Drilling Engineering Drillstring

8/9/2019 Drilling Engineering Drillstring

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CHAPTER 2:

2.1 DRILLSTRING

DRILLING ENGINEERING I

(CGE577)

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At the end of this lesson, students should be able to:

Describe the functions of Drillstring

Describe Drillpipe Stresses & Failures

Describe Drillpipe Inspection

Describe the functions of BHA components

Design the Drillstring

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Introduction to DRILLSTRING…

Enjoy the video!!!


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DRILLSTRING

• Drillstring is the combination of

drillpipe, drillcollars, kelly and other

accessories used to bring and turn the

drill bit during drilling process

• The drillcollars, reamers, stabillizers,

jar and other accessories just above

the drill bit is called the Bottomhole

Assembly (BHA)

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FUNCTIONS OF DRILLSTRING

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• Drillpipe is the main component of drillstring (90 -95% of the entire length ofdrillstring)

• It is a tubular steel pipe that is welded with threaded ends called tooljoints.

• Each length of drillpipe is called the joint/ single.

• Each joint of drillpipe has standard dimension specified by API.

• The joints is manufactured not at uniform length – the exact length must bemeasured at the rigsite since the only way in which the driller knows the depth of thedrillbit is by knowing the the length of the drillstring and its components.

Drillpipe

API Range Joint Length (ft)(Do not include tool joint length)

1 18-22

2 27-30 (most common)

3 38-45

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Drillpipe

There are also variety of O.D, weightsand material grades of drillpipe

The specification for drill pipe stringis written in:

[O.D, weight, grade and range]

E.g: 5 ½” 25.6 lb/ft Grade X Range 2

These specifications influence the

burst, collapse, tensile and torsionalstrength of the drillpipe.

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O.D Nominal Weight

(lb/ft)

I.D

2 -3/8 6.65 1.815

2 -7/8 10.40 2.151

3 -½ 9.50 2.992

3 -½ 13.30 2.764

5 15.50 4.602

5 16.25 4.408

5 -½ 25.6 4.000

5 -½ 21.90 4.776

5 -½ 24.70 4.670

Drillpipe dimensions


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Drillpipe

Properties of drillpipe (after API-RP-7G)Ref: Drilling Engineering, J.J Azar & G. Robello Samuel, 2007

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The weight and dimensions of the drillpipe given is nominal value and does notrepresent the real value.

The true weight (approx. weight) of a drillpipe is a function of the connectiontype, grade of the drillpipe and weight per foot of the drillpipe.

Technically, the weight of drillpipe is lower when it is immersed in the drillingfluid (wet weight) as compared to its weight when suspended in air (weight inair) due to buoyant force (buoyant factor


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Exercise 1

What is the wet weight of a joint of drillpipe (30ft) when immersed in a drilling fluid with a density of12 ppg. Given:

Ans: 527 lbs

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Size andConnection

Nominal Weight

(lb/ft)

Grade Approx. Weight

(lb/ft)

4-1/2 IF 19.5 G 105 21.5


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Tool Joints

• Tool joints are manufactured separately and

usually made of steel that has higher strength

of the pipe body.

• They are located at each end of a length of

drillpipe.

• The tool joints have the box (female

connection) and pin (male connection).

• When making up connection, the rig tongs must

be engaged around tool joints instead of main

joint body.

• Tool joints have greater wall thickness than

main body (ID is lesser) and can sustain high

torque during make up connection.• Frequent engagement of pins & boxes can

damage threads if done harshly. Therefore,

connections should be greased properly to

extend the lifetime.

Pin (male connection

of tooljoints)

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Box Pin


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Heavy Wall Drillpipe (HWDP)

Also called Heavy Weight Drillpipe

Has greater wall thickness than ordinarydrillpipe

Has longer tool joint to increase stiffnessof the pipe to better withstand loads acting

on drillstring. Used at the base of the drillpipe where the

stress concentration is greatest

HWDP can absorb stresses beingtransferred from the stiff drill collars tothe flexible drillpipe due to the differencein cross section

HWDP can also absorb stresses due torotation and cutting action of the bit whichmay result in a vertical bouncing effect

See Video

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HWDP Standard

drillpipe

Ref: Drilling Engineering, J.J Azar & G. Robello Samuel, 2007

http://localhost/var/www/apps/conversion/tmp/scratch_4/Video/heavy%20Wall%20Drill%20Pipe%20-%20YouTube.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Video/heavy%20Wall%20Drill%20Pipe%20-%20YouTube.flv


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Drillpipe Stresses & Failures

Drillstring is subjected to a few kind ofstresses and failures.

These failures tend to break thedrillstring into two parts (twist off).

When this happened, drilling has tostop and the retrievel of the lower partof the string is very difficult and time

consuming

The common stresses are tension,torque and cyclic stress fatigue.

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Drillpipe Stresses

Tension Torque Cyclic Stress Fatigue

The weight of the suspendeddrillstring expose each jointof drillpipe to severalthousand of tensile load and become higher when it is

being pulled out from adifficult hole condition.Eg: dogleg due to overpull ordrag.

The twisting force whilethe drillstring is rotatedduring drilling process.

The poor borehole

condition increase theamount of the torque ortwisting force on each joint.

Happens when the point of bending on the drillstring isexposed to repeated tension andcompression stresses and maycause fatigue in the wall of the

pipe especially while drilling indeviated hole.

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Fatigue due to cyclic stress


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Drillpipe Failures

Corrosion on drillstring can be due to the dissolvedgases, dissolved salts and acids in the water basedmud

The most common gases are oxygen, carbon dioxide

and hydrogen sulphide. Oxygen can cause erosion tothe drillpipe, the carbon dioxide forms the corrosivecarbonic acid and corrodes steel while the hydrogensulphide introduces hydrogen embrittlement.

The dissolved salts expedite the corrosion processdue to its conductivity whereas the organic acidsreduce the pH of the fluid and thus increasehydrogen embrittlement.

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CO2 Corrosion

Erosion

Hydrogen Embrittlement


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Drillpipe Inspection

A joint of drillpipe will be used in a number of wells and will undergo a some degree of wearand will not be able to withstand the same loadsas when it is new.

The drillpipe must be inspected by time to

monitor degree of damage and predict the loadthat it can withstand.

The service life of drillpipe is hard to define because no two boreholes undergo the sameconditions.

Approximately service life of drillpipe may beused up to 25 wells which are about 10,000 ftdeep depending on well conditions.

Longer service life for drilling soft area ascompared with hard or deviated drilling areas.

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EMI Inspection


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The inspection method of drillpipe can be done using non-destructive testssuch as the optical, magnetic particle, magnetic induction, ultrasonic andGamma Ray technique to determine the degree of damage of the pipe and anydefects.

Optical - visual inspection of possible noticeable external damages eg major dents,external corrosion, and straightness.

Magnetic particles - magnetize pipe ends / tool joint and observe attraction of ferrousparticles to fatigue cracks by UV light.

Magnetic induction - detect disturbances in magnetic flux fields by pits, notches andcracks.

Ultra sonic - pulse echo technique used to determine wall thickness

Hardness testing - Portable Hardness Testers are utilized to perform hardness readingon the pipe.

The pipe will be classified to its degree of damage in term of drillpipe wallthickness after each inspection.

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Drill pipe is classed according to wear in the following categories:-

Most operators specify that their Drilling Contractors supply only Premium Pipe

Premium pipe is typically 80% as strong as new pipe and then a further factor of safety

of 85% is applied to calculate the maximum allowable pull permitted in normal Drilling

Operations

Different Grades of steel are used in the manufacture of Drill pipe with increasing percentage of Alloy Content

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Class 1 New

Premium (wall thickness not less than 80% of Class 1)

Class 2 (wall thickness not less than 70% of Class 1)

Class 3 (any imperfections or damage exceeding Class 2)


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Drillpipe

Typical New Drill pipe Properties are as follows:-

Size Weight

lbs/ft

Grade

Tensile Strength(lbs)

Torsional Strengthft/lbs

3 ½” 13.3 E 271,569 18,551

3 ½” 13.3 G 380,197 25,972

3 ½” 13.3 S 488,825 33,392

5” 19.5 E 395,595 41,1675” 19.5 G 553,833 57,633

5” 19.5 S 712,070 74,100


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Exercise 2

In a 6” hole using 3 ½” x 13.3 #premium drill pipe, we anticipate needingto pull 320,000 lbs tension.

What grade of pipe do we need if company policy dictates that we can pull

85% of yield strength

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Solution

Maximum pull on each grade of 3 ½” x 13.3 # pipe will be:-

Grade TensileStrength

PremiumFactor

SafetyFactor

Max Pull in lbs

Grade E 271,569 x 0.8 x 0.85 = 184,666Grade G 380,197 x 0.8 x 0.85 = 258,533

Grade S 488,825 x 0.8 x 0.85 = 332,401

Therefore, the suitable grade to be used is Grade ‘S’ Drill pipe


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The Bottom Hole Assembly (BHA) consists of drillcollar and other types of components just above the bitsuch as the reamers, stabilizers, jars etc.

Drill collars are the pipe that have wall thicker than thedrillpipe with larger O.D and smaller I.D. Typical O.D for

drill collar are 9” connected to 5” drillpipe. The functions of the drill collar are to give additional

weight to the drillstring, keep the drillstring intension to reduce fatigue failures and to providestiffness for BHA for directional control.

Like drillpipe, most common length for a drill collar isRange 2 (30 – 32 ft).

Among the special types of drill collars are Slick DrillCollar, Flex Drill Collar, Spiral Drill Collar, SquareDrill Collar and Non-Magnetic Drill Collar. See Video.

Bottomhole Assembly (BHA)

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Length of Drill Collar

The required weight on bit (WOB) determine the required length of the drill collar fora particular drilling process and can be calculated from;

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Example 1

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Solution

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Exercise 3

If highest ROP for a 12-¼″ bit can be achieved when 25,000 lbs WOBis applied to the bit, Calculate the length of drillcollars that would berequired to provide 25,000 lbs WOB and keep the drillpipe in tensionin 12ppg mud.

a) Calculate the weight in air of 10000 ft of 5″ 19.5 lb/ft Grade G drillpipe with 4 ½″ IF connections = 215,000 lbs

b) Calculate the wet weight of this string in 12 ppg mud = 175, 655 lbs

c) Calculate the length of 9-½″ x 2-13/16″ drillcollars

Given:

Buoyancy factor for steel = 0.817

DF = 1.2

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Solution

Calculated length of drillcollar = 166 ft

Please also note that additional length is required to ensurethe drill pipe in tension when drilling in order to overcome

the buoyant force on the drill pipe.

Weight in air of drillpipe – Weight in mud of drillpipe

buoyant weight per foot of drill collar.

= (215,000 – 175,655)/ 220.4 x 0.817 = 219 ft

Therefore, the total length of drill collars = 166ft + 219 ft = 385 ft


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Stabilizers are pipe with straight orspiral blades on the external surface

The stabilizers helps to reduce buckling and stresses on drill

collars and prevent wall sticking.See Video

Roller reamer is a kind of stabilizer with rollers embedded on the blades. Acts as stabilizers anduseful in maintaining gauge hole.See Video

BHA Components

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http://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Drilling%20Part%2020%20%20%20Stabilizer%20-%20YouTube.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Drilling%20Part%2019%20%20%20Rotary%20Reamer%20-%20YouTube.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Drilling%20Part%2019%20%20%20Rotary%20Reamer%20-%20YouTube.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Drilling%20Part%2020%20%20%20Stabilizer%20-%20YouTube.flv


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Shock sub (vibration dampener) is normally placedabove the bit to absorb the vibration when drillingthrough hard rock.

Subs (substitutes) are crossovers pipes to connects

components which cannot be connected together because of differences in thread type or sizes

Drilling jars are used to free pipe if it is stuck duringdrilling. There are mechanical jar (downward blow atsurface) and hydraulic jar (upward blow). Normally

placed at top of drill collars. See Video1 & Video2

BHA Components

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Shock Sub

http://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Drilling%20Jar%20-%20YouTube.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Weatherford%20Drilling%20Jars%20-%20YouTube.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Weatherford%20Drilling%20Jars%20-%20YouTube.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Video/Drilling%20Jar%20-%20YouTube.flv


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DRILLSTRING DESIGN

Involves determination of length, weights and grades of drillpipe fordrilling process

Depends on several factors such as hole depth and size, desired safetyfactor in tension and margin of overpull, length and weight of drill

collars and desired drillpipe size and inspection class Design criteria to choose a suitable drill string:

1. Tension

2. Collapse

3. Shock Loading

4. Torsion

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1. Tension

Tension describes the pulling force exerted by each end of a string which isopposite of compression

The greatest tension (P) on the drillstring occurs at the top joint at themaximum drilled depth.

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1. Tension (Cont’d)

Margin of Overpull (MOP) is defined as the additional allowed tension to be applied tofree stuck pipe.

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1. Tension (Cont’d)

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Safety Factor during drilling:


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1. Tension Design Procedure

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Legend:

Pt = Theoritical tension load given by the manufacturer

MOP = Margin of overpull, lbs

BF = Buoyancy factor

Wdp = weight of drillpipe, lbs/ft

Wdc = weight of drillcollar, lbs/ft

Ldc = Length of drillcollar, ft


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Exercise 4

Tension:

A drillstring consists of 600 ft of 8-1/4 in x 2-13/16 in drill collars andthe rest is a 5 in, 19.5 lbm/ft Grade X95 drillpipe. If the required MOPis 100,000 lbs and mud weight is 10ppg, calculate the maximum depth

of hole that can be drilled when:

a. Using new drillpipe = 16909 ft

b. Using Class 2 drillpipe having a yield strength of 394 600lb = 11106 ft

Please consider 90% safety factor of yield strength.

See Drillpipe properties

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http://localhost/var/www/apps/conversion/tmp/scratch_4/Drillpipe%20properties.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Drillpipe%20properties.pdf


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2. Collapse

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2. Collapse (Cont’d)

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• Safety factor of 1.125 is normally used


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Exercise 5

Collapse:

If 10,000 ft of the drillpipe in Exercise 4 is used,

a. Determine the maximum collapse pressure that can be encounteredand the resulting safety factor. Assume maximum collapse pressureoccurs when the drillpipe is 100% empty. SF = 2.3

b. If the fluid level inside the drillpipe drops to 6000ft below the rotarytable, determine the new safety factor. SF = 3.8

Given mud density is 10 ppg (75 pcf).

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http://localhost/var/www/apps/conversion/tmp/scratch_4/Drillpipe%20properties.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Drillpipe%20properties.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Drillpipe%20properties.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Drillpipe%20properties.pdfhttp://localhost/var/www/apps/conversion/tmp/scratch_4/Drillpipe%20properties.pdf


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3. Shock Loading

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• Shock loading arises whenever slips are set on moving drillpipe and can

contribute to parting of pipe in marginal design

• Shock loading gives additional tensile force (Fs) on drillpipe and can be

calculated as follows:

- W dp is the weight of drillpipe per unit length

• A minimum safety factor of 1.6 should be applied to the tension loads if

shock loading is not accounted for.

• If the shock loading is quantified and included in the load calculation, a

minimum safety factor of 1.3 can be used.


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Exercise 6

Shock Load:

Assuming that 10,000 ft of Grade X95 has been selected from Exercise 4,Determine:

a. The safety factor during drilling SF= 1.83

b. The magnitude of shock loading (Fs), Fs = 62400 lbc. The safety factor when shock loading is included, SF = 1.46

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4. Torsion

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Exercise 7

Torsion:

The following data refer to a drillstring stuck at the drill collars:

Drillpipe: 10,000 ft, 5 in. OD, 4.276 in. ID, 19.5 lb/ft, Grade E,Class 2 having a tensile strength of 311,540 lb

DrillCollars: 600 ft, total weight 80,000 lb.

Make up torque for drillpipe tool joints = 20,000 lb-ft

100% free point = 9,900 ft

Determine the maximum torque that can be applied at the surface without exceeding the minimum torsional yield strength of drillpipe

25468 lb-ft

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Assignment 1

Design a drill string for a hole size of 8-1/2 in., to 15,000ft, given thefollowing additional data:

MOP = 25,000 lbs

Drill collar size: 6 in. x 3 in.; 72 lbs/ft; 720 ft

Use Class II drill pipe of E,G or higher grade

Mud weight = 12 lbs/gal

Percentage of yield = 90%

* Submission dateline: 13 October 2014 before 5pm.. Submission should be made through i-learncourse forum (CGE577)…

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Component Description Length (ft) Depth (ft) Weight (lbs)

Drill Collar 6 in. x 3 in.; 72 lbs/ft 720 15,000 - ? ?

Drillpipe ? ? ? to surface ?

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Drilling Engineering Drillstring

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