In the Chinese oil and gas industry, two statistical facts define the structural challenge of large-diameter surface sections in deep wells: drill string fractures occur at a 68% probability in large-diameter boreholes, and washout failures occur at 59% probability — the highest failure rates of any borehole size category. When a large-diameter drill collar drops to the bottom of a surface section and becomes wedged against the formation wall, fishing is extraordinarily difficult. A single large-diameter fishback operation can consume weeks of rig time and millions of yuan in cost — far exceeding the savings from any shortcut in the drill string inspection and management programme that might have prevented the failure.
Despite this, the engineering literature on large-diameter borehole drill collar failure remains sparse. Most published research addresses drill string integrity in standard-diameter wellbores or in the deep sections of ultra-deep wells. The upper surface section — where the largest OD drill collars are deployed, where formation alternation between soft and hard lithologies is most severe, and where drill string weight is lowest — has received comparatively little attention. The consequence is a quantifiable gap in operators' safety management systems: the inspection intervals, acceptable wear limits, and operational thresholds that apply to standard-diameter operations do not directly translate to large-diameter borehole conditions.
This article presents the complete failure analysis of the Ø228.6 mm drill collar box thread fracture that occurred during pullout of hole in the Ø406.4 mm second-spud section of Well Z201-A, Yuxi block, Sichuan basin — a case study conducted by CNPC Chuanqing Drilling Engineering Company. The analysis integrates fracture surface morphology, bending strength ratio calculation from as-worn OD measurements, three- dimensional drill string dynamic simulation across multiple borehole diameters, and wellhead torque measurement correlated with downhole vibration modelling. Together, these four analytical threads establish a multi-cause failure model and a set of quantified safety management thresholds applicable to large-diameter borehole operations throughout the Sichuan-Chongqing basin. For operators specifying non-magnetic drill collars for directional BHAs deployed in these large-diameter environments, the wear limits and BSR criteria established here define the in-service structural standards these tools must maintain.
Well Z201-A: Case Background and Failure Event
Well Z201-A is a horizontal well in the Yuxi block of the Sichuan basin, designed to a total depth of 6,757 m. The well's casing programme begins with a first-spud Ø660.4 mm surface section drilled to 220 m through Jurassic Shaximiao Formation sandstone-mudstone. The second-spud section is a Ø406.4 mm intermediate borehole drilled to 1,913 m, where the rate of penetration declined significantly and a trip out of hole was initiated. During this pullout, the box thread of a 9-inch (Ø228.6 mm) drill collar fractured. The collar had previously served on two other wells in the same platform — Z201B and Z201HC — during their first-spud Ø660.4 mm sections, accumulating 487.5 cumulative operating hours and 8 trips across both wells before the second-spud deployment at Z201-A. The same collar had also undergone thread recutting repair after earlier service on two additional wells following initial manufacture in 2019.
The second-spud BHA configuration at Z201-A was: Ø406.4 mm PDC bit + Ø244.5 mm 1.25° bent motor (Ø398 mm stabilizer) + Ø228.6 mm float sub + Ø228.6 mm non-magnetic drill collar (1 joint) + Ø228.6 mm directional hanging sub + Ø400 mm centralizer + Ø228.6 mm drill collar (3 joints) + Ø228.6 mm bypass valve + Ø203.2 mm drill collar (6 joints) + Ø139.7 mm heavy-wall drill pipe (6 joints) + Ø139.7 mm drill pipe. Drilling parameters were: WOB 120 kN, rotary speed 55 RPM, flow rate 60 L/s. The fractured collar was positioned immediately above the Ø400 mm centralizer — a location the failure analysis would subsequently identify as the primary stress concentration site in the BHA.
Fracture Surface Morphology: Three-Zone Fatigue Pattern
The cross-sectional fracture face of the failed Ø228.6 mm collar exhibits the classic three-zone morphology of a fatigue fracture, clearly distinguishable by visual examination. This morphological classification allows reconstruction of the complete failure progression sequence from initial crack initiation through crack propagation to final overload fracture.
The fracture morphology confirms fatigue as the primary failure mechanism — not a single-event overload. The 63% propagation zone fraction indicates that the crack had been growing for a significant portion of the collar's recent service life before the final fracture event. This is consistent with the absence of a significant standpipe pressure drop during the drilling run — the crack existed but had not yet penetrated the bore wall — and explains why the failure was not detected through routine surface pressure monitoring. Only downhole acoustic or vibration-based tool health monitoring, or per-trip ultrasonic NDT of the box thread, would have had the sensitivity to detect this propagating crack before failure.
Four Contributing Causes: The Multi-Factor Failure Model
The investigation identifies four independent but interacting causes, each of which alone would have elevated fatigue risk; their simultaneous occurrence in this case created the conditions for relatively rapid crack propagation and early final fracture.
Bending Strength Ratio Calculation and Allowable Wear Limits
BSR Formula for Drill Collar Thread Connections
R_BS = [(D⁴ − b⁴) / D] / [(R⁴ − d⁴) / R]
- R_BS — Bending Strength Ratio (dimensionless; API standard: 2.5, allowable range 1.9–3.2)
- D — Box thread critical cross-section OD (at box-end critical section), mm
- b — Box thread critical cross-section bore ID, mm
- R — Pin thread root OD at 0.75 in from shoulder face, mm
- d — Pin bore (flow channel) ID, mm
For the Z201-A failed collar, after OD wear from 228.6 mm to 226.0 mm, the computed BSR fell to 2.18 — 12.8% below the API standard of 2.5 and inside the allowable range (1.9–3.2) but well below the optimum. The direct consequence of reduced BSR is that the box section carries a disproportionately higher bending stress fraction compared to the pin, accelerating fatigue crack initiation at the box thread root — exactly the failure mode observed in this case.
Allowable Minimum OD After Wear — API and Field Reference Values
The following table provides the minimum acceptable OD values for worn drill collars before retirement or regrinding. These values define the threshold below which BSR degradation becomes structurally unacceptable and the collar must be removed from service.
| Collar Nominal OD (mm) | Nominal OD (in) | Nominal Bore ID (mm) | Thread Type | Min Acceptable OD After Wear (mm) | Max Allowable Wear (mm) |
|---|---|---|---|---|---|
| 120.7 | 4¾ | 50.8 | NC35 | ≥118 | 2.7 |
| 127.0 | 5 | 57.2 | NC38 | ≥122 | 5.0 |
| 158.8 | 6¼ | 71.4 | NC46 | ≥156 | 2.8 |
| 165.1 | 6½ | 71.4 | NC46 | ≥163 | 2.1 |
| 177.8 | 7 | 71.4 | NC50 | ≥175 | 2.8 |
| 203.2 | 8 | 71.4 | 6⅝REG | ≥201 | 2.2 |
| 228.6 | 9 | 71.4 | 7⅝REG | ≥227 | 1.6 |
Red row: The collar size that failed in this case study (Ø228.6 mm). The failed collar had worn to 226.0 mm — 1.0 mm below the minimum allowable value of 227 mm. This collar should have been retired before the Z201-A deployment.
Drill String Mechanics: The Centralizer as Primary BHA Stress Concentration
3D Dynamic Finite Element Model
The drill string dynamics model discretises the BHA into Euler-Bernoulli beam elements along the axial direction. Each element has two nodes; each node has six degrees of freedom (3 translational: x, y, z; 2 lateral rotational: θ_y, θ_z; 1 torsional: θ_x). The governing equation of motion for the assembled drill string system is the Lagrangian equation of the beam element:
[M]{Ü} + [C]{U̇} + [K]{U} = {F}
- [M] — Mass matrix
- [C] — Damping matrix
- [K] — Stiffness matrix
- {Ü}, {U̇}, {U} — Generalised acceleration, velocity, displacement vectors
- {F} — External force vector (WOB, torque, formation contact, fluid forces)
Centralizer Stress Concentration: Why the Collar Above Is Always the Weak Point
Simulation of the BHA in three different borehole diameters (Ø660.4 mm, Ø444.5 mm, Ø323.8 mm) reveals a consistent structural pattern: the centralizer acts as a bending stiffness discontinuity in the BHA. The centralizer blade contact with the borehole wall locally restrains lateral displacement of the collar at that position, while the collar sections above and below the centralizer remain free to deflect. The result is that the collar immediately above (and below) the centralizer is the location of maximum curvature — the bending starts here — and therefore the location of maximum equivalent stress and maximum bending moment. The non-destructive test failure of several collars in the Z201-A BHA was concentrated at the position immediately above the centralizer, and the fracture occurred at exactly that location.
In the Ø660.4 mm simulation, drill tool bending deformation was significantly higher than in the Ø444.5 mm and Ø323.8 mm cases — with maximum deformations reaching 0.06 m vs. approximately 0.03 m and 0.01 m respectively. The larger annular clearance in a large-diameter borehole allows the drill collar to undergo greater lateral displacement before it contacts the borehole wall, resulting in longer free-deflection spans and therefore higher curvature and bending stress at the restrained positions (centralizer locations).
Downhole Vibration: Measurement, Calculation, and Damage Quantification
Equivalent Stress Formula — Fourth Strength Theory
σ = √[ (|σ_p| + |σ_w|)² + 3(τ_n² + τ_m²) ]
- σ_p — Axial stress (tension/compression from string weight and WOB), MPa
- σ_w — Bending stress (from borehole curvature and lateral deflection): σ_w = 32M_w × d_o / [π(d_o⁴ − d_i⁴)], MPa
- τ_n — Net pressure shear stress: τ_n = A_i(p_od − p_id) / [π(d_o⁴ − d_i⁴)], MPa
- τ_m — Torsional shear stress: τ_m = 16T × d_o / [π(d_o⁴ − d_i⁴)], MPa
- M_w — Bending moment (N·m); T — Torque (kN·m)
- d_o, d_i — Collar outer and inner diameter (mm)
- p_od, p_id — External and internal fluid pressure (MPa)
Exponential Stress Increase Above Ø444.5 mm
Applying this formula across four borehole diameters reveals the non-linear relationship between borehole size and drill collar equivalent stress that defines the special risk of large-diameter operations.
Torque Fluctuation vs. WOB Fluctuation: Which Matters More?
Stress amplitude calculations across four borehole sizes reveal that torque fluctuation (stick-slip) consistently produces larger stress amplitude increases than equivalent WOB fluctuation (bit bounce). This has direct operational implications: when downhole conditions are deteriorating, controlling stick-slip torque variation is more important for drill collar fatigue life than controlling bit bounce. The relative influence of both becomes smaller as borehole size increases, however, because the base bending stress level in large-diameter boreholes is so high that the incremental stress from either torque or WOB fluctuation is relatively small in comparison — meaning that in Ø660.4 mm boreholes, the collar fatigue state is dominated by static bending stress, and reducing vibration alone is insufficient to make the operating environment safe without also ensuring that BSR and collar OD are within specification.
Large-Diameter Borehole Drill String Safety Management: Quantified Measures
Drilling Technology Measures
The following operational measures address the four contributing causes identified in the failure analysis. They are sequenced by borehole size because the Ø444.5 mm threshold separates two qualitatively different risk regimes with different appropriate responses.
For Ø444.5 mm and larger boreholes: Strengthen NDT inspection and collar rotation on every trip. Each trip should include rotation of the neutral point and the collar immediately above and below the centralizer; simultaneously, perform per-joint ultrasonic thread NDT on all BHA components during every trip in and out of hole. Remove any component that fails inspection immediately. Limit all BHA component service time (pure drilling time plus reaming time) to a maximum of 450 cumulative hours; if continued use beyond 450 hours is required, apply the full large-diameter borehole inspection and rotation standard.
For Ø444.5 mm and smaller boreholes: Base inspection and rotation decisions on the stick-slip and bit bounce severity thresholds defined above. If WOB fluctuation exceeds 80 kN, torque fluctuation exceeds 8 kN·m, or both exceed the combined threshold simultaneously for sustained periods, rotate the neutral point collars and the centralizer-adjacent collars, and perform per-joint NDT. If vibration remains below these thresholds, focus NDT on the neutral point and centralizer-adjacent joints; other BHA sections can be inspected on a stand-by-stand basis.
Vibration control: For wells equipped with real-time three-axis downhole vibration measurement and telemetry, use the vibration severity grade transmitted in real time to adaptively adjust drilling parameters, maintaining vibration below the severe classification threshold. For wells without downhole vibration sensors, use wellhead torque and WOB variability as vibration proxies. If severe stick-slip cannot be controlled in large-diameter surface sections by parameter adjustment, switch from PDC to roller cone bits until sufficient drill collar weight is in the hole (approximately 500 m) to stabilize BHA dynamics, then reintroduce the PDC for rate-of-penetration optimization.
Optimised BHA Configurations for Large-Diameter Operations
The dual-centralizer BHA configuration is recommended as the standard for all large-diameter borehole operations, replacing single-centralizer designs. Dual centralizers improve BHA centralization, reduce individual centralizer contact forces, reduce the bending moment step-change at any single restrained position, and lower the maximum equivalent stress at the centralizer-adjacent collar location.
Drill String Management Measures
The paper establishes a full-lifecycle drill string management model with three principal requirements. First, every collar entering the well must have its full service history recorded — total operating time, deployment positions, and downhole conditions (including borehole size and vibration severity) — to allow accurate assessment of accumulated fatigue damage. This enables data-driven retirement decisions rather than purely time-based or OD-based retirement criteria applied without service context.
Second, all drill collars must be precisely dimensioned upon arrival at the wellsite. OD measurements must be taken with a calibrated gauge and compared against the minimum allowable values in the wear table. Any collar below the minimum OD threshold — or calculated to have BSR below the API standard of 2.5 after wear — is prohibited from running in the hole. This is not optional: the Z201-A case demonstrates that a collar measuring 226.0 mm against a minimum of 227 mm will fail under large-diameter borehole loading conditions.
Third, thread management requires strict cleaning of all thread connections before each deployment, using compressed air purging and manual cleaning, combined with high-temperature-rated copper-based thread compound (such as LB 8008TM). Make-up torque must be monitored with a real-time torque display instrument to ensure neither over-torque nor under-torque conditions, both of which degrade thread joint fatigue performance. DS-1 drill string inspection standards should be applied at every collar repair visit, ensuring that prior fatigue damage is identified and cleared — specifically including thread recutting that fully removes any heat-affected or work-hardened material from the thread root zone where fatigue cracks initiate.
Frequently Asked Questions
Q: The paper identifies Ø444.5 mm as the exponential stress increase threshold. Does this mean that standard drill collar inspection intervals are acceptable for Ø406.4 mm boreholes, even though the Z201-A failure occurred in a Ø406.4 mm section?
The Ø444.5 mm threshold refers to the average equivalent stress behaviour during active drilling in the respective borehole. The Z201-A failure occurred during a Ø406.4 mm second-spud section, but the primary fatigue damage accumulation occurred during the collar's prior service in the Ø660.4 mm first-spud sections on Z201B and Z201HC wells. The fracture therefore represents residual fatigue damage carried over from large-diameter borehole service materialising as final fracture under the additional loading of the Ø406.4 mm second-spud deployment. This is precisely why full-lifecycle service history tracking is essential: a collar that has served in a Ø660.4 mm borehole cannot be evaluated using Ø406.4 mm operating parameters alone. The accumulated fatigue from the high-stress large-diameter service must be accounted for in the total remaining life assessment.
Q: Why does the centralizer create a stress concentration immediately above it rather than at the contact point itself? And does the same effect apply to stabilizers and reamers?
The centralizer creates a lateral restraint at its position — the blades contact the borehole wall and resist lateral displacement of the BHA at that point. Immediately above the centralizer, the collar transitions from a laterally restrained condition (at the centralizer) to a laterally unrestrained condition (in the open annulus above). This transition is a bending stiffness discontinuity: the BHA must accommodate the full lateral deflection resulting from formation contact, gravity, and wellbore curvature over the span between the bit or lower restraint and the upper centralizer. The collar just above the centralizer is at the beginning of this deflection span and therefore experiences the steepest bending curvature gradient — hence maximum bending moment and stress. The same physical mechanism applies to stabilizers and reamers, which create equivalent restraint conditions. The practical consequence is that any BHA component immediately adjacent to a lateral restraint element — whether centralizer, stabilizer, or reamer — is the primary fatigue failure candidate and should be the focus of NDT inspection priority on every trip.
Q: The non-magnetic drill collar in this BHA is positioned above the directional motor and immediately below the first centralizer. Does the non-magnetic collar bear the same centralizer-adjacent stress as a standard steel collar, or is its stress state different?
The non-magnetic collar bears the same centralizer-adjacent bending stress as a standard steel collar in the same BHA position — the stress is determined by the geometry of the BHA (centralizer spacing, collar diameter, borehole clearance) and the applied loads, not by the material grade. What changes between non-magnetic austenitic steel and standard steel collars is the fatigue strength available to resist that stress. Austenitic non-magnetic grades such as P550, P650, and the TWZ and HS series are designed to provide high fatigue strength while maintaining non-magnetic permeability — but their fatigue limits must still exceed the applied stress amplitude at the centralizer- adjacent position to provide adequate service life. The combination of the stress analysis in this paper (establishing the applied stress level as a function of borehole diameter and BHA configuration) and the BSR analysis in the companion paper by Wang Qinghua et al. (CNOOC, 2025) therefore provides a complete structural specification basis for non-magnetic drill collar selection in large-diameter directional BHA applications: the applied stress determines the required fatigue strength, and the BSR determines the acceptable wear limit before retirement.
Q: The paper recommends switching to a roller cone bit when severe stick-slip cannot be controlled in a large-diameter surface section. Why does bit type affect stick-slip severity, and at what depth does it become safe to return to PDC?
PDC bits in alternating soft-hard lithologies are prone to severe stick-slip because the cutting action is continuous and the cutter engagement changes abruptly as the bit transitions between hard and soft rock intervals. The shear cutting mechanism of a PDC bit generates periodic torque spikes as the cutters alternately engage hard layers and release into soft ones. Roller cone bits have a crushing and indenting mechanism that is less sensitive to formation hardness variation and typically generates lower peak torque levels in alternating lithologies — at the cost of a lower rate of penetration. The paper recommends returning to PDC once the drill collar weight in the hole reaches approximately 500 m worth of drill collars. At this depth, the drill collar assembly has sufficient weight to dampen lateral vibration and stabilise BHA dynamics, reducing the sensitivity of the bit to stick-slip excitation. The 500 m threshold is based on empirical field observation from the Sichuan-Chongqing basin rather than a universal analytical formula — operators in different basins with different formation characteristics may find a different threshold depth is appropriate for their specific conditions.
Large-Diameter Drill Collars Built to the Standard the Data Demands
The failure analysis of Well Z201-A establishes a clear and measurable structural standard for drill collars deployed in large-diameter Sichuan-Chongqing basin operations: OD must be maintained above the minimum wear threshold, BSR must remain at or above the API standard of 2.5, and the material must provide adequate fatigue strength at the stress levels — up to 140 MPa average equivalent stress in Ø660.4 mm boreholes — that characterise these severe operating environments. For non-magnetic drill collars that must additionally satisfy the magnetic permeability requirements of MWD survey instrument protection, the combined structural and magnetic specification is demanding. ShunFu Metal's non-magnetic drill collar product range — including P550, P650, and TWZ and HS series grades — is developed specifically for these high-performance directional BHA applications. To review material certifications, mechanical property data, and minimum OD specifications for our NMDC product range, visit gaslinepipe.com or contact our technical team directly.
