Basic Principles of Failure Analysis for Roller Cone Bits [Abstract]

In most oilfield organizations the quality assurance function is designed to continually improve the ability to produce products and services that meet or exceed our customer’s requirements.

This means, the manufacturing processes much consistently produce drill bits using high quality materials within the specified API and IADC tolerances. Also quality engineering expends includes reliability considerations, defined as “the probability to perform a required drilling function without a failure under stated operating conditions for a specific period of time”.

For roller cone  rock bits, drilling operators expect the tool to meet the specified parameters upon delivery, but to function throughout what is considered a reasonable lifetime. As roller cone bit parts become more complex, the reliability requirement of individual internal and external components increases.

Despite highly a controlled manufacturing process and high-quality components, a roller cone bit fails before reaching its specified life expectancy. In this situation failure analysis is performed to fully understand the failure and take steps to solve the issues. The forensic analysis is an extremely complex process that involves mechanics, physics, metallurgy and electrochemistry competencies. Engineers also look at the, specific manufacturing process, and perform stress and design analysis in addition to fracture mechanics.

It is essential that reliability engineers adhere to a methodical scientific process without application or personal bias to ensure the root cause of bit failure is accurately identified. Making premature conclusions about bit failure can lead to additional problems in the field.

Unforeseen Roller cone bit failure negatively affects all aspects of the drilling operation and can increase non-productive time resulting in higher drilling costs. The study of basic principle of failure analysis throughout reliability engineering responds to each of these influences and helps our design engineers determine and increase the useful lifetime of roller cone bits, improving the manufacturing process and field services.

Roller cone  bits( TCI Tricone bits)  failure modes are based on multiple unknown factors while drilling a specific application. There are several types of failure modes affecting the drilling performance of roller cone bits. The most common and well recognized in oilfield drilling are:

1. Seal / bearing failure
2. Cutting structure failure
3. Leg breakage

This paper outlines the basic principles of roller cone bit failure analysis and describes the expected bit life cycle based on the most common failure modes, root causes, sequence of events and bit dull condition.