Drill String Failure

Drill String Failure Prevention CASING

Objectives On completion of this module you will be able to: Indentify different types of Drill String failure Understand the factors that influence the life of Drill String components Describe the prevention measures to prevent DS failure and extend its life

Introduction

Premature and unexpected failures of drill strings cause great losses in time and material.

Reducing drill string failures will improve rig operating performance and reduce expenses

The “ADIOS”* Elements Attributes: These are the metallurgical properties and dimensions that are built into each drill string component at manufacturing. Design: Drill string design is selecting components and configuring assemblies to accomplish the drilling objective. Inspection: Drill string components, unless new, have been exposed to handling damage and an unknown amount of cumulative fatigue damage. Operation: The Drilling operation presents many opportunities to overload and misuse the drill String. Surroundings: The chemical and mechanical environment surrounding the drill String can have major effect on failure probability.

* TH Hill

What is a Drill String Failure? What is a Drill String Failure? a. When a component cannot perform its function b. Complete separation (parting) c. Leak (washout) Location? a. Tube body, Tool Joint or Threads b. Any drillString component

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Failure Types Mechanisms which can cause failures:

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Tension

Torsion

Sulfide Stress Cracking

Fatigue

Other Causes

DS Failure Mechanisms Group 1 Mechanisms (Overload failures): Acts only if stresses in a component exceeds some fairly high stress threshold

Tension Torsion Collapse Pressure Burst Pressure Combined Tension and Torsion Combined Tension and Collapse UTC

DS Failure Mechanisms Group 2 Mechanisms: Can occurs at low stress level Fatigue Split Box Sulfide Stress Cracking (Corrosion failure) Stress Corrosion Cracking (Corrosion failure)

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Failure Study

Failure Mechanism

Fatigue Torsion SSC/SCC Tension Other 0%

20%

40% % of All Failures

60%

80%

Overload and Fatigue Overload: A condition in which the bulk stress in a component exceeds yield strength at the weakest point in the component.

Fatigue Damage that accumulates when a component undergoes cyclic stress. At some point, cumulative damage results in the formation of a fatigue crack which can grow under continuing stress cycles until failure occurs. UTC

Tensile Failures

Tensile failures occur when the tensile load exceeds the capacity of the weakest component in the drill String.

Occasionally the pin will fail if the connection was made up beyond recommended torque.

How do you recognize a Tensile failure?

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Tensile Failure

Appearance : Jagged and Necked down

Orientation: 45 deg to pipe axis

Pin stretched due excess tension and/or high make up torque

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Box do not fail in tension

Responding to Tensile Failures

Select drill pipe that is capable of carrying the anticipated loads plus a Margin of Over-pull plus a design factor.

Use a marking system that shows tube weight and grade. Check pin markings to make sure that the weight and grade are correct.

Make sure that the rig weight indicator is calibrated properly and does not exceed the allowable tensile load.

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Torsional Failures API Standard tool joints are 80% as strong in torsion as the tube to which they are attached. Therefore in all cases, torsional failures will occur in tool joints.

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Torsional Failures

Torsional stress limit is exceeded.

Failures occur in form of stretched pin or belled box (swelling).

Torsional failures usually occur in the tool joint.

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Responding to Torsional Failures

Select tool joint ID and OD so that the maximum makeup torque exceeds the maximum anticipated torsion.

Check tool joints to ensure that they meet with all the dimensional requirements.

Make sure torque application device is working and calibrated properly.

Use API tool joint compound with a FF between 0.95 and 1.05 or compensate the applied torque accordingly.

Make up connections to recommended torque.

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Increase of Make Up Torque

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Combination of Tension/Torsion These failures are most likely to happen while fishing or pulling on stuck pipe.

Burst and Collapse Failures

Drill pipe tubes may burst or collapse if pressure loading exceeds capacity.

Burst is more likely to happen when pipe is high in the hole

Collapse is most likely to happen deep in hole, evacuated for drill String testing.

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Wear If during drilling significant wear is expected then tools can be run to measure wall thickness reduction. Collapse and burst pressures will be determined by the thinnest part of the wall, tensile strength by the remaining cross sectional area. Burst strength Determined by minimum wall thickness Tensile strength determined by remaining area.

Wear Prevention

Reducing side force by minimizing DLS (especially high up in the hole) and using drillpipe protectors.

Using drilling fluids containing solids (weighted)

Always using sharp tong dies

Minimizing rotating hours (use down-hole motors)

Run a “casing friendly” hardbanding material on tool joints

Weld Related Failures With the obvious exception of tool joint to tube welds, welded components in the drill string should be avoided. Welding alters the mechanical properties unless the component is re-heat treated.

Group 2 Mechanism Can occur at low stress levels: Fatigue Split box Sulfide Stress Cracking Stress Corrosion Cracking

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Fatigue - contributing factors Sources of Cyclic Loads

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Fatigue damaged is caused by repeated stress cycles.

Usually occurred when the string is rotated and at the same time it is bent or buckled.

Fatigue may result from excessive vibration

Stress Concentrators Stress concentrators….The accelerators of fatigue:

Stress concentrators focus and magnify the cyclic stress at local points.

These points become the origin of fatigue cracks, which act as their own concentrators, to speed crack growth to ultimate failure.

Internal upsets, thread roots, slip cuts and corrosion pits are the most common stress concentrators

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Fatigue Under cycle loading, microscopic damage at high stress points… A microscopic crack forms… The crack grows under continuing stress cycles until a failure occurs.

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Recognizing Fatigue Failures

A fatigue crack will be smooth and planar, unless the surface is altered by erosion or mechanical damage.

The crack will be oriented perpendicular to the axis of the pipe or connection.

Fatigue cracks will originate at high stress concentrators namely, internal upsets, slip cuts and corrosion pits.

A fatigue crack surface will clearly show mode of attack. Ratchet marks appear when small multiple cracks join to form a large one.

Fatigue in connection Shape and Appearance: Flat planar shape. Maybe accompanied by ragged area where component parted in tension

Location BHA Connections…Near last engaged thread roots

Orientation Perpendicular to the pipe axis

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Recognizing Stress Concentrators

Slip cuts

Upsets

Recognizing Stress Concentrators Cyclic loading causes very small cracks. With repeated cycles, the cracks grow. Fatigue is cumulative. Fatigue cracks occur in a 90 degree plane to axis of pipe.

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Prevention of Fatigue Failures Fatigue cannot be eliminated: REDUCE THE NUMBER AND SEVERITY OF CYCLIC AND STRESS CONCENTRATORS

Do not buckle Drill-pipe / Jar

Plan the trajectory with the lowest dogleg severity

Ensure good rig site operation practices

Check BSR and SR, stress relief features

Chose the right connection type (NC)

Follow inspection program

Consider rotating the string more slowly, by means of introducing a mud motor (if hole cleaning and directional objectives allow).

Corrosion Corrosion occurs due to electrochemical reactions with corrosive agents. Corrosion rate increases when:

Higher temperature. Rates double for each 31°C.

Higher flow rate, especially if abrasive solids present.

Higher concentration of corrosive agents (O2, H2S, CO2).

Corrosion rate decreases when:

Reducing dissolved O2

Reducing dissolved CO2

Increasing pH to > 9

Add coatings and inhibitors

Corrosion Corrosion reduces the wall thickness of tubular.

There are three patterns of corrosion;

Uniform wall thickness reduction

Localized patterns of metal loss

Pitting

SSC / H2S Embrittlement Exposure of high tensile steels to partial pressures of H2S greater than 0.05 psi at less than a threshold pressure (which varies by steel grade) can lead to catastrophic failure. The metal becomes brittle and will break suddenly and without warning.

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Sulfide Stress Cracking

Sulfide Stress Cracking Occurs in H2S environment Fe+ + + H2S ⇒ FeS + 2H +

Elemental hydrogen (H +) migrates into steel and collects at high stress points 2H + + 2e ⇒ H 2

Elemental hydrogen combines to form molecular hydrogen (H 2) causing a crack.

Preventing Corrosion Corrosive attention usually falls into one or more of the areas below:

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OXYGEN

pH

CO2 AND CHLORIDES

HYDROGEN SULFIDE

BARRIERS and INHIBITORS

Preventing SSC Failures Keep H2S out of the mud system by: i) drilling overbalanced ii) keeping pH high iii) using H2S scavengers iv) using an oil based mud Control the Metallurgy Use a different grade pipe

Why Inspect Connections/tubes? Guarantee the integrity of our connections Avoid lost in hole Avoid tool damage such as flooding & washouts To assess threads for repair Customer requirements

Inspection Methods

Ultrasonic (wall thickness)

Magnetic Particle (cracks in thread roots and stress relief features)

Liquid (Dye) Penetrant (thread roots and stress relief features)

Electromagnetic (DP)

Visual

Follow an Inspection Program

What is a good program?

There is no “Perfect” answer

DS-1 is a guide but not a policy

Areas to consider when creating a program Severity of the drilling conditions Safety and environmental impact of a failure Cost impact of a failure Risk tolerance of management

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References

API RP 7G Drill String Design and Op Limits

API SPEC 7 Specifications for Rotary Drilling Elements

API SPEC 5D Specifications for Drill Pipe

SLB Drill String Design manual

TH Hill DS-1 Drill String Design

Drill String Failure

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