10 Drill String Design

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© 2005 PetroSkills LLC, All Rights Reserved

DRILL STRING DESIGN



© 2005 PetroSkills LLC, All Rights Reserved2

Drill String Design

Common grades of drill pipe with

yield strength

Grade

Minimum Yield

psi

Maximum Yield

psi

Tensile Strength

psi

E 75,000(517 MPa) 105,000(724 MPa) 85,000(586 MPa)

X95,000

(655 MPa)

125,000

(862 MPa)

105,000

(724 MPa)

G 105,000(724 MPa)

135,000(931 MPa)

115,000(793 MPa)

S

135,000

(931 MPa)

165,000

(1138 MPa)

145,000

(1000 MPa)




Drill String Design




Drill String Design

The API recognizes four classesof drill pipe

New

Premium ClassClass 2

Class 3

Pipe is rarely considered newIf it has been run in the hole, it isconsidered premium class




Drill String Design

The DS-1 standard has anadditional class of drill pipe

It is premium class, reduced TSR

The tool joints do not meet the

standards for API tool joints and have

a reduce torsional strength

Pressure and tensile ratings are the

same, but torsional strength is lower




Drill String Design

Premium class assumes thatthere is 80% wall thickness

remaining on the tube and that

the reduction in wall thicknesscomes from the outside diameter

of the tube


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Drill String Design

A reduction in outside diameterremoves the most steel and

results in the lowest strength

5”, 19.5# As = 5.2746 in2

20% outer wall As = 4.1538 in2

20% inner wall As = 4.2855 in2

5” by 4.276”

4.8552” by 4.276” 5” by 4.4208”



Drill String Design

The worn OD can be determinedfrom the following formula

The worn OD can be used to

calculate tensile strength andcombined torsion and tension

limits

D D D t D pw p i h i



Drill String Design

Class 2 has at least 70% of thewall thickness remaining withthe loss on the OD

Class 2 is seldom used fordrilling except small rigs withlimited hook load capability

Class 3 is less than 70% wallthickness remaining and isconsidered junk



Drill String Design

Drill string design starts at thebottom of the hole with the BHA

The drill collars must provide

enough weight for the bit withthe top of the drill collarsremaining in tension (not

buckled, the top of the drillcollars are actually not in axialtension)



Drill String Design

A design factor (DF) is used tomake sure sufficient drill collars

are available for estimated

weight on bit

Typical design factors are 10% to

15% or 1.10 to 1.15 in equation

10-43 BW

DF W L

f

c



Drill String Design

Example shows how to determine thenumber of drill collars

Maximum anticipated weight on 8 3/4 inch

(222.3 mm) bit is 50,000 lbs (22,400 daN)Drill collar size is 6 1/2 inch (165.1 mm) by 2

13/16 inch (71.4 mm)

Mud weight is 11.5 ppg (1380 kg/m3)

Excess collars should be 10% (DF = 1.10) to

insure the drill pipe remains in tension



Drill String Design

The number of 30 foot (9.14 m)collars to be run

First determine the weight per

foot of the drill collars in air

2267.2 i pf DDW

mkg W

W

f

f

/665

footper lbs928125.25.667.2 22



Drill String Design

Determine the buoyancy factor

Calculate the length of the drill

collars

mB 015.01

83.05.11015.01

B

BW

DF W L

f

c

m219.5feet720

83.09210.150000 c L



Drill String Design

Determine the number of collarsand weight of collars in drilling

fluid

720/30 = 24 drill collars

daN643,24

979,5483.0923024

tc

tc

W

lbsW





Drill String Design

With BHA components, most of

the bending will occur in the

connections

BHA connections are subjectedto bending and fatigue from

buckling



Drill String Design

The BSR is a ratio of the relativestiffness of the box and pin for a

given connection

A typical target BSR is 2.5, +/-

0.25

Graphs showing the BSR can befound in the API RP 7G



Drill String Design

8

8.5

9

9.5

10

10.5

11

11.5

11.522.533.5

Bending Strength Ratio

O u t s i d e D i a m e t e

r , i n c h e s

8 5/8 H90

NC 77

8 5/8 REG

7 5/8 H90

NC70

7 5/8 REG

6 5/8 FH

5 1/2 IF7 H90

NC 61

5.25

5.75

6.25

6.75

7.25

7.75

8.25

8.75

11.522.533.5

Bending Strength Ratio

O u t s i d e D i a m e t e

r , i n c h e s

6 5/8 H90

6 5/8 REG

5 1/2 FH

NC56

5 1/2 H90

5 1/2 REG

NC50

5 H90

4 1/2 H90

NC46

4 1/2 FH

NC44

4 H90

NC40



Drill String Design

The BSR for a given connectionis a function of the ID of the pin

and OD of the box

In theory, high BSR’s will cause

accelerated pin failures and low

BSR’s will cause accelerated box

failures



Drill String Design

The DS-1 standard gives the followingrecommended BSR’s

Drill Collar OD

Traditional

API BSRRange

DS-1

RecommendedBSR Range

< 6 inches (152

mm)2.25 to 2.75 1.80 to 2.50

6 to 7 7/8 inches

(152 - 200 mm) 2.25 to 2.75 2.25 to 2.75

> or = to 8 inches

(203 mm)

2.25 to 2.75 2.50 to 3.20



Drill String Design

Many operators will placeapproximately 6 joints of HWDP

on top of the drill collars as a

transition to the drill pipe

It may help reduce drill pipe

failures at the top of the drill

collars



Drill String Design

HWDP

Drill collars

Neutral Point

Otheroperators

will rotate

the drillpipe from

the top of

the drill

collars on

trips




Drill String Design

If jars are placed in a verticalwell, they are at the top of the

collars and then additional

collars (3 to 4) are run above the jars

In a vertical well, the jars should

not be run in compression




Drill String Design

Jars are oftenrun in

compression in

directional wells

HWDP

Drill collars

Neutral Point

Jars

Drill collars




Drill String Design

Drill pipe is usually designedwith a design factor plus

overpull

A normal design factor is 1.10 or

10%

Overpull can range from 50,000to 100,000 lbs (22,411 – 44,822

daN)




Drill String Design

The overpull is the amount thatcan be pulled on the pipe over

and above the drill string weight

If the drill pipe consists of more

than one weight or grade of pipe,

the overpull is balanced between

the two strings



D ill St i D i




Drill String Design

Calculate the worn OD of the drill

string

Calculate the cross sectional

area of the drill pipe

i hi p pw Dt DDD

mm)(123.32"855.4276.480.0276.45 pw D

22

4 i pw s DD A

)mm2679(in152.4276.4855.44

2222

s A

D ill St i D i




Drill String Design

Calculate the tensile strength ofthe grade E pipe

Calculate the tensile strength of

the grade G pipe

s pst AY T

kg141,067pounds000,311152.4000,75 st T

kg197,767pounds000,436152.4000,105 st T

D ill St i D i




Drill String Design



D ill St i D i




Drill String Design

The maximum weight of grade Ethat can be used with 100,000

pounds over pull is:

The maximum length of grade E

drill pipe that can be used is:

kg58,069lbs021,12810000054979283000max W

m)(2001feet656550.19

128021max L

D ill St i D i




Drill String Design

The maximum pull on the gradeG with the 1.10 design factor

would be:

kg179,623pounds000,39610.1

436000max P

D ill St i D i




Drill String Design

The maximum weight of grade Gthat can be used with 100,000


kg266,51

pounds021,11312802110000054979396000

max

max

W

W

D ill St i D i




Drill String Design

The maximum length of grade Gdrill pipe that can be used is:

m1766feet579550.19

113000

max

L

D ill St i D i




Drill String Design

The drill string would consist ofthe following:

720 feet of drill collars (219.5 m)

6565 feet (2001 m) of 5”, 19.50#/ft,

(127.0 mm, 29.02 kg/m) grade E drill

pipe and

4715 feet (1437 m) of 5”, 19.50#/ft,(127.0 mm, 29.02 kg/m) grade G drill

pipe

D ill St i D i




Drill String Design

Drill Pipe

Drill collars

HWDP

Neutral Point

Top of Grade E

Drill String Design




Drill String Design

Class Problem – Design the drill string

Drill 12 ¼” (311.2 mm) hole to 10,000’ (3048 m) Maximum bit weight is 65,000 lbs (29,000 daN)

Collars 8” OD (203.2 mm) by 2 13/16” ID (71.4mm)

4 ½”, 16.60 (114.3 mm, 24.7 kg/m) Grade E,Premium

4 ½”, 16.60 (114.3 mm, 24.7 kg/m) Grade S-135, Premium

Use 1.10 design factor for collarsDesign factor of 1.10 with overpull of 75,000 lbs

(34,000 daN) for drill pipe

Mud weight is 9.5 ppg (1140 kg/m3)

Drill String Design




Drill String Design


570 feet (173.7 m) of drill collars (19)

5300 feet (1615 m) of 4 1/2”,

16.60#/ft (114.3 mm, 24.7 kg/m),

grade E drill pipe and

4130 feet (1259 m) of 4 1/2”,16.60#/ft (114.3 mm, 24.7 kg/m),

grade S drill pipe

Drill String Design




Drill String Design

First determine the weight per foot

of the drill collars in air

2267.2 i pf DDW

kg/m)(223lbs/ft1508125.2867.2 22 f W

Drill String Design




Drill String Design

Determine the buoyancy factor

Calculate the length of the drill

collars

mB 015.01

86.05.9015.01 B

BW DF W L

f

c m)(168.9feet554

86.015010.165000 c L

Drill String Design




Drill String Design

Determine the number of collarsand weight of collars in drillingfluid

554/30 = 18.5 drill collarsUse 19 drill collars

Get the tensile strength of pipefrom the API spec

kg353,33lbs530,7386.01503019 tc W

Drill String Design




Drill String Design

Drill String Design




Drill String Design

The maximum pull on the gradeE with the 1.10 design factor

would be:

DF

T P st max

kg)(107,281pounds513,23610.1

165,260max P

Drill String Design




Drill String Design

The maximum weight of grade Ethat can be used with 75,000


The maximum length of grade Edrill pipe that can be used is:

kg908,39

pounds983,87000,75530,73513,236

max

max

W

W

m1515feet530060.16

983,87max L

Drill String Design




Drill String Design

The maximum pull on the gradeS with the 1.10 design factor

would be:

kg106,193

pounds725,42510.1

297,468

max

max

P

P

Drill String Design




Drill String Design

The maximum weight of grade Sthat can be used with 75,000

pounds (34,019 kg) over pull is:

kg825,85

pounds212,189983,87000,75530,73725,425

max

max

W

W

Drill String Design




Drill String Design

The maximum length of grade Sdrill pipe that can be used is:

m3474 feet398,11

50.16

212,189

max L

Drill String Design




Drill String Design


570 feet (173.7 m) of drill collars (19)

5300 feet (1615 m) of 4 1/2”,16.60#/ft (114.3 mm, 24.7 kg/m),

grade E drill pipe and

4130 feet (1259 m) of 4 1/2”,16.60#/ft (114.3 mm, 24.7 kg/m),

grade S drill pipe

Drill String Design




Drill String Design

In directional wells, the pipeweight available for bit weight is

a function of the inclination

W

Inclination, IBit Weight

= W cos I

Normal Force

= W sin I

Drill String Design




Drill String Design

Most of the drill

collars are often

replaced by hevi-

wate drill pipe

(HWDP).

Helps reduce torque

and drag by reducingstring weight

Fewer drill collar

connection failures

Drill String Design




Drill String Design

The body of a drill collar is

stiffer than the connectionand bending occurs in the

connection

In HWDP, bending occurs

in the body and not at the

connection, so fewer

connection failures are

experienced



Drill String Design




Drill String Design

Therefore, drill pipe can be run in

compression in a directional wellwithout causing buckling provided the

compressive load is less than the

critical buckling load

Basic Units

English Units

F E Ag I

r

crit 2 I sin

)(

sin)/)((1082.9 445

OD D

I B ft Wt IDOD F

h

crit



Drill String Design




Drill String Design

-

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

0 10 20 30 40 50 60 70 80 90

Inclination

C r i t i c a l B u c k l i n

g

L o a d ,

l b f

4 1/2" in 8 1/2" Hole

4 1/2" in 12 1/4" Hole

5" in 8 1/2" Hole

5" in 12 1/4" Hole

Drill String Design




Drill String Design

Buckling is actually a little more

complicated

It also depends upon the curvature of

the wellbore, internal pressure and

external pressure

The DS-1 Standard has more detailed

buckling calculations

Drill String Design




Drill String Design

The pipe weight and grade can be

identified by the pin end, tool joint

Drill String Design




Drill String Design

Pipe weight

codes can be

found in Table

17

Grade Grade Code

E-75 E

X-95 XG-105 G

S-135 S

Drill String Design




Drill String DesignPipe Weight Code

Pipe Grade Code

Pipe Weight Code

Pipe Grade Code

Pipe Weight Code

Pipe Grade Code

Standard Weight

Grade E Drill Pipe

Heavy Weight

Grade E Drill Pipe

Standard Weight

High Strength Drill Pipe

Heavy Weight

High Strength Drill Pipe

Old API markings for tool joints

Drill String Design




Drill String Design

Mill slot and groove method of drill string identification

Standard Weight Grade E-75 Drill Pipe Standard Weight Grade X-95 Drill Pipe

Standard Weight Grade G-105 Drill Pipe Standard Weight Grade S-135 Drill Pipe

Drill String Design




Drill String Design

Mill slot and groove method of drill string identification

Heavy Weight Grade S-135 Drill PipeHeavy Weight Grade G-105 Drill Pipe

Heavy Weight Grade E-75 Drill Pipe Heavy Weight Grade X-95 Drill Pipe

Pipe Weight Code

Pipe Grade Code

Drill pipe identification is not strictly

followed and you have to look



Drill String Design




Drill String Design

The equation for calculating themaximum allowable torque

based on tension is as follows:

English

SI

5.0

2

22096167.0

s

p

pw

T A

T Y

D

J Q

5.0

2

22

6 )9800(101536.1

s

p

pw

T A

T Y

D

J Q

Drill String Design




Drill String Design

0

10,000

20,000

30,000

40,000

50,000

60,000

0 100,000 200,000 300,000 400,000 500,000 600,000

Tension, pounds

T o r q u e ,

f t - l b

Grade E Grade X Grade G Grade S

250,000 lbs (112,000 daN)

19,242 ft-lbs (26,119 N-m)

Drill String Design




Drill String Design

Drill string inspectionPeriodically the drill string must be

inspected to make sure that the pipe

and BHA components are still fit forpurpose

There are no API standards for drill

string inspection; however, some ofthe industry uses the inspection

standards in the DS-1



Drill String Design




g g

Service Category 2Routine drilling conditions where the

established practice is to perform

minimal inspection and failureexperience is low

Drill String Design




g g

Service Category 3Mid-range drilling conditions where a

standard inspection program is

justifiedIf a failure occurs, the risk of

significant fishing cost or losing part

of the hole is minimal

Drill String Design




g g

Service Category 4Drilling conditions more difficult than

Category 3

Significant fishing cost or losing partof the hole is likely in the event of a

drill stem failure

Drill String Design




g g

Service Category 5Severe drilling conditions

Several factors combine to make the

cost of a possible failure very high

The service category is selected

by the operator depending upon

their risk analysis

Drill String Design




g g

Directional wells will have ahigher service category than

vertical wells

Very deep, very high pressureand sour wells are beyond the

scope of the DS-1 Standard



Drill String Design




g g

The BHA components should beinspected before they are picked

up

After the components have beenpicked up, additional inspections

should be carried out at regular

intervals

Drill String Design



g g

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10 Drill String Design