STRING FAILURE: The main causes of drill string failure are: a Fatigue failure.

b. Washout. c. Twist off d. Tensile failure. e. Collapse. f. Burst.

g. Down hole vibrations. h. Slip crushing.

a) Fatigue failure

z Mostly drill pipe failures are caused by fatigue.

z Fatigue is the combined effect of tension, torsion and bending.

z The cyclic reversal of stress that results in as the string is rotated.

z Fatigue is accelerated when string is rotated in a section of directional & crooked hole.

z Failure of the drill pipe due to fatigue takes place in the pipe body generally in the area where slip is set.

z Fatigue fractures are progressive beginning as micro cracks that grow under the action of cyclic stress.

z The rate of propagation is related to the applied cyclic load. Since the crack develops from the inside of the drill pipe and no plastic deformation occurs these cracks are very difficult to detect.

z Fatigue results in washouts and twist off, in drill collars it takes place in the connection with the pin being left in the box.

z Fatigue results in washouts and twist off. Fatigue also results in heat checking of tool joints.

z Tool joints which are rotated under high lateral force against the wall of the hole may be damaged as a result of frictional heat checking.

z The heat generated at the surface of the tool joint by the friction with the wall of the hole under high radial thrust may raise the temperature of the tool joint steel above its critical temperature.

z The hardness of the affected surface is normally 3/16” below the OD.

z The heat checking in the presence of mud causes alternate heating and quenching.

z This results in numerous irregular heat check cracks often accompanied by longer axial cracks sometimes extending through the full section of the joint and wash outs may occur.

z Surface imperfections caused by slip marks, cuts, tong marks, grooves caused by rubber protectors, welding and down hole notches caused by junk greatly affect the fatigue limit.

b) Washout

z A washout is a place where a small opening result in forcing the drilling fluid through pipe.

z It is usually the result of a fatigue crack penetrating the wall of the pipe.

z Wash out may also be caused by a damaged shoulder of box and/or a damaged pin.

z Wearing out and tool joint gets worn out and connection not made up to its recommended torque.

c) Twist off

Usually caused by the fatigue crack extending around the pipe and causing the pipe to break. This type of failure usually occurs in the following manner :

z Most failures occur when rotating or when picking the pipe off bottom immediately after drilling rather than pulling on stuck pipe.

z Most failures occur within 1m of the tool joint on either end of the pipe.

z Failure that originate from the outside of the pipe are usually associated with slip marks or other surface damages such as gouges, welding arc spots, marks made by drill pipe protectors, etc. Progressive growth is indicated in such damages.

z In case of stuck pipe, failure frequently occurs in a location where a fatigue crack has developed but has not progressed to the point of failure.

d) Tensile failure

z The drill string can fail due to tension alone i.e. the total weight of the drill stem member together exceeds the pipe yield value. The design of the drill stem for static tensile load requires sufficient strength in the top most member of each size, weight, grade and class of drill pipe to support the buoyed weight of all hanging load below it. Rated tensile capacity is the product of the minimum yield strength and its cross sectional area. The actual tensile strength will be more because the yield strength is normally high than the minimum specified tensile strength.

z The tensile failure will in most cases be located between the upsets.

z Although this type of failure is usually near the top of the string, but variation in wall Fthickness and tensile strength between different pipes can cause pipe to fail somewhere lower in the string.

z Tensile failure of the tool joint is rare because the tool joint has a greater cross sectional area than the pipe body.

z The exception is when a slim undersized connection is used or the tensile capacity of a pin neck is weakened by higher make up torque.

z In tensile failures the pipe body usually bottle necked near the fracture.

z Tension factures surfaces often show extensive plastic deformation. Facture surfaces will be oriented 45 degrees to the axis of the pipe.

Tensile failure usually occurs if

z Error in the weight indicator.

z While pulling accidentally or purposely more than the rated capacity.

z The wrong size, weight or grade of string is in the hole due to improper design or due to mix up during tripping.

z The pipe is of lower class than assumed because of improper inspection or excessive wear since last inspection.

e) Collapse failure: Drill pipe may be subjected to an external pressure higher than the internal

pressure. This condition usually occurs during drill stem testing and may result in collapse of the drill pipe. The collapse pressure is the maximum in the lower most drill pipe. The drill pipe will be mashed flat or into a half moon shape.

f) Burst failure: Also this type of failure is extremely rare but it can occur in any operation with a

high differential pressure from inside the pipe, for example when well testing or fracturing.

g) Down hole vibrations

z Although some down hole vibrations are inevitable, severe down hole vibration can cause drill string fatigue (washout / twist off), crooked drill string, premature bit failure and reduced penetration rates.

z These vibrations cause three component of motion in the drill string and bit axially, torsional and lateral).

z All three dynamic motions may coexist and one motion may cause the other.

z There are number of mechanisms which can cause severe down hole vibrations. For mechanisms, their symptoms and methods of control are described below :

i) Slap stick

z Non uniform bit rotation in which the bit slows or even stops rotating momentarily, causing the drill string to torque up and then spin free.

z This mechanism sets up the primary torsional vibration in the string. Primary symptoms are surface torque fluctuations (greater than 15% of average), increased MWD shock counts, cutter impact damage, drill string washout/ twist off, connections over-torque or back-off.

z This can be controlled by reducing WOB & increasing RPM, modify mud lubricity, reduce stabilizer torque (change blade design or no. of blades, use non rotating stabiliser or roller reamer), adjust stabiliser placement, smooth well profile and rotary feed back system.

ii) Drill string whirl

z The BHA (or drill pipe) gears around the hole.

z The violent action slams the bit against the hole.

z The mechanism can cause torsional and lateral vibrations.

z Symptoms are: drill string washout/twist-offs, localized tool joint or stabiliser wear, increased average drilling and off-bottom torque.

z To control this, lift bit off bottom and stop rotation, drill with reduced RPM.

z Avoid drill collar weight in excess of 1.15 to 1.25 times WOB, use packed hole assembly, reduce stabiliser torque, adjust stabiliser placement, modify mud properties, consider drilling with down hole motor.

iii) Bit whirl

z Eccentric rotation of a bit about a point other than its geometric centre.

z This causes high frequency lateral vibrations of the bit and drill string. Symptoms are: cutter damage, uneven bit gauge wear, over-gauge hole & reduced ROP.

z To control this, lift bit off bottom and stop rotation, then reduce RPM & increase WOB, consider changing bit (flatter profile, anti whirl), use slower RPM when tagging bottom and during reaming, pick off bottom before stopping rotary use stabilised BHA with full gauge near-bit stabiliser or reamer.

iv) Bit bounce

z Large WOB fluctuations causing the bit to repeatedly lift off & impact the formation.

z This mechanism often occurs while drilling with roller cone bits in hard formation. Symptoms are large axial vibrations (shaking of hoisting equipment) large WOB fluctuations, cutter and/or bearing impact damage, fatigue cracks and reduced ROP.

z To control this run shock subs, adjust WOB/RPM, consider changing bit style, change length of BHA.

h) Slip crushing: A majority of the drill pipe failures occur in the slip area. These type of failures are

z Highly concentrated stresses originating from axial and transverse loads that are not equally distributed over the full gripping surface of the slip.

z Improper handling methods which result in abnormal markings and stressing in the slip area.

Drill pipe failure in the slip area can be prevented by

z Maintaining rotary master bushing and slips to correct API specifications and by good handling techniques. Do not use new master bushing in a worn rotary table and vice- versa. When wear or non uniform gripping of the slip dies is observed, the entire set of dies on the slip must be changed. Never use re-sharpened dies.

z Maximum axial and transverse loads do not act at the same section in the slip area. The critical section occurs at the zone of maximum crushing pressure, and at this point the axial load is less than the hook load. The calculated axial load verses the transverse load factor must always be considered when designing a drill string with excessive hook loads. The presence of transverse load diminishes the total load in pure tension

which causes yielding.

z Do not overlook the effect of transverse load. The transverse load acts as a compressive force on drill pipe and, with adverse conditions of equipment or handling techniques, bottlenecking or crushing will occur when excessive hook load is prevails. This condition exists because the collapse strength of the drill pipe in the slip area is exceeded.

z Use thick wall tube drill pipe.

Proper handling techniques

z Do not stop the downward movement of the drill string with the slips instead of the brake. This result in the area immediately below the last gripping surface of the rotary slip to an increased tensile load, which if the weight is sufficient will elongate the drill pipe in this area. This elongation yields the pipe and renders it useless.

z If the slips are caught in the tool joint area, there is a possibility of damage to the slip. Inspect the slip immediately when this happens and carry out repairs if necessary.

z Do not let the slips ride the pipe.

z Always use slip of correct specifications of tubulars.

z Always use back up tong when breaking or making connection.

z Never allow the pipe to rotate in slip.