Drill String Failure

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 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 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.

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)

c. Leak (washout)

Location?

a. Tube body, Tool Joint or Threads

b. Any drillString component

Failure Types

Mechanisms which can cause failures:

Tension

Torsion

Torsion

Sulfide Stress Cracking

Fatigue

Group 1 Mechanisms (

Overload failures

)

:

Acts only if stresses in a component exceeds some

fairly high stress threshold

DS Failure Mechanisms

Tension

Torsion

Collapse Pressure

Group 2 Mechanisms:

Can occurs at low stress level

Fatigue

Split Box

DS Failure Mechanisms

Split Box

Sulfide Stress Cracking (Corrosion failure)

Failure Study

Overload:

A condition in which the bulk stress in a component

exceeds yield strength at the weakest point in the

component.

Overload and Fatigue

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.

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.

Tensile Failures

Tensile Failure

Appearance : Jagged and Necked down

Orientation: 45 deg to pipe axis

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

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

Responding to Tensile Failures

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.

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.

Torsional Failures

Torsional Failures

Torsional stress limit is exceeded.

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

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.

Responding to Torsional Failures

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.

Combination of Tension/Torsion

These failures are most likely to happen while fishing or

pulling on stuck pipe.

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.

Burst and Collapse Failures

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.

remaining cross sectional area.

Determined by minimum wall thickness Burst strength 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

Always using sharp tong dies

Minimizing rotating hours (use down-hole motors)

With the obvious exception of tool joint to tube welds, welded components in the drill string should be avoided.

Weld Related Failures

Welding alters the mechanical properties unless the component is re-heat treated.

Group 2 Mechanism

Can occur at low stress levels:

Fatigue

Split box

Split box

Sulfide Stress Cracking

Stress Corrosion Cracking

Fatigue - contributing factors

Sources of Cyclic Loads



Fatigue damaged is caused by repeated

stress cycles.



Usually occurred when the string is rotated

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….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.

Stress Concentrators

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

Fatigue



Under cycle loading, microscopic damage at high stress

points…



A microscopic crack forms…



The crack grows under continuing stress cycles until a



The crack grows under continuing stress cycles until a

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,

Recognizing Fatigue Failures

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

Location

BHA Connections…Near last engaged thread roots

Orientation

Recognizing Stress Concentrators

Slip cuts

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.

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

Prevention of Fatigue Failures

Ensure good rig site operation practices

Check BSR and SR, stress relief features

Chose the right connection type (NC)

Corrosion

Higher temperature. Rates double for each 31°C.

Higher flow rate, especially if abrasive solids present.

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

Higher concentration of corrosive agents (O2, H2S, CO2). Corrosion rate decreases when:

Reducing dissolved O2

Reducing dissolved CO2

Increasing pH to > 9

Corrosion

Corrosion reduces the wall thickness of tubular.

There are three patterns of corrosion;

Uniform wall thickness reduction

Localized patterns of metal loss

Localized patterns of metal loss

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.

Sulfide Stress Cracking

Occurs in H2S environment

Elemental hydrogen (H +) migrates into steel and

+ + +

₊

_⇒

₊

H

FeS

S

H

Fe

2

Elemental hydrogen (H +) migrates into steel and

collects at high stress points

Elemental hydrogen combines to form molecular

hydrogen (H 2) causing a crack.

2

2

Preventing Corrosion

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

OXYGEN

pH

CO2 AND CHLORIDES

CO2 AND CHLORIDES

HYDROGEN SULFIDE

Preventing SSC Failures

Keep H2S out of the mud system by: i) drilling overbalanced

ii) keeping pH high

iii) using H2S scavengers iii) using H2S scavengers iv) using an oil based mud

Control the Metallurgy

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

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)

Electromagnetic (DP)

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

References

API SPEC 7 Specifications for Rotary Drilling Elements

API SPEC 5D Specifications for Drill Pipe

SLB Drill String Design manual

SLB Drill String Design manual