Unit 3 - Drillstring Flashcards
What does the drillstring consist of ?
Drillpipe, drillcollars, kelly, stabilisers etc.
What is the Bottom Hole Assembly (BHA)?
Drillcollars + Equipment, which is made up just above the bit
What are the functions of the drillstring?
- suspend the bit - transmit rotary torque from kelly to bit - provide conduit for circulation of drill fluid
What is the name of the threaded connections on a joint of drillpipe?
Tooljoints - on one end is the pin and on the other end the box
Why are all drillpipe joints measured?
Because length is always slightly different → need to know exact length for exact position of drillstring
To which stresses is the drillpipe exposed?
- Tension - Torque - Cyclic Stress Fatigue → stresses are also induced by vibration, abrasive friction and bouncing bit of bottom
How come that drillpipe experiences cyclic stress fatigue?
- 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
What are the main reasons for drillstring corrosions?
mainly due to dissolved gases, salts and acids in wellbore
Examples for gases for corrosion ?
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
What is the problem with dissolved salts for drillstring?
- they increase conductivity and therefore the rates of corrosion - may come from make up water, formation fluid, drilling fluid
How come that hydrogen sulfide causes problems with the drillstring?
- 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
What testing methods are there to determine the degree of damage on a drillpipe?
- optical - magnetic particle - magnetic induction - ultra sonic
Summary of Inspection methods for drillpipe?
What happens following the inspection of drillpipe?
They are classified in terms of wear or damage degree
What does it mean if a drillpipe is condsidered “ Grade 1 or Premium”?
It is a new pipe or an used pipe with a minimum of 80% of the original wall thickness
What is the purpose of a tooljoint?
Provide screw thread to connect joints of pipe together
How are tooljoints nowadays connected to the drillpipe?
Flash-weld onto the pipe
Why do tooljoints often have a hard material layer on their surface ?
To protect it from abrasive wear as drillstring is rotated in borehole
What are additional problems, which the tooljoints experience but the drillpipe doesn’t?
- 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
What does HWDP stand for?
Heavy Wall/Weight Drillpipe
Where is HWDP mostly used?
At base of drillpipe, where stress concentration is greatest
Why is the stress concentration at the base of the drillpipe the greatest?
- 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
What is the purpose of HWDP?
To absorb stresses, that are transferred from stiff drill collars to relatively flexible drillpipe
What is the difference in construction of HWDP to normal drillpipe?
- increased wall thickness
- longer tool joints
- more hard facing
- may have a long central upset section
What is a drillcollar?
Tubular, with a much larger OD, generally smaller ID than drillpipe → way thicker wall than drillpipe
What are the functions of a drillcollar?
- 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
Why are tooljoints on drill collars not necessary?
because drill collars already have large wall thickness
Why do drillcollars receive stresses?
- Buckling and bending forces
- tension
- vibrations
- alternate compression and tension
Where is differential sticking more likely to happen?
- highly porous formations, where large overbalanced mud is used and well is highly deviated
How do you prevent differential sticking with a drillcollar?
- reduce contact area of collar against wellbore
(example: spiral grooves in collar)
→ spiral drillcollar
When do you use a square drillcollar?
For maximum stabilisation of BHA → 1/16” small than bit size
What are monel collars used for?
- they are made of special non-magnetic steel alloy
- they can isolate directional survey instruments from magnetic distortion due to the steel drillstring
What are stabilisers?
- pieces of pipe with blade on external surface
What are the different functions of stabilisers in vertical holes?
- 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
Where are sabilisers usually place in the drillstring?
Right above bit
What are roller reamers?
- 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
What is a shock sub and what is its task?
- 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
What is a sub (substitute)?
- 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)
What is the purpose of a drilling jar ?
deliver strong blow to free pipe if stuck in hole
What are the four requirements, that must be met, when designing a drillstring?
- burst/collapse/tensile strength of drillstring components are all not allowed to be exceeded
- bending stresses within the drillstring must be minimised
- drillcollars must be able to provide all of the required weight for drilling
- BHA must be stabilised to control the direction of well
Where are drilling jars usually positioned?
At the top of the drillcollars
What are the main factors that are considered when selecting the drillpipe?
- collapse load
- tensile load
