Unit 3 - Drillstring

Question 1

What does the drillstring consist of ?

Answer 1

Drillpipe, drillcollars, kelly, stabilisers etc.

Question 2

What is the Bottom Hole Assembly (BHA)?

Answer 2

Drillcollars + Equipment, which is made up just above the bit

Question 3

What are the functions of the drillstring?

Answer 3

• suspend the bit - transmit rotary torque from kelly to bit - provide conduit for circulation of drill fluid

Question 4

What is the name of the threaded connections on a joint of drillpipe?

Answer 4

Tooljoints - on one end is the pin and on the other end the box

Question 5

Why are all drillpipe joints measured?

Answer 5

Because length is always slightly different → need to know exact length for exact position of drillstring

Question 6

To which stresses is the drillpipe exposed?

Answer 6

• Tension - Torque - Cyclic Stress Fatigue → stresses are also induced by vibration, abrasive friction and bouncing bit of bottom

Question 7

How come that drillpipe experiences cyclic stress fatigue?

Answer 7

• in deviated holes, wall of pipe is exposed to compressive and tensile forces at points of bending in hole → as string is rotated, each joint sustains a cycle of compressive and tensile forces → con result in fatigue in wall of pipe

Question 8

What are the main reasons for drillstring corrosions?

Answer 8

mainly due to dissolved gases, salts and acids in wellbore

Question 9

Examples for gases for corrosion ?

Answer 9

Oxygen - in all drilling fluids → (<1 ppm can already be damaging) Carbon dioxide - introduced to wellbore through drilling fluid or formation fluid → forms carbonic acid → corrodes steel

Question 10

What is the problem with dissolved salts for drillstring?

Answer 10

• they increase conductivity and therefore the rates of corrosion - may come from make up water, formation fluid, drilling fluid

Question 11

How come that hydrogen sulfide causes problems with the drillstring?

Answer 11

• causes “hydrogen embrittlement” or “ sulfide stress cracking” →hydrogen is absorbed on surface of steel in presence of sulfide → with sufficient concentration, cracks can be formed → will accelerate failure of pipe if already under stress or notched

Question 12

What testing methods are there to determine the degree of damage on a drillpipe?

Answer 12

• optical - magnetic particle - magnetic induction - ultra sonic

Question 13

Summary of Inspection methods for drillpipe?

Answer 13

Question 14

What happens following the inspection of drillpipe?

Answer 14

They are classified in terms of wear or damage degree

Question 15

What does it mean if a drillpipe is condsidered “ Grade 1 or Premium”?

Answer 15

It is a new pipe or an used pipe with a minimum of 80% of the original wall thickness

Question 16

What is the purpose of a tooljoint?

Answer 16

Provide screw thread to connect joints of pipe together

Question 17

How are tooljoints nowadays connected to the drillpipe?

Answer 17

Flash-weld onto the pipe

Question 18

Why do tooljoints often have a hard material layer on their surface ?

Answer 18

To protect it from abrasive wear as drillstring is rotated in borehole

Question 19

What are additional problems, which the tooljoints experience but the drillpipe doesn’t?

Answer 19

• During trips out of hole, elevator supports string weight underneath shoulder of tool joint
• frequent engagement of pins and boxes can damage the threads
• threaded pin end of pipe is often left exposed and therefore exposed to possible damage

Question 20

What does HWDP stand for?

Answer 20

Heavy Wall/Weight Drillpipe

Question 21

Where is HWDP mostly used?

Answer 21

At base of drillpipe, where stress concentration is greatest

Question 22

Why is the stress concentration at the base of the drillpipe the greatest?

Answer 22

• Since drillcollars and drillpipe have a difference in cross section, they have a different stiffness
• rotation and cutting action of bit can result in vertical bouncing effect

Question 23

What is the purpose of HWDP?

Answer 23

To absorb stresses, that are transferred from stiff drill collars to relatively flexible drillpipe

Question 24

What is the difference in construction of HWDP to normal drillpipe?

Answer 24

• increased wall thickness
• longer tool joints
• more hard facing
• may have a long central upset section

Question 25

What is a drillcollar?

Answer 25

Tubular, with a much larger OD, generally smaller ID than drillpipe → way thicker wall than drillpipe

Question 26

What are the functions of a drillcollar?

Answer 26

• provide enough weight on bit for efficient drilling
• keep drillstring in tension, thereby reducing bending stresses and failure due to fatigue
• provide stiffness in BHA for directional control

Question 27

Why are tooljoints on drill collars not necessary?

Answer 27

because drill collars already have large wall thickness

Question 28

Why do drillcollars receive stresses?

Answer 28

• Buckling and bending forces
• tension
• vibrations
• alternate compression and tension

Question 29

Where is differential sticking more likely to happen?

Answer 29

• highly porous formations, where large overbalanced mud is used and well is highly deviated

Question 30

How do you prevent differential sticking with a drillcollar?

Answer 30

• reduce contact area of collar against wellbore
  (example: spiral grooves in collar)

→ spiral drillcollar

Question 31

When do you use a square drillcollar?

Answer 31

For maximum stabilisation of BHA → 1/16” small than bit size

Question 32

What are monel collars used for?

Answer 32

• they are made of special non-magnetic steel alloy
• they can isolate directional survey instruments from magnetic distortion due to the steel drillstring

Question 33

What are stabilisers?

Answer 33

• pieces of pipe with blade on external surface

Question 34

What are the different functions of stabilisers in vertical holes?

Answer 34

• reduce buckling/bending stresses on drill collars
• allow higher WOB since string stays concentric
• increase bit life by reducing wobble (→ all three cones are loaded equally)
• help prevent wall sticking
• act as key seat wiper when placed at top of collars

Question 35

Where are sabilisers usually place in the drillstring?

Answer 35

Right above bit

Question 36

What are roller reamers?

Answer 36

• consist of stabiliser blades with rollers embedded into surface of blade
• act as stabilisers → useful in maintaining gauge hole → also reams out potential hole problems

Question 37

What is a shock sub and what is its task?

Answer 37

• a vibration dampener
• reduces stress due to bouncing, when bit drills through hard rock
• absorb vertical vibrations by either using strong steel spring or resilient rubber element

Question 38

What is a sub (substitute)?

Answer 38

• short joint of pipe, which acts as a crossover → connect components, which can’t otherwise be screwed together because of difference in thread type or size)

Question 39

What is the purpose of a drilling jar ?

Answer 39

deliver strong blow to free pipe if stuck in hole

Question 40

What are the four requirements, that must be met, when designing a drillstring?

Answer 40

1. burst/collapse/tensile strength of drillstring components are all not allowed to be exceeded
2. bending stresses within the drillstring must be minimised
3. drillcollars must be able to provide all of the required weight for drilling
4. BHA must be stabilised to control the direction of well

Question 41

Where are drilling jars usually positioned?

Answer 41

At the top of the drillcollars

Question 42

What are the main factors that are considered when selecting the drillpipe?

Answer 42

• collapse load
• tensile load