Fearnley Procter Group
Drillstring Failure Prevention

Fearnley Procter UK Ltd

Standards and Publications

NS-7 Drill String Failure Prevention Supervisor's Course Training Manual

NS-7 Drill String Failure Prevention Supervisor's Course Training Manual A Drill String failure occurs if a component breaks, leaks or cannot perform its intended function. When a failure occurs, we may be inclined to think first of "bad pipe", or that the pipe was misused. Such simple explanations are usually not correct. In fact, many factors can contribute to drill string failures and rarely is a failure caused by a single problem. Drilling conditions can differ greatly from one point in the string to another, even at the same time in the same hole. A dozen different mechanisms can attack the drill string and conditions, that prevent one type of attack may actually accelerate another.

However, drill string failures can be easily understood by grouping their causes into five main categories. No matter what failure mechanism was involved, the causes can be traced to problems or weaknesses in one or more of these areas:

 

This course manual goes into detail on each of the above topics with further sub sections.

Responding to Drill String Failures

If a drill string washout or twistoff occurs, the first step toward preventing a second failure is to understand what caused the first. This section covers how to identify the most common failure mechanisms and, in broad terms, what to do about each.

 

Properties of Drill String Components

Drill string properties are the metallurgical, mechanical and dimensional attributes that are built into each component in the drill string. The need for good properties in the drill string is the most basic one of all. If we start with bad material, all of our careful design, inspection and handling efforts will be wasted. If we want everything to stay together, we must start with "the right stuff".

Minimum Yield Strength: Determines minimum rated capacity in torsion, tension, burst and collapse pressure for a component of a given size

Minimum Tensile Strength: Determines minimum parting load in torsion, tension, burst and collapse for a component of a given size

Maximum Yield Strength: Helps ensure that material isn't too hard and brittle

Minimum Ductility (Elongation): Ensures a minimum amount of plastic stretch after yield but before parting

Minimum Toughness: Ensures a minimum resistance to fatigue crack extension. Ensures that a component can support at least a through wall crack without parting (leak before break)

Drill Pipe Internal Upset Geometry: Determines the stress concentration effect of the change in wall section at the internal upset on a drill pipe tube. This in turn affects the fatigue life of the tube, higher stress = shorter life

Rotary Shouldered Connections

Some of the most confusing things about rotary shouldered connections are their names. What is so internally flush about that 4.1/2 IF connection? Every one we ever saw was internally upset! Also, where is the other hole in an "Extra Hole" (XH) connection? (It has only one hole as far as we can tell). Maybe it makes an extra hole in the ground. But who wants two holes in the ground when one will do nicely? It just runs up the mud bill.

Design

The objectives of the design effort should be:

 

Inspection

How many times have we heard someone say "Give me an API inspection"? It sounds good, but there is simply no such thing as an API drill string inspection. API RP7G gives many useful performance properties for drill pipe and connections. It tells us what the attributes of a "Premium Class" piece of pipe are. It does not however give instructions on how to inspect a piece of pipe to determine whether or not it meets those attributes! Therefore, we must supplement what's available in API. In total, inspecting our drillstring must include all three of the following tasks:

 

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