1. Drill String Inspection
A component is designed to perform certain functions. The user buys a product with every expectation that it performs the assigned function well and gives trouble-free service for a stipulated period of time (reliability). Reliability comes through improving the quality level of the component. Quality is also related to defects which are generated during service. Inspection is used for routine monitoring of various items during service to predict and assess the remaining life of the component while retaining its structural integrity.
Reliability of inspection depends on standards. Standards are used to ensure reproducible results during inspection/ testing, no matter when, where or who conducts the inspection. Standards also helps to compare results and to take decisions on acceptance/rejection.
Drill String inspection standards:
i) API RP 7G: Recommended practice for Drill Stem Design and Operating Limits
ii) Standard DS-1 Volume 3: Drill Stem Inspection
Primary objectives of this standard are:
a) To publish procedures for inspecting used drill string components that, when employed, will optimize the uniformity and economic benefit of inspection to the inspection buyer. “Inspection buyer” means the party at economic risk in the event of a drill stem failure.
b) To provide an engineered approach for accepting or rejecting used drill stem components during inspection on the basis of their fitness for the intended purpose instead of on arbitrary attributes.
c) To provide the user with methodology and data needed for drill stem design.
d) To establish uniform benchmarks against which the internal process quality control of inspection and threading companies can be evaluated by their customers.
Major differences
API RP 7G:
■ Recommendations only
■ Drill Stem Design & operating limits
■ Non specific
■ Limited options
Standard DS1:
■ Standards for Inspection- Accepting & Rejecting used drill Pipes
■ Drill Stem Inspection covering wide range of components
■ Specific Procedures & Methodology
■ Flexible options
■ Training & Qualifications
■ Calibration mandatory requirement
■ Quality Control measures outlined for equipment & processes
Process comparison
Most of the time API Inspection loosely called as "Standard Rack Inspection". This "Standard Rack Inspection" (or Its rough equivalent, since its meaning varied by company and location) was adopted by the DS1 sponsor committee as DS1 Category 3.
The cost of a Category 5 inspection is roughly twice that of the normal program represented by Category 3 or Standard Rack Inspection. DS1 Category 5 is intended for drill string components that are to be used in extremely adverse conditions where the potential cost of failure is very large.
DS1 Category 5 includes the Ultrasonic End Area Inspection of Upset and Slip areas of the Drill pipe, where the accumulated fatigue at both the areas together contribute 96% probability of failure.
2. Inspection Capabilities
API Recommended Practice 7G Address
13.2.1 Limitations of Inspection Capability
Most failures of drill pipe result from some form of metal fatigue. A fatigue is one which originates as a result of repeated or fluctuating stresses having maximum values less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress. The rate of propagation is related to the applied cyclic loads and under certain conditions may be extremely rapid. The failure does not normally exhibit extensive plastic deformation and is therefore difficult to detect until such time as considerable damage has occurred. There is no accepted means of inspecting to determine the amount of accumulated fatigue damage or the remaining life in the pipe at a given stress level.
Presently accepted means of inspection are limited to location of cracks, pits, and other surface marks: measurement of remaining wall thickness; measurement of outside diameter; and calculation of remaining cross-sectional area. Recent industry statistics confirm that a major percentage of tube body in-service failures occur near the upset run out or within the slip area. Special attention to these critical failure areas should be performed during inspection to facilitate crack detection in drill strings which have been subjected to abnormally high bending stresses. Drill pipe which has just been inspected and found free of cracks may develop cracks after very short additional service through the addition of damage to previously accumulated fatigue damage.
3. Detecting Fatigue:
No reliable method exists to quantify the amount of fatigue damage a component may have accumulated. Rather, present inspection technology is limited to looking for fatigue cracks, and even when these inspections are performed properly, they can still occasionally fail to find small cracks.
Most of the fatigue life of a drill string component will have been used up by the time a crack has formed and grown large enough to be detected by inspection, so a fatigue crack, once detected, is cause for immediate rejection of the component.
4. Drivers Affecting
Fatigue Life:
How quickly a fatigue crack will form and propagate to failure depends on many variables. The four major drivers are:
(a) Mean tensile stress: Higher mean stress shortens fatigue life, other things equal.
(b) Cyclic stress excursions about the mean stress: Larger stress excursions shorten fatigue life, other things equal.
(c) Corrosiveness of the mud system: More corrosive environments shorten fatigue life, other things equal.
(d) Fracture toughness of the material: Tougher material will have a longer fatigue life and support a larger crack before final failure, other thing equal (Fracture toughness, rather than being a driver of fatigue damage, should be thought of as a retarder).
5. Possible Inspection Results:
The component being examined is actually either good or bad, depending on its condition and the acceptance criteria in effect. The inspector examines the part and decides whether to accept or reject it. Therefore, four possibilities exist. Two correct decision and two errors
Type 1 Error: (Accepting bad material) increase failure probability and may add catastrophic failure costs to the well costs.
Type 2 Error: (Rejecting good material) increase inspection costs, cause undue hardship on drilling contractors and rental companies, and lead to logistical cost to unnecessarily replace good equipment.
