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Sucker Rod Failure Analysis a Special Report from Norris

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Sucker Rod Failure Analysis a Special Report from Norris Sucker Rod Failure Analysis A Special Report from Norris A NORRIS A ______ D O V E R ) COMPANY A NORRIS A DOVER) COMPANY Failure Mechanisms........................................................ 1 Design and Operating Failures ....................................... 5 Mechanical Failures ........................................................ 8 Bent Rod Failures ............................................................ 8 Surface Damage Failures ............................................... 9 Connection Failures ........................................................ 10 Corrosion-fatigue Failures................................................ 14 Acid Corrosion ....................................................... 15 Chloride Corrosion ............................................... 16 CO2 Corrosion ...................................................... 16 Dissimilar Metals Corrosion .................................. 17 H2S Corrosion ...................................................... 17 Microbiologically Influenced Corrosion (MIC) ....... 18 Acid Producing Bacteria ............................. 18 Sulfate Reducer Bacteria ........................... 19 Oxygen (02) Enhanced Corrosion ........................ 19 Stray Current Corrosion ....................................... 20 Under-Deposit Corrosion....................................... 20 Manufacturing Defects ..................................................... 20 Failure Analysis Request Form ........................................ 23 2 © 2000-2007 Norris/A Dover Company All rights reserved Root Cause Failure Analysis is Essential ures. The database failure history should in- for Failure Frequency Reduction in Wells clude information on the failure type, location, with Artificial Lift. depth, root cause and the corrective action measures implemented. Sucker rods can be caused to fail pre- A NORRIS maturely. Understanding the effects of seem- A DOVER) COMPANY ingly minor damage to rod strings, and know- ing how that damage can produce catastro- phic failures, is very important for production personnel. Sucker rod failure analysis is chal- Most failures associated with artificial lenging and you need to be able to look past lift systems can be attributed to one of three the obvious and seek clues from the not so primary downhole components—subsurface obvious. All production personnel should have pump, sucker rod or tubing string. A subsur- adequate knowledge and training in failure face pump, sucker rod or tubing failure is de- root cause analysis. Understanding how to fined as any catastrophic event requiring ser- identify failures and their contributing factors vicing personnel to pull or change-out one or allows us an understanding of what is required more of these components. By this definition, to correct the root cause of the failure. Every the failure frequency rate is the total number of step that can be taken to eliminate premature component failures occurring per well, per sucker rod failures must be taken. On-going year. Marginally producing wells with high fail- training programs concerning sucker rods ure frequency rates are often classified as should include formal and informal forums that "problem" wells and effective failure manage- advocate following the recommendations of ment practices can mean the difference be- manufacturers for artificial lift design, storage tween operating and plugging these wells. & transportation, care & handling, running & Failure management includes preventing, rerunning and makeup and breakout proce- identifying, implementing and recording the dures. A variety of training schools are cur- "real" root cause of each failure and is central rently available and, with advanced notice, to overall cost-effective asset management. most can be tailored to meet the specific For the purpose of this photo essay, we will needs of production personnel. deal only with sucker rod failures. Cost-effective failure management be- Failure Mechanisms gins with prevention, and the time to stop the next failure is now—prior to an incident! Sim- All sucker rod, pony rod and coupling ply fishing and hanging the well on after a failures are either tensile or fatigue failures. sucker rod failure will not prevent failure recur- Tensile failures occur when the applied load rence. In fact, most failures continue with in- exceeds the tensile strength of the rod. The creasing frequency until the entire rod string load will concentrate at some point in the rod must be pulled and replaced. Achievable fail- string, create a necked-down appearance ure frequency reductions require accurate fail- around the circumference of the rod, and frac- ure root cause analysis and the implementa- ture occurs where the cross-section is re- tion of corrective action measures to prevent duced. This rare failure mechanism only oc- failure recurrence. A database capable of curs when pulling too much load on the rod querying the well "servicing" history is needed string—such as attempting to unseat a stuck to track and identify failure trends. Once a fail- pump. To avoid tensile failures, the maximum ure trend is identified, remedial measures weight indicator pull for a rod string in "like should be implemented during well servicing operations to prevent premature rod string fail- 3 © 2000-2007 Norris/A Dover Company All rights reserved new" condition should never exceed 90% of (extrusion / protrusion) must be carefully the yield strength for the known size and cleaned and thoroughly examined. Fatigue grade of the smallest diameter sucker rod. failures have visible or macroscopic identifying For unknown sucker rod conditions, sizes or characteristics on the fracture surface, which grades a sufficient de-rating factor should be help to identify the location of the stress raiser. applied to the maximum weight pulled. All Ratchet marks and beach marks are arguably other sucker rod, pony rod and coupling fail- two of the most important features in fatigue ures are fatigue failures. failure identification. Ratchet marks are lines that result from the intersection and connec- Fatigue failures are progressive and tion of multiple stress fatigue cracks while begin as small stress cracks that grow under beach marks indicate the successive position the action of cyclic stresses. The stresses as- of the advancing fatigue crack. Ratchet marks sociated with this failure have a maximum are parallel to the overall direction of crack value that is less than the tensile strength of growth and lead to the initiation point of the the sucker rod steel. Since the applied load is failure. Beach marks are elliptical or semi- distributed nearly equally over the full cross- elliptical rings radiating outward from the frac- sectional area of the rod string, any damage ture origin and indicate successive positions of that reduces the cross-sectional area will in- the advancing stress fatigue crack growth. crease the load or stress at that point and is a stress raiser. A small stress fatigue crack forms at the base of the stress raiser and propagates perpendicular to the line of stress, or axis of the rod body. As the stress fatigue crack gradually advances, the mating fracture surfaces opposite the advancing crack front try to separate under load and these surfaces be- come smooth and polished from chafing. As the fatigue crack progresses, it reduces the effective cross-sectional area of the sucker rod until not enough metal remains to support the Figure 1 is an example of fatigue and load, and the sucker rod simply fractures in tensile failure mechanisms. The two exam- two. The fracture surfaces of a typical fatigue ples on the right are tensile failures. A tensile failure have a fatigue portion, tensile tear por- failure is characterized by a reduction in the tion and final shear tear. diameter of the cross-sectional area at the point of fracture. Typical tensile failures have cup-cone fracture halves. The second exam- ple from the right is typical in appearance for Fatigue failures are initiated by a multi- tensile failures. Fractures from tensile failures tude of stress raisers. Stress raisers are visi- rupture, or shear, on 45° angles to the stresses ble or microscopic discontinuities that cause applied. A good example of the shear is the an increase in local stress on the rod string characteristic cup-con fracture surfaces of a during load. Typical visible stress raisers on typical tensile failure. The rod body on the sucker rods, pony rods, and couplings are right is an excellent example of needing to bends, corrosion, cracks, mechanical damage, look past the obvious for the not so obvious. threads and wear or any combination of the A stress fatigue crack is primarily responsible preceding. This increased stress effect is the for this failure even though fracture occurred most critical when the discontinuity on the rod while trying to unseat the pump. Visual exami- string is transverse (normal) to the principle tensile stress. In determining the origin of a stress raiser in a fatigue failure, the fatigue portion opposite the final shear tear 4 © 2000-2007 Norris/A Dover Company All rights reserved nation of the fracture surface reveals a small, gether since this almost always destroys semi-elliptical, stress fatigue crack. This (smears) microscopic features that aid in de- sucker rod had preexisting, transverse stress termining the failure cause. To avoid me- fatigue cracks, from in-service stresses. One chanical damage, fracture surfaces should of the stress fatigue cracks opened
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