. Threaded connections are often rated acc ordi ng to their Threaded joint ejkiency which is the tensile strength of the joint divided by the tens tensile ile strength of the pipe body. Althoug h the joint joint effi efficien ciency cy of the AP I LCSG con nec tor is greater than than tthe he CSG connector, neithe r ar e 10 0% efficient. Becausee of th Becaus thee taperi tapering ng on the th read s, a s well as the 6 0 included angle of the threads, the threaded end of the casing sometimes begins ttoo yield and to col laps e (F ig . 7.4). This can produce an unzippering effect and, upon failure, the pin appears to jump out of the coupling. In addition to thi thiss jump-out, fracture of the pin or co upl ing also can occur. The API API BCS BCSG G is shown iinn F ig. 7. 3b . Th e joint efficiency of thi thiss connector is 100 % iinn mo st c ase s. T he basic thread thread de desi sign gn is simi similar lar to that of the API Rou nd Th read in that iitt is tapered. However, longer c oupling and thre ad run-out are used and the thread shape is squarer, so the unzipperi unzi ppering ng ttend endenc encyy is gre greatly atly re duc ed. Fiv e thread s ar e cut to to the inch, aand nd tthe he thread taper is %- in. /ft fo r casin g sizes up ttoo 7% in. aand nd 1 in./f in./ftt for 16-in. o r larg er casings. As wit withat h API Threa the provides plac ementtheofsealing threa d compound the Round roots of the ds, teeth mechanism. It is is not not,, howev er, a good ch oice w hen a leakproof connection is needed. The AP APII XC XCSG SG con connec nector tor iiss shown in Fig. 7 . 3 ~ . t differs from tthe he ot other her AP I conn ecto rs in that ;;tt is an integraljoint (i.e., the ox is machined on the pipe wall). On an integral-joint connection, the pipe wall must be thicker near the ends of the casing to provide the necessary meta metall to m mach achin inee a str stronger onger c onn ecto r. T he O D of an XCSG connect connector or iiss significantly less than the o ther API couplings, thus providing an altern ativ e when the l a r s s t possible casi casing ng size is rrun un in a re stric ted-c lear anc e situ ation. Also, Also, onl onlyy ha half lf as many threaded con nec tion s exis t; therefore, there there are fewer potential sites for leakage. Ho wever, the minimum ID will be less for the XCSG connector.

Fig. 7.4-J 7.4-Joint oint

pull-out failure mode for API round thread.

.

 

CASING DESIGN

3 5

available on on AP APII connections. Among the special featu features res offered are the following items.

1. Flush joints for maximum clearance. 2 Smooth bores through connectors for reduced turbulence. 3 Thre Threads ads designed for fast make-up wit with h low tendency to cross-thread. metal-to-metal seals for improved pressure 4. Multiple metal-to-metal integrity. 5 Multiple shoulders for improved torque strength. 6. High compressive strengths for special loading situations. 7. Resilien Resilientt rings for secondary pressure seals and connector corrosion protection. Several examples of premium non-API connectors are shown in Figs. 7.5 through 7.7, which illustrate the special features listed above.

7 3 API Casing Performance Properties

Fig. 7.5 Arm Armco co

seal lock connector.

The sealing mechanism used in the XCSG connector a

is metal-to-metal seal between the pin and the box Fig. 7.3~). his connector does not depend only on a thread compound for sealing, although a compound is still needed for lubrication. Because of the required thicker pipe walls near the ends and the closer machining tolerances needed for the metal-to-meta metal-to-metall sseal, eal, XCSG connectors a are re much more expensive than the other API connectors. In addition to the API connections, many proprietary connections are available that offer premium features not

a) IJ-4s CONNECTOR INTE GRAL JOINT CONNECTOR CONNECTOR))

Fig. 7.6 Samp Sample le

The most important performance properties of casing include its rated values for axial tension burst pressure and collapse pressure. Axial tension loading results primarily from the weight of the casing string suspended below the joint of interest. Body yield stre strength ngth is the tensional force required to cause the pipe bod body y to exceed e xceed its elastic limit. Similar Similarly, ly, ~ o i n ttrength ttrength is the minimum tensional force required to cause joint failure Fig. 7.8a). Burst pressure rating is the calculated minimum internal pressure that will tause the casing to rupture in the absence of external pressure and axial loading loading Fig. 7.8b). Collapse pressure rating is the minimum external pressure that will cause the casing ca sing walls to collapse iin n the absence of internal pressure and axial axial loading Fig. 7 . 8 ~ ) . API provides recommended formulas4 for computing these performance properties. 7l3 1 Tension

Pipe-body strength in tension can be computed by use of the simplified free-body diagram shown in Fig. 7.9. The

b) TC-4s CONNECTOR THRE ADED AND COUPLED CONNECTOR CONNECTOR))

c ) FL-4S CONNECTOR FLUSH INTEGRAL JOINT)

Atlas Bradford connectors with resilient seals and smooth bores.

 

APPLIED DRILLING ENGINEERING

111 N C T C O ON NN NEC ECTO TOR R FO R CON ND DUC CTO TO R C A SI SIN NG I N ON ON - C RO RO S S T H R E AD AD D E SI SI GN GN )

21 EX TE R N A L U PS ET G EO TM ER M A L SER VI C E HIGH COMPRE SSIVE STRENGTH INTEG RAL JO I N T)

Fig. ?.?-Sample

3) C T S C O NN NN E CT CT O R

I C O U PLED TR I PLE SEA L WITH SMOOTH BORE)

41 TR IP LE SEAL CONN NE ECTOR I N T EG R A L C O N N EC TO R FOR NON UPS ET PIP E1

5 ) F J I F J - P C O NN NN E C CT TOR FLUSH INTEGRAL JOINT1

Hydril two-step two-step connectors with three m etal- to-metal seals.

force F tending to pull apart the pipe is resisted resisted by the strength of the pipe walls, which exert a counterforce, F2.F2 F2. F2 s given by F 2 = ayieldAs, ayieldAs,

J OAIINLTU R E

where ayield is the minimum yield strength and A is the cross-sectional cross-s ectional area of steel. Thu s, the p ipe-body strength is given by

-

.

(7. 1)

The pipe-body strength computed with Eq. 7.1 is the minimum force that would be expected to cause permanent deformation of the pipe. The expected minimum force required to pull the pipe in two would be significantl can tlyy higher than thi thiss value. H ow eve r. the nom inal w all thicknes thicknesss rather than than the minimum acc epta ble wall thicknesss is nes is use usedd in Eq. 7.1. Becau se the min imu m a cceptab le walll thickness is 87.5 of the nominal wall thickne ss, wal the absen absence ce of permanent permanent deform ation canno t be assured. Joint-strength formulas based on theoretical considerations and partially on empirical observations have been accepted ormulas for y API omputing or API he minimum ound Thread oint fracture onnections, orce and and tthe he minimum joi joint nt pull-out for ce are p resented (Fi g. 7.10a). The lower values are recommended for use in casing design. Similarly, for Buttress connections, formulass are mula are presented for minimum pipe-thread strength and for minimum coupling-thread strength (Fig. 7. lob). Three for formulas mulas are presente presentedd for Extrem e-line c onn ectio ns, depending pendi ng on whether the st steel eel are a is minim al in the box . pin, o r pipe body (Fig . 7.1 OC).

OF STRING

0 1 TEN SION

F A I L U R E I N P I P E BO BO DY DY OR OR

/ INTERN

JO~NT

\

L

PRESSURE

b)

BURST

L U R E FROM

lNT

E RNAL PRESSURE

8p z GXTERNAL

KRESSURE

\

L LL A AP PS SE E c ) C O LL

FAILURE FROM EXTE RN AL PRESSURE

Fig. 7.8-Tension,

burst, and collapse modes of fail failure. ure.

 

CASING DESIGN

.

Area under last perfect thread .

Tensional lorce for fracture F~en

O

95AlPoull

Tens~ onal orce for lolnt pull-out

I

l

l

(a) Round Thread Connector

Area of Steel in Pipe B

I

y

Area of Steel In Coupllng Asc

=

'[d z

d,?~

4

Tens~onal orce for Pipe Thread Failure

F2

u

yiel

Fig 7.9-Tens 7.9-Tensional ional

A

s

force balan balance ce on pipe body.

Tens ~ona l orce for for Coupllng Th Thread read Failure F

=

0

95A,,n,1~ -

Example 7 1 Compute the body-yield body-yield strength for 20-in., K 55 casing with a nominal wall thickness of 0.635 in. and and a nominal weig weight ht per foot of 133 lbflft. lbflf t.

(b) Buttress Thread Connector

Tens~onal orce lor Plpe Failure .

Solution. This pipe has a minimum yield strength of

=

F

55,000 psi and an ID of

7

I

4

(d:

-

dZ)

Tens~onalForce Force for Box F K u r e

Thus, the cross-sectional area of steel is

.

Tens~onal orce for Pln Failure

A =-(202

4

-18 .73 ~)= 38. 63 sq in.

and and minimum pipe-body yield is predicted predi cted by Eq. 7 . 1 at an axial load of Fp =55,000 38.63)=2,125,000

7 3 2

lbf.

urst Pressure

AS shown in the simplified free-body diagram of Fig. 7.11, tendency for strength the force, to burst l pipe string the is resisted by the of F the walls, wallsa, casing which exert exe rt a count counterforce, erforce, F 2 . The force, F1 which results from the internal pressure, pbr, acting on the projected area (LdS) is given by

The resisting force, F 2 , resulting resulting from the steel steel strength, a,, acting over the steel area (tL) is given by

 L

-

.

-

- .

.

(c) Extreme-L~ne onne onnector ctor . -

--

.

.

API Fig 7.10-API

- ..

-

. .

.

.

.

.

jo joint int-s -str tren ength gth f o r r n ~ l a . ~ , ~

Totaling forces for static conditions gives

Substituting the Substituting the appropriate a ppropriate expressions expres sions for F1 and F2 and and solving for the burst pressure rating, pb,, pb,, yields

This equation is valid only for thin-wall pipes with d,/t values greater than those of most casing strings. ~ a r l o w ' s quation q ~uation for thick-wall pipe is identical to the above equation for thin-wall pipe if if the OD, d, , is used in in place of of the ID, d. Barlow's equation equation results from

 

  PPLIED PPLIED DRILLING ENGINEERING

Fig 7.1 1 Free body

Fig 7.12 Two dimensional

diagram for casing burst.

wall stress.

a nonrigorous solution but is a fairly accurate (slightly conservative) conservati ve) thick-wal thick-walll fo rmula. The A PI burst-press burst-pressure ure rating is based on Barlow 's equation. Use of 87 .5 of the minimum yield strength for steel, a takes into account the minimum allowable wall thickness and gives

ample, the casing cross section sshown hown in Fig. 7.12 with any external pressure, p and internal pressure, p Application of the classical elasticity theory for this twodimensional problem at any radius, r between the inner radius, r ; and outer radius, r gives6

API recommends use of this equation with wall thickness rounded rounded t o the neares nearestt 0.0 01 in. and the results rounded to the nearest 10 ps psi. i.

and

Example 7 2 Compute the burst-pressure rating for 20-in., K-55 casing with a nom inal wall thickness of 0.635 in. and a nominal weight per foot of 133 lbftft. Solution. The burst-pressure rating is computed by use of Eq. 7.2.

where a and a are the radial and tangenti tangential al stresses at radius r . For both collapse and burst conditions, stress will be a maximum in the tangential direction. If it is assume d that the pipe is subjected only to an external pressure, p then for r = r Eq. 7.3b reduces to

p b =0.875 2) 55,000) 0.635)/20.00=3,056 psi.

Rounded to the nearest nearest 10 psi, this value bec becomes omes 3,0 60 psi. Th is bur burst-pres st-pressure sure rating correspon ds to the minimum expected internal pressure at which permanent pipe deformation could take place, if the pipe is subjected to no external pressure or axial loads.

Use of the effective compressive yield strength for ut and rearrang ed term s reduces the above equation to the following formulas for collapse pressure rating, p .

7.3.3 Collapse Pressure

The collapse ooff steel pi pipe pe from external pressure is a much more complex phenomenon than pipe burst from internaltpressure. A simplified free-body diagram analysis, such as the the one shown in Fig. 7.11, does not lead to useful result results. s. H owev er, a mo re complex, classical elas elasticiticity theory can be used to establish the radial stress and tangenti tang ential al hoop stress in the pipe wall. Consider, fo r ex-

It can also be show n that that Eq . 7.3b reduces to E q. 7.2 when the pipe is subjected only to internal pressure. pressure. The proof of this is left as a student exercise. Th e coll collapse apse that occu rs in approximate agreement wit withh Eq. 7.4a is called yield-strength collapse. It has been shown experimentally that yield-strength collapse occurs

 

CASINGDESIGN TABLE TABL E 7.4-EMPIRIC 7.4-EMPIRICAL AL COEFFICIENTS COEFFICIENTS USED FOR COLLAPSE-PRESSURE DETERMINATION4 Empirical Coefficients Grade

-50 J-K 55 -60 -70 C-75 E L-80 N-80 C-90 C-95

F 2.976 2.991 3.005 3.037 3.054 3.071 3.106 3.124

F 0.0515 0.0541 0.0566 0.0617 0.0642 0.0667 0.0718 0.0743

F

F

F

where F l y 2 , and F3 are given in Table 7.4. Values computed by Eq. 7.4b 7.4 b for zero ze ro axial stress are shown in Table 7.5. The Th e effective yield yield strength, strength, uyiel uy iel d)es, equal to the minimum yield strength when the axial stress is zero. Table 7.4 is based on the minimum yield strength. d, lt,, collapse can occur at lower At high values of d,lt pressure pres suress than predicted by by Eq. E q. 7.4a 7.4 a because of a geometric instability. insta bility. Application Application of elastic stability stability theory7 theor y7 leads to the following collapse formula:

.-lo0 P-105 P-110 -1 -120 20 -125

3.143 3.162 3.181 3.219 3.239

-1 30 -1 -135 35 -1 -140 40 -1 -150 50 -155 -1 -160 60 -1 -170 70 -180

3.258 3.278 3.297 3.336 3.356 3.375 3.412 3.449

0.0768 0.0794 0.0819 0.0870 0.0895 0.0920 0.0946 0.0971 0.1021 0.1047 0.1072 0.1123 0.1173

After an adjustment for statistical variations in the properties of manufactured pipe is applied, this equation becomes4 46.95 x l o6 cr

on only ly for the lower range of d,lt d,l t values applicable for oilwelll casing. The upper we u pper limit li mit of the yield-qtrength yield-qtrength collapse range rang e is calculated with

t

dn1t)) dnltdn1t dnlt- i) 2

. . . . . . . . . . . . . . . . . . . 7.5a)

Collapse that occurs n approximate agreement with Eq. 7.5a is called elastic collapse. The applicable range of d,l t values recommended recommended by API for elastic collapse are given give n in Table 7.5. 7 .5. The lower limit of of the elastic elas tic collapse collapse range is calculated by

Grades indicated without without lener designation are not API grades bul are arades in use or arades beina considered for use and are shown for information purposes.

d

=

d, /t) =

a ~2)2 +8[F2 +F3/ uyi eld)el + F1 -2)

2+F21FI 3F2 F1

...................

7.5b)

where F1 and F2 are given in Table 7.4. The transition from yield-strength yield-s trength collapse to elastic collapse is not sharp but covers a significant range of d,lt values. Based on the results of many many experimental tests,

2[F2 +F31 uyield)e

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4b)

TABLE TA BLE 7.5-RANG 7.5-RANGE E OF d / t FOR VARIOUS COLLAPSE-PRESSURE REGIONS WHEN AXIAL STRESS IS ZERO4

Grade

I +Yield StrengthCollapse

+Plastic+ Collapse

+Transition+ Collapse

+Elastic+ Collapse

I

H-40 -50 J-K-55 D -60 -70 E C-75 L-80 N-80 C-90 C-95 -1 -100 00 P-105 P-110 -1 -120 20 -1 -125 25 -1 -130 30 -135 -140 -150 -1 -155 55 -1 -160 60 -1 -170 70 -180 Grade s indicated without letter letter designation designation are not API grades but are grades in use or grades being cons idered for use an d are shown for information purposes.

 

PPLIED DRILLING ENGINEERING

API has adopted two additional collapse-pressure equations to to cover the transition region. A plastic collapse rating for d n l r values just above the yield-strength collapse region is predicted with

The upper limit of the plastic collapse range is calculated by

signif icantly by axial tension or significantly o r compression compressio n and by bending stresses. Thus the table values for the the performance properties often must be corrected before they are used in a casing design application. The generally accepted relationship for the effect of ax ial stress on colla se or burst was presented by by Holmquist and Nadiag in 1939. Application of classical distortion energy theory to casing gives the following equation.

where a , , a , , and a , are the principal principal radial tangential

where F , through F 5 are given in Table 7.4.

mamition collapse region between the plastic collapse and elastic collapse regions region s is defined by use of

and vertical vertical stresses stresse s respectively. The application of the the distortional energy theorem is based on the yield stress value and the surface that is developed denotes the onset of yield not a physical physic al failure of the casing. casin g. After regrouping Eq. 7.8 takes the form of either an ellipse or a circle.

Values of d n l t computed with Eq. 7.6b for zero axial stress are shown in Table 7.5.

Example

Compute the collapse-pressure rating for 20-in. K-55 K-55 casing with with a nominal wall thickness of 0.635 in. and a nominal weight per foot of 133 lbflft. 7 3

Solution. This pipe has a d n l t ratio given by

Table 7.5 indicates that this value for d n l r falls in the range specified for transition collapse. collap se. Thus the collapsepressure rating can be computed with Eq. 7.7.

The ellipse of plasticity was chosen for this book because it is more commonly-uscd in in current drilling engineering practice. Recall that that the radial and tangential tangential stresses of Eq. 7.9a were defined previously by Eqs. 7.3a and 7.3b. The maximum stress will occur at the inner pipe wall. Substitution of r = r i in Eq. 7.3a gives a value of - p ; ) for the radial stress at this point. Use of this value in Eq. 7.9a and rearranged terms yields

Rounded to the Rounded the nearest 10 psi this value becomes becomes 1 490 psi. This collapse-pressure rating corresponds to the minimum mu m expected external pressure at which the pipe would collapse if the pipe were subjected to no internal pressure or axial loads. Solving this quadratic equation ;gives

7 3 4 Casing Performance Summary The values for tensional strength streng th burst resistance and collapse resistance given given in Table T able 7 .6 were computed in accordance accor dance with theoretical and empirical formulas adopted by API. The last entry in this table corresponds to the casing properties properties determined in Examples 7.1 through 7.3 7.3.. Such tables generally are found to be extremely useful and convenient for casing design applications. 7 3 5 Effect of Combined Stress

The performance properties given in Table 7.6 apply only for zero axial tension and no pipe bending. Unfortunately many many of the casing casing performance performance properties are altered

This is the equatiG for the ellipse of plasticity shown in Fig. 7.13. With substitution substi tution of Eq. 7.3b 7. 3b with r = r i for a , , Eq. 7.11 defines the combinations of internal pressure external pressure press ure and axial stress that will result in a yield strength mode of failure. It can be shown that for p i O and a z =0 Eq. 7. 1 1 reduce reducess to Eq. 7.4a. The proof of this is left as a student exercise.

 

CASING DESIGN a

- = ( G ) x ~ ~ ~

+ Pi

The ID of the casing is 4.548 in. Evaluation of the terms present in Eq. 7.11 for nominal conditions of zero axial stress and internal pressure gives

(=> 1

+Pi

(

2

o2

Pi-Pe

) (=)

0

10

20

30

40

50

60

7

80

90

a,

100

pi

=o

yiel

Use of these terms in Eq. 7.1 1 or Fig. 7.1 3 yields -P 12,649 p,

;

12,649.

Note that after it is rounded to the nearest 10 psi, this value agrees with that that given b by yE Eq. q. 7. 7.4 4 and shown in in Table 7.6. For in-service conditions of a, a,=4 =40, 0,00 000 0

=) I 1~ Fig 7.13 Ellipse

o

i

ps psii and

10,000 psi psi,,

plasticity.

Examination of the ellipse of plasticity Fig. 7.13) Examination shows that axial tension has a detrimental effect on collapse-pressure rating and and a beneficial effect on burstpressure rating. In contrast, axial compression has a detrimental detri mental effect on burst-pressure rating and a beneficial effect on collapse-pressure rating. In casing-design practice, i t is customary to apply the ellipse of plasticity only when a detrimental effect would be observed.

Use of these terms in Eq. Eq . 7.1 1 or Fig. 7.13 yields

7 4

Example Compute the nominal rating for 5.5-in., N-80 casing with acollapse-pressure nominal wall thickness of 0.476 in. and a nominal weight per foot of 26 Ibflft. In addition, determine the collapse pressure for inservice conditions in which the pipe is subjected to a 40,000-psi axial tension stress and a 10,000-psi internal pressure. Assume a yield strength mode of failure. Solution. For a yield yield strength mode of failure, Eqs. 7.3b 7.1 1 can be applie applied. d. Us Use e of Eq. 7.3b wi with th r= r ; gives

p,

10,000+0.5284 12,649)= 16,6 16,684 84 psi psi..

This analysis indicates that, because of the combined stresses present, the pressure difference external pressure minus internal pressure) required for collapse failure was reduce reduced d to 5?,8 5?,84 4 of the nominal colla collapse pse press pressure ure rating given in Table 7.5.

and

In casing design practice, the ellipse of plasticity cannot be applied unless the assumption of a yield-strength mode of of failure is known to be valid. For a ssimple imple stress state in which the internal pressure and axial tension are

 

APPLIED DRILLING ENGINEERING

TABLE 7.6 7.6 MINIMU MINIMUM M

PERFORMANCE PROPERTIES OF CASING

Threaded and Coupled

Extreme Line

Outside Nominal Outside Diameter Outside Pipe Weight Diameter Special Diameter Body Size Threads Wall Inside Drift of Clearance Drift of Box Collapse Yield Outside and Diameter Coupling Thickness Di Diameter ameter Diameter Coupling Coupling Diameter Powertight Resistance Strength (in.) (Ibml (Ibmlft) ft) Grade (in.) (in.) (in.) (in.) (in.) (in.) (in. (in.)) (psi) (1,000 Ibf Ibf)) 9.50

H-40

0.205

4.090

3.965

5.000

2,760

111

9.50

J-55

0.205

4.090 4.090

3.965

5.000

3,310

152

5

10.50 11.60

J-55 J-55

0.224 0.250

4.052 4.000

3.927 3.875

5.000 5.000

4.875 4.875

4,010 4,960

165 184 184

9.50 10.50

K-55 K-55

0.205 0.2 0.224 24

4.090 4.052

3.965 3.927

5.000 5.000

4.875

3,310 4,010

152 165

11.60 11.60 13.50

K-55 C-75 C-75

0.250 0.250 0.290

4.000 4.000 3.920

3.875 3.875 3.795

5.000 5.000 5.000

4.875 4.875 4.875

4,960 6,100 8,140

184 250 288

11.60 13.50

L-80 L-80

0.250 0.290

4.000 3.920

3.875 3.795

5.000 5.000

4.875 4.875

6,350 8,540

267 307

11.60 13.50

N-80 N-80

0.250 0.2 0.290 90

4.000 3.920

3.875 3.795

5.000 5.000

4.875 4.875

6,350 8,540

267 307

11.60 13.50

C-90 '2-90

0.250 0.290

4.000 3.920

3.875 3.795

5.000 5.000

4.875 4.875

6,820 9,300

300 345

11.60 13.50

C-95 C-95

0.250 0.290

4.000 3.920

3.875 3.795

5.000 5. 5.000 000

4.875 4.875

7,030 9,660

31 7 364

11.60 13.50 15.10

P-110 P-110 P-110

0.250 0.290 0.337

4.000 3.920 3.826

3.875 3.795 3.701

5.000 5.000 5.000

4.875 4.875 4.875

7,580 10,680 14,350

367 422 485

11.50 13.00 15.00

J-55 J-55 J-55

0.220 0.253 0.296

4.560 4.494 4.408

4.43 4.435 5 4.369 4.283

5.563 5.563 5.563 5.563

5.375 5.375

5.360

3,060 4,140 5,560

182 208 241

5.375 5.375

4.151

5.360

3,060 4,140 5,560

182 208 241

4.151

-

11.50 13.00 15.00

K-55 K-55 K-55

0.220 0.253 0.296

4.560 4.494 4.408

4.435 4.369 4.283

5.563 5.563 5.563

15.00 18.00 21.40 23.20 24.10

C-75 C-75 C-75 C-75 C-75

0.296 0.362 0.437 0.478 0.500,

4.408 4.276 4.126 4.044 4.000

4.283 4.151 4.15 1 4.001 3.919 3.875

5.563 5.563 5.563 5.563 5.563

5.375 5.375 5.375 5.375 5.375.

4.151 4.151

5.360 5.360 5.360

6,940 9,960 11,970 12,970 13,500

328 396 470 509 530

15.00 18.00 21.40 23.20 24.10

L-80 L-80 L-80 L-80 L-80

0.296 0.362 0.437 0.478 0.500

4.408 4.276 4.126 4.044 4.000

4.283 4.151 4.001 3.919 3.875

5.563 5.563 5.563 5.563 5.563

5.375 5.375 5.375 5.375 5.375

4.151 4.151

5.360 5.360

7,250 10,500 12,760 13,830 14,400

350 422 501 543 566

15.00 18.00 21.40 23.20 24.10

N-80 N-80 N-80 N-80 N-80

0.296 0.362 0.437 0.478 0.500

4.408 4.276 4.126 4.044 4.000

4.283 4.151 4.001 3.919 3.875 3.875

5.563 5.563 5.563 5.563 5.563

5.375 5.375 5.375 5.375 5.375

4.151 4.151

5.360 5.360

7,250 10,500 12,760 13,830 14,400

350 422 501 543 566

15.00 18.00

C-90 '2-90

0.296 0.362

5.375 5.375 5.375 5.375 5.375

7,840 11,530

394 475

0.437 0.478 0.500

5.563 5.563 5.563 5.563 5.563

5.366 5.366

C-90 C-90 C-90

4.283 4.151 4.001 3.919 3.875

4.151 4.151

21.40 23.20 24.10

4.408 4.276 4.126 4.044 4.000

14,360 15,560 16,200

564 61 1 636

15.00 18.00 21.40 23.20 24.10

C-95 C-95 C-95 C-95 C-95

0.296 0.362 0.437 0.478 0.500

4.408 4.276 4.126 4.044 4.000

4.283 4.151 4.001 3.919 3.875

5.563 5.563 5.563 5.563 5.563

5.375 5.375 5.375 5.375 5.375

4.151 4.151 4.151

5.360 5.360

8,110 12,030 15,160 16,430 17,100

416 50 1 595 645 672

15.00 18.00 21.40 23.20 24.10

P-110 P-110 P-110 P-110 P-110

0.296 0.362 0.437 0.478 0.47 8 0.500

4.408 4.276 4.126 4.044 4.000

: 4.283

5.563 5.563 5.563 5.563 5.563

5.375 5.375 5.375 5.375 5.375

4