Drill Pipe: Descriptions and Specifications















The drill pipe can be classified according to many parameters such as length, diameter, weight, upset and grade of steel. There are many factors which can influence drill pipe selection:

- Hole size
- Circulating and drilling parameters
- Casing and cementing requirements
- Hole size
- Hoisting capacity
- Pipe availability

Drill Pipe Size and Weight

The drill pipe can be available in three length ranges:

- Range 1 ( 18-22 ft )
- Range 2 ( 27-30 ft )
- Range 3 ( 38-45 ft )

Range 2 is the most commonly used.


Drill Pipe Strength

The strength of the drill pipe can be defined according to:

- Pipe body yield strength: it can be presented by the resistance of the pipe body to the axial tension
- Collapse Resistance: it is presented by the ability of the drill pipe to resist to the external pressure
- Internal yield pressure: it represents the ability of the pipe to withstand the internal pressure
- Torsional Yield: it is the resistance to the torque

The level of these types of strength is related to the grade of steel used to manufacture the pipe and other parameters such as the drill pipe size and weight per unit of length.


Drill Pipe Upset

The drill pipe can be also categorized according to its upset area ( shape of pipe at the ends). The upset area is thicker than the rest of the pipe to allow for the welding on the tool joints. The upset can be internal, external or internal-external upset. The upset used in the welding area depends on the pipe diameter.
In an internal upset, the required welding area thickness is provided by decreasing the pipe inside diameter at the upset area, the outside diameter remains the same.
In an external upset is got by increasing the pipe outside diameter at the upset area, the inside diameter remains the same.
In an internal-external upset, extra wall thickness at the upset area is reached by decreasing the inside diameter and increasing the outside diameter.


Fig 1- Drill Pipe Upset Types

Tool Joints

Tools joints are the thickest and the strongest part of the drill pipe joints. They can be defined as pin-box connections with rounded threads and they are manufactured separately from the drill pipe body. The tool joints, at later stage, will be welded on the upset area. The most important parameter when designing the tool joint is the torsional yield strength because the most common problem which tool joints can face is the thread damage. The tool joint performance specifications should include the torsional yield strength and the recommended make up torque required for connections which is related to the type, size, outside diameter, inside diameter and the tool joint condition.


Fig 2 - Pin and Box Tool Joints 


There are many types of tool joints such as internal flush (IF), full hole (FH) and slim hole (SH).

The tool joints can be identified by a code contains 5 information stenciled at the pin connection (Example: ZZ 6 70 N E ). The information are represented as follows:

- First marking designates the company which has manufactured the tool joint
- Second marking indicates the month of fabrication of the tool joint
- Third marking indicates the year of fabrication
- Fourth marking is the code of the pipe body manufactureur 
- Fifth marking is the pipe grade code ( example: E, X-95,G-105,S-135 )

FIG 3 - Tool Joint Marking 


Drill Pipe Fatigue failure

The drill pipe while drilling operations is subjected to many forces and stresses and can fail when the stresses are greater at a certain limits. The steel is weaker under dynamic conditions than static conditions but it is capable of resisting to dynamic loading or cycles of stresses for unlimited cycles if the forces are below a certain limit which is called the endurance limit.

Fatigue failure can be defined as the failure which is caused by the cyclic stresses. The fatigue failure happens progressively, it starts like a submicroscopic cracks which increase gradually till becoming visible cracks. The drill pipe failure can be recognized as a circumferential damage. As a rule of thumb, the fatigue strength can be roughly estimated as the half of the tensile strength. The drill pipe during operations is under many types of loading: tension, compression, torsion and bending. The failure can be due to one or a combination of these loadings.

Drilling directional wells can increase the risk of drill pipe failure due to the high level of cyclic stresses applied on it. One portion of drill pipe shifts from tension to compression while rotating the drill string. These cycles of tension-compression and with the contribution of the bending increases the risk of failure.

Drill Pipe and Tool Joints Inspection

The Drill pipe specifications are very important but cannot give us information about the pipe condition and for this reason a physical inspection is crucial in drilling operations. The inspection can be simple as visual inspection to more sophisticated inspection using a sonic and gamma ray tools.

Fatigue is considered as the main source of drill pipe failure. It can be defined as the failure originates from a cyclic or fluctuating stresses at maximum values (less than the tensile strength of the used metal). These stresses are caused by bending, torsion, vibration and tension and can be increased by corrosion and erosion. 

Drill pipe inspection can involve:

- Magnetic particle inspection to locate cracks
- Measurement of wall thickness
- Outside diameter measurement

Tool joints inspection involves:

- Outside diameter measurement
- Checking shoulders and looking for any type of wear
- Checking the thread profile and looking for any over torqued areas and galled threads.



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