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1 Chair of Drilling Engineering - University Leoben

PE DESIGN, PART A - DRILLING ENGINEERING: Casing Design

PETROLEUM ENGINEERING SUMMER COURSE

2015

Petroleum Engineering Design PART A

Drilling Engineering

CHAPTER 4

Casing Design



Content

•  Introduc.on •  Design Factors, Design Scenarios •  API Pipe Body Internal Yield Pressure (Burst) •  API Collapse •  API Biaxial Design

Introduc.on



Casing Design

There are two different jobs that casing must be designed for:

1.  To allow to drill safely the well and resist any forces or condi.on.

2.  To act throughout the life of the well to meet the objec.ves of the well without requiring a work-‐over.

Ref: Lecture Notes Drilling Engineering 2, MUL, 2000

Casing Pipe Specifica.ons

•  Grade of Material -‐ Casing Quality: API has adopted a casing grade designa.on to define the strength or quality of casing steel. A leWer is followed by a number that indicates the minimum yield strength. This minimum (longitudinal) yield strength is defined as the stress required to produce a total elonga.on of the 0.5% of the test specimen. It is determine by an extensometer during a tensile test.

Cost



Extensometer Test

Extensometer Test

0,5%



Casing Cost per Unit Weight

0

50

100

150

200

250

Q125 LTC/STC P110 LTC/STC C95/C90/L80 LTC/STC N80 LTC/STC J55/K55/H40 LTC/STC

Price pe

r unit w

eight com

pared to H40 (%

)

Casing Price per Unit Weight for Different QualiPes, STC and LTC Couplings

H40 LTC/STC

lb/^ kg/m PRICE ($/m) PRICE ($/ton)

H-‐40/17 -‐ STC 17 25,30 28,23 1116

H-‐40/20 -‐ STC 20 29,76 33,22 1116

J-‐55/20 -‐ STC 20 29,76 33,63 1130

J-‐55/23 -‐ BTC 23 34,23 47,58 1390

J-‐55/23 -‐ LTC 23 34,23 39,36 1150

J-‐55/23 -‐ STC 23 34,23 38,68 1130

N-‐80/23 -‐ BTC 23 34,23 65,38 1910

N-‐80/23 -‐ LTC 23 34,23 61,61 1800

J-‐55/26 -‐ BTC 26 38,69 53,78 1390

J-‐55/26 -‐ LTC 26 38,69 44,50 1150

J-‐55/26 -‐ STC 26 38,69 43,72 1130

N-‐80/26 -‐ BTC 26 38,69 73,90 1910

N-‐80/26 -‐ LTC 26 38,69 69,65 1800

P-‐110/26 -‐ BTC 26 38,69 89,38 2310

P-‐110/26 -‐ LTC 26 38,69 83,58 2160

N-‐80/29 -‐ BTC 29 43,16 82,43 1910

N-‐80/29 -‐ LTC 29 43,16 77,68 1800

P-‐110/29 -‐ BTC 29 43,16 99,69 2310

P-‐110/29 -‐ LTC 29 43,16 93,22 2160

N-‐80/32 -‐ BTC 32 47,62 90,96 1910

N-‐80/32 -‐ LTC 32 47,62 85,72 1800

P-‐110/32 -‐ BTC 32 47,62 110,01 2310

P-‐110/32 -‐ LTC 32 47,62 102,86 2160

N-‐80/35 -‐ BTC 35 52,09 99,48 1910

N-‐80/35 -‐ LTC 35 52,09 93,75 1800

P-‐110/35 -‐ BTC 35 52,09 120,32 2310

P-‐110/35 -‐ LTC 35 52,09 112,51 2160

N-‐80/38 -‐ BTC 38 56,55 108,01 1910

N-‐80/38 -‐ LTC 38 56,55 101,79 1800

P-‐110/38 -‐ BTC 38 56,55 130,63 2310

P-‐110/38 -‐ LTC 38 56,55 122,15 2160

Casing Pipe Prices for 7” OD, 2003 1,000 meters 7 inch 23lb/ft (34.23 kg/m) N80 LTC costs 62,000 US$ in 2003



Steel Price Development (2/2)

1000 meters 7 inch 23lb/ft (34,23 kg/m) N80 LTC costs 110,000 US$ in 2013

Casing costs of total well costs: Soft rock 50% Medium Hard 45% Hard 35%

Drilling/tripping Costs Soft rock 15% Medium Hard 30% Hard 40-50%

2014

Cost Comparison of Wells



Spec 5CT, Casing and Tubing Covers seamless and welded casing and tubing, couplings, and connectors in all grades. Process of manufacture; chemical and mechanical property requirements; methods of tes.ng; and dimensions are included.

•  1/3 of worldwide installa.ons are according API specifica.ons. •  1/3 of installa.ons have size, steel grade and wall thickness of API

casing but have special non API connectors, such as VAM connec.ons. •  1/3 are of non API steel grade and may have non API connectors.

API and Non-‐API Casing

Note: Non-‐API casing, tubing and connec.ons generally have manufacturing tolerances equal to or superior API specifica.ons

•  API Round threads in short (STC) and long (LTC) design

•  API BuWress squared threads (BTC) are longer and stronger connec.ons

•  API Integral joint (extreme line, XL) with smaller ID, OD are strong connec.ons but expensive

Standard API Casing Connec.ons



VAM Big Omega Connec.on Big Omega is a large diameter connection with improved running performances, for conductor pipes, surfaces and intermediate casings, up to 26". Big Omega is a very strong coupled connection. The pin thread is cut directly into the pipe. Standard coupling ODs are identical with API; they may be increased for higher grades and larger wall thickness if matched internal pressure resistance is required. The standard coupling length is 10 5/8" (269,9 mm)as with API. A make-up arrestor positions the coupling accurately on the mill end. Pin to pin torque shoulder for positive torque stop on the field end allows over-torque and compression resistance. Ref: www.vam-‐usa.com

Design Steps •  Decide on objec.ves

–  Size and number of tubulars, poten.al for drilling beyond planned total depth, semng depths, failure consequences

•  Iden.fy life.me loads –  Load means anything ac.ng upon pipe such as a force like

tension, compression, bending, pressures, torsion, weight, fric.on, corrosion, dynamic forces during opera.ons, or environment like temperature, H2S, CO2 => discuss design scenarios

•  Sa.sfy management guidelines –  Economics, risk versus cost

•  Create criteria •  Make computa.ons •  Select casing



Simple Calcula.on Principles

•  Es.mate loads (pipe internal, external pressures, tensional load, bending load, torsional load)

•  Define design factors (DF) •  Compare:

PipeProperty (from API Tables) ≥ DF Load−Backload( )Uni-‐axial Bi-‐axial Tri-‐axial

Design Factors Design Scenarios



Principal Mechanical Loads on Casing

•  External pressure (collapse) –  pressure of mud or forma.on fluids on the empty casing, pressure

of expanding salts

•  Tension –  weight of the casing string in empty hole

•  Biaxial stress (collapse plus tension) –  reduc.on of yield strength due to tensional load (above TOC)

•  Bending –  bending stress at dog-‐legs

•  Internal pressure (burst) –  pressure of fluids inside the casing when the annulus is empty (e.g.

above TOC)

Maximum Loads Versus Depth

Stress

Dep

th

Tension- Tensile stress due to weight of string is highest at top

Burst - Assume full reservoir pressure all along the wellbore (may subtract the gas gradient if known)

Collapse - Hydrostatic pressure increases with depth



API Design Factors (typical)

•  Collapse 1.125

•  Tension 1.8

•  Burst 1.1

Required

10,000 psi

100,000 lbf

10,000 psi

Design

11,250 psi

180,000 lbf

11,000 psi

Worst Case Collapse Loads on Casing

•  Assume evacuated casing. •  Apply mud hydrosta.c outside of the casing. •  Conserva.ve design does not account for

fluid hydrosta.c inside the casing (par.ally filled). However, this assump.on can be true under some circumstances.

•  Collapse design factor, DFe = 1 -‐ 1,125 •  On the other hand, collapse resistance is

reduced in bent sec.ons and under tensional load. Thus, worst-‐case design would give addi.onal safety.

Pe

Pi = 0?



Worst Case (Conserva.ve) Burst Load on Casing

•  Assume forma.on pressure (e.g. a shut-‐in gas kick while drilling next sec.on) exerted over the length of the casing string.

•  Conserva.ve design does not account for external fluids or fracturing of the forma.on while a kick migrates upwards along the open hole sec.on (compare the discussed scenarios).

•  Burst design factor, SFi = 1 -‐ 1,33 •  On the other hand, burst resistance capability

may be reduced by casing wear and longitudinal defects. Thus, conserva.ve design would give addi.onal safety.

Pi = Pf?

Pe = 0?

Conserva.ve Tensile Load on Casing

•  Assume full air weight of the casing. •  But conserva.ve design does not account for a wellbore

containing fluid. In mud, air weight can be reduced by mul.ply it with a buoyancy factor.

•  Also, in inclined wells the casing string lays at the low side of

the wellbore which reduces its weight. •  Tension design factor SFt = 1,6 – 1,8 •  On the other hand, tensile strength may be significantly

reduced by bending loads created by crooked wellbores or by improper connec.on make-‐up.

steel

mudBFFactorBuoyancyρρ

−=1,



Worst-‐Case Design Scenario for Surface/Intermediate Casing Collapse

Pe = Pform

(Pe = Pmud hyd@casing point)

Assume: Lost Circula.on Pe Pi

Air

Cement

Mud

Lost Circulation

Pi = 0

Alterna.ve Design Scenario for Surface Casing Burst

Pi

Cement

Next Mud

Kick Frac

Pe = 0

Assume: Kick

Pi = ρfrac g Dshoe -‐ ρgas g Dshoe

Pe Pi

BOP



Alterna.ve Design Scenario for Intermediate Casing Collapse

Pe = Pform

(Pe = Pmud hyd@casing point)

Assume: Lost Circula.on

Pe Pi

Air

Cement

Next Mud

Lost Circulation

Pi = Pmud hyd (H2) H2

H1

Alterna.ve Design Scenario Intermediate Casing Burst

Pe = 0

Assume: Kick Pe Pi

Gas

Cement

Next Mud Frac

Pi = ρfrac g Dshoe -‐ ρgas g Dshoe

BOP

Kick



Worst Case Design Scenario Produc.on Casing Collapse

Pi = 0

Pe = Pform

(Pe = Pmud hyd@casing point )

Extremes: Floa.ng, par.ally filled casing, plugged perfora.ons

Cement

Mud

Tubing

Packer (Pi = Pgas hyd inside casing )

Gas Pe

Plugged Perforations Pi

Completion Fluid

Worst Case Design Scenario Produc.on Casing Burst

Assume: Tubing Leakage

Pe = 0

Pi = Pform-Phydrostatic gas

Cement

Mud

Tubing

Packer

Gas

Pe

Completion Fluid

Pi Leaky Tubing



API Pipe Body Internal Yield Pressure (Burst)

API Pipe Body Internal Yield •  According to API, the following formula is used

to calculate pipe body internal yield pressure at minimum yield strength:

⎥⎦

⎤⎢⎣

⎡=

D

tYP p2

875.0

YP = minimum yield strength of pipe, psi t = wall thickness, in. D = Outside diameter, in.

p p Internal Pressure

For example: 7 in., 23 lb/^, N80

P = 0.8752 *80, 000 * 0.317

7

!

"#$

%&= 6340psi



API Internal Yield (Burst) Pressure

Example 1: Design For Burst

•  Design a 7” Casing string to 10,000 ^. •  Pore pressure gradient = 0.5 psi/^ •  Design factor, SFi = 1.1 •  Design for burst only, assume a worst case scenario. For the selected casing quality, check the safety for tension (SFt=1.8), neglect buoyancy.



Example 1: Solu.on

1. Calculate probable reservoir pressure.

psi 000 , 5 ft 000 , 10 * ft psi 5 . 0 p res = =

3. Select the appropriate csg. grade and wt. from Casing tables 7 in., 23 lb/^, N80 has minimum required burst resistance, however, weight in air is 230.000 lb, with a safety of 1.8 the tensional load is 414.000 lb, joint strength o.k. Note that a L-‐80, with same pressure data, is a slightly beWer choice for H2S environment.

2. Calculate required pipe internal yield pressure ra.ng

psi 500 , 5 1 . 1 * 000 , 5 SF * p p i res i = = =

API Collapse Resistance



Collapse Failure


•  There are four different types of collapse pressure, each with its own equa.on for calcula.ng the collapse resistance according to API:

1.  Yield strength collapse 2.  Plas.c collapse 3.  Transi.on collapse 4.  Elas.c collapse



4 API Collapse Pressures •  If axial tension is zero:

J-55 14.81 25.01 37.31

N-80 13.38 22.47 31.02

P-110 12.44 20.41 26.22

D/t RaPo

PlasPc TransiPon ElasPc Yield Strength

API Collapse Pressure Formulas (1/2) •  Calculate D/t to determine proper equaPon to use for calcula.ng the collapse pressure, P

Plas.c Collapse , PP:

PYP = 2Yp

Dt

!

"#

$

%&−1

Dt

!

"#

$

%&

2

(

)

****

+

,

----

CB

t

DA

YP pp −

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

−

⎟⎠

⎞⎜⎝

⎛=

YP = minimum yield strength of pipe, psi

Yield Strength Collapse, PYP:



API Collapse Pressure Formulas (2/2)

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

−

⎟⎠

⎞⎜⎝

⎛= G

t

DF

YP pT

2

6

1

1095.46

⎥⎦

⎤⎢⎣

⎡−⎟⎠

⎞⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛

×=

t

D

t

DPE

Transi.on Collapse, PT:

Elas.c Collapse, PE:

A, B, C, F, G

A = 2.8762+1.0679×10−3Yp+ 2.1301×10−5 Yp( )2− 5.3132×10−8 Yp( )3

B = 0.026233+ 5.0609×10−4Yp

C = 30.867Yp−1.0483×10−2 Yp( )2+3.6989×10−5 Yp( )3

− 465.93

F =3,17×108 * B / A( )2

Yp*1000 * 1−B / A( )3

G = F B / A( )



Use Yield Pressure Formula if Condi.on is True

Use Plas.c Collapse Formula if Condi.on is True



Use Transi.on Collapse Formula if Condi.on is True

Use Elas.c Collapse Formula if Condi.on is True



Example 2: Uniaxial Collapse Determine the collapse strength of a 5 1/2” O.D., 14.00 lb/^ J-‐55 casing under zero axial loads.

1.  Calculate the D/t ra.o: 2.  Check the mode of collapse: 3.  The plas.c collapse is calculated from:

D

t=

5.500

0.244= 22.54

For J-‐55 casing with 14.81 < D/t < 25.01 the mode of failure is plasPc collapse.

Note, tables rounds off to 3,120 psi

Pp =Yp

A

D / t−B

"

#$

%

&'−C = 55,000

2.990

22.541− 0.0541

(

)*+

,-−1, 205=3,115psi




API Biaxial Design

The Biaxial Load

•  The biaxial load concept is necessary to inves.gate the combined loads of tension/compression and collapse.

•  This combined load is of special importance in the area above the cement head.

•  When the casing is empty and a tensional stress exists between surface and cement head, there must be an correc.on of the collapse resistance.



Collapse and Axial Stress

4  The collapse pressure resistance of a pipe depends on the axial stress

4  The collapse resistance of a pipe is reduced when tensional loads are present

4  The collapse resistance of a pipe is increased when compression loads are present

Collapse and Axial Stress •  Yield strength -‐ with axial stress: use this equa.on to

correct the yield strength Yp (API collapse equa.ons)

⎪⎭

⎪⎬

⎫

⎪⎩

⎪⎨

⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛−

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−=

P

a

P

aPPA YY

YYσσ

5.075.01

2/12

YPA = yield strength of axial stress equivalent grade, psi YP = minimum yield strength of pipe, psi σa = axial stress, psi (tension is posi.ve)

⎪⎭

⎪⎬

⎫

⎪⎩

⎪⎨

⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛+

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−=

P

a

P

aPPA YY

YYσσ

5.075.01

2/12

Tension -‐ collapse

Compression -‐ collapse



Burst and Axial Stress

4  The burst pressure resistance of a pipe depends on the axial stress

4  The burst resistance of a pipe is increased when tensional loads are present

4  The burst resistance of a pipe is reduced when compression loads are present

p

Biaxial Stress Equa.ons -‐ Summary



Example 3: Biaxial Collapse (1/3) •  Determine the collapse strength for a 5 1/2” O.D., 14.00

lb/^, J-‐55 casing under axial load of 100,000 lbs.

Pp

a

p

aPA Y

YYY

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎟⎠

⎞⎜⎜⎝

⎛−=

σσ5.075.01

2

( )psi

Area

Faa 820,24

012.55.54

000,100

22=

−==π

σ

YPA = yield strength of axial stress equivalent grade, psi YP = minimum yield strength of pipe, psi σa = axial stress, psi (tension is posi.ve)

The axial tension will reduce the collapse pressure as follows:

Example 3: Biaxial Collapse (2/3) •  The axial tension reduces the collapse pressure ra.ng to:

YPA = 1− 0.7524,820

55, 000

"

#$

%

&'

2

− 0.524,820

55, 000

"

#$

%

&'

(

)

**

+

,

--55, 000

= 38,216 psi

Here the axial load decreased the J-‐55 ra.ng to an equivalent “J-‐38.2” ra.ng (less than H-‐40!)



Example 3: Biaxial Collapse (3/3)

Pp ≈ 2, 551 psi

…compared to 3,117 psi with no axial stress!

•  The plas.c collapse is then:

∴Pp =YPA

A

D / t−B

#

$%

&

'(−C

= 38, 2162.9452

22.54− 0.0456

)

*+,

-.− 701.43= 2, 551 psi

Quality A B C 38,216 2,9452 0,0456 701,43

Appendix API Casing Tables



API 5CT 5 1/2 in. Casing Performance Properties (1/2)




API 5CT 7 in. Casing Performance Properties (1/3)





Conversion Table

Meters 3,2808 Feet

Centimeters (cm) 0,3937 Inches

Millimeters (mm) 0,03937 Inches

Metric Tons 2204,6 Pounds

Decanewtons (daN) 0,22481 Wt Indicator (Lbs)

Newton 0,224809 Pounds Kilograms (Kg) 2,2046 Pounds

Kilogram/cubic meter 16,0184633 Pound/Cubic Foot Kg/M 0,67196 Weight (Lb/Ft)

Kg/M3 0,3505 Pounds per Barrel

Liters 0,00629 Barrels

Cubic Meters 6,2898 Barrels

Liters 0,2642 Gallons

Cubic Meters 264,173 Gallons

Liters/Stroke 0,00629 Barrels/Stroke

Cubic Meters/Stroke 6,2898 Barrels/Stroke

Liters/Minute 0,2642 Gallons/Minute

Liters/Minute 0,00629 Barrels/ Minute

Cubic Meters/Minute 6,2898 Barrels/Minute

Liters/Meter (L/M) 0,0019171 BBL/ Ft Capacity

Cubic Meters/Meter 1,917 BBL/Ft Capacity

Liters/Meter (L/M) 0,0019171 BBL/ Displacement

Cubic Meters/Meter 1,9171 BBL/Displacement

KPa/M 0,044207 Gradient PSI/Ft

Bar/M 4,4207 Gradient PSI/Ft

Kilograms/Liter (Kg/L) 8,3454 Mud Weight PPG

Kilograms/Cubic Mtr 0,0083454 Mud Weight PPG

Specific Gravity (SG) 8,3454 Mud Weight PPG

Kg/M3 6,24279 Mud Weight (Lb/Ft3)

Celsius Degrees 1.8 + 32 Farenheit Degree

Pascals (Pa) 0,000145 PSI

Kilopascals (KPa) 0,14504 PSI

Bar 14,50377 Psi



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