Drill String Definition & Components Guide

We can define the drill string as a group of pipes and tools. These components allow us to deliver the rotation, the weight, and the fluids from the surface to the bottom of the drilled hole. It is an important part of the rotary drilling process in oil and gas wells. To understand more about it, we will have to go through two subjects :

  1. Its Components
  2. Its Design

Drill String Functions

The Drill string components serve several purposes and functions, which include the following:

  • The drill string Provides a fluid conduit from the drilling rig (drilling rigs types) to the Drilling Bits.
  • Also, it had another function: imparting rotary motion to the Drilling Bits.
  • Because of the existence of drill collars, it will provide the Weight On Bit (WOB) necessary to drill the hole.
  • Lower and raise the Drilling Bits in the well.
  • Drill String functions are a lot more, but it depends on what you must do. It can allow formation evaluation and testing, but special tools will do this for such applications.


drill string components diagram

Its components fall under three main topics:

  1. Bottom Hole Assembly
  2. Transition pipe, which is often heavy-weight drill pipe (HWDP)
  3. Drill Pipe

Bottom Hole Assembly:

The Bottom Hole Assembly (BHA) is one of the drill string components that consist of a 

  • Drilling Bit, which is used to break up the rock formations; 
  • Drill Collars are heavy enough to apply weight to the drill bit.
  • Drilling Stabilizers, which centralize the drill string in the hole.
  • Reamers are used to enlarge the drilled hole for certain reasons.
  • Jars are used to free any stuck that may happen during drilling.
  • Shock subs are used to reduce the vertical drill string oscillation (bouncing).
  • The hole opener.

The Bottom Hole Assembly may also contain other components such as a downhole mud motor and rotary steerable system (RSS), measurement while drilling (MWD), and logging while drilling (LWD) tools. The components are joined together using rugged threaded connections. Short “subs” are used to connect items with dissimilar threads.

Transition Pipes:


This component is mainly heavy-weight drill pipe (HWDP). In the past, the two main components of the Drill String were the Pipes and the Collars. The transition Zone is the point where the relatively small OD and flexible Drill Pipe connect to the large stiff Drill Collars.

Oilfield studies have shown that almost all Drill Pipe fatigue failures result from an accumulation of fatigue damage occurring when the Drill Pipe joints are run in the Transition Zone or are stressed above the endurance limit in crooked holes. Downhole data has indicated that the large change in diameter at the transition zone caused accelerated fatigue damage due to the concentration of cyclic bending stress reversals in the bottom joints of the flexible Drill Pipe since the stiff Drill Collars bend very little from these stress reversals.

Drill String Component | Drill Pipes:

Generally, the longest component of the Drill String is the D/Ps. Furthermore, each joint of the drill pipe consists of the tube body and the Tool Joint (connection). Drill pipe joints are available in three length ranges:

  • The length of range 1: 18-22 ft
  • The length of range 2: 27-30 ft
  • The length of range 3: 38-45 ft

Drill Pipes Range 2 is the most common.

Tube OD, nominal weight, pipe grade, type of tool joint, thread connection, and classification generally describe drill pipe components. For example, a 5″€ drill pipe can be described as,

5″19.5Grade EXHNC50Premium Tube
ODNominal WeightMinimum Yield StrengthTool Joint UpsetConnection ThreadWear Classification

Each description item is explained in the API DRILL PIPES DESCRIPTION & SPECS Article.

Drilling String Design

Ideally, the drill string components should be designed to deliver sufficient weight to the Bit and provide sufficient torsional and tensile strength to withstand vigorous and dynamic drilling conditions. The components should also withstand burst and collapse pressure loads and be designed to minimize hole stability problems. Many factors must be considered in its design, such as:

If you want more details about the design, you will find it in the DRILL STRING DESIGN Article.  

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